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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 21, SJR: 2.156, h-index: 61)
Geophysical Research Letters     Full-text available via subscription   (Followers: 46, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 3, SJR: 2.4, h-index: 109)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 0.126, h-index: 2)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 20)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 5)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 22)
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: 23)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 4, SJR: 2.16, h-index: 82)
Radio Science     Full-text available via subscription   (Followers: 2, SJR: 0.527, h-index: 47)
Reviews of Geophysics     Full-text available via subscription   (Followers: 19, SJR: 8.837, h-index: 87)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 0.496, h-index: 16)
Tectonics     Full-text available via subscription   (Followers: 7, SJR: 2.16, h-index: 79)
Water Resources Research     Full-text available via subscription   (Followers: 147, SJR: 1.769, h-index: 110)
Journal Cover Global Biogeochemical Cycles
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     ISSN (Print) 0886-6236 - ISSN (Online) 1944-9224
     Published by American Geophysical Union (AGU) Homepage  [17 journals]   [SJR: 2.4]   [H-I: 109]
  • Non‐growing‐season soil respiration is controlled by freezing
           and thawing processes in the summer‐monsoon‐dominated Tibetan
           alpine grassland
    • Authors: Yonghui Wang; Huiying Liu, Haegeun Chung, Lingfei Yu, Zhaorong Mi, Yan Geng, Xin Jing, Shiping Wang, Hui Zeng, Guangmin Cao, Xinquan Zhao, Jin‐Sheng He
      Pages: n/a - n/a
      Abstract: The Tibetan alpine grasslands, sharing many features with arctic tundra ecosystems, have a unique non‐growing‐season climate that is usually dry and without persistent snow cover. Pronounced winter warming recently observed in this ecosystem may significantly alter the non‐growing‐season carbon cycle processes such as soil respiration (Rs), but detailed measurements to assess the patterns, drivers of and potential feedbacks on Rs have not been made yet. We conducted a 4‐year study on Rs using a unique Rs‐measuring system, composed of an automated soil CO2 flux sampling system and a custom‐made container, to facilitate measurements in this extreme environment. We found that in the non‐growing season: 1) cumulative Rs was 82–89 g C m−2, accounting for 11.8‐13.2% of the annual total Rs; 2) surface soil freezing controlled the diurnal pattern of Rs and bulk soil freezing induced lower reference respiration rate (R0) and temperature sensitivity (Q10) than those in the growing season (0.40‐0.53 vs. 0.84‐1.32 µmol CO2 m−2 s−1 for R0 and 2.5‐2.9 vs. 2.9‐5.6 for Q10); and 3) the intra‐annual variation in cumulative Rs were controlled by accumulated surface soil temperature. We found that in the summer‐monsoon‐dominated Tibetan alpine grassland, surface soil freezing, bulk soil freezing and accumulated surface soil temperature are the day‐, season‐, and year‐scale drivers of the non‐growing‐season Rs, respectively. Our results suggest that warmer winters can trigger carbon loss from this ecosystem because of higher Q10 of thawed than frozen soils.
      PubDate: 2014-09-12T18:28:16.608988-05:
      DOI: 10.1002/2013GB004760
       
  • Climate change reduces the capacity of northern peatlands to absorb the
           atmospheric carbon dioxide: the different responses of bogs and fens
    • Authors: Jianghua Wu; Nigel T. Roulet
      Pages: n/a - n/a
      Abstract: The carbon (C) storage of northern peatlands is equivalent to ~34‐46% of the ~795 T g C currently held in the atmosphere as CO2. Most studies report that northern peatlands are a sink of between 20 and 60 g CO2‐C m‐2 yr‐1. Since peatland hydrology and biogeochemistry are very closely related to climate, there is concern whether northern peatlands will continue to function as C sinks with climate change. We used a coupled land surface scheme and peatland C model, called CLASS3W‐MWM, to examine the sensitivity of peatland C to climate change. Based on the data available to constrain our model, we simulated the C dynamics of the Mer Bleue (MB) bog in eastern Canada and the Degerö Stormyr (DS) poor fen in northern Sweden for four IPCC climate change scenarios, i.e. A1B, A2, B1 and Commit, over four time periods, i.e. present day, 2030, 2060 and 2100. When the simulated future C fluxes were compared to the baseline fluxes under the present climate conditions we found fens were much more sensitive to climate change than bogs. Gross primary production (GPP) at MB significantly increased by 4‐44% up to 2100 for all scenarios except Commit. GPP at DS significantly decreased by 34‐39% for A1B and A2, and slightly increased by 6‐10% for B1 and Commit. Total ecosystem respiration (TER) significantly increased by 7‐57% for MB and 4‐34% for DS up to 2100 for all scenarios except Commit. Net ecosystem production (NEP), therefore, significantly decreased. The bog, however, was still a C sink up to 2100, though much reduced, but the fen switched to a C source for A1B and A2 scenarios. Additional experiments where we climatically transplanted the study peatlands or forced vegetation changes when the fen became too dry showed similar but less dramatic results as the standard runs. Our results indicate that northern peatlands should be included in the C‐coupled climate model to fully understand the response of C cycling in terrestrial ecosystems to climate change and to reduce the uncertainties for projecting the future climate.
      PubDate: 2014-09-05T22:18:57.801166-05:
      DOI: 10.1002/2014GB004845
       
