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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 26, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 55, 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: 7)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 25)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 15)
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: 3, 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: 85, 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]
  • Issue Information
    • Abstract: Cover: In Paulot et al. [doi 10.1002/2015GB005106], atmospheric distribution of total ammonia (NHx) in surface air over the ocean. Annual mean values from the GEOS‐Chem simulation driven by COBALT‐HA seawater [NHx(sw)] (background) are compared to ship measurements (circles). (top left) The model contribution of ocean emissions to [NHx]. (top right) The fractional neutralization of non‐sea‐salt sulfate by ammonia as expressed by the molar ratio of NHx to non‐sea‐salt total sulfate (nss‐SO4T). (bottom left) The concentrations of aerosol NH+ and (bottom right) gas phase NH3. The observations are averaged onto a 7.5° × 7.5° grid for readability. For NH+, we exclude observed values with nss SO4T exceeding 1 µg m‐3 to limit the influence of continental sources. See pp. 1165–1178.
      PubDate: 2015-09-15T14:40:04.722555-05:
      DOI: 10.1002/gbc.20199
  • Seasonal and interannual variability of sea‐air CO2 fluxes in the
           tropical Atlantic affected by the Amazon River plume
    • Abstract: CO2 fugacities obtained from a merchant ship sailing from France to French Guyana were used to explore the seasonal and interannual variability of the sea‐air CO2 exchange in the western tropical North Atlantic (TNA; 5‐14°N, 41‐52°W). Two distinct oceanic water masses were identified in the area associated to the main surface currents, i.e. the North Brazil Current (NBC) and the North Equatorial Current (NEC). The NBC was characterized by permanent CO2 oversaturation throughout the studied period, contrasting with the seasonal pattern identified in the NEC. The NBC retroflection was the main contributor to the North Equatorial Counter Current (NECC), thus spreading into the central TNA the Amazon River plume and the CO2–rich waters probably originated from the equatorial upwelling. Strong CO2 undersaturation was associated to the Amazon River plume. Total inorganic carbon drawdown due to biological activity was estimated to be 154 µmol kg−1 within the river plume. As a consequence, the studied area acted as a net sink of atmospheric CO2 (from −72.2±10.2 mmol m−2 month−1 in February to 14.3±4.5 mmol m−2 month−1 in May). This contrasted with the net CO2 efflux estimated by the main global sea‐air CO2 flux climatologies. Interannual SST changes in the TNA caused by large‐scale climatic events could determine the direction and intensity of the sea‐air CO2 fluxes in the NEC. Positive temperature anomalies observed in the TNA led to an almost permanent CO2 outgassing in the NEC in 2010.
      PubDate: 2015-09-09T17:12:27.960764-05:
      DOI: 10.1002/2015GB005110
  • Pathways and transformations of dissolved methane and dissolved inorganic
           carbon in Arctic tundra watersheds: Evidence from analysis of stable
    • Authors: Heather M. Throckmorton; Jeffrey M. Heikoop, Brent D. Newman, Garrett L. Altmann, Mark S. Conrad, Jordan D. Muss, George B. Perkins, Lydia J. Smith, Margaret S. Torn, Stan D. Wullschleger, Cathy J. Wilson
      Abstract: Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO2) and methane (CH4) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem‐scale CH4 and CO2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4 and DIC balance by estimating mechanisms of CH4 and DIC production and transport pathways and oxidation of subsurface CH4. We observed a shift from acetoclastic (July) towards hydrogenotropic (September) methanogenesis at sites located towards the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape‐scale effect on CH4 production mechanism. The majority of subsurface CH4 was transported upward by plant‐mediated transport and ebullition, predominantly bypassing the potential for CH4 oxidation. Thus, surprisingly CH4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4 produced at the frost table, contributing to < 0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e‐ acceptors. This work highlights the importance of spatial and temporal variability of CH4 production at the watershed scale, and suggests broad scale investigations are required to build better regional or pan‐Arctic representations of CH4 and CO2 production.
      PubDate: 2015-09-09T17:11:53.88415-05:0
      DOI: 10.1002/2014GB005044
  • Is atmospheric phosphorus pollution altering global alpine lake
    • Authors: Janice Brahney; Natalie Mahowald, Daniel S. Ward, Ashley P. Ballantyne, Jason C. Neff
      Abstract: Anthropogenic activities have significantly altered atmospheric chemistry and changed the global mobility of key macronutrients. Here, we show that contemporary global patterns in nitrogen (N) and phosphorus (P) emissions drive large hemispheric variation in precipitation chemistry. These global patterns of nutrient emission and deposition (N:P) are in turn closely reflected in the water chemistry of naturally oligotrophic lakes (r2=0.81, p
      PubDate: 2015-09-09T15:43:31.716955-05:
      DOI: 10.1002/2015GB005137
  • Biogenic carbon fluxes from global agricultural production and consumption
    • Authors: Julie Wolf; Tristram O. West, Yannick L. Le Page, G. Page Kyle, Xuesong Zhang, G. James Collatz, Marc L. Imhoff
      Abstract: Quantification of biogenic carbon fluxes from agricultural lands is needed to generate comprehensive bottom‐up estimates of net carbon exchange for global and regional carbon monitoring. We estimated global agricultural carbon fluxes associated with annual crop net primary production (NPP), harvested biomass, and consumption of biomass by humans and livestock. These estimates were combined for a single estimate of net carbon exchange (NCE) and spatially distributed to 0.05 degree resolution using MODIS satellite land cover data. Global crop NPP in 2011 was estimated at 5.25 ± 0.46 Pg C yr−1, of which 2.05 ± 0.05 Pg C yr−1 was harvested and 0.54 Pg C yr−1 was collected from crop residues for livestock fodder. Total livestock feed intake in 2011 was 2.42 ± 0.21 Pg C yr−1, of which 2.31 ± 0.21 Pg C yr−1 was emitted as CO2, 0.07 ± 0.01 Pg C yr−1 was emitted as CH4, and 0.04 Pg C yr−1 was contained within milk and egg production. Livestock grazed an estimated 1.27 Pg C yr−1 in 2011, which constituted 52.4% of total feed intake. Global human food intake was 0.57 ± 0.03 Pg C yr−1 in 2011, the majority of which was respired as CO2. Completed global cropland carbon budgets accounted for the ultimate use of ca. 80% of harvested biomass. The spatial distribution of these fluxes may be used for global carbon monitoring, estimation of regional uncertainty, and for use as input to Earth system models.
      PubDate: 2015-09-08T16:36:24.193624-05:
      DOI: 10.1002/2015GB005119
  • Disentangling the contribution of multiple land covers to
    • Abstract: In less than 15 years, the Amazon region experienced three major droughts. Links between droughts and fires have been demonstrated for the 1997/98, 2005 and 2010 droughts. In 2010, emissions of 510±120 Tg C were associated to fire alone in Amazonia. Existing approaches have, however, not yet disentangled the proportional contribution of multiple land cover sources to this total. We develop a novel integration of multi‐sensor and multi‐temporal satellite‐derived data on land cover, active fires and burned area, and an empirical model of fire‐induced biomass loss to quantify the extent of burned areas and resulting biomass loss for multiple land‐covers in Mato Grosso state (MT), southern Amazonia ‐ the 2010 drought most impacted region. We show that 10.77% (96,855 km2) of MT burned. We estimated a gross carbon emission of 56.21 ± 22.5 TgC from direct combustion of biomass, with an additional 29.4 ± 10 Tg C committed to be emitted in the following years due to dead wood decay. It is estimated that old growth forest fires in the whole Brazilian Legal Amazon (BLA) have contributed to 14.81 Tg of C (11.75 Tg C to 17.87 Tg C) emissions to the atmosphere during the 2010 fire season, with an affected area of 27,555 km2. Total C loss from the 2010 fires in MT state and old growth forests fires in the BLA represent, respectively, 77% (47% to 107%) and 86% (68.2% to 103%) of Brazil's National Plan on Climate Change annual target for Amazonia C emissions reductions from deforestation.
