Publisher: Geological Society of America   (Total: 4 journals)   [Sort by number of followers]

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Bulletin of the Geological Society of America     Hybrid Journal   (Followers: 35, SJR: 2.329, CiteScore: 4)
Geology     Full-text available via subscription   (Followers: 57, SJR: 3.114, CiteScore: 4)
Geosphere     Open Access   (Followers: 2, SJR: 1.752, CiteScore: 3)
Lithosphere     Open Access   (Followers: 3, SJR: 1.892, CiteScore: 3)
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Geology
Journal Prestige (SJR): 3.114
Citation Impact (citeScore): 4
Number of Followers: 57  
 
  Full-text available via subscription Subscription journal
ISSN (Print) 0091-7613 - ISSN (Online) 1943-2682
Published by Geological Society of America Homepage  [4 journals]
  • Biogeodynamics: Coupled evolution of the biosphere, atmosphere, and
           lithosphere

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      Abstract: There is broad consensus that tectonic and magmatic processes play a role in the evolution of life and the composition of the atmosphere. Tectonic and magmatic processes provide a suite of bio-essential nutrients that are carried into the hydrosphere through sedimentary processes. Tectonic processes facilitate the subsequent recycling and concentration of these nutrients for continued biospheric utilization. The burgeoning field of “biogeodynamics” aims to test hypotheses about such connections (Fig. 1). The past 50 years brought the plate tectonics revolution into full swing. The next 50 years are in a prime position to expand phylogenic consensus, augmenting secular evolution of geologic processes and expanding the links between the disparate processes controlling the evolution of the biosphere, atmosphere, and the solid Earth.
      PubDate: Mon, 01 Aug 2022 00:00:00 GMT
       
  • Recurrent tectonic activity in northeastern Brazil during Pangea breakup:
           Constraints from U-Pb carbonate dating

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      Abstract: AbstractCarbonate U-Pb dating of samples from rift-bounding faults of intracontinental basins in the Borborema province, northeastern Brazil, indicate recurrent tectonic activity during Pangea breakup lasting for >150 m.y. from the Late Triassic to the Paleocene, reactivating inherited strike-slip Neoproterozoic–Cambrian shear zones. Triassic ages indicate that brittle deformation started some 80 m.y. before previously known, most likely related to rifting along the incipient Central Atlantic. The subsequent Cretaceous opening of the South Atlantic caused renewed fault activity during rifting and basin development. Furthermore, recurrent Cenozoic tectonic activity along the rift-bounding faults is indicated, suggesting that structural inheritance of the Neoproterozoic–Cambrian continental-scale Borborema shear zone system has been responsible for accommodation of recurrent tectonic stress from Mesozoic rifting to the present day.
      PubDate: Thu, 12 May 2022 00:00:00 GMT
       
  • Permeability of subducted oceanic crust revealed by eclogite-facies vugs

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      Abstract: AbstractWe report the finding of rare eclogite-facies vugs forming millimeter- to centimeter-sized pockets in meta-ophiolites from the western European Alps. Euhedral garnet crystals covering the vug walls display oscillatory chemical zoning for a wide range of major and trace elements, including Cr, Mn, and rare earth elements. Thermodynamic modeling revealed that closed-system fluid production through the breakdown of prograde glaucophane, lawsonite, and chlorite between 505 °C and 525 °C can successfully explain porosity creation of ~4% and the mineralogical properties of the vugs. Available geologic and geochronologic constraints indicate that the eclogitization of the downgoing mafic crust spanned a window of at least 1 m.y. These observations can only be explained by the presence of extremely low permeability values (<10−22 m2) to keep the fluid confined at the meter scale within vugs on such time scales. Our field-based report of eclogite porosity provides the first in situ confirmation of previous experimental data and geophysical estimates on active margins. A substantial amount of fluid trapped in this porosity may be carried deeper than expected into Earth's mantle, with implications for volatile recycling budgets.
      PubDate: Thu, 12 May 2022 00:00:00 GMT
       
  • U-Pb dating reveals multiple Paleoproterozoic orogenic events (Hamersley
           orogenic cycle) along the southern Pilbara margin (Australia) spanning the
           onset of atmospheric oxygenation

