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Publisher: AGU   (Total: 17 journals)   [Sort by number of followers]

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Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 26, SJR: 2.439, h-index: 91)
Geophysical Research Letters     Full-text available via subscription   (Followers: 106, SJR: 3.323, h-index: 185)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 14, SJR: 3.22, h-index: 136)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 4, SJR: 4.444, h-index: 18)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 119)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 28)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 50)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 49)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 104)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 43)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 122)
Paleoceanography     Full-text available via subscription   (Followers: 5, SJR: 3.067, h-index: 100)
Radio Science     Full-text available via subscription   (Followers: 37, SJR: 1.072, h-index: 59)
Reviews of Geophysics     Full-text available via subscription   (Followers: 34, SJR: 8.833, h-index: 107)
Space Weather     Full-text available via subscription   (Followers: 16, SJR: 1.341, h-index: 26)
Tectonics     Full-text available via subscription   (Followers: 14, SJR: 2.628, h-index: 96)
Water Resources Research     Full-text available via subscription   (Followers: 78, SJR: 2.661, h-index: 144)
Journal Cover Global Biogeochemical Cycles
  [SJR: 3.22]   [H-I: 136]   [14 followers]  Follow
    
   Full-text available via subscription Subscription journal  (Not entitled to full-text)
   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by AGU Homepage  [17 journals]
  • Nitrogen dynamic in Eurasian coastal Arctic ecosystem: Insight from
           nitrogen isotope
    • Authors: Benoit Thibodeau; Dorothea Bauch, Maren Voss
      Abstract: Primary productivity is limited by the availability of nitrogen (N) in most of the coastal Arctic, as a large portion of N is released by the spring freshet and completely consumed during the following summer. Thus, understanding the fate of riverine nitrogen is critical to identify the link between dissolved nitrogen dynamic and coastal primary productivity to foresee upcoming changes in the Arctic seas, such as increase riverine discharge and permafrost thaw. Here, we provide a field-based study of nitrogen dynamic over the Laptev Sea shelf based on isotope geochemistry. We demonstrate that while most of the nitrate found under the surface fresh water layer is of remineralized origin, some of the nitrate originates from atmospheric input and was probably transported at depth by the mixing of brine-enriched denser water during sea-ice formation. Moreover, our results suggest that riverine dissolved organic nitrogen (DON) represents up to 6 times the total riverine release of nitrate and that about 62 to 76% of the DON is removed within the shelf waters. This is a crucial information regarding the near-future impact of climate change on primary productivity in the Eurasian coastal Arctic.
      PubDate: 2017-04-24T00:26:14.70952-05:0
      DOI: 10.1002/2016GB005593
       
  • Global patterns of woody residence time and its influence on model
           simulation of aboveground biomass
    • Authors: Bao-Lin Xue; Qinghua Guo, Tianyu Hu, Jingfeng Xiao, Yuanhe Yang, Guoqiang Wang, Shengli Tao, Yanjun Su, Jin Liu, Xiaoqian Zhao
      Abstract: Woody residence time (τw) is an important parameter that expresses the balance between mature forest recruitment/growth and mortality. Using field data collected from the literature, this study explored the global forest τw and investigated its influence on model simulations of aboveground biomass (AGB) at a global scale. Specifically, τw was found to be related to forest age, annual temperature and precipitation at a global scale, but its determinants were different among various plant function types. The estimated global forest τw based on the filed data showed large spatial heterogeneity, which plays an important role in model simulation of AGB by a Dynamic Global Vegetation Model (DGVM). τw could change the resulting AGB in 10-fold based on a site-level test using the Monte Carlo method. At the global level, different parameterization schemes of the Integrated Biosphere Simulator using the estimated τw resulted in a two-fold change in the AGB simulation for 2100. Our results highlight the influences of various biotic and abiotic variables on forest τw. The estimation of τw in our study may help improve the model simulations and reduce the parameter's uncertainty over the projection of future AGB in the current DGVM or Earth System Models. A clearer understanding of the responses of τw to climate change and the corresponding sophisticated description of forest growth/mortality in model structure is also needed for the improvement of carbon stock prediction in future studies.
      PubDate: 2017-04-23T22:52:26.852908-05:
      DOI: 10.1002/2016GB005557
       
