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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: 78, SJR: 2.189, h-index: 121)
Journal Cover   Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [5 followers]  Follow
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   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • Biogeochemical drivers of the fate of riverine mercury discharged to the
           global and Arctic oceans
    • Authors: Yanxu Zhang; Daniel J. Jacob, Stephanie Dutkiewicz, Helen M. Amos, Michael S. Long, Elsie M. Sunderland
      Abstract: Rivers discharge 28±13 Mmol a−1 of mercury (Hg) to ocean margins, an amount comparable to atmospheric deposition to the global oceans. Most of the Hg discharged by rivers is sequestered by burial of benthic sediment in estuaries or the coastal zone, but some is evaded to the atmosphere and some is exported to the open ocean. We investigate the fate of riverine Hg by developing a new global 3‐D simulation for Hg in the MIT ocean general circulation model (MITgcm). The model includes plankton dynamics and carbon respiration (DARWIN project model) coupled to inorganic Hg chemistry. Results are consistent with observed spatial patterns and magnitudes of surface ocean Hg concentrations. We use observational constraints on seawater Hg concentrations and evasion to infer that most Hg from rivers is sorbed to refractory organic carbon and preferentially buried. Only 6% of Hg discharged by rivers (1.8 Mmol a−1) is transported to the open ocean on a global basis. This fraction varies from a low of 2.6% in East Asia due to the barrier imposed by the Korean Peninsula and Japanese Archipelago, up to 25% in eastern North America facilitated by the Gulf Stream. In the Arctic Ocean, low tributary particle loads and efficient degradation of particulate organic carbon by deltaic microbial communities favors a more labile riverine Hg pool. Evasion of Hg to the Arctic atmosphere is indirectly enhanced by heat transport during spring freshet that accelerates sea‐ice melt and ice rafting. Discharges of 0.23 Mmol Hg a −1 from Arctic rivers can explain the observed summer maximum in the Arctic atmosphere and this magnitude of releases is consistent with recent observations. Our work indicates that rivers are major contributors to Hg loads in the Arctic Ocean.
      PubDate: 2015-06-25T13:54:46.531055-05:
      DOI: 10.1002/2015GB005124
  • Quantifying the relative importance of land cover change from climate and
           land‐use in the representative concentration pathways
    • Authors: T. Davies‐Barnard; P. J. Valdes, J. S. Singarayer, A. J. Wiltshire, C. D. Jones
      Abstract: Climate change is projected to cause substantial alterations in vegetation distribution, but these have been given little attention in comparison to land‐use in the Representative Concentration Pathway (RCP) scenarios. Here we assess the climate‐induced land cover changes (CILCC) in the RCPs, and compare them to land‐use land cover change (LULCC). To do this, we use an ensemble of simulations with and without LULCC in earth system model HadGEM2‐ES for RCP2.6, RCP4.5 and RCP8.5. We find that climate change causes an expansion poleward of vegetation that affects more land area than LULCC in all of the RCPs considered here. The terrestrial carbon changes from CILCC are also larger than for LULCC. When considering only forest, the LULCC is larger, but the CILCC is highly variable with the overall radiative forcing of the scenario. The CILCC forest increase compensates 90% of the global anthropogenic deforestation by 2100 in RCP8.5, but just 3% in RCP2.6. Overall, bigger land cover changes tend to originate from LULCC in the shorter term or lower radiative forcing scenarios, and from CILCC in the longer term and higher radiative forcing scenarios. The extent to which CILCC could compensate for LULCC raises difficult questions regarding global forest and biodiversity offsetting, especially at different timescales. This research shows the importance of considering the relative size of CILCC to LULCC, especially with regard to the ecological effects of the different RCPs.
      PubDate: 2015-06-22T14:24:52.982011-05:
      DOI: 10.1002/2014GB004949
  • Transports and budgets of anthropogenic CO2 in the tropical North Atlantic
           in 1992–93 and 2010–11
    • Authors: Patricia Zunino; Fiz F. Pérez, Noelia M. Fajar, Elisa F. Guallart, Aida Ríos, Josep L. Pelegrí, A. Hernández‐Guerra
      Abstract: The meridional transport of anthropogenic CO2 (Cant) in the tropical North Atlantic (TNA) is investigated using data from transoceanic sections along 7.5°N and 24.5 °N, carried out in the early 1990s and 2010s. The net Cant transport across both sections is northward. At 7.5°N, this transport increased from 315 ± 47 kmol s−1 in 1993 to 493 ± 51 kmol s−1 in 2010; similarly, across 24.5°N it grew from 530 ± 46 kmol s−1 in 1992 to 662 ± 49 kmol s−1 in 2011. These changes result from modifications in the intermediate and deep circulation patterns, as well as from Cant increase within the thermocline waters. In deep waters, lateral advection causes a net Cant input of 112 ± 60 kmol s−1 (234 ± 65 kmol s−1) in 1992–93 (2010–11); within these deep waters, the storage rate of Cant is not statistically different from the net Cant input, 139 ± 21 kmol s−1 (188 ± 21 kmol s−1) in 1992–93 (2010–11). The Cant increase in deep waters is due to the large injection of Cant across the 24.5°N by the Deep Western Boundary Current and the northward recirculation of North Atlantic Deep Water along 7.5°N. In contrast, a large net Cant output in the upper layer is caused by the Florida Current. Despite this net Cant output, the Cant accumulates at a rate of 215 ± 24 kmol s−1 (291 ± 24 kmol s−1) referenced to year 1993 (2010). From the two Cant budgets, we infer a Cant air‐sea flux of 0.23 ± 0.02 Pg yr−1in the TNA, much larger than previous estimates.
      PubDate: 2015-06-22T09:12:46.349955-05:
      DOI: 10.1002/2014GB005075
  • A new look at ocean carbon remineralization for estimating
           deep‐water sequestration
    • Authors: Lionel Guidi; Louis Legendre, Gabriel Reygondeau, Julia Uitz, Lars Stemmann, Stephanie A. Henson
      Abstract: The “biological carbon pump” causes carbon sequestration in deep waters by downward transfer of organic matter, mostly as particles. This mechanism depends to a great extent on the uptake of CO2 by marine plankton in surface waters, and subsequent sinking of particulate organic carbon (POC) through the water column. Most of the sinking POC is remineralized during its downward transit and modest changes in remineralization have substantial feedbacks on atmospheric CO2 concentrations, but little is known about global variability in remineralization. Here we assess this variability based on modern underwater particle imaging combined with field POC flux data, and discuss the potential sources of variations. We show a significant relationship between remineralization and the size structure of the phytoplankton assemblage. We obtain the first regionalized estimates of remineralization in biogeochemical provinces, where these estimates range between ‐50 and +100% of the commonly used globally uniform remineralization value. We apply the regionalized values to satellite‐derived estimates of upper ocean POC export to calculate regionalized and ocean‐wide deep carbon fluxes and sequestration. The resulting value of global organic carbon sequestration at 2000 m is 0.33 Pg C y‐1, and 0.72 Pg C y‐1 at the depth of the top of the permanent pycnocline, which is up to 3 times higher than the value resulting from the commonly‐used approach based on uniform remineralization and constant sequestration depth. These results stress that variable remineralization length scale and sequestration depth should be used to model ocean carbon sequestration and feedbacks on the atmosphere.