      PubDate: 2015-09-08T01:27:25.003184-05:
      DOI: 10.1002/2014GB005008
  • Controls on biogenic silica burial in the Southern Ocean
    • Authors: Zanna Chase; Karen Kohfeld, Katsumi Matsumoto
      Abstract: Understanding the controls on opal export in the Southern Ocean can inform both the prediction of how the leakage of silicic acid from the Southern Ocean responds to climate and the interpretation of paleo‐proxies. We have compiled a database of 185 230Thorium‐normalized opal burial rates and 493 opal concentration measurements in Southern Ocean sediments and matched these with environmental climatologies. By subdividing the Southern Ocean on the basis of oceanographic regions and interpolating the opal burial rates, we estimate a total biogenic Si burial south of 40°S of 2.3 ± 1.0 Tmol Si year −1. In both the seasonally ice‐covered and permanently ice‐free regions we can predict 73% of opal burial from surface‐ocean properties. Where sea‐ice is present for at least part of the year, the length of the ice‐free season determines the upper limit of opal burial in the underlying sediments. In the ice‐free regions of the Southern Ocean, the supply of silicic acid through winter mixing is the most important factor. Our results do not support a strong role of iron in controlling opal burial. We do however find that satellite‐derived net primary production increases with increasing (modelled) dust delivery. These findings support the decoupling between carbon and opal fluxes in the Southern Ocean. When corrected for opal dissolution, the observed opal fluxes are in reasonable agreement with fluxes simulated using an ocean biogeochemical model. However, the results suggest current preservation algorithms for opal could be improved by incorporating the composition of particle flux, not only its magnitude.
      PubDate: 2015-09-07T14:47:40.598719-05:
      DOI: 10.1002/2015GB005186
  • Intense nitrogen cycling in permeable intertidal sediment revealed by a
           nitrous oxide hotspot
    • Authors: Charles A. Schutte; Samantha B. Joye, Alicia M. Wilson, Tyler Evans, Willard S. Moore, Karen Casciotti
      Abstract: Approximately 40% of the total global rate of nitrogen fixation is the result of human activities, and most of this anthropogenic nitrogen is used to fertilize agricultural fields. Approximately 23% of the applied agricultural nitrogen is delivered to the coastal zone, often reducing water quality and driving eutrophication. Nitrogen cycling in coastal sediments can mitigate eutrophication by removing bioavailable nitrogen. However, some of these processes generate nitrous oxide, a potent greenhouse gas, as a byproduct. Here we report the discovery of a nitrous oxide production hotspot in shallow barrier island sands. Nitrous oxide concentrations, production and consumption rates, vertical diffusion fluxes, and flux to the atmosphere were measured across triplicate depth profiles. Using a mass balance approach, rates of net nitrous oxide production were estimated to be 40 µmol m−2 day−1. This production was driven by a hotspot of nitrate consumption that removed bioavailable nitrogen from the coastal environment at a rate of 10 mmol m−2 day−1, a rate that is comparable with the highest rates of denitrification reported for coastal sediments.
      PubDate: 2015-09-03T05:44:20.669359-05:
      DOI: 10.1002/2014GB005052
  • Responses of carbon uptake and oceanic pCO2 to climate change in the North
           Atlantic: A model study with the Bergen Earth System Model
    • Abstract: Several model studies diagnose the carbon uptake of the North Atlantic as most sensitive to climate change when considered per unit‐area. Yet, the main drivers of the modeled sensitivity and the share of biological production and physical transport are under debate. In order to contribute to this ongoing discussion, two simulations with the Bergen Earth System Model were carried out for period 1850‐2099. One of the simulations (COU) includes the radiative effect of rising CO2 (i.e. climate change), while the second simulation (BGC) excludes this effect. The modeled carbon fluxes show substantially different responses to climate change for different parts of the North Atlantic. Based on these differences, we divide the North Atlantic into 2 regions, namely the subpolar gyre (SPG) and the rest of the North Atlantic (rNAT*, covering mainly the subtropical gyre). The highest climate sensitivity is found in the SPG‐region (accounting for an uptake reduction of 8.06 Pg C over the period 1850‐2099), while the response of the rNAT*‐region is moderate (reduction of 4.00 Pg C). We show that the changing CO2‐fluxes in both SPG‐ and rNAT*‐region are driven by increasing oceanic pCO2. The pCO2‐changes in the rNAT*‐region are caused by both changing physical and biogeochemical processes, while changes in DIC and alkalinity are the primary contributor to the high climate sensitivity of the SPG‐region. We identify a reduced biological production to be responsible for the modeled response of DIC and alkalinity, yet the differences between biological contribution and contributions of ocean circulation and CO2 uptake are small, highlighting our need for a better understanding of the marine biological cycle.
      PubDate: 2015-08-29T10:23:14.124043-05:
      DOI: 10.1002/2015GB005109
  • Increased influence of nitrogen limitation on CO2 emissions from future
           land use and land‐use change
    • Authors: Prasanth Meiyappan; Atul K. Jain, Joanna I. House
      Abstract: In the latest projections of future greenhouse gas emissions for the Intergovernmental Panel on Climate Change (IPCC), few Earth System Models included the effect of nitrogen limitation, a key process limiting forest regrowth. Few included forest management (wood harvest). We estimate the impacts of nitrogen limitation on the CO2 emissions from land use and land‐use change (LULUC), including wood harvest, for the period 1900‐2100. We use a land‐surface model that includes a fully coupled carbon and nitrogen cycle, and accounts for forest regrowth processes following agricultural abandonment and wood harvest. Future projections are based on the four Representation Concentration Pathways used in the IPCC Fifth Assessment Report, and we account for uncertainty in future climate for each scenario based on ensembles of climate model outputs. Results show that excluding nitrogen limitation will underestimate global LULUC emissions by 34‐52 PgC (20‐30%) during the 20th century (range across three different historical LULUC reconstructions) and by 128‐187 PgC (90‐150%) during the 21st century (range across the four IPCC scenarios). The full range for estimated LULUC emissions during the 21st century including climate model uncertainty is 91 to 227 PgC (with nitrogen limitation included). The underestimation increases with time because: (1) Projected annual wood harvest rates from forests summed over the 21st century are 380‐1080% higher compared to those of the 20th century, resulting in more regrowing secondary forests, (2) Nitrogen limitation reduces the CO2 fertilization effect on net primary production of regrowing secondary forests following wood harvest and agricultural abandonment, and (3) Nitrogen limitation effect is aggravated by the gradual loss of soil nitrogen from LULUC disturbance. Our study implies that: (1) Nitrogen limitation of CO2 uptake is substantial and sensitive to nitrogen inputs, (2) If LULUC emissions are larger than previously estimated in studies without nitrogen limitation, then meeting the same climate mitigation target would require an equivalent additional reduction of fossil fuel emissions, (3) The effectiveness of land‐based mitigation strategies will critically depend on the interactions between nutrient limitations and secondary forests resulting from LULUC, and (4) It is important for terrestrial biosphere models to consider nitrogen constraint in estimates of the strength of future land carbon uptake.
      PubDate: 2015-08-26T06:23:53.930286-05:
      DOI: 10.1002/2015GB005086
  • Silicon pools in human impacted soils of temperate zones
    • Abstract: Besides well‐known effects of climate and parent material on silicate weathering the role of land use change as a driver in the global silicon cycle is not well known. Changes in vegetation cover have altered reservoirs of silicon and carbon in plants and soils. This has potential consequences for plant‐Si availability, agricultural yields and coastal eutrophication, as Si is a beneficial element for many crop plants and an essential nutrient for diatom growth. We here examined the role of sustained and intensive land use and human disturbance on silicon (Si) pool distribution in soils with similar climatological and bulk mineralogical characteristics. We show that land use impacts both biogenic and non‐biogenic Si pools. While biogenic Si strongly decreases along the land use change gradient (from forest to croplands), pedogenic silica fractions (e.g. pedogenic clays) increase in top soils with a long duration of cultivation and soil disturbance. Our results suggest that non‐biogenic Si pools might compensate for the loss of reactive biogenic silicon in temperate zones.