  • Modeling radiocarbon constraints on the dilution of dissolved organic
           carbon in the deep ocean.
    • Authors: Jamie D. Wilson; Sandra Arndt
      Abstract: The recalcitrance of dissolved organic carbon (DOC) that leads to its accumulation in the deep ocean is typically considered a function of its reactivity. Yet, recent experimental evidence has shown that DOC from the deep ocean, if concentrated, can support significant microbial growth. This supports an alternative hypothesis that [DOC] may become too dilute to support microbial growth. The radiocarbon signature of DOC is a key constraint on the DOC cycling that allows testing of the plausibility of this hypothesis. Here, we use a box model of diluted DOC in the deep ocean and its radiocarbon signature that is constrained on the basis of the new experimental evidence, as well as current knowledge of deep ocean DOC cycling to quantitatively test the dilution hypothesis. We explore the uncertainty in model results across a range of plausible dilution thresholds, additional processes, and fluxes of DOC to the deep ocean. Results show that the model is able to predict the observed radiocarbon signature for a dilution threshold close to the observed deep ocean [DOC] and for fluxes close to published estimates. Sensitivity analysis shows that this result is highly sensitive to variations in the dilution threshold, and the assumption that diluted DOC is able to survive ocean overturning. The experimental findings can be alternatively reconciled over a large range of different conditions assuming a small pool of diluted DOC with a modern radiocarbon signature, consistent with recent observations, and offering a parsimonious interpretation of dilution with existing hypotheses on DOC recalcitrance.
      PubDate: 2017-04-19T06:22:19.587587-05:
      DOI: 10.1002/2016GB005520
       
  • Strong biotic influences on regional patterns of climate regulation
           services
    • Authors: H. M. Serna-Chavez; N. G. Swenson, M. D. Weiser, E. E. Loon, W. Bouten, M. D. Davidson, P. M. Bodegom
      Abstract: Climate regulation services from forests are an important leverage in global-change mitigation treaties. Like most ecosystem services, climate regulation is the product of various ecological phenomena with unique spatial features. Elucidating which abiotic and biotic factors relate to spatial patterns of climate regulation services advances our understanding of what underlies climate-mitigation potential, and its variation within and across ecosystems. Here we quantify and contrast the statistical relations between climate regulation services (albedo and evapotranspiration, primary productivity and soil carbon) and abiotic and biotic factors. We focus on 16,955 forest plots in a regional extent across the eastern United States. We find the statistical effects of forest litter and understory carbon on climate regulation services to be as strong as those of temperature-precipitation interactions. These biotic factors likely influence climate regulation through changes in vegetation and canopy density, radiance scattering, and decomposition rates. We also find a moderate relation between leaf nitrogen traits and primary productivity at this regional scale. The statistical relation between climate regulation and temperature-precipitation ranges, seasonality, and climatic thresholds highlights a strong feedback with global climate change. Our assessment suggests the expression of strong biotic influences on climate regulation services at a regional, temperate extent. Biotic homogenization and management practices manipulating forests structure and succession will likely strongly impact climate-mitigation potential. The identity, strength, and direction of primary influences differed for each process involved in climate regulation. Hence, different abiotic and biotic factors are needed to monitor and quantify the full climate-mitigation potential of temperate forest ecosystems.
      PubDate: 2017-04-18T07:16:10.659312-05:
      DOI: 10.1002/2017GB005627
       
  • One possible uncertainty in CMIP5 projections of low-oxygen water volume
           in the Eastern Tropical Pacific
    • Authors: M. Shigemitsu; A. Yamamoto, A. Oka, Y. Yamanaka
      Abstract: Using the results from nine Earth system models submitted to the Coupled Model Intercomparison Project Phase 5 (CMIP5), we identify the Eastern Tropical Pacific (ETP) as the region with the greatest uncertainty of future changes in oxygen-deficient (
      PubDate: 2017-04-17T08:50:24.915209-05:
      DOI: 10.1002/2016GB005447
       
  • Picoplankton contribution to biogenic silica stocks and production rates
           in the Sargasso Sea
    • Authors: Jeffrey W. Krause; Mark A. Brzezinski, Stephen B. Baines, Jackie L. Collier, Benjamin S. Twining, Daniel C. Ohnemus
      Abstract: Picocyanobacteria in the Sargasso Sea accumulate significant amounts of Si, a finding which questions how we interpret previous regional measurements of biogenic silica (bSi) production and the role of diatoms in the open ocean. The picoplankton (
      PubDate: 2017-04-13T10:45:14.871372-05:
      DOI: 10.1002/2017GB005619
       
