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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 26, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 54, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 6, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 7)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 25)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 15)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
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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: 102, SJR: 2.189, h-index: 121)
Journal Cover   Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [6 followers]  Follow
   Full-text available via subscription Subscription journal
   ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
   Published by American Geophysical Union (AGU) Homepage  [17 journals]
  • An impulse response function for the ‘long tail’ of excess
           atmospheric CO2 in an Earth system model
    • Authors: N. S. Lord; A. Ridgwell, M. C. Thorne, D. J. Lunt
      Abstract: The ultimate fate of (fossil fuel) CO2 emitted to the atmosphere is governed by a range of sedimentological and geological processes operating on timescales of up to the ca. hundred thousand year response of the silicate weathering feedback. However, how the various geological CO2 sinks might saturate and feedbacks weaken in response to increasing total emissions is poorly known. Here, we explore the relative importance and timescales of these processes using a 3D ocean‐based Earth system model. We first generate an ensemble of 1 Myr duration CO2 decay curves spanning cumulative emissions of up to 20,000 PgC. To aid characterization and understanding of the model response to increasing emission size, we then generate an impulse response function description for the long‐term fate of CO2 in the model. Our analysis is consistent with a progressively increasing fraction of total emissions that are removed from the atmosphere as emissions increase, via carbonate weathering and burial, due to the ocean carbon sink becoming saturated, together with a lengthening of the timescale of removal. However, we find that in our model the ultimate CO2 sink – silicate weathering feedback – is approximately invariant with respect to cumulative emissions, both in terms of its importance (it removes the remaining excess ~7% of total emissions from the atmosphere) and timescale (~270 kyr). Because a simple pulse‐response description leads to initially large predictive errors for a realistic time‐varying carbon release, we also develop a convolution‐based description of atmospheric CO2 decay which can be used as a simple and efficient means of making long‐term carbon cycle perturbation projections.
      PubDate: 2015-11-18T05:40:17.504919-05:
      DOI: 10.1002/2014GB005074
  • Nitrogen cycling in the secondary nitrite maximum of the Eastern Tropical
           North Pacific off Costa Rica
    • Authors: Carolyn Buchwald; Alyson E. Santoro, Rachel H. R. Stanley, Karen L. Casciotti
      Abstract: Nitrite is a central intermediate in the marine nitrogen cycle and represents a critical juncture where nitrogen can be reduced to the less bioavailable N2 gas or oxidized to nitrate and retained in a more bioavailable form. We present an analysis of rates of microbial nitrogen transformations in the oxygen deficient zone (ODZ) within the eastern tropical north Pacific ocean (ETNP). We determined rates using a novel one‐dimensional model using the distribution of nitrite and nitrate concentrations, along with their natural abundance nitrogen (N) and oxygen (O) isotope profiles. We predict rate profiles for nitrate reduction, nitrite reduction, and nitrite oxidation throughout the ODZ, as well as the contributions of anammox to nitrite reduction and nitrite oxidation. Nitrate reduction occurs at a maximum rate of 25 nM d−1 at the top of the ODZ, at the same depth as the maximum rate of nitrite reduction, 15 nM d−1. Nitrite oxidation occurs at maximum rates of 10 nM d−1 above the secondary nitrite maximum (SNM), but also in the secondary nitrite maximum, within the ODZ. Anammox contributes to nitrite oxidation within the ODZ, but cannot account for all of it. Nitrite oxidation within the ODZ that is not through anammox is also supported by microbial gene abundance profiles. Our results suggest the presence of nitrite oxidation within the ETNP ODZ, with implications for the distribution and physiology of marine nitrite‐oxidizing bacteria, and for total nitrogen loss in the largest marine ODZ.
      PubDate: 2015-11-17T02:17:04.312454-05:
      DOI: 10.1002/2015GB005187
  • Cadmium regeneration within the North Atlantic
    • Authors: Saeed Roshan; Jingfeng Wu
      Abstract: Cadmium (Cd) incorporated into the benthic microfossils has been widely used in reconstructing the past water circulation in the North Atlantic. This requires a major control by conservative mixing over regeneration process on the seawater cadmium distribution in the North Atlantic. Through coupling the recently‐reported Cd data at depths below 300 m with quantitative water mass analysis along the GA03 transect, we tested two models for cadmium cycling within the North Atlantic: conservative mixing alone and conservative mixing plus regeneration which is termed as regenerative mixing. The results show that the regenerative mixing model reproduces the observations (Slope=0.99 and R2=0.97) much better than the conservative mixing model (Slope=0.99 and R2=0.88). The regenerative mixing model was applied to estimate the amount of dissolved cadmium regenerated “within” the North Atlantic. This regionally‐regenerated cadmium contributes to ~10% and >50% of the total cadmium in the North Atlantic at depths >1000 m and 300–1000 m, respectively, indicating that the microfossil‐reconstructed seawater Cd can be attributed to the mixing from the South Atlantic to the North Atlantic with high accuracy for depths below 1000 m, whilst within 300–1000 m cadmium is more controlled by the regeneration process than the mixing process, complicating the attribution of the microfossil‐reconstructed seawater Cd to the changes in water mass geometry within this 300–1000 m depth range. A regionally and vertically constant Cd/PO43− regeneration ratio ~ 262 pM/µmol/kg was derived from the regenerative mixing model which is comparable with ratios estimated in the recent studies for the North Atlantic.
      PubDate: 2015-11-11T14:36:51.303273-05:
      DOI: 10.1002/2015GB005215
  • Issue Information
    • Abstract: No abstract is available for this article.
      PubDate: 2015-11-11T01:13:13.80031-05:0
      DOI: 10.1002/gbc.20201
  • Marine Biological Production from In Situ Oxygen Measurements on a
           Profiling Float in the Subarctic Pacific Ocean
    • Authors: Seth M. Bushinsky; Steven Emerson
      Abstract: Evaluating the organic carbon flux from the surface ocean to the interior (the marine biological pump) is essential for predictions of ocean carbon cycle feedback to climate change. One approach for determining these fluxes is to measure the concentration of oxygen in the upper ocean over a seasonal cycle, calculate the net O2 flux using an upper ocean model, and then use a stoichiometric relationship between oxygen evolved and organic carbon produced. Applying this tracer in a variety of ocean areas over seasonal cycles requires accurate O2 measurements on autonomous vehicles. Here we demonstrate this approach using an O2 sensor on a profiling float that is periodically calibrated against atmospheric pO2. Using accurate data and a model that includes all physical and biological processes influencing oxygen, we determine an annual net community production (ANCP) of 0.7 ± 0.5 mol C m−2 yr−1 in the Northeast Pacific Ocean (50°N, 145°W) from June 2012 to June 2013. There is a strong seasonal cycle in net biological oxygen production with wintertime fluxes caused by bubble processes critical to determining the annual flux. Approximately 50% of net autotrophic production during summer months is consumed by net respiration during the winter. The result is a biological pump in the subarctic Pacific Ocean that is less than that determined by similar methods in the subtropics to the south. This estimate is significantly lower than that predicted by satellite remote sensing and global circulation models.
