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Earth and Planetary Science Letters
Journal Prestige (SJR): 3.166
Citation Impact (citeScore): 5
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  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 0012-821X
Published by Elsevier Homepage  [3185 journals]
  • Seismic signal from waves on Titan's seas
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Simon C. Stähler, Mark P. Panning, Céline Hadziioannou, Ralph D. Lorenz, Steve Vance, Knut Klingbeil, Sharon Kedar Seismology is the main tool for inferring the deep interior structures of Earth and potentially also of other planetary bodies in the solar system. Terrestrial seismology is influenced by the presence of the ocean-generated microseismic signal, which sets a lower limit on the earthquake detection capabilities but also provides a strong energy source to infer the interior structure on scales from local to continental. Titan is the only other place in the solar system with permanent surface liquids and future lander missions there might carry a seismic package. Therefore, the presence of microseisms would be of great benefit for interior studies, but also for detecting storm-generated waves on the lakes remotely. We estimated the strength of microseismic signals on Titan, based on wind speeds predicted from modeled global circulation models interior structure. We find that storms of more than 2 m/s wind speed, would create a signal that is globally observable with a high-quality broadband sensor and observable to a thousand kilometer distance with a space-ready seismometer, such as the InSight instruments currently operating on the surface of Mars.Graphical abstractGraphical abstract for this article
       
  • Fluorine and chlorine fractionation during magma ocean crystallization:
           Constraints on the origin of the non-chondritic F/Cl ratio of the Earth
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Hideharu Kuwahara, Takanori Kagoshima, Ryoichi Nakada, Nobuhiro Ogawa, Asuka Yamaguchi, Yuji Sano, Tetsuo Irifune Previous studies have reported that the relative abundances of volatile elements in the silicate Earth are non-chondritic. The abundance and distribution of volatile elements in terrestrial planets would have been predominantly controlled by planetary formation processes, including core-mantle separation, magma ocean crystallization, and volatility-dependent high-temperature fractionation. Thus, the current abundance patterns of volatile elements in the silicate fraction of terrestrial planets are the key to understanding the accretional history of terrestrial volatiles and the chemical differentiation of terrestrial planets. Although the origin of the non-chondritic ratios of volatile elements in terrestrial planets has been previously studied, it is still a matter of debate. In this study, we focused on the super-chondritic F/Cl ratio of the bulk silicate Earth and experimentally investigated the silicate mineral-melt partitioning of fluorine and chlorine at pressures from 18 GPa to 25 GPa. Our experimental results show that fluorine is moderately compatible with mantle minerals, whereas chlorine is highly incompatible. These results support the formation of a solid mantle with high F/Cl ratios, and a residual magma ocean and steam atmosphere with low F/Cl ratios during magma ocean crystallization. Thus, the F/Cl ratio in the residual solid parts of terrestrial planets would have become relatively enriched following escape of volatile elements from the planetary surface into outer space. This model is consistent with the collisional erosion hypothesis of primordial crusts and atmospheres, and current observations on the abundance and distribution of terrestrial fluorine and chlorine.
       
  • Asthenospheric flow beneath the Carpathian-Pannonian region: Constraints
           from shear wave splitting analysis
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Wenkai Song, Youqiang Yu, Cong Shen, Fan Lu, Fansheng Kong Systematic shear wave splitting (SWS) study was conducted to illuminate the mantle deformation and dynamics in the Carpathian-Pannonian region (CPR) where its tectonic mechanism is still enigmatic and actively debated. We have obtained a total of 1170 SWS measurements using seismic data recorded by 59 broadband seismic stations. The resulting SWS measurements mostly possess NW-SE fast orientations and are mostly oblique or even orthogonal to the trends of major geological features and the absolute plate motions. Spatial coherency of the splitting parameters reveals that the center of the observed anisotropy is most likely originated from the asthenosphere at the depth of about 250 km. Thus, the uniformly NW-SE fast orientations is more likely to represent an asthenospheric flow, which has partially or completely overprinted the anisotropic fabrics developed during the formation of the Pannonian basin. Such an asthenospheric flow is regionally deflected along the edges of continental keels beneath the South Carpathians and the Vrancea zone where complex anisotropy is well-determined and range-parallel fast orientations are revealed based on two-layered fitting of the SWS measurements. Our results combined with previous SWS measurements suggest that the revealed NW-SE asthenospheric flow is actually a key component of the toroidal flow induced by the Adriatic subduction, which dominantly promotes the current upper mantle deformations beneath the CPR and its adjacent regions.
       
  • Understanding the emplacement of Martian volcanic rocks using petrofabrics
           of the nakhlite meteorites
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Luke Daly, Sandra Piazolo, Martin R. Lee, Sammy Griffin, Peter Chung, Fabrizio Campanale, Benjamin E. Cohen, Lydia J. Hallis, Patrick W. Trimby, Raphael Baumgartner, Lucy V. Forman, Gretchen K. Benedix In order to validate calculated ages of the Martian crust we require precise radiometric dates from igneous rocks where their provenance on the Martian surface is known. Martian meteorites have been dated precisely and quantitatively, but the launch sites are currently unknown. Inferring the formation environment of a correlated suite of Martian meteorites can constrain the nature and complexity of the volcanic system they formed from. The nakhlite meteorites are such a suite of augite-rich rocks that sample the basaltic crust of Mars, and as such can provide unique insights into its volcanic processes. Using electron backscatter diffraction we have determined the shape-preferred and crystallographic-preferred orientation petrofabrics of four nakhlites (Governador Valadares, Lafayette, Miller Range 03346 and Nakhla) in order to understand the conditions under which their parent rocks formed. In all samples, there is a clear link between the shape-preferred orientation (SPO) and crystallographic-preferred orientation (CPO) of augite phenocrysts. This relationship reveals the three-dimensional shape of the augite crystals using CPO as a proxy for 3D SPO, and also enables a quantitative 3-dimensional petrofabric analysis. All four nakhlites exhibit a foliation defined by the CPO of the augite axis in a plane, although individual meteorites show subtle textural variations. Nakhla and Governador Valadares display a weak CPO lineation within their axis foliation that is interpreted to have developed in a combined pure shear/simple shear flow regime, indicative of emplacement of their parent rock as a subaerial hyperbolic lava flow. By contrast, the foliation dominated CPO petrofabrics of Lafayette and Miller Range 03346 suggest formation in a pure shear dominated regime with little influence of hyperbolic flow. These CPO petrofabrics are indicative of crystal settling in the stagnant portion of cooling magma bodies, or the flattening area of spreading lava flows. The CPO foliation of Lafayette's is substantially weaker than Miller Range 03346, probably due to its higher phenocryst density causing grain-grain interactions that hindered fabric development. The CPO petrofabrics identified can also be used to determine the approximate plane of the Martian surface and the line of magma flow to within ∼20°. Our results suggest that the nakhlite launch crater sampled a complex volcanic edifice that was supplied by at least three distinct magmatic systems limiting the possible locations these rocks could have originated from on Mars.Graphical abstractGraphical abstract for this article
       
  • Incipient subduction at the contact with stretched continental crust: The
           Puysegur Trench
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Michael Gurnis, Harm Van Avendonk, Sean P.S. Gulick, Joann Stock, Rupert Sutherland, Erin Hightower, Brandon Shuck, Jiten Patel, Ethan Williams, Dominik Kardell, Erich Herzig, Benjamin Idini, Kenny Graham, Justin Estep, Luke Carrington A seismic Benioff zone and plate kinematics show Puysegur Trench south of New Zealand transitioning to subduction. Because the local structure and its influence on subduction initiation is poorly understood, we conducted a seismic survey with ocean bottom seismometers and multichannel seismic profiles. Our early results show that the overriding Pacific Plate beneath the Solander Basin is composed of block-faulted and thinned continental crust, and the inner trench wall of northern Puysegur Ridge is composed of folded and faulted sediment. The megathrust interface has been imaged and shows ∼500 m of downgoing, undisturbed sediments. Combining plate kinematic history with seismic velocity-inferred density, we show that the density difference across the plate boundary changed as oblique strike-slip plate motion juxtaposed dense oceanic crust with thinned continental crust. The density difference rapidly increased 18 to 15 Ma, coincident with subduction initiation, suggesting that compositional differences have a large influence on subduction initiation.
       
  • Magmatic evolution biases basaltic records of mantle chemistry towards
           melts from recycled sources
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): David A. Neave, Olivier Namur, Oliver Shorttle, François Holtz The chemistry of erupted magmas provides a crucial window into the composition and structure of Earth's convecting mantle. However, magmatic evolution in the crust makes it challenging to reconstruct mantle properties from volcanic rocks in important but incompletely understood ways. Here we investigate how mantle-derived compositional variability in primary oceanic basalts determines their phase equilibria relations and the nature of the geochemical signals they record. By performing experiments on synthetic analogues of compositionally extreme primitive lavas from the Reykjanes Peninsula of Iceland at realistic magma storage conditions (300 MPa, 1140–1260 °C), we show that melts from enriched mantle domains retain higher melt fractions as they cool than those generated by melting of typical fertile lherzolite (i.e. they crystallise less mass over any interval of decreasing temperature). These melt fraction differences arise because plagioclase crystallisation is suppressed in Na- and H2O-rich but Ca- and Al-poor liquids derived from enriched source lithologies. Thus, compositional characteristics inherited from the mantle have a first-order control on the efficiency with which cooling basalts crystallise. This means that enriched melts will be more likely to survive crustal processing than depleted melts. Basalt chemistry will therefore be disproportionately influenced by melts from volumetrically minor enriched lithologies compared with melts from the upper mantle's most common lithology, lherzolite, systematically biasing basaltic records towards melts from recycled mantle sources.We combine our experimental observations from Iceland with thermodynamic simulations on mid-ocean ridge basalt compositions and show that mantle-derived variability in crystallisation efficiency can explain two enigmatic features of the global oceanic basalt record: firstly, the anomalous over-enrichment of incompatible elements during the differentiation of mid-ocean ridge basalts, which may reflect a progressive bias towards enriched compositions as differentiation proceeds; and secondly, the frequently documented cargoes of highly anorthitic plagioclase crystals carried by evolved and enriched liquids from which they cannot have crystallised. These crystals can now be understood as the solidified remnants of depleted, lherzolite-derived melts that have been entrained into melt mixtures from more enriched sources. Increases in the degree of enrichment of cumulate rocks sampled from progressively shallower horizons of the oceanic crust can also be interpreted in terms of enriched melts surviving crustal processing in preference to depleted melts.
       
