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

Geochemistry, Geophysics, Geosystems     Full-text available via subscription   (Followers: 25, SJR: 2.56, h-index: 69)
Geophysical Research Letters     Full-text available via subscription   (Followers: 53, SJR: 3.493, h-index: 157)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 3.239, h-index: 119)
J. of Advances in Modeling Earth Systems     Open Access   (Followers: 2, SJR: 1.944, h-index: 7)
J. of Geophysical Research : Atmospheres     Partially Free   (Followers: 22)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 14)
J. of Geophysical Research : Planets     Full-text available via subscription   (Followers: 13)
J. of Geophysical Research : Solid Earth     Full-text available via subscription   (Followers: 26)
J. of Geophysical Research : Space Physics     Full-text available via subscription   (Followers: 15)
Paleoceanography     Full-text available via subscription   (Followers: 3, SJR: 3.22, h-index: 88)
Radio Science     Full-text available via subscription   (Followers: 3, SJR: 0.959, h-index: 51)
Reviews of Geophysics     Full-text available via subscription   (Followers: 20, SJR: 9.68, h-index: 94)
Space Weather     Full-text available via subscription   (Followers: 3, SJR: 1.319, h-index: 19)
Tectonics     Full-text available via subscription   (Followers: 9, SJR: 2.748, h-index: 85)
Water Resources Research     Full-text available via subscription   (Followers: 82, SJR: 2.189, h-index: 121)
Journal Cover   Global Biogeochemical Cycles
  [SJR: 3.239]   [H-I: 119]   [5 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]
  • Issue Information
    • Abstract: Cover: In Randerson et al. [doi 10.1002/2014GB005079], surface air temperature changes (in units of K) and climate change impacts on vertically integrated carbon stocks (kg C m–2) at 2100 and 2300 from simulations by the Community Earth System Model. By 2300, climate‐induced reductions in ocean carbon were largest in the Atlantic and Arctic Oceans, whereas on land impacts were largest in tropical forests of Central and South America. See pp. 744–759.
      PubDate: 2015-07-15T13:02:09.747653-05:
      DOI: 10.1002/gbc.20197
       
  • Soluble iron inputs to the Southern Ocean through recent andesitic to
           rhyolitic volcanic ash eruptions from the Patagonian Andes
    • Authors: L.E. Simonella; M.E. Palomeque, P.L. Croot, A. Stein, M. Kupczewski, A. Rosales, M.L. Montes, F. Colombo, M.G. García, G. Villarosa, D.M. Gaiero
      Abstract: Patagonia, due to its geographic position and the dominance of westerly winds, is a key area that contributes to the supply of nutrients to the Southern Ocean, both through mineral dust as well as the periodic deposits of volcanic ash. Here we evaluate the characteristics of Fe dissolved (into soluble and colloidal species) from volcanic ash for three recent southern Andes volcanic eruptions having contrasting features and chemical compositions. Contact between cloud waters (wet deposition) and end‐members of andesitic (Hudson volcano) and rhyolitic (Chaitén volcano) materials was simulated. Results indicate higher Fe release and faster liberation rates in the andesitic material. Fe release during particle‐seawater interaction (dry deposition), have higher rates in rhyolitic type ashes. Rhyolitic ashes under acidic conditions release Fe in higher amounts and at a slower rate, while in those samples containing mostly glass shards, Fe release was lower and faster. The 2011 Puyehue eruption was observed by a dust monitoring station. Puyehue‐type eruptions can contribute soluble Fe to the ocean via dry or wet deposition, nearly reaching the limit required for phytoplankton growth. In contrast, the input of Fe after processing by an acidic eruption plume could raise the amount of dissolved Fe in surface ocean waters several times, above the threshold required to initiate phytoplankton blooms. A single eruption like the Puyehue one represents more than half of the yearly Fe‐flux contributed by dust.
      PubDate: 2015-07-14T10:58:03.974088-05:
      DOI: 10.1002/2015GB005177
       
  • Short‐term variability in euphotic zone biogeochemistry and primary
           productivity at Station ALOHA: A case study of summer 2012
    • Authors: Samuel T. Wilson; Benedetto Barone, Francois Ascani, Robert R. Bidigare, Matthew J. Church, Daniela A. Valle, Sonya T. Dyhrman, Sara Ferrón, Jessica N. Fitzsimmons, Laurie W. Juranek, Zbigniew S. Kolber, Ricardo M. Letelier, Sandra Martínez‐Garcia, David P. Nicholson, Kelvin J. Richards, Yoshimi M. Rii, Mónica Rouco, Donn A. Viviani, Angelicque E. White, Jonathan P. Zehr, David M. Karl
      Abstract: Time‐series observations are critical to understanding the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time‐series (HOT) program has maintained near‐monthly sampling at Station ALOHA (22° 45′N, 158° 00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and identified ecosystem variability over seasonal to interannual time‐scales. To further extend the temporal resolution of these near‐monthly time‐series observations, an extensive field campaign was conducted during July‐September 2012 at Station ALOHA with near‐daily sampling of upper water‐column biogeochemistry, phytoplankton abundance, and activity. The resulting dataset provides biogeochemical measurements at high temporal resolution and documented two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea‐surface salinity minimum feature which was prominent in the upper water‐column (0–50 m) for a period of approximately 30 days. The shipboard observations during July‐September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea‐surface salinity minimum feature in the NPSG.
      PubDate: 2015-07-14T10:31:10.947074-05:
      DOI: 10.1002/2015GB005141
       