  • Global Dry Deposition of Nitrogen Dioxide and Sulfur Dioxide Inferred from
           Space‐Based Measurements
    • Authors: C. R. Nowlan; R. V. Martin, S. Philip, L. N. Lamsal, N. Krotkov, E. A. Marais, S. Wang, Q. Zhang
      Pages: n/a - n/a
      Abstract: A method is developed to estimate global NO2 and SO2 dry deposition fluxes at high spatial resolution (0.1° × 0.1°) using satellite measurements from the Ozone Monitoring Instrument (OMI) on the Aura satellite, in combination with simulations from the GEOS‐Chem global chemical transport model. These global maps for 2005–2007 provide a dataset for use in examining global and regional budgets of deposition. In order to properly assess SO2 on a global scale, a method is developed to account for the geospatial character of background offsets in retrieved satellite columns. Globally, annual dry deposition to land estimated from OMI as NO2 contributes 1.5 ± 0.5 Tg of nitrogen and as SO2 contributes 13.7 ± 4.0 Tg of sulfur. Differences between OMI‐inferred NO2 dry deposition fluxes and those of other models and observations vary from excellent agreement to an order of magnitude difference, with OMI typically on the low end of estimates. SO2 dry deposition fluxes compare well with in situ CASTNET‐network‐inferred flux over North America (slope = 0.98, r = 0.71). The most significant NO2 dry deposition flux to land per area occurs in the Pearl River Delta, China at 13.9 kg N ha−1 yr−1, while SO2 dry deposition has a global maximum rate of 72.0 kg S ha−1 yr−1 to the east of Jinan in China's Shandong province. Dry deposition fluxes are explored in several urban areas, where NO2 contributes on average 9–36% and as much as 85% of total NOy dry deposition.
      PubDate: 2014-09-03T16:50:40.164708-05:
      DOI: 10.1002/2014GB004805
       
  • Iron sources and dissolved‐particulate interactions in the seawater
           of the Western Equatorial Pacific, iron isotope perspectives
    • Authors: M. Labatut; F. Lacan, C. Pradoux, J. Chmeleff, A. Radic, J.W. Murray, F. Poitrasson, A.M. Johansen, F. Thil
      Pages: n/a - n/a
      Abstract: This work presents iron isotope data in the western equatorial Pacific. Marine aerosols and top core margin sediments display a slightly heavy Fe isotopic composition (δ56Fe) of 0.33 ± 0.11‰ (2SD) and 0.14 ± 0.07‰, respectively. Samples reflecting the influence of Papua New Guinea runoff (Sepik River and Rabaul volcano water) are characterized by crustal values. In seawater, Fe is mainly supplied in the particulate form and is found with a δ56Fe between −0.49 and 0.34 ± 0.07‰. The particulate Fe seems to be brought mainly by runoff and transported across continental shelves and slopes. Aerosols are suspected to enrich the surface Vitiaz Strait waters while hydrothermal activity enriched likely New Ireland waters. Dissolved Fe isotopic ratios are found between ‐0.03 to 0.53 ± 0.07‰. They are almost systematically heavier than the corresponding particulate Fe, and the difference between the signature of both phases is similar for most samples Δ56FeDFe – PFe = + 0.27 ± 0.25‰ (2SD). This is interpreted as an equilibrium isotopic fractionation revealing exchange fluxes between both phases. The dissolved phase being heavier than the particles suggest that the exchanges result in a net non‐reductive release of dissolved Fe. This process seems to be locally significantly more intense than Fe reductive dissolution documented along reducing margins. It may therefore constitute a very significant iron source to the ocean, thereby influencing the actual estimation of the iron residence time and sinks. The underlying processes could also apply to other elements.
      PubDate: 2014-09-03T08:36:44.426075-05:
      DOI: 10.1002/2014GB004928
       