  • Effect of spin transition of iron on the thermal conductivity of (Fe,
           Al)-bearing bridgmanite
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Yoshiyuki Okuda, Kenji Ohta, Ryosuke Sinmyo, Kei Hirose, Takashi Yagi, Yasuo Ohishi Thermal conductivity of bridgmanite (Bdg) is the important physical property controlling the heat transfer inside the Earth. Here we report room temperature lattice thermal conductivity of (Fe,Al)-bearing Bdg with chemical compositions of Mg0.848Fe0.090Al0.206Si0.856O3 and Mg0.718Fe0.123Al0.281Si0.878O3 measured up to 125 GPa and 74 GPa, respectively, using the pulsed light heating thermoreflectance technique in a diamond anvil cell. We found that the lattice thermal conductivity of these Bdg samples show abnormal reduction in the pressure range of 20-40 GPa at 300 K, which is probably due to the spin transition of Fe3+ in octahedral Si-site (B-site). We propose that the lattice thermal conductivity of Bdg is reduced by 46 ± 16% when Fe is in the mixed spin state, which may form a thermal insulating layer in the Earth's mid lower mantle. In addition, we provide a thermal conductivity model of Bdg, taking into account the effect of compositional difference and the spin transition of Fe. Our conductivity model indicates that the thermal conductivity of Bdg in the pyrolitic lower mantle is more than twice as high as that in the descending MORB, which is likely to create heterogeneity of lateral heat flux through the Earth's core-mantle boundary.
       
  • Highly siderophile element geochemistry and Re–Os isotopic systematics
           of carbonatites: Insights from Tamil Nadu, India
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Lukáš Ackerman, Ladislav Polák, Tomáš Magna, Vladislav Rapprich, Jana Ďurišová, Dewashish Upadhyay Carbonatite metasomatism has been widely implicated for worldwide mafic mantle suites but so far, no combined data have been available for highly siderophile element systematics (HSE – Os, Ir, Ru, Pt, Pd, Re) and Re–Os isotopic compositions in carbonatites themselves. We present the first systematic survey of the HSE and Re–Os isotopic compositions in a suite of well-characterized Neoproterozoic carbonatites, silicocarbonatites and associated silicate rocks (pyroxenites, monzogabbros, syenites) from south India in order to place constraints on the HSE systematics in carbonatite magmas, anchoring possible mantle sources of carbonatites and relationship to the ambient crustal lithologies as well as preliminary constraints on carbonatite metasomatism in Earth's mantle. The most plausible explanation for generally low HSE contents in calciocarbonatites from Tamil Nadu (∑HSE < 1.22 ppb) involves a low-degree (
       
  • Carbon storage in Fe-Ni-S liquids in the deep upper mantle and its
           relation to diamond and Fe-Ni alloy precipitation
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Zhou Zhang, Tian Qin, Anne Pommier, Marc M. Hirschmann To better understand the role of sulfide in C storage in the upper mantle, we construct a thermodynamic model for Fe-Ni-S-C sulfide melts and consider equilibrium between sulfide melts, mantle silicates, Fe-Ni alloy, and diamond. The sulfide melt model is based upon previous parameterization of Fe-Ni-S melts calibrated at 100 kPa, which we have extended to high pressure based on volumetric properties of end-member components. We calculate the behavior of C in the sulfide melt from empirical parameterization of experimental C solubility data. We calculate the continuous compositional evolution of Fe-Ni sulfide liquid and associated effects on carbon storage at pressure and redox conditions corresponding to mantle depths of 60 to 410 km. Equilibrium and mass balance conditions were solved for coexisting Fe-Ni-S melt and silicate minerals (olivine [(Mg,Fe,Ni)2SiO4], pyroxene [(Mg,Fe)SiO3]) in a mantle with 200 ppmw S. With increasing depth and decreasing oxygen fugacity (fO2), the calculated melt (Fe+Ni)/S atomic ratio increases from 0.8–1.5 in the shallow oxidized mantle to 2.0–10.5 in the reduced deep upper mantle (>8 GPa), with Fe-Ni alloy saturation occurring at>10 GPa. Compared to previous calculations for the reduced deep upper mantle, alloy saturation occurs at greater depth owing to the capacity of sulfide melt to dissolve metal species, thereby attenuating the rise of Fe and Ni metal activities. The corresponding carbon storage capacity in the metal-rich sulfide liquid rises from negligible below 6 GPa to 8-20 ppmw at 9 GPa, and thence increases sharply to 90-110 ppmw at the point of alloy saturation at 10-12 GPa. The combined C storage capacity of liquid and solid alloy reaches 110-170 ppmw at 14 GPa. Thus, in the deep upper mantle, all carbon in depleted sources (10–30 ppmw C) can be stored in the sulfide liquid, and alloy and sulfide liquids host a significant fraction of the C in enriched sources (30–500 ppmw C). Application of these results to the occurrences of inferred metal-rich sulfide melts in the Fe-Ni-S-C system and inclusions in diamonds from the mantle transition zone suggests that oxidization of a reduced metal-rich sulfide melt is an efficient mechanism for deep-mantle diamond precipitation, owing to the strong effect of (Fe+Ni)/S ratio on carbon solubility in Fe-Ni-S melts. This redox reaction likely occurs near the boundary between oxidized subducted slabs and the reduced ambient peridotitic mantle.
       
  • Radial thermo-chemical structure beneath Western and Northern Pacific from
           seismic waveform inversion
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Frédéric Deschamps, Kensuke Konishi, Nobuaki Fuji, Laura Cobden The Earth's deep mantle seismic structure is dominated by two large low shear-wave velocity provinces (LLSVPs) located beneath Africa and the Pacific. While the existence of these structures has been attested by many studies and data sets, their detailed nature, purely thermal or thermo-chemical, is still a matter of debate. Discriminating between these hypotheses requires constraints independent from seismic velocity structure. Seismic shear-wave attenuation, measured by the quality factor QS, strongly depends on temperature but not (or weakly) on composition. It may bring key information on temperature, resolving in turn the trade-off between temperature and composition. Here, we invert seismic waveform data for radial models of shear-wave velocity anomalies (dlnVS), and QS at two different locations in the Pacific, and from a depth of 2000 km down to the core-mantle boundary (CMB). We show that Western Pacific (WP) models, sampling the western tip of the Pacific LLSVP and the Caroline plume, cannot be explained by thermal anomalies alone, but require excess in iron of ∼4.0% from the CMB up to 2600 km. By contrast, Northern Pacific models (NP), if unaffected by seismic anisotropy, may have a purely thermal origin. Based on these observations, we build radial thermo-chemical models at WP and NP.
       
  • Spectral behavior of sulfides in simulated daytime surface conditions of
           Mercury: Supporting past (MESSENGER) and future missions (BepiColombo)
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): I. Varatharajan, A. Maturilli, J. Helbert, G. Alemanno, H. Hiesinger To detect the mineral diversity of a planet's surface, it is essential to study the spectral variations over a broad wavelength range at relevant simulated laboratory conditions. The MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) mission to Mercury discovered that irrespective of its formation closest to the Sun, Mercury is richer in volatiles than previously expected. This is especially true for sulfur (S), with an average abundance of 4 wt%. It has been proposed that sulfur in the interior of Mercury can be brought to the surface through volcanic activity in the form of sulfides as slag deposits in Mercury hollows and pyroclastic deposits. However, comprehensive spectral library of sulfide minerals measured under vacuum conditions in a wide spectral range (0.2–100 μm) was lacking. This affects the detectability and understanding of the distribution, abundance, and type of sulfides on Mercury using remote-sensing spectral observations. In the case of Mercury, the effect of thermal weathering affecting the spectral behavior of these sulfides must be studied carefully for their effective detection. In this study, we present a spectral library of synthetic sulfides including MgS, FeS, CaS, CrS, TiS, NaS, and MnS. For each sample, we performed emissivity measurements in the thermal infrared range (TIR: ∼7–14 μm) for sample temperatures from 100 °C–500 °C, covering the daytime temperature cycle on Mercury's surface. In addition, for each sample we measured the spectral reflectance of fresh and thermally processed sulfides over a wide spectral range (0.2–100 μm) and at four different phase angles, 26°, 40°, 60°, 80°. This spectral library facilitates the detection of sulfides by past and future missions to Mercury by any optical spectrometer of any spectral range. Specifically, the emissivity measurements in this study will support the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS) instrument on the ESA/JAXA BepiColombo mission, which will study the surface mineralogy over a wavelength range of 7–14 μm at a spatial resolution of 500 m/pixel. The measured reflectance of these sulfides in 0.2–100 μm at various phase angles will support the interpretation of measurements from past (MDIS, MASCS on MESSENGER) and future missions (SIMBIO-SYS on BepiColombo).
       