      PubDate: 2015-08-26T06:19:23.293441-05:
      DOI: 10.1002/2014GB005049
  • Century‐scale patterns and trends of global pyrogenic carbon
           emissions and fire impacts on the terrestrial carbon balance
    • Authors: Yang Jia; Hanqin Tian, Bao Tao, Wei Ren, Chaoqun Lu, Shufen Pan, Yuhang Wang, Yongqiang Liu
      Abstract: Fires have consumed a large amount of terrestrial organic carbon, and significantly influenced terrestrial ecosystems and the physical climate system over the past century. Although biomass burning has been widely investigated at a global level in recent decades via satellite observations, less work has been conducted to examine the century‐scale changes in global fire regimes and fire impacts on the terrestrial carbon balance. In this study, we investigated global pyrogenic carbon emissions and fire impacts on the terrestrial carbon fluxes from 1901 to 2010 by using a process‐based land ecosystem model. Our results show a significant declining trend in global pyrogenic carbon emissions between the early 20th century and the mid‐1980s, but a significant upward trend between the mid‐1980s and the 2000s as a result of more frequent fires in ecosystems with high carbon storage, such as peatlands and tropical forests. Over the past 110 years, average pyrogenic carbon emissions were estimated to be 2.43 Pg C year‐1 (1 Pg = 1015 g), and global average combustion rate (defined as carbon emissions per unit area burned) was 537.85 g C m‐2 burned area. Due to the impacts of fires, the net primary productivity and carbon sink of global terrestrial ecosystems were reduced by 4.14 Pg C year‐1 and 0.57 Pg C year‐1, respectively. Our study suggests that special attention should be paid to fire activities in the peatlands and tropical forests in the future. Practical management strategies, such as minimizing forest logging and reducing the rate of cropland expansion in the humid regions, are in need to reduce fire risk and mitigate fire‐induced greenhouse gases emissions.
      PubDate: 2015-08-25T16:19:25.689752-05:
      DOI: 10.1002/2015GB005160
  • On the Southern Ocean CO2 uptake and the role of the biological carbon
           pump in the 21st century
    • Abstract: We use a suite of eight ocean biogeochemical/ecological general circulation models from the MAREMIP and CMIP5 archives to explore the relative roles of changes in winds (positive trend of Southern Annular Mode, SAM) and in warming‐ and freshening‐driven trends of upper ocean stratification in altering export production and CO2 uptake in the Southern Ocean at the end of the 21st century. The investigated models simulate a broad range of responses to climate change, with no agreement ona dominance of either the SAM or the warming signal south of 44 ∘ S. In the southernmost zone, i.e., south of 58∘ S, they concur on an increase of biological export production, while between 44 and 58∘ S the models lack consensus on the sign of change in export. Yet, in both regions, the models show an enhanced CO2 uptake during spring and summer. This is due to a larger CO 2 (aq) drawdown by the same amount of summer export production at a higher Revelle factor at the end of the 21st century. This strongly increases the importance of the biological carbon pump in the entire Southern Ocean. In the temperate zone, between 30 and 44∘ S all models show a predominance of the warming signal and a nutrient‐driven reduction of export production. As a consequence, the share of the regions south of 44∘ S to the total uptake of the Southern Ocean south of 30∘ S is projected to increase at the end of the 21st century from 47 to 66% with a commensurable decrease to the north. Despite this major reorganization of the meridional distribution of the major regions of uptake, the total uptake increases largely in line with the rising atmospheric CO2. Simulations with the MITgcm‐REcoM2 model show that this is mostly driven by the strong increase of atmospheric CO2, with the climate‐driven changes of natural CO2 exchange offsetting that trend only to a limited degree (∼10%) and with negligible impact of climate effects on anthropogenic CO2 uptake when integrated over a full annual cycle south of 30∘S.
      PubDate: 2015-08-21T13:41:12.268511-05:
      DOI: 10.1002/2015GB005140
  • An observational assessment of the influence of mesoscale and submesoscale
           heterogeneity on ocean biogeochemical reactions
    • Abstract: Numerous observations demonstrate that considerable spatial variability exists in components of the marine planktonic ecosystem at the mesoscale and submesoscale (100 km ‐1 km). The causes and consequences of physical processes at these scales (‘eddy advection’) influencing biogeochemistry have received much attention. Less studied, the non‐linear nature of most ecological and biogeochemical interactions means that such spatial variability has consequences for regional estimates of processes including primary production and grazing, independent of the physical processes. This effect has been termed ‘eddy reactions’. Models remain our most powerful tools for extrapolating hypotheses for biogeochemistry to global scales and to permit future projections. The spatial resolution of most climate and global biogeochemical models means that processes at the mesoscale and submesoscale are poorly resolved. Modelling work has previously suggested that the neglected ‘eddy reactions’ may be almost as large as the mean field estimates in some cases. This study seeks to quantify the relative size of eddy and mean reactions observationally, using in situ and satellite data. For primary production, grazing and zooplankton mortality the eddy reactions are between 7% and 15% of the mean reactions. These should be regarded as preliminary estimates to encourage further observational estimates, and not taken as a justification for ignoring eddy reactions. Compared to modelling estimates, there are inconsistencies in the relative magnitude of eddy reactions and in correlations which are a major control on their magnitude. One possibility is that models exhibit much stronger spatial correlations than are found in reality, effectively amplifying the magnitude of eddy reactions.
      PubDate: 2015-08-13T13:02:36.87994-05:0
      DOI: 10.1002/2015GB005129
  • Seasonal variations, origin and fate of settling diatoms in the Southern
           Ocean tracked by silicon isotope records in deep sediment traps
    • Abstract: The Southern Ocean plays a pivotal role in the control of atmospheric CO2 levels, via both physical and biological sequestration processes. The biological carbon transfer to the ocean interior is tightly coupled to the availability of other elements, especially iron as a trace limiting nutrient and dissolved silicon (DSi) as the mineral substrate that allows diatoms to dominate primary production. Importantly, variations in the silicon cycling are large but not well understood. Here, we use δ30Si measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, AZ; Polar Frontal, PFZ and Subantarctic, SAZ) of the open Southern Ocean. Variations in the exported flux of biogenic silica (BSi) and its δ30Si composition reveal a range of insights, including that i) the sinking rate of BSi exceeds 200 m d−1 in summer in the AZ, yet decreases to very low values in winter that allow particles to remain in the water column through to the following spring, ii) occasional vertical mixing events affect the δ30Si composition of exported BSi in both the SAZ and AZ, iii) the δ30Si signature of diatoms is well conserved through the water column despite strong BSi and POC attenuation at depth, and is closely linked to the Si consumption in surface waters. With the strong coupling observed between BSi and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on production and export.
      PubDate: 2015-08-13T13:02:17.351269-05:
      DOI: 10.1002/2015GB005180
  • The imbalance of new and export production in the Western Antarctic
           Peninsula, a potentially “leaky” ecosystem
    • Authors: Michael R. Stukel; Elizabeth Asher, Nicole Couto, Oscar Schofield, Stefani Strebel, Philippe Tortell, Hugh W. Ducklow
      Abstract: To quantify the balance between new production and vertical nitrogen export of sinking particles, we measured nitrate uptake, net nitrate drawdown, ΔO2 /Ar‐based net community production, sediment trap flux, and 234Th export at a coastal site near Palmer Station, Antarctica during the phytoplankton growing season from October 2012 to March 2013. We also measured nitrate uptake and 234Th export throughout the northern western Antarctic Peninsula (WAP) region on a cruise in January 2013. We used a non‐steady state 234Th equation with temporally‐varying upwelling rates and an irradiance‐based phytoplankton production model to correct our export and new production estimates in the complex coastal site near Palmer Station. Results unequivocally showed that nitrate uptake and net community production were significantly greater than the sinking particle export on region‐wide spatial scales and season‐long temporal scales. At our coastal site, new production (105±17.4 mg N m−2 d−1, mean±st.err.) was 5.3 times greater than vertical nitrogen export (20.4±2.4 mg N m−2 d‐1). On the January cruise in the northern WAP, new production (47.9±14.4 mg N m−2 d‐1) was 2.4 times greater than export (19.9±1.4 mg N m−2 d−1). Much of this imbalance can be attributed to diffusive losses of particulate nitrogen from the surface ocean due to diapycnal mixing, indicative of a “leaky” WAP ecosystem. If these diffusive losses are common in other systems where new production exceeds export, it may be necessary to revise current estimates of the ocean's biological pump.
