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Publisher: American Geophysical Union (AGU)   (Total: 17 journals)

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 53, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 205, SJR: 2.189, h-index: 121)
Journal Cover   Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [7 followers]  Follow
    
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • A meta‐analysis of oceanic DMS and DMSP cycling processes:
           Disentangling the summer paradox
    • Authors: Martí Galí; Rafel Simó
      Abstract: The biogenic volatile compound dimethylsulfide (DMS) is produced in the ocean mainly from the ubiquitous phytoplankton osmolyte dimethylsulfoniopropionate (DMSP). In the upper mixed layer, DMS concentration and the daily averaged solar irradiance are roughly proportional across latitudes and seasons. This translates into a seasonal mismatch between DMS and phytoplankton biomass at low latitudes, termed the “DMS summer paradox”, which remains difficult to reproduce with biogeochemical models. Here we report on a global meta‐analysis of DMSP and DMS cycling processes and their relationship to environmental factors. We show that DMS seasonality reflects progressive changes in a short‐term dynamic equilibrium, set by the quotient between gross DMS production rates and the sum of biotic and abiotic DMS consumption rate constants. Gross DMS production is the principal driver of DMS seasonality, due to the synergistic increases towards summer in two of its underlying factors: phytoplankton DMSP content (linked to species succession) and short‐term community DMSP‐to‐DMS conversion yields (linked to physiological stress). We also show that particulate DMSP transformations (linked to grazing‐induced phytoplankton mortality) generally contribute a larger share of gross DMS production than dissolved‐phase DMSP metabolism. The summer paradox is amplified by a decrease in microbial DMS consumption rate constants towards summer. However, this effect is partially compensated by a concomitant increase in abiotic DMS loss rate constants. Besides seasonality, we identify consistent covariation between key sulfur cycling variables and trophic status. These findings should improve the modeling projections of the main natural source of climatically active atmospheric sulfur.
      PubDate: 2015-03-12T17:22:41.039729-05:
      DOI: 10.1002/2014GB004940
       
  • Controls on the silicon isotope distribution in the ocean: New diagnostics
           from a data‐constrained model
    • Authors: Mark Holzer; Mark A. Brzezinski
      Abstract: The global distributions of the silicon isotopes within silicic acid are estimated by adding isotope fractionation to an optimized, data‐constrained model of the oceanic silicon cycle that is embedded in a data‐assimilated steady circulation. Including fractionation during opal dissolution improves the model's ability to capture the approximately linear relation between isotope ratio, δ30Si, and inverse silicic‐acid concentration observed in the deep Atlantic. To quantify the importance of hydrographic control on the isotope distribution, δ30Si is partitioned into contributions from preformed and regenerated silicic acid, further partitioned according to euphotic‐zone origin. We find that the large‐scale features of the isotope distribution in the Atlantic basin are dominated by preformed silicic acid, with regenerated silicic acid being important for setting vertical gradients. In the Pacific and Indian Oceans, preformed and regenerated silicic acid make roughly equally important contributions to the pattern of the isotope ratio, with gradients of the preformed and regenerated contributions tending to cancel each other in the deep Pacific. The Southern‐Ocean euphotic zone is the primary origin of both the preformed and regenerated contributions to δ30Si. Nearly the entire preformed part of δ30Si is of Southern‐Ocean and North‐Atlantic origin. The regenerated part of lta30Si in the Atlantic basin also has a contribution of central‐Atlantic (∼ 40∘ S – 40∘N) origin that is comparable in magnitude to the North‐Atlantic contribution. In other basins, the central Pacific and Indian Ocean are the second largest contributors to the regenerated part of δ30Si.
      PubDate: 2015-03-12T16:56:27.931535-05:
      DOI: 10.1002/2014GB004967
       