  • The geochemistry of Tibetan lavas: Spatial and temporal relationships,
           tectonic links and geodynamic implications
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Petr V. Yakovlev, Alberto Saal, Marin K. Clark, Chang Hong, Nathan A. Niemi, Soumen Mallick The continental lithosphere is created and modified at convergent plate boundaries through complex, and interdependent, tectonic and magmatic processes. Evolution of the continental lithosphere modifies conditions within the mantle wedge, and therefore, the generation and composition of magmas at subduction zones. One approach to unraveling interactions between the continental lithosphere and asthenosphere in subduction zones is through the evolution of pre-, syn- and post-collisional mafic volcanism (i.e., MgO ≥ 6 wt%). Such lavas are most likely to contain characteristic geochemical fingerprints of the composition and physical condition of the mantle source at the time of melting. Here, we combine new analyses of thirty-one samples from the Hoh Xil Basin of the northern Tibetan Plateau with a compilation of published geochemical and geochronological data for late Mesozoic and Cenozoic magmas from the Indo-Eurasian collisional orogen. Geochronology of the Hoh Xil lavas reveals unrecognized early (∼27 Ma) alkali basaltic magmatism in northern Tibet that likely triggered melting of the mafic lower and metasedimentary upper continental crust, generating trachyanadesites-trachydacites and rhyolites, respectively. Based on a combined analysis of new and published compositional, age and spatial patterns of mafic lavas (MgO ≥ 6 wt%), we delineate two distinct periods of Tibetan mafic magmatism, with a major compositional change occurring at roughly 46 Ma. Early (pre-46 Ma) mafic lavas erupted only in central and southern Tibet, and were generated by melting within the mantle wedge during the northward subduction of the Neo-Tethyan oceanic slab. Late (post-46 Ma) mafic lavas were produced by the partial melting of metasomatized subcontinental lithospheric mantle. The latter group is spatially restricted to central Tibet between 46-30 Ma. After 30 Ma, lavas derived from melting of the lithospheric mantle expanded both northward and southward, eventually encompassing the entire Tibetan Plateau by 20 Ma. These observations suggest that melting of the lithospheric mantle beneath Tibet initiated earlier, and was more widespread, than previously thought. Our results also suggest that widespread melting of the mantle lithosphere across Tibet post-dates the cessation of upper crustal shortening in northern Tibet at ∼30 Ma, and precedes the initiation of crustal extension at ∼15-10 Ma. Hence, we suggest that plateau-wide melting of metasomatized subcontinental mantle lithosphere initiated following the collision of India with Eurasia, at ∼46 Ma. This melting was induced by detachment of the Neo-Tethyan slab concomitant with thinning of the continental mantle lithosphere. Such melting has continued until the present, with the exception of volcanic quiescence in south-central Tibet since ∼10 Ma. This quiescence likely arose from the underthrusting of Indian lithosphere beneath Eurasia. Although melting of the mantle lithosphere beneath Asia has previously been interpreted as an abrupt event, we propose that mantle lithosphere thinning and removal has been part of a continuum melting process during the Indo-Asian orogen that reflects an evolution from mantle wedge to mantle lithosphere melting accompanying the transition from subduction to collision.
       
  • Molybdenum and boron isotope evidence for fluid-fluxed melting of
           intraplate upper mantle beneath the eastern North China Craton
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Hong-Yan Li, Jie Li, Jeffrey G. Ryan, Xiang Li, Rui-Peng Zhao, Liang Ma, Yi-Gang Xu Fluid-fluxed melting of the mantle is the principal mechanism for the generation of arc magmas above subduction zones, but it has rarely been documented as important in intraplate settings. Here, we present new molybdenum (Mo) isotopic data from a suite of well-characterized Cenozoic basalts from the eastern North China Craton (NCC) to constrain the mechanisms of mantle melting in the region. These basalts represent mixtures of three components, namely, nephelinites and basanites that variably mixed with alkali basalts. The alkali basalts have relatively low δ98/95Mo ranging from −0.56‰ to −0.22‰ relative to the NIST3134 standard, suggesting the presence of dehydrated pelagic sediments in their mantle source rocks. Interestingly, the basanites range to higher δ98/95Mo (−0.04 ± 0.02‰) compared with the nephelinites (−0.31‰ to −0.22‰) with no associated changes in their radiogenic isotopes, and they show trends of decreasing Dy/Yb, Ce/Mo and δ11B with increasing Ba/Nb and B/Nb ratios. These correlations indicate that Mo behaves as a fluid-mobile trace element in the intraplate mantle beneath the NCC, and that the mantle sources of the nephelinites and basanites were infiltrated by this fluid component. Existing data show that this fluid component has lower H2O/Ce ratios (110–130) than that of mid-ocean ridge basalts (H2O/Ce ≈ 200). This result, together with the low-SiO2 contents, relative depletion of Zr–Hf–Ti, and high δ66Zn characteristics of these strongly alkaline rocks, suggests derivation from a carbonated mantle source. Mo and B isotope systematics thus reveal carbonated fluid-fluxed mantle melting occurred beneath the eastern NCC. Increasing fluid inputs led to increases in the degree of melting of the mantle source that generated melts ranging in composition from nephelinite through basanite. The origin of such a fluid flux is enigmatic in an intraplate setting, but could be related to the decarbonization of subducted slabs stalled in the deep mantle.
       
  • Dating of tsunami boulders from Ishigaki Island, Japan, with a modified
           viscous remanent magnetization approach
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Tetsuro Sato, Norihiro Nakamura, Kazuhisa Goto, Yuho Kumagai, Hiroyuki Nagahama, Koji Minoura, Xiang Zhao, David Heslop, Andrew P. Roberts Reworked boulders are expected to acquire a viscous remanent magnetization (VRM) approximately parallel to Earth's modern magnetic field. The magnitude of such a VRM depends on several factors, including the time since reworking and ambient temperature, for which there are well-known theoretical relationships. VRM unblocking temperature can, therefore, be a powerful tool for determining the reworking age of boulders and can be used to assess the timing of geological hazards such as landslides and tsunami events. In this study, VRM unblocking temperatures for twenty-seven samples from four coral tsunami boulders on Ishigaki Island, Japan, are compared with three candidate time–temperature relationships for VRM acquisition. For the Pullaiah nomogram, which is applicable to single-domain magnetite particle assemblages, nineteen samples from four boulders agree well with the expected unblocking temperature derived from previously reported 14C ages. Two samples have low unblocking temperatures and six samples appear to have anomalously high VRM unblocking. The Walton nomogram, which is used for lognormal grain-size distributions, relates acquisition temperatures required to produce equal magnetic (paleo)intensities and cannot explain the high unblocking temperatures; it produces younger predicted ages than the youngest boulder 14C age. We find that an alternative time–temperature relationship, defined by a stretched exponential law, has the potential to yield reworking ages for the anomalous boulders that are consistent with the 14C ages. We suggest that future VRM dating can be undertaken using a combination of the Pullaiah nomogram and the stretched exponential law.
       
  • Enigmatic upper-plate sliver transport paused by megathrust earthquake and
           afterslip
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): T.E. Hobbs, A.V. Newman, M. Protti How does deformation after an earthquake affect megathrust stresses? Five years of surface velocities following the 2012 moment magnitude 7.6 Nicoya, Costa Rica earthquake uniquely capture the lithospheric recovery. During a four-year period, seaward afterslip transitions to relocked, landward interseismic motion that matches the velocity field seen before the 2012 event. Locking reinitiates temporarily but is interrupted by late 2014 and is followed soon by a period of never-before-described, exclusively trench-parallel motion associated with a slow slip event and no resolvable megathrust locking. We present a conceptual model in which low postseismic megathrust coupling (little locking) generates partitioned slip: trench-normal motion on the megathrust during afterslip and trench-parallel motion during this never before seen transient. High coupling (strong locking) during the interseismic period drives oblique, convergent surface motions. This challenges the paradigm that megathrusts are either always partitioned or always oblique, contradicts the tectonic escape hypothesis in Central American, and introduces a new time-dependent megathrust fault behavior. Given that most subduction zones are oblique, explaining these observations is critical to characterizing stress accumulation.Graphical abstractGraphical abstract for this article
       
  • Glacier melt runoff controls bedload transport in Alpine catchments
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): F. Comiti, L. Mao, D. Penna, A. Dell'Agnese, M. Engel, S. Rathburn, M. Cavalli Research on factors affecting sediment regime in glacierized catchments under warming climates is still scarce despite its societal relevance. In particular, coarse bedload transport has never been quantitatively related to water runoff origin (snowmelt vs glacier melt), which provides important information on the role of different sediment sources (glaciers vs hillslopes and channel bed). Drawing on data from multiple spatial and temporal scales in a paradigmatic Alpine glacierized catchment, we show that glacier melt flows play a key role in coarse sediment transport dynamics. Bedload concentration measured during glacier melt flows is up to 6 orders of magnitude larger than during snowmelt. At the catchment scale and within the channel, however, minimal aggradation and degradation was detected over almost a decade. In addition, sedimentation rates at a hydropower weir, inferred from flushing frequency during the last four decades, are tightly associated to summer air temperature and not to precipitation trends, and most of sediment export occurred in July-August. However, sediment flushing frequency has been decreasing since the late 1990s despite very warm summers in the following decades. Collectively, these findings indicate that sediment is dominantly sourced from within glacier-covered areas and that transport rates are thus dictated by seasonal and multi-annual glacial dynamics. As glacier melt flows decrease due to ice mass loss, our results suggest that, for similar basins, a progressive shift from supply-limited (driven by glacier activity) to transport-limited (during rainfall-induced events) sediment transport will occur, disrupting the current near-equilibrium channel conditions.
       
  • Large true polar wander in a sea level model with application to the
           Neoproterozoic snowball Earth events
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Yonggang Liu A sea level model which is normally used to calculate sea level changes during glacial-interglacial cycles is modified to solve the approximate nonlinear Liouvelle equation, in order to calculate ‘large’ true polar wander (TPW) that may be induced by ice sheet loading. The purpose is to understand when the TPW will be too large to be solved properly by the linear model, and to properly calculate the TPW induced by snowball Earth events. It is found that the relative error for TPW calculated with a linear model will be>10% when the TPW exceeds ∼2° for ice sheets that develop near the poles, but remains
       
  • Age constraints of Mercury's polar deposits suggest recent delivery of ice
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Ariel N. Deutsch, James W. Head, Gregory A. Neumann Surface ice at the poles of Mercury appears as several-m-thick deposits that are composed of nearly pure water. We provide new age estimates of the surfaces of Mercury's polar deposits from combined analyses of Poisson statistics and direct observations of crater densities within permanently shadowed, radar-bright regions imaged by the MESSENGER spacecraft. These age estimates conservatively suggest that ice was delivered to Mercury within the last ∼330 Myr. The geologically young ages suggest that the surfaces have been recently refreshed, and this may be accomplished by the delivery of ice in a young impactor or impactors. A single, recent impactor is more consistent with the relative purity of the ice, as suggested by the Earth-based radar observations. In contrast to ice on Mercury, observations of the lunar poles are suggestive of a highly patchy distribution of surface frost. The patchiness of lunar polar deposits is consistent with long exposure times to the space weathering environment. Given enough time, the polar deposits on Mercury may age into a more heterogeneous spatial distribution, similar to that on the Moon.
       