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      Abstract: AbstractThe early Paleoproterozoic was a time of unprecedented change in Earth's climate and surface environment. The key to resolving some of the controversies surrounding the timing and causes of these changes lies with supracrustal sequences, such as the 2.45–2.22 Ga Turee Creek Group in the southern Pilbara craton, northwestern Australia. The group preserves a predominantly siliciclastic sequence; however, its precise age, tectonic setting, and postdepositional history are disputed. Although it is interpreted to have been deposited in a foreland basin setting shortly after 2.45 Ga, the oldest well-recognized deformational event, marked by northward folding and thrusting, is the 2.20–2.15 Ga Ophthalmia orogeny. Evidence for a pre-Ophthalmia fabric-forming tectonic event north of the Archean Sylvania Inlier, southeast Pilbara craton, which is marked by tight to isoclinal folding, has been largely overlooked. In this area, we report in situ U-Pb geochronology of authigenic monazite and xenotime in shale with a well-developed tectonic cleavage from the ca. 2.63 Ga Jeerinah Formation. Monazite porphyroblasts, which are locally wrapped by strain fringes aligned in a tectonic cleavage, yielded weighted mean 207Pb/206Pb ages at 2370 ± 11 Ma and 2312 ± 8 Ma, whereas xenotime, which overprints a crenulation cleavage, gave a weighted mean 207Pb/206Pb age of 2291 ± 11 Ma, constraining fabric development to between 2.31 Ga and 2.29 Ga. Our results confirm the existence of a pre-Ophthalmia deformational event in the southeastern Pilbara craton, herein referred to as the Sylvania orogeny, which is part of an ~300 m.y. interval (2.45–2.15 Ga) of northward-directed compression (“Hamersley orogenic cycle”). This orogenic cycle is marked by east-west and northwest-southeast folding, cleavage development, veining, hydrothermal gold mineralization, and isotopic resetting across the southern Pilbara craton. Our results indicate that the syn–Great Oxidation Event Turee Creek Group was deposited in one or more foreland basins after 2.45 Ga. Our results provide a new tectonostratigraphic and geodynamic framework for understanding the timing and origin of geochemical records in a key succession deposited during an interval of global environmental change.
      PubDate: Thu, 12 May 2022 00:00:00 GMT
       
  • Cenozoic delamination of the southwestern Yangtze craton owing to
           densification during subduction and collision

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      Abstract: AbstractIt is widely thought that oceanic subduction can trigger cratonic keel delamination, but the southwestern Yangtze craton (SYC; southwestern China) lost its lower keel during Cenozoic continental collision. The upper mantle beneath the thinned SYC contains its incompletely delaminated keel, which has high-velocity seismic anomalies. Combining geophysical observations with the geochemistry of Eocene mafic potassic lavas derived from the SYC mantle at different depths, we suggest that the deep (~130 km) delaminated lithosphere was more fertile and dense, with low-forsterite (Fo; molar 100 × Mg/[Mg + Fe] = 91.3) and high-δ18O (5.9‰) olivine, than the shallow (~55 km) intact lithosphere (Fo = 94.2; δ18O = 5.2‰), although both were rehydrated and oxidized. The deep keel underwent strong refertilization and densification owing to the addition of Fe-rich basaltic melts during earlier oceanic subduction. Subduction-driven refertilization and subsequent collision-driven cooling caused cratonic keel delamination due to compositional and thermal densification rather than hydration- or oxidation-induced rheological weakening. Our study provides an example of Cenozoic cratonic keel delamination in a collisional orogen and highlights the key roles of compositional and thermal densification in delamination during subduction and collision.
      PubDate: Mon, 09 May 2022 00:00:00 GMT
       
  • Evidence for benthic oxygen production in Neoarchean lacustrine
           stromatolites

