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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 53, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 15)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 69, 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]
  • New model for capturing the variations of fertilizer‐induced
           emission factors of N2O
    • Authors: Feng Zhou; Ziyin Shang, Zhenzhong Zeng, Shilong Piao, Philippe Ciais, Peter A. Raymond, Xuhui Wang, Rong Wang, Minpeng Chen, Changliang Yang, Shu Tao, Yue Zhao, Qian Meng, Shuoshuo Gao, Qi Mao
      Abstract: Accumulating evidence indicates that N2O emission factors (EFs) vary with nitrogen additions and environmental variations. Yet the impact of the latter was often ignored by previous EF determinations. We developed piecewise statistical models (PMs) to explain how the N2O EFs in agricultural soils depend upon various predictors such as climate, soil attributes, and agricultural management. The PMs are derived from a new Bayesian Recursive Regression Tree algorithm. The PMs were applied to the case of EFs from agricultural soils in China, a country where large EF spatial gradients prevail. The results indicate substantial improvements of the PMs compared with other EF determinations. First, PMs are able to reproduce a larger fraction of the variability of observed EFs for upland grain crops (84%, n=381) and paddy rice (91%, n=161) as well as the ratio of EFs to nitrogen application rates (73%, n=96). The superior predictive accuracy of PMs is further confirmed by evaluating their predictions against independent EF measurements (n=285) from outside China. Results show that the PMs calibrated using Chinese data can explain 75% of the variance. Hence the PMs could be reliable for upscaling of N2O EFs and fluxes for regions that have a phase‐space of predictors similar to China. Results from the validated models also suggest that climatic factors regulate the heterogeneity of EFs in China, explaining 69% and 85% of their variations for upland grain crops and paddy rice, respectively. The corresponding N2O EFs in 2008 are 0.84±0.18% (as N2O‐N emissions divided by the total N input) for upland grain crops and 0.65±0.14% for paddy rice, the latter being twice as large as the IPCC Tier 1 defaults. Based upon these new estimates of EFs, we infer that only 22% of current arable land could achieve a potential reduction of N2O emission of 50%.
      PubDate: 2015-05-20T06:32:07.117159-05:
      DOI: 10.1002/2014GB005046
       
  • Metals and Metalloids in Precipitation Collected during CHINARE Campaign
           from Shanghai, China to Zhongshan Station, Antarctica: Spatial Variability
           and Source Identification
    • Authors: G. Shi; J. Teng, H. Ma, Y. Li, B. Sun
      Abstract: Metals and metalloids in continental precipitation have been widely observed, but the data over open oceans are still very limited. Investigation of metals and metalloids in marine precipitation is of great significance to understand global transport of these elements in the atmosphere and their input fluxes to the oceans. So, shipboard sampling of precipitation was conducted during a Chinese National Antarctic Research Expedition (CHINARE) campaign from Shanghai, China to Zhongshan Station, East Antarctica, and 22 samples (including 17 rainfall and 5 snowfall events) were collected and analyzed for concentrations of Pb, Ni, Cr, Cu, Co, Hg, As, Cd, Sb, Se, Zn, Mn and Ti. Results show that concentrations of both metals and metalloids vary considerably along the cruise, with higher concentrations at coastal sites and lower values on the south Indian Ocean. Although only soluble fractions were determined for elements, concentrations in this study are generally comparable to the reported values of marine rain. Enrichment factor analysis shows that most of metals and metalloids are enriched versus crustal sources, even in the samples collected from remote south Indian Ocean. In addition, metals and metalloids in precipitation are also very enriched above sea‐salt abundance, indicating that impacts of sea salt aerosols on their concentrations are negligible. Main sources of metals and metalloids were explored with the aid of multivariate statistical analyses. The results show that human emissions have far‐reaching distribution, which may exert an important influence on the solubility of elements in precipitation. This investigation provides valuable information on spatial variation and possible sources of trace elements in precipitation over the open oceans corresponding to understudied region.
      PubDate: 2015-05-14T05:34:17.232202-05:
      DOI: 10.1002/2014GB005060
       
  • Towards a parameterization of global‐scale organic carbon
           mineralization kinetics in surface marine sediments
    • Authors: K. Stolpovsky; A. W. Dale, K. Wallmann
      Abstract: An empirical function is derived for predicting the rate‐depth profile of particulate organic carbon (POC) degradation in surface marine sediments including the bioturbated layer. The rate takes the form of a power law analogous to the Middelburg function. The functional parameters were optimized by simulating measured benthic O2 and NO3− fluxes at 185 stations worldwide using a diagenetic model. The novelty of this work rests with the finding that the vertically‐resolved POC degradation rate in the bioturbated zone can be determined using a simple function where the POC rain rate is the governing variable. Although imperfect, the model is able to fit 71 % of paired O2 and NO3− fluxes to within 50% of measured values. It further provides realistic geochemical concentration‐depth profiles, NO3− penetration depths and apparent first‐order POC mineralization rate constants. The model performs less well on the continental shelf due to the high heterogeneity there. When applied to globally resolved maps of rain rate, the model predicts a global denitrification rate of 182 ± 88 Tg yr−1 of N and a POC burial rate of 107 ± 52 Tg yr−1 of C with a mean carbon burial efficiency of 6.1%. These results are in very good agreement with published values. Our proposed function is conceptually simple, requires less parameterization than multi‐G type models and is suitable for non‐steady state applications. It provides a basis for more accurately simulating benthic nutrient fluxes and carbonate dissolution rates in Earth system models.
      PubDate: 2015-05-14T05:33:54.810907-05:
      DOI: 10.1002/2015GB005087
       
  • Issue Information
    • Abstract: Cover: In Lovenduski et al. [doi 10.1002/2014GB004933], model‐estimated trends in the sea‐air flux of (first column) natural, (second column) anthropogenic, and (third column) contemporary CO2 over (first row) the simulated period (1958–2007) and (second row) the observed period (1981–2007). Units are mol C m‐2 yr‐2, and positive indicates CO2 outgassing trend. Only those trends with significance = 95% are shown. Black lines mark the edges of the model biomes used the study. From south to north, these biomes are the Southern Ocean seasonally ice covered biome (SO‐ICE) and the Southern Ocean subpolar seasonally stratified biome (SO‐SPSS). See pp. 416–426.
      PubDate: 2015-05-11T15:20:19.131922-05:
      DOI: 10.1002/gbc.20195
       