      PubDate: 2015-06-18T19:25:21.63808-05:0
      DOI: 10.1002/2014GB005063
  • Processes Controlling the Distributions of Cd and PO4 in the Ocean
    • Authors: Paul Quay; Jay Cullen, William Landing, Peter Morton
      Abstract: Depth profiles of dissolved Cd and PO4 from a global data compilation including recent GEOTRACES and CLIVAR cruises were used to derive the Cd/P of particles exported from the surface layer and the results indicate lowest values in the North Atlantic (0.17±0.05), highest in the Southern (0.56±0.24) and intermediate in the South Indian (0.31±0.14) and North Pacific (0.36±0.08) Ocean basins. The Cd/P of exported particles in high nutrient‐low chlorophyll (HNLC) regions is twice that for particles exported in non‐HNLC regions as is the fractionation effect during biological uptake of Cd and PO4 and these trends primarily determine the spatial trends of dissolved Cd/PO4 observed in the surface ocean. In deep waters the lowest dissolved Cd/PO4 of 0.23±0.07 is found in the North Atlantic Ocean and the result primarily of low Cd/PO4 of North Atlantic Deep Water (0.23). In contrast, deep waters in the Southern Ocean have significantly higher dissolved Cd/PO4 (0.30±.06) which is a result of the Cd/PO4 of upwelled deep water from the South Pacific and South Indian (0.27) and the high Cd/P of degrading particles. A multi‐box model that accounts for the impacts of particle degradation and thermohaline circulation in the deep sea yields dissolved Cd and PO4 inter‐basin trends close to observations. Model experiments illustrate the dependence of the dissolved Cd/PO4 of the deep sea on the extent of HNLC conditions in the Southern Ocean and the impact on reconstructing paleo PO4 concentrations from a Cd proxy.
      PubDate: 2015-06-17T16:39:28.617165-05:
      DOI: 10.1002/2014GB004998
  • Why are biotic iron pools uniform across high‐ and low‐iron
           pelagic ecosystems'
    • Authors: P.W. Boyd; R.F. Strzepek, M.J. Ellwood, D.A. Hutchins, S.D. Nodder, B.S. Twining, S.W. Wilhelm
      Abstract: Dissolved iron supply is pivotal in setting global phytoplankton productivity and pelagic ecosystem structure. However, most studies of the role of iron have focussed on carbon biogeochemistry within pelagic ecosystems, with less effort to quantify the iron biogeochemical cycle. Here, we compare mixed‐layer biotic iron inventories from a low‐iron (~0.06 nmol L−1) subantarctic (FeCycle study) and a seasonally high‐iron (~0.6 nmol L−1) subtropical (FeCycle II study) site. Both studies were quasi‐Lagrangian, and had multi‐day occupation, common sampling protocols, and indirect estimates of biotic iron (from a limited range of available published biovolume/carbon/iron quotas). Biotic iron pools were comparable (~100± 30 pmol L−1) for low‐ and high‐iron waters, despite a ten‐fold difference in dissolved iron concentrations. Consistency in biotic iron inventories (~80±24 pmol L−1, largely estimated using a limited range of available quotas) was also conspicuous for three Southern Ocean polar sites. Insights into the extent to which uniformity in biotic iron inventories was driven by the need to apply common iron quotas obtained from laboratory cultures were provided from FeCycle II. The observed two‐ to three‐fold range of iron quotas during the evolution of FeCycle II subtropical bloom was much less than reported from laboratory monocultures. Furthermore, the iron recycling efficiency varied by fourfold during FeCycle II, increasing as stocks of new iron were depleted, suggesting that quotas and iron recycling efficiencies together set biotic iron pools. Hence, site‐specific differences in iron recycling efficiencies (which provide 20‐50% and 90% of total iron supply in high and low iron waters, respectively) helps offset the differences in new iron inputs between low‐ and high‐iron sites. Future parameterisation of iron in biogeochemical models must focus on the drivers of biotic iron inventories, including the differing iron requirements of the resident biota, and the subsequent fate (retention/export/recycling) of the biotic iron.
      PubDate: 2015-06-17T08:03:43.288577-05:
      DOI: 10.1002/2014GB005014
  • Origin and fluxes of nitrous oxide along a latitudinal transect in western
           North Pacific: Controls and regional significance
    • Authors: Florian Breider; Chisato Yoshikawa, Hitomi Abe, Sakae Toyoda, Naohiro Yoshida
      Abstract: Nitrous oxide (N2O) is an atmospheric trace gas playing an important role in both radiative forcing and stratospheric ozone depletion. The oceans are the second most important natural source of N2O. The magnitude of the flux of this source is poorly constrained. Moreover the relative importance of the microbial processes leading to the formation or the consumption of N2O in oceans remains unclear. We present here fluxes, isotope and isotopomer signatures of N2O measured at three stations located along a latitudinal transect in subtropical and subarctic Western North Pacific. These results indicate that about 30% to 55% of the oceanic flux of N2O to the atmosphere originates from the deep euphotic and shallow aphotic zones. The sea‐to‐air fluxes of N2O calculated using an isotope mass balance model indicate that the emission rate of N2O in subarctic waters is about two times higher than in oligotrophic subtropical waters suggesting that nutrient‐rich water coming from the western subarctic gyre stimulates the N2O production. Moreover, isotopomer analysis has revealed that in shallow water N2O originates from nitrification and nitrifier‐denitrification processes and its distribution in the water column is partly controlled by the incident solar radiation. The results of this study contribute to better constrain the global N2O budget and provide important information to better predict the future evolution of the oceanic emissions of N2O.