      PubDate: 2015-11-10T13:41:13.929491-05:
      DOI: 10.1002/2015GB005251
  • Mechanisms and predictability of multi‐year ecosystem variability in
           the North Pacific
    • Authors: M. O. Chikamoto; A. Timmermann, Y. Chikamoto, H. Tokinaga, N. Harada
      Abstract: Aleutian Low variations provide vorticity, buoyancy and heat‐flux forcing to the North Pacific Ocean, which in turn cause changes in ocean circulation, mixed layer characteristics and sea‐ice coverage. In this process the white noise atmospheric characteristics are integrated dynamically and thermodynamically to generate red noise ocean spectra. Using the Community Earth System Model (version 1.0.3) we study the resulting biogeochemical and ecosystem responses in the North Pacific. We find that ocean dynamical variables have an impact on the tendencies of key nutrients and biological production, which leads to a further reddening of biogeochemical spectra resulting in potential predictability on timescales of 2‐4 years. However, this low‐pass filtering does not apply to all biogeochemical variables and is regionally dependent. It is shown that phytoplankton biomass in the Central North Pacific adjusts to the much shorter‐term variability associated with changes in mixed layer depth, light availability and zooplankton grazing, thus limiting the predictability of phytoplankton anomalies to about 1 year. In the eastern North Pacific the slow advection of anomalous nutrient concentrations leads to longer persistence of phytoplankton variability and increased potential predictability of up to 3 years.
      PubDate: 2015-11-07T04:30:58.861851-05:
      DOI: 10.1002/2015GB005096
  • Ammonia and nitrite oxidation in the Eastern Tropical North Pacific
    • Abstract: Nitrification plays a key role in the marine nitrogen (N) cycle, including in oceanic oxygen minimum zones (OMZs), which are hot spots for denitrification and anaerobic ammonia oxidation (anammox). Recent evidence suggests that nitrification links the source (remineralized organic matter) and sink (denitrification and anammox) of fixed N directly in the steep oxycline in the OMZs. We performed shipboard incubations with 15N tracers to characterize the depth distribution of nitrification in the Eastern Tropical North Pacific (ETNP). Additional experiments were conducted to investigate photoinhibition. Allylthiourea (ATU) was used to distinguish the contribution of archaeal and bacterial ammonia oxidation. The abundance of archaeal and β‐proteobacterial ammonia monooxygenase gene subunit A (amoA) was determined by qPCR. The rates of ammonia and nitrite oxidation showed distinct subsurface maxima, with the latter slightly deeper than the former. The ammonia oxidation maximum coincided with the primary nitrite concentration maximum, archaeal amoA gene maximum, and the subsurface nitrous oxide maximum. Negligible rates of ammonia oxidation were found at anoxic depths, where high rates of nitrite oxidation were measured. Archaeal amoA gene abundance was generally one to two orders of magnitude higher than bacterial amoA gene abundance, and inhibition of ammonia‐oxidizing bacteria with 10 μM ATU did not affect ammonia oxidation rates, indicating the dominance of archaea in ammonia oxidation. These results depict highly dynamic activities of ammonia and nitrite oxidation in the oxycline of the ETNP OMZ.
      PubDate: 2015-11-07T04:14:14.168913-05:
      DOI: 10.1002/2015GB005278
  • Changing Amazon biomass and the role of atmospheric CO2 concentration,
           climate and land use
    • Authors: Andrea D. Almeida Castanho; David Galbraith, Ke Zhang, Michael T. Coe, Marcos H. Costa, Paul Moorcroft
      Abstract: The Amazonian tropical evergreen forest is an important component of the global carbon budget. Its forest floristic composition, structure and function are sensitive to changes in climate, atmospheric composition and land use. In this study biomass and productivity simulated by three DGVMs (IBIS, ED2 and JULES) for the period 1970–2008 are compared with observations from forest plots (RAINFOR). The spatial variability in biomass and productivity simulated by the DGVMs is low in comparison to the field observations in part because of poor representation of the heterogeneity of vegetation traits within the models. We find that over the last four decades the CO2 fertilization effect dominates a long‐term increase in simulated biomass in undisturbed Amazonian forests, while land use change dominates a reduction in AGB, of similar magnitude to the CO2 biomass gain, in the south and southeastern Amazonia. Climate extremes exert a strong effect on the biomass on short time scales, but the models are incapable of reproducing the observed impacts of extreme drought on forest biomass. We find that future improvements in the accuracy of DGVM predictions will require improved representation of four key elements: 1) spatially variable plant traits; 2) soil and nutrients mediated processes; 3) extreme event mortality; 4) sensitivity to climatic variability. Finally, continued long‐term observations and ecosystem‐scale experiments (e.g. FACE experiments) are essential for a better understanding of the changing dynamics of tropical forests.
      PubDate: 2015-11-06T06:40:57.29023-05:0
      DOI: 10.1002/2015GB005135
  • Impact of oceanic circulation changes on atmospheric δ13CO2
    • Authors: L. Menviel; A. Mouchet, K. J. Meissner, F. Joos, M. H. Engl
      Abstract: δ13CO2 measured in Antarctic ice cores provides constraints on oceanic and terrestrial carbon cycle processes linked with millennial‐scale changes in atmospheric CO2. However, the interpretation of δ13CO2 is not straightforward. Using carbon isotope‐enabled versions of the LOVECLIM and Bern3D models, we perform a set of sensitivity experiments in which the formation rates of North Atlantic Deep Water (NADW), North Pacific Deep Water (NPDW), Antarctic Bottom Water (AABW) and Antarctic Intermediate Water (AAIW) are varied. We study the impact of these circulation changes on atmospheric δ13CO2 as well as on the oceanic δ13C distribution. In general, we find that the formation rates of AABW, NADW, NPDW and AAIW are negatively correlated with changes in δ13CO2: namely strong oceanic ventilation decreases atmospheric δ13CO2. However, since large scale ocean circulation reorganizations also impact nutrient utilization and the Earth's climate, the relationship between atmospheric δ13CO2 levels and ocean ventilation rate is not unequivocal. In both models atmospheric δ13CO2 is very sensitive to changes in AABW formation rates: increased AABW formation enhances the transport of low δ13C waters to the surface and decreases atmospheric δ13CO2. By contrast, the impact of NADW changes on atmospheric δ13CO2 is less robust and might be model dependent. This results from complex interplay between global climate, carbon cycle, and the formation rate of NADW, a water body characterized by relatively high δ13C.
      PubDate: 2015-11-02T13:36:18.722583-05:
      DOI: 10.1002/2015GB005207
  • Tracing terrestrial DOC in the Baltic Sea ‐ a 3‐D model study
    • Abstract: The fate of terrestrial organic matter brought to the coastal seas by rivers, and its role in the global carbon cycle, are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semi‐enclosed sea, by releasing it as a tracer in a 3‐D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways; one by applying a decay time on the order of 10 years to the whole pool of DOCter, and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land‐sea‐atmosphere cycling of carbon in coastal‐ and shelf seas that are heavily influenced by riverine DOC.