      PubDate: 2015-08-12T14:46:29.623846-05:
      DOI: 10.1002/2015GB005211
  • Strong dependence of CO2 emissions from anthropogenic land cover change on
           initial land cover and soil carbon parametrization
    • Abstract: The quantification of sources and sinks of carbon from land use and land cover changes (LULCC) is uncertain. We investigated how the parametrization of LULCC and of organic matter decomposition, as well as initial land cover affect the historical and future carbon fluxes in an Earth System Model (ESM). Using the land component of the Max‐Planck‐Institute ESM, we found that the historical (1750–2010) LULCC flux varied up to 25% depending on the fraction of biomass which enters the atmosphere directly due to burning or is used in short‐lived products. We found an uncertainty in the decadal LULCC fluxes of the recent past due to the parametrization of decomposition and direct emissions of 0.6 Pg C yr−1, which is three times larger than the un‐certainty previously attributed to model and method in general. Pre‐industrial natural land cover had a larger effect on decadal LULCC fluxes than the aforementioned parameter sensitivity (1.0 Pg C yr−1). Re‐gional differences between reconstructed and dynamically‐computed land cover, in particular at low‐latitudes, led to differences in historical LULCC emissions of 84–114 Pg C, globally. This effect is larger than the effects of forest regrowth, shifting cultivation or climate feedbacks and comparable to the effect of differences among studies in the terminology of LULCC. In general, we find that the practice of calibrating the net land carbon balance to provide realistic boundary conditions for the climate component of an ESM hampers the applicability of the land component outside its primary field of application.
      PubDate: 2015-08-12T14:44:48.224991-05:
      DOI: 10.1002/2014GB004988
  • Climate extremes dominate seasonal and interannual variations in carbon
           export from the Mississippi River Basin
    • Abstract: Knowledge about the annual and seasonal patterns of organic and inorganic carbon (C) exports from the major rivers of the world to the coastal ocean are essential for our understanding and potential management of the global C budget so as to limit anthropogenic modification of global climate. Unfortunately our predictive understanding of what controls the timing, magnitude and quality of carbon export is still rudimentary. Here we use a process‐based coupled hydrologic/ecosystem biogeochemistry model (the Dynamic Land Ecosystem Model, DLEM) to examine how climate variability and extreme events, changing land use, and atmospheric chemistry have affected the annual and seasonal patterns of C exports from the Mississippi River basin to the Gulf of Mexico. Our process‐based simulations estimate that the average annual exports of dissolved organic C (DOC), particulate organic C (POC), and dissolved inorganic C (DIC) in the 2000s was 2.6 ± 0.4 Tg C yr−1, 3.4 ± 0.3 Tg C yr−1 and 18.8 ± 3.4 Tg C yr−1, respectively. Although land‐use change was the most important agent of change in C export over the past century, climate variability and extreme events (such as flooding and drought) were primarily responsible for seasonal and interannual variations in C export from the basin. The maximum seasonal export of DIC occurred in summer while for maximum DOC and POC occurred in winter. Relative to the 10‐year average (2001–2010), our modeling analysis indicates that the years of maximal and minimal C export co‐occurred with wet and dry years (2008: 32% above average and 2006: 32% below average). Given IPCC‐predicted changes in climate variability and the severity of rain events and droughts of wet and dry years for the remainder of the 21st Century, our modeling results suggest major changes in the riverine link between the terrestrial and oceanic realms, which are likely to have a major impact on carbon delivery to the coastal ocean.
      PubDate: 2015-08-06T12:38:46.634104-05:
      DOI: 10.1002/2014GB005068
  • The multiple fates of sinking particles in the North Atlantic Ocean
    • Authors: James R. Collins; Bethanie R. Edwards, Kimberlee Thamatrakoln, Justin E. Ossolinski, Giacomo R. DiTullio, Kay D. Bidle, Scott C. Doney, Benjamin A. S. Van Mooy
      Abstract: The direct respiration of sinking organic matter by attached bacteria is often invoked as the dominant sink for settling particles in the mesopelagic ocean. However, other processes, such as enzymatic solubilization and mechanical disaggregation, also contribute to particle flux attenuation by transferring organic matter to the water column. Here, we use observations from the North Atlantic Ocean, coupled to sensitivity analyses of a simple model, to assess the relative importance of particle‐attached microbial respiration compared to the other processes that can degrade sinking particles. The observed carbon fluxes, bacterial production rates, and respiration by water column and particle‐attached microbial communities each spanned more than an order of magnitude. Rates of substrate‐specific respiration on sinking particle material ranged from 0.007 ± 0.003 to 0.173 ± 0.105 d‐1. A comparison of these substrate‐specific respiration rates with model results suggested sinking particle material was transferred to the water column by various biological and mechanical processes nearly 3.5 times as fast as it was directly respired. This finding, coupled with strong metabolic demand imposed by measurements of water column respiration (729.3 ± 266.0 mg C m‐2 d‐1, on average, over the 50 to 150 m depth interval), suggested a large fraction of the organic matter evolved from sinking particles ultimately met its fate through subsequent remineralization in the water column. At three sites, we also measured very low bacterial growth efficiencies and large discrepancies between depth‐integrated mesopelagic respiration and carbon inputs.
      PubDate: 2015-08-04T01:03:40.41616-05:0
      DOI: 10.1002/2014GB005037
  • Small phytoplankton drive high summertime carbon and nutrient export in
           the Gulf of California and Eastern Tropical North Pacific
    • Abstract: Summertime carbon, nitrogen and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, PC and PN concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol·dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. Highest C fixation rates were associated with diatom dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton and diazotroph dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including
      PubDate: 2015-08-02T23:05:00.189407-05:
      DOI: 10.1002/2015GB005134
  • Phosphorus budget in the water‐agro‐food system at nested
           scales in two contrasted regions of the world (ASEAN‐8 and
    • Abstract: Phosphorus (P) plays a strategic role in agricultural production as well as in the occurrence of freshwater and marine eutrophication episodes throughout the world. Moreover, the scarcity and uneven distribution of minable P resources is raising concerns about the sustainability of long‐term exploitation. In this paper we analyze the P cycle in anthropic systems with an original multi‐scale approach (world region, country, and large basin scales) in two contrasting world regions representative of different trajectories in socioeconomic development for the 1961–2009 period: Europe (EU‐27)/France and the Seine River Basin, and Asia (ASEAN‐8)/Vietnam and the Red River Basin. Our approach highlights different trends in the agricultural and food production systems of the two regions. Whereas crop production increased until the 1980s in Europe and France and has stabilized thereafter, in ASEAN‐8 and Vietnam it began to increase in the 1980s and it is still rising today. These trends are related to the increasing use of fertilizers, although in European countries the amount of fertilizers sharply decreased after the 1980s. On average, the total P delivered from rivers to the sea is three times higher for ASEAN‐8 (300 kgP km‐2 yr‐1) than for EU‐27 countries (100 kgP km‐2 yr‐1) and is twice as high in the Red River (200 kgP km‐2 yr‐1) than in the Seine River (110 kgP km‐2 yr‐1), with agricultural losses to water in ASEAN‐8 three times higher than in EU‐27. Based on the P flux budgets, this study discusses early warnings and management options according to the particularities of the two world regions, newly integrating the perspective of surface water quality with agricultural issues (fertilizers, crop production, and surplus), food/feed exchanges, and diet, defining the so‐called water‐agro‐food system.
      PubDate: 2015-07-30T05:51:42.093853-05:
      DOI: 10.1002/2015GB005147
  • The mutual importance of anthropogenically and climate induced changes in
           global vegetation cover for future land carbon emissions in the
           MPI‐ESM CMIP5 simulations
    • Authors: R. Schneck; C. H. Reick, J. Pongratz, V. Gayler
      Abstract: Based on the MPI‐ESM simulations for the Coupled Model Intercomparison Project Phase 5 (CMIP5) and on simulations with the submodel Cbalone we disentangle the influence of natural and anthropogenic vegetation changes on land carbon emissions for the years 1850 till 2300. According to our simulations, climate induced changes in distribution and productivity of natural vegetation strongly mitigates future carbon emissions from anthropogenic landuse and landcover change (LULCC). Depending on the assumed scenario, the accumulated carbon emissions until the year 2100 are reduced between 22 and 49% and until 2300 between 45 and 261%. The carbon storage due to climate‐induced vegetation change is generally stronger in combination with LULCC. This is because in our simulations anthropogenic vegetation changes support more productive Plant Functional Types (PFTs). After stopping LULCC in the year 2100 the refilling of depleted C pools on formerly transformed land takes (dependent on the scenario) timescales of centuries.