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      Abstract: AbstractThe evolution of oxygenic photosynthesis fundamentally altered the global environment, but the history of this metabolism prior to the Great Oxidation Event (GOE) at ca. 2.4 Ga remains unclear. Increasing evidence suggests that non-marine microbial mats served as localized “oxygen oases” for hundreds of millions of years before the GOE, though direct examination of redox proxies in Archean lacustrine microbial deposits remains relatively limited. We report spatially distinct patterns of positive and negative cerium (Ce) anomalies in lacustrine stromatolites from the 2.74 Ga Ventersdorp Supergroup (Hartbeesfontein Basin, South Africa), which indicate that dynamic redox conditions within ancient microbial communities were driven by oxygenic photosynthesis. Petrographic analyses and rare earth element signatures support a primary origin for Ce anomalies in stromatolite oxides. Oxides surrounding former bubbles entrained in mats (preserved as fenestrae) exhibit positive Ce anomalies, while oxides in stromatolite laminae typically contain strong negative Ce anomalies. The spatial patterns of Ce anomalies in Ventersdorp stromatolites are most parsimoniously explained by localized Ce oxidation and scavenging around oxygen bubbles produced by photosynthesis in microbial mats. Our new data from Ventersdorp stromatolites supports the presence of oxygenic photosynthesis ~300 m.y. before the GOE, and add to the growing evidence for early oxygen oases in Archean non-marine deposits.
      PubDate: Mon, 09 May 2022 00:00:00 GMT
       
  • A newly discovered subglacial lake in East Antarctica likely hosts a
           valuable sedimentary record of ice and climate change

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      Abstract: AbstractThe Princess Elizabeth Land sector of the East Antarctic Ice Sheet is a significant reservoir of grounded ice and is adjacent to regions that experienced great change during Quaternary glacial cycles and Pliocene warm episodes. The existence of an extensive subglacial water system in Princess Elizabeth Land (to date only inferred from satellite imagery) bears the potential to significantly impact the thermal and kinematic conditions of the overlying ice sheet. We confirm the existence of a major subglacial lake, herein referred to as Lake Snow Eagle (LSE), for the first time using recently acquired aerogeophysical data. We systematically investigated LSE's geological characteristics and bathymetry from two-dimensional geophysical inversion models. The inversion results suggest that LSE is located along a compressional geologic boundary, which provides reference for future characterization of the geologic and tectonic context of this region. We estimate LSE to be ~42 km in length and 370 km2 in area, making it one of the largest subglacial lakes in Antarctica. Additionally, the airborne ice-penetrating radar observations and geophysical inversions reveal a layer of unconsolidated water-saturated sediment around and at the bottom of LSE, which—given the ultralow rates of sedimentation expected in such environments—may archive valuable records of paleoenvironmental changes and the early history of East Antarctic Ice Sheet evolution in Princess Elizabeth Land.
      PubDate: Mon, 09 May 2022 00:00:00 GMT
       
  • The influence of glacial isostatic adjustment on continental shelf
           stratigraphic correlation

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      Abstract: AbstractGlacial isostatic adjustment (GIA) imparts geographic variability in the amplitude and timing of local sea-level (LSL) change arising from glacial-interglacial oscillations relative to a global mean signal (eustasy). We modeled how GIA manifests in the stratigraphic record across four shelf-perpendicular transects moving progressively more distal to the Quaternary North American ice complex, subject to varying amounts of GIA during glacial-interglacial cycles. Along each transect, we obtained LSL histories for nine sites between 1 m and 250 m water depth from the output of a gravitationally self-consistent GIA model run from marine oxygen isotope stage (MIS) 11 to the present. We paired each site's unique LSL history with 50 identical annual sedimentation models to create a library of 400-k.y.-duration synthetic stratigraphic columns (each assuming no tectonics). Comparison of the suite of synthetic stratigraphic columns between transects for a given bathymetric depth reveals latitudinal differences in the stratigraphically determined number, magnitude, and age of glacial-interglacial cycles, as inferred from stratigraphic sequence count, apparent water-depth change, and age of preserved deglacial transgression. We conclude that, for many field locales, extraction of primary information about the number, scale, and duration of pre-Cenozoic glacial-interglacial cycles from continental shelf stratigraphic records near ice sheets demands a deconvolution of the GIA signal.
      PubDate: Mon, 09 May 2022 00:00:00 GMT
       