  • Observing multi‐decadal trends in Southern Ocean CO2 uptake: What
           can we learn from an ocean model'
    • Authors: Nicole S. Lovenduski; Amanda R. Fay, Galen A. McKinley
      Abstract: We use output from a hindcast simulation (1958–2007) of an ocean biogeochemical and ecological model to inform an observational strategy for detection of a weakening Southern Ocean CO2 sink from surface ocean pCO2 data. Particular emphasis isplaced on resolving disparate conclusions about the Southern Ocean CO2 sink that have been drawn from surface ocean pCO2 observation studies in the past. We find that long‐term trends in ΔpCO2 (pCO2oc ‐ pCO2atm) can be used as a proxy for changes in the strength of the CO2 sink, but must be interpreted with caution, as they are calculated from small differences in the oceanic and atmospheric pCO2 trends. Large interannual, decadal, and multi‐decadal variability in ΔpCO2 persists throughout the simulation, suggesting that one must consider a range of start and end years for trend analysis before drawing conclusions about changes in the CO2 sink. Winter‐mean CO2 flux trends are statistically indistinguishable from annual‐mean trends, arguing for inclusion of all available pCO2oc data in future analyses of the CO2 sink. The weakening of the CO2 sink emerges during the observed period of our simulation (1981–2007) in the subpolar seasonally stratified biome (4∘C < average climatological temperature < 9∘C); the weakening is most evident during periods with positive trends in the Southern Annular Mode. With perfect temporal and spatial coverage, 13 years of pCO2oc data would be required to detect a weakening CO2 sink in this biome. Given available data, it is not yet possible to detect a weakening of the Southern Ocean CO2 sink with much certainty, due to imperfect data coverage and high variability in Southern Ocean surface pCO2.
      PubDate: 2015-03-11T16:44:53.674191-05:
      DOI: 10.1002/2014GB004933
       
  • Geographic variability in organic carbon stock and accumulation rate in
           sediments of East and Southeast Asian seagrass meadows
    • Authors: Toshihiro Miyajima; Masakazu Hori, Masami Hamaguchi, Hiromori Shimabukuro, Hiroshi Adachi, Hiroya Yamano, Masahiro Nakaoka
      Abstract: Organic carbon (OC) stored in the sediments of seagrass meadows has been considered a globally significant OC reservoir. However, the sparsity and regional bias of studies on long‐term OC accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global OC sink. We evaluated the amount and accumulation rate of OC in sediment of seagrass meadows and adjacent areas in East and Southeast Asia. In temperate sites, the average OC concentration in the top 30 cm of sediment was higher in seagrass meadows (780–1080 mmol g−1) than in sediments without seagrass cover (52–430 mmol g−1). The average OC in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140–440 mmol g−1. Carbon isotope mass balancing suggested that the contribution of seagrass‐derived carbon to OC stored in sediments was often relatively minor (temperate: 10–40%; subtropical: 35–82%; tropical: 4–34%) and correlated to the habitat type, being particularly low in estuarine habitats. Stock of OC in the top meter of sediment of all the studied meadows ranged from 38–120 Mg ha−1. The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32–1.34 mm yr−1). The long‐term OC accumulation rates calculated from the sediment accumulation rate and the top 30 cm average OC concentration for the seagrass meadows (24–101 kg ha−1 yr−1) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha−1 yr−1 on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.
      PubDate: 2015-03-08T18:11:13.059669-05:
      DOI: 10.1002/2014GB004979
       
  • The relation of mixed‐layer net community production to
           phytoplankton community composition in the Southern Ocean
    • Authors: Nicolas Cassar; Simon W. Wright, Paul G. Thomson, Thomas W. Trull, Karen J. Westwood, Miguel Salas, Andrew Davidson, Imojen Pearce, Diana M. Davies, Richard J. Matear
      Abstract: Surface ocean productivity mediates the transfer of carbon to the deep ocean and in the process regulates atmospheric CO2 levels. A common axiom in oceanography is that large phytoplankton contribute disproportionally to the transfer of carbon to the deep ocean because of their greater ability to escape grazing pressure,build biomass and sink. In the present study, we assessed the relationship of net community production to phytoplankton assemblages and plankton size distribution in the Subantarctic Zone (SAZ) and northern reaches of the Polar Frontal Zone (PFZ) in the Australian sector of the Southern Ocean. We reanalyzed and synthesized previously published estimates of O2/Ar‐net community oxygen production (NCP) and triple‐O2 isotopes‐gross primary oxygen production (GPP) along with microscopic and pigment analyses of the microbial community. Overall, we found the axiom that large phytoplankton drive carbon export was not supported in this region. Mixed‐layer depth‐integrated NCP was correlated to particulate organic carbon (POC) concentration in the mixed layer. While lower NCP/GPP and NCP/POC values were generally associated with communities dominated by smaller plankton size (as would be expected), these communities did not preclude high values for both properties. Vigorous NCP in some regions occurred in the virtual absence of large phytoplankton (and specifically diatoms) and in communities dominated by nanoplankton and picoplankton. We also observed a positive correlation between NCP and the proportion of the phytoplankton community grazed by microheterotrophs, supporting the mediating role of grazers in carbon export. The novel combination of techniques allowed us to determine how NCP relates to upper ocean ecosystem characteristics and may lead to improved models of carbon export.
      PubDate: 2015-03-08T18:11:11.548602-05:
      DOI: 10.1002/2014GB004936
       