      PubDate: 2015-06-17T08:03:26.97157-05:0
      DOI: 10.1002/2014GB004977
  • Biological and physical controls on N2, O2 and CO2 distributions in
           contrasting Southern Ocean surface waters
    • Authors: Philippe D. Tortell; Henry C. Bittig, Arne Körtzinger, Elizabeth M. Jones, Mario Hoppema
      Abstract: We present measurements of pCO2, O2 concentration, biological oxygen saturation (ΔO2/Ar) and N2 saturation (ΔN2) in Southern Ocean surface waters during austral summer, 2010–2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chla) concentrations in regions of frontal mixing and sea‐ice melt. pCO2 and ΔO2 /Ar exhibited large spatial gradients (range 90 to 450 µatm and −10 to 60%, respectively) and co‐varied strongly with Chla. However, the ratio of biological O2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time‐scales of O2 and CO2 air‐sea equilibration. We measured significant oceanic CO2 uptake, with a mean air‐sea flux (~ −10 mmol m−2 d−1) that significantly exceeded regional climatological values. N2 was mostly supersaturated in surface waters (mean ΔN2 of +2.5 %), while physical processes resulted in both supersaturation and undersaturation of mixed layer O2 (mean ΔO2phys = 2.1 %). Box model calculations were able to reproduce much of the spatial variability of ΔN2 and ΔO2phys along the cruise track, demonstrating significant effects of air‐sea exchange processes (e.g. atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment‐corrected surface ΔO2 /Ar data, ranged from ~ −40 to > 300 mmol O2 m−2 d−1 and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and stratified regions of sea‐ice melt, reflecting physical controls on surface water light fields and nutrient availability.
      PubDate: 2015-06-16T06:18:50.485864-05:
      DOI: 10.1002/2014GB004975
  • Issue Information
    • Abstract: Cover: In Colbourn et al. [doi 10.1002/2014GB005054], illustration of the long‐term (geological) carbon cycle fluxes. Shown are the longterm fluxes in the GENIE model at steady state. In red are sources of CO2 to the atmosphere or ocean, and in dark blue are sinks of CO2. See pp. 583–596.
      PubDate: 2015-06-15T14:30:08.738675-05:
      DOI: 10.1002/gbc.20196
  • Flux and budget of black carbon (BC) in the continental shelf seas
           adjacent to Chinese high BC emission source regions
    • Authors: Yin Fang; Yingjun Chen, Chongguo Tian, Tian Lin, Limin Hu, Guopei Huang, Jianhui Tang, Jun Li, Gan Zhang
      Abstract: This study conducted the first comprehensive investigation of sedimentary black carbon (BC) concentration, flux and budget in the continental shelves of “Bohai Sea (BS) and Yellow Sea (YS)”, based on measurements of BC in 191 surface sediments, 36 riverine water and 2 seawater samples, as well as the reported dataset of the atmospheric samples from seven coastal cities in the Bohai Rim. BC concentrations in these matrices were measured using the method of thermal/optical reflectance. The spatial distribution of the BC concentration in surface sediments was largely influenced by the regional hydrodynamic conditions, with high values mainly occurring in the central mud areas where fine‐grained particles (median diameters >6 Φ (i.e.,
      PubDate: 2015-06-10T10:42:21.623058-05:
      DOI: 10.1002/2014GB004985
  • Multi‐decadal trends of oxygen and their controlling factors in the
           western North Pacific
    • Authors: Daisuke Sasano; Yusuke Takatani, Naohiro Kosugi, Toshiya Nakano, Takashi Midorikawa, Masao Ishii
      Abstract: The rate of change of dissolved oxygen (O2) concentrations was analyzed over 1987–2011 for the high‐frequency repeat section along 165°E in the western North Pacific. Significant trends towards decreasing O2 were detected in the northern subtropical to subtropical‐subarctic transition zones over a broad range of isopycnal horizons. On 25.3σθ between 25°N‐30°N in North Pacific Subtropical Mode Water, the rate of O2 decrease reached −0.45 ± 0.16 µmol kg−1 yr−1. It is largely attributed to a deepening of isopycnal horizons and to a reduction in oxygen solubility associated with ocean warming. In North Pacific Intermediate Water, the rate of O2 decrease was elevated (−0.44 ± 0.14 µmol kg−1 yr−1 on 26.8σθ) and was associated with net increases in apparent oxygen utilization in the source waters. On 27.3σθ in the subtropical Oxygen Minimum Layer (OML) between 32.5°N‐35°N, the rate of O2 decrease was significant (−0.22 ± 0.05 µmol kg−1 yr−1). It was likely due to the increases in westward transport of low‐oxygen water. These various drivers controlling changes in O2 along the 165°E section are the same as those acting along 137°E (analyzed previously), and also account for the differences in the rate of O2 decrease between these sections. Additionally, in the tropical OML near 26.8σθ between 5°N‐10°N, significant trends toward increasing O2 were detected in both sections (+0.36 ± 0.04 µmol kg−1 yr−1 in the 165°E section). These results demonstrate that warming and circulation changes are causing multi‐decadal changes in dissolved O2 over wide expanses of the western North Pacific.
      PubDate: 2015-06-08T21:58:52.516376-05:
      DOI: 10.1002/2014GB005065
    • Authors: T. S. Catalá; I. Reche, M. Álvarez, S. Khatiwala, E.F. Guallart, V. M. Benítez‐Barrios, A. Fuentes‐Lema, C. Romera‐Castillo, M. Nieto‐Cid, C. Pelejero, E. Fraile‐Nuez, E. Ortega‐Retuerta, C. Marrasé, X. A. Álvarez‐Salgado
      Abstract: The omnipresence of chromophoric dissolved organic matter (CDOM) in the open ocean enables its use as a tracer for biochemical processes throughout the global overturning circulation. We made an inventory of CDOM optical properties, ideal water age (τ) and apparent oxygen utilization (AOU) along the Atlantic, Indian and Pacific Ocean waters sampled during the Malaspina 2010 expedition. A water mass analysis was applied to obtain intrinsic, hereinafter archetypal, values of τ, AOU, oxygen utilisation rate (OUR), and CDOM absorption coefficients, spectral slopes and quantum yield for each one of the 22 water types intercepted during this circumnavigation. Archetypal values of AOU and OUR have been used to trace the differential influence of water mass ageing and ageing rates, respectively, on CDOM variables. Whereas the absorption coefficient at 325nm (a325) and the fluorescence quantum yield at 340 nm (Φ340) increased, the spectral slope over the wavelength range 275–295 nm (S275–295) and the ratio of spectral slopes over the ranges 275 –295 nm and 350–400 nm (SR) decreased significantly with water mass ageing (AOU). Combination of the slope of the linear regression between archetypal AOU and a325 with the estimated global OUR allowed us to obtain a CDOM turnover time of 634 ± 120 years, which exceeds the flushing time of the dark ocean (>200 m) by 46%. This positive relationship supports the assumption of in situ production and accumulation of CDOM as a by‐product of microbial metabolism as water masses turn older. Furthermore, our data evidence that global‐scale CDOM quantity (a325) is more dependant on ageing (AOU), whereas CDOM quality (S275–295, SR, Φ340) is more dependent on ageing rate (OUR).