  • The impact of Neogene grassland expansion and aridification on the
           isotopic composition of continental precipitation
    • Authors: C.P. Chamberlain; M.J. Winnick, H.T. Mix, S.D. Chamberlain, K. Maher
      Pages: n/a - n/a
      Abstract: The late Cenozoic was a time of global cooling, increased aridity, and expansion of grasslands. In the last two decades numerous records of oxygen isotopes have been collected to assess plant ecological changes, understand terrestrial paleoclimate, and to determine the surface history of mountain belts. The δ18O values of these records, in general, increase from the mid‐Miocene to the Recent. We suggest that these records record an increase in aridity and expansion of grasslands in mid‐latitude continental regions. We use a non‐dimensional isotopic vapor transport model coupled with a soil water isotope model to evaluate the role of vapor recycling and transpiration by different plant functional types. This analysis shows that increased vapor recycling associated with grassland expansion along with bio‐mechanistic changes in transpiration by grasses themselves conspire to lower the horizontal gradient in the δ18O of atmospheric vapor as an air mass moves into continental interiors. The resulting signal at a given inland site is an increase in δ18O of precipitation with the expansion of grasslands and increasing aridity, matching the general observed trend in terrestrial Cenozoic δ18O records. There are limits to the isotopic effect that are induced by vapor recycling, which we refer to here as a “hydrostat”. In the modern climate, this hydrostatic limit occurs at approximately the boundary between forest and grassland ecosystems.
      PubDate: 2014-08-27T05:34:01.097622-05:
      DOI: 10.1002/2014GB004822
       
  • Close coupling of N‐cycling processes expressed in stable isotope
           data at the redoxcline of the Baltic Sea
    • Authors: Claudia Frey; Joachim W. Dippner, Maren Voss
      Pages: n/a - n/a
      Abstract: Over the past decades, the hypoxic state of the central Baltic Sea has deteriorated because of eutrophication, but little is known about the extent to which related factors such as nitrogen removal have been altered. The Baltic Sea is a stratified semi‐enclosed basin with a large, anoxic bottom‐water mass in its central Gotland Basin and highly active microbial nitrogen transformation processes at the redoxcline, the interface between oxic and anoxic waters. In this study, we identified and quantified the dominant transformation processes of reactive nitrogen by exploiting fine‐resolution profiles of δ15NNO3, δ18ONO3, and δ15NNH4 through the pelagic redoxcline between 60 and 140 m. Our results showed increasing δ15NNO3 and δ18ONO3 values with decreasing nitrate concentrations, but the associated low apparent isotope effect (ϵ = ~5 ‰), as inferred from a closed system Rayleigh model, was not consistent with the high ϵ (~25 ‰) characteristic of denitrification in the water column. These findings could be explained by substrate limitation. The observed δ18ONO3:δ15NNO3 ratio of 1.38:1 rather than the usual 1:1 ratio typical for denitrification‐dominated systems could be explained by the occurrence of both nitrification and denitrification We then developed a numeric reaction–diffusion model, according to which a realistic denitrification rate of 14 nmol N L−1 d−1 was estimated and a nitrification rate of 6.6 nmol N L−1 d−1 confirmed. Our study demonstrates the value of stable isotope data for investigating nitrogen transformation processes but also highlights that care is needed in interpreting systems with closely coupled processes such as those at ocean redoxclines.
      PubDate: 2014-08-23T08:45:32.293311-05:
      DOI: 10.1002/2013GB004642
       
  • Global patterns of ecosystem carbon flux in forests: A biometric
           data‐based synthesis
    • Authors: Bing Xu; Yuanhe Yang, Pin Li, Haihua Shen, Jingyun Fang
      Pages: n/a - n/a
      Abstract: Forest ecosystems function as a significant carbon sink for atmospheric carbon dioxide. However, our understanding of global patterns of forest carbon fluxes remains controversial. Here we examined global patterns and environmental controls of forest carbon balance using biometric measurements derived from 243 sites and synthesized from 81 publications around the world. Our results showed that both production and respiration increased with mean annual temperature and exhibited unimodal patterns along a gradient of precipitation. However, net ecosystem production (NEP) initially increased and subsequently declined along gradients of both temperature and precipitation. Our results also indicated that ecosystem production increased during stand development but eventually leveled off, whereas respiration was significantly higher in mature and old forests than in young forests. The residual variation of carbon flux along climatic and age gradients might be explained by other factors such as atmospheric CO2 elevation and disturbances (e.g., forest fire, storm damage and selective harvest). Heterotrophic respiration (Rh) was positively associated with net primary production (NPP), but the Rh‐NPP relationship differed between natural and planted forests: Rh increased exponentially with NPP in natural forests but tended toward saturation with increased NPP in planted forests. Comparison of biometric measurements with eddy‐covariance observations revealed that ecosystem carbon balance derived from the latter generated higher overall NEP estimates. These results suggest that the eddy‐covariance observations may overestimate the strength of carbon sinks, and thus biometric measurements need to be incorporated into global assessments of the forest carbon balance.
      PubDate: 2014-08-19T09:14:25.565813-05:
      DOI: 10.1002/2013GB004593
       