      PubDate: 2015-10-28T21:30:23.634165-05:
      DOI: 10.1002/2014GB005078
  • Significant mixed layer nitrification in a natural iron‐fertilized
           bloom of the Southern Ocean
    • Abstract: Nitrification, the microbially mediated oxidation of ammonium into nitrate, is generally expected to be low in the Southern Ocean mixed layer. This paradigm assumes that nitrate is mainly provided through vertical mixing and assimilated during the vegetative season, supporting the concept that nitrate uptake is equivalent to the new primary production (i.e., primary production which is potentially available for export). Here we show, that nitrification is significant (~40 to 80% of the seasonal nitrate uptake) in the naturally iron‐fertilized bloom over the southeast Kerguelen Plateau. Hence, a large fraction of the nitrate‐based primary production is regenerated, instead of being exported. It appears that nitrate assimilation (light‐dependent) and nitrification (partly light‐inhibited) are spatially separated between the upper and lower parts, respectively, of the deep surface mixed layers. These deep mixed layers, extending well below the euphotic layer, allow nitrifiers to compete with phytoplankton for the assimilation of ammonium. The high contributions of nitrification to nitrate uptake are in agreement with both low export efficiency (i.e., the percentage of primary production that is exported) and low seasonal nitrate drawdown despite high nitrate assimilation.
      PubDate: 2015-10-21T00:38:33.053801-05:
      DOI: 10.1002/2014GB005051
  • Terrestrial pyrogenic carbon export to fluvial ecosystems: Lessons learned
           from the White Nile watershed of East Africa
    • Abstract: Pyrogenic carbon (PyC) is important because of its role in the global organic C (OC) cycle and in modifying soil properties. However, our understanding of PyC movement from terrestrial to fluvial ecosystems is not robust. This study examined (i) whether erosion or subsurface transport was more important for PyC export from headwaters, (ii) whether PyC was exported preferentially to total OC (TOC), and (iii) whether the movement of PyC from terrestrial to aquatic ecosystems provides an explanation for the coupling of PyC and non‐PyC observed in rivers at a global scale. In the Guineo‐Congolian highland forest region of western Kenya, duplicate catchments with sizes of 1–12 ha were equipped with stream gauges in primary forest and adjacent mixed agricultural landscapes that were cleared by fire 10, 16 or 62 years before. Stream water samples were taken weekly throughout one year and compared with runoff to assess PyC movement. Additional stream samples were taken from all major tributaries of the White Nile watershed of Lake Victoria. PyC was not preferentially eroded relative to TOC or non‐PyC, as topsoil (0–0.15 m) PyC concentrations (6.3±0.3% of TOC; means and standard errors) were greater than runoff sediment (1.9±0.4%) and dissolved PyC concentrations (2.0±0.4%, n=252). In addition, PyC proportions in eroded sediment were lower than and uncorrelated (r2=0.04; P=0.14) with topsoil PyC. An enrichment of PyC was found with depth in the soil, from 6.3±0.3% of TOC in the topsoil (0–0.15 m) to 12.3±0.3% of TOC at 1–2 m. Base‐flow PyC proportions of TOC correlated well with subsoil PyC (r2=0.57; P0.05). Similar PyC proportions were found in the studied headwater streams (2.7±0.2%), their downstream inflow into Lake Victoria (3.7%), the other nine major rivers into Lake Victoria (4.9±0.8%) and its outflow into the White Nile (1.1%). A strong positive correlation between dissolved PyC and non‐PyC (r2=0.91; P
      PubDate: 2015-10-19T13:30:18.341743-05:
      DOI: 10.1002/2015GB005095
  • Low particulate carbon to nitrogen ratios in arctic surface waters
    • Abstract: During the Canada Three Oceans and Joint Ocean Ice Study projects in the summers of 2007 and 2008, we measured particulate organic carbon to nitrogen ratios (POC:PON) throughout the euphotic zone in subarctic and arctic waters. Depth‐integrated values averaged 2.65 (±0.19) in the Beaufort Sea and Canada Basin (BS‐CB domain), and were much lower than both the Redfield ratio (6.6) and the average ratios (3.9 to 5.6) measured across other arctic‐subarctic domains. Average uptake ratios of C and N (ρC:ρN) were also lower (0.87±0.14) in BS‐CB than in the other four domains (2.10 to 3.51). Decreasing POC:PON ratios were associated with low concentrations of phytoplankton C, reduced abundance of biogenic silica (bSiO2), a smaller relative contribution of the >5 µm fraction to total chlorophyll a and a larger relative contribution of small flagellates (
      PubDate: 2015-10-15T00:48:11.614376-05:
      DOI: 10.1002/2015GB005200
  • Black carbon aerosol dynamics and isotopic composition in Alaska linked
           with boreal fire emissions and depth of burn in organic soils
    • Authors: G. O. Mouteva; C. I. Czimczik, S. M. Fahrni, E. B. Wiggins, B. M. Rogers, S. Veraverbeke, X. Xu, G. M. Santos, J. Henderson, C.E. Miller, J. T. Randerson
      Abstract: Black carbon (BC) aerosol emitted by boreal fires has the potential to accelerate losses of snow and ice in many areas of the Arctic, yet the importance of this source relative to fossil fuel BC emissions from lower latitudes remains uncertain. Here we present measurements of the isotopic composition of BC and organic carbon (OC) aerosols collected at two locations in interior Alaska during the summer of 2013, as part of NASA's Carbon in Arctic Reservoirs Vulnerability Experiment. We isolated BC from fine air particulate matter (PM2.5) and measured its radiocarbon (Δ14C) content with accelerator mass spectrometry (AMS). We show that fires were the dominant contributor to variability in carbonaceous aerosol mass in interior Alaska during the summer by comparing our measurements with satellite data, measurements from an aerosol network, and predicted concentrations from a fire inventory coupled to an atmospheric transport model. The Δ14C of BC from boreal fires was 131 ± 52‰ in year 2013 when the Δ14C of atmospheric CO2 was 23 ± 3‰, corresponding to a mean fuel age of 20 years. Fire‐emitted OC had a similar Δ14C (99 ± 21‰) as BC, but during background (low fire) periods OC (45 to 51‰) was more positive than BC (−354 to −57‰). We also analyzed the carbon and nitrogen elemental and stable isotopic composition of the PM2.5. Fire‐emitted aerosol had an elevated carbon to nitrogen (C/N) ratio (29 ± 2) and δ15N (16 ± 4‰). Aerosol Δ14C and δ13C measurements were consistent with a mean depth of burning in organic soil horizons of 20 cm (and a range of 8 to 47 cm). Our measurements of fire‐emitted BC and PM2.5 composition constrain the endmember of boreal forest fire contributions to aerosol deposition in the Arctic and may ultimately reduce uncertainties related to the impact of a changing boreal fire regime on the climate system.