  • Ventilation versus biology: What is the controlling mechanism of nitrous
           oxide distribution in the North Atlantic?
    • Authors: Mercedes Paz; Maribel I. García-Ibáñez, Reiner Steinfeldt, Aida F. Rios, Fiz F. Pérez
      Abstract: The extent to which water mass mixing and ocean ventilation contribute to nitrous oxide (N2O) distribution at the scale of oceanic basins is poorly constrained. We used novel N2O and chlorofluorocarbon (CFC) measurements along with multiparameter water mass analysis to evaluate the impact of water mass mixing and Atlantic meridional overturning circulation (AMOC) on N2O distribution along the OVIDE section, extending from Portugal to Greenland. The biological N2O production has a stronger impact on the observed N2O concentrations in the water masses travelling northwards in the upper limb of the AMOC than those in recently ventilated cold water masses in the lower limb, where N2O concentrations reflect the colder temperatures. The high N2O tongue, with concentrations as high as 16 nmol kg-1, propagates above the isopycnal surface delimiting the upper and lower AMOC limbs, which extends from the eastern North Atlantic Basin to the Iceland Basin and coincides with the maximum N2O production rates. Water mixing and basin scale remineralization account for 72% of variation in the observed distribution of N2O. The mixing-corrected stoichiometric ratio N2O:O2 for the North Atlantic basin of 0.06 nmol/µmol is in agreement with ratios of N2O:O2 for local N2O anomalies, suggesting than up to 28% of N2O production occurs in the temperate and subpolar Atlantic, an overlooked region for N2O cycling. Overall, our results highlight the importance of taking into account mixing, O2 undersaturation when water masses are formed and the increasing atmospheric N2O concentrations when parameterizing N2O:O2 and biological N2O production in the global oceans.
      PubDate: 2017-04-13T10:45:05.988366-05:
      DOI: 10.1002/2016GB005507
       
  • Modest net autotrophy in the oligotrophic ocean
    • Authors: Robert T. Letscher; J. Keith Moore
      Abstract: The metabolic state of the oligotrophic subtropical ocean has long been debated. Net community production (NCP) represents the balance of autotrophic carbon fixation with heterotrophic respiration. Many in vitro NCP estimates based on oxygen incubation methods and the corresponding scaling relationships used to predict the ecosystem metabolic balance have suggested the ocean gyres to be net heterotrophic, however all in situ NCP methods find net autotrophy. Reconciling net heterotrophy requires significant allochthonous inputs of organic carbon to the oligotrophic gyres to sustain a preponderance of respiration over in situ production. Here we use the first global ecosystem-ocean circulation model that contains representation of the three allochthonous carbon sources to the open ocean, to show that the five oligotrophic gyres exhibit modest net autotrophy throughout the seasonal cycle. Annually integrated rates of NCP vary in the range ~1.5 - 2.2 mol O2 m-2 yr-1 across the five gyre systems, however seasonal NCP rates are as low as ~1 ± 0.5 mmol O2 m-2 d-1 for the North Atlantic. Volumetric NCP rates are heterotrophic below the 10% light level, however become net autotrophic when integrated over the euphotic zone. Observational uncertainties when measuring these modest autotrophic NCP rates as well as the metabolic diversity encountered across space and time complicate the scaling up of in vitro measurements to the ecosystem scale and may partially explain the previous reports of net heterotrophy. The oligotrophic ocean is autotrophic at present; however it could shift towards seasonal heterotrophy in the future as rising temperatures stimulate respiration.
      PubDate: 2017-04-05T06:05:47.714444-05:
      DOI: 10.1002/2016GB005503
       
  • Annual net community production in the subtropical Pacific Ocean from
           in-situ oxygen measurements on profiling floats
    • Authors: Bo Yang; Steven R. Emerson, Seth M. Bushinsky
      Abstract: Annual net community production (ANCP) in the subtropical Pacific Ocean was determined using annual oxygen measurements from Argo profiling floats with an upper water column oxygen mass balance model. ANCP was determined to be from 2.0 to 2.4 mol C m-2 yr-1 in the western subtropical North Pacific, 2.4 mol C m-2 yr-1 in the eastern subtropical North Pacific, and near zero in the subtropical South Pacific. Error analysis with the main sources of uncertainty being the accuracy of oxygen measurements and the parameterization of bubble fluxes in winter suggested an uncertainty of ~ 0.3 mol C m-2 yr-1 in subtropical Pacific. The results are in good agreement with previous observations in locations where ANCP has been determined before. These are the first results from the western subtropical North Pacific and subtropical South Pacific where ANCP have not been evaluated before. ANCP for the subtropical South Pacific is significantly lower than in all other open ocean locations where it has been determined by mass balance. Comparison of our observations with net biological carbon export estimated from remote sensing algorithms indicate that observations from the subtropical North Pacific are higher than the satellite estimates but those in the subtropical South Pacific are lower than satellite-determined carbon export.
      PubDate: 2017-03-31T08:55:43.785926-05:
      DOI: 10.1002/2016GB005545
       