  • A tropically hot mid-Cretaceous North American Western Interior Seaway

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      Abstract: AbstractThe mid-Cretaceous thermal maximum (KTM) during Cenomanian to Santonian times from ca. 100 to 83 Ma is considered among Earth's warmest sustained intervals of the Phanerozoic. The time interval is also characterized by major paleoceanographic changes in the form of an oceanic anoxic event and the flooding of epicontinental seaways, such as the Western Interior Seaway in North America. We report carbonate clumped isotope (Δ47) paleotemperatures (TΔ47) of the KTM measured from Cenomanian oyster fossils of the Western Interior Seaway. Following screening of specimens for carbonate diagenesis and exclusion of geographic zones with evidence consistent with solid-state Δ47 reordering, a mean TΔ47 of 28–34 °C (95% confidence interval for the standard error of mean) for primary oyster calcite quantifies extreme mid-latitude warmth in North America. When combined with existing Campanian and Maastrichtian marine TΔ47 records, the new data constrain Late Cretaceous temperature trends underlying the evolution of North American faunal and stratigraphic records. These TΔ47 data from the peak KTM highlight the potential of this proxy to quantitatively resolve the upper thermal limits of Phanerozoic greenhouse climates.
      PubDate: Mon, 09 May 2022 00:00:00 GMT
       
  • Early incubation and prolonged maturation of large ignimbrite magma
           bodies: Evidence from the Southern Rocky Mountain volcanic field,
           Colorado, USA

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      Abstract: AbstractClusters of early central volcanoes in the mid-Cenozoic Southern Rocky Mountain volcanic field (SRMVF; southwestern Colorado, USA) record sites of initial magmatic focusing that led to assembly of sizable upper-crustal magma bodies capable of generating large ignimbrites. Peak growth at precursor andesitic volcanoes was followed by extended periods (0.5 to >2 m.y.) of reduced eruptive activity during inferred prolonged incubation of the crustal reservoir prior to eruption of ignimbrites at the San Juan magmatic locus, as exemplified by the 5000 km3 Fish Canyon Tuff and associated La Garita caldera. After a magma system became thermally mature and compositionally evolved, additional large ignimbrites could erupt more rapidly from polycyclic calderas. In contrast, incubation times for smaller ignimbrite magmas, as at Crater Lake, Oregon, were briefer than for San Juan systems. Plutonic counterparts to the temporal-compositional assembly of arc-ignimbrite magmas are exemplified by incrementally emplaced granitoid intrusions like the Mesozoic Tuolumne complex in the Sierra Nevada.
      PubDate: Mon, 09 May 2022 00:00:00 GMT
       
  • Multiple phyla, one time resolution' Similar time averaging in benthic
           foraminifera, mollusk, echinoid, crustacean, and otolith fossil
           assemblages

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      Abstract: AbstractTime averaging of fossil assemblages determines temporal precision of paleoecological and geochronological inferences. Taxonomic differences in intrinsic skeletal durability are expected to produce temporal mismatch between co-occurring species, but the importance of this effect is difficult to assess due to lack of direct estimates of time averaging for many higher taxa. Moreover, burial below the taphonomic active zone and early diagenetic processes may alleviate taxonomic differences in disintegration rates in subsurface sediments. We compared time averaging across five phyla of major carbonate producers co-occurring in a sediment core from the northern Adriatic Sea shelf. We dated individual bivalve shells, foraminiferal tests, tests and isolated plates of irregular and regular echinoids, crab claws, and fish otoliths. In spite of different skeletal architecture, mineralogy, and life habit, all taxa showed very similar time averaging varying from ~1800 to ~3600 yr (interquartile age ranges). Thus, remains of echinoids and crustaceans—two groups with multi-elemental skeletons assumed to have low preservation potential—can still undergo extensive age mixing comparable to that of the co-occurring mollusk shells. The median ages of taxa differed by as much as ~3700 yr, reflecting species-specific timing of seafloor colonization during the Holocene transgression. Our results are congruent with sequestration models invoking taphonomic processes that minimize durability differences among taxa. These processes together with temporal variability in skeletal production can overrule the effects of durability in determining temporal resolution of multi-taxic fossil assemblages.
      PubDate: Mon, 09 May 2022 00:00:00 GMT
       
  • An explosive volcanic origin identified for dark sand in Aeolis Dorsa,
           Mars