  • Response of the Amazon carbon balance to the 2010 drought derived with
           CarbonTracker South America
    • Authors: I.T. Laan‐Luijkx; I.R. Velde, M.C. Krol, L.V. Gatti, L.G. Domingues, C.S.C. Correia, J.B. Miller, M. Gloor, T.T. Leeuwen, J.W. Kaiser, C. Wiedinmyer, S. Basu, C. Clerbaux, W. Peters
      Abstract: Two major droughts in the past decade had large impacts on carbon exchange in the Amazon. Recent analysis of vertical profile measurements of atmospheric CO2 and CO by Gatti et al. [2014] suggests that the 2010 drought turned the normally close‐to‐neutral annual Amazon carbon balance into a substantial source of nearly 0.5 PgC/yr, revealing a strong drought response. In this study, we revisit this hypothesis and interpret not only the same CO2/CO vertical profile measurements, but also additional constraints on carbon exchange such as satellite observations of CO, burned area, and fire hotspots. The results from our CarbonTracker South America data assimilation system suggest that carbon uptake by vegetation was indeed reduced in 2010, but that the magnitude of the decrease strongly depends on the estimated 2010 and 2011 biomass burning emissions. We have used fire products based on burned area (GFED4), satellite‐observed CO columns (IASI), fire radiative power (GFASv1) and fire hotspots (FINNv1), and found an increase in biomass burning emissions in 2010 compared to 2011 of 0.16 to 0.24 PgC/yr. We derived a decrease of biospheric uptake ranging from 0.08 to 0.26 PgC/yr, with the range determined from a set of alternative inversions using different biomass burning estimates. Our numerical analysis of the 2010 Amazon drought results in a total reduction of carbon uptake of 0.24 to 0.50 PgC/yr and turns the balance from carbon sink to source. Our findings support the suggestion that the hydrological cycle will be an important driver of future changes in Amazonian carbon exchange.
      PubDate: 2015-07-02T14:44:12.074049-05:
      DOI: 10.1002/2014GB005082
       
  • Combined Impact of Catchment Size, Land Cover and Precipitation on
           Streamflow and Total Dissolved Nitrogen: A Global Comparative Analysis
    • Authors: Erika L. Gallo; Tom Meixner, Hadi Aoubid, Kathleen A. Lohse, Paul D. Brooks
      Abstract: Nitrogen (N) loading is a global stressor to fresh and salt water systems with cascading effects on ecosystem processes [Galloway et al., 2003; Galloway et al., 2004]. However, it is unclear if generalized global response patterns exist between discharge and N sourcing and retention with respect to land cover and precipitation. Using data compiled from 78 catchments from across the world, we identified how discharge and total dissolved nitrogen (TDN) vary with precipitation and land cover; and how TDN yields deviate from a generalized global response pattern. Area‐weighted discharge regressions indicate that catchment size and the absence of vegetation largely control hydrologic responses. TDN concentrations and yields varied significantly (p
      PubDate: 2015-07-02T14:43:33.096705-05:
      DOI: 10.1002/2015GB005154
       
  • Water mass mixing is the dominant control on the zinc distribution in the
           North Atlantic Ocean
    • Authors: Saeed Roshan; Jingfeng Wu
      Abstract: Dissolved zinc (dZn) concentration was determined in the North Atlantic during the US GEOTRACES 2010&2011 cruise (GOETRACES GA03). A relatively poor linear correlation (R2=0.756) was observed between dZn and silicic acid (Si) the slope of which was 0.0577 nM:µmol/kg. We attribute the relatively poor dZn‐Si correlation to the following processes: a. differential regeneration of zinc relative to silicic acid; b. mixing of multiple water masses that have different Zn/Si; c. zinc sources such as sedimentary or hydrothermal. To quantitatively distinguish these possibilities, we use the results of Optimum Multi‐Parameter Water Mass Analysis by Jenkins et al. [2015] to model the zinc distribution below 500m. We hypothesized two scenarios: conservative mixing and regenerative mixing. The first scenario (conservative) could be modeled to results in a correlation with observations with a R2=0.846. In the second scenario, we took a Si‐related regeneration into account which could model the observations with a R2=0.867. Through this regenerative mixing scenario, we estimated a Zn/Si=0.0548 nM:µmol/kg that may be more realistic than linear regression slope due to accounting for process b. However, this did not improve the model substantially (R2=0.867 vs.0.846) which may indicate the insignificant effect of remineralization on the zinc distribution in this region. The relative weakness in the model‐observation correlation (R2~0.85 for both scenarios) imply that processes a and c may be plausible. Furthermore, dZn in the upper 500m exhibited a very poor correlation with apparent oxygen utilization (AOU) suggesting a minimal role for the organic matter‐associated remineralization process.
      PubDate: 2015-07-01T12:37:23.505278-05:
      DOI: 10.1002/2014GB005026
       