  • Carbonate buffering and metabolic controls on carbon dioxide in rivers
    • Authors: Edward G. Stets; David Butman, Cory P. McDonald, Sarah Stackpoole, Michael D. DeGrandpre, Robert G. Striegl
      Abstract: Multiple processes support the significant efflux of carbon dioxide (CO2) from rivers and streams. Attribution of CO2 oversaturation will lead to better quantification of the freshwater carbon cycle and provide insights into the net cycling of nutrients and pollutants. CO2 production is closely related to O2 consumption because of the metabolic linkage of these gases. However, this relationship can be weakened due to dissolved inorganic carbon (DIC) inputs from groundwater, carbonate buffering, calcification, and anaerobic metabolism. CO2 and O2 concentrations and other water quality parameters were analyzed in two datasets: a synoptic field study, and nationwide water quality monitoring data. CO2 and O2 concentrations were strongly negatively correlated in both datasets (ρ = -0.67 and ρ = -0.63, respectively), although the correlations were weaker in high alkalinity environments. In nearly all samples, the molar oversaturation of CO2 was a larger magnitude than molar O2 undersaturation. We used a dynamically coupled O2-CO2 model to show that lags in CO2 air-water equilibration are a likely cause of this phenomenon. Lags in CO2 equilibration also impart landscape-scale differences in the behavior of CO2 between high- and low-alkalinity watersheds. Although the concept of carbonate buffering and how it creates lags in CO2 equilibration with the atmosphere is well understood, it has not been sufficiently integrated into our understanding of CO2 dynamics in freshwaters. We argue that the consideration of carbonate equilibria and its effects on CO2 dynamics are primary steps in understanding the sources and magnitude of CO2 oversaturation in rivers and streams.
      PubDate: 2017-03-20T07:00:23.159463-05:
      DOI: 10.1002/2016GB005578
       
  • Calculating the balance between atmospheric CO2 drawdown and organic
           carbon oxidation in subglacial hydrochemical systems
    • Authors: Joseph A. Graly; James I. Drever, Neil F. Humphrey
      Abstract: In order to constrain CO2 fluxes from biogeochemical processes in subglacial environments, we model the evolution of pH and alkalinity over a range of subglacial weathering conditions. We show that subglacial waters reach or exceed atmospheric pCO2 levels when atmospheric gases are able to partially access the subglacial environment. Subsequently, closed system oxidation of sulfides is capable of producing pCO2 levels well in excess of atmosphere levels without any input from the decay of organic matter. We compared this model to published pH and alkalinity measurements from 21 glaciers and ice sheets. Most subglacial waters are near atmospheric pCO2 values. The assumption of an initial period of open system weathering requires substantial organic carbon oxidation in only 4 of the 21 analyzed ice bodies. If the subglacial environment is assumed to be closed from any input of atmospheric gas, large organic carbon inputs are required in nearly all cases. These closed system assumptions imply that order of 10 gm-2y-1 of organic carbon are removed from a typical subglacial environment – a rate too high to represent soil carbon built up over previous interglacial periods and far in excess of fluxes of surface deposited organic carbon. Partial open system input of atmospheric gases is therefore likely in most subglacial environments. The decay of organic carbon is still important to subglacial inorganic chemistry where substantial reserves of ancient organic carbon are found in bedrock. In glaciers and ice sheets on silicate bedrock, substantial long term drawdown of atmospheric CO2 occurs.
      PubDate: 2017-03-15T05:00:33.495732-05:
      DOI: 10.1002/2016GB005425
       
  • Atmospheric methane variability: Centennial scale signals in the Last
           Glacial Period
    • Authors: Rachael H. Rhodes; Edward J. Brook, Joseph R. McConnell, Thomas Blunier, Louise C. Sime, Xavier Fain, Robert Mulvaney
      Abstract: In order to understand atmospheric methane (CH4) biogeochemistry now and in the future, we must apprehend its natural variability, that without anthropogenic influence. Samples of ancient air trapped within ice cores provide the means to do this. Here we analyze the ultra-high resolution CH4 record of the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core 67.2–9.8 ka and find novel, atmospheric CH4 variability at centennial timescales throughout the record. This signal is characterized by recurrence intervals within a broad 80–500 yr range but we find that age scale uncertainties complicate the possible isolation of any periodic frequency. Lower signal amplitudes in the Last Glacial relative to the Holocene may be related to incongruent effects of firn-based signal smoothing processes. Within interstadial and stadial periods, the peak-to-peak signal amplitudes vary in proportion to the underlying millennial scale oscillations in CH4 concentration—the relative amplitude change is constant. We propose that the centennial CH4 signal is related to tropical climate variability that influences predominantly low latitude wetland CH4 emissions.
      PubDate: 2017-03-14T20:55:39.203347-05:
      DOI: 10.1002/2016GB005570
       