  • Global Patterns and controls of soil organic carbon dynamics as simulated
           by multiple terrestrial biosphere models: current status and future
           directions
    • Authors: Hanqin Tian; Chaoqun Lu, Jia Yang, Kamaljit Banger, Deborah N. Huntinzger, Christopher R. Schwalm, Anna M. Michalak, Robert Cook, Philippe Ciais, Daniel Hayes, Maoyi Huang, Akihiko Ito, Atul Jain, Huimin Lei, Jiafu Mao, Shufen Pan, Wilfred M. Post, Shushi Peng, Benjamin Poulter, Wei Ren, Daniel Ricciuto, Kevin Schaefer, Xiaoying Shi, Bo Tao, Weile Wang, Yaxing Wei, Qichun Yang, Bowen Zhang, Ning Zeng
      Abstract: Soil is the largest organic carbon (C) pool of terrestrial ecosystems, and C loss from soil accounts for a large proportion of land‐atmosphere C exchange. Therefore, a small change in soil organic C (SOC) can affect atmospheric carbon dioxide (CO2) concentration and climate change. In the past decades, a wide variety of studies have been conducted to quantify global SOC stocks and soil C exchange with the atmosphere through site measurements, inventories, and empirical/process‐based modeling. However, these estimates are highly uncertain and identifying major driving forces controlling soil C dynamics remains a key research challenge. This study has compiled century‐long (1901–2010) estimates of SOC storage and heterotrophic respiration (Rh) from ten terrestrial biosphere models (TBMs) in the Multi‐scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) and two observation‐based datasets. The ten‐TBM ensemble shows that global SOC estimate range from 425 to 2111 Pg C (1 Pg = 1015 g) with a median value of 1158 Pg C in 2010. The models estimate a broad range of Rh from 35 to 69 Pg C yr−1 with a median value of 51 Pg C yr−1 during 2001–2010. The largest uncertainty in SOC stocks exists in the 40–65°N latitude whereas the largest differences in Rh between models are in the tropics. The modeled SOC change during 1901–2010 ranges from −70 Pg C to 86 Pg C, but in some models the SOC change has a different sign from the change of total C stock, implying very different contribution of vegetation and soil pools in determining the terrestrial C budget among models. The model ensemble‐estimated mean residence time of SOC shows a reduction of 3.4 years over the past century, which is primarily caused by the increment in proportion of labile substrate which accelerate C cycling through the land biosphere. All the models agreed that climate and land use changes decreased SOC stocks while elevated atmospheric CO2 and nitrogen deposition over intact ecosystems increased SOC stocks – even though the responses varied significantly among models. Model representations of temperature and moisture sensitivity, nutrient limitation and land use partially explain the divergent estimates of global SOC stocks and soil C fluxes in this study. In addition, a major source of systematic error in model estimations relates to non‐modeled SOC storage in wetlands and peatlands, as well as to old C storage in deep soil layers.
      PubDate: 2015-05-11T05:31:54.306869-05:
      DOI: 10.1002/2014GB005021
       
  • Temperature, oxygen, and vegetation controls on decomposition in a James
           Bay peatland
    • Authors: Michael Philben; James Holmquist, Glen MacDonald, Dandan Duan, Karl Kaiser, Ronald Benner
      Abstract: The biochemical composition of a peat core from James Bay Lowland, Canada was used to assess the extent of peat decomposition and diagenetic alteration. Our goal was to identify environmental controls on peat decomposition, particularly its sensitivity to naturally occurring changes in temperature, oxygen exposure time, and vegetation. All three varied substantially during the last 7000 yr, providing a natural experiment for evaluating their effects on decomposition. The bottom 50 cm of the core formed during the Holocene Climatic Optimum (~7000‐4000 yr B.P.), when mean annual air temperature was likely 1‐2°C warmer than present. A reconstruction of the water table level using testate amoebae indicated oxygen exposure time was highest in the subsequent upper portion of the core between 150 and 225 cm depth (from ~2560‐4210 yr B.P) and the plant community shifted from mostly Sphagnum to vascular plant dominance. Several independent biochemical indices indicated decomposition was greatest in this interval. Hydrolysable amino acid yields, hydroxyproline yields, and acid:aldehyde ratios of syringyl lignin phenols were higher, while hydrolysable neutral sugar yields and carbon:nitrogen ratios were lower in this zone of both vascular plant vegetation and elevated oxygen exposure time. Thus, peat formed during the Holocene Climatic Optimum did not appear to be more extensively decomposed than peat formed during subsequent cooler periods. Comparison with a core from the West Siberian Lowland, Russia, indicates oxygen exposure time and vegetation are both important controls on decomposition, while temperature appears to be of secondary importance. The low apparent sensitivity of decomposition to temperature is consistent with recent observations of a positive correlation between peat accumulation rates and mean annual temperature, suggesting contemporary warming could enhance peatland carbon sequestration, although this could be offset by an increasing contribution of vascular plants to the vegetation.
      PubDate: 2015-05-06T14:47:47.620213-05:
      DOI: 10.1002/2014GB004989
       
  • Spatial and temporal contrasts in the distribution of crops and pastures
           across Amazonia: A new agricultural land‐use dataset from census
           data since 1950
    • Authors: P. Imbach; M. Manrow, E. Barona, A. Barretto, G. Hyman, P Ciais
      Abstract: Amazonia holds the largest continuous area of tropical forests with intense land use change dynamics inducing water, carbon and energy feedbacks with regional and global impacts. Much of our knowledge of land‐use change in Amazonia comes from studies of the Brazilian Amazon, which accounts for two thirds of the region. Amazonia outside of Brazil has received less attention because of the difficulty of acquiring consistent data across countries. We present here an agricultural statistics database of the entire Amazonia region, with a harmonized description of crops and pastures in geospatial format, based on administrative boundary data at the municipality level. The spatial coverage includes countries within Amazonia and spans censuses and surveys from 1950 to 2012. Harmonized crop and pasture types are explored by grouping annual and perennial cropping systems, C3 and C4 photosynthetic pathways, planted and natural pastures, and main crops. Our analysis examined the spatial pattern of ratios between classes of the groups and their correlation with the agricultural extent of crops and pastures within administrative units of the Amazon, by country and census/survey dates. Significant correlations were found between all ratios and the fraction of agricultural lands of each administrative unit, with the exception of planted to natural pastures ratio and pasture lands extent. Brazil and Peru in most cases have significant correlations for all ratios analyzed even for specific census and survey dates. Results suggested improvements and potential applications of the database for carbon, water, climate and land use change studies are discussed. The database presented here provides an Amazon‐wide improved data set on agricultural dynamics with expanded temporal and spatial coverage.
      PubDate: 2015-05-05T13:31:30.542301-05:
      DOI: 10.1002/2014GB004999
       