      PubDate: 2015-10-08T08:28:43.770648-05:
      DOI: 10.1002/2015GB005247
  • Nutrient cycling in the Atlantic basin: the evolution of nitrate isotope
           signatures in water masses
    • Authors: R.E. Tuerena; R.S. Ganeshram, W. Geibert, A. E. Fallick, J. Dougans, A. Tait, S.F. Henley, E.M.S. Woodward
      Abstract: A basin‐wide transect of nitrate isotopes (δ15NNO3, δ18ONO3), across the UK‐GEOTRACES 40°S transect in the South Atlantic is presented. This dataset is used to investigate Atlantic nutrient cycling and the communication pathways of nitrogen cycling processes in the global ocean. Intermediate waters formed in the subantarctic are enriched in δ15NNO3 and δ18ONO3 from partial utilization of nitrate by phytoplankton and distant denitrification processes, transporting heavy isotope signatures to the subtropical Atlantic. Water mass modification through the Atlantic is investigated by comparing data from 40°S (South Atlantic) and 30°N (North Atlantic). This reveals that nitrate in the upper intermediate waters is regenerated as it transits through the subtropical Atlantic, as evidenced by decreases in δ18ONO3. We document diazotrophy producing high N:P particle ratios (18–21:1) for remineralization, which is further confirmed by a decrease in δ15NNO3 through the subtropical Atlantic. These modifications influence the isotopic signatures of the North Atlantic Deep Water (NADW) which is subsequently exported from the Atlantic to the Southern Ocean. This study reveals the dominance of recycling processes and diazotrophy on nitrate cycling in the Atlantic. These processes provide a source of low δ15NNO3 to the Southern Ocean via the NADW, to counteract enrichment in δ15NNO3 from water column denitrification in the Indo/Pacific basins. We hence identify the Southern Ocean as a key hub through which denitrification and N2 fixation communicate in the ocean through deep water masses. Therefore, the balancing of the oceanic N budget and isotopic signatures require timescales of oceanic mixing.
      PubDate: 2015-10-05T17:26:21.032599-05:
      DOI: 10.1002/2015GB005164
  • Global deep ocean oxygenation by enhanced ventilation in the Southern
           Ocean under long‐term global warming
    • Abstract: Global warming is expected to decrease ocean oxygen concentrations by less solubility of surface ocean and change in ocean circulation. The associated expansion of the oxygen minimum zone would have adverse impacts on marine organisms and ocean biogeochemical cycles. Oxygen reduction is expected to persist for a thousand years or more, even after atmospheric carbon dioxide stops rising. However, long‐term changes in ocean oxygen and circulation are still unclear. Here, we simulate multi‐millennium changes in ocean circulation and oxygen under doubling and quadrupling of atmospheric carbon dioxide, using a fully coupled atmosphere–ocean general circulation model and an offline biogeochemical model. In the first 500 years, global oxygen concentration decreases, consistent with previous studies. Thereafter, however, the oxygen concentration in the deep ocean globally recovers and overshoots at the end of the simulations, despite surface oxygen decrease and weaker Atlantic meridional overturning circulation. This is because, after the initial cessation, the recovery and overshooting of deep ocean convection in the Weddell Sea enhance ventilation and supply oxygen‐rich surface waters to deep ocean. Another contributor to deep ocean oxygenation is seawater warming, which reduces the export production and shifts the organic matter remineralization to the upper water column. Our results indicate that the change in ocean circulation in the Southern Ocean potentially drives millennial‐scale oxygenation in deep ocean, which is opposite to the centennial‐scale global oxygen reduction and general expectation.
      PubDate: 2015-10-05T01:33:52.826934-05:
      DOI: 10.1002/2015GB005181
  • Contrasted geographical distribution of N2 fixation rates and nifH
           phylotypes in the Coral and Solomon Seas (South‐Western Pacific)
           during austral winter conditions
    • Abstract: Biological dinitrogen (N2) fixation and the distribution of diazotrophic phylotypes were investigated during two cruises in the Coral Sea and the Solomon Sea (South Western Pacific) during austral winter conditions. N2 fixation rates were measurable at every station, but integrated (0–150 m) rates were an order of magnitude higher in the Solomon Sea (30 to 5449 µmol N m−2 d−1) compared to those measured in the Coral Sea (2 to 109 µmol N m−2 d−1). Rates measured in the Solomon Sea were in the upper range (100–1000 µmol m−2 d−1) or higher than rates compiled in the global MAREDAT database [Luo et al., 2012], indicating that this region has some of the highest N2 fixation rates reported in the global ocean. While unicellular diazotrophic cyanobacteria from Group A (UCYN‐A1 and UCYN‐A2) and the proteobacteria γ‐24774A11 dominated in the Coral Sea and were correlated with N2 fixation rates (p
      PubDate: 2015-10-05T01:32:25.063002-05:
      DOI: 10.1002/2015GB005117
  • Leaky nitrogen cycle in pristine African montane rainforest soil
    • Abstract: Many pristine humid tropical forests show simultaneously high nitrogen (N) richness and sustained loss of bioavailable N forms. To better understand this apparent up‐regulation of the N cycle in tropical forests, process‐based understanding of soil N transformations, in geographically diverse locations, remains paramount. Field based evidence is limited and entirely lacking for humid tropical forests on the African continent. This study aimed at filling both knowledge gaps by monitoring N losses and by conducting an in situ 15N labelling experiment in the Nyungwe tropical montane forest in Rwanda. Here we show that this tropical forest shows high nitrate (NO3‐) leaching losses, confirming findings from other parts of the world. Gross N transformation rates point to an open soil N cycle with mineralized N nitrified rather than retained via immobilization; gross immobilization of NH4+ and NO3‐ combined accounted for 37 % of gross mineralization, and plant N uptake is dominated by ammonium (NH4+). This study provided new process understanding of soil N cycling in humid tropical forests and added geographically independent evidence that humid tropical forests are characterized by soil N dynamics and N inputs sustaining bioavailable N loss.
      PubDate: 2015-10-02T02:27:26.283649-05:
      DOI: 10.1002/2015GB005144
  • Carbon isotope ratios suggest no additional methane from boreal wetlands
           during the rapid Greenland Interstadial 21.2
    • Abstract: Samples from two Greenland ice cores (NEEM and NGRIP) have been measured for methane carbon isotope ratios (δ13C‐CH4) to investigate the CH4 mixing ratio anomaly during Greenland Interstadial (GI) 21.2 (85,000 years before present). This extraordinarily rapid event occurred within 150 years, comprising a CH4 mixing ratio pulse of 150 ppb (∼25 %). Our new measurements disclose a concomitant shift in δ13C‐CH4 of 1 %0. Keeling Plot Analyses reveal the δ13C of the additional CH4 source constituting the CH4 anomaly as ‐56.8±2.8 %0, which we confirm by the means of a previously published box model. We propose tropical wetlands as the most probable additional CH4 source during GI‐21.2 and present independent evidence that suggests tropical wetlands in South America and Asia have played a key role. We find no evidence that boreal CH4 sources, such as permafrost degradation, contributed significantly to the atmospheric CH4 increase, despite the pronounced warming in the Northern Hemisphere during GI‐21.2.