  • Influence of water depth on the carbon sequestration capacity of
           seagrasses
    • Authors: Oscar Serrano; Paul S Lavery, Mohammad Rozaimi, Miguel Ángel Mateo
      Pages: n/a - n/a
      Abstract: The actual estimates of carbon stocks beneath seagrass meadows worldwide are derived from few data, resulting in a tendency to generalize global carbon stocks from a very limited number of seagrass habitats. We surveyed Posidonia oceanica and Posidonia sinuosa meadows along depth‐induced gradients of light availability to assess the variability in their sedimentary organic carbon (Corg) stocks and accretion rates. This study showed a 4‐fold decrease in Corg stocks from 2‐4 m to 6‐8 m depth P. sinuosa meadows (averaging 7.0 and 1.8 kg m‐2, respectively; top meter of sediment) and a 14‐ to 16‐fold decrease from shallow (2 m) to deep (32 m) P. oceanica meadows (200 and 19 kg m‐2 average, respectively; top 2.7 m of sediment). The average Corg accretion rates in shallow P. sinuosa meadows were higher (10.5 g m‐2 y‐1) than in deeper meadows (2.1 g m‐2 y‐1). The reduction of sedimentary Corg stocks and accretion rates along depth‐related gradients of light reduction suggest that irradiance, controlling plant productivity, meadow density and sediment accretion rates, is a key environmental factor affecting Corg storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows at Balearic Islands (Spain), containing the highest areal Corg stocks of all seagrasses (estimated in up to 691‐770 kg m‐2 in 8‐13 m‐thick deposits). Seagrass communities are experiencing worldwide decline, and reduced irradiance (following e.g. eutrophication or sediment regime alterations) will lead to photo‐acclimation responses (i.e. reduced plant productivity and shoot density), which may impact the carbon sequestration capacity of seagrasses.
      PubDate: 2014-08-16T00:45:05.685159-05:
      DOI: 10.1002/2014GB004872
       
  • Recent variability of the global ocean carbon sink
    • Authors: P. Landschützer; N. Gruber, D.C.E. Bakker, U. Schuster
      Pages: n/a - n/a
      Abstract: We present a new observation‐based estimate of the global oceanic carbon dioxide (CO2) sink and its temporal variation on a monthly basis from 1998 through 2011 and at a spatial resolution of 1° × 1°. This sink estimate rests upon a neural network‐based mapping of global surface ocean observations of the partial pressure of CO2 (pCO2) from the Surface Ocean CO2 Atlas (SOCAT) database. The resulting pCO2 has small biases when evaluated against independent observations in the different ocean basins, but larger randomly distributed differences exist particularly in the high latitudes. The seasonal climatology of our neural network‐based product agrees overall well with the Takahashi et al. [2009] climatology, although our product produces a stronger seasonal cycle at high latitudes. From our global pCO2 product, we compute a mean net global ocean (excluding the Arctic Ocean and coastal regions) CO2 uptake flux of ‐1.42 ± 0.53 Pg C yr−1, which is in good agreement with ocean inversion based estimates. Our data indicate a moderate level of interannual variability in the ocean carbon sink (±0.12 Pg C yr−1, 1σ) from from 1998 through 2011, mostly originating from the equatorial Pacific Ocean, and associated with the El Niño Southern Oscillation (ENSO). Accounting for steady‐state riverine and Arctic Ocean carbon fluxes our estimate further implies a mean anthropogenic CO2 uptake of ‐1.99 ± 0.59 Pg C yr−1 over the analysis period. From this estimate plus the most recent estimates for fossil fuel emissions and atmospheric CO2 accumulation, we infer a mean global land sink of ‐2.82 ± 0.85 Pg C yr−1 over the 1998 through 2011 period with strong interannual variation.
      PubDate: 2014-08-13T06:13:08.92576-05:0
      DOI: 10.1002/2014GB004853
       
  • Biogeochemical control of marine productivity in the Mediterranean Sea
           during the last 50 years
    • Authors: Diego Macias; Elisa Garcia‐Gorriz, Chiara Piroddi, Adolf Stips
      Abstract: The temporal dynamics of biogeochemical variables derived from a coupled 3D model of the Mediterranean Sea are evaluated for the last 50 years (1960–2010) against independent data on fisheries catches per unit effort (CPUE) for the same time period. Concordant patterns are found in the time‐series of all of the biological variables (from the model and from fisheries statistics), with low values at the beginning of the series, a later increase, with maximum levels reached at the end of the 1990's, and a posterior stabilization. Spectral analysis of the annual biological time series reveals coincident low‐frequency signals in all of them. The first, more energetic signal peaking around the year 2000, while the second, less energetic signal peaks near 1982. Almost identical low‐frequency signals are found in the nutrient loads of the rivers and in the integrated nutrient levels in the surface marine ecosystem. Nitrate concentration shows a maximum level in 1998, with a later stabilization to present day values, coincident with the first low‐frequency signal found in the biological series. Phosphate shows maximum concentrations around 1982 and a posterior sharp decline, in concordance with the second low‐frequency signal observed in the biological series. That result seems to indicate that the control of marine productivity (plankton to fish) in the Mediterranean is principally mediated through bottom‐up processes that could be traced back to the characteristics of riverine discharges. The high sensitivity of CPUE time series to environmental conditions might be another indicator of the overexploitation of this marine ecosystem.
      PubDate: 2014-08-06T05:02:35.541354-05:
      DOI: 10.1002/2014GB004846
       