  • Fate of the Amazon River dissolved organic matter in the tropical Atlantic
           Ocean
    • Authors: Patricia M. Medeiros; Michael Seidel, Nicholas D. Ward, Edward J. Carpenter, Helga R. Gomes, Jutta Niggemann, Alex V. Krusche, Jeffrey E. Richey, Patricia L. Yager, Thorsten Dittmar
      Abstract: Constraining the fate of dissolved organic matter (DOM) delivered by rivers is key to understand the global carbon cycle, since DOM mineralization directly influences air‐sea CO2 exchange and multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet, the fate of this material in the ocean remains unclear. Here, we investigate the molecular composition and transformations of DOM in the Amazon River‐ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50‐76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially‐derived organic carbon from the continental margin, where it can be entrained in the large scale circulation and potentially contribute to the long‐term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean.
      PubDate: 2015-04-25T09:01:10.100395-05:
      DOI: 10.1002/2015GB005115
       
  • Mercury Species Concentrations and Fluxes in the Central Tropical Pacific
           Ocean
    • Authors: Kathleen M. Munson; Carl H. Lamborg, Gretchen J. Swarr, Mak A. Saito
      Abstract: The formation of the toxic and bioaccumulating monomethylmercury (MMHg) in marine systems is poorly understood, due in part to sparse data from many ocean regions. We present dissolved mercury (Hg) speciation data from 10 stations in the North and South Equatorial Pacific spanning large water mass differences and gradients in oxygen utilization. We also compare the mercury content in suspended particles from 6 stations and sinking particles from 3 stations to constrain local Hg sources and sinks. Concentrations of total Hg (THg) and methylated Hg in the surface and intermediate waters of the Equatorial and South Pacific suggest Hg cycling distinct from that of the North Pacific gyre. Maximum concentrations of 180 fM for both MMHg and dimethylmercury (DMHg) are observed in the Equatorial Pacific. South of the Equator, concentrations of MMHg and DMHg are less than 100 fM. Sinking fluxes of particulate THg can reasonably explain the shape of dissolved THg profiles, but those of MMHg are too low to account for dissolved MMHg profiles. However, methylated Hg species are lower than predicted from remineralization rates based on North Pacific data, consistent with limitation of methylation in Equatorial and South Pacific waters. Full water column depths profiles were also measured for the first time in these regions. Concentrations of THg are elevated in deep waters of the North Pacific, compared to those in the intermediate and surface waters, and taper off in the South Pacific. Comparisons with previous measurements from nearby regions suggest little enrichment of THg or MMHg over the past 20 years.
      PubDate: 2015-04-25T08:50:55.37441-05:0
      DOI: 10.1002/2015GB005120
       
  • N‐loss isotope effects in the Peru oxygen minimum zone studied using
           a mesoscale eddy as a natural tracer experiment
    • Authors: Annie Bourbonnais; Mark A. Altabet, Chawalit N. Charoenpong, Jennifer Larkum, Haibei Hu, Hermann W. Bange, Lothar Stramma
      Abstract: Mesoscale eddies in Oxygen Minimum Zones (OMZ's) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N‐loss as well as the ocean's N isotope budget. They also represent ‘natural tracer experiments’ with intensified biogeochemical signals that can be exploited to understand the large‐scale processes that control N‐loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus the heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−) and biogenic N2 associated with an anticyclonic eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2‐N concentrations were also the highest near the eddy's center (up to ~40 µmol L−1). δ15N‐N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N‐loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N‐loss was calculated to be only ~13‐14‰ (as compared to canonical values of ~20‐30‰) assuming a closed system and only slightly higher assuming an open system (16‐19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε for NO3− reduction to NO2− and NO2− reduction to N2 of ~16‐21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2‐, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for NO3− and NO2− reduction as well as N‐loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N‐loss.
      PubDate: 2015-04-25T08:41:06.583857-05:
      DOI: 10.1002/2014GB005001
       
  • A comparison of plot‐based, satellite and Earth system model
           estimates of tropical forest net primary production
    • Authors: Cory C. Cleveland; Philip Taylor, K. Dana Chadwick, Kyla Dahlin, Christopher E. Doughty, Yadvinder Malhi, W. Kolby Smith, Benjamin W. Sullivan, William R. Wieder, Alan R. Townsend
      Abstract: Net primary production (NPP) by plants represents the largest annual flux of carbon dioxide (CO2) from the atmosphere to the terrestrial biosphere, playing a critical role in the global carbon (C) cycle and the Earth's climate. Rates of NPP in tropical forests are thought to be among the highest on Earth, but debates about the magnitude, patterns and controls of NPP in the tropics highlight uncertainty in our understanding of how tropical forests may respond to environmental change. Here, we compared tropical NPP estimates generated using three common approaches: 1) field‐based methods scaled from plot‐level measurements of plant biomass; 2) radiation‐based methods that model NPP from satellite‐derived radiation absorption by plants; 3) and biogeochemical model‐based methods. For undisturbed tropical forests as a whole, the three methods produced similar NPP estimates (i.e., ~ 10 Pg C y−1). However, the three different approaches produced vastly different patterns of NPP both in space and through time, suggesting that our understanding of tropical NPP is poor, and that our ability to predict the response of NPP in the tropics to environmental change is limited. To address this shortcoming, we suggest the development of an expanded, high density, permanent network of sites where NPP is continuously evaluated using multiple approaches. Well‐designed NPP megatransects that include a high density plot network would signficantly increase the accuracy and certainty in the observed rates and patterns of tropical NPP, and improve the reliability of Earth system models used to predict NPP – carbon cycle – climate interactions into the future.
      PubDate: 2015-04-23T23:45:38.815374-05:
      DOI: 10.1002/2014GB005022
       