      PubDate: 2015-07-24T14:08:12.197437-05:
      DOI: 10.1002/2014GB004959
  • Recent Amazon Climate as background for possible ongoing and future
           changes of Amazon humid forests
    • Abstract: Recent analyses of Amazon runoff and gridded precipitation data suggest an intensification of the hydrological cycle over the past few decades in the following sense: wet‐season precipitation and peak river runoff (since ∼ 1980) as well as annual‐mean precipitation (since ∼ 1990) have increased while dry‐season precipitation and minimum runoff have slightly decreased. There has also been an increase in the frequency of anomalously severe floods and droughts. Here we extend and expand these analyses to characterize recent climate state and change, as a background for possible ongoing and future changes of these forests. The contrasting recent changes in wet and dry season precipitation have continued and are generally consistent with changes in catchment‐level peak and minimum river runoff as well as a positive trend of water vapour inflow into the basin. Consistent with the river records the increased vapour inflow is concentrated to the wet season. Temperature has been rising by 0.7∘C since 1980 with more pronounced warming during dry months. Suggestions for the cause of the observed changes of the hydrological cycle come from patterns in tropical sea surface temperatures (SST's). Tropical and North Atlantic SST's have increased rapidly and steadily since 1990, while Pacific SST's have shifted from a negative Pacific Decadal Oscillation (PDO) phase (approximately pre 1990) with warm eastern Pacific temperatures to a positive phase with cold eastern Pacific temperatures. These SST conditions have been shown to be associated with an increase in precipitation over most of the Amazon except the south and south‐west. If ongoing changes continue we expect these to be generally beneficial for forests in those regions where there is an increase in precipitation with the exception of floodplain forests. An increase in flood‐pulse height and duration could lead to increased mortality at higher levels of the floodplain and, over the long term, to a lateral shift of the zonally stratified floodplain forest communities. Negative effects on forests are mainly expected in the south‐west and south, which have become slightly drier and hotter, consistent with tree mortality trends observed at the RAINFOR forest plot census network.
      PubDate: 2015-07-22T09:05:53.202115-05:
      DOI: 10.1002/2014GB005080
  • Soluble iron inputs to the Southern Ocean through recent andesitic to
           rhyolitic volcanic ash eruptions from the Patagonian Andes
    • Abstract: Patagonia, due to its geographic position and the dominance of westerly winds, is a key area that contributes to the supply of nutrients to the Southern Ocean, both through mineral dust as well as the periodic deposits of volcanic ash. Here we evaluate the characteristics of Fe dissolved (into soluble and colloidal species) from volcanic ash for three recent southern Andes volcanic eruptions having contrasting features and chemical compositions. Contact between cloud waters (wet deposition) and end‐members of andesitic (Hudson volcano) and rhyolitic (Chaitén volcano) materials was simulated. Results indicate higher Fe release and faster liberation rates in the andesitic material. Fe release during particle‐seawater interaction (dry deposition), have higher rates in rhyolitic type ashes. Rhyolitic ashes under acidic conditions release Fe in higher amounts and at a slower rate, while in those samples containing mostly glass shards, Fe release was lower and faster. The 2011 Puyehue eruption was observed by a dust monitoring station. Puyehue‐type eruptions can contribute soluble Fe to the ocean via dry or wet deposition, nearly reaching the limit required for phytoplankton growth. In contrast, the input of Fe after processing by an acidic eruption plume could raise the amount of dissolved Fe in surface ocean waters several times, above the threshold required to initiate phytoplankton blooms. A single eruption like the Puyehue one represents more than half of the yearly Fe‐flux contributed by dust.
      PubDate: 2015-07-14T10:58:03.974088-05:
      DOI: 10.1002/2015GB005177
  • Short‐term variability in euphotic zone biogeochemistry and primary
           productivity at Station ALOHA: A case study of summer 2012
    • Abstract: Time‐series observations are critical to understanding the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time‐series (HOT) program has maintained near‐monthly sampling at Station ALOHA (22° 45′N, 158° 00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and identified ecosystem variability over seasonal to interannual time‐scales. To further extend the temporal resolution of these near‐monthly time‐series observations, an extensive field campaign was conducted during July‐September 2012 at Station ALOHA with near‐daily sampling of upper water‐column biogeochemistry, phytoplankton abundance, and activity. The resulting dataset provides biogeochemical measurements at high temporal resolution and documented two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea‐surface salinity minimum feature which was prominent in the upper water‐column (0–50 m) for a period of approximately 30 days. The shipboard observations during July‐September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea‐surface salinity minimum feature in the NPSG.
      PubDate: 2015-07-14T10:31:10.947074-05:
      DOI: 10.1002/2015GB005141
  • Criteria for rejection of papers without review
    • First page: 1123
      PubDate: 2015-07-25T16:26:25.340715-05:
      DOI: 10.1002/2015GB005234
  • A thank you to our GBC Reviewers
    • First page: 1124
      PubDate: 2015-07-27T00:39:32.325531-05:
      DOI: 10.1002/2015GB005233
  • Global oceanic emission of ammonia: constraints from seawater and
           atmospheric observations
    • Authors: F. Paulot; D. J. Jacob, M. T. Johnson, T. G. Bell, A. R. Baker, W. C. Keene, I. D. Lima, S. C. Doney, C. A. Stock
      First page: 1165
      Abstract: Current global inventories of ammonia emissions identify the ocean as the largest natural source. This source depends on seawater pH, temperature, and the concentration of total seawater ammonia (NHx(sw)), which reflects a balance between remineralization of organic matter, uptake by plankton, and nitrification. Here, we compare [NHx(sw)] from two global ocean biogeochemical models (BEC and COBALT) against extensive ocean observations. Simulated [NHx(sw)] are generally biased high. Improved simulation can be achieved in COBALT by increasing the plankton affinity for NHx within observed ranges. The resulting global ocean emissions is 2.5 TgN a−1, much lower than current literature values(7–23 TgN a−1), including the widely used GEIA inventory (8 TgN a−1). Such a weak ocean source implies that continental sources contribute more than half of atmospheric NHx over most of the ocean in the Northern hemisphere. Ammonia emitted from oceanic sources is insufficient to neutralize sulfate aerosol acidity, consistent with observations. There is evidence over the Equatorial Pacific for a missing source of atmospheric ammonia that could be due to photolysis of marine organic nitrogen at the ocean surface or in the atmosphere. Accommodating this possible missing source yields a global ocean emission of ammonia in the range 2–5 TgN a−1, comparable in magnitude to other natural sources from open fires and soils.
      PubDate: 2015-07-17T20:01:20.713827-05:
      DOI: 10.1002/2015GB005106
  • Ocean nutrient pathways associated with the passage of a storm
    • Authors: Anna Rumyantseva; Natasha Lucas, Tom Rippeth, Adrian Martin, Stuart C. Painter, Timothy J. Boyd, Stephanie Henson
      First page: 1179
      Abstract: Storms that affect ocean surface layer dynamics and primary production are a frequent occurrence in the open North Atlantic Ocean. In this study we use an interdisciplinary dataset collected in the region to quantify nutrient supply by two pathways associated with a storm event: entrainment of nutrients during a period of high wind forcing and subsequent shear‐spiking at the pycnocline due to interactions of storm generated inertial currents with wind. The post‐storm increase in surface layer nitrate (by ~20 mmol m−2) was predominantly driven by the first pathway: nutrient intrusion during the storm. Alignment of post‐storm inertial currents and surface wind stress caused shear instabilities at the ocean pycnocline, forming the second pathway for nutrient transport into the euphotic zone. During the alignment period, pulses of high turbulent nitrate flux through the pycnocline (up to 1 mmol m−2 day−1; approximately 25 times higher than the background flux) were detected. However, the impact of the post‐storm supply was an order of magnitude lower than during the storm due to the short duration of the pulses. Cumulatively, the storm passage was equivalent to 2.5‐5 % of the nitrate supplied by winter convection and had a significant effect compared to previously reported (sub)‐mesoscale dynamics in the region. As storms occur frequently, they can form an important component in local nutrient budgets.