      PubDate: 2015-10-01T16:39:32.486254-05:
      DOI: 10.1002/2014GB005007
  • Multi‐molecular tracers of terrestrial carbon transfer across the
           pan‐Arctic: 14C characteristics of sedimentary carbon components and
           their environmental controls
    • Abstract: Distinguishing the sources, ages and fate of various terrestrial organic carbon (OC) pools mobilized from heterogeneous arctic landscapes is key to assessing climatic impacts on the fluvial release of carbon from permafrost. Through molecular 14C measurements, including novel analyses of suberin‐ and/or cutin‐derived diacids (DAs) and hydroxy fatty acids (FAs), we compared the radiocarbon characteristics of a comprehensive suite of terrestrial markers (including plant wax lipids, cutin, suberin, lignin and hydroxy phenols) in the sedimentary particles from nine major arctic and sub‐arctic rivers in order to establish a benchmark assessment of the mobilization patterns of terrestrial OC pools across the pan‐Arctic. Terrestrial lipids, including suberin‐derived longer‐chain DAs (C24,26,28), plant wax FAs (C24,26,28) and n‐alkanes (C27,29,31), incorporated significant inputs of aged carbon, presumably from deeper soil horizons. Mobilization and translocation of these “old” terrestrial carbon components was dependent on non‐linear processes associated with permafrost distributions. By contrast, shorter‐chain (C16,18) DAs and lignin phenols (as well as hydroxy phenols in rivers outside eastern Eurasian Arctic) were much more enriched in 14C, suggesting incorporation of relatively young carbon supplied by runoff processes from recent vegetation debris and surface layers. Furthermore, the radiocarbon content of terrestrial markers is heavily influenced by specific OC sources and degradation status. Overall, multi‐tracer molecular 14C analysis sheds new light on the mobilization of terrestrial OC from arctic watersheds. Our findings of distinct ages for various terrestrial carbon components may aid in elucidating fate of different terrestrial OC pools in the face of increasing arctic permafrost thaw.
      PubDate: 2015-09-29T15:52:30.772414-05:
      DOI: 10.1002/2015GB005204
  • Pathways and transformations of dissolved methane and dissolved inorganic
           carbon in Arctic tundra watersheds: Evidence from analysis of stable
    • Authors: Heather M. Throckmorton; Jeffrey M. Heikoop, Brent D. Newman, Garrett L. Altmann, Mark S. Conrad, Jordan D. Muss, George B. Perkins, Lydia J. Smith, Margaret S. Torn, Stan D. Wullschleger, Cathy J. Wilson
      Abstract: Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO2) and methane (CH4) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem‐scale CH4 and CO2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4 and DIC balance by estimating mechanisms of CH4 and DIC production and transport pathways and oxidation of subsurface CH4. We observed a shift from acetoclastic (July) towards hydrogenotropic (September) methanogenesis at sites located towards the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape‐scale effect on CH4 production mechanism. The majority of subsurface CH4 was transported upward by plant‐mediated transport and ebullition, predominantly bypassing the potential for CH4 oxidation. Thus, surprisingly CH4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4 produced at the frost table, contributing to < 0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e‐ acceptors. This work highlights the importance of spatial and temporal variability of CH4 production at the watershed scale, and suggests broad scale investigations are required to build better regional or pan‐Arctic representations of CH4 and CO2 production.
      PubDate: 2015-09-09T17:11:53.88415-05:0
      DOI: 10.1002/2014GB005044
  • Responses of carbon uptake and oceanic pCO2 to climate change in the North
           Atlantic: A model study with the Bergen Earth System Model
    • First page: 1567
      Abstract: Several model studies diagnose the carbon uptake of the North Atlantic as most sensitive to climate change when considered per unit‐area. Yet, the main drivers of the modeled sensitivity and the share of biological production and physical transport are under debate. In order to contribute to this ongoing discussion, two simulations with the Bergen Earth System Model were carried out for period 1850‐2099. One of the simulations (COU) includes the radiative effect of rising CO2 (i.e. climate change), while the second simulation (BGC) excludes this effect. The modeled carbon fluxes show substantially different responses to climate change for different parts of the North Atlantic. Based on these differences, we divide the North Atlantic into 2 regions, namely the subpolar gyre (SPG) and the rest of the North Atlantic (rNAT*, covering mainly the subtropical gyre). The highest climate sensitivity is found in the SPG‐region (accounting for an uptake reduction of 8.06 Pg C over the period 1850‐2099), while the response of the rNAT*‐region is moderate (reduction of 4.00 Pg C). We show that the changing CO2‐fluxes in both SPG‐ and rNAT*‐region are driven by increasing oceanic pCO2. The pCO2‐changes in the rNAT*‐region are caused by both changing physical and biogeochemical processes, while changes in DIC and alkalinity are the primary contributor to the high climate sensitivity of the SPG‐region. We identify a reduced biological production to be responsible for the modeled response of DIC and alkalinity, yet the differences between biological contribution and contributions of ocean circulation and CO2 uptake are small, highlighting our need for a better understanding of the marine biological cycle.
      PubDate: 2015-08-29T10:23:14.124043-05:
      DOI: 10.1002/2015GB005109
  • Intense nitrogen cycling in permeable intertidal sediment revealed by a
           nitrous oxide hotspot
    • Authors: Charles A. Schutte; Samantha B. Joye, Alicia M. Wilson, Tyler Evans, Willard S. Moore, Karen Casciotti
      First page: 1584
      Abstract: Approximately 40% of the total global rate of nitrogen fixation is the result of human activities, and most of this anthropogenic nitrogen is used to fertilize agricultural fields. Approximately 23% of the applied agricultural nitrogen is delivered to the coastal zone, often reducing water quality and driving eutrophication. Nitrogen cycling in coastal sediments can mitigate eutrophication by removing bioavailable nitrogen. However, some of these processes generate nitrous oxide, a potent greenhouse gas, as a byproduct. Here we report the discovery of a nitrous oxide production hotspot in shallow barrier island sands. Nitrous oxide concentrations, production and consumption rates, vertical diffusion fluxes, and flux to the atmosphere were measured across triplicate depth profiles. Using a mass balance approach, rates of net nitrous oxide production were estimated to be 40 µmol m−2 day−1. This production was driven by a hotspot of nitrate consumption that removed bioavailable nitrogen from the coastal environment at a rate of 10 mmol m−2 day−1, a rate that is comparable with the highest rates of denitrification reported for coastal sediments.