  • Source and sink carbon dynamics and carbon allocation in the Amazon basin
    • Authors: Christopher E. Doughty; D. B. Metcalfe, C. A. J. Girardin, F. F. Amezquita, L. Durand, W. Huaraca Huasco, J. E. Silva‐Espejo, A. Araujo‐Murakami, M. C. Costa, A. C. L. Costa, W. Rocha, P. Meir, D. Galbraith, Y. Malhi
      Abstract: Changes to the carbon cycle in tropical forests could affect global climate, but predicting such changes has been previously limited by lack of field based data. Here we show seasonal cycles of the complete carbon cycle for fourteen, one hectare intensive carbon cycling plots which we separate into three regions: humid lowland, highlands and dry lowlands. Our data highlight three trends: (1) there is differing seasonality of total NPP with the highlands and dry lowlands peaking in the dry season and the humid lowland sites peaking in the wet season; (2) seasonal reductions in wood NPP are not driven by reductions in total NPP but by carbon during the dry season being preferentially allocated towards either roots or canopy NPP; and (3) there is a temporal decoupling between total photosynthesis and total carbon usage or plant carbon expenditure (PCE). This decoupling indicates the presence of non‐structural carbohydrates (NSC) which may allow growth and carbon to be allocated when it is most ecologically beneficial rather when it is most environmentally available.
      PubDate: 2015-04-23T23:44:45.222501-05:
      DOI: 10.1002/2014GB005028
       
  • Patterns and drivers of change in organic carbon burial across a diverse
           landscape: insights from 116 Minnesota lakes
    • Authors: Robert D. Dietz; Daniel R. Engstrom, N. J. Anderson
      Abstract: Lakes may store globally significant quantities of organic carbon (OC) in their sediments, but the extent to which burial rates vary across space and time is not well described. Using 210Pb‐dated sediment cores, we explored patterns of OC burial in 116 lakes spanning several ecoregions and land‐use regimes in Minnesota, USA during the past 150–200 years. Rates for individual lakes (across all time periods) range from 3 to 204 g C m−2 yr−1 (median 33 g C m−2 yr−1) and show strong geographic separation in accordance with the degree of catchment disturbance and nutrient enrichment. Climate and basin morphometry exercise subordinate control over OC burial patterns, and diagenetic gradients introduce little bias to estimated temporal trends. Median burial rates in agricultural lakes exceed urban lakes and have increased fourfold since Euro‐American settlement. The greatest increase in OC burial occurred prior to the widespread adoption of industrial fertilizers, during an era of land clearance and farmland expansion. Northern boreal lakes, impacted by historical logging and limited cottage development yet comparatively undisturbed by human activity, bury OC at rates 3X lower than agricultural lakes and exhibit much smaller increases in OC burial. Scaling up modern OC burial estimates to the entire state, we find that Minnesota lakes annually store 0.40 Tg C in their sediments, equal to 1.5% of annual statewide CO2 emissions from fossil fuel combustion. During the period of Euro‐American settlement (ca. 1860–2000), cumulative OC burial amounted to 36 Tg C, 40% of which can be attributed to anthropogenic enhancement.
      PubDate: 2015-04-23T18:08:56.659453-05:
      DOI: 10.1002/2014GB004952
       
  • Seasonal to decadal variations of sea surface pCO2 and sea‐air CO2
           flux in the equatorial oceans over 1984‐2013: A basin‐scale
           comparison of the Pacific and Atlantic Oceans
    • Authors: Xiujun Wang; Raghu Murtugudde, Eric Hackert, Jing Wang, Jim Beauchamp
      Abstract: The equatorial Pacific and Atlantic Oceans release significant amount of CO2 each year. Not much attention has been paid to evaluating the similarities and differences between these two basins in terms of temporal variability. Here, we employ a basin scale, fully coupled physical‐biogeochemical model to study the spatial and temporal variations in sea surface pCO2 and air‐sea CO2 flux over the period of 1984‐2013 in the equatorial Pacific and Atlantic Oceans. The model reproduces the overall spatial and temporal variations in the carbon fields for both basins, including higher values to the south of the equator than to the north, the annual maximum sea surface pCO2 in boreal spring and the annual peak in sea‐to‐air CO2 flux in boreal fall in the upwelling regions. The equatorial Pacific reveals a large interannual variability in sea surface pCO2, which is associated with the El Niño‐Southern Oscillation (ENSO). As a contrast, there is a strong seasonality but little interannual variability in the carbon fields of the equatorial Atlantic. The former is driven by the variability of dissolved inorganic carbon but the latter by sea surface temperature. Our model estimates an average sea‐to‐air CO2 flux of 0.521±0.204 Pg C yr‐1 for the tropical Pacific (18ºS‐18ºN, 150ºE‐90ºW), which is in good agreement with the observation‐based estimate (0.51±0.24 Pg C yr‐1). On average, sea‐to‐air CO2 flux is 0.214±0.03 Pg C yr‐1 in the tropical Atlantic (10ºS‐10ºN), which compares favorably with observational estimates.
      PubDate: 2015-04-23T08:55:24.424942-05:
      DOI: 10.1002/2014GB005031
       
  • Multi‐century changes in ocean and land contributions to
           climate‐carbon feedbacks
    • Authors: J. T. Randerson; K. Lindsay, E. Munoz, W. Fu, J. K. Moore, F. M. Hoffman, N. M. Mahowald, S. C. Doney
      Abstract: Improved constraints on carbon cycle responses to climate change are needed to inform mitigation policy, yet our understanding of how these responses may evolve after 2100 remains highly uncertain. Using the Community Earth System Model (v1.0), we quantified climate‐carbon feedbacks from 1850 to 2300 for the Representative Concentration Pathway 8.5 and its extension. In three simulations, land and ocean biogeochemical processes experienced the same trajectory of increasing atmospheric CO2. Each simulation had a different degree of radiative coupling for CO2 and other greenhouse gases and aerosols, enabling diagnosis of feedbacks. In a fully coupled simulation, global mean surface air temperature increased by 9.3 K from 1850 to 2300, with 4.4 K of this warming occurring after 2100. Excluding CO2, warming from other greenhouse gases and aerosols was 1.6 K by 2300, near a 2 K target needed to avoid dangerous anthropogenic interference with the climate system. Ocean contributions to the climate‐carbon feedback increased considerably over time, and exceeded contributions from land after 2100. The sensitivity of ocean carbon to climate change was found to be proportional to changes in ocean heat content, as a consequence of this heat modifying transport pathways for anthropogenic CO2 inflow and solubility of dissolved inorganic carbon. By 2300 climate change reduced cumulative ocean uptake by 330 Pg C, from 1410 Pg C to 1080 Pg C. Land fluxes similarly diverged over time, with climate change reducing stocks by 232 Pg C. Regional influence of climate change on carbon stocks was largest in the North Atlantic Ocean and tropical forests of South America. Our analysis suggests that after 2100, oceans may become as important as terrestrial ecosystems in regulating the magnitude of climate‐carbon feedbacks.
      PubDate: 2015-04-21T17:01:21.272412-05:
      DOI: 10.1002/2014GB005079
       