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      Abstract: AbstractDark, windblown (eolian) sand on Mars has produced significant geologic effects throughout Martian history. Although local and regional sand sources have been identified, a primary origin, or genesis, for Martian sand has not been demonstrated. This knowledge gap was recently heightened by the discovery of widespread sand motion, implying breakdown of grains to sub-sand sizes. To address the question of sand genesis, we investigated the source(s) of sand in Aeolis Dorsa (AD), the westernmost Medusae Fossae Formation, using comparisons to sand potentially sourced from multiple regions, each connoting a different sand genesis. Our methods included comparison of (1) AD sand mineralogies with those of possible sand source features, and (2) mapped AD sand deposits and inferred emplacement directions with modeled sand deposit locations and transport pathways. The results point to a time-transgressive unit, interpreted as pyroclastic, as a source of dark sand. High-resolution images of this unit reveal outcrops with dark sand weathering out of lithified bedrock. Given the extent of interpreted pyroclastic deposits on Mars, this sand genesis mechanism is likely widespread today and operated throughout Martian history. Whereas this work identified olivine-rich sand, a range of original pyroclastic lithologies would account for the mineralogic variability of dune fields on Mars. These findings can be tested through analyses of other pyroclastic deposits and potentially by data from the NASA Curiosity rover in nearby Gale crater.
      PubDate: Fri, 06 May 2022 00:00:00 GMT
       
  • Diapirism of carbonate platforms subducted into the upper mantle

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      Abstract: AbstractSubduction of lithospheric plates at convergent margins leads to transport of materials once close to or at the surface of Earth to great depths. Some of them later return to the surface by magmatism or degassing, whereas others end up being stored in the mantle for long periods of time. The fate of carbon-bearing minerals in subduction is of particular interest because they can arbitrate the long-term availability of CO2 at the surface. However, there are major gaps in the understanding of even the most fundamental processes that modulate carbon pathways at mantle depths. We use geodynamic models to understand carbonate pathways upon subduction in the form of large carbonate platforms, which were common in the Tethys realm of Europe. We conducted a series of geodynamic forward models for a 1-km-thick carbonate platform entering subduction. We show that most of the carbonate load detaches from the subducting slab and rises up diapirically through the mantle wedge and eventually mixes with the mantle lithosphere. A smaller fraction gets accreted under the forearc, whereas an even smaller fraction descends deeper into the mantle. The cold diapiric plume has a significant role in retarding silicate mantle melting above these subduction zones and promoting the formation of small-volume carbonate-rich melts and, in some cases, alkaline silica-undersaturated silicate melts. We propose that large amounts of CO2 can be stored as carbonate in the shallow uppermost lithospheric mantle.
      PubDate: Fri, 06 May 2022 00:00:00 GMT
       
  • Extensive evidence for a last interglacial Laurentide outburst (LILO)
           event

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      Abstract: AbstractA catastrophic last interglacial Laurentide outburst (LILO) event approximately 125,000 years ago (125 ka) may have contributed to abrupt climate change during the last interglacial. It has been proposed that this event was an analog of the Holocene 8.2 ka event. We characterize in detail the (1) provenance, (2) timing, and (3) delivery mechanism of a layer of red sediments deposited across much of the northwestern Atlantic Ocean at 125 ka. Our observations provide strong support for the occurrence of a LILO event that was analogous to the 8.2 ka event in all three aspects, and likely surpassed it in magnitude. The freshwater discharge associated with the 125 ka LILO event may explain a series of abrupt global changes, including a reduction of the North Atlantic Deep Water and reinvigoration of the Antarctic Bottom Water. Our findings suggest that the mechanism that triggered the LILO event may be an integral part of the deglacial sequence of events, during which the final collapse of the contiguous Laurentide Ice Sheet took place 3.5–4 k.y. after full interglacial temperature was reached in the middle and high northern latitudes.
      PubDate: Fri, 06 May 2022 00:00:00 GMT
       
  • Late glacial–Holocene record of Southern Hemisphere westerly wind
           dynamics from the Falkland Islands, South Atlantic Ocean