      PubDate: 2015-07-16T01:09:58.494909-05:
      DOI: 10.1002/2015GB005097
  • Sea‐air CO2 exchange in the western Arctic coastal ocean
    • First page: 1190
      Abstract: The biogeochemical seascape of the western Arctic coastal ocean is in rapid transition. Changes in sea ice cover will be accompanied by alterations in sea‐air carbon dioxide (CO2) exchange, of which the latter has been difficult to constrain owing to sparse temporal and spatial datasets. Previous assessments of sea‐air CO2 flux have targeted specific sub‐regional areas of the western Arctic coastal ocean. Here a holistic approach is taken to determine the net sea‐air CO2 flux over this broad region. We compiled and analyzed an extensive dataset of nearly 600,000 surface seawater CO2 partial pressure (pCO2) measurements spanning 2003 through 2014. Using space‐time co‐located, reconstructed atmospheric pCO2 values coupled with the seawater pCO2 dataset, monthly climatologies of sea‐air pCO2 differences (∆pCO2) were created on a 0.2° latitude x 0.5° longitude grid. Sea‐air CO2 fluxes were computed using the ∆pCO2 grid and gas transfer rates calculated from a climatology of wind speed second moments. Fluxes were calculated with and without the presence of sea ice, treating sea ice as an imperfect barrier to gas exchange. This allowed for carbon uptake by the western Arctic coastal ocean to be assessed under existing and reduced sea ice cover conditions, in which carbon uptake increased 30% over the current 10.9 ± 5.7 Tg C (1 Tg = 1012 g) yr−1 of sea ice adjusted exchange in the region. This assessment extends beyond previous sub‐regional estimates in the region in an all‐inclusive manner, and points to key unresolved aspects that must be targeted by future research.
      PubDate: 2015-07-16T01:10:26.0701-05:00
      DOI: 10.1002/2015GB005153
  • Understanding large‐extent controls of soil organic carbon storage
           in relation to soil depth and soil‐landscape systems
    • First page: 1210
      Abstract: In this work we aimed at developing a conceptual framework in which we improve our understanding of the controlling factors for soil organic carbon (SOC) over vast areas at different depths. We postulated that variability in SOC may be better explained by modelling SOC within soil‐landscape systems (SLSs). The study was performed in mainland France and explanatory SOC models were developed for the sampled topsoil (0–30 cm) and subsoil (>30 cm), using both directed and undirected data mining techniques. With this study we demonstrated that there is a shift in controlling factors both in space and depth which were mainly related to 1) typical SLSs characteristics and 2) human induced changes to SLSs. The controlling factors in relation to depth alter from predominantly biotic to more abiotic with increasing depth. Especially, water availability, soil texture and physical protection control deeper stored SOC. In SLSs with similar SOC levels, different combinations of physical protection, the input of organic matter and climatic conditions largely determined the SOC level. The SLSs approach provided the means to partition the data into datasets that were having homogenous conditions with respect to this combination of controlling factors. This information may provide important information on the carbon storage and sequestration potential of a soil.
      PubDate: 2015-07-21T16:28:21.636805-05:
      DOI: 10.1002/2015GB005178
  • Relevance of methodological choices for accounting of land use change
           carbon fluxes
    • Authors: Eberhard Hansis; Steven J. Davis, Julia Pongratz
      First page: 1230
      Abstract: Accounting for carbon fluxes from land use and land cover change (LULCC) generally requires choosing from multiple options of how to attribute the fluxes to regions and to LULCC activities. Applying a newly‐developed and spatially‐explicit bookkeeping model BLUE (bookkeeping of land use emissions) we quantify LULCC fluxes and attribute them to land‐use activities and countries by a range of different accounting methods. We present results with respect to a Kyoto Protocol‐like “commitment” accounting period, using land use emissions of 2008‐12 as example scenario. We assess the effect of accounting methods that vary (1) the temporal evolution of carbon stocks, (2) the state of the carbon stocks at the beginning of the period, (3) the temporal attribution of carbon fluxes during the period, and (4) treatment of LULCC fluxes that occurred prior to the beginning of the period. We show that the methodological choices result in grossly different estimates of carbon fluxes for the different attribution definitions.
      PubDate: 2015-07-21T16:02:13.785754-05:
      DOI: 10.1002/2014GB004997
  • Influence of ENSO and the NAO on Terrestrial Carbon Uptake in the
           Texas‐northern Mexico Region
    • Authors: Nicholas C. Parazoo; Elizabeth Barnes, John Worden, Anna. B. Harper, Kevin W Bowman, Christian Frankenberg, Sebastian Wolf, Marcy Litvak, Trevor F. Keenan
      First page: 1247
      Abstract: Climate extremes such as drought and heat waves can cause substantial reductions in terrestrial carbon uptake. Advancing projections of the carbon uptake response to future climate extremes depends on (1) identifying mechanistic links between the carbon cycle and atmospheric drivers, (2) detecting and attributing uptake changes, and (3) evaluating models of land response and atmospheric forcing. Here, we combine model simulations, remote sensing products, and ground observations to investigate the impact of climate variability on carbon uptake in the Texas‐northern Mexico region. Specifically, we (1) examine the relationship between drought, carbon uptake, and variability of ENSO and the NAO using JULES biosphere simulations from 1950–2012, (2) quantify changes in carbon uptake during record drought conditions in 2011, and (3) evaluate JULES carbon uptake and soil moisture in 2011 using observations from remote sensing and a network of flux towers in the region. Long term simulations reveal systematic decreases in regional‐scale carbon uptake during negative phases of ENSO and NAO, including amplified reductions of gross primary production (GPP) (−0.42 ± 0.18 Pg C yr‐1) and net ecosystem production (NEP) (−0.14 ± 0.11 Pg C yr‐1) during strong La Niña years. The 2011 mega‐drought caused some of the largest declines of GPP (−0.50 Pg C yr‐1) and NEP (−0.23 Pg C yr‐1) in our simulations. In 2011, consistent declines were found in observations, including high correlation of GPP and surface soil moisture (r = 0.82 ± 0.23, p = 0.012) in remote sensing based products. These results suggest a large‐scale response of carbon uptake to ENSO and NAO, and highlight a need to improve model predictions of ENSO and NAO in order to improve predictions of future impacts on the carbon cycle and the associated feedbacks to climate change.
      PubDate: 2015-07-28T15:17:02.797609-05:
      DOI: 10.1002/2015GB005125
  • Decoupling of net community and export production on submesoscales in the
           Sargasso Sea
    • Authors: M. L. Estapa; D. A. Siegel, K. O. Buesseler, R. H. R. Stanley, M. W. Lomas, N. B. Nelson
      First page: 1266
      Abstract: Determinations of the net community production (NCP) in the upper ocean and the particle export production (EP) should balance over long time and large spatial scales. However, recent modeling studies suggest that a horizontal decoupling of flux‐regulating processes on submesoscales (≤10 km) could lead to imbalances between individual determinations of NCP and EP. Here we sampled mixed‐layer biogeochemical parameters and proxies for NCP and EP during ten, high spatial‐resolution (~2 km) surface transects across strong physical gradients in the Sargasso Sea. We observed strong biogeochemical and carbon flux variability in nearly all transects. Spatial coherence among measured biogeochemical parameters within transects was common, but rarely did the same parameters co‐vary consistently across transects. Spatial variability was greater in parameters associated with higher trophic levels, such as chlorophyll in > 5.0 µm particles, and variability in EP exceeded that of NCP in nearly all cases. Within sampling transects, coincident EP and NCP determinations were uncorrelated. However when averaged over each transect (30 to 40 km in length), we found NCP and EP to be significantly and positively correlated (R = 0.72, p = 0.04). Transect‐averaged EP determinations were slightly smaller than similar NCP values (Type‐II regression slope of 0.93, std = 0.32); but not significantly different from a 1:1 relationship. The results show the importance of appropriate sampling scales when deriving carbon flux budgets from upper ocean observations.