      PubDate: 2015-06-02T20:46:12.123557-05:
      DOI: 10.1002/2014GB005048
  • On the influence of “non‐Redfield” dissolved organic
           nutrient dynamics on the spatial distribution of N2 fixation and the size
           of the marine fixed nitrogen inventory
    • Authors: Christopher J. Somes; Andreas Oschlies
      Abstract: Dissolved organic nitrogen (DON) and phosphorus (DOP) represent the most abundant form of their respective nutrient pool in the surface layer of the oligotrophic oceans and play an important role in nutrient cycling and productivity. Since DOP is generally more labile than DON, it provides additional P that may stimulate growth of N2‐fixing diazotrophs that supply fixed nitrogen to balance denitrification in the ocean. In this study, we introduce semi‐recalcitrant components of DON and DOP as state variables in an existing global ocean–atmosphere‐sea ice‐biogeochemistry model of intermediate complexity to assess their impact on the spatial distribution of N2‐fixation and the size of the marine fixed nitrogen inventory. Large‐scale surface datasets of global DON and Atlantic Ocean DOP are used to constrain the model. Our simulations suggest that both preferential DOP remineralization and phytoplankton DOP uptake are important “non‐Redfield” processes (i.e., deviate from molar N:P=16) that need to be accounted for to explain the observed patterns of DOP. Additional non‐Redfield DOP sensitivity experiments testing DOM production rate uncertainties that best reproduce the observed spatial patterns of DON and DOP stimulate additional N2‐fixation that increases the size of the global marine fixed nitrogen inventory by 4.7±1.7% compared to the simulation assuming Redfield DOM stoichiometry that underestimates the observed nitrogen inventory. The extra 8 Tg yr−1 of N2‐fixation stimulated in the Atlantic Ocean is mainly responsible for this increase due to its large spatial separation from water column denitrification, which buffers any potential nitrogen surplus in the Pacific Ocean. Our study suggests that the marine fixed nitrogen budget is sensitive to non‐Redfield DOP dynamics because access to the relatively labile DOP pool expands the ecological niche for N2‐fixing diazotrophs.
      PubDate: 2015-05-25T05:59:17.465865-05:
      DOI: 10.1002/2014GB005050
  • Environmental controls on the biogeography of diazotrophy and
           Trichodesmium in the Atlantic Ocean
    • Authors: J. T. Snow; C. Schlosser, E.M.S. Woodward, M.M. Mills, E. P. Achterberg, C.A. Mahaffey, T.S. Bibby, C.M. Moore
      Abstract: The cyanobacterium Trichodesmium is responsible for a significant proportion of the annual 'new' nitrogen introduced into the global ocean. Despite being arguably the best studied marine diazotroph, the factors controlling the distribution and growth of Trichodesmium remain a subject of debate, with sea surface temperature, the partial pressure of CO2 and nutrients including iron (Fe) and phosphorus (P), all suggested to be important. Synthesising data from 7 cruises collectively spanning large temporal and spatial scales across the Atlantic Ocean, including 2 previously unreported studies crossing the largely under‐sampled South Atlantic gyre, we assessed the relationship between proposed environmental drivers and both community N2 fixation rates and the distribution of Trichodesmium. Simple linear regression analysis would suggest no relationship between any of the sampled environmental variables and N2 fixation rates. However, considering the concentrations of iron and phosphorus together within a simplified resource‐ratio framework, illustrated using an idealised numerical model, indicates the combined effects these nutrients have on Trichodesmium and broader diazotroph biogeography, alongside the reciprocal maintenance of different biogeographic provinces of the (sub)‐tropical Atlantic in states of Fe or P oligotrophy by diazotrophy. The qualitative principles of the resource‐ratio framework are argued to be consistent with both the previously described North–south Atlantic contrast in Trichodesmium abundance and the presence and consequence of a substantial non‐Trichodesmium diazotrophic community in the western South Atlantic subtropical gyre. A comprehensive, observation‐based explanation of the interactions between Trichodesmium and the wider diazotrophic community with iron and phosphorus in the Atlantic Ocean is thus revealed.
      PubDate: 2015-05-23T16:25:44.14183-05:0
      DOI: 10.1002/2015GB005090
  • New model for capturing the variations of fertilizer‐induced
           emission factors of N2O
    • Authors: Feng Zhou; Ziyin Shang, Zhenzhong Zeng, Shilong Piao, Philippe Ciais, Peter A. Raymond, Xuhui Wang, Rong Wang, Minpeng Chen, Changliang Yang, Shu Tao, Yue Zhao, Qian Meng, Shuoshuo Gao, Qi Mao
      Abstract: Accumulating evidence indicates that N2O emission factors (EFs) vary with nitrogen additions and environmental variations. Yet the impact of the latter was often ignored by previous EF determinations. We developed piecewise statistical models (PMs) to explain how the N2O EFs in agricultural soils depend upon various predictors such as climate, soil attributes, and agricultural management. The PMs are derived from a new Bayesian Recursive Regression Tree algorithm. The PMs were applied to the case of EFs from agricultural soils in China, a country where large EF spatial gradients prevail. The results indicate substantial improvements of the PMs compared with other EF determinations. First, PMs are able to reproduce a larger fraction of the variability of observed EFs for upland grain crops (84%, n=381) and paddy rice (91%, n=161) as well as the ratio of EFs to nitrogen application rates (73%, n=96). The superior predictive accuracy of PMs is further confirmed by evaluating their predictions against independent EF measurements (n=285) from outside China. Results show that the PMs calibrated using Chinese data can explain 75% of the variance. Hence the PMs could be reliable for upscaling of N2O EFs and fluxes for regions that have a phase‐space of predictors similar to China. Results from the validated models also suggest that climatic factors regulate the heterogeneity of EFs in China, explaining 69% and 85% of their variations for upland grain crops and paddy rice, respectively. The corresponding N2O EFs in 2008 are 0.84±0.18% (as N2O‐N emissions divided by the total N input) for upland grain crops and 0.65±0.14% for paddy rice, the latter being twice as large as the IPCC Tier 1 defaults. Based upon these new estimates of EFs, we infer that only 22% of current arable land could achieve a potential reduction of N2O emission of 50%.