  • Potential controls of isoprene in the surface ocean
    • Authors: S. C. Hackenberg; S. J. Andrews, R. Airs, S. R. Arnold, H. A. Bouman, R. J. W. Brewin, R. J. Chance, D. Cummings, G. Dall'Olmo, A. C. Lewis, J. K. Minaeian, K. M. Reifel, A. Small, G. A. Tarran, G. H. Tilstone, L. J. Carpenter
      Abstract: Isoprene surface ocean concentrations and vertical distribution, atmospheric mixing ratios and calculated sea-to-air fluxes spanning approximately 125 degrees of latitude (80 °N - 45 °S) over the Arctic and Atlantic Oceans are reported. Oceanic isoprene concentrations were associated with a number of concurrently monitored biological variables including chlorophyll a (Chl a), photo-protective pigments, integrated primary production (intPP) and cyanobacterial cell counts, with higher isoprene concentrations relative to all respective variables found at sea surface temperatures greater than 20 °C. The correlation between isoprene and the sum of photo-protective carotenoids, which is reported here for the first time, was the most consistent across all cruises. Parameterisations based on linear regression analyses of these relationships perform well for Arctic and Atlantic data, producing a better fit to observations than an existing Chl a–based parameterisation. Global extrapolations of isoprene surface water concentrations using satellite-derived Chl a and intPP were able to reproduce general trends in the in situ data and absolute values within a factor of 2 between 60% and 85% of the time, depending on the data set and algorithm used.
      PubDate: 2017-03-13T03:15:26.729357-05:
      DOI: 10.1002/2016GB005531
       
  • Sensitivity of land-use change emission estimates to historical land-use
           and land-cover mapping
    • Authors: Shushi Peng; Philippe Ciais, Fabienne Maignan, Wei Li, Jinfeng Chang, Tao Wang, Chao Yue
      Abstract: The carbon emissions from land-use and land-cover change (ELUC) are an important anthropogenic component of the global carbon budget. Yet, these emissions have a large uncertainty. Uncertainty in historical land-use and land-cover change (LULCC) maps and their implementation in global vegetation models is one of the key sources of the spread of ELUC calculated by global vegetation models. In this study, we used the ORCHIDEE terrestrial biosphere model to investigate how the different transition rules to define the priority of conversion from natural vegetation to agricultural land affect the historical reconstruction of plant functional types (PFTs) and ELUC. First, we reconstructed ten sets of historical PFT maps using different transition rules and two methods. Then, we calculated ELUC from these ten different historical PFT maps and an additional published PFT reconstruction, using the difference between two sets of simulations (with and without LULCC). The total area of forest loss is highly correlated with the total simulated ELUC (R2 = 0.83, P 
      PubDate: 2017-03-10T15:00:31.224518-05:
      DOI: 10.1002/2015GB005360
       
  • N2O production and consumption from stable isotopic and concentration data
           in the Peruvian Coastal Upwelling System
    • Authors: Annie Bourbonnais; Robert T. Letscher, Hermann W. Bange, Vincent Échevin, Jennifer Larkum, Joaquim Mohn, Naohiro Yoshida, Mark A. Altabet
      Abstract: The ocean is an important source of nitrous oxide (N2O) to the atmosphere; yet, the factors controlling N2O production and consumption in oceanic environments are still not understood nor constrained. We measured N2O concentrations and isotopomer ratios, as well as O2, nutrient and biogenic N2 concentrations and the isotopic compositions of nitrate and nitrite at several coastal stations during two cruises off the Peru coast (~5-15°S, 75-81°W) in December 2012 and January 2013. N2O concentrations varied from below equilibrium values in the Oxygen Deficient Zone (ODZ) to up to 190 nmol L-1 in surface waters. We used a 3D-reaction-advection-diffusion model to evaluate the rates and modes of N2O production in oxic waters and rates of N2O consumption versus production by denitrification in the ODZ. Intramolecular Site Preference (SP) in N2O isotopomer was relatively low in surface waters (generally -3 to 14‰) and together with modeling results, confirmed the dominance of nitrifier- or incomplete denitrifier-denitrification, corresponding to an efflux of up to 0.6 Tg N year-1 off the Peru coast. Other evidences, e.g., the absence of a relationship between ΔN2O and apparent O2 utilization and significant relationships between nitrate, a substrate during denitrification, and N2O isotopes, suggest that N2O production by incomplete denitrification or nitrifier-denitrification decoupled from aerobic organic matter remineralization are likely pathways for extreme N2O accumulation in newly upwelled surface waters. We observed imbalances between N2O production and consumption in the ODZ, with the modeled proportion of N2O consumption relative to production generally increasing with biogenic N2. However, N2O production appeared to occur even where there was high N-loss at the shallowest stations.
      PubDate: 2017-03-07T10:05:27.389288-05:
      DOI: 10.1002/2016GB005567
       