  • Transports and budgets of anthropogenic CO2 in the tropical North Atlantic
           in 1992–93 and 2010–11
    • Authors: Patricia Zunino; Fiz F. Pérez, Noelia M. Fajar, Elisa F. Guallart, Aida Ríos, Josep L. Pelegrí, A. Hernández‐Guerra
      Abstract: The meridional transport of anthropogenic CO2 (Cant) in the tropical North Atlantic (TNA) is investigated using data from transoceanic sections along 7.5°N and 24.5 °N, carried out in the early 1990s and 2010s. The net Cant transport across both sections is northward. At 7.5°N, this transport increased from 315 ± 47 kmol s−1 in 1993 to 493 ± 51 kmol s−1 in 2010; similarly, across 24.5°N it grew from 530 ± 46 kmol s−1 in 1992 to 662 ± 49 kmol s−1 in 2011. These changes result from modifications in the intermediate and deep circulation patterns, as well as from Cant increase within the thermocline waters. In deep waters, lateral advection causes a net Cant input of 112 ± 60 kmol s−1 (234 ± 65 kmol s−1) in 1992–93 (2010–11); within these deep waters, the storage rate of Cant is not statistically different from the net Cant input, 139 ± 21 kmol s−1 (188 ± 21 kmol s−1) in 1992–93 (2010–11). The Cant increase in deep waters is due to the large injection of Cant across the 24.5°N by the Deep Western Boundary Current and the northward recirculation of North Atlantic Deep Water along 7.5°N. In contrast, a large net Cant output in the upper layer is caused by the Florida Current. Despite this net Cant output, the Cant accumulates at a rate of 215 ± 24 kmol s−1 (291 ± 24 kmol s−1) referenced to year 1993 (2010). From the two Cant budgets, we infer a Cant air‐sea flux of 0.23 ± 0.02 Pg yr−1in the TNA, much larger than previous estimates.
      PubDate: 2015-06-22T09:12:46.349955-05:
      DOI: 10.1002/2014GB005075
       
  • A new look at ocean carbon remineralization for estimating
           deep‐water sequestration
    • Authors: Lionel Guidi; Louis Legendre, Gabriel Reygondeau, Julia Uitz, Lars Stemmann, Stephanie A. Henson
      Abstract: The “biological carbon pump” causes carbon sequestration in deep waters by downward transfer of organic matter, mostly as particles. This mechanism depends to a great extent on the uptake of CO2 by marine plankton in surface waters, and subsequent sinking of particulate organic carbon (POC) through the water column. Most of the sinking POC is remineralized during its downward transit and modest changes in remineralization have substantial feedbacks on atmospheric CO2 concentrations, but little is known about global variability in remineralization. Here we assess this variability based on modern underwater particle imaging combined with field POC flux data, and discuss the potential sources of variations. We show a significant relationship between remineralization and the size structure of the phytoplankton assemblage. We obtain the first regionalized estimates of remineralization in biogeochemical provinces, where these estimates range between ‐50 and +100% of the commonly used globally uniform remineralization value. We apply the regionalized values to satellite‐derived estimates of upper ocean POC export to calculate regionalized and ocean‐wide deep carbon fluxes and sequestration. The resulting value of global organic carbon sequestration at 2000 m is 0.33 Pg C y‐1, and 0.72 Pg C y‐1 at the depth of the top of the permanent pycnocline, which is up to 3 times higher than the value resulting from the commonly‐used approach based on uniform remineralization and constant sequestration depth. These results stress that variable remineralization length scale and sequestration depth should be used to model ocean carbon sequestration and feedbacks on the atmosphere.
      PubDate: 2015-06-18T19:25:21.63808-05:0
      DOI: 10.1002/2014GB005063
       
  • Why are biotic iron pools uniform across high‐ and low‐iron
           pelagic ecosystems'
    • Authors: P.W. Boyd; R.F. Strzepek, M.J. Ellwood, D.A. Hutchins, S.D. Nodder, B.S. Twining, S.W. Wilhelm
      Abstract: Dissolved iron supply is pivotal in setting global phytoplankton productivity and pelagic ecosystem structure. However, most studies of the role of iron have focussed on carbon biogeochemistry within pelagic ecosystems, with less effort to quantify the iron biogeochemical cycle. Here, we compare mixed‐layer biotic iron inventories from a low‐iron (~0.06 nmol L−1) subantarctic (FeCycle study) and a seasonally high‐iron (~0.6 nmol L−1) subtropical (FeCycle II study) site. Both studies were quasi‐Lagrangian, and had multi‐day occupation, common sampling protocols, and indirect estimates of biotic iron (from a limited range of available published biovolume/carbon/iron quotas). Biotic iron pools were comparable (~100± 30 pmol L−1) for low‐ and high‐iron waters, despite a ten‐fold difference in dissolved iron concentrations. Consistency in biotic iron inventories (~80±24 pmol L−1, largely estimated using a limited range of available quotas) was also conspicuous for three Southern Ocean polar sites. Insights into the extent to which uniformity in biotic iron inventories was driven by the need to apply common iron quotas obtained from laboratory cultures were provided from FeCycle II. The observed two‐ to three‐fold range of iron quotas during the evolution of FeCycle II subtropical bloom was much less than reported from laboratory monocultures. Furthermore, the iron recycling efficiency varied by fourfold during FeCycle II, increasing as stocks of new iron were depleted, suggesting that quotas and iron recycling efficiencies together set biotic iron pools. Hence, site‐specific differences in iron recycling efficiencies (which provide 20‐50% and 90% of total iron supply in high and low iron waters, respectively) helps offset the differences in new iron inputs between low‐ and high‐iron sites. Future parameterisation of iron in biogeochemical models must focus on the drivers of biotic iron inventories, including the differing iron requirements of the resident biota, and the subsequent fate (retention/export/recycling) of the biotic iron.
      PubDate: 2015-06-17T08:03:43.288577-05:
      DOI: 10.1002/2014GB005014
       