      PubDate: 2015-05-20T06:32:07.117159-05:
      DOI: 10.1002/2014GB005046
  • Metals and Metalloids in Precipitation Collected during CHINARE Campaign
           from Shanghai, China to Zhongshan Station, Antarctica: Spatial Variability
           and Source Identification
    • Authors: G. Shi; J. Teng, H. Ma, Y. Li, B. Sun
      Abstract: Metals and metalloids in continental precipitation have been widely observed, but the data over open oceans are still very limited. Investigation of metals and metalloids in marine precipitation is of great significance to understand global transport of these elements in the atmosphere and their input fluxes to the oceans. So, shipboard sampling of precipitation was conducted during a Chinese National Antarctic Research Expedition (CHINARE) campaign from Shanghai, China to Zhongshan Station, East Antarctica, and 22 samples (including 17 rainfall and 5 snowfall events) were collected and analyzed for concentrations of Pb, Ni, Cr, Cu, Co, Hg, As, Cd, Sb, Se, Zn, Mn and Ti. Results show that concentrations of both metals and metalloids vary considerably along the cruise, with higher concentrations at coastal sites and lower values on the south Indian Ocean. Although only soluble fractions were determined for elements, concentrations in this study are generally comparable to the reported values of marine rain. Enrichment factor analysis shows that most of metals and metalloids are enriched versus crustal sources, even in the samples collected from remote south Indian Ocean. In addition, metals and metalloids in precipitation are also very enriched above sea‐salt abundance, indicating that impacts of sea salt aerosols on their concentrations are negligible. Main sources of metals and metalloids were explored with the aid of multivariate statistical analyses. The results show that human emissions have far‐reaching distribution, which may exert an important influence on the solubility of elements in precipitation. This investigation provides valuable information on spatial variation and possible sources of trace elements in precipitation over the open oceans corresponding to understudied region.
      PubDate: 2015-05-14T05:34:17.232202-05:
      DOI: 10.1002/2014GB005060
  • Towards a parameterization of global‐scale organic carbon
           mineralization kinetics in surface marine sediments
    • Authors: K. Stolpovsky; A. W. Dale, K. Wallmann
      Abstract: An empirical function is derived for predicting the rate‐depth profile of particulate organic carbon (POC) degradation in surface marine sediments including the bioturbated layer. The rate takes the form of a power law analogous to the Middelburg function. The functional parameters were optimized by simulating measured benthic O2 and NO3− fluxes at 185 stations worldwide using a diagenetic model. The novelty of this work rests with the finding that the vertically‐resolved POC degradation rate in the bioturbated zone can be determined using a simple function where the POC rain rate is the governing variable. Although imperfect, the model is able to fit 71 % of paired O2 and NO3− fluxes to within 50% of measured values. It further provides realistic geochemical concentration‐depth profiles, NO3− penetration depths and apparent first‐order POC mineralization rate constants. The model performs less well on the continental shelf due to the high heterogeneity there. When applied to globally resolved maps of rain rate, the model predicts a global denitrification rate of 182 ± 88 Tg yr−1 of N and a POC burial rate of 107 ± 52 Tg yr−1 of C with a mean carbon burial efficiency of 6.1%. These results are in very good agreement with published values. Our proposed function is conceptually simple, requires less parameterization than multi‐G type models and is suitable for non‐steady state applications. It provides a basis for more accurately simulating benthic nutrient fluxes and carbonate dissolution rates in Earth system models.
      PubDate: 2015-05-14T05:33:54.810907-05:
      DOI: 10.1002/2015GB005087
  • Global Patterns and controls of soil organic carbon dynamics as simulated
           by multiple terrestrial biosphere models: current status and future
    • Authors: Hanqin Tian; Chaoqun Lu, Jia Yang, Kamaljit Banger, Deborah N. Huntinzger, Christopher R. Schwalm, Anna M. Michalak, Robert Cook, Philippe Ciais, Daniel Hayes, Maoyi Huang, Akihiko Ito, Atul Jain, Huimin Lei, Jiafu Mao, Shufen Pan, Wilfred M. Post, Shushi Peng, Benjamin Poulter, Wei Ren, Daniel Ricciuto, Kevin Schaefer, Xiaoying Shi, Bo Tao, Weile Wang, Yaxing Wei, Qichun Yang, Bowen Zhang, Ning Zeng
      Abstract: Soil is the largest organic carbon (C) pool of terrestrial ecosystems, and C loss from soil accounts for a large proportion of land‐atmosphere C exchange. Therefore, a small change in soil organic C (SOC) can affect atmospheric carbon dioxide (CO2) concentration and climate change. In the past decades, a wide variety of studies have been conducted to quantify global SOC stocks and soil C exchange with the atmosphere through site measurements, inventories, and empirical/process‐based modeling. However, these estimates are highly uncertain and identifying major driving forces controlling soil C dynamics remains a key research challenge. This study has compiled century‐long (1901–2010) estimates of SOC storage and heterotrophic respiration (Rh) from ten terrestrial biosphere models (TBMs) in the Multi‐scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) and two observation‐based datasets. The ten‐TBM ensemble shows that global SOC estimate range from 425 to 2111 Pg C (1 Pg = 1015 g) with a median value of 1158 Pg C in 2010. The models estimate a broad range of Rh from 35 to 69 Pg C yr−1 with a median value of 51 Pg C yr−1 during 2001–2010. The largest uncertainty in SOC stocks exists in the 40–65°N latitude whereas the largest differences in Rh between models are in the tropics. The modeled SOC change during 1901–2010 ranges from −70 Pg C to 86 Pg C, but in some models the SOC change has a different sign from the change of total C stock, implying very different contribution of vegetation and soil pools in determining the terrestrial C budget among models. The model ensemble‐estimated mean residence time of SOC shows a reduction of 3.4 years over the past century, which is primarily caused by the increment in proportion of labile substrate which accelerate C cycling through the land biosphere. All the models agreed that climate and land use changes decreased SOC stocks while elevated atmospheric CO2 and nitrogen deposition over intact ecosystems increased SOC stocks – even though the responses varied significantly among models. Model representations of temperature and moisture sensitivity, nutrient limitation and land use partially explain the divergent estimates of global SOC stocks and soil C fluxes in this study. In addition, a major source of systematic error in model estimations relates to non‐modeled SOC storage in wetlands and peatlands, as well as to old C storage in deep soil layers.