  • Measurement of the 13C isotopic signature of methane emissions from
           Northern European wetlands
    • Authors: Rebecca E. Fisher; James L. France, David Lowry, Mathias Lanoisellé, Rebecca Brownlow, John A. Pyle, Michelle Cain, Nicola Warwick, Ute M. Skiba, Julia Drewer, Kerry J. Dinsmore, Sarah R. Leeson, Stéphane J.-B. Bauguitte, Axel Wellpott, Sebastian J. O'Shea, Grant Allen, Martin W. Gallagher, Joseph Pitt, Carl J. Percival, Keith Bower, Charles George, Garry D. Hayman, Tuula Aalto, Annalea Lohila, Mika Aurela, Tuomas Laurila, Patrick M. Crill, Carmody K. McCalley, Euan G. Nisbet
      Abstract: Isotopic data provide powerful constraints on regional and global methane emissions and their source profiles. However, inverse modeling of spatially-resolved methane flux is currently constrained by a lack of information on the variability of source isotopic signatures. In this study, isotopic signatures of emissions in the Fennoscandian Arctic have been determined in chambers over wetland, in the air 0.3 to 3 m above the wetland surface and by aircraft sampling from 100 m above wetlands up to the stratosphere. Overall the methane flux to atmosphere has a coherent δ13C isotopic signature of -71 ± 1‰, measured in situ on the ground in wetlands. This is in close agreement with δ13C isotopic signatures of local and regional methane increments measured by aircraft campaigns flying through air masses containing elevated methane mole fractions. In contrast results from wetlands in Canadian boreal forest further south gave isotopic signatures of -67 ± 1 ‰.Wetland emissions dominate the local methane source measured over the European Arctic in summer. Chamber measurements demonstrate a highly variably methane flux and isotopic signature, but the results from air sampling within wetland areas show that emissions mix rapidly immediately above the wetland surface and methane emissions reaching the wider atmosphere do indeed have strongly coherent C isotope signatures. The study suggests that for boreal wetlands (>60°N) global and regional modeling can use an isotopic signature of -71‰ to apportion sources more accurately, but there is much need for further measurements over other wetlands regions to verify this.
      PubDate: 2017-03-07T09:15:30.83873-05:0
      DOI: 10.1002/2016GB005504
       
  • Calculating surface ocean pCO2 from biogeochemical Argo floats equipped
           with pH: an uncertainty analysis
    • Authors: N. L. Williams; L. W. Juranek, R. A. Feely, K. S. Johnson, J. L. Sarmiento, L. D. Talley, A. G. Dickson, A. R. Gray, R. Wanninkhof, J. L. Russell, S. C. Riser, Y. Takeshita
      Abstract: More than 74 biogeochemical profiling floats that measure water column pH, oxygen, nitrate, fluorescence, and backscattering at 10-day intervals have been deployed throughout the Southern Ocean. Calculating the surface ocean partial pressure of carbon dioxide (pCO2sw) from float pH has uncertainty contributions from the pH sensor, the alkalinity estimate, and carbonate system equilibrium constants, resulting in a relative standard uncertainty in pCO2sw of 2.4% (or 10 µatm at pCO2sw of 400 µatm). The calculated pCO2sw from several floats spanning a range of oceanographic regimes are compared to existing climatologies. In some locations, such as the Subantarctic zone, the float data closely match the climatologies, but in the Polar Antarctic Zone significantly higher pCO2sw are calculated in the wintertime implying a greater air-sea CO2 efflux estimate. Our results based on four representative floats suggest that despite their uncertainty relative to direct measurements the float data can be used to improve estimates for air-sea carbon flux, as well as to increase knowledge of spatial, seasonal, and interannual variability in this flux.
      PubDate: 2017-03-03T11:40:26.959603-05:
      DOI: 10.1002/2016GB005541
       
  • The export and fate of organic matter in the ocean: New constraints from
           combining satellite and oceanographic tracer observations
    • Authors: Tim DeVries; Thomas Weber
      Abstract: The ocean's biological pump transfers carbon from the surface euphotic zone into the deep ocean, reducing the atmospheric CO2 concentration. Despite its climatic importance, there are large uncertainties in basic metrics of the biological pump. Previous estimates of the strength of the biological pump, as measured by the amount of organic carbon exported from the euphotic zone, range from about 4–12 Pg C yr−1. The fate of exported carbon, in terms of how efficiently it is transferred into the deep ocean, is even more uncertain. Here we present a new model of the biological pump that assimilates satellite and oceanographic tracer observations to constrain rates and patterns of organic matter production, export and remineralization in the ocean. The data-assimilated model predicts a global particulate organic carbon (POC) flux out of the euphotic zone of ∼9 Pg C yr−1. The particle export ratio (the ratio of POC export to net primary production) is highest at high latitudes and lowest at low latitudes, but low-latitude export is greater than predicted by previous models, in better agreement with observed patterns of long-term carbon export. Particle transfer efficiency (Teff) through the mesopelagic zone is controlled by temperature and oxygen, with highest Teff for high-latitude regions and oxygen minimum zones. In contrast, Teff in the deep ocean (below 1000 m) is controlled by particle sinking speed, with highest deep-ocean Teff below the subtropical gyres. These results emphasize the utility of both remote sensing and oceanographic tracer observations for constraining the operation of the biological pump.
      PubDate: 2017-02-27T08:46:39.852157-05:
      DOI: 10.1002/2016GB005551
       