  • Nitrogen and phosphorus fluxes from watersheds of the Northeast U.S. from
           1930–2000: Role of anthropogenic nutrient inputs, infrastructure,
           and runoff.
    • Authors: Rebecca L. Hale; Nancy B. Grimm, Charles J. Vörösmarty, Balazs Fekete
      Abstract: An ongoing challenge for society is to harness the benefits of nutrients, nitrogen (N) and phosphorus (P), while minimizing their negative effects on ecosystems. While there is a good understanding of the mechanisms of nutrient delivery at small scales, it is unknown how nutrient transport and processing scale up to larger watersheds and whole regions over long time periods. We used a model that incorporates nutrient inputs to watersheds, hydrology, and infrastructure (sewers, waste‐water treatment plants, and reservoirs) to reconstruct historic nutrient yields for the northeastern U.S. from 1930 to 2002. Over the study period, yields of nutrients increased significantly from some watersheds and decreased in others. As a result, at the regional scale, the total yield of N and P from the region did not change significantly. Temporal variation in regional N and P yields was correlated with runoff coefficient, but not with nutrient inputs. Spatial patterns of N and P yields were best predicted by nutrient inputs, but the correlation between inputs and yields across watersheds decreased over the study period. The effect of infrastructure on yields was minimal relative to the importance of soils and rivers. However, infrastructure appeared to alter the relationships between inputs and yields. The role of infrastructure changed over time and was important in creating spatial and temporal heterogeneity in nutrient input‐yield relationships.
      PubDate: 2015-02-25T07:58:55.328816-05:
      DOI: 10.1002/2014GB004909
       
  • Carbon dynamics of the Weddell Gyre, Southern Ocean
    • Authors: Peter J. Brown; Loïc Jullion, Peter Landschützer, Dorothee C. E. Bakker, Alberto C. Naveira Garabato, Michael P. Meredith, Sinhue Torres‐Valdés, Andrew Watson, Mario Hoppema, Brice Loose, Elizabeth M. Jones, Maciej Telszewski, Steve D. Jones, Rik Wanninkhof
      Abstract: The accumulation of carbon within the Weddell Gyre, and its exchanges across the gyre boundaries are investigated with three recent full‐depth oceanographic sections enclosing this climatically‐important region. The combination of carbon measurements with ocean circulation transport estimates from a box inverse analysis reveal that deep water transports associated with Warm Deep Water (WDW) and Weddell Sea Deep Water dominate the gyre's carbon budget, while a dual‐cell vertical overturning circulation leads to both upwelling and the delivery of large quantities of carbon to the deep ocean. Historical sea surface pCO2 observations, interpolated using a neural network technique, confirm the net summertime sink of 0.044 to 0.058 ± 0.010 Pg C yr‐1 derived from the inversion. However, a wintertime outgassing signal similar in size results in a statistically insignificant annual air‐to‐sea CO2 flux of 0.002 ± 0.007 Pg C yr‐1 (mean 1998‐2011) to 0.012 ± 0.024 Pg C yr‐1 (mean 2008‐2010) to be diagnosed for the Weddell Gyre. A surface layer carbon balance, independently derived from in situ biogeochemical measurements reveals that freshwater inputs and biological drawdown decrease surface ocean inorganic carbon levels more than they are increased by WDW entrainment, resulting in an estimated annual carbon sink of 0.033 ± 0.021 Pg C yr‐1. Although relatively less efficient for carbon uptake than the global oceans, the summertime Weddell Gyre suppresses the winter outgassing signal, while its biological pump and deep water formation act as key conduits for transporting natural and anthropogenic carbon to the deep ocean where they can reside for long timescales.
      PubDate: 2015-02-19T00:54:45.332305-05:
      DOI: 10.1002/2014GB005006
       