  • Impacts and effects of mesoscale ocean eddies on ocean carbon storage and
           atmospheric pCO2
    • Authors: D. R. Munday; H. L. Johnson, D. P. Marshall
      Pages: n/a - n/a
      Abstract: An idealised numerical ocean model is used to investigate the sensitivity of the partial pressure of atmospheric carbon dioxide (pCO2) to changes in surface wind stress when mesoscale eddies are permitted in the flow. When wind stress increases, pCO2 increases, and vice versa. The introduction of mesoscale eddies reduces the overall sensitivity of pCO2 by changing the sensitivity of ocean carbon storage due to the saturation state of carbon dioxide, the net air‐sea disequilibrium, soft tissue carbon, and the carbonate pump. However, a full carbon pump decomposition shows different responses for different ocean carbon storage terms. For example, air‐sea disequilibrium is actually more sensitive to increased winds at eddy‐permitting resolution, whereas soft tissue carbon is much less sensitive to wind changes in an eddy‐permitting ocean. Changes in pycnocline depth and the strength of both upper and lower cells of the meridional overturning circulation affect this sensitivity.
      PubDate: 2014-08-02T01:12:58.013371-05:
      DOI: 10.1002/2014GB004836
       
  • Sensitivity of ocean oxygenation to variations in tropical zonal wind
           stress magnitude
    • Authors: Nina N. Ridder; Matthew H. England
      Pages: n/a - n/a
      Abstract: Ocean oxygenation has been observed to have changed over the past few decades and is projected to change further under global climate change due to an interplay of several mechanisms. In this study we isolate the effect of modified tropical surface wind stress conditions on the evolution of ocean oxygenation in a numerical climate model. We find that ocean oxygenation varies inversely with low‐latitude surface wind stress. Approximately one third of this response is driven by SST anomalies; the remaining two thirds result from changes in ocean circulation and marine biology. Global mean O2 concentration changes reach maximum values of +4 μM and ‐3.6 μM in the two most extreme perturbation cases of ‐30% and +30% wind change, respectively. Localised changes lie between +92 μM under 30% reduced winds and ‐56 μM for 30% increased winds. Overall we find that the extent of the global low‐oxygen volume varies with the same sign as the wind perturbation; namely weaker winds reduce the low oxygen volume on the global scale and vice versa for increased trade winds. We identify two regions, one in the Pacific Ocean off Chile, the other in the Indian Ocean off Somalia, that are of particular importance for the evolution of oxygen minimum zones in the global ocean.
      PubDate: 2014-08-01T13:27:04.415115-05:
      DOI: 10.1002/2013GB004708
       
  • Understanding the seasonal dynamics of phytoplankton biomass and the deep
           chlorophyll maximum in oligotrophic environments: a Bio‐Argo float
           investigation
    • Authors: Alexandre Mignot; Hervé Claustre, Julia Uitz, Antoine Poteau, Fabrizio D'Ortenzio, Xiaogang Xing
      Pages: n/a - n/a
      Abstract: We deployed four Bio‐Argo profiling floats in various oligotrophic locations of the Pacific subtropical gyres and Mediterranean Sea to address the seasonal phytoplankton dynamics in the euphotic layer and explore its dependence on light regime dynamics. Results show that there is a similar phytoplankton biomass seasonal pattern in the four observed oceanic regions. In the lower part of the euphotic layer, the seasonal displacement of the deep chlorophyll maximum (DCM) is light driven. During winter, the chlorophyll a concentration ([Chl a]) always increases in the upper euphotic‐mixed layer. This increase always results from a photoacclimation to the reduced irradiance. Depending on the location, however, the concentration can also be associated with an actual increase in biomass. The winter increase in [Chl a] results in an increase in irradiance attenuation that impacts the position of the isolume (level where the daily integrated photon flux is constant) and DCM, which becomes shallower. In summer when the [Chl a] in the upper layer decreases along with light attenuation, the DCM deepens and becomes closer to (and sometimes reaches) the nitracline, which enhances the phytoplankton biomass at the DCM. The bio‐optical mechanisms and their relationship to light regimes that are revealed by the times series appear to be generic and potentially characteristic of all of the areas where a DCM forms, which is 50% of the open ocean.
      PubDate: 2014-08-01T08:56:39.801395-05:
      DOI: 10.1002/2013GB004781
       