  • Microbial nitrogen dynamics in organic and mineral soil horizons along a
           latitudinal transect in Western Siberia
    • Authors: Birgit Wild; Jörg Schnecker, Anna Knoltsch, Mounir Takriti, Maria Mooshammer, Norman Gentsch, Robert Mikutta, Ricardo J. Eloy Alves, Antje Gittel, Nikolay Lashchinskiy, Andreas Richter
      Abstract: Soil N availability is constrained by the breakdown of N‐containing polymers such as proteins to oligo‐peptides and amino acids that can be taken up by plants and microorganisms. Excess N is released from microbial cells as ammonium (N mineralization), which in turn can serve as substrate for nitrification. According to stoichiometric theory, N mineralization and nitrification are expected to increase in relation to protein depolymerization with decreasing N limitation, and thus from higher to lower latitudes and from topsoils to subsoils. To test these hypotheses, we compared gross rates of protein depolymerization, N mineralization and nitrification (determined using 15N pool dilution assays) in organic topsoil, mineral topsoil and mineral subsoil of seven ecosystems along a latitudinal transect in Western Siberia, from tundra (67°N) to steppe (54°N). The investigated ecosystems differed strongly in N transformation rates, with highest protein depolymerization and N mineralization rates in middle and southern taiga. All N transformation rates decreased with soil depth following the decrease in organic matter content. Related to protein depolymerization, N mineralization and nitrification were significantly higher in mineral than in organic horizons, supporting a decrease in microbial N limitation with depth. In contrast, we did not find indications for a decrease in microbial N limitation from arctic to temperate ecosystems along the transect. Our findings thus challenge the perception of ubiquitous N limitation at high latitudes, but suggest a transition from N to C limitation of microorganisms with soil depth, even in high latitude systems such as tundra and boreal forest.
      PubDate: 2015-04-09T21:51:19.381845-05:
      DOI: 10.1002/2015GB005084
       
  • The timescale of the silicate weathering negative feedback on atmospheric
           CO2
    • Authors: G. Colbourn; A. Ridgwell, T. M. Lenton
      Abstract: The ultimate fate of CO2 added to the ocean–atmosphere system is chemical reaction with silicate minerals and burial as marine carbonates. The timescale of this silicate weathering negative feedback on atmospheric pCO2 will determine the duration of perturbations to the carbon cycle, be they geological release events or the current anthropogenic perturbation. However, there has been little previous work on quantifying the time‐scale of the silicate weathering feedback, with the primary estimate of 300–400 kyr being traceable to an early box model study by Sundquist [1991]. Here we employ a representation of terrestrial rock weathering in conjunction with the ‘GENIE’ Earth System Model to elucidate the different timescales of atmospheric CO2 regulation whilst including the main climate feedbacks on CO2 uptake by the ocean. In this coupled model, the main dependencies of weathering – runoff, temperature and biological productivity – were driven from an energy‐moisture balance atmosphere model and parameterized plant productivity. Long‐term projections (1 Myr) were conducted for idealized scenarios of 1000 and 5000 PgC fossil fuel emissions and their sensitivity to different model parameters was tested. By fitting model output to a series of exponentials we determined the e‐folding timescale for atmospheric CO2 draw‐down by silicate weathering to be ~240 kyr (range 170–380 kyr), significantly less than existing quantifications. Although the time‐scales for re‐equilibration of global surface temperature and surface ocean pH are similar to that for CO2, a much greater proportion of the peak temperature anomaly persists on this longest time‐scale; ~21% compared to ~10% for CO2.
      PubDate: 2015-04-07T20:58:49.986963-05:
      DOI: 10.1002/2014GB005054
       
  • Spatial patterns in CO2 evasion from the global river network
    • Authors: Ronny Lauerwald; Goulven G. Laruelle, Jens Hartmann, Philippe Ciais, Pierre A.G. Regnier
      Abstract: CO2 evasion from rivers (FCO2) is an important component of the global carbon budget. Here, we present the first global maps of CO2 partial pressures (pCO2) in rivers of stream order 3 and higher and the resulting FCO2 at 0.5° resolution constructed with a statistical model. Our statistical model based upon a GIS based approach is used to derive a pCO2 prediction function trained on data from 1182 sampling locations. While data from Asia and Africa are scarce and the training data set is dominated by sampling locations from the Americas, Europe, and Australia, the sampling locations cover the full spectrum from high to low latitudes. The predictors of pCO2 are net primary production, population density and slope gradient within the river catchment as well as mean air temperature at the sampling location (r2=0.47). The predicted pCO2 map was then combined with spatially explicit estimates of stream surface area Ariver and gas exchange velocity k calculated from published empirical equations and data sets to derive the FCO2 map. Using Monte Carlo simulations, we assessed the uncertainties of our estimates. At the global scale, we estimate an average river pCO2 of 2400 (2019–2826) µatm and a FCO2 of 650 (483–846) Tg C yr−1 (5th and 95th percentile of confidence interval). Our global CO2 evasion is substantially lower than the recent estimate of 1800 Tg C yr−1 [Raymond et al., 2013] although the training set of pCO2 is very similar in both studies, mainly due to lower tropical pCO2 estimates in the present study. Our maps reveal strong latitudinal gradients in pCO2, Ariver and FCO2. The zone between 10°N and 10°S contributes about half of the global CO2 evasion. Collection of pCO2 data in this zone, in particular for African and South East Asian rivers is a high priority to reduce uncertainty on FCO2.
      PubDate: 2015-04-07T06:21:35.630659-05:
      DOI: 10.1002/2014GB004941
       