  • Dissolved Fe and Al in the upper 1000m of the eastern Indian Ocean: a
           high‐resolution transect along 95°E from the Antarctic margin
           to the Bay of Bengal
    • Authors: Maxime M. Grand; Christopher I. Measures, Mariko Hatta, William T. Hiscock, William M. Landing, Peter L. Morton, Clifton S. Buck, Pamela M. Barrett, Joseph A. Resing
      Abstract: A high‐resolution section of dissolved iron (dFe) and aluminium (dAl) was obtained along ~95°E in the upper 1000m of the eastern Indian Ocean from the Antarctic margin (66°S) to the Bay of Bengal (18°N) during the US‐CLIVAR‐CO2 Repeat Hydrography I08S and I09N sections (February‐April 2007). In the Southern Ocean, low concentrations of dAl (
      PubDate: 2015-02-17T21:16:28.327974-05:
      DOI: 10.1002/2014GB004920
       
  • Dust deposition in the eastern Indian Ocean: the ocean perspective from
           Antarctica to the Bay of Bengal
    • Authors: Maxime M. Grand; Christopher I. Measures, Mariko Hatta, William T. Hiscock, Clifton S. Buck, William M. Landing
      Abstract: Atmospheric deposition is an important but still poorly constrained source of trace micronutrients to the open ocean because of the dearth of in situ measurements of total deposition (i.e., wet + dry deposition) in remote regions. In this work, we discuss the upper‐ocean distribution of dissolved Fe and Al in the eastern Indian Ocean along a 95°E meridional transect spanning the Antarctic margin to the Bay of Bengal. We use the mixed layer concentration of dissolved Al in conjunction with empirical data in a simple steady state model to produce 75 estimates of total dust deposition that we compare with historical observations and atmospheric model estimates. Except in the northern Bay of Bengal where the Ganges‐Brahmaputra river plume contributes to the inventory of dissolved Al, the surface distribution of dissolved Al along 95°E is remarkably consistent with the large‐scale gradients in mineral dust deposition and multiple source regions impacting the eastern Indian Ocean. The lowest total dust deposition fluxes are calculated for the Southern Ocean (66±60 mg m‐2 yr‐1) and the highest for the northern end of the south Indian subtropical gyre (up to 940 mg m‐2 yr‐1 at 18°S) and in the southern Bay of Bengal (2,500±570 mg m‐2 yr‐1). Our total deposition fluxes, which have an uncertainty on the order of a factor 3.5, are comparable with the composite atmospheric model data of Mahowald et al. (2005), except in the south Indian subtropical gyre where models may underestimate total deposition. Using available measurements of the solubility of Fe in aerosols, we confirm that dust deposition is a minor source of dissolved Fe to the Southern Ocean and show that aeolian deposition of dissolved Fe in the southern Bay of Bengal may be comparable to that observed underneath the Saharan dust plume in the Atlantic Ocean.
      PubDate: 2015-02-16T16:34:08.73026-05:0
      DOI: 10.1002/2014GB004898
       
  • Preferential remineralization of dissolved organic phosphorus and
           non‐Redfield DOM dynamics in the global ocean: Impacts on marine
           productivity, nitrogen fixation, and carbon export
    • Authors: Robert T. Letscher; J. Keith Moore
      Abstract: Selective removal of nitrogen (N) and phosphorus (P) from the marine dissolved organic matter (DOM) pool has been reported in several regional studies. Because DOM is an important advective/mixing pathway of carbon (C) export from the ocean surface layer and its non‐Redfieldian stoichiometry would affect estimates of marine export production per unit N and P, we investigated the stoichiometry of marine DOM and its remineralization globally using a compiled DOM dataset. Marine DOM is enriched in C and N compared to Redfield stoichiometry, averaging 317:39:1 and 810:48:1 for C:N:P within the degradable and total bulk pools, respectively. Dissolved organic phosphorus (DOP) is found to be preferentially remineralized about twice as rapidly with respect to the enriched C:N stoichiometry of marine DOM. Biogeochemical simulations with the Biogeochemical Elemental Cycling model using Redfield and variable DOM stoichiometry corroborate the need for non‐Redfield dynamics to match the observed DOM stoichiometry. From our model simulations, preferential DOP remineralization is found to increase the strength of the biological pump by ~9% versus the case of Redfield DOM cycling. Global net primary productivity increases ~10% including an increase in marine nitrogen fixation of ~26% when preferential DOP remineralization and direct utilization of DOP by phytoplankton are included. The largest increases in marine nitrogen fixation, NPP, and carbon export are observed within the western subtropical gyres, suggesting the lateral transfer of P in the form of DOP from the productive eastern and poleward gyre margins may be important for sustaining these processes downstream in the subtropical gyres.
      PubDate: 2015-02-13T07:02:48.931038-05:
      DOI: 10.1002/2014GB004904
       