  • Kelp DNA records late Holocene paleoseismic uplift of coastline,
           southeastern New Zealand
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Elahe Parvizi, Dave Craw, Jonathan M. Waters Holocene paleoseismic activity on the Akatore Fault zone, southeastern New Zealand, has caused uplift of a 23 km section of coastline by several metres. Prominent relict shoreline terraces are preserved at 6 m and 3 m above the present sea level, and the latter terrace was formed 1000-1400 yrs BP. The main fault strand farther inland has 6 m of late Holocene vertical offset, but the relationships between coastal offsets and fault offsets are not understood. There is no preserved geological evidence on the coastline to distinguish between incremental uplift (e.g., numerous centimetre-scale events) and major, metre-scale, uplift events: a distinction that is important for evaluating regional paleoseismicity. We have used genetic characterisation of populations of live kelp, Durvillaea antarctica growing along the shoreline to investigate whether or not there has been a catastrophic uplift event, greater than the two metre tidal range, that was sufficient to extirpate intertidal kelp populations. Our results show that all kelp along the southeastern New Zealand coastline belongs to the same distinctive clade of D. antarctica, and inhabits the same ecological niches on exposed rocks and reefs. However, the kelp population on the uplifted coastline is genetically uniform, and clearly distinct from those of flanking shorelines. This regional genetic anomaly is consistent with a major extirpation event, followed by recolonisation from source populations some 30-120 km to the northeast of the uplifted coast. The uplift event caused vertical crustal movement with similar amounts of uplift on the fault plane and the coastline 3 km away. This approach, using genetics of intertidal biota, has potential applications for evaluation of paleoseismicity of other tectonically active shorelines around the world.Graphical abstractGraphical abstract for this article
       
  • Earthquake rupture dependence on hypocentral location along the Nicoya
           Peninsula subduction megathrust
    • Abstract: Publication date: 15 August 2019Source: Earth and Planetary Science Letters, Volume 520Author(s): Hongfeng Yang, Suli Yao, Bing He, Andrew V. Newman Although interseismic locking distributions have been used in qualitatively evaluating the future earthquake potential, quantitatively estimating how an earthquake may rupture through the locked interface is a more useful tool for quantifying both seismic and tsunami hazards. Here, we investigate rupture scenarios from interseismic locking models along the megathrust interface below Nicoya Peninsula, Costa Rica using spontaneous rupture simulations. We first estimate initial stress from locking, then initiate spontaneous ruptures at different nucleation points and observe the eventual earthquake magnitudes and slip distribution. We find that ∼40% of nucleations tested develop into large earthquakes of Mw>7.2 based on present interseismic locking models. Of these events, those nucleated from deeper depths have a tendency for larger-amplitude shallow slip, suggesting increased tsunami potential. Furthermore, irrespective of the input locking models we do not observe rupture scenarios of earthquakes with intermediate magnitudes between 6 and 7, a result consistent with observations in Nicoya. The results of hypocentre-dependent earthquake magnitudes and tsunamigenic potential not only pose challenges in estimating rupture extents from locking models, but also underscore the significance of quantitatively evaluating seismic and tsunami hazard in subduction zones.
       
  • Spontaneous generation of ductile shear zones by thermal softening:
           Localization criterion, 1D to 3D modelling and application to the
           lithosphere
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Dániel Kiss, Yuri Podladchikov, Thibault Duretz, Stefan M. Schmalholz The generation of ductile shear zones is essential for the formation of tectonic plate boundaries, such as subduction or strike-slip zones. However, the primary mechanism of ductile strain localization is still contentious. We study here the spontaneous generation of ductile shear zones by thermal softening using thermo-mechanical numerical simulations for linear and power-law viscous flow in one-dimension (1D), 2D and 3D. All models are velocity-driven. The 1D model exhibits bulk simple shear whereas the 2D and 3D models exhibit bulk pure shear. The initial conditions include a small temperature perturbation in otherwise homogeneous material. We use a series of 1D simulations to determine a new analytical formula which predicts the temperature evolution inside the shear zone. This temperature prediction requires knowledge of only the boundary velocity, flow law and thermal parameters, but no a priori information about the shear zone itself, such as thickness, stress and strain rate. The prediction is valid for 1D, 2D and 3D shear zones in bulk pure and simple shear. The results show that shear heating dominates over conductive cooling if the relative temperature increase is>50 °C. The temperature variation induced by the shear zone is nearly one order of magnitude wider than the corresponding finite strain variation so that no significant temperature variation occurs between shear zone and wall rock. Applying typical flow laws for lithospheric rocks shows that shear zone generation by thermal softening occurs for typical plate tectonic velocities of few cm.yr−1 or strain rates between 10−16 and 10−14 s−1. Shear stresses larger than 200 MPa can already cause strain localization. The results indicate that thermal softening is a feasible mechanism for spontaneous ductile shear zone generation in the lithosphere and may be one of the primary mechanisms of lithospheric strain localization.
       
  • Evidence for seasonality in early Eocene high latitude sea-surface
           temperatures
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Andrew Davies, Stephen J. Hunter, Benjamin Gréselle, Alan M. Haywood, Chris Robson Specific challenges still exist in our understanding of past greenhouse climate states. Whilst climate model simulations using atmospheric CO2 concentrations consistent with proxy estimates broadly align with lower latitude proxy temperature estimates, they struggle to reproduce the warming implied by proxies at higher latitudes, especially in the marine realm. This inconsistency has often led to the conclusion that climate models are insufficiently sensitive. Here, we analyse the distribution of photozoan and heterozoan carbonates, which provide important constraints for latitudinal sea surface temperature (SST) gradients, to assess data/model mismatches for the early Eocene Climatic Optimum. The carbonate facies distribution is compared against quantitative geochemical proxy temperature estimates (δ18O, Mg/Ca, clumped isotopes and TEX86) and a new HadCM3L climate simulation. Good correspondence exists between the simulated cold-month SSTs and photozoan carbonates, indicating HadCM3L is effectively reconstructing meridional temperature gradients into mid-latitudes. Whilst there is good agreement between simulated mean annual SSTs and geochemical proxy estimates in low latitudes, the δ18O, Mg/Ca and TEX86 estimates instead align with warm-month SSTs at higher latitudes. In light of the carbonate facies evidence, and consistency between our simulation and available terrestrial proxy temperature estimates, this study supports previous claims that a warm season bias exists in many middle and high latitude SST estimates. This helps resolve the discrepancy between climate simulations and marine proxies and shows that climate models and data might be more closely aligned than is appreciated. Further, we demonstrate that simple, and widely available, proxies can play a fundamental role in contextualising wider paleoclimate uncertainties.
       
  • Thin lithosphere beneath the central Appalachian Mountains: Constraints
           from seismic attenuation beneath the MAGIC array
    • Abstract: Publication date: Available online 31 May 2019Source: Earth and Planetary Science LettersAuthor(s): Joseph S. Byrnes, Maximiliano Bezada, Maureen D. Long, Margaret H. Benoit The passive margin of the eastern coast of the United States is known to be geologically active, with recently rejuvenated topography, intraplate seismicity, and volcanism of Eocene age. This study uses seismic data from the Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment to constrain lateral variations in the attenuation of teleseismic P waves beneath the central Appalachian Mountains to shed light on the structure and dynamics of the upper mantle at this “active” passive margin. We use a Monte Carlo approach to estimate variations in attenuation along with both data and model uncertainties. The quality factor of the upper mantle dramatically decreases over a distance of less than 50 km on the western side of the central Appalachian Mountains, where a low-velocity anomaly has been previously inferred. Extrinsic factors such as scattering or focusing are rejected as explanations for the observations on the basis of finite-difference waveform modeling experiments. The peak in attenuation beneath the crest of the Appalachian Mountains requires that near- to super-solidus conditions occur in the upper mantle and is co-located with volcanism of Eocene age. Our preferred interpretation is that the attenuation reflects the removal of the mantle lithosphere via delamination beneath the mountains, followed by ongoing small-scale convection.
       
  • Beyond Byerlee friction, weak faults and implications for slip behavior
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): C. Collettini, T. Tesei, M.M. Scuderi, B.M. Carpenter, C. Viti Some faults are considered strong because their strength is consistent with the Coulomb criterion under Byerlee's friction, 0.6
       
  • Reactive fluid infiltration along fractures: Textural observations coupled
           to in-situ isotopic analyses
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Florence Bégué, Lukas P. Baumgartner, Anne-Sophie Bouvier, Martin Robyr Fluid-rock interaction occurred in hydrothermal veins in dolomitic wall pendants during cooling of the Bruffione granodiorite (Adamello batholith, Italy). These veins formed through fluid infiltration along a central fracture, and the fluid reacted with the dolomite to precipitate forsterite + calcite in cm-wide replacement zones through SiO2 metasomatism. Textural observations have been coupled to in-situ stable isotope and chemical analysis of dolomite, calcite and forsterite to show that stable isotope exchange occurs only through mineral reaction and recrystallization. We identified two distinct reaction regimes; (1) a regime dominated by net transfer reactions, which is characterized by the complete dissolution of dolomite and the formation of the new vein minerals (i.e. forsterite + calcite) close to the open fracture. The driver of the reaction is the large chemical disequilibrium introduced by infiltration of fluid of magmatic origin. And (2) a regime dominated by the dissolution and reprecipitation of dolomite more distal to the central fracture. The dolomite protolith is partially dissolved along grain boundaries, but also trough the grains which are being fragmented, and reprecipitation of newer generations of dolomite rims occur. A progressive change in fluid composition is detected with increasing distance from the vein. In comparison to the first regime, the system is rock-dominated, and only small amounts of fluids and small chemical disequilibrium at the trace element level are suggested to be the drivers for this reaction. A fundamental understanding of mineral reactions in the presence of fluids is necessary for many areas of science and industry, and recrystallization reactions driven by very small chemical disequilibrium as described here are important to be identified and integrated into reactive transport models. In fact, for this case study, the fluid-rock interaction volume would be largely underestimated if only the net transfer reaction zone is considered. This could be crucial for safety evaluations in technical applications.
       