  • A revised global estimate of dissolved iron fluxes from marine sediments
    • Authors: A. W. Dale; L. Nickelsen, F. Scholz, C. Hensen, A. Oschlies, K. Wallmann
      Abstract: Literature data on benthic dissolved iron (DFe) fluxes (µmol m−2 d−1), bottom water oxygen concentrations (O2BW, μM) and sedimentary carbon oxidation rates (COX, mmol m−2 d−1) from water depths ranging from 80 to 3700 m were assembled. The data were analyzed with a diagenetic iron model to derive an empirical function for predicting benthic DFe fluxes: DFeflux=γ⋅TANHCOXO2BW where γ (=170 µmol m−2 d−1) is the maximum flux for sediments at steady state located away from river mouths. This simple function unifies previous observations that COX and O2BW are important controls on DFe fluxes. Upscaling predicts a global DFe flux from continental margin sediments of 109 ± 55 Gmol yr−1, of which 72 Gmol yr−1 is contributed by the shelf (2000 m) of 41 ± 21 Gmol yr−1 is unsupported by empirical data. Previous estimates of benthic DFe fluxes derived using global iron models are far lower (ca. 20–30 Gmol yr−1). This can be attributed to (i) inadequate treatment of the role of oxygen on benthic DFe fluxes, and (ii) improper consideration of continental shelf processes due to coarse spatial resolution. Globally‐averaged DFe concentrations in surface waters simulated with an intermediate‐complexity Earth system climate model (UVic ESCM) were a factor of two higher with the new function. We conclude that (i) the DFe flux from marginal sediments has been underestimated in the marine iron cycle, and (ii) iron scavenging in the water column is more intense than currently presumed.
      PubDate: 2015-04-07T06:20:08.893961-05:
      DOI: 10.1002/2014GB005017
       
  • Atmospheric observations inform CO2 flux responses to enviroclimatic
           drivers
    • Authors: Yuanyuan Fang; Anna M. Michalak
      Abstract: Understanding the response of the terrestrial biospheric carbon cycle to variability in enviroclimatic drivers is critical for predicting climate‐carbon interactions. Here we apply an atmospheric‐inversion‐based framework to assess the relationships between the spatiotemporal patterns of net ecosystem exchange (NEE) and those of enviroclimatic drivers. We show that those relationships can be directly observed at 1°×1° 3‐hourly resolution from atmospheric CO2 measurements for four of seven large biomes in North America, namely (i) boreal forests and taiga, (ii) temperate coniferous forests, (iii) temperate grasslands, savannas, shrublands, and (iv) temperate broadleaf and mixed forests. We find that shortwave radiation plays a dominant role during the growing season over all four biomes. Specific humidity and precipitation also play key roles and are associated with decreased uptake (or increased sources). The explanatory power of specific humidity is especially strong during transition seasons, while that of precipitation appears during both the growing and dormant seasons. We further find that the ability of four prototypical Terrestrial Biospheric Models (TBMs) to represent the spatiotemporal variability of NEE improves as the influence of radiation becomes more dominant, implying that TBMs have better skill in representing the impact of radiation relative to other drivers. Even so, we show that TBMs underestimate the strength of the relationship to radiation and do not fully capture its seasonality. Furthermore, the TBMs appear to misrepresent the relationship to precipitation and specific humidity at the examined scales, with relationships that are not consistent in terms of sign, seasonality, or significance relative to observations. More broadly, we demonstrate the feasibility of directly probing relationships between NEE and enviroclimatic drivers at scales with no direct measurements of NEE, opening the door to the study of emergent processes across scales and to the evaluation of their scaling within TBMs.
      PubDate: 2015-04-07T06:19:33.867268-05:
      DOI: 10.1002/2014GB005034
       
  • Modeling the fate of methane hydrates under global warming
    • Authors: Kerstin Kretschmer; Arne Biastoch, Lars Rupke, Ewa Burwicz
      Abstract: Large amounts of methane hydrate locked up within marine sediments are vulnerable to climate change. Changes in bottom water temperatures may lead to their destabilization and the release of methane into the water column or even the atmosphere. In a multi‐model approach, the possible impact of destabilizing methane hydrates onto global climate within the next century is evaluated. The focus is set on changing bottom water temperatures to infer the response of the global methane hydrate inventory to future climate change. Present and future bottom water temperatures are evaluated by the combined use of hindcast high‐resolution ocean circulation simulations and climate modeling for the next century. The changing global hydrate inventory is computed using the parameterized transfer function recently proposed by Wallmann et al. (2012). We find that the present‐day world's total marine methane hydrate inventory is estimated to be 1146Gt of methane carbon. Within the next 100 years this global inventory may be reduced by ~ 0.03% (releasing ~ 473Mt methane from the seafloor). Compared to the present‐day annual emissions of anthropogenic methane, the amount of methane released from melting hydrates by 2100 is small and will not have a major impact on the global climate. On a regional scale, ocean bottom warming over the next 100 years will result in a relatively large decrease in the methane hydrate deposits, with the Arctic and Blake Ridge region, offshore South Carolina, being most affected.
      PubDate: 2015-03-30T01:37:32.035211-05:
      DOI: 10.1002/2014GB005011
       
  • The effects of secular calcium and magnesium concentration changes on the
           thermodynamics of seawater acid/base chemistry: Implications for Eocene
           and Cretaceous ocean carbon chemistry and buffering
    • Authors: Mathis P. Hain; Daniel M. Sigman, John A. Higgins, Gerald H. Haug
      Abstract: Reconstructed changes in seawater calcium and magnesium concentration ([Ca2+], [Mg2+]) predictably affect the ocean's acid/base and carbon chemistry. Yet, inaccurate formulations of chemical equilibrium “constants” are currently in use to account for these changes. Here we develop an efficient implementation of the MIAMI Ionic Interaction Model (Millero and Pierrot, 1998) to predict all chemical equilibrium constants required for carbon chemistry calculations under variable [Ca2+] and [Mg2+]. We investigate the impact of [Ca2+] and [Mg2+] on the relationships among the ocean's pH, CO2, dissolved inorganic carbon (DIC), saturation state of CaCO3 (Ω) and buffer capacity. Increasing [Ca2+] and/or [Mg2+] enhances “ion pairing,” which increases seawater buffering by increasing the concentration ratio of total to “free” (uncomplexed) carbonate ion. An increase in [Ca2+], however, also causes a decline in carbonate ion to maintain a given Ω, thereby overwhelming the ion pairing effect and decreasing seawater buffering. Given the reconstructions of Eocene [Ca2+] and [Mg2+] ([Ca2+]~20mM; [Mg2+]~30mM), Eocene seawater would have required essentially the same DIC as today to simultaneously explain a similar‐to‐modern Ω and the estimated Eocene atmospheric CO2 of ~1000ppm. During the Cretaceous, at ~4x modern [Ca2+], ocean buffering would have been at a minimum. Overall, during times of high seawater [Ca2+], CaCO3 saturation, pH and atmospheric CO2 were more susceptible to perturbations of the global carbon cycle. For example, given both Eocene and Cretaceous seawater [Ca2+] and [Mg2+], a doubling of atmospheric CO2 would require less carbon addition to the ocean/atmosphere system than under modern seawater composition. Moreover, declining seawater buffering since the Cretaceous may have been a driver of evolution by raising energetic demands of biologically controlled calcification and CO2 concentration mechanisms that aid photosynthesis.
      PubDate: 2015-03-24T00:55:28.090141-05:
      DOI: 10.1002/2014GB004986
       