  • The impact of atmospheric pCO2 on carbon isotope ratios of the atmosphere
           and ocean
    • Authors: Eric D. Galbraith; Eun Young Kwon, Daniele Bianchi, Mathis P. Hain, Jorge L. Sarmiento
      Abstract: It is well known that the equilibration timescale for the isotopic ratios 13C/12C and 14C/12C in the ocean mixed layer is on the order of a decade, two orders of magnitude slower than for oxygen. Less widely‐appreciated is the factthat the equilibration timescale is quite sensitive to the speciation of Dissolved Inorganic Carbon (DIC) in the mixed layer, scaling linearly with the ratio DIC/CO 2, which varies inversely with atmospheric pCO 2. Although this effect is included in models that resolve the role of carbon speciation in air‐sea exchange, its role is often unrecognized, and it is not commonly considered in the interpretation of carbon isotope observations. Here, we use a global 3‐dimensional ocean model to estimate the redistribution of the carbon isotopic ratios between the atmosphere and ocean due solely to variations in atmospheric pCO 2. Under Last Glacial Maximum (LGM) pCO 2, atmospheric Δ14 C is increased by ≈ 30 due to the speciation change, all else being equal, raising the surface reservoir age by about 250 years throughout most of the ocean. For 13 C, enhanced surface disequilibrium under LGM pCO2 causes the upper ocean, atmosphere and North Atlantic Deep Water δ13C to become at least 0.2 higher relative to deep waters ventilated by the Southern Ocean. Conversely, under high pCO2, rapid equilibration greatly decreases isotopic disequilibrium. As a result, during geological periods of high pCO2, vertical δ13C gradients may have been greatly weakened as a direct chemical consequence of the high pCO2, masquerading as very well‐ventilated or biologically‐deadÔStrangeloveÕ oceans. The ongoing anthropogenic rise of pCO2 is accelerating the equilibration of the carbon isotopes in the ocean, lowering atmospheric Δ14C and weakening δ13C gradients within the ocean to a degree that is similar to the traditional fossil fuel ’Suess’ effect.
      PubDate: 2015-02-09T08:26:52.665271-05:
      DOI: 10.1002/2014GB004929
       
  • Issue Information
    • Abstract: Cover: Holzer and Brzezinski [doi:10.1002/2014GB004967] added isotope fractionation to a steady‐state data‐constrained model of the oceanic silicon cycle to calculate the global distribution of the isotopes of dissolved silicon, plotted here as zonal averages for the global ocean (GLB) and for each basin (ATL, PAC, IND). The δ30Si isotope ratio was partitioned into preformed and regenerated contributions traced back to their origin in the euphotic zone. This analysis showed that regenerated silicic acid, and possibly fractionation on opal dissolution, control the deep vertical gradients of δ30Si, such as the observed weak vertical gradients in the deep South Pacific that are captured in the PAC panel. See pp. 267–287.
       
  • Atmospheric observations inform CO2 flux responses to enviroclimatic
           drivers
    • Abstract: Understanding the response of the terrestrial biospheric carbon cycle to variability in enviroclimatic drivers is critical for predicting climate‐carbon interactions. Here we apply an atmospheric‐inversion‐based framework to assess the relationships between the spatiotemporal patterns of net ecosystem exchange (NEE) and those of enviroclimatic drivers. We show that those relationships can be directly observed at 1°×1° 3‐hourly resolution from atmospheric CO2 measurements for four of seven large biomes in North America, namely (i) boreal forests and taiga, (ii) temperate coniferous forests, (iii) temperate grasslands, savannas, shrublands, and (iv) temperate broadleaf and mixed forests. We find that shortwave radiation plays a dominant role during the growing season over all four biomes. Specific humidity and precipitation also play key roles and are associated with decreased uptake (or increased sources). The explanatory power of specific humidity is especially strong during transition seasons, while that of precipitation appears during both the growing and dormant seasons. We further find that the ability of four prototypical Terrestrial Biospheric Models (TBMs) to represent the spatiotemporal variability of NEE improves as the influence of radiation becomes more dominant, implying that TBMs have better skill in representing the impact of radiation relative to other drivers. Even so, we show that TBMs underestimate the strength of the relationship to radiation and do not fully capture its seasonality. Furthermore, the TBMs appear to misrepresent the relationship to precipitation and specific humidity at the examined scales, with relationships that are not consistent in terms of sign, seasonality, or significance relative to observations. More broadly, we demonstrate the feasibility of directly probing relationships between NEE and enviroclimatic drivers at scales with no direct measurements of NEE, opening the door to the study of emergent processes across scales and to the evaluation of their scaling within TBMs.
       