  • Origin and fluxes of nitrous oxide along a latitudinal transect in western
           North Pacific: Controls and regional significance
    • Authors: Florian Breider; Chisato Yoshikawa, Hitomi Abe, Sakae Toyoda, Naohiro Yoshida
      Abstract: Nitrous oxide (N2O) is an atmospheric trace gas playing an important role in both radiative forcing and stratospheric ozone depletion. The oceans are the second most important natural source of N2O. The magnitude of the flux of this source is poorly constrained. Moreover the relative importance of the microbial processes leading to the formation or the consumption of N2O in oceans remains unclear. We present here fluxes, isotope and isotopomer signatures of N2O measured at three stations located along a latitudinal transect in subtropical and subarctic Western North Pacific. These results indicate that about 30% to 55% of the oceanic flux of N2O to the atmosphere originates from the deep euphotic and shallow aphotic zones. The sea‐to‐air fluxes of N2O calculated using an isotope mass balance model indicate that the emission rate of N2O in subarctic waters is about two times higher than in oligotrophic subtropical waters suggesting that nutrient‐rich water coming from the western subarctic gyre stimulates the N2O production. Moreover, isotopomer analysis has revealed that in shallow water N2O originates from nitrification and nitrifier‐denitrification processes and its distribution in the water column is partly controlled by the incident solar radiation. The results of this study contribute to better constrain the global N2O budget and provide important information to better predict the future evolution of the oceanic emissions of N2O.
      PubDate: 2015-06-17T08:03:26.97157-05:0
      DOI: 10.1002/2014GB004977
       
  • Biological and physical controls on N2, O2 and CO2 distributions in
           contrasting Southern Ocean surface waters
    • Authors: Philippe D. Tortell; Henry C. Bittig, Arne Körtzinger, Elizabeth M. Jones, Mario Hoppema
      Abstract: We present measurements of pCO2, O2 concentration, biological oxygen saturation (ΔO2/Ar) and N2 saturation (ΔN2) in Southern Ocean surface waters during austral summer, 2010–2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chla) concentrations in regions of frontal mixing and sea‐ice melt. pCO2 and ΔO2 /Ar exhibited large spatial gradients (range 90 to 450 µatm and −10 to 60%, respectively) and co‐varied strongly with Chla. However, the ratio of biological O2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time‐scales of O2 and CO2 air‐sea equilibration. We measured significant oceanic CO2 uptake, with a mean air‐sea flux (~ −10 mmol m−2 d−1) that significantly exceeded regional climatological values. N2 was mostly supersaturated in surface waters (mean ΔN2 of +2.5 %), while physical processes resulted in both supersaturation and undersaturation of mixed layer O2 (mean ΔO2phys = 2.1 %). Box model calculations were able to reproduce much of the spatial variability of ΔN2 and ΔO2phys along the cruise track, demonstrating significant effects of air‐sea exchange processes (e.g. atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment‐corrected surface ΔO2 /Ar data, ranged from ~ −40 to > 300 mmol O2 m−2 d−1 and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and stratified regions of sea‐ice melt, reflecting physical controls on surface water light fields and nutrient availability.
      PubDate: 2015-06-16T06:18:50.485864-05:
      DOI: 10.1002/2014GB004975
       
  • Flux and budget of black carbon (BC) in the continental shelf seas
           adjacent to Chinese high BC emission source regions
    • Authors: Yin Fang; Yingjun Chen, Chongguo Tian, Tian Lin, Limin Hu, Guopei Huang, Jianhui Tang, Jun Li, Gan Zhang
      Abstract: This study conducted the first comprehensive investigation of sedimentary black carbon (BC) concentration, flux and budget in the continental shelves of “Bohai Sea (BS) and Yellow Sea (YS)”, based on measurements of BC in 191 surface sediments, 36 riverine water and 2 seawater samples, as well as the reported dataset of the atmospheric samples from seven coastal cities in the Bohai Rim. BC concentrations in these matrices were measured using the method of thermal/optical reflectance. The spatial distribution of the BC concentration in surface sediments was largely influenced by the regional hydrodynamic conditions, with high values mainly occurring in the central mud areas where fine‐grained particles (median diameters >6 Φ (i.e.,
      PubDate: 2015-06-10T10:42:21.623058-05:
      DOI: 10.1002/2014GB004985
       
  • Multi‐decadal trends of oxygen and their controlling factors in the
           western North Pacific
    • Authors: Daisuke Sasano; Yusuke Takatani, Naohiro Kosugi, Toshiya Nakano, Takashi Midorikawa, Masao Ishii
      Abstract: The rate of change of dissolved oxygen (O2) concentrations was analyzed over 1987–2011 for the high‐frequency repeat section along 165°E in the western North Pacific. Significant trends towards decreasing O2 were detected in the northern subtropical to subtropical‐subarctic transition zones over a broad range of isopycnal horizons. On 25.3σθ between 25°N‐30°N in North Pacific Subtropical Mode Water, the rate of O2 decrease reached −0.45 ± 0.16 µmol kg−1 yr−1. It is largely attributed to a deepening of isopycnal horizons and to a reduction in oxygen solubility associated with ocean warming. In North Pacific Intermediate Water, the rate of O2 decrease was elevated (−0.44 ± 0.14 µmol kg−1 yr−1 on 26.8σθ) and was associated with net increases in apparent oxygen utilization in the source waters. On 27.3σθ in the subtropical Oxygen Minimum Layer (OML) between 32.5°N‐35°N, the rate of O2 decrease was significant (−0.22 ± 0.05 µmol kg−1 yr−1). It was likely due to the increases in westward transport of low‐oxygen water. These various drivers controlling changes in O2 along the 165°E section are the same as those acting along 137°E (analyzed previously), and also account for the differences in the rate of O2 decrease between these sections. Additionally, in the tropical OML near 26.8σθ between 5°N‐10°N, significant trends toward increasing O2 were detected in both sections (+0.36 ± 0.04 µmol kg−1 yr−1 in the 165°E section). These results demonstrate that warming and circulation changes are causing multi‐decadal changes in dissolved O2 over wide expanses of the western North Pacific.
      PubDate: 2015-06-08T21:58:52.516376-05:
      DOI: 10.1002/2014GB005065
       