  • Long‐term variability of phytoplankton carbon biomass in the
           Sargasso Sea
    • Authors: Philip J. Wallhead; Véronique C. Garçon, John R. Casey, Michael W. Lomas
      Pages: n/a - n/a
      Abstract: Time series of phytoplankton carbon biomass are scarce yet may provide important insights into ocean productivity and carbon export to depth via the oceanic biological pump. We combine recent flow‐cytometric measurements with pigment concentrations and other standard measurements to reconstruct taxon‐specific phytoplankton carbon biomass in the Sargasso Sea over 22 years, using a multiple regression approach. The reconstructed series reveal an increasing trend (~3% per year) in total phytoplankton carbon, apparently driven by increasing nutrient supply by vertical mixing associated with a shift to a negative phase in the winter North Atlantic Oscillation index. Also, the reconstructed eukaryote biomass fraction shows a multiannual shift from ~45% in the early 1990s/late 2000s to ~70% in the late 1990s/early 2000s. We hypothesize that a multiannual shift in the seasonal pattern of mixing may have stimulated and restructured the eukaryote community while suppressing prokaryote populations by increasing photodamage and grazing mortality.
      PubDate: 2014-07-26T08:59:00.902149-05:
      DOI: 10.1002/2013GB004797
       
  • Worldwide retention of nutrient silicon by river damming: From sparse data
           set to global estimate
    • Authors: Taylor Maavara; Hans H. Dürr, Philippe Van Cappellen
      Pages: n/a - n/a
      Abstract: Damming of rivers represents a major anthropogenic perturbation of the hydrological cycle, with the potential to profoundly modify the availability of nutrient silicon (Si) in streams, lakes and coastal areas. A global assessment of the impact of dams on river Si fluxes, however, is limited by the sparse data set on Si budgets for reservoirs. To alleviate this limitation, we use existing data on dissolved Si (DSi) retention by dams to calibrate a mechanistic model for the biogeochemical cycling of DSi and reactive particulate Si (PSi) in reservoir systems. The model calibration yields a relationship between the annual in‐reservoir siliceous primary productivity and the external DSi supply. With this relationship and an estimate of catchment Si loading, the model calculates the total reactive Si (RSi = DSi + PSi) retention for any given reservoir. A Monte Carlo analysis accounts for the effects of variations in reservoir characteristics, and generates a global relationship that predicts the average reactive Si retention in reservoirs as a function of the water residence time. This relationship is applied to the GRanD database to estimate Si retention by damming worldwide. According to the results, dams retain 163 Gmol yr−1 (9.8 Tg SiO2 yr−1) of DSi and 372 Gmol yr−1 (22.3 Tg SiO2 yr−1) of RSi, or 5.3% of the global RSi loading to rivers.
      PubDate: 2014-07-25T11:44:57.012629-05:
      DOI: 10.1002/2014GB004875
       
  • Evidence for production and lateral transport of dissolved organic
           phosphorus in the eastern subtropical North Atlantic
    • Authors: Sarah Reynolds; Claire Mahaffey, Vassil Roussenov, Richard G. Williams
      Pages: n/a - n/a
      Abstract: The concentration of phosphate and dissolved organic phosphorus (DOP) is chronically low and limits phytoplankton growth in the subtropical North Atlantic relative to other ocean basins. Transport of phosphate and DOP from the productive flanks of the gyre to its interior has been hypothesized as an important phosphorus supply pathway. During a cruise in the eastern Atlantic in spring 2011, the rates of phosphate uptake, alkaline phosphatase (APA) and DOP production were measured in the northwest African shelf region, subtropics and tropics. Rates of DOP production were 6‐fold higher in the shelf region (43 ± 41 nM d−1) relative to the subtropics (6.9 ± 4.4 nM d−1). In contrast, APA production was 3‐fold higher in the subtropics (8.0 ± 7.3 nM d−1), indicative of enhanced DOP utilization, relative to the shelf region (2.6 ± 2.1 nM d−1). Hence, observations suggest net production of DOP in the shelf region and either net consumption of DOP or a near balance in DOP production and consumption in the gyre interior. Eddy‐permitting model experiments demonstrate that (i) DOP accounts for over half the total phosphorus in surface waters, (ii) DOP is transported westward from the shelf region by a combination of gyre and eddy circulations, and (iii) advected DOP supports up to 70% of the particle export over much of the subtropical gyre. Our combined observational and modeling study supports the view that the horizontal transport of DOP from the shelf region is an important mechanism supplying phosphorus to the surface subtropical North Atlantic.
      PubDate: 2014-07-23T08:18:23.989691-05:
      DOI: 10.1002/2013GB004801
       