  • A revised global estimate of dissolved iron fluxes from marine sediments
    • Abstract: Literature data on benthic dissolved iron (DFe) fluxes (µmol m−2 d−1), bottom water oxygen concentrations (O2BW, μM) and sedimentary carbon oxidation rates (COX, mmol m−2 d−1) from water depths ranging from 80 to 3700 m were assembled. The data were analyzed with a diagenetic iron model to derive an empirical function for predicting benthic DFe fluxes: DFeflux=γ⋅TANHCOXO2BW where γ (=170 µmol m−2 d−1) is the maximum flux for sediments at steady state located away from river mouths. This simple function unifies previous observations that COX and O2BW are important controls on DFe fluxes. Upscaling predicts a global DFe flux from continental margin sediments of 109 ± 55 Gmol yr−1, of which 72 Gmol yr−1 is contributed by the shelf (2000 m) of 41 ± 21 Gmol yr−1 is unsupported by empirical data. Previous estimates of benthic DFe fluxes derived using global iron models are far lower (ca. 20–30 Gmol yr−1). This can be attributed to (i) inadequate treatment of the role of oxygen on benthic DFe fluxes, and (ii) improper consideration of continental shelf processes due to coarse spatial resolution. Globally‐averaged DFe concentrations in surface waters simulated with an intermediate‐complexity Earth system climate model (UVic ESCM) were a factor of two higher with the new function. We conclude that (i) the DFe flux from marginal sediments has been underestimated in the marine iron cycle, and (ii) iron scavenging in the water column is more intense than currently presumed.
       
  • Spatial patterns in CO2 evasion from the global river network
    • Abstract: CO2 evasion from rivers (FCO2) is an important component of the global carbon budget. Here, we present the first global maps of CO2 partial pressures (pCO2) in rivers of stream order 3 and higher and the resulting FCO2 at 0.5° resolution constructed with a statistical model. Our statistical model based upon a GIS based approach is used to derive a pCO2 prediction function trained on data from 1182 sampling locations. While data from Asia and Africa are scarce and the training data set is dominated by sampling locations from the Americas, Europe, and Australia, the sampling locations cover the full spectrum from high to low latitudes. The predictors of pCO2 are net primary production, population density and slope gradient within the river catchment as well as mean air temperature at the sampling location (r2=0.47). The predicted pCO2 map was then combined with spatially explicit estimates of stream surface area Ariver and gas exchange velocity k calculated from published empirical equations and data sets to derive the FCO2 map. Using Monte Carlo simulations, we assessed the uncertainties of our estimates. At the global scale, we estimate an average river pCO2 of 2400 (2019–2826) µatm and a FCO2 of 650 (483–846) Tg C yr−1 (5th and 95th percentile of confidence interval). Our global CO2 evasion is substantially lower than the recent estimate of 1800 Tg C yr−1 [Raymond et al., 2013] although the training set of pCO2 is very similar in both studies, mainly due to lower tropical pCO2 estimates in the present study. Our maps reveal strong latitudinal gradients in pCO2, Ariver and FCO2. The zone between 10°N and 10°S contributes about half of the global CO2 evasion. Collection of pCO2 data in this zone, in particular for African and South East Asian rivers is a high priority to reduce uncertainty on FCO2.
       
  • The timescale of the silicate weathering negative feedback on atmospheric
           CO2
    • Abstract: The ultimate fate of CO2 added to the ocean–atmosphere system is chemical reaction with silicate minerals and burial as marine carbonates. The timescale of this silicate weathering negative feedback on atmospheric pCO2 will determine the duration of perturbations to the carbon cycle, be they geological release events or the current anthropogenic perturbation. However, there has been little previous work on quantifying the time‐scale of the silicate weathering feedback, with the primary estimate of 300–400 kyr being traceable to an early box model study by Sundquist [1991]. Here we employ a representation of terrestrial rock weathering in conjunction with the ‘GENIE’ Earth System Model to elucidate the different timescales of atmospheric CO2 regulation whilst including the main climate feedbacks on CO2 uptake by the ocean. In this coupled model, the main dependencies of weathering – runoff, temperature and biological productivity – were driven from an energy‐moisture balance atmosphere model and parameterized plant productivity. Long‐term projections (1 Myr) were conducted for idealized scenarios of 1000 and 5000 PgC fossil fuel emissions and their sensitivity to different model parameters was tested. By fitting model output to a series of exponentials we determined the e‐folding timescale for atmospheric CO2 draw‐down by silicate weathering to be ~240 kyr (range 170–380 kyr), significantly less than existing quantifications. Although the time‐scales for re‐equilibration of global surface temperature and surface ocean pH are similar to that for CO2, a much greater proportion of the peak temperature anomaly persists on this longest time‐scale; ~21% compared to ~10% for CO2.
       