  • WATER MASS AGE AND AGEING DRIVING CHROMOPHORIC DISSOLVED ORGANIC MATTER IN
           THE DARK GLOBAL OCEAN
    • Authors: T. S. Catalá; I. Reche, M. Álvarez, S. Khatiwala, E.F. Guallart, V. M. Benítez‐Barrios, A. Fuentes‐Lema, C. Romera‐Castillo, M. Nieto‐Cid, C. Pelejero, E. Fraile‐Nuez, E. Ortega‐Retuerta, C. Marrasé, X. A. Álvarez‐Salgado
      Abstract: The omnipresence of chromophoric dissolved organic matter (CDOM) in the open ocean enables its use as a tracer for biochemical processes throughout the global overturning circulation. We made an inventory of CDOM optical properties, ideal water age (τ) and apparent oxygen utilization (AOU) along the Atlantic, Indian and Pacific Ocean waters sampled during the Malaspina 2010 expedition. A water mass analysis was applied to obtain intrinsic, hereinafter archetypal, values of τ, AOU, oxygen utilisation rate (OUR), and CDOM absorption coefficients, spectral slopes and quantum yield for each one of the 22 water types intercepted during this circumnavigation. Archetypal values of AOU and OUR have been used to trace the differential influence of water mass ageing and ageing rates, respectively, on CDOM variables. Whereas the absorption coefficient at 325nm (a325) and the fluorescence quantum yield at 340 nm (Φ340) increased, the spectral slope over the wavelength range 275–295 nm (S275–295) and the ratio of spectral slopes over the ranges 275 –295 nm and 350–400 nm (SR) decreased significantly with water mass ageing (AOU). Combination of the slope of the linear regression between archetypal AOU and a325 with the estimated global OUR allowed us to obtain a CDOM turnover time of 634 ± 120 years, which exceeds the flushing time of the dark ocean (>200 m) by 46%. This positive relationship supports the assumption of in situ production and accumulation of CDOM as a by‐product of microbial metabolism as water masses turn older. Furthermore, our data evidence that global‐scale CDOM quantity (a325) is more dependant on ageing (AOU), whereas CDOM quality (S275–295, SR, Φ340) is more dependent on ageing rate (OUR).
      PubDate: 2015-06-02T20:46:12.123557-05:
      DOI: 10.1002/2014GB005048
       
  • On the influence of “non‐Redfield” dissolved organic
           nutrient dynamics on the spatial distribution of N2 fixation and the size
           of the marine fixed nitrogen inventory
    • Authors: Christopher J. Somes; Andreas Oschlies
      Abstract: Dissolved organic nitrogen (DON) and phosphorus (DOP) represent the most abundant form of their respective nutrient pool in the surface layer of the oligotrophic oceans and play an important role in nutrient cycling and productivity. Since DOP is generally more labile than DON, it provides additional P that may stimulate growth of N2‐fixing diazotrophs that supply fixed nitrogen to balance denitrification in the ocean. In this study, we introduce semi‐recalcitrant components of DON and DOP as state variables in an existing global ocean–atmosphere‐sea ice‐biogeochemistry model of intermediate complexity to assess their impact on the spatial distribution of N2‐fixation and the size of the marine fixed nitrogen inventory. Large‐scale surface datasets of global DON and Atlantic Ocean DOP are used to constrain the model. Our simulations suggest that both preferential DOP remineralization and phytoplankton DOP uptake are important “non‐Redfield” processes (i.e., deviate from molar N:P=16) that need to be accounted for to explain the observed patterns of DOP. Additional non‐Redfield DOP sensitivity experiments testing DOM production rate uncertainties that best reproduce the observed spatial patterns of DON and DOP stimulate additional N2‐fixation that increases the size of the global marine fixed nitrogen inventory by 4.7±1.7% compared to the simulation assuming Redfield DOM stoichiometry that underestimates the observed nitrogen inventory. The extra 8 Tg yr−1 of N2‐fixation stimulated in the Atlantic Ocean is mainly responsible for this increase due to its large spatial separation from water column denitrification, which buffers any potential nitrogen surplus in the Pacific Ocean. Our study suggests that the marine fixed nitrogen budget is sensitive to non‐Redfield DOP dynamics because access to the relatively labile DOP pool expands the ecological niche for N2‐fixing diazotrophs.
      PubDate: 2015-05-25T05:59:17.465865-05:
      DOI: 10.1002/2014GB005050
       
  • New model for capturing the variations of fertilizer‐induced
           emission factors of N2O
    • Authors: Feng Zhou; Ziyin Shang, Zhenzhong Zeng, Shilong Piao, Philippe Ciais, Peter A. Raymond, Xuhui Wang, Rong Wang, Minpeng Chen, Changliang Yang, Shu Tao, Yue Zhao, Qian Meng, Shuoshuo Gao, Qi Mao
      Abstract: Accumulating evidence indicates that N2O emission factors (EFs) vary with nitrogen additions and environmental variations. Yet the impact of the latter was often ignored by previous EF determinations. We developed piecewise statistical models (PMs) to explain how the N2O EFs in agricultural soils depend upon various predictors such as climate, soil attributes, and agricultural management. The PMs are derived from a new Bayesian Recursive Regression Tree algorithm. The PMs were applied to the case of EFs from agricultural soils in China, a country where large EF spatial gradients prevail. The results indicate substantial improvements of the PMs compared with other EF determinations. First, PMs are able to reproduce a larger fraction of the variability of observed EFs for upland grain crops (84%, n=381) and paddy rice (91%, n=161) as well as the ratio of EFs to nitrogen application rates (73%, n=96). The superior predictive accuracy of PMs is further confirmed by evaluating their predictions against independent EF measurements (n=285) from outside China. Results show that the PMs calibrated using Chinese data can explain 75% of the variance. Hence the PMs could be reliable for upscaling of N2O EFs and fluxes for regions that have a phase‐space of predictors similar to China. Results from the validated models also suggest that climatic factors regulate the heterogeneity of EFs in China, explaining 69% and 85% of their variations for upland grain crops and paddy rice, respectively. The corresponding N2O EFs in 2008 are 0.84±0.18% (as N2O‐N emissions divided by the total N input) for upland grain crops and 0.65±0.14% for paddy rice, the latter being twice as large as the IPCC Tier 1 defaults. Based upon these new estimates of EFs, we infer that only 22% of current arable land could achieve a potential reduction of N2O emission of 50%.
      PubDate: 2015-05-20T06:32:07.117159-05:
      DOI: 10.1002/2014GB005046
       