  • Radium isotopes as a tracer of sediment‐water column exchange in the
           North Sea
    • Authors: W. J. Burt; H. Thomas, J. Pätsch, A. M. Omar, C. Schrum, U. Daewel, H. Brenner, H. J. W. Baar
      Pages: n/a - n/a
      Abstract: Sediment‐water column exchange plays an important role in coastal biogeochemistry. We utilize short‐lived radium isotopes (224Ra and 223Ra) to understand and quantify the dominant processes governing sediment‐water column exchange throughout the North Sea. Our comprehensive survey, conducted in September 2011, represents the first of its kind conducted in the North Sea. We find that two main sources regulate surface Ra distributions: minor coastal input from rivers and shallow mudflats, and North Sea sediments as the dominant source. Porewaters show 100‐fold larger activities than the water column. North Sea sediment characteristics such as porosity and mean grain size, as well as turbulence at the sediment‐water interface, are the dominant factors contributing to variability of Ra efflux. Ra inventory and mass‐balance approaches consistently yield high benthic Ra effluxes in the southern North Sea, driven by strong tidal and wind mixing, which in turn cause high sediment irrigation rates. These results exceed incubation‐based Ra flux estimates, and the majority of previously reported Ra flux estimates for other regions. Ra‐based estimates of benthic alkalinity fluxes compare well to observed values and the high rates of Ra efflux imply a potentially significant exchange of other products of sedimentary reactions, including carbon and nutrient species. Passive tracer simulations lend strong support to the Ra source attribution and imply seasonal variation in the surface water Ra distribution depending on stratification conditions.
      PubDate: 2014-07-18T08:26:53.862128-05:
      DOI: 10.1002/2014GB004825
       
  • Decadal (1994–2008) change in the carbon isotope ratio in the
           eastern South Pacific Ocean
    • Authors: Young Ho Ko; Kitack Lee, Paul D. Quay, Richard A. Feely
      Pages: n/a - n/a
      Abstract: We determined the 14‐year change in the anthropogenic CO2 inventory in the eastern South Pacific Ocean along the 110˚W meridian from 67˚S to 21˚N, using seawater δ13C datasets collected in 1994 and 2008. The vertical integral of the 14‐year δ13C change was assessed in five latitude bands, and found to be greatest (–14.7‰ m yr–1) in the subpolar band (38˚S–55˚S) and smallest (–3.0‰ m yr–1) in the tropical band (21˚N–18˚S). The δ13C change in each of the latitudinal bands was primarily caused by inputs of anthropogenic CO2 via air‐sea exchange and transport. More than 50% of the total anthropogenic CO2 was added to the subpolar band via the northward movement of Antarctic Intermediate Water (AAIW) from the south, and the remaining 50% was added via air‐sea exchange. We also calculated the ratio of the temporal change in δ13C to the change in dissolved inorganic carbon, which is a measure of the efficiency of oceanic uptake of anthropogenic CO2. The ratio for AAIW in 1994 (–0.017‰ (µmol kg–1) –1) was greater than that in 2008 (–0.010‰ (µmol kg–1) –1) based on the change in preformed δ13C and DIC, indicating reduced efficiency of CO2 uptake by the Southern Ocean in 2008 relative to that in 1994. AAIW remained at the surface for a shorter period in 2008 relative to 1994, and thus would have taken up less atmospheric CO2 prior to subduction. The projected reduction in this ratio indicates a weakening of CO2 uptake by the Southern Ocean in the future.
      PubDate: 2014-07-11T01:05:36.589329-05:
      DOI: 10.1002/2013GB004786
       
  • Late summer net community production in the central Arctic Ocean using
           multiple approaches
    • Abstract: Large‐scale patterns of net community production (NCP) were estimated during the late summer cruise ARK‐XXVI/3 (TransArc, Aug/Sep 2011) to the central Arctic Ocean. Several approaches were used based on: (i) continuous measurements of surface water oxygen to argon ratios (O2/Ar), (ii) underway measurements of surface partial pressure of carbon dioxide (pCO2), (iii) discrete samples of dissolved inorganic carbon (DIC), and (iv) dissolved inorganic nitrogen and phosphate. The NCP estimates agreed well within the uncertainties associated with each approach. The highest late summer NCP (up to 6 mol C m−2) was observed in the marginal sea ice zone region. Low values (
       