      PubDate: 2015-09-03T05:44:20.669359-05:
      DOI: 10.1002/2014GB005052
  • Controls on biogenic silica burial in the Southern Ocean
    • Authors: Zanna Chase; Karen Kohfeld, Katsumi Matsumoto
      First page: 1599
      Abstract: Understanding the controls on opal export in the Southern Ocean can inform both the prediction of how the leakage of silicic acid from the Southern Ocean responds to climate and the interpretation of paleo‐proxies. We have compiled a database of 185 230Thorium‐normalized opal burial rates and 493 opal concentration measurements in Southern Ocean sediments and matched these with environmental climatologies. By subdividing the Southern Ocean on the basis of oceanographic regions and interpolating the opal burial rates, we estimate a total biogenic Si burial south of 40°S of 2.3 ± 1.0 Tmol Si year −1. In both the seasonally ice‐covered and permanently ice‐free regions we can predict 73% of opal burial from surface‐ocean properties. Where sea‐ice is present for at least part of the year, the length of the ice‐free season determines the upper limit of opal burial in the underlying sediments. In the ice‐free regions of the Southern Ocean, the supply of silicic acid through winter mixing is the most important factor. Our results do not support a strong role of iron in controlling opal burial. We do however find that satellite‐derived net primary production increases with increasing (modelled) dust delivery. These findings support the decoupling between carbon and opal fluxes in the Southern Ocean. When corrected for opal dissolution, the observed opal fluxes are in reasonable agreement with fluxes simulated using an ocean biogeochemical model. However, the results suggest current preservation algorithms for opal could be improved by incorporating the composition of particle flux, not only its magnitude.
      PubDate: 2015-09-07T14:47:40.598719-05:
      DOI: 10.1002/2015GB005186
  • Biogenic carbon fluxes from global agricultural production and consumption
    • Authors: Julie Wolf; Tristram O. West, Yannick L. Le Page, G. Page Kyle, Xuesong Zhang, G. James Collatz, Marc L. Imhoff
      First page: 1617
      Abstract: Quantification of biogenic carbon fluxes from agricultural lands is needed to generate comprehensive bottom‐up estimates of net carbon exchange for global and regional carbon monitoring. We estimated global agricultural carbon fluxes associated with annual crop net primary production (NPP), harvested biomass, and consumption of biomass by humans and livestock. These estimates were combined for a single estimate of net carbon exchange (NCE) and spatially distributed to 0.05 degree resolution using MODIS satellite land cover data. Global crop NPP in 2011 was estimated at 5.25 ± 0.46 Pg C yr−1, of which 2.05 ± 0.05 Pg C yr−1 was harvested and 0.54 Pg C yr−1 was collected from crop residues for livestock fodder. Total livestock feed intake in 2011 was 2.42 ± 0.21 Pg C yr−1, of which 2.31 ± 0.21 Pg C yr−1 was emitted as CO2, 0.07 ± 0.01 Pg C yr−1 was emitted as CH4, and 0.04 Pg C yr−1 was contained within milk and egg production. Livestock grazed an estimated 1.27 Pg C yr−1 in 2011, which constituted 52.4% of total feed intake. Global human food intake was 0.57 ± 0.03 Pg C yr−1 in 2011, the majority of which was respired as CO2. Completed global cropland carbon budgets accounted for the ultimate use of ca. 80% of harvested biomass. The spatial distribution of these fluxes may be used for global carbon monitoring, estimation of regional uncertainty, and for use as input to Earth system models.
      PubDate: 2015-09-08T16:36:24.193624-05:
      DOI: 10.1002/2015GB005119
  • Seasonal and interannual variability of sea‐air CO2 fluxes in the
           tropical Atlantic affected by the Amazon River plume
    • First page: 1640
      Abstract: CO2 fugacities obtained from a merchant ship sailing from France to French Guyana were used to explore the seasonal and interannual variability of the sea‐air CO2 exchange in the western tropical North Atlantic (TNA; 5‐14°N, 41‐52°W). Two distinct oceanic water masses were identified in the area associated to the main surface currents, i.e. the North Brazil Current (NBC) and the North Equatorial Current (NEC). The NBC was characterized by permanent CO2 oversaturation throughout the studied period, contrasting with the seasonal pattern identified in the NEC. The NBC retroflection was the main contributor to the North Equatorial Counter Current (NECC), thus spreading into the central TNA the Amazon River plume and the CO2–rich waters probably originated from the equatorial upwelling. Strong CO2 undersaturation was associated to the Amazon River plume. Total inorganic carbon drawdown due to biological activity was estimated to be 154 µmol kg−1 within the river plume. As a consequence, the studied area acted as a net sink of atmospheric CO2 (from −72.2±10.2 mmol m−2 month−1 in February to 14.3±4.5 mmol m−2 month−1 in May). This contrasted with the net CO2 efflux estimated by the main global sea‐air CO2 flux climatologies. Interannual SST changes in the TNA caused by large‐scale climatic events could determine the direction and intensity of the sea‐air CO2 fluxes in the NEC. Positive temperature anomalies observed in the TNA led to an almost permanent CO2 outgassing in the NEC in 2010.
      PubDate: 2015-09-09T17:12:27.960764-05:
      DOI: 10.1002/2015GB005110
  • Climatological distribution of aragonite saturation state in the global
    • First page: 1656
      Abstract: Aragonite saturation state (Ωarag) in surface and subsurface waters of the global oceans was calculated from up‐to‐date (through the year of 2012) ocean station dissolved inorganic carbon (DIC) and total alkalinity (TA) data. Surface Ωarag in the open ocean was always supersaturated (Ω>1), ranging between 1.1 and 4.2. It was above 2.0 (2.0‐4.2) between 40°N and 40°S, but decreased towards higher latitude to below 1.5 in polar areas. The influences of water temperature on the TA/DIC ratio, combined with the temperature effects on inorganic carbon equilibrium and apparent solubility product (K’sp), explain the latitudinal differences in surface Ωarag. Vertically, Ωarag was highest in the surface mixed layer (SML). Higher hydrostatic pressure, lower water temperature, and more CO2 buildup from biological activity in the absence of air‐sea gas exchange helped maintain lower Ωarag in the deep ocean. Below the thermocline, aerobic decomposition of organic matter along the pathway of global thermohaline circulation played an important role in controlling Ωarag distributions. Seasonally, surface Ωarag above 30° latitudes was about 0.06 to 0.55 higher during warmer months than during colder months in the open‐ocean waters of both hemispheres. Decadal changes of Ωarag in the Atlantic and Pacific Oceans showed that Ωarag in waters shallower than 100 m depth decreased by 0.10±0.09 (−0.40±0.37% yr−1) on average from the decade spanning 1989–1998 to the decade spanning 1998–2010.