      PubDate: 2015-05-11T05:31:54.306869-05:
      DOI: 10.1002/2014GB005021
  • Temperature, oxygen, and vegetation controls on decomposition in a James
           Bay peatland
    • Authors: Michael Philben; James Holmquist, Glen MacDonald, Dandan Duan, Karl Kaiser, Ronald Benner
      Abstract: The biochemical composition of a peat core from James Bay Lowland, Canada was used to assess the extent of peat decomposition and diagenetic alteration. Our goal was to identify environmental controls on peat decomposition, particularly its sensitivity to naturally occurring changes in temperature, oxygen exposure time, and vegetation. All three varied substantially during the last 7000 yr, providing a natural experiment for evaluating their effects on decomposition. The bottom 50 cm of the core formed during the Holocene Climatic Optimum (~7000‐4000 yr B.P.), when mean annual air temperature was likely 1‐2°C warmer than present. A reconstruction of the water table level using testate amoebae indicated oxygen exposure time was highest in the subsequent upper portion of the core between 150 and 225 cm depth (from ~2560‐4210 yr B.P) and the plant community shifted from mostly Sphagnum to vascular plant dominance. Several independent biochemical indices indicated decomposition was greatest in this interval. Hydrolysable amino acid yields, hydroxyproline yields, and acid:aldehyde ratios of syringyl lignin phenols were higher, while hydrolysable neutral sugar yields and carbon:nitrogen ratios were lower in this zone of both vascular plant vegetation and elevated oxygen exposure time. Thus, peat formed during the Holocene Climatic Optimum did not appear to be more extensively decomposed than peat formed during subsequent cooler periods. Comparison with a core from the West Siberian Lowland, Russia, indicates oxygen exposure time and vegetation are both important controls on decomposition, while temperature appears to be of secondary importance. The low apparent sensitivity of decomposition to temperature is consistent with recent observations of a positive correlation between peat accumulation rates and mean annual temperature, suggesting contemporary warming could enhance peatland carbon sequestration, although this could be offset by an increasing contribution of vascular plants to the vegetation.
      PubDate: 2015-05-06T14:47:47.620213-05:
      DOI: 10.1002/2014GB004989
  • Spatial and temporal contrasts in the distribution of crops and pastures
           across Amazonia: A new agricultural land‐use dataset from census
           data since 1950
    • Authors: P. Imbach; M. Manrow, E. Barona, A. Barretto, G. Hyman, P Ciais
      Abstract: Amazonia holds the largest continuous area of tropical forests with intense land use change dynamics inducing water, carbon and energy feedbacks with regional and global impacts. Much of our knowledge of land‐use change in Amazonia comes from studies of the Brazilian Amazon, which accounts for two thirds of the region. Amazonia outside of Brazil has received less attention because of the difficulty of acquiring consistent data across countries. We present here an agricultural statistics database of the entire Amazonia region, with a harmonized description of crops and pastures in geospatial format, based on administrative boundary data at the municipality level. The spatial coverage includes countries within Amazonia and spans censuses and surveys from 1950 to 2012. Harmonized crop and pasture types are explored by grouping annual and perennial cropping systems, C3 and C4 photosynthetic pathways, planted and natural pastures, and main crops. Our analysis examined the spatial pattern of ratios between classes of the groups and their correlation with the agricultural extent of crops and pastures within administrative units of the Amazon, by country and census/survey dates. Significant correlations were found between all ratios and the fraction of agricultural lands of each administrative unit, with the exception of planted to natural pastures ratio and pasture lands extent. Brazil and Peru in most cases have significant correlations for all ratios analyzed even for specific census and survey dates. Results suggested improvements and potential applications of the database for carbon, water, climate and land use change studies are discussed. The database presented here provides an Amazon‐wide improved data set on agricultural dynamics with expanded temporal and spatial coverage.
      PubDate: 2015-05-05T13:31:30.542301-05:
      DOI: 10.1002/2014GB004999
  • N‐loss isotope effects in the Peru oxygen minimum zone studied using
           a mesoscale eddy as a natural tracer experiment
    • Authors: Annie Bourbonnais; Mark A. Altabet, Chawalit N. Charoenpong, Jennifer Larkum, Haibei Hu, Hermann W. Bange, Lothar Stramma
      Abstract: Mesoscale eddies in Oxygen Minimum Zones (OMZ's) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N‐loss as well as the ocean's N isotope budget. They also represent ‘natural tracer experiments’ with intensified biogeochemical signals that can be exploited to understand the large‐scale processes that control N‐loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus the heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−) and biogenic N2 associated with an anticyclonic eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2‐N concentrations were also the highest near the eddy's center (up to ~40 µmol L−1). δ15N‐N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N‐loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N‐loss was calculated to be only ~13‐14‰ (as compared to canonical values of ~20‐30‰) assuming a closed system and only slightly higher assuming an open system (16‐19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε for NO3− reduction to NO2− and NO2− reduction to N2 of ~16‐21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2‐, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for NO3− and NO2− reduction as well as N‐loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N‐loss.
      PubDate: 2015-04-25T08:41:06.583857-05:
      DOI: 10.1002/2014GB005001
  • Multi‐century changes in ocean and land contributions to
           climate‐carbon feedbacks
    • Authors: J. T. Randerson; K. Lindsay, E. Munoz, W. Fu, J. K. Moore, F. M. Hoffman, N. M. Mahowald, S. C. Doney
      Abstract: Improved constraints on carbon cycle responses to climate change are needed to inform mitigation policy, yet our understanding of how these responses may evolve after 2100 remains highly uncertain. Using the Community Earth System Model (v1.0), we quantified climate‐carbon feedbacks from 1850 to 2300 for the Representative Concentration Pathway 8.5 and its extension. In three simulations, land and ocean biogeochemical processes experienced the same trajectory of increasing atmospheric CO2. Each simulation had a different degree of radiative coupling for CO2 and other greenhouse gases and aerosols, enabling diagnosis of feedbacks. In a fully coupled simulation, global mean surface air temperature increased by 9.3 K from 1850 to 2300, with 4.4 K of this warming occurring after 2100. Excluding CO2, warming from other greenhouse gases and aerosols was 1.6 K by 2300, near a 2 K target needed to avoid dangerous anthropogenic interference with the climate system. Ocean contributions to the climate‐carbon feedback increased considerably over time, and exceeded contributions from land after 2100. The sensitivity of ocean carbon to climate change was found to be proportional to changes in ocean heat content, as a consequence of this heat modifying transport pathways for anthropogenic CO2 inflow and solubility of dissolved inorganic carbon. By 2300 climate change reduced cumulative ocean uptake by 330 Pg C, from 1410 Pg C to 1080 Pg C. Land fluxes similarly diverged over time, with climate change reducing stocks by 232 Pg C. Regional influence of climate change on carbon stocks was largest in the North Atlantic Ocean and tropical forests of South America. Our analysis suggests that after 2100, oceans may become as important as terrestrial ecosystems in regulating the magnitude of climate‐carbon feedbacks.
      PubDate: 2015-04-21T17:01:21.272412-05:
      DOI: 10.1002/2014GB005079
  • 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
    • Authors: Mathis P. Hain; Daniel M. Sigman, John A. Higgins, Gerald H. Haug
      First page: 517
      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.