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      Abstract: AbstractThe Southern Hemisphere westerly wind belt (SHWW) is a major feature of Southern Hemisphere, midlatitude climate that is closely linked with the sequestration and release of CO2 in the Southern Ocean. Past changes in the strength and position of this wind belt are poorly resolved, particularly across the Pleistocene-Holocene transition, a time period associated with fluctuations in atmospheric temperatures and CO2 levels. We used dust geochemistry, particle size measurements, and paleoecological analyses from a peat sequence in the Falkland Islands, South Atlantic Ocean, to describe changes in the SHWW between 16.0 and 6.5 ka (thousands of years before CE 1950). Wind strength was low at ~51°S before and during the Antarctic Cold Reversal (ACR, 14.9–13.0 ka), intensified between 13.1 and 12.1 ka as atmospheric temperatures increased, and then weakened, reaching a minimum between 12.1 and 10.9 ka during the Early Holocene thermal maximum. Northwesterly air masses became more dominant from 12.0 to 10.2 ka, and wind strength remained low until our record was affected by a storm surge or tsunami ca. 7.8 ka. These data indicate a southward shift in the latitude of the SHWW, from north of 51°S prior to and during the ACR, at ~51°S before the onset of the Holocene, and south of 51°S during the early Holocene thermal maximum. This pattern suggests that the latitude of the SHWW was coupled with atmospheric temperatures through the Pleistocene-Holocene transition.
      PubDate: Fri, 06 May 2022 00:00:00 GMT
       
  • Claspers in the mid-Cambrian Olenoides serratus indicate horseshoe
           crab–like mating in trilobites

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      Abstract: AbstractSexual reproduction represents a fundamental aspect of animal biology, but the diversity of reproductive strategies among early Paleozoic metazoans remains obscure. Direct evidence of reproductive strategies comes from exceptionally preserved egg masses in Cambrian and Ordovician euarthropods such as waptiids and trilobites, but anatomical or behavioral adaptations for mating in these taxa are all but unknown. We demonstrate the presence of functionally specialized appendages in the mid-Cambrian (Wuliuan) Burgess Shale trilobite Olenoides serratus. New material of O. serratus preserves significantly modified and reduced endopodites underneath the seventh thoracic and first pygidial tergites. Comparison with extant euarthropods indicates that these specialized limbs are functional analogs to claspers, which are used by sexually mature males to grasp females prior to or during mating. The claspers in O. serratus were most likely used by the male to hold onto the posterior pygidial spines of the female during amplex, similar to the strategy observed in the horseshoe crab Limulus polyphemus. The new appendicular data from O. serratus provide the first direct evidence for trilobite sexual dimorphism based on the non-biomineralized ventral anatomy. Our findings illuminate the reproductive biology of trilobites and indicate that complex mating behaviors observed in extant euarthropods originated during the Cambrian explosion.
      PubDate: Fri, 06 May 2022 00:00:00 GMT
       
  • 830-million-year-old microorganisms in primary fluid inclusions in halite

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      Abstract: AbstractPrimary fluid inclusions in bedded halite from the 830-m.y.-old Browne Formation of central Australia contain organic solids and liquids, as documented with transmitted light and ultraviolet–visible (UV-vis) petrography. These objects are consistent in size, shape, and fluorescent response with cells of prokaryotes and eukaryotes and with organic compounds. This discovery shows that microorganisms from saline depositional environments can remain well preserved in halite for hundreds of millions of years and can be detected in situ with optical methods alone. This study has implications for the search for life in both terrestrial and extraterrestrial chemical sedimentary rocks.
      PubDate: Fri, 06 May 2022 00:00:00 GMT
       
  • Nd isotopic evidence for enhanced mafic weathering leading to Ordovician
           cooling

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      Abstract: AbstractIt remains unclear whether waning of the volcanic degassing CO2 source or enhancement of the mafic (Ca, Mg-silicate) weathering CO2 sink, or both, caused global cooling leading to the Ordovician greenhouse–icehouse transition. We present a uniquely age-constrained and integrated Middle–Late Ordovician (470–450 Ma) continental weathering isotopic proxy data set (87Sr/86Sr and εNd(t)) from carbonate rocks of the Antelope Range of central Nevada, USA, paired with published paleotemperature proxy measurements (δ18O) of conodont apatite from the same locality. This suite of proxy records signals an increase in mafic weathering of the Taconic mountains (eastern United States) at ca. 463 Ma, which forced a period of global cooling. We adapt a 87Sr/86Sr and pCO2 mass balance approach to model CO2 drawdown during the Ordovician, and show that a combined decrease in volcanic degassing and increase in mafic weathering approximately halves pCO2 in agreement with δ18O trends and paleotemperature reconstructions.
      PubDate: Fri, 06 May 2022 00:00:00 GMT
       
  • Time scales of open-system processes in a complex and heterogeneous
           mush-dominated plumbing system