  • Microbial nitrogen dynamics in organic and mineral soil horizons along a
           latitudinal transect in Western Siberia
    • Abstract: Soil N availability is constrained by the breakdown of N‐containing polymers such as proteins to oligo‐peptides and amino acids that can be taken up by plants and microorganisms. Excess N is released from microbial cells as ammonium (N mineralization), which in turn can serve as substrate for nitrification. According to stoichiometric theory, N mineralization and nitrification are expected to increase in relation to protein depolymerization with decreasing N limitation, and thus from higher to lower latitudes and from topsoils to subsoils. To test these hypotheses, we compared gross rates of protein depolymerization, N mineralization and nitrification (determined using 15N pool dilution assays) in organic topsoil, mineral topsoil and mineral subsoil of seven ecosystems along a latitudinal transect in Western Siberia, from tundra (67°N) to steppe (54°N). The investigated ecosystems differed strongly in N transformation rates, with highest protein depolymerization and N mineralization rates in middle and southern taiga. All N transformation rates decreased with soil depth following the decrease in organic matter content. Related to protein depolymerization, N mineralization and nitrification were significantly higher in mineral than in organic horizons, supporting a decrease in microbial N limitation with depth. In contrast, we did not find indications for a decrease in microbial N limitation from arctic to temperate ecosystems along the transect. Our findings thus challenge the perception of ubiquitous N limitation at high latitudes, but suggest a transition from N to C limitation of microorganisms with soil depth, even in high latitude systems such as tundra and boreal forest.
       
  • Isotopic evidence for nitrification in the Antarctic winter mixed layer
    • Abstract: We report wintertime nitrogen and oxygen isotope ratios (δ15N and δ18O) of seawater nitrate in the Southern Ocean south of Africa. Depth profile and underway surface samples collected in July 2012 extend from the subtropics to just beyond the Antarctic winter sea‐ice edge. We focus here on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile δ15N data yields estimates for the isotope effect (the degree of isotope discrimination) of nitrate assimilation (1.6‐3.3‰) that are significantly lower than commonly observed in the summertime Antarctic (5‐8‰). δ18O data from the same depth profiles and lateral δ15N variations within the mixed layer, however, imply O and N isotope effects that are more similar to those suggested by summertime data. These findings point to active nitrification (i.e., regeneration of organic matter to nitrate) within the Antarctic winter mixed layer. Nitrite removal from samples reveals a low δ15N for nitrite in the winter mixed layer (‐40‰ to ‐20‰), consistent with nitrification, but does not remove the observation of an anomalously low δ15N for nitrate. The winter data, and the nitrification they reveal, explain the previous observation of an anomalously low δ15N for nitrate in the temperature minimum layer (remnant winter mixed layer) of summertime depth profiles. At the same time, the wintertime data require a low δ15N for the combined organic N and ammonium in the autumn mixed layer that is available for wintertime nitrification, pointing to intense N recycling as a pervasive condition of the Antarctic in late summer.
       
  • Predicting long‐term carbon sequestration in response to CO2
           enrichment: How and why do current ecosystem models differ'
    • Abstract: Large uncertainty exists in model projections of the land carbon (C) sink response to increasing atmospheric CO2. Free‐Air CO2 Enrichment (FACE) experiments lasting a decade or more have investigated ecosystem responses to a step change in atmospheric CO2 concentration. To interpret FACE results in the context of gradual increases in atmospheric CO2 over decades to centuries, we used a suite of seven models to simulate the Duke and Oak Ridge FACE experiments extended for 300 years of CO2 enrichment. We determine key modelling assumptions that drive divergent projections of terrestrial C uptake and evaluate whether these assumptions can be constrained by experimental evidence. All models simulated increased terrestrial C pools resulting from CO2 enrichment, though there was substantial variability in quasi‐equilibrium C sequestration and rates of change. In two of two models that assume that plant nitrogen (N) uptake is solely a function of soil N supply, the NPP response to elevated CO2 became progressively N limited. In four of five models that assume that N uptake is a function of both soil N supply and plant N demand elevated CO2 led to reduced ecosystem N losses and thus progressively relaxed nitrogen limitation. Many allocation assumptions resulted in increased wood allocation relative to leaves and roots which reduced the vegetation turnover rate and increased C sequestration. In addition, self‐thinning assumptions had a substantial impact on C sequestration in two models. Accurate representation of N process dynamics (in particular N uptake), allocation, and forest self‐thinning are key to minimising uncertainty in projections of future C sequestration in response to elevated atmospheric CO2.
       