  • On spreading modes and magma supply at slow and ultraslow mid-ocean ridges
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Mathilde Cannat, Daniel Sauter, Luc Lavier, Manon Bickert, Ekéabino Momoh, Sylvie Leroy The ultraslow eastern Southwest Indian Ridge (SWIR) offers an opportunity to study the effect of magma supply on an ultraslow mid-ocean ridge starting from quasi-melt-free detachment-dominated spreading, and transitioning to volcanic spreading as one nears prominent axial volcanos. Detachments in the quasi-melt-free mode extend along-axis 60 to 95 km and have a lifetime of 0.6 to 1.5 myrs. They cut into their predecessor's footwall with an opposite polarity, causing part of the footwall lithosphere to experience further deformation, hydrothermal alteration, sparse magmatism and possibly thermal rejuvenation, in a hanging wall position. The accretion of the oceanic lithosphere in this context therefore occurs in two distinct stages over the lifetime of two successive detachment faults. We examine the transition from this nearly amagmatic detachment-dominated mode to the more common volcanic mode of spreading, showing that it occurs along-axis over distances ≤30 km. It involves a significant thinning of the axial lithosphere and a gradual decrease of the amount of tectonic displacement on faults, as the magmatic contribution to the divergence of the two plates increases. We develop a conceptual model of this transition, in which magma plays a double role: it fills the space between the diverging plates, thus reducing the need for displacement along faults, and it modifies the thermal state and the rheology of the plate boundary, affecting its thickness and its tectonic response to plate divergence. Based on a comparison of the ultraslow eastern SWIR, with the faster spreading Mid-Atlantic Ridge, we show that the activation of the volcanic, or of the detachment-dominated modes of spreading is connected with the volume of magma supplied per increment of plate separation, over a range of axial lithosphere thickness, and therefore over a range of the M ratio defined by (Buck et al., 2005) as the relative contribution of magma and faults to plate divergence (M is smaller, for a given volume of melt per increment of plate separation, if the plate is thicker). We therefore propose that M does not fully explain the variability in faulting styles observed at slow and ultraslow ridges and propose that rheological changes induced by magma also play a key role (melt itself is weak, hydrothermally altered gabbro-peridotite mixtures are weak, and melt heat sustains more vigorous hydrothermal circulation), resulting in contrasted potentials for strain localization, footwall flexure on faults and the development of detachment faults.
       
  • Thermodynamic constraints on carbonate stability and carbon volatility
           during subduction
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): J.S. Gorce, M.J. Caddick, R.J. Bodnar The breakdown of carbonate minerals at high pressure is frequently cited as an important mechanism that leads to carbon release from subducted rocks. However, carbonate minerals in the subducting slab are predicted to be stable to depths that are greater than arc-generating magma depths of approximately 150 km, implying that breakdown of carbonate phases in dehydrated MORB may not be a major contributor to arc volcano carbon budgets. To account for this discrepancy, previous studies have suggested that addition of H2O-rich fluids promotes the breakdown of carbonate-rich lithologies, thus generating volatile C species that could be incorporated into arc magmas. Here, we explore the feasibility of H2O-mediated decarbonation with a simple thermodynamic model. We calculate equilibrium mineral assemblages and accompanying fluid H2O/CO2 ratios for typical subducted lithologies, assuming a range of subduction zone geotherms, and explore the implications of addition of external fluids that are generated from deserpentinization of ultramafic lithologies at various stages. Results suggest that the liberation of C along volcanic arcs is facilitated by either the breakdown of carbonate minerals due to thermodynamically favorable conditions in hotter subduction systems, or by the breakdown of carbonate minerals during periods of higher fluid productivity associated with deserpentinization at appropriate depths along colder subduction geotherms. A comparison of C fluxes measured at volcanic arcs shows that colder subduction zones generate higher C fluxes, implying that the depth at which deserpentinization reactions occur strongly controls the availability of aqueous fluids for slab decarbonation, and that fluid availability represents the dominant control on carbon volatility during subduction.
       
  • Unexpected far-field hydrological response to a great earthquake
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Yan Zhang, Chi-Yuen Wang, Li-Yun Fu, Bo Zhao, Yuchuan Ma Permeability, an important crustal property that controls transport processes in Earth's shallow crust, is known to change during large earthquakes, with significant implications on the groundwater resources and contaminant transport. Analysis of the response of groundwater in wells to Earth tides has been increasingly used to detect such changes, but often multiple interpretations of the response changes are possible, making it difficult to ascertain its hydrological implications. Here we use data from a groundwater well in North China to illustrate the use of barometric response as a method to resolve this uncertainty. The tidal response of water level in this well shows abrupt coseismic increases of both phase shift and amplitude ratio after the far-field great 2011 Mw 9.1 Tohoku earthquake, which may be interpreted either as a large increase in the horizontal permeability of a confined aquifer or as an occurrence in the vertical permeability of an unconfined aquifer. Using data for the water-level response to barometric pressure we show that the aquifer was confined both before and after the Tohoku earthquake; thus the coseismic changes of the tidal response in this well are more likely to be due to an increase in the horizontal permeability. Interpretation of the tidal response shows a hereto unidentified change of poroelastic property in the far field of an earthquake, which stayed constant for ∼4 years after the earthquake until silt-cleaning in the well interrupted data collection. Thus the dynamic shaking during the Tohoku earthquake may have caused permanent deformation in the shallow crust at epicentral distances>1500 km.
       
  • Barium-isotopic constraints on the origin of post-Marinoan barites
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Peter W. Crockford, Boswell A. Wing, Adina Paytan, Malcolm S.W. Hodgskiss, Kimberley K. Mayfield, Justin A. Hayles, Julia E. Middleton, Anne-Sofie C. Ahm, David T. Johnston, Fabricio Caxito, Gabriel Uhlein, Galen P. Halverson, Benjamin Eickmann, Marta Torres, Tristan J. Horner Measurements of triple oxygen isotope ratios in barite horizons within post-Marinoan cap carbonates have provided some of the most compelling evidence that the Marinoan glaciation was a Snowball Earth event. However, the origin of these barite horizons remains unresolved. To constrain the Ba sources, and thus formation mechanisms of these horizons, we analyzed the Ba isotope composition of post-Marinoan barite deposits from Northwest Canada, Northern Norway, Brazil and South China. We augment these analyses with a Ba isotope survey of almost 100 modern and ancient additional barite measurements, including samples from pelagic (or ‘marine’), hydrothermal, terrestrial, Proterozoic stratiform and cold seep environments. Unlike modern cold seep or terrestrial barites, we find that globally-distributed post-Marinoan barites exhibit a relatively narrow isotopic range, suggesting a well-mixed, effectively limitless Ba source. Moreover, post-Marinoan deposits exhibit a similar mean Ba isotope composition to modern marine barites, which we interpret as evidence of a marine Ba source. Considered alongside existing geochemical, geological, and new Ba isotope survey data, we conclude that Ba in barite horizons was sourced from a well-mixed, Ba-replete but SO4-poor reservoir that accumulated during the Marinoan Snowball Earth interval. This deep Ba reservoir was then transported upward—either by ocean circulation or dolomitization of underlying cap carbonates—and was brought into contact with continental weathering-derived sulfate in a post-glacial meltwater surface layer. Thus, in addition to providing a plausible mechanism for generating globally-synchronous deposition of post-Marinoan barite horizons that reconciles all existing geochemical and geological data, our results demonstrate the utility of Ba isotopes to interrogate the origin of enigmatic barite deposits throughout the sedimentary record.
       
  • Thin lithosphere beneath the central Appalachian Mountains: A combined
           seismic and magnetotelluric study
    • Abstract: Publication date: Available online 27 May 2019Source: Earth and Planetary Science LettersAuthor(s): Rob. L. Evans, Margaret H. Benoit, Maureen D. Long, James Elsenbeck, Heather A. Ford, Jasmine Zhu, Xavier Garcia A joint analysis of magnetotelluric and Sp receiver function data, collected along a profile across the central Appalachians, highlights variations in regional lithospheric structure. While the interpretation of each data set by itself is non-unique, we identify three distinct features that are consistent with both the resistivity model and the receiver function image: 1) thin lithosphere beneath the Appalachian Mountains, 2) somewhat thicker lithosphere to the east of the mountains beneath the Coastal Plain, and 3) a lithosphere-asthenosphere boundary that deepens to the west of the mountains. In some regions, the correspondence between seismic velocity discontinuities and resistivity mark the base of the lithosphere, while in other locations we see seismic discontinuities that are contained within the lithosphere. At the western end of our profile a transition from highly resistive lithosphere to more conductive mantle represents the transition across the Grenville front. The thickness of lithosphere beneath the Grenville terrain is ∼140 km. Lithosphere at the eastern end of the profile has a thickness that is not well constrained by our coverage, but is at least 110 km thick. This lithosphere can be associated with a broader region of high resistivity material seen to extend further south. Directly beneath the Appalachian Mountains, lithospheric thickness is inferred to be as thin as ∼80 km, based on observations of elevated mantle conductivities and a westward-dipping seismic converter. Electrical conductivities in the uppermost asthenospheric mantle are sufficiently high (>0.1 S/m) to require the presence of a small volume of partial melt. The location of these elevated conductivities is close (offset ∼50 km to the west) to Eocene volcanic outcrops in and around Harrisonburg, VA. Our observations speak to mechanisms of intraplate volcanism where there is no divergent or convergent plate motion to trigger mantle upwelling or obvious fluid release, either of which can facilitate melting. Instead, we suggest that small scale mantle convection related either to pre-existing lithospheric thickness variations, or to lithospheric loss through delamination, coupled with relative plate motion with respect to the underlying asthenosphere, can trigger small amounts of melting. This melt migrates upslope, along the base of the lithosphere, potentially thermally eroding the lithosphere resulting in further thinning.
       