  • A meta‐analysis of oceanic DMS and DMSP cycling processes:
           Disentangling the summer paradox
    • Authors: Martí Galí; Rafel Simó
      Abstract: The biogenic volatile compound dimethylsulfide (DMS) is produced in the ocean mainly from the ubiquitous phytoplankton osmolyte dimethylsulfoniopropionate (DMSP). In the upper mixed layer, DMS concentration and the daily averaged solar irradiance are roughly proportional across latitudes and seasons. This translates into a seasonal mismatch between DMS and phytoplankton biomass at low latitudes, termed the “DMS summer paradox”, which remains difficult to reproduce with biogeochemical models. Here we report on a global meta‐analysis of DMSP and DMS cycling processes and their relationship to environmental factors. We show that DMS seasonality reflects progressive changes in a short‐term dynamic equilibrium, set by the quotient between gross DMS production rates and the sum of biotic and abiotic DMS consumption rate constants. Gross DMS production is the principal driver of DMS seasonality, due to the synergistic increases towards summer in two of its underlying factors: phytoplankton DMSP content (linked to species succession) and short‐term community DMSP‐to‐DMS conversion yields (linked to physiological stress). We also show that particulate DMSP transformations (linked to grazing‐induced phytoplankton mortality) generally contribute a larger share of gross DMS production than dissolved‐phase DMSP metabolism. The summer paradox is amplified by a decrease in microbial DMS consumption rate constants towards summer. However, this effect is partially compensated by a concomitant increase in abiotic DMS loss rate constants. Besides seasonality, we identify consistent covariation between key sulfur cycling variables and trophic status. These findings should improve the modeling projections of the main natural source of climatically active atmospheric sulfur.
      PubDate: 2015-03-12T17:22:41.039729-05:
      DOI: 10.1002/2014GB004940
       
  • Geographic variability in organic carbon stock and accumulation rate in
           sediments of East and Southeast Asian seagrass meadows
    • Authors: Toshihiro Miyajima; Masakazu Hori, Masami Hamaguchi, Hiromori Shimabukuro, Hiroshi Adachi, Hiroya Yamano, Masahiro Nakaoka
      First page: 397
      Abstract: Organic carbon (OC) stored in the sediments of seagrass meadows has been considered a globally significant OC reservoir. However, the sparsity and regional bias of studies on long‐term OC accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global OC sink. We evaluated the amount and accumulation rate of OC in sediment of seagrass meadows and adjacent areas in East and Southeast Asia. In temperate sites, the average OC concentration in the top 30 cm of sediment was higher in seagrass meadows (780–1080 mmol g−1) than in sediments without seagrass cover (52–430 mmol g−1). The average OC in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140–440 mmol g−1. Carbon isotope mass balancing suggested that the contribution of seagrass‐derived carbon to OC stored in sediments was often relatively minor (temperate: 10–40%; subtropical: 35–82%; tropical: 4–34%) and correlated to the habitat type, being particularly low in estuarine habitats. Stock of OC in the top meter of sediment of all the studied meadows ranged from 38–120 Mg ha−1. The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32–1.34 mm yr−1). The long‐term OC accumulation rates calculated from the sediment accumulation rate and the top 30 cm average OC concentration for the seagrass meadows (24–101 kg ha−1 yr−1) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha−1 yr−1 on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.
      PubDate: 2015-03-08T18:11:13.059669-05:
      DOI: 10.1002/2014GB004979
       
  • Observing multi‐decadal trends in Southern Ocean CO2 uptake: What
           can we learn from an ocean model'
    • Authors: Nicole S. Lovenduski; Amanda R. Fay, Galen A. McKinley
      First page: 416
      Abstract: We use output from a hindcast simulation (1958–2007) of an ocean biogeochemical and ecological model to inform an observational strategy for detection of a weakening Southern Ocean CO2 sink from surface ocean pCO2 data. Particular emphasis isplaced on resolving disparate conclusions about the Southern Ocean CO2 sink that have been drawn from surface ocean pCO2 observation studies in the past. We find that long‐term trends in ΔpCO2 (pCO2oc ‐ pCO2atm) can be used as a proxy for changes in the strength of the CO2 sink, but must be interpreted with caution, as they are calculated from small differences in the oceanic and atmospheric pCO2 trends. Large interannual, decadal, and multi‐decadal variability in ΔpCO2 persists throughout the simulation, suggesting that one must consider a range of start and end years for trend analysis before drawing conclusions about changes in the CO2 sink. Winter‐mean CO2 flux trends are statistically indistinguishable from annual‐mean trends, arguing for inclusion of all available pCO2oc data in future analyses of the CO2 sink. The weakening of the CO2 sink emerges during the observed period of our simulation (1981–2007) in the subpolar seasonally stratified biome (4∘C < average climatological temperature < 9∘C); the weakening is most evident during periods with positive trends in the Southern Annular Mode. With perfect temporal and spatial coverage, 13 years of pCO2oc data would be required to detect a weakening CO2 sink in this biome. Given available data, it is not yet possible to detect a weakening of the Southern Ocean CO2 sink with much certainty, due to imperfect data coverage and high variability in Southern Ocean surface pCO2.
      PubDate: 2015-03-11T16:44:53.674191-05:
      DOI: 10.1002/2014GB004933
       