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      Abstract: AbstractThe architecture of a mush-dominated plumbing system in active volcanic areas conditions the magma pathways feeding eruptions. Open-system processes along these pathways and the associated time scales are directly related to monitoring data and eruptive behavior. Despite crystal mush–dominated systems being common in active volcanoes, previous studies have not focused on the integration of data from the mush sectors feeding different eruptions, supplying a partial view of the pre-eruptive magmatic processes and hindering the interpretation of the monitoring signals during unrest periods. We focus on the Marsili seamount (Tyrrhenian Sea), where the mineral data document processes within a magmatic system vertically extended throughout the local oceanic crust and made of a mush framework spotted with eruptible melt- and crystal-rich pockets. We undertook a study of Marsili olivine crystals that constrains the time scales of three pre-eruptive scenarios, dominated by open-system processes: (1) disaggregation of the deep Marsili volcano mush zone that occurred over a time scale of years prior to the eruption; (2) rapid ascent (days) of mantle-derived basaltic magma that, in some cases, intercepts shallow plagioclase-rich pockets; and (3) multiple mixing events between melt- and crystal-rich mush zones occurring approximately 1–2 mo and 0.5–3 yr before the eruption. Our results highlight the importance of contemporaneously studying eruptions in different locations on a volcano edifice for a better comprehension on how mush-dominated plumbing systems work as a whole and how this must be considered during the interpretation of monitoring data.
      PubDate: Tue, 26 Apr 2022 00:00:00 GMT
       
  • Possible bipolar global expression of the P3 and P4 glacial events of
           eastern Australia in the Northern Hemisphere: Marine diamictites and
           glendonites from the middle to upper Permian in southern Verkhoyanie,
           Siberia

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      Abstract: AbstractThree intervals of glaciomarine diamictites with extensive glendonites in middle to upper Permian sediments were found in the Kobyume River, southern Verkhoyanie, Russia. The successions are biostratigraphically constrained as middle to upper Permian. The middle Permian diamictite horizons extend over a large area with a lateral distance of >1000 km. The upper Permian diamictites developed only locally. The diamictites are interpreted as glaciomarine sediments containing ice-rafted debris. Two glacial episodes in Siberia temporally correspond to the P3 (middle Permian) and P4 (late Permian) glacial events of eastern Australia, strongly suggesting a global bipolar climate and well-developed climatic belts during the middle to late Permian.
      PubDate: Tue, 26 Apr 2022 00:00:00 GMT
       
  • Thermal and compositional anomalies in a detailed xenolith-based
           lithospheric mantle profile of the Siberian craton and the origin of
           seismic midlithosphere discontinuities

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      Abstract: AbstractThe fine structure and thermal state of >200-km-thick cratonic lithosphere remain poorly explored because of insufficient sampling and uncertainties in pressure (P) and temperature (T) estimates. We report exceptionally detailed thermal and compositional profiles of the continental lithospheric mantle (CLM) in the Siberian craton based on petrographic, in situ chemical, and P-T data for 92 new garnet peridotite xenoliths from the Udachnaya kimberlite, as well as literature data. The thermal profile is complex, with samples indicating model conductive geotherms between 40 and 35 mW/m2 at ~55–130 km, colder (35 mW/m2 geotherm) mantle from 140 to 190 km, and hotter layers at the CLM base (190–230 km) and at ~135 km. The latter, previously unidentified, anomalous midlithospheric horizon has rocks up to 150 °C hotter than the 35 mW/m2 geotherm, that are rich in garnet and clinopyroxene, have low Mg#, and have melt-equilibrated rare earth element patterns. We posit that this horizon formed in a depth range where ascending melts stall (e.g., via loss of volatiles and redox change), heat wall-rock harzburgites, and transform them to lherzolites or wehrlites. This may explain some seismic midlithosphere discontinuities (MLDs) in cratons. By contrast, we found no rocks rich in metasomatic volatile-rich amphibole, phlogopite, or carbonate matching the MLD, nor layers composed of peridotites with distinct melt-extraction degrees. The CLM below 190 km contains both coarse and variably deformed rocks heated and reworked (Mg#Ol down to 0.86) by localized lithosphere-asthenosphere interaction.
      PubDate: Tue, 26 Apr 2022 00:00:00 GMT
       
 
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