  • Predicted diurnal variation of the deuterium to hydrogen ratio in water at
           the surface of Mars caused by mass exchange with the regolith
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Renyu Hu Regolith on Mars exchanges water with the atmosphere on a diurnal basis and this process causes significant variation in the abundance of water vapor at the surface. While previous studies of regolith-atmosphere exchange focus on the abundance, recent in-situ experiments and remote sensing observations measure the isotopic composition of the atmospheric water. We are therefore motivated to investigate isotopic water exchange between the atmosphere and the regolith and determine its effect on the deuterium to hydrogen ratio (D/H) of the atmosphere. We model transport of water in the regolith and regolith-atmosphere exchange by solving a transport equation including regolith adsorption, condensation, and diffusion. The model calculates equilibrium fractionation between HDO and H2O in each of these processes. The fractionation in adsorption is caused by the difference in the latent heat of adsorption, and that of condensation is caused by the difference in the vapor pressure. Together with a simple, bulk-aerodynamic boundary layer model, we simulate the diurnal variation of the D/H near the planetary surface. We find that the D/H can vary by 300–1400‰ diurnally in the equatorial and mid-latitude locations, and the magnitude is greater at a colder location or season. The variability is mainly driven by adsorption and desorption of regolith particles, and its diurnal trend features a drop in the early morning, a rise to the peak value during the daytime, and a second drop in the late afternoon and evening, tracing the water vapor flow into and out from the regolith. The predicted D/H variation can be tested with in-situ measurements. As such, our calculations suggest stable isotope analysis to be a powerful tool in pinpointing regolith-atmosphere exchange of water on Mars.
       
  • Relative paleointensity estimates from magnetic anisotropy: Proof of
           concept
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Stuart A. Gilder, Kuang He, Michael Wack, Josef Ježek Relative paleointensity data from sedimentary rocks play an important role to decipher the workings of the geodynamo and to correct for atmospheric cosmogenic radionucleide production, so it is important to understand how sediments acquire remanent magnetizations and to better assess the quality of relative paleointensity data. We present experimental results from sediments deposited in controlled magnetic fields to observe the changes in magnetic anisotropy as a function of applied field strength going from near Earth-like values to almost full saturation. Relative paleointensity values followed a very well defined power law through the entire range of applied field intensities. Magnetic remanence fabrics evolved from oblate with maximum anisotropy axes in the sedimentary plane at low field strengths to prolate with maximum anisotropy axes parallel to the applied field direction at high fields. Anisotropy of magnetic susceptibility also evolved with field strength, but in a much less coherent manner than anisotropy of magnetic remanence. The experiments used well-characterized, natural sediments containing single domain magnetite, which made it possible to numerically model the data. The model matches the field dependency of both relative paleointensity and magnetic fabric development using a simple assumption that a large proportion (∼80%) of the remanence carriers in the sediments are unable to align with the magnetic field while a small fraction are free to align. Anisotropy of magnetic remanence thus holds promise to improve and assess relative paleointensity estimates and helps improve theoretical treatment of magnetic recording in sediments.
       
  • Low and seasonally variable salinity in the Pennsylvanian equatorial
           Appalachian Basin
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Marie Y. Jimenez, Linda C. Ivany, Emily J. Judd, Gregory Henkes Use of the δ18O thermometer in deep time investigations is complicated by uncertainty in the oxygen isotopic composition of seawater and an increasing potential for diagenetic alteration with age. These concerns are particularly important when considering that δ18O values from Paleozoic marine carbonates tend to be low and increasingly depleted with age. Demonstrating the preservation of original chemistry and thus eliminating diagenesis as a factor is a primary hurdle in reconstructing climatic conditions in deep time. Here, we report stable isotope data from serially sampled, Middle Pennsylvanian mollusks from the tropical Appalachian Basin of Kentucky, USA. X-ray diffraction and scanning electron microscopy indicate an aragonite mineralogy with retention of primary microtextures, elemental chemistry is consistent with the shells of living mollusks, and δ18O data reveal regular cyclic variation over ontogeny, together suggesting that original shell carbonate is preserved and records environmental conditions over the life history of the animal. However, values are depleted, centering around −4.6‰, and intraannual variation is significant, spanning up to 2.2‰, calling for significant summertime runoff of fresh water to the basin in which the shell-producing organisms grew. Our data extend a documented trend toward more depleted carbonate values eastward across the midcontinent sea with increasing distance from Panthalassa, reflecting a salinity gradient driven largely by isotopically depleted and seasonal runoff from the Central Pangaean Mountains. Mollusk data support the primary nature of low δ18O values from brachiopod shells in the midcontinent, but they also highlight the problems with calculating paleotemperatures from δ18O values of carbonates precipitated in epicontinental seas by assuming a marine seawater composition. While carbonate clumped isotope paleothermometry is a potential solution to this seawater δ18O ‘problem’, we demonstrate that such data from these aragonitic samples are related to burial diagenesis and not the Pennsylvanian paleoenvironment. The bias toward samples from such settings in the Paleozoic, because continental margins are more likely to be deformed, suggests that postulated regional and global temperature histories should be treated with caution until epeiric oxygen isotope seawater values can be better constrained.Graphical abstractGraphical abstract for this article
       
  • Stranded landscapes in the humid tropics: Earth's oldest land surfaces
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Paulo M. Vasconcelos, Kenneth A. Farley, John Stone, Thiago Piacentini, L. Keith Fifield Defying common expectations, some of the oldest landscapes on Earth are found in the humid tropics. 40Ar/39Ar dating shows that Fe-rich regolith that armors the ∼700–1000 m elevation banded iron-formation plateaus at Urucum, Mato Grosso do Sul, Brazil, results from chemical weathering that began at least 70 million years ago. Cosmogenic 3He concentrations in residual hematite clasts and the underlying saprolite on one plateau imply an extraordinarily slow surface lowering rate of 0.07–0.10 m Ma−1 maintained over the last ∼17–70 Ma. Cosmogenic 10Be and 26Al concentrations in a quartz vein in the same plateau suggest a similarly low surface erosion rate of ∼0.11–0.18 m Ma−1 over the past few million years. Cosmogenic 3He concentrations in goethite cementing a ∼200 m elevation indurated pediment ∼3 km northwest of the plateau escarpment yields a minimum exposure age of ∼2.6 Ma, revealing that some of the low-lying areas had already been incised and locally stabilized by goethite cementation by then. In contrast, 36Cl measurements on limestone outcrops at ∼170 m elevation and ∼6.5 km northwest of the Urucum plateau reveal short-term erosion rates of 7.1 ± 0.4 and 10.1 ± 0.5 m Ma−1, showing that areas not stabilized by goethite cementation are undergoing erosion at a relatively fast pace. The ∼700 m of relief at Urucum is consistent with differential denudation over ∼70 Ma controlled by lithological differences in chemical and physical rates of weathering and erosion. Effective chemical weathering and protracted low rates of erosion for the high-elevation surface result in armoring by a resilient residue of hematite blocks that are relatively insoluble and too heavy to transport. Armoring by the surface lag of large chemically resilient hematite blocks and bedded hematite saprolite focuses meteoric water flow into the subsurface and along faults. Preferential subsurface flow along the permeable pathways between insoluble hematite blocks, combined with segmentation of the Urucum Surface into isolated catchments too small to host active drainage systems, brings physical erosion to a stand-still. The only effective erosive processes on the Urucum plateau are subsurface chemical erosion and scarp retreat. Non-steady-state landscapes in which elevation differences result from prolonged differential denudation are a common feature of Earth's southern hemisphere cratons, and they inherit their present form from underlying lithologies coupled with geomorphic processes active since the Mesozoic.
       
  • The effects of back-arc spreading on arc magmatism
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Valentina Magni Mantle flow is a key feature that can affect melt production and composition in and around subduction zones. In this study, I use three-dimensional numerical models to investigate the role of mantle flow on subduction-related volcanism during back-arc extension. Results show that for a time interval of about 10-15 Myr during back-arc basin formation and spreading a wide convection cell brings mantle that has already partially depleted at the back-arc to the mantle wedge. Before and after this phase, the mantle reaching the sub-arc melting region is fertile. Thus, changes in back-arc activity and mantle flow pattern can be responsible for changes in magmatic composition and the amount of magmatism at the arc. These results are consistent with many examples of present-day subduction zones, in which phases of actively spreading back-arc correspond to a gap or a decrease of the arc volcanic activity.
       
  • The influence of seawater calcium ions on coral calcification mechanisms:
           Constraints from boron and carbon isotopes and B/Ca ratios in Pocillopora
           damicornis
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Sharmila J. Giri, Peter K. Swart, Ali Pourmand In order to better understand the response of coral calcification to changes in seawater chemistry, fragments of the coral Pocillopora damicornis were grown in seawater with varying [Ca2+]. Using a combined trace element (B/Ca) and stable isotope (δ11B, δ13C) approach, this paper explores the effect of seawater calcium concentrations ([Ca2+]SW) on coral calcification mechanisms and describes the manipulation of the extracellular calcifying fluid (ECF) pH (pHECF), saturation state (ΩECF) and dissolved inorganic carbon (DIC) speciation of the ECF. In these experiments, increases in [Ca2+]SW did not significantly influence calcification rates but caused the skeletal B/Ca ratio to significantly increase. To explain these results we propose a mechanism by which [CO32−]ECF is influenced by [Ca2+]SW. This mechanism suggests that the pHECF will be lowest in our highest [Ca2+] treatment and using this relationship we can place constraints on the pHECF. The difference in pHECF in our treatments is supported in part by a slight, but insignificant decrease in skeletal δ11B and δ13C values. We propose a novel dual-proxy approach to estimate pHECF, where calcification occurs at a pHECF> 9. This work emphasizes the importance of multi-element proxy approaches in understanding pH up-regulation and CO32− concentrations during calcification of corals.
       