  • Silicon cycling in Lake Baikal
    • Authors: V.N. Panizzo; G.E.A. Swann, A.W. Mackay, E. Vologina, L. Alleman, L. Andre, V.H. Pashley, M.S.A. Horstwood
      Abstract: Constraining the continental silicon cycle is a key requirement in attempts to understand both nutrient fluxes to the ocean and linkages between silicon and carbon cycling over different timescales. Silicon isotope data of dissolved silica (δ30SiDSi) are presented here from Lake Baikal and its catchment in central Siberia. As well as being the world's oldest and voluminous lake, Lake Baikal lies within the seventh largest drainage basin in the world and exports significant amounts of freshwater into the Arctic Ocean. Data from river waters accounting for c. 92% of annual river inflow to the lake suggest no seasonal alteration or anthropogenic impact on river δ30SiDSi composition. The absence of a change in δ30SiDSi within the Selenga Delta, through which 62% of riverine flow passes, suggest a net balance between biogenic uptake and dissolution in this system. A key feature of this study is the use of δ30SiDSi to examine seasonal and spatial variations in DSi utilisation and export across the lake. Using an open system model against deep water δ30SiDSi values from the lake, we estimate that 20-24% of DSi entering Lake Baikal is exported into the sediment record. Whilst highlighting the impact that lakes may have upon the sequestration of continental DSi, mixed layer δ30SiDSi values from 2003 and 2013 show significant spatial variability in the magnitude of spring bloom nutrient utilisation with lower rates in the north relative to south basin.
      PubDate: 2017-02-27T06:25:33.739123-05:
      DOI: 10.1002/2016GB005518
       
  • Net community production and carbon export during the late summer in the
           Ross Sea, Antarctica
    • Authors: Hans. B. DeJong; Robert B. Dunbar, David A. Koweek, David A. Mucciarone, Sarah K. Bercovici, Dennis A. Hansell
      Abstract: The phytoplankton bloom in the Ross Sea is the largest in spatial extent and one of the most productive in Antarctica, yet the fate of the summer bloom remains poorly understood. Here we present carbon system data from the first biogeochemical process cruise to be conducted in both the western and central Ross Sea during late summer (February-March 2013). Using one-dimensional carbon budgets, we found evidence for substantial positive net community production (425 ± 204 mmol C m-2 d-1) during the late summer in Terra Nova Bay (TNB) of the western Ross Sea, which was rapidly exported to below 200 m. In addition, seasonally integrated carbon export was higher in diatom-dominated TNB (7.3 ± 0.9 mol C m-2) compared to the Phaeocystis antarctica-dominated central Ross Sea (3.4 ± 0.8 mol C m-2). Substantial late summer productivity and export may be a widespread phenomenon in Antarctic coastal regions that is not accounted for in regional carbon models.
      PubDate: 2017-02-18T02:55:42.493681-05:
      DOI: 10.1002/2016GB005417
       
  • Correlations of surface ocean pCO2 to satellite chlorophyll on monthly to
           interannual timescales
    • Authors: Amanda R. Fay; Galen A. McKinley
      Abstract: On the mean, ocean carbon uptake is linked to biological productivity, but how biological variability impacts carbon uptake is poorly quantified. Our ability to diagnose past change, understand present variability and predict the future state of the global carbon cycle requires improving mechanistic understanding in this area. Here, we make use of co-located pCO2 and temperature data, a merged surface ocean color product, and physical fields from an ocean state estimate to assess relationships between surface ocean biology and the carbon cycle on seasonal, monthly anomaly, and interannual timescales over the period 1998-2014. Using a correlation analysis on spatial scales from local to basin-scale biomes, we identify the timescales on which ocean productivity could be directly modifying ocean carbon uptake. On seasonal timescales outside of the equatorial Pacific, biome-scale correlations are negative between chlorophyll and pCO2. Though this relationship is pervasive, the underlying mechanisms vary across timescales and biomes. Consistent with previous findings, biological activity is a significant driver of pCO2 seasonality only in the subpolar biomes. For monthly anomalies acting on top of the mean seasonality, productivity and pCO2 changes are significantly correlated in the subpolar North Pacific and Southern Ocean. Only in the Southern Ocean are correlations consistent with a dominant role for biology in the surface ocean carbon cycle on all timescales.
      PubDate: 2017-02-18T02:40:38.240114-05:
      DOI: 10.1002/2016GB005563
       