      PubDate: 2015-03-24T00:55:28.090141-05:
      DOI: 10.1002/2014GB004986
  • Spatial patterns in CO2 evasion from the global river network
    • Authors: Ronny Lauerwald; Goulven G. Laruelle, Jens Hartmann, Philippe Ciais, Pierre A.G. Regnier
      First page: 534
      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.
      PubDate: 2015-04-07T06:21:35.630659-05:
      DOI: 10.1002/2014GB004941
  • Atmospheric observations inform CO2 flux responses to enviroclimatic
    • Authors: Yuanyuan Fang; Anna M. Michalak
      First page: 555
      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.
      PubDate: 2015-04-07T06:19:33.867268-05:
      DOI: 10.1002/2014GB005034
  • Microbial nitrogen dynamics in organic and mineral soil horizons along a
           latitudinal transect in Western Siberia
    • Authors: Birgit Wild; Jörg Schnecker, Anna Knoltsch, Mounir Takriti, Maria Mooshammer, Norman Gentsch, Robert Mikutta, Ricardo J. Eloy Alves, Antje Gittel, Nikolay Lashchinskiy, Andreas Richter
      First page: 567
      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.
      PubDate: 2015-04-09T21:51:19.381845-05:
      DOI: 10.1002/2015GB005084
  • The timescale of the silicate weathering negative feedback on atmospheric
    • Authors: G. Colbourn; A. Ridgwell, T. M. Lenton
      First page: 583
      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.
      PubDate: 2015-04-07T20:58:49.986963-05:
      DOI: 10.1002/2014GB005054
  • Seasonal to decadal variations of sea surface pCO2 and sea‐air CO2
           flux in the equatorial oceans over 1984‐2013: A basin‐scale
           comparison of the Pacific and Atlantic Oceans
    • Authors: Xiujun Wang; Raghu Murtugudde, Eric Hackert, Jing Wang, Jim Beauchamp
      First page: 597
      Abstract: The equatorial Pacific and Atlantic Oceans release significant amount of CO2 each year. Not much attention has been paid to evaluating the similarities and differences between these two basins in terms of temporal variability. Here, we employ a basin scale, fully coupled physical‐biogeochemical model to study the spatial and temporal variations in sea surface pCO2 and air‐sea CO2 flux over the period of 1984‐2013 in the equatorial Pacific and Atlantic Oceans. The model reproduces the overall spatial and temporal variations in the carbon fields for both basins, including higher values to the south of the equator than to the north, the annual maximum sea surface pCO2 in boreal spring and the annual peak in sea‐to‐air CO2 flux in boreal fall in the upwelling regions. The equatorial Pacific reveals a large interannual variability in sea surface pCO2, which is associated with the El Niño‐Southern Oscillation (ENSO). As a contrast, there is a strong seasonality but little interannual variability in the carbon fields of the equatorial Atlantic. The former is driven by the variability of dissolved inorganic carbon but the latter by sea surface temperature. Our model estimates an average sea‐to‐air CO2 flux of 0.521±0.204 Pg C yr‐1 for the tropical Pacific (18ºS‐18ºN, 150ºE‐90ºW), which is in good agreement with the observation‐based estimate (0.51±0.24 Pg C yr‐1). On average, sea‐to‐air CO2 flux is 0.214±0.03 Pg C yr‐1 in the tropical Atlantic (10ºS‐10ºN), which compares favorably with observational estimates.
      PubDate: 2015-04-23T08:55:24.424942-05:
      DOI: 10.1002/2014GB005031
  • Modeling the fate of methane hydrates under global warming
    • Authors: Kerstin Kretschmer; Arne Biastoch, Lars Rupke, Ewa Burwicz
      First page: 610
      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.
      PubDate: 2015-03-30T01:37:32.035211-05:
      DOI: 10.1002/2014GB005011
  • A comparison of plot‐based, satellite and Earth system model
           estimates of tropical forest net primary production
    • Authors: Cory C. Cleveland; Philip Taylor, K. Dana Chadwick, Kyla Dahlin, Christopher E. Doughty, Yadvinder Malhi, W. Kolby Smith, Benjamin W. Sullivan, William R. Wieder, Alan R. Townsend
      First page: 626
      Abstract: Net primary production (NPP) by plants represents the largest annual flux of carbon dioxide (CO2) from the atmosphere to the terrestrial biosphere, playing a critical role in the global carbon (C) cycle and the Earth's climate. Rates of NPP in tropical forests are thought to be among the highest on Earth, but debates about the magnitude, patterns and controls of NPP in the tropics highlight uncertainty in our understanding of how tropical forests may respond to environmental change. Here, we compared tropical NPP estimates generated using three common approaches: 1) field‐based methods scaled from plot‐level measurements of plant biomass; 2) radiation‐based methods that model NPP from satellite‐derived radiation absorption by plants; 3) and biogeochemical model‐based methods. For undisturbed tropical forests as a whole, the three methods produced similar NPP estimates (i.e., ~ 10 Pg C y−1). However, the three different approaches produced vastly different patterns of NPP both in space and through time, suggesting that our understanding of tropical NPP is poor, and that our ability to predict the response of NPP in the tropics to environmental change is limited. To address this shortcoming, we suggest the development of an expanded, high density, permanent network of sites where NPP is continuously evaluated using multiple approaches. Well‐designed NPP megatransects that include a high density plot network would signficantly increase the accuracy and certainty in the observed rates and patterns of tropical NPP, and improve the reliability of Earth system models used to predict NPP – carbon cycle – climate interactions into the future.
      PubDate: 2015-04-23T23:45:38.815374-05:
      DOI: 10.1002/2014GB005022
  • Source and sink carbon dynamics and carbon allocation in the Amazon basin
    • Authors: Christopher E. Doughty; D. B. Metcalfe, C. A. J. Girardin, F. F. Amezquita, L. Durand, W. Huaraca Huasco, J. E. Silva‐Espejo, A. Araujo‐Murakami, M. C. Costa, A. C. L. Costa, W. Rocha, P. Meir, D. Galbraith, Y. Malhi
      First page: 645
      Abstract: Changes to the carbon cycle in tropical forests could affect global climate, but predicting such changes has been previously limited by lack of field based data. Here we show seasonal cycles of the complete carbon cycle for fourteen, one hectare intensive carbon cycling plots which we separate into three regions: humid lowland, highlands and dry lowlands. Our data highlight three trends: (1) there is differing seasonality of total NPP with the highlands and dry lowlands peaking in the dry season and the humid lowland sites peaking in the wet season; (2) seasonal reductions in wood NPP are not driven by reductions in total NPP but by carbon during the dry season being preferentially allocated towards either roots or canopy NPP; and (3) there is a temporal decoupling between total photosynthesis and total carbon usage or plant carbon expenditure (PCE). This decoupling indicates the presence of non‐structural carbohydrates (NSC) which may allow growth and carbon to be allocated when it is most ecologically beneficial rather when it is most environmentally available.