      PubDate: 2015-07-18T16:01:30.024777-05:
      DOI: 10.1002/2014GB004913
  • Sources of new nitrogen in the Indian Ocean
    • First page: 1283
      Abstract: Quantifying the different sources of nitrogen (N) within the N cycle is crucial to gain insights in oceanic phytoplankton production. To understand the controls of primary productivity and the associated capture of CO2 through photosynthesis in the southeastern Indian Ocean we compiled the physical and biogeochemical data from 4 voyages conducted in 2010, 2011, 2012 and 2013. Overall higher NH4+ assimilation rates (~530 μmol m−2 h−1) relative to NO3− assimilation rates (~375 μmol m−2 h−1) suggest that the assimilation dynamics of C are primarily regulated by microbial regeneration in our region. N2 fixation rates did not decline when other source of dissolved inorganic nitrogen (DIN) were available, although the assimilation of N2 is a highly energetic process. Our data showed that the diazotrophic community assimilated ~2 nmol N L−1 h−1 at relative elevated NH4+ assimilation rates ~12 nmol L−1 h−1 and NO3− assimilation rates ~6 nmol L−1 h−1. The small diffusive deep‐water NO3− fluxes could not support the measured NO3− assimilation rates and consequently point towards another source of dissolved inorganic NO3−. Highest NO2− values coincided consistently with shallow lower dissolved O2layers (100–200 m; 100‐180μmol L−1). These results suggest that nitrification above the pycnocline could be a significant component of the N cycle in the eastern Indian Ocean. In our analysis we provide a conceptual understanding how NO3− in the photic zone could be derived from new N through N2‐fixation. We conclude with the hypothesis that N injected through N2 fixation can be recycled within the photic zone as NH4+, and sequentially oxidized to NO2− and NO3− in shallow lower DO layers.
      PubDate: 2015-07-28T15:18:20.12004-05:0
      DOI: 10.1002/2015GB005194
  • Plant nutrients do not covary with soil nutrients under changing climatic
    • First page: 1298
      Abstract: Nitrogen (N) and phosphorus (P) play vital roles in plant growth and development. Yet, how climate regimes and soil fertility influence plant N and P stoichiometry is not well understood, especially in the belowground plant parts. Here we investigated plant above‐ and belowground N and P concentrations ([N] and [P]) and their stoichiometry in three dominant genera along a 2200‐km long climatic gradient in northern China. Results showed that temperature explained more variation of [N] and [P] in C4 plants, whereas precipitation exerted a stronger influence on [N] and [P] in C3 plants. Both plant above‐ and belowground [N] and [P] increased with decreasing precipitation and increasing temperature yet were negatively correlated with soil [N] and [P]. Plant N:P ratios were unrelated with all climate and soil variables. Plant above‐ and belowground [N] followed an allometric scaling relationship but the allocation of [P] was isometric. These results imply that internal processes stabilize plant N:P ratios and hence tissue N:P ratios may not be an effective parameters for predicting plant nutrient limitation. Our results also imply that past positive relationships between plant and nutrient stocks may be challenged under changing climatic conditions. While any modeling would need to be able to replicate currently observed relationships, it is conceivable that some relationships, such as those between temperature or rainfall and carbon:nutrient ratios, should be different under changing climatic conditions.
      PubDate: 2015-07-30T05:51:57.005544-05:
      DOI: 10.1002/2015GB005089
  • Multi‐molecular tracers of terrestrial carbon transfer across the
           pan‐Arctic: 14C characteristics of sedimentary carbon components and
           their environmental controls
    • Abstract: Distinguishing the sources, ages and fate of various terrestrial organic carbon (OC) pools mobilized from heterogeneous arctic landscapes is key to assessing climatic impacts on the fluvial release of carbon from permafrost. Through molecular 14C measurements, including novel analyses of suberin‐ and/or cutin‐derived diacids (DAs) and hydroxy fatty acids (FAs), we compared the radiocarbon characteristics of a comprehensive suite of terrestrial markers (including plant wax lipids, cutin, suberin, lignin and hydroxy phenols) in the sedimentary particles from nine major arctic and sub‐arctic rivers in order to establish a benchmark assessment of the mobilization patterns of terrestrial OC pools across the pan‐Arctic. Terrestrial lipids, including suberin‐derived longer‐chain DAs (C24,26,28), plant wax FAs (C24,26,28) and n‐alkanes (C27,29,31), incorporated significant inputs of aged carbon, presumably from deeper soil horizons. Mobilization and translocation of these “old” terrestrial carbon components was dependent on non‐linear processes associated with permafrost distributions. By contrast, shorter‐chain (C16,18) DAs and lignin phenols (as well as hydroxy phenols in rivers outside eastern Eurasian Arctic) were much more enriched in 14C, suggesting incorporation of relatively young carbon supplied by runoff processes from recent vegetation debris and surface layers. Furthermore, the radiocarbon content of terrestrial markers is heavily influenced by specific OC sources and degradation status. Overall, multi‐tracer molecular 14C analysis sheds new light on the mobilization of terrestrial OC from arctic watersheds. Our findings of distinct ages for various terrestrial carbon components may aid in elucidating fate of different terrestrial OC pools in the face of increasing arctic permafrost thaw.
  • Coralline algae are a globally significant pool of marine dimethylated
    • Abstract: Marine algae are key sources of the biogenic sulphur compound dimethylsulphoniopropionate (DMSP), a vital component of the marine sulphur cycle. Autotrophic ecosystem engineers such as red coralline algae support highly diverse and biogeochemically active ecosystems and are known to be high DMSP producers, but their importance in the global marine sulphur cycle has not yet been appreciated. Using a global sampling approach, we show that red coralline algae are a globally significant pool of DMSP in the oceans, estimated to be ~110 x 1012 moles worldwide during the summer months. Latitude was a major driver of observed regional‐scale variations, with peaks in polar and tropical climate regimes, reflecting the varied cellular functions for DMSP (e.g. as a cryoprotectant and antioxidant). A temperate coralline algal bed was investigated in more detail to also identify local‐scale temporal variations. Here, water column DMSP was driven by water temperature, and to a lesser extent, cloud cover; two factors which are also vital in controlling coralline algal growth. This study demonstrates that coralline algae harbour a large pool of dimethylated sulphur, thereby playing a significant role in both the sulphur and carbon marine biogeochemical cycles. However, coralline algal habitats are severely threatened by projected climate change; a loss of this habitat may thus detrimentally impact oceanic sulphur and carbon biogeochemical cycling.
  • Dissolved organic carbon pools and export from the coastal ocean
    • Abstract: The distribution of dissolved organic carbon (DOC) concentration across coastal waters was characterised based on the compilation of 3510 individual estimates of DOC in coastal waters worldwide. We estimated the DOC concentration in the coastal waters that directly exchange with open ocean waters in two different ways, as the DOC concentration at the edge of the shelf break and as the DOC concentration in coastal waters with salinity close to the average salinity in the open ocean. Using these estimates of DOC concentration in the coastal waters that directly exchange with open ocean waters, the mean DOC concentration in the open ocean and the estimated volume of water annually exchanged between coastal and open ocean, we estimated a median ± SE (and average ± SE) global DOC export from coastal to open ocean waters ranging from 4.4 ± 1.0 Pg C yr−1 to 27.0 ± 1.8 Pg C yr−1 (7.0 ± 5.8 Pg C yr−1 to 29.0 ± 8.0 Pg C yr−1) depending on the global hydrological exchange. These values correspond to a median and mean median (and average) range between 14.7 ± 3.3 to 90.0 ± 3.3 (23.3 ± 19.3 to 96.7± 26.7) Gg C yr−1 per km of shelf break, which is consistent with the range between 1.4 to 66.1 Gg C yr−1 per km of shelf break of available regional estimates of DOC export. The estimated global DOC export from coastal to open ocean waters is also consistent with independent estimates of the net metabolic balance of the coastal ocean. The DOC export from the coastal to the open ocean is likely to be a sizeable flux and is likely to be an important term in the carbon budget of the open ocean, potentially providing an important subsidy to support heterotrophic activity in the open ocean.
  • Explicitly representing soil microbial processes in Earth system models
    • Abstract: Microbes influence soil organic matter (SOM) decomposition and the long‐term stabilization of carbon (C) in soils. We contend that by revising the representation of microbial processes and their interactions with the physicochemical soil environment, Earth system models (ESMs) will make more realistic global C cycle projections. Explicit representation of microbial processes presents considerable challenges due to the scale at which these processes occur. Thus, applying microbial theory in ESMs requires a framework to link micro‐scale process‐level understanding and measurements to macro‐scale models used to make decadal‐ to century‐long projections. Here, we review the diversity, advantages, and pitfalls of simulating soil biogeochemical cycles using microbial‐explicit modeling approaches. We present a roadmap for how to begin building, applying, and evaluating reliable microbial‐explicit model formulations that can be applied in ESMs. Drawing from experience with traditional decomposition models we suggest: (1) guidelines for common model parameters and output that can facilitate future model intercomparisons; (2) development of benchmarking and model‐data integration frameworks that can be used to effectively guide, inform, and evaluate model parameterizations with data from well‐curated repositories; and (3) the application of scaling methods to integrate microbial‐explicit soil biogeochemistry modules within ESMs. With contributions across scientific disciplines, we feel this roadmap can advance our fundamental understanding of soil biogeochemical dynamics and more realistically project likely soil C response to environmental change at global scales.