  • Temperature, oxygen, and vegetation controls on decomposition in a James
           Bay peatland
    • Authors: Michael Philben; James Holmquist, Glen MacDonald, Dandan Duan, Karl Kaiser, Ronald Benner
      First page: 729
      Abstract: The biochemical composition of a peat core from James Bay Lowland, Canada was used to assess the extent of peat decomposition and diagenetic alteration. Our goal was to identify environmental controls on peat decomposition, particularly its sensitivity to naturally occurring changes in temperature, oxygen exposure time, and vegetation. All three varied substantially during the last 7000 yr, providing a natural experiment for evaluating their effects on decomposition. The bottom 50 cm of the core formed during the Holocene Climatic Optimum (~7000‐4000 yr B.P.), when mean annual air temperature was likely 1‐2°C warmer than present. A reconstruction of the water table level using testate amoebae indicated oxygen exposure time was highest in the subsequent upper portion of the core between 150 and 225 cm depth (from ~2560‐4210 yr B.P) and the plant community shifted from mostly Sphagnum to vascular plant dominance. Several independent biochemical indices indicated decomposition was greatest in this interval. Hydrolysable amino acid yields, hydroxyproline yields, and acid:aldehyde ratios of syringyl lignin phenols were higher, while hydrolysable neutral sugar yields and carbon:nitrogen ratios were lower in this zone of both vascular plant vegetation and elevated oxygen exposure time. Thus, peat formed during the Holocene Climatic Optimum did not appear to be more extensively decomposed than peat formed during subsequent cooler periods. Comparison with a core from the West Siberian Lowland, Russia, indicates oxygen exposure time and vegetation are both important controls on decomposition, while temperature appears to be of secondary importance. The low apparent sensitivity of decomposition to temperature is consistent with recent observations of a positive correlation between peat accumulation rates and mean annual temperature, suggesting contemporary warming could enhance peatland carbon sequestration, although this could be offset by an increasing contribution of vascular plants to the vegetation.
      PubDate: 2015-05-06T14:47:47.620213-05:
      DOI: 10.1002/2014GB004989
       
  • Multi‐century changes in ocean and land contributions to
           climate‐carbon feedbacks
    • Authors: J. T. Randerson; K. Lindsay, E. Munoz, W. Fu, J. K. Moore, F. M. Hoffman, N. M. Mahowald, S. C. Doney
      First page: 744
      Abstract: Improved constraints on carbon cycle responses to climate change are needed to inform mitigation policy, yet our understanding of how these responses may evolve after 2100 remains highly uncertain. Using the Community Earth System Model (v1.0), we quantified climate‐carbon feedbacks from 1850 to 2300 for the Representative Concentration Pathway 8.5 and its extension. In three simulations, land and ocean biogeochemical processes experienced the same trajectory of increasing atmospheric CO2. Each simulation had a different degree of radiative coupling for CO2 and other greenhouse gases and aerosols, enabling diagnosis of feedbacks. In a fully coupled simulation, global mean surface air temperature increased by 9.3 K from 1850 to 2300, with 4.4 K of this warming occurring after 2100. Excluding CO2, warming from other greenhouse gases and aerosols was 1.6 K by 2300, near a 2 K target needed to avoid dangerous anthropogenic interference with the climate system. Ocean contributions to the climate‐carbon feedback increased considerably over time, and exceeded contributions from land after 2100. The sensitivity of ocean carbon to climate change was found to be proportional to changes in ocean heat content, as a consequence of this heat modifying transport pathways for anthropogenic CO2 inflow and solubility of dissolved inorganic carbon. By 2300 climate change reduced cumulative ocean uptake by 330 Pg C, from 1410 Pg C to 1080 Pg C. Land fluxes similarly diverged over time, with climate change reducing stocks by 232 Pg C. Regional influence of climate change on carbon stocks was largest in the North Atlantic Ocean and tropical forests of South America. Our analysis suggests that after 2100, oceans may become as important as terrestrial ecosystems in regulating the magnitude of climate‐carbon feedbacks.
      PubDate: 2015-04-21T17:01:21.272412-05:
      DOI: 10.1002/2014GB005079
       