  • Global and Regional Fluxes of Carbon from Land Use and Land-Cover Change
           1850-2015
    • Authors: R. A. Houghton; Alexander A. Nassikas
      Abstract: The net flux of carbon from land use and land-cover change (LULCC) is an important term in the global carbon balance. Here we report a new estimate of annual fluxes from 1850 to 2015, updating earlier analyses with new estimates of both historical and current rates of LULCC and including emissions from draining and burning of peatlands in Southeast Asia. For most of the 186 countries included we relied on data from FAO to document changes in the areas of croplands and pastures since 1960 and changes in the areas of forests and “other land” since 1990. For earlier years we used other sources of information. We used a bookkeeping model that prescribed changes in carbon density of vegetation and soils for 20 types of ecosystems and five land uses. The total net flux attributable to LULCC over the period 1850-2015 is calculated to have been 145 ± 16 PgC (one standard deviation). Most of the emissions were from the tropics (102 ± 5.8 PgC), generally increasing over time to a maximum of 2.10 PgC yr-1 in 1997. Outside the tropics emissions were roughly constant at 0.5 PgC yr-1 until 1940, declined to zero around 1970, and then became negative. For the most recent decade (2006-2015) global net emissions from LULCC averaged 1.11 (±0.35) PgC yr-1, consisting of a net source from the tropics (1.41 ± 0.17 PgC yr-1), a net sink in northern mid-latitudes (-0.28 ± 0.21 PgC yr-1), and carbon neutrality in southern mid-latitudes.
      PubDate: 2017-02-15T12:36:00.450841-05:
      DOI: 10.1002/2016GB005546
       
  • A Global Ocean Climatology of Preindustrial and Modern Ocean δ13C
    • Authors: M. Eide; A. Olsen, U. Ninnemann, T. Johannessen
      Abstract: We present a global ocean climatology of dissolved inorganic carbon δ13C (‰) corrected for the 13C-Suess effect, preindustrial δ13C. This was constructed by first using Olsen and Ninnemann's [2010] back-calculation method on data from 25 World Ocean Circulation Experiment cruises to reconstruct the preindustrial δ13C on sections spanning all major oceans. Next, we developed five multilinear regression equations, one for each major ocean basin, which were applied on the World Ocean Atlas data to construct the climatology. This reveals the natural δ13C distribution in the global ocean. Compared to the modern distribution, the preindustrial δ13C spans a larger range of values. The maxima, of up to 1.8‰, occurs in the subtropical gyres of all basins, in the upper and intermediate waters of the North Atlantic, as well as in mode waters with a Southern Ocean origin. Particularly strong gradients occur at intermediate depths, revealing a strong potential for using δ13C as a tracer for changes in water mass geometry at these levels. Further, we identify a much tighter relationship between δ13C and Apparent Oxygen Utilization (AOU) than between δ13C and phosphate. This arises because, in contrast to phosphate, AOU and δ13C are both partly reset when waters are ventilated in the Southern Ocean, and underscores that δ13C is a highly robust proxy for past changes in ocean oxygen content and ocean ventilation. Our global preindustrial δ13C climatology is openly accessible, and can be used for example for improved model evaluation and interpretation of sediment δ13C records.
      PubDate: 2017-02-13T00:25:35.504076-05:
      DOI: 10.1002/2016GB005473
       
  • A Global Estimate of the Full Oceanic 13C Suess Effect Since the
           Preindustrial
    • Authors: M. Eide; A. Olsen, U. Ninnemann, T. Eldevik
      Abstract: We present the first estimate of the full global ocean 13C Suess effect since preindustrial times, based on observations. This has been derived by first using the method of Olsen and Ninnemann [2010] to calculate 13C Suess effect estimates on sections spanning the world ocean, which were next mapped on a global 1°x1° grid. We find a strong 13C Suess effect in the upper 1000 m of all basins, with strongest decrease in the Subtropical Gyres of the Northern Hemisphere, where δ13C of dissolved inorganic carbon has decreased by more than 0.8‰ since the industrial revolution. At greater depths, a significant 13C Suess effect can only be detected in the northern parts of the North Atlantic Ocean. The relationship between the 13C Suess effect and the concentration of anthropogenic carbon varies strongly between water masses, reflecting the degree to which source waters are equilibrated with the atmospheric 13C Suess effect before sinking. Finally, we estimate a global ocean inventory of anthropogenic CO2 of 92 ± 46 Gt C. This provides an estimate that is almost independent of, and consistent, within the uncertainties, with previous estimates.
      PubDate: 2017-02-13T00:25:34.277328-05:
      DOI: 10.1002/2016GB005472
       
  • Issue Information
    • Pages: 435 - 435
      Abstract: No abstract is available for this article.
      PubDate: 2017-04-18T03:04:27.144226-05:
      DOI: 10.1002/gbc.20451
       
 
 
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