  • The biogeochemical cycling of zinc and zinc isotopes in the North Atlantic
           Ocean
    • Abstract: Zinc (Zn) is a marine micronutrient, with an overall oceanic distribution mirroring the major macronutrients, especially silicate. Seawater Zn isotope ratios (δ66Zn) are a relatively new oceanographic parameter which may offer insights into the biogeochemical cycling of Zn. To date, the handful of published studies of seawater δ66Zn show the global deep ocean to be both remarkably homogenous (~ + 0.5‰) and isotopically heavier than the marine sources of Zn (+0.1 to +0.3‰). Here, we present the first high‐resolution oceanic section of δ66Zn, from the US GEOTRACES GA03 North Atlantic Transect, from Lisbon to Woods Hole. Throughout the surface ocean, biological uptake and release of isotopically light Zn, together with scavenging of heavier Zn, leads to large variability in δ66Zn. In the ocean below 1000 m, δ66Zn is generally homogenous (0.50 ± 0.14‰; 2SD), though deviations from +0.5‰ allow us to identify specific sources of Zn. The Mediterranean Outflow is characterized by δ66Zn of +0.1 to +0.3‰, whilst margin sediments are a source of isotopically light Zn (‐0.5 to ‐0.8‰), which we attribute to release of non‐regenerated biogenic Zn. Mid‐Atlantic Ridge hydrothermal vents are also a source of light Zn (close to ‐0.5‰), though Zn is not transported far from the vents. Understanding the biogeochemical cycling of Zn in the modern ocean begins to address the imbalance between the light δ66Zn signature of marine sources and the globally homogenous deep oceans (δ66Zn of +0.5‰) on long timescales, with overall patterns pointing to sediments as an important sink for isotopically light Zn throughout the oceans.
       
  • Heterotrophic bacteria are major nitrogen fixers in the euphotic zone of
           the Indian Ocean
    • Abstract: Diazotrophy in the Indian Ocean is poorly understood compared to that in the Atlantic and Pacific Oceans. We first examined the basin‐scale community structure of diazotrophs and their nitrogen fixation activity within the euphotic zone during the northeast monsoon period along about 69°E from 17°N to 20°S in the oligotrophic Indian Ocean, where a shallow nitracline (49–59 m) prevailed widely and the sea surface temperature (SST) was above 25 °C. Phosphate was detectable at the surface throughout the study area. The dissolved iron concentration and the ratio of iron to nitrate + nitrite at the surface were significantly higher in the Arabian Sea than in the equatorial and southern Indian Ocean. Nitrogen fixation in the Arabian Sea (24.6–47.1 μmolN m−2 d−1) was also significantly greater than that in the equatorial and southern Indian Ocean (6.27–16.6 μmolN m−2 d−1), indicating that iron could controll diazotrophy in the Indian Ocean. Phylogenetic analysis of nifH showed that most diazotrophs belonged to the Proteobacteria, and that cyanobacterial diazotrophs were absent in the study area except in the Arabian Sea. Furthermore, nitrogen fixation was not associated with light intensity throughout the study area. These results are consistent with nitrogen fixation in the Indian Ocean being largely performed by heterotrophic bacteria and not by cyanobacteria. The low cyanobacterial diazotrophy was attributed to the shallow nitracline, which is rarely observed in the Pacific and Atlantic oligotrophic oceans. Because the shallower nitracline favored enhanced upward nitrate flux, the competitive advantage of cyanobacterial diazotrophs over non‐diazotrophic phytoplankton was not as significant as it is in other oligotrophic oceans.
       
  • Issue Information
    • Abstract: No abstract is available for this article.
       
  • Isotopic evidence for a marine ammonium source in rainwater at Bermuda
    • Abstract: Emissions of anthropogenic nitrogen (N) to the atmosphere have increased tenfold since preindustrial times, resulting in increased N deposition to terrestrial and coastal ecosystems. The sources of N deposition to the ocean, however, are poorly understood. Two years of event‐based rainwater samples were collected on the island of Bermuda in the western North Atlantic, which experiences both continent‐ and ocean‐influenced air masses. The rainwater ammonium concentration ranged from 0.36 to 24.6 μM, and the ammonium δ15N from ‐12.5 to 0.7‰; and neither have a strong relationship with air mass history (6.0 ± 4.2 μM, ‐4.1 ± 2.6‰ in marine air masses, and 5.9 ± 3.2 μM, ‐5.8 ± 2.5‰ in continental air masses; numerical average ± standard deviation). A simple box model suggests that the ocean can account for the concentration and isotopic composition of ammonium in marine rainwater, consistent with the lack of correlation between ammonium δ15 N and air mass history. If so, ammonium deposition reflects the cycling of N between the ocean and the atmosphere, rather than representing a net input to the ocean. The δ15N data appear to require that most of the ammonium/a flux to the ocean is by dissolution in surface waters rather than atmospheric deposition. This suggests that the atmosphere and surface ocean are near equilibrium with respect to air/sea gas exchange, implying that anthropogenic ammonia will equilibrate near the coast and not reach the open marine atmosphere. Whereas ~90% of the ammonium deposition to the global ocean has previously been attributed to anthropogenic sources, the evidence at Bermuda suggests that the anthropogenic contribution could be much smaller.
       
 
 
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