  • Metals and Metalloids in Precipitation Collected during CHINARE Campaign
           from Shanghai, China to Zhongshan Station, Antarctica: Spatial Variability
           and Source Identification
    • Authors: G. Shi; J. Teng, H. Ma, Y. Li, B. Sun
      First page: 760
      Abstract: Metals and metalloids in continental precipitation have been widely observed, but the data over open oceans are still very limited. Investigation of metals and metalloids in marine precipitation is of great significance to understand global transport of these elements in the atmosphere and their input fluxes to the oceans. So, shipboard sampling of precipitation was conducted during a Chinese National Antarctic Research Expedition (CHINARE) campaign from Shanghai, China to Zhongshan Station, East Antarctica, and 22 samples (including 17 rainfall and 5 snowfall events) were collected and analyzed for concentrations of Pb, Ni, Cr, Cu, Co, Hg, As, Cd, Sb, Se, Zn, Mn and Ti. Results show that concentrations of both metals and metalloids vary considerably along the cruise, with higher concentrations at coastal sites and lower values on the south Indian Ocean. Although only soluble fractions were determined for elements, concentrations in this study are generally comparable to the reported values of marine rain. Enrichment factor analysis shows that most of metals and metalloids are enriched versus crustal sources, even in the samples collected from remote south Indian Ocean. In addition, metals and metalloids in precipitation are also very enriched above sea‐salt abundance, indicating that impacts of sea salt aerosols on their concentrations are negligible. Main sources of metals and metalloids were explored with the aid of multivariate statistical analyses. The results show that human emissions have far‐reaching distribution, which may exert an important influence on the solubility of elements in precipitation. This investigation provides valuable information on spatial variation and possible sources of trace elements in precipitation over the open oceans corresponding to understudied region.
      PubDate: 2015-05-14T05:34:17.232202-05:
      DOI: 10.1002/2014GB005060
       
  • Global Patterns and controls of soil organic carbon dynamics as simulated
           by multiple terrestrial biosphere models: current status and future
           directions
    • Authors: Hanqin Tian; Chaoqun Lu, Jia Yang, Kamaljit Banger, Deborah N. Huntinzger, Christopher R. Schwalm, Anna M. Michalak, Robert Cook, Philippe Ciais, Daniel Hayes, Maoyi Huang, Akihiko Ito, Atul Jain, Huimin Lei, Jiafu Mao, Shufen Pan, Wilfred M. Post, Shushi Peng, Benjamin Poulter, Wei Ren, Daniel Ricciuto, Kevin Schaefer, Xiaoying Shi, Bo Tao, Weile Wang, Yaxing Wei, Qichun Yang, Bowen Zhang, Ning Zeng
      First page: 775
      Abstract: Soil is the largest organic carbon (C) pool of terrestrial ecosystems, and C loss from soil accounts for a large proportion of land‐atmosphere C exchange. Therefore, a small change in soil organic C (SOC) can affect atmospheric carbon dioxide (CO2) concentration and climate change. In the past decades, a wide variety of studies have been conducted to quantify global SOC stocks and soil C exchange with the atmosphere through site measurements, inventories, and empirical/process‐based modeling. However, these estimates are highly uncertain and identifying major driving forces controlling soil C dynamics remains a key research challenge. This study has compiled century‐long (1901–2010) estimates of SOC storage and heterotrophic respiration (Rh) from ten terrestrial biosphere models (TBMs) in the Multi‐scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) and two observation‐based datasets. The ten‐TBM ensemble shows that global SOC estimate range from 425 to 2111 Pg C (1 Pg = 1015 g) with a median value of 1158 Pg C in 2010. The models estimate a broad range of Rh from 35 to 69 Pg C yr−1 with a median value of 51 Pg C yr−1 during 2001–2010. The largest uncertainty in SOC stocks exists in the 40–65°N latitude whereas the largest differences in Rh between models are in the tropics. The modeled SOC change during 1901–2010 ranges from −70 Pg C to 86 Pg C, but in some models the SOC change has a different sign from the change of total C stock, implying very different contribution of vegetation and soil pools in determining the terrestrial C budget among models. The model ensemble‐estimated mean residence time of SOC shows a reduction of 3.4 years over the past century, which is primarily caused by the increment in proportion of labile substrate which accelerate C cycling through the land biosphere. All the models agreed that climate and land use changes decreased SOC stocks while elevated atmospheric CO2 and nitrogen deposition over intact ecosystems increased SOC stocks – even though the responses varied significantly among models. Model representations of temperature and moisture sensitivity, nutrient limitation and land use partially explain the divergent estimates of global SOC stocks and soil C fluxes in this study. In addition, a major source of systematic error in model estimations relates to non‐modeled SOC storage in wetlands and peatlands, as well as to old C storage in deep soil layers.
      PubDate: 2015-05-11T05:31:54.306869-05:
      DOI: 10.1002/2014GB005021
       
  • N‐loss isotope effects in the Peru oxygen minimum zone studied using
           a mesoscale eddy as a natural tracer experiment
    • Authors: Annie Bourbonnais; Mark A. Altabet, Chawalit N. Charoenpong, Jennifer Larkum, Haibei Hu, Hermann W. Bange, Lothar Stramma
      First page: 793
      Abstract: Mesoscale eddies in Oxygen Minimum Zones (OMZ's) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N‐loss as well as the ocean's N isotope budget. They also represent ‘natural tracer experiments’ with intensified biogeochemical signals that can be exploited to understand the large‐scale processes that control N‐loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus the heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−) and biogenic N2 associated with an anticyclonic eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2‐N concentrations were also the highest near the eddy's center (up to ~40 µmol L−1). δ15N‐N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N‐loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N‐loss was calculated to be only ~13‐14‰ (as compared to canonical values of ~20‐30‰) assuming a closed system and only slightly higher assuming an open system (16‐19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε for NO3− reduction to NO2− and NO2− reduction to N2 of ~16‐21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2‐, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for NO3− and NO2− reduction as well as N‐loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N‐loss.
      PubDate: 2015-04-25T08:41:06.583857-05:
      DOI: 10.1002/2014GB005001
       