  • Thermal response of the western tropical Atlantic to slowdown of the
           Atlantic Meridional Overturning Circulation
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Stefano Crivellari, Cristiano Mazur Chiessi, Henning Kuhnert, Christoph Häggi, Gesine Mollenhauer, Jens Hefter, Rodrigo Portilho-Ramos, Enno Schefuß, Stefan Mulitza The western tropical Atlantic plays an important role in the interhemispheric redistribution of heat during millennial-scale changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The proper evaluation of this role depends on a clear understanding of sea surface temperature (SST) variations during AMOC slowdown periods like Heinrich Stadials (HS) in the western tropical Atlantic. However, published SST records from the western tropical Atlantic between ca. 4°S and 7°N show inconsistencies that are apparently related to the employed temperature proxy (i.e., Mg/Ca versus alkenone unsaturation index U37k′). In general, while Mg/Ca values indicate warming during Heinrich Stadials, U37k′ values show cooling. To assess this issue, we sampled core GeoB16224-1 retrieved off French Guiana (i.e., 6°39.38′N) and reconstructed water temperatures at high resolution using Mg/Ca on the foraminifera species Globigerinoides ruber, U37k′, TEX86 and modern analogue technique (MAT) transfer functions using planktonic foraminifera assemblages calibrated for 50 m water depth. Our results show that Mg/Ca and TEX86 values recorded an increase in SST related to AMOC slowdown. Conversely, U37k′ and MAT values registered a decrease in temperatures during HS3 and HS1. Our U37k′ and Mg/Ca results thus confirm the previously reported inconsistency for the period between 48–13 cal ka BP. We suggest that several non-thermal physiological effects probably imparted a negative temperature bias on the U37k′ temperatures during Heinrich Stadials. However, MAT-based temperatures show similar variability with U37k′-based temperatures. Hence, we also suggest that during severe slowdown periods of the AMOC, a steeper meridional temperature gradient together with a southward shift of the Intertropical Convergent Zone produced not only an increase in SST but also a stronger upper water column stratification and a shoaling of the thermocline, decreasing subsurface temperatures. Our new high resolution temperature records allow a better characterization of the thermal response of the upper water column in the tropical western Atlantic to slowdown events of the AMOC, reconciling previously discrepant records.
       
  • Effect of iron content on thermal conductivity of olivine with
           implications for cooling history of rocky planets
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Youyue Zhang, Takashi Yoshino, Akira Yoneda, Masahiro Osako The influence of Fe concentration on heat transport properties of olivine was investigated to understand the cooling history of rocky planets such as Mercury, Mars and asteroids. Thermal conductivity (λ) and thermal diffusivity (κ) were measured simultaneously for olivine polycrystal with different Fe contents (Fo, Fo90, Fo70, Fo50, Fo31 and Fo0) up to 10 GPa and 1100 K by a pulse heating method. With increasing Fe in olivine, thermal conductivity of olivine first decreases and then slightly increases. The minimum λ was found to be at composition near Fo31; the absolute λ value of Fo31 is about 65% lower than that of Fo. Small amounts of Fe in olivine can strongly reduce the thermal conductivity at low temperature; λ value of Fo90 is about 50% of Fo at room temperature. Thermal conductivities of polycrystalline olivine have smaller absolute values and weaker pressure and temperature dependences, compared with those of natural single crystal olivine determined by previous studies. Heat capacity of Fo70 and Fo50 calculated from λ and κ is independent of pressure and is controlled by nearly constant thermal expansion coefficient with increasing temperature. Smaller λ of olivine aggregate with high Fe content would produce a warmer mantle and, in turn, possibly a thicker crust in the Fe-rich Mars, while heat in the Fe-poor Mercury can escape faster than the other terrestrial planets. Olivine-dominant asteroids with high Fe concentration could have longer cooling history and lower thermal inertia on the surface.
       
  • Ancient high Pt/Os crustal contaminants can explain radiogenic 186Os in
           some intraplate magmas
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): James M.D. Day, Brian O'Driscoll The origin of variations in 186Os/188Os ratios amongst mantle-derived basaltic and komatiitic lavas remains controversial, with opposing models arguing for deep core-mantle versus shallow mantle sources. Crustal contamination has generally not been favored due to the low Os contents of such sources, meaning that variations in 186Os/188Os would require involvement of extremely high proportions of crustal material. Here we re-examine crustal contamination as an effective means for generating significant 186Os/188Os variations in Earth materials. Using chromitites and peridotites from the Stillwater, Muskox and Rum layered intrusions, we show that radiogenic 186Os/188Os ratios are correlated with 187Os/188Os ratios and can only be explained by shallow-level mixing processes and crustal contamination. The samples have δ186Os ([{(186Os/188Ossample[t]/186Os/188OsPM(t)) − 1} × 1000], where the modern primitive mantle [PM] 186Os/188Os is 0.1198388) values ranging between 0.04 to 0.15 for the ∼2.7 Ga Stillwater Igneous Complex, −0.05 to 0.17 for the ∼1.27 Ga Muskox Intrusion, and 0.02 to 0.13 for the ∼0.06 Ga Rum Layered Suite. The highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents of the chromitites and peridotites can be modeled through high sulfide-melt partitioning (typically>8000) and emphasize the role of S-saturation and HSE scavenging. Considering the high sulfide-melt partitioning and accounting for high silicate melt to sulfide melt ratios (R-factors), it is possible to explain the variations in 186Os–187Os in layered intrusions using calculated Os isotope crustal evolution growth models. These calculations indicate that
       
  • High-resolution 10Be and paleomagnetic recording of the last polarity
           reversal in the Chiba composite section: Age and dynamics of the
           Matuyama–Brunhes transition
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Quentin Simon, Yusuke Suganuma, Makoto Okada, Yuki Haneda, ASTER Team We present new magnetic (direction and relative paleointensity) and beryllium isotope (10Be, 9Be and 10Be/9Be ratio) results covering the last geomagnetic reversal, i.e., the Matuyama–Brunhes transition (MBT), from the Chiba composite section (CbCS), east-central Japan. The very high sedimentation rates (>90 cm/ka) of the studied site, a candidate site for the global boundary stratotype section and point (GSSP) of the Lower–Middle Pleistocene boundary, allow the acquisition of a dataset of exceptional resolution. Coupled cosmogenic 10Be and magnetic results measured on the same samples demonstrate that the magnetization acquisition conserved the timing of rapid geomagnetic features, allowing accurate paleomagnetic interpretations. A polarity switch (PS) capturing most of the angular deviation occurring between 771.9 and 773.9 ka was followed by a directional instability rebound (IC1) until 768.5 ka. This sequence with 5.4 ka duration was characterized by a weak dipole field as low as 2.3 ± 0.3 × 1022 Am2. Four rapid episodes of directional instabilities preceding and following the PS-IC1 phase completed the reversal sequence. The asymmetry observed between the long-term dipole decay and sharp recovery, and rapid oscillations (
       
  • The essence of time – fertile skarn formation in the Variscan
           Orogenic Belt
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Mathias Burisch, Axel Gerdes, Lawrence D. Meinert, Richard Albert, Thomas Seifert, Jens Gutzmer Extending from Northern America to Central China the Variscan belt is a Paleozoic Orogen exceptionally well endowed in magmatic-hydrothermal ore deposits, including skarn deposits. Yet, the genesis of fertile skarns and their distinction from barren equivalents in orogenic zones is only poorly constrained. Here, we present innovative U-Pb laser-ablation inductively-coupled-plasma mass-spectrometry geochronology of garnet from different skarns in the Erzgebirge, a classic metallogenic province in central Europe. Garnet ages obtained not only constrain the timing of fertile skarn formation and associated Sn, W, Fe, Zn, Cu and In mineralization, but also clearly distinguish these from barren skarn bodies. We show that barren skarns formed during times of peak regional metamorphism at ∼340 Ma whereas mineralized skarns are temporally associated with late-orogenic magmatism at ∼325-313 Ma as well as post-orogenic magmatism at ∼308-295 Ma. The recognition of discrete mineralization events associated with the largest and economically most important skarn deposits provides valuable insight into the punctuated evolution of magmatic-hydrothermal systems in ancient collisional orogens on a regional scale; this has important implications to direct future mineral exploration.
       
  • Blueschist from the Mariana forearc records long-lived residence of
           material in the subduction channel
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): R. Tamblyn, T. Zack, A.K. Schmitt, M. Hand, D. Kelsey, L. Morrissey, S. Pabst, I.P. Savov From ca. 50 Ma to present, the western Pacific plate has been subducting under the Philippine Sea plate, forming the oceanic Izu-Bonin-Mariana (IBM) subduction system. It is the only known location where subduction zone products are presently being transported to the surface by serpentinite-mud volcanoes. A large serpentine mud “volcano” forms the South Chamorro Seamount and was successfully drilled by ODP during Leg 195. This returned mostly partially serpentinized harzburgites enclosed in serpentinite muds. In addition, limited numbers of small (1 mm–1 cm) fragments of rare blueschists were also discovered. U–Pb dating of zircon and rutile from one of these blueschist clasts give ages of 51.1 ± 1.2 Ma and 47.5 ± 2.0 Ma, respectively. These are interpreted to date prograde high-pressure metamorphism. Mineral equilibria modelling of the blueschist clast suggests the mineral assemblage formed at conditions of ∼1.6 GPa and ∼590 °C. We interpret that this high-pressure assemblage formed at a depth of ∼50 km within the subduction channel and was subsequently exhumed and entrained into the South Chamorro serpentinite volcano system at depths of ∼27 km. Consequently, we propose that the material erupted from the South Chamarro Seamount may be sampling far greater depths within the Mariana subduction system than previously thought. The apparent thermal gradient implied by the pressure–temperature modelling (∼370 °C/GPa) is slightly warmer than that predicted by typical subduction channel numerical models and other blueschists worldwide. The age of the blueschist suggests it formed during the arc initiation stages of the proto-Izu-Bonin-Mariana arc, with the P–T conditions recording thermally elevated conditions during initial stages of western Pacific plate subduction. This indicates the blueschist had prolonged residence time in the stable forearc as the system underwent east-directed rollback. The Mariana blueschist shows that subduction products can remain entrained in subduction channels for many millions of years prior to exhumation.
       