      PubDate: 2015-09-18T17:56:54.932035-05:
      DOI: 10.1002/2015GB005198
  • Recent decadal trends in global phytoplankton composition
    • Authors: Cecile S. Rousseaux; Watson W. Gregg
      First page: 1674
      Abstract: Identifying major trends in biogeochemical composition of the oceans is essential to improve our understanding of biological responses to climate forcing. Using the NASA Ocean Biogeochemical Model (NOBM) combined with ocean color remote sensing data assimilation, we assessed the trends in phytoplankton composition (diatoms, cyanobacteria, coccolithophores and chlorophytes) at a global scale for the period 1998–2012. We related these trends in phytoplankton to physical conditions (surface temperature, surface photosynthetically available radiation [PAR] and mixed layer depth [MLD]) and nutrients (iron, silicate and nitrate). We found a significant global decline in diatoms (−1.22% y−1, P
      PubDate: 2015-09-23T12:32:24.616201-05:
      DOI: 10.1002/2015GB005139
  • Coupling atmospheric mercury isotope ratios and meteorology to identify
           sources of mercury impacting a coastal urban‐industrial region near
           Pensacola, Florida, USA
    • Authors: Jason D. Demers; Laura S. Sherman, Joel D. Blum, Frank J. Marsik, J. Timothy Dvonch
      First page: 1689
      Abstract: Identifying the anthropogenic and natural sources of mercury (Hg) emissions contributing to atmospheric mercury on local, regional, and global scales continues to be a grand challenge. The relative importance of various direct anthropogenic emissions of mercury, in addition to natural geologic sources and re‐emission of previously released and deposited mercury, differs regionally and temporally. In this study, we used local, mesoscale, and synoptic scale meteorological analysis to couple the isotopic composition of ambient atmospheric mercury with potential sources of mercury contributing to a coastal urban‐industrial setting near a coal‐fired power plant in Pensacola, Florida, USA. We were able to broadly discern four influences on the isotopic composition of ambient atmospheric mercury impacting this coastal urban‐industrial region: (1) local to regional urban‐industrial anthropogenic emissions (mean δ202Hg = 0.44 ± 0.05‰, 1SD, n = 3); (2) marine‐influenced sources derived from the Gulf of Mexico (mean δ202Hg = 0.77 ± 0.15‰, 1SD, n = 4); (3) continental sources associated with north‐northwesterly flows from within the planetary boundary layer (mean δ202Hg = 0.65 ± 0.04‰, 1SD, n = 3); and (4) continental sources associated with north‐northeasterly flows at higher altitudes (i.e., 2000 m above ground level; mean δ202Hg = 1.10 ± 0.21‰, 1SD, n = 8). Overall, these data, in conjunction with previous studies, suggest that the background global atmospheric mercury pool is characterized by moderately positive δ202Hg values; that urban‐industrial emissions drive the isotopic composition of ambient atmospheric mercury toward lower δ202Hg values; and that air‐surface exchange dynamics across vegetation and soils of terrestrial ecosystems drive the isotopic composition of ambient atmospheric mercury toward higher positive δ202Hg values. The data further suggest that mass independent fractionation (MIF) of both even‐mass‐ and odd‐mass‐number isotopes, likely generated by photochemical reactions in the atmosphere or during re‐emission from terrestrial and aquatic ecosystems, can be obscured by mixing with anthropogenic emissions having different MIF signatures.
      PubDate: 2015-09-18T17:56:28.425594-05:
      DOI: 10.1002/2015GB005146
  • Anthropogenic CO2 uptake, transport, storage, and dynamical controls in
           the ocean imposed by the meridional overturning circulation: A modeling
    • Authors: H. Nakano; M. Ishii, K. B. Rodgers, H. Tsujino, G. Yamanaka
      First page: 1706
      Abstract: Using an ocean carbon cycle model embedded in an ocean general circulation model, we examine how the budget of anthropogenic CO2(Cant) is controlled by ocean dynamics. To complement recent studies showing only vertically integrated budgets, we provide a step‐by‐step description by making use of three different coarse‐grainings of the full vertical resolution of the ocean model in our budget analysis. For the 11 sub‐domains of the global ocean, these coarse‐grainings are (1) a one‐layer (vertically integrated) budget, (2) a three‐layer budget, and (3) an 11‐layer budget. We largely focus on the Pacific circulation. We identify and quantify substantial carbon transport associated with the subtropical cells (STCs), which are dominant contributors to the meridional overturning circulation in the upper ocean in the tropics and subtropics, as playing a fundamental role in governing the ocean interior distribution of Cant. The upper branch of the STCs transports Cant‐rich water from the tropics to the subtropics, contributing to the precondition for the high Cant inventory in mode waters. The lower branch of the STCs carries about two‐thirds of the transported Cant back to the tropics, while it largely excludes Subtropical Mode Waters. This work implies that the re‐emergence of Cant through recirculation within the STCs may lead to a reduced capacity for further Cant uptake via gas exchange into the surface ocean, potentially contributing to a positive carbon‐climate feedback.
      PubDate: 2015-09-21T10:01:24.027658-05:
      DOI: 10.1002/2015GB005128
  • Dissolved organic carbon pools and export from the coastal ocean
    • First page: 1725
      Abstract: The distribution of dissolved organic carbon (DOC) concentration across coastal waters was characterised based on the compilation of 3510 individual estimates of DOC in coastal waters worldwide. We estimated the DOC concentration in the coastal waters that directly exchange with open ocean waters in two different ways, as the DOC concentration at the edge of the shelf break and as the DOC concentration in coastal waters with salinity close to the average salinity in the open ocean. Using these estimates of DOC concentration in the coastal waters that directly exchange with open ocean waters, the mean DOC concentration in the open ocean and the estimated volume of water annually exchanged between coastal and open ocean, we estimated a median ± SE (and average ± SE) global DOC export from coastal to open ocean waters ranging from 4.4 ± 1.0 Pg C yr−1 to 27.0 ± 1.8 Pg C yr−1 (7.0 ± 5.8 Pg C yr−1 to 29.0 ± 8.0 Pg C yr−1) depending on the global hydrological exchange. These values correspond to a median and mean median (and average) range between 14.7 ± 3.3 to 90.0 ± 3.3 (23.3 ± 19.3 to 96.7± 26.7) Gg C yr−1 per km of shelf break, which is consistent with the range between 1.4 to 66.1 Gg C yr−1 per km of shelf break of available regional estimates of DOC export. The estimated global DOC export from coastal to open ocean waters is also consistent with independent estimates of the net metabolic balance of the coastal ocean. The DOC export from the coastal to the open ocean is likely to be a sizeable flux and is likely to be an important term in the carbon budget of the open ocean, potentially providing an important subsidy to support heterotrophic activity in the open ocean.
      PubDate: 2015-09-22T05:36:10.479536-05:
      DOI: 10.1002/2014GB005056
  • Disentangling the contribution of multiple land covers to
    • First page: 1739
      Abstract: In less than 15 years, the Amazon region experienced three major droughts. Links between droughts and fires have been demonstrated for the 1997/98, 2005 and 2010 droughts. In 2010, emissions of 510±120 Tg C were associated to fire alone in Amazonia. Existing approaches have, however, not yet disentangled the proportional contribution of multiple land cover sources to this total. We develop a novel integration of multi‐sensor and multi‐temporal satellite‐derived data on land cover, active fires and burned area, and an empirical model of fire‐induced biomass loss to quantify the extent of burned areas and resulting biomass loss for multiple land‐covers in Mato Grosso state (MT), southern Amazonia ‐ the 2010 drought most impacted region. We show that 10.77% (96,855 km2) of MT burned. We estimated a gross carbon emission of 56.21 ± 22.5 TgC from direct combustion of biomass, with an additional 29.4 ± 10 Tg C committed to be emitted in the following years due to dead wood decay. It is estimated that old growth forest fires in the whole Brazilian Legal Amazon (BLA) have contributed to 14.81 Tg of C (11.75 Tg C to 17.87 Tg C) emissions to the atmosphere during the 2010 fire season, with an affected area of 27,555 km2. Total C loss from the 2010 fires in MT state and old growth forests fires in the BLA represent, respectively, 77% (47% to 107%) and 86% (68.2% to 103%) of Brazil's National Plan on Climate Change annual target for Amazonia C emissions reductions from deforestation.