  • Towards a parameterization of global‐scale organic carbon
           mineralization kinetics in surface marine sediments
    • Authors: K. Stolpovsky; A. W. Dale, K. Wallmann
      First page: 812
      Abstract: An empirical function is derived for predicting the rate‐depth profile of particulate organic carbon (POC) degradation in surface marine sediments including the bioturbated layer. The rate takes the form of a power law analogous to the Middelburg function. The functional parameters were optimized by simulating measured benthic O2 and NO3− fluxes at 185 stations worldwide using a diagenetic model. The novelty of this work rests with the finding that the vertically‐resolved POC degradation rate in the bioturbated zone can be determined using a simple function where the POC rain rate is the governing variable. Although imperfect, the model is able to fit 71 % of paired O2 and NO3− fluxes to within 50% of measured values. It further provides realistic geochemical concentration‐depth profiles, NO3− penetration depths and apparent first‐order POC mineralization rate constants. The model performs less well on the continental shelf due to the high heterogeneity there. When applied to globally resolved maps of rain rate, the model predicts a global denitrification rate of 182 ± 88 Tg yr−1 of N and a POC burial rate of 107 ± 52 Tg yr−1 of C with a mean carbon burial efficiency of 6.1%. These results are in very good agreement with published values. Our proposed function is conceptually simple, requires less parameterization than multi‐G type models and is suitable for non‐steady state applications. It provides a basis for more accurately simulating benthic nutrient fluxes and carbonate dissolution rates in Earth system models.
      PubDate: 2015-05-14T05:33:54.810907-05:
      DOI: 10.1002/2015GB005087
       
  • Processes Controlling the Distributions of Cd and PO4 in the Ocean
    • Authors: Paul Quay; Jay Cullen, William Landing, Peter Morton
      First page: 830
      Abstract: Depth profiles of dissolved Cd and PO4 from a global data compilation including recent GEOTRACES and CLIVAR cruises were used to derive the Cd/P of particles exported from the surface layer and the results indicate lowest values in the North Atlantic (0.17±0.05), highest in the Southern (0.56±0.24) and intermediate in the South Indian (0.31±0.14) and North Pacific (0.36±0.08) Ocean basins. The Cd/P of exported particles in high nutrient‐low chlorophyll (HNLC) regions is twice that for particles exported in non‐HNLC regions as is the fractionation effect during biological uptake of Cd and PO4 and these trends primarily determine the spatial trends of dissolved Cd/PO4 observed in the surface ocean. In deep waters the lowest dissolved Cd/PO4 of 0.23±0.07 is found in the North Atlantic Ocean and the result primarily of low Cd/PO4 of North Atlantic Deep Water (0.23). In contrast, deep waters in the Southern Ocean have significantly higher dissolved Cd/PO4 (0.30±.06) which is a result of the Cd/PO4 of upwelled deep water from the South Pacific and South Indian (0.27) and the high Cd/P of degrading particles. A multi‐box model that accounts for the impacts of particle degradation and thermohaline circulation in the deep sea yields dissolved Cd and PO4 inter‐basin trends close to observations. Model experiments illustrate the dependence of the dissolved Cd/PO4 of the deep sea on the extent of HNLC conditions in the Southern Ocean and the impact on reconstructing paleo PO4 concentrations from a Cd proxy.
      PubDate: 2015-06-17T16:39:28.617165-05:
      DOI: 10.1002/2014GB004998
       
  • Quantifying the relative importance of land cover change from climate and
           land‐use in the representative concentration pathways
    • Authors: T. Davies‐Barnard; P. J. Valdes, J. S. Singarayer, A. J. Wiltshire, C. D. Jones
      First page: 842
      Abstract: Climate change is projected to cause substantial alterations in vegetation distribution, but these have been given little attention in comparison to land‐use in the Representative Concentration Pathway (RCP) scenarios. Here we assess the climate‐induced land cover changes (CILCC) in the RCPs, and compare them to land‐use land cover change (LULCC). To do this, we use an ensemble of simulations with and without LULCC in earth system model HadGEM2‐ES for RCP2.6, RCP4.5 and RCP8.5. We find that climate change causes an expansion poleward of vegetation that affects more land area than LULCC in all of the RCPs considered here. The terrestrial carbon changes from CILCC are also larger than for LULCC. When considering only forest, the LULCC is larger, but the CILCC is highly variable with the overall radiative forcing of the scenario. The CILCC forest increase compensates 90% of the global anthropogenic deforestation by 2100 in RCP8.5, but just 3% in RCP2.6. Overall, bigger land cover changes tend to originate from LULCC in the shorter term or lower radiative forcing scenarios, and from CILCC in the longer term and higher radiative forcing scenarios. The extent to which CILCC could compensate for LULCC raises difficult questions regarding global forest and biodiversity offsetting, especially at different timescales. This research shows the importance of considering the relative size of CILCC to LULCC, especially with regard to the ecological effects of the different RCPs.
      PubDate: 2015-06-22T14:24:52.982011-05:
      DOI: 10.1002/2014GB004949
       
  • Biogeochemical drivers of the fate of riverine mercury discharged to the
           global and Arctic oceans
    • Authors: Yanxu Zhang; Daniel J. Jacob, Stephanie Dutkiewicz, Helen M. Amos, Michael S. Long, Elsie M. Sunderland
      First page: 854
      Abstract: Rivers discharge 28±13 Mmol a−1 of mercury (Hg) to ocean margins, an amount comparable to atmospheric deposition to the global oceans. Most of the Hg discharged by rivers is sequestered by burial of benthic sediment in estuaries or the coastal zone