  • Disentangling interglacial sea level and global dynamic topography:
           Analysis of Madagascar
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Simon N. Stephenson, Nicky J. White, Tao Li, Laura F. Robinson Global inventories of stable sea-level markers for the peak of the last interglacial period, Marine Isotopic Stage (MIS) 5e, play a pivotal role in determining sea-level changes and in testing models of glacial isostatic adjustment. Here, we present surveying and radiometric dating results for emergent terraces from northern Madagascar, which is generally regarded as a stable equatorial site. Fossil coral specimens were dated using conventional and open-system corrected uranium series methods. Elevation of the upper (undated) terrace decreases from 33.8 m to 29.5 m over a distance of 35 km. An intermediate terrace has an average radiometric age of 130.7±13.2 ka (i.e. MIS 5e). Its elevation decreases from 9.3 m to 2.8 m over a distance of 80 km. The record of the lowest terrace is fragmentary and consists of beach rock containing rare corals with ages of 1.6–3.8 ka. The spatial gradient of the MIS 5e marker is inconsistent with glacio-isostatic adjustment calculations. Instead, we propose that variable elevations of this marker around Madagascar, and possibly throughout the Indian Ocean, at least partly reflect spatial patterns of dynamic topography generated by sub-plate mantle convection.
       
  • Reevaluating the carbon sink due to sedimentary carbonate formation in
           modern marine sediments
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Harold J. Bradbury, Alexandra V. Turchyn Previous attempts to quantify the amount of sedimentary carbonate precipitation in modern marine sediments have been derived from the flux of calcium into the sediments due to diffusion under assumed steady state, the application of Fick's first law, and then extrapolation of these site-specific rates to the global ocean sediment column. This approach is limited, however, as much of the ocean floor has not been sampled. We take a machine learning approach to update and refine the estimate of the amount of sedimentary carbonate precipitation, as well as define whether sedimentary carbonate precipitation is driven by organoclastic microbial sulfate reduction or anaerobic methane oxidation. We identify areas where there is sedimentary carbonate formation using machine learning, based upon oceanic physical and chemical properties including bathymetry, temperature, water depth, distance from shore, and tracers of primary production, and data from the global ODP/IODP database. Our results suggest that the total amount of sedimentary carbonate formation is much lower than previous estimates, at 1.35±0.5×1011 mol C/yr. We suggest that this rate is a lower estimate and discuss why machine-learning approaches may always produce lower-bound estimates of global processes. Our calculations suggest that the formation of sedimentary carbonate today is mainly driven by anaerobic methane oxidation (77%), with the remainder attributed to organoclastic sulfate reduction. We use our machine-learning results to speculate the impact that sedimentary carbonate precipitation may have had on the carbon isotope composition of the surface dissolved inorganic carbon reservoir over Earth history.
       
  • Late-Quaternary secular variation data from Mexican volcanoes
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Ahmed Nasser Mahgoub, Erick Juárez-Arriaga, Harald Böhnel, Claus Siebe, Francisco Javier Pavón-Carrasco We present 32 new paleomagnetic directions and 21 absolute paleomagnetic intensities (PI) from 33 volcanoes, sampled at 66 sites and covering the last 46 ka. Of these, 29 were radiocarbon-dated, 3 by thermo-luminescence, and one is of historical age (AD 1793). Rock magnetic experiments show that the dominant minerals are magnetite and titanomagnetite of low to intermediate titanium content, and of pseudo single domain size. Paleodirections were determined using stepwise demagnetization protocols, and PI by the IZZI or Thellier-Coe protocols, and strict selection criteria were applied to ensure the reliability of the data. Previously published data were evaluated to fulfill similar quality criteria as our data, resulting in: 65 directional data point; 18 paleointensity entries from lavas; 59 archeointensities are found to be acceptable. All accepted data come from a region within an 860 km radius around Mexico City. Paleosecular variation (PSV) curves of the past geomagnetic full-vector were established by using the bootstrap-resampling algorithm combined with cubic P-Splines for smoothing and interpolation. Because of the uneven time distribution of the data, this was done for two successive periods, between AD 2000 – 1600 BC and 1600 BC - 44000 BC. The full-vector PSV curves are unique for the American continents because of their temporal extension end because they include both, directions and absolute PI. The recent PSV curves, the last 3600 years, are of high resolutions which certainly have implication for both regional and global geomagnetic field models. Fast PSV change centered around AD 700 and AD 900, separated by a period of much slower PSV were observed, and rapid PI changes were found around 400 BC and AD 1000. In the period between 1600 BC and 44000 BC, PSV curves are well defined only within some periods and otherwise only define general PSV trends. Several intensity peaks with values up to 65 μT (VADM 15⋅1022 Am2) were observed around 250 BC, 12000 BC, 15000 BC and 27000 BC, and intensity lows around AD 700, 1200 BC, 24000-17000 BC, and 34000 BC. Abnormal directions were found around 26000 BC, with negative inclinations down to about -30°, which may be related to the Mono Lake or Rockall event. A comparison with other PSV curves is complicated by the large distance to Europe and the Hawaiian islands, with longitude differences of 99° and 57°, respectively. Lake sediment data from southern Texas show more differences than agreements, suggesting that either of the data may be affected by systematic errors.
       
  • Anelastic torsional oscillations in Jupiter's metallic hydrogen region
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): K. Hori, R.J. Teed, C.A. Jones We consider torsional Alfvén waves which may be excited in Jupiter's metallic hydrogen region. These axisymmetric zonal flow fluctuations have previously been examined for incompressible fluids in the context of Earth's liquid iron core. Theoretical models of the deep-seated Jovian dynamo, implementing radial changes of density and electrical conductivity in the equilibrium model, have reproduced its strong, dipolar magnetic field. Analysing such models, we find anelastic torsional waves travelling perpendicular to the rotation axis in the metallic region on timescales of at least several years. Being excited by the more vigorous convection in the outer part of the dynamo region, they can propagate both inwards and outwards. When being reflected at a magnetic tangent cylinder at the transition to the molecular region, they can form standing waves. Identifying such reflections in observational data could determine the depth at which the metallic region effectively begins. Also, this may distinguish Jovian torsional waves from those in Earth's core, where observational evidence has suggested waves mainly travelling outwards from the rotation axis. These waves can transport angular momentum and possibly give rise to variations in Jupiter's rotation period of magnitude no greater than tens of milliseconds. In addition these internal disturbances could give rise to a 10% change over time in the zonal flows at a depth of 3000 km below the surface.
       
  • Atmospheric carbon dioxide, ice sheet and topographic constraints on
           palaeo moisture availability in Asia
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): D. Zoura, D.J. Hill, A.M. Dolan, S.J. Hunter, Z. Tang, A.M. Haywood Today, the hydrological regime in East and South Asia is dominated by the monsoons, whilst central Asia is characterized as arid. Studies that have examined the onset of aridity and the intensification of the monsoons in Asia have generated significant debate, especially in respect to the timing of monsoon onset and how this relates to the potential causal mechanisms. The uplift of the Tibetan Plateau, the retreat of the Paratethys Sea, and the global cooling after the Eocene/Oligocene transition are all considered major drivers of Asian aridity and monsoonal intensification. However, little is known about each of these factor's contribution to the development of modern monsoon behaviour. Here, for the first time, we perform sensitivity simulations of a fully coupled ocean–atmosphere climate model (HadCM3) to investigate the effect of the Greenland and Antarctic ice-sheets formation, atmospheric carbon dioxide (CO2) variability, and Tibetan Plateau uplift on East Central Asian aridity and monsoon driven precipitation. We focus on three individual regions, the South Asian Monsoon, the East Asian Monsoon and the Arid East Central Asia and we present the annual precipitation cycle and the moisture availability over each region. Our results show that of the parameters investigated the primary control on Asian hydroclimate is the topography of the Tibetan Plateau. Furthermore, our results highlight that the significance of each forcing depends on the component of the hydrological region and factors studied, a factor that proxy interpretation need to take into consideration.
       
  • Constraining olivine abundance and water content of the mantle at the
           410-km discontinuity from the elasticity of olivine and wadsleyite
    • Abstract: Publication date: 1 August 2019Source: Earth and Planetary Science Letters, Volume 519Author(s): Wenzhong Wang, Michael J. Walter, Ye Peng, Simon Redfern, Zhongqing Wu Velocity and density jumps across the 410-km seismic discontinuity generally indicate olivine contents of ∼30 to 50 vol.% on the basis of the elastic properties of anhydrous olivine and wadsleyite, which is considerably less than the ∼60% olivine in the widely accepted pyrolite model for the upper mantle. A possible explanation for this discrepancy is that water dissolved in olivine and wadsleyite affects their elastic properties in ways that can reconcile the pyrolitic model with seismic observations. In order to more fully constrain the olivine content of the upper mantle near the 410-km discontinuity, and to place constraints on the mantle water content at this depth, we determined the full elasticity of hydrous wadsleyite at the P-T conditions of the discontinuity based on density functional theory calculations. Together with previous determinations for the effect of water on olivine elasticity, we simultaneously modeled the density and seismic velocity jumps (Δρ, ΔVP, ΔVS) across the olivine-wadsleyite transition. Our models allow for several scenarios that can well reproduce the density and seismic velocity jumps across the 410-km discontinuity when compared to globally averaged seismic models. When the water content of olivine and wadsleyite is assumed to be equal as in a simple binary system, our modeling indicates a best fit for low water contents (
       
 
 
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