      PubDate: 2015-09-08T01:27:25.003184-05:
      DOI: 10.1002/2014GB005008
  • Surface distribution of dissolved trace metals in the oligotrophic ocean
           and their influence on phytoplankton biomass and productivity
    • First page: 1763
      Abstract: The distribution of bioactive trace metals has the potential to enhance or limit primary productivity and carbon export in some regions of the world ocean. To study these connections, the concentrations of Cd, Co, Cu, Fe, Mo, Ni, and V were determined for 110 surface water samples collected during the Malaspina 2010 Circumnavigation Expedition (MCE). Total dissolved Cd, Co, Cu, Fe, Mo, Ni, and V concentrations averaged 19.0 ± 5.4 pM, 21.4 ± 12 pM, 0.91 ± 0.4 nM, 0.66 ± 0.3 nM, 88.8 ± 12 nM, 1.72 ± 0.4 nM, 23.4 ± 4.4 nM, respectively, with the lowest values detected in the Central Pacific and increased values at the extremes of all transects near coastal zones. Trace metal concentrations measured in surface waters of the Atlantic Ocean during the MCE were compared to previously published data for the same region. The comparison revealed little temporal changes in the distribution of Cd, Co, Cu, Fe, and Ni over the last 30 years. We utilized a multivariable linear regression model to describe potential relationships between primary productivity and the hydrological, biological, trace‐ and macro‐nutrient data collected during the MCE. Our statistical analysis shows that primary productivity in the Indian Ocean is best described by chlorophyll a, NO3, Ni, temperature, SiO4 and Cd. In the Atlantic Ocean, primary productivity is correlated with chlorophyll a, NO3, PO4, mixed layer depth, Co, Fe, Cd, Cu, V, and Mo. The variables salinity, temperature, SiO4, NO3, PO4, Fe, Cd, and V were found to best predict primary productivity in the Pacific Ocean. These results suggest that some of the lesser‐studied trace elements (e.g., Ni, V, Mo, Cd) may play a more important role in regulating oceanic primary productivity than previously thought and point to the need for future experiments to verify their potential biological functions.
      PubDate: 2015-09-23T07:09:55.895147-05:
      DOI: 10.1002/2015GB005149
  • Explicitly representing soil microbial processes in Earth system models
    • First page: 1782
      Abstract: Microbes influence soil organic matter (SOM) decomposition and the long‐term stabilization of carbon (C) in soils. We contend that by revising the representation of microbial processes and their interactions with the physicochemical soil environment, Earth system models (ESMs) will make more realistic global C cycle projections. Explicit representation of microbial processes presents considerable challenges due to the scale at which these processes occur. Thus, applying microbial theory in ESMs requires a framework to link micro‐scale process‐level understanding and measurements to macro‐scale models used to make decadal‐ to century‐long projections. Here, we review the diversity, advantages, and pitfalls of simulating soil biogeochemical cycles using microbial‐explicit modeling approaches. We present a roadmap for how to begin building, applying, and evaluating reliable microbial‐explicit model formulations that can be applied in ESMs. Drawing from experience with traditional decomposition models we suggest: (1) guidelines for common model parameters and output that can facilitate future model intercomparisons; (2) development of benchmarking and model‐data integration frameworks that can be used to effectively guide, inform, and evaluate model parameterizations with data from well‐curated repositories; and (3) the application of scaling methods to integrate microbial‐explicit soil biogeochemistry modules within ESMs. With contributions across scientific disciplines, we feel this roadmap can advance our fundamental understanding of soil biogeochemical dynamics and more realistically project likely soil C response to environmental change at global scales.
      PubDate: 2015-09-22T05:36:46.856239-05:
      DOI: 10.1002/2015GB005188
  • The mutual importance of anthropogenically and climate induced changes in
           global vegetation cover for future land carbon emissions in the
           MPI‐ESM CMIP5 simulations
    • Authors: R. Schneck; C. H. Reick, J. Pongratz, V. Gayler
      First page: 1816
      Abstract: Based on the MPI‐ESM simulations for the Coupled Model Intercomparison Project Phase 5 (CMIP5) and on simulations with the submodel Cbalone we disentangle the influence of natural and anthropogenic vegetation changes on land carbon emissions for the years 1850 till 2300. According to our simulations, climate induced changes in distribution and productivity of natural vegetation strongly mitigates future carbon emissions from anthropogenic landuse and landcover change (LULCC). Depending on the assumed scenario, the accumulated carbon emissions until the year 2100 are reduced between 22 and 49% and until 2300 between 45 and 261%. The carbon storage due to climate‐induced vegetation change is generally stronger in combination with LULCC. This is because in our simulations anthropogenic vegetation changes support more productive Plant Functional Types (PFTs). After stopping LULCC in the year 2100 the refilling of depleted C pools on formerly transformed land takes (dependent on the scenario) timescales of centuries.
      PubDate: 2015-07-24T14:08:12.197437-05:
      DOI: 10.1002/2014GB004959
  • Coralline algae are a globally significant pool of marine dimethylated
    • Authors: Heidi L Burdett; Angela D Hatton, Nicholas A Kamenos
      First page: 1845
      Abstract: Marine algae are key sources of the biogenic sulphur compound dimethylsulphoniopropionate (DMSP), a vital component of the marine sulphur cycle. Autotrophic ecosystem engineers such as red coralline algae support highly diverse and biogeochemically active ecosystems and are known to be high DMSP producers, but their importance in the global marine sulphur cycle has not yet been appreciated. Using a global sampling approach, we show that red coralline algae are a globally significant pool of DMSP in the oceans, estimated to be ~110 x 1012 moles worldwide during the summer months. Latitude was a major driver of observed regional‐scale variations, with peaks in polar and tropical climate regimes, reflecting the varied cellular functions for DMSP (e.g. as a cryoprotectant and antioxidant). A temperate coralline algal bed was investigated in more detail to also identify local‐scale temporal variations. Here, water column DMSP was driven by water temperature, and to a lesser extent, cloud cover; two factors which are also vital in controlling coralline algal growth. This study demonstrates that coralline algae harbour a large pool of dimethylated sulphur, thereby playing a significant role in both the sulphur and carbon marine biogeochemical cycles. However, coralline algal habitats are severely threatened by projected climate change; a loss of this habitat may thus detrimentally impact oceanic sulphur and carbon biogeochemical cycling.
      PubDate: 2015-09-29T21:45:42.358226-05:
      DOI: 10.1002/2015GB005274
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