  • Surface distribution of dissolved trace metals in the oligotrophic ocean
           and their influence on phytoplankton biomass and productivity
    • Abstract: The distribution of bioactive trace metals has the potential to enhance or limit primary productivity and carbon export in some regions of the world ocean. To study these connections, the concentrations of Cd, Co, Cu, Fe, Mo, Ni, and V were determined for 110 surface water samples collected during the Malaspina 2010 Circumnavigation Expedition (MCE). Total dissolved Cd, Co, Cu, Fe, Mo, Ni, and V concentrations averaged 19.0 ± 5.4 pM, 21.4 ± 12 pM, 0.91 ± 0.4 nM, 0.66 ± 0.3 nM, 88.8 ± 12 nM, 1.72 ± 0.4 nM, 23.4 ± 4.4 nM, respectively, with the lowest values detected in the Central Pacific and increased values at the extremes of all transects near coastal zones. Trace metal concentrations measured in surface waters of the Atlantic Ocean during the MCE were compared to previously published data for the same region. The comparison revealed little temporal changes in the distribution of Cd, Co, Cu, Fe, and Ni over the last 30 years. We utilized a multivariable linear regression model to describe potential relationships between primary productivity and the hydrological, biological, trace‐ and macro‐nutrient data collected during the MCE. Our statistical analysis shows that primary productivity in the Indian Ocean is best described by chlorophyll a, NO3, Ni, temperature, SiO4 and Cd. In the Atlantic Ocean, primary productivity is correlated with chlorophyll a, NO3, PO4, mixed layer depth, Co, Fe, Cd, Cu, V, and Mo. The variables salinity, temperature, SiO4, NO3, PO4, Fe, Cd, and V were found to best predict primary productivity in the Pacific Ocean. These results suggest that some of the lesser‐studied trace elements (e.g., Ni, V, Mo, Cd) may play a more important role in regulating oceanic primary productivity than previously thought and point to the need for future experiments to verify their potential biological functions.
  • Recent decadal trends in global phytoplankton composition
    • Abstract: Identifying major trends in biogeochemical composition of the oceans is essential to improve our understanding of biological responses to climate forcing. Using the NASA Ocean Biogeochemical Model (NOBM) combined with ocean color remote sensing data assimilation, we assessed the trends in phytoplankton composition (diatoms, cyanobacteria, coccolithophores and chlorophytes) at a global scale for the period 1998–2012. We related these trends in phytoplankton to physical conditions (surface temperature, surface photosynthetically available radiation [PAR] and mixed layer depth [MLD]) and nutrients (iron, silicate and nitrate). We found a significant global decline in diatoms (−1.22% y−1, P
  • Coupling atmospheric mercury isotope ratios and meteorology to identify
           sources of mercury impacting a coastal urban‐industrial region near
           Pensacola, Florida, USA
    • Abstract: Identifying the anthropogenic and natural sources of mercury (Hg) emissions contributing to atmospheric mercury on local, regional, and global scales continues to be a grand challenge. The relative importance of various direct anthropogenic emissions of mercury, in addition to natural geologic sources and re‐emission of previously released and deposited mercury, differs regionally and temporally. In this study, we used local, mesoscale, and synoptic scale meteorological analysis to couple the isotopic composition of ambient atmospheric mercury with potential sources of mercury contributing to a coastal urban‐industrial setting near a coal‐fired power plant in Pensacola, Florida, USA. We were able to broadly discern four influences on the isotopic composition of ambient atmospheric mercury impacting this coastal urban‐industrial region: (1) local to regional urban‐industrial anthropogenic emissions (mean δ202Hg = 0.44 ± 0.05‰, 1SD, n = 3); (2) marine‐influenced sources derived from the Gulf of Mexico (mean δ202Hg = 0.77 ± 0.15‰, 1SD, n = 4); (3) continental sources associated with north‐northwesterly flows from within the planetary boundary layer (mean δ202Hg = 0.65 ± 0.04‰, 1SD, n = 3); and (4) continental sources associated with north‐northeasterly flows at higher altitudes (i.e., 2000 m above ground level; mean δ202Hg = 1.10 ± 0.21‰, 1SD, n = 8). Overall, these data, in conjunction with previous studies, suggest that the background global atmospheric mercury pool is characterized by moderately positive δ202Hg values; that urban‐industrial emissions drive the isotopic composition of ambient atmospheric mercury toward lower δ202Hg values; and that air‐surface exchange dynamics across vegetation and soils of terrestrial ecosystems drive the isotopic composition of ambient atmospheric mercury toward higher positive δ202Hg values. The data further suggest that mass independent fractionation (MIF) of both even‐mass‐ and odd‐mass‐number isotopes, likely generated by photochemical reactions in the atmosphere or during re‐emission from terrestrial and aquatic ecosystems, can be obscured by mixing with anthropogenic emissions having different MIF signatures.
  • Climatological distribution of aragonite saturation state in the global
    • Abstract: Aragonite saturation state (Ωarag) in surface and subsurface waters of the global oceans was calculated from up‐to‐date (through the year of 2012) ocean station dissolved inorganic carbon (DIC) and total alkalinity (TA) data. Surface Ωarag in the open ocean was always supersaturated (Ω>1), ranging between 1.1 and 4.2. It was above 2.0 (2.0‐4.2) between 40°N and 40°S, but decreased towards higher latitude to below 1.5 in polar areas. The influences of water temperature on the TA/DIC ratio, combined with the temperature effects on inorganic carbon equilibrium and apparent solubility product (K’sp), explain the latitudinal differences in surface Ωarag. Vertically, Ωarag was highest in the surface mixed layer (SML). Higher hydrostatic pressure, lower water temperature, and more CO2 buildup from biological activity in the absence of air‐sea gas exchange helped maintain lower Ωarag in the deep ocean. Below the thermocline, aerobic decomposition of organic matter along the pathway of global thermohaline circulation played an important role in controlling Ωarag distributions. Seasonally, surface Ωarag above 30° latitudes was about 0.06 to 0.55 higher during warmer months than during colder months in the open‐ocean waters of both hemispheres. Decadal changes of Ωarag in the Atlantic and Pacific Oceans showed that Ωarag in waters shallower than 100 m depth decreased by 0.10±0.09 (−0.40±0.37% yr−1) on average from the decade spanning 1989–1998 to the decade spanning 1998–2010.
  • Anthropogenic CO2 uptake, transport, storage, and dynamical controls in
           the ocean imposed by the meridional overturning circulation: A modeling
    • Abstract: Using an ocean carbon cycle model embedded in an ocean general circulation model, we examine how the budget of anthropogenic CO2(Cant) is controlled by ocean dynamics. To complement recent studies showing only vertically integrated budgets, we provide a step‐by‐step description by making use of three different coarse‐grainings of the full vertical resolution of the ocean model in our budget analysis. For the 11 sub‐domains of the global ocean, these coarse‐grainings are (1) a one‐layer (vertically integrated) budget, (2) a three‐layer budget, and (3) an 11‐layer budget. We largely focus on the Pacific circulation. We identify and quantify substantial carbon transport associated with the subtropical cells (STCs), which are dominant contributors to the meridional overturning circulation in the upper ocean in the tropics and subtropics, as playing a fundamental role in governing the ocean interior distribution of Cant. The upper branch of the STCs transports Cant‐rich water from the tropics to the subtropics, contributing to the precondition for the high Cant inventory in mode waters. The lower branch of the STCs carries about two‐thirds of the transported Cant back to the tropics, while it largely excludes Subtropical Mode Waters. This work implies that the re‐emergence of Cant through recirculation within the STCs may lead to a reduced capacity for further Cant uptake via gas exchange into the surface ocean, potentially contributing to a positive carbon‐climate feedback.
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
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