  • The effects of secular calcium and magnesium concentration changes on the
           thermodynamics of seawater acid/base chemistry: Implications for Eocene
           and Cretaceous ocean carbon chemistry and buffering
    • Abstract: Reconstructed changes in seawater calcium and magnesium concentration ([Ca2+], [Mg2+]) predictably affect the ocean's acid/base and carbon chemistry. Yet, inaccurate formulations of chemical equilibrium “constants” are currently in use to account for these changes. Here we develop an efficient implementation of the MIAMI Ionic Interaction Model (Millero and Pierrot, 1998) to predict all chemical equilibrium constants required for carbon chemistry calculations under variable [Ca2+] and [Mg2+]. We investigate the impact of [Ca2+] and [Mg2+] on the relationships among the ocean's pH, CO2, dissolved inorganic carbon (DIC), saturation state of CaCO3 (Ω) and buffer capacity. Increasing [Ca2+] and/or [Mg2+] enhances “ion pairing,” which increases seawater buffering by increasing the concentration ratio of total to “free” (uncomplexed) carbonate ion. An increase in [Ca2+], however, also causes a decline in carbonate ion to maintain a given Ω, thereby overwhelming the ion pairing effect and decreasing seawater buffering. Given the reconstructions of Eocene [Ca2+] and [Mg2+] ([Ca2+]~20mM; [Mg2+]~30mM), Eocene seawater would have required essentially the same DIC as today to simultaneously explain a similar‐to‐modern Ω and the estimated Eocene atmospheric CO2 of ~1000ppm. During the Cretaceous, at ~4x modern [Ca2+], ocean buffering would have been at a minimum. Overall, during times of high seawater [Ca2+], CaCO3 saturation, pH and atmospheric CO2 were more susceptible to perturbations of the global carbon cycle. For example, given both Eocene and Cretaceous seawater [Ca2+] and [Mg2+], a doubling of atmospheric CO2 would require less carbon addition to the ocean/atmosphere system than under modern seawater composition. Moreover, declining seawater buffering since the Cretaceous may have been a driver of evolution by raising energetic demands of biologically controlled calcification and CO2 concentration mechanisms that aid photosynthesis.
       
  • Modeling the fate of methane hydrates under global warming
    • Abstract: Large amounts of methane hydrate locked up within marine sediments are vulnerable to climate change. Changes in bottom water temperatures may lead to their destabilization and the release of methane into the water column or even the atmosphere. In a multi‐model approach, the possible impact of destabilizing methane hydrates onto global climate within the next century is evaluated. The focus is set on changing bottom water temperatures to infer the response of the global methane hydrate inventory to future climate change. Present and future bottom water temperatures are evaluated by the combined use of hindcast high‐resolution ocean circulation simulations and climate modeling for the next century. The changing global hydrate inventory is computed using the parameterized transfer function recently proposed by Wallmann et al. (2012). We find that the present‐day world's total marine methane hydrate inventory is estimated to be 1146Gt of methane carbon. Within the next 100 years this global inventory may be reduced by ~ 0.03% (releasing ~ 473Mt methane from the seafloor). Compared to the present‐day annual emissions of anthropogenic methane, the amount of methane released from melting hydrates by 2100 is small and will not have a major impact on the global climate. On a regional scale, ocean bottom warming over the next 100 years will result in a relatively large decrease in the methane hydrate deposits, with the Arctic and Blake Ridge region, offshore South Carolina, being most affected.
       
  • Detecting the progression of ocean acidification from the saturation state
           of CaCO3 in the subtropical South Pacific
    • Abstract: Progression of ocean acidification in the subtropical South Pacific was investigated by using high‐quality data from trans‐Pacific zonal section at 17°S (World Ocean Circulation Experiment section P21) collected in 1994 and 2009. During this 15‐year period, the CaCO3 saturation state of seawater with respect to calcite (Ωcal) and aragonite (Ωarg) in the upper water column (
       
 
 
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