      PubDate: 2015-04-23T23:44:45.222501-05:
      DOI: 10.1002/2014GB005028
  • Mercury Species Concentrations and Fluxes in the Central Tropical Pacific
    • Authors: Kathleen M. Munson; Carl H. Lamborg, Gretchen J. Swarr, Mak A. Saito
      First page: 656
      Abstract: The formation of the toxic and bioaccumulating monomethylmercury (MMHg) in marine systems is poorly understood, due in part to sparse data from many ocean regions. We present dissolved mercury (Hg) speciation data from 10 stations in the North and South Equatorial Pacific spanning large water mass differences and gradients in oxygen utilization. We also compare the mercury content in suspended particles from 6 stations and sinking particles from 3 stations to constrain local Hg sources and sinks. Concentrations of total Hg (THg) and methylated Hg in the surface and intermediate waters of the Equatorial and South Pacific suggest Hg cycling distinct from that of the North Pacific gyre. Maximum concentrations of 180 fM for both MMHg and dimethylmercury (DMHg) are observed in the Equatorial Pacific. South of the Equator, concentrations of MMHg and DMHg are less than 100 fM. Sinking fluxes of particulate THg can reasonably explain the shape of dissolved THg profiles, but those of MMHg are too low to account for dissolved MMHg profiles. However, methylated Hg species are lower than predicted from remineralization rates based on North Pacific data, consistent with limitation of methylation in Equatorial and South Pacific waters. Full water column depths profiles were also measured for the first time in these regions. Concentrations of THg are elevated in deep waters of the North Pacific, compared to those in the intermediate and surface waters, and taper off in the South Pacific. Comparisons with previous measurements from nearby regions suggest little enrichment of THg or MMHg over the past 20 years.
      PubDate: 2015-04-25T08:50:55.37441-05:0
      DOI: 10.1002/2015GB005120
  • Fate of the Amazon River dissolved organic matter in the tropical Atlantic
    • Authors: Patricia M. Medeiros; Michael Seidel, Nicholas D. Ward, Edward J. Carpenter, Helga R. Gomes, Jutta Niggemann, Alex V. Krusche, Jeffrey E. Richey, Patricia L. Yager, Thorsten Dittmar
      First page: 677
      Abstract: Constraining the fate of dissolved organic matter (DOM) delivered by rivers is key to understand the global carbon cycle, since DOM mineralization directly influences air‐sea CO2 exchange and multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet, the fate of this material in the ocean remains unclear. Here, we investigate the molecular composition and transformations of DOM in the Amazon River‐ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50‐76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially‐derived organic carbon from the continental margin, where it can be entrained in the large scale circulation and potentially contribute to the long‐term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean.
      PubDate: 2015-04-25T09:01:10.100395-05:
      DOI: 10.1002/2015GB005115
  • A revised global estimate of dissolved iron fluxes from marine sediments
    • Authors: A. W. Dale; L. Nickelsen, F. Scholz, C. Hensen, A. Oschlies, K. Wallmann
      First page: 691
      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.
      PubDate: 2015-04-07T06:20:08.893961-05:
      DOI: 10.1002/2014GB005017
  • Patterns and drivers of change in organic carbon burial across a diverse
           landscape: insights from 116 Minnesota lakes
    • Authors: Robert D. Dietz; Daniel R. Engstrom, N. J. Anderson
      First page: 708
      Abstract: Lakes may store globally significant quantities of organic carbon (OC) in their sediments, but the extent to which burial rates vary across space and time is not well described. Using 210Pb‐dated sediment cores, we explored patterns of OC burial in 116 lakes spanning several ecoregions and land‐use regimes in Minnesota, USA during the past 150–200 years. Rates for individual lakes (across all time periods) range from 3 to 204 g C m−2 yr−1 (median 33 g C m−2 yr−1) and show strong geographic separation in accordance with the degree of catchment disturbance and nutrient enrichment. Climate and basin morphometry exercise subordinate control over OC burial patterns, and diagenetic gradients introduce little bias to estimated temporal trends. Median burial rates in agricultural lakes exceed urban lakes and have increased fourfold since Euro‐American settlement. The greatest increase in OC burial occurred prior to the widespread adoption of industrial fertilizers, during an era of land clearance and farmland expansion. Northern boreal lakes, impacted by historical logging and limited cottage development yet comparatively undisturbed by human activity, bury OC at rates 3X lower than agricultural lakes and exhibit much smaller increases in OC burial. Scaling up modern OC burial estimates to the entire state, we find that Minnesota lakes annually store 0.40 Tg C in their sediments, equal to 1.5% of annual statewide CO2 emissions from fossil fuel combustion. During the period of Euro‐American settlement (ca. 1860–2000), cumulative OC burial amounted to 36 Tg C, 40% of which can be attributed to anthropogenic enhancement.
      PubDate: 2015-04-23T18:08:56.659453-05:
      DOI: 10.1002/2014GB004952
  • Water mass mixing is the dominant control on the zinc distribution in the
           North Atlantic Ocean
    • Abstract: Dissolved zinc (dZn) concentration was determined in the North Atlantic during the US GEOTRACES 2010&2011 cruise (GOETRACES GA03). A relatively poor linear correlation (R2=0.756) was observed between dZn and silicic acid (Si) the slope of which was 0.0577 nM:µmol/kg. We attribute the relatively poor dZn‐Si correlation to the following processes: a. differential regeneration of zinc relative to silicic acid; b. mixing of multiple water masses that have different Zn/Si; c. zinc sources such as sedimentary or hydrothermal. To quantitatively distinguish these possibilities, we use the results of Optimum Multi‐Parameter Water Mass Analysis by Jenkins et al. [2015] to model the zinc distribution below 500m. We hypothesized two scenarios: conservative mixing and regenerative mixing. The first scenario (conservative) could be modeled to results in a correlation with observations with a R2=0.846. In the second scenario, we took a Si‐related regeneration into account which could model the observations with a R2=0.867. Through this regenerative mixing scenario, we estimated a Zn/Si=0.0548 nM:µmol/kg that may be more realistic than linear regression slope due to accounting for process b. However, this did not improve the model substantially (R2=0.867 vs.0.846) which may indicate the insignificant effect of remineralization on the zinc distribution in this region. The relative weakness in the model‐observation correlation (R2~0.85 for both scenarios) imply that processes a and c may be plausible. Furthermore, dZn in the upper 500m exhibited a very poor correlation with apparent oxygen utilization (AOU) suggesting a minimal role for the organic matter‐associated remineralization process.
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