  • Isotopic evidence for nitrification in the Antarctic winter mixed layer
    • Authors: Sandi M. Smart; Sarah E. Fawcett, Sandy J. Thomalla, Mira A. Weigand, Chris J. C. Reason, Daniel M. Sigman
      First page: 427
      Abstract: We report wintertime nitrogen and oxygen isotope ratios (δ15N and δ18O) of seawater nitrate in the Southern Ocean south of Africa. Depth profile and underway surface samples collected in July 2012 extend from the subtropics to just beyond the Antarctic winter sea‐ice edge. We focus here on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile δ15N data yields estimates for the isotope effect (the degree of isotope discrimination) of nitrate assimilation (1.6‐3.3‰) that are significantly lower than commonly observed in the summertime Antarctic (5‐8‰). δ18O data from the same depth profiles and lateral δ15N variations within the mixed layer, however, imply O and N isotope effects that are more similar to those suggested by summertime data. These findings point to active nitrification (i.e., regeneration of organic matter to nitrate) within the Antarctic winter mixed layer. Nitrite removal from samples reveals a low δ15N for nitrite in the winter mixed layer (‐40‰ to ‐20‰), consistent with nitrification, but does not remove the observation of an anomalously low δ15N for nitrate. The winter data, and the nitrification they reveal, explain the previous observation of an anomalously low δ15N for nitrate in the temperature minimum layer (remnant winter mixed layer) of summertime depth profiles. At the same time, the wintertime data require a low δ15N for the combined organic N and ammonium in the autumn mixed layer that is available for wintertime nitrification, pointing to intense N recycling as a pervasive condition of the Antarctic in late summer.
      PubDate: 2015-04-22T01:43:17.412055-05:
      DOI: 10.1002/2014GB005013
       
  • The relation of mixed‐layer net community production to
           phytoplankton community composition in the Southern Ocean
    • Authors: Nicolas Cassar; Simon W. Wright, Paul G. Thomson, Thomas W. Trull, Karen J. Westwood, Miguel Salas, Andrew Davidson, Imojen Pearce, Diana M. Davies, Richard J. Matear
      First page: 446
      Abstract: Surface ocean productivity mediates the transfer of carbon to the deep ocean and in the process regulates atmospheric CO2 levels. A common axiom in oceanography is that large phytoplankton contribute disproportionally to the transfer of carbon to the deep ocean because of their greater ability to escape grazing pressure,build biomass and sink. In the present study, we assessed the relationship of net community production to phytoplankton assemblages and plankton size distribution in the Subantarctic Zone (SAZ) and northern reaches of the Polar Frontal Zone (PFZ) in the Australian sector of the Southern Ocean. We reanalyzed and synthesized previously published estimates of O2/Ar‐net community oxygen production (NCP) and triple‐O2 isotopes‐gross primary oxygen production (GPP) along with microscopic and pigment analyses of the microbial community. Overall, we found the axiom that large phytoplankton drive carbon export was not supported in this region. Mixed‐layer depth‐integrated NCP was correlated to particulate organic carbon (POC) concentration in the mixed layer. While lower NCP/GPP and NCP/POC values were generally associated with communities dominated by smaller plankton size (as would be expected), these communities did not preclude high values for both properties. Vigorous NCP in some regions occurred in the virtual absence of large phytoplankton (and specifically diatoms) and in communities dominated by nanoplankton and picoplankton. We also observed a positive correlation between NCP and the proportion of the phytoplankton community grazed by microheterotrophs, supporting the mediating role of grazers in carbon export. The novel combination of techniques allowed us to determine how NCP relates to upper ocean ecosystem characteristics and may lead to improved models of carbon export.
      PubDate: 2015-03-08T18:11:11.548602-05:
      DOI: 10.1002/2014GB004936
       
  • Detecting the progression of ocean acidification from the saturation state
           of CaCO3 in the subtropical South Pacific
    • Authors: Akihiko Murata; Kazuhiko Hayashi, Yuichiro Kumamoto, Ken‐ichi Sasaki
      First page: 463
      Abstract: Progression of ocean acidification in the subtropical South Pacific was investigated by using high‐quality data from trans‐Pacific zonal section at 17°S (World Ocean Circulation Experiment section P21) collected in 1994 and 2009. During this 15‐year period, the CaCO3 saturation state of seawater with respect to calcite (Ωcal) and aragonite (Ωarg) in the upper water column (
      PubDate: 2015-04-24T02:04:18.918809-05:
      DOI: 10.1002/2014GB004908
       
  • Predicting long‐term carbon sequestration in response to CO2
           enrichment: How and why do current ecosystem models differ'
    • Authors: Anthony P. Walker; Sönke Zaehle, Belinda E. Medlyn, Martin G. De Kauwe, Shinichi Asao, Thomas Hickler, William Parton, Daniel M. Ricciuto, Ying‐Ping Wang, David Wårlind, Richard J. Norby
      First page: 476
      Abstract: Large uncertainty exists in model projections of the land carbon (C) sink response to increasing atmospheric CO2. Free‐Air CO2 Enrichment (FACE) experiments lasting a decade or more have investigated ecosystem responses to a step change in atmospheric CO2 concentration. To interpret FACE results in the context of gradual increases in atmospheric CO2 over decades to centuries, we used a suite of seven models to simulate the Duke and Oak Ridge FACE experiments extended for 300 years of CO2 enrichment. We determine key modelling assumptions that drive divergent projections of terrestrial C uptake and evaluate whether these assumptions can be constrained by experimental evidence. All models simulated increased terrestrial C pools resulting from CO2 enrichment, though there was substantial variability in quasi‐equilibrium C sequestration and rates of change. In two of two models that assume that plant nitrogen (N) uptake is solely a function of soil N supply, the NPP response to elevated CO2 became progressively N limited. In four of five models that assume that N uptake is a function of both soil N supply and plant N demand elevated CO2 led to reduced ecosystem N losses and thus progressively relaxed nitrogen limitation. Many allocation assumptions resulted in increased wood allocation relative to leaves and roots which reduced the vegetation turnover rate and increased C sequestration. In addition, self‐thinning assumptions had a substantial impact on C sequestration in two models. Accurate representation of N process dynamics (in particular N uptake), allocation, and forest self‐thinning are key to minimising uncertainty in projections of future C sequestration in response to elevated atmospheric CO2.
      PubDate: 2015-04-27T03:33:13.108617-05:
      DOI: 10.1002/2014GB004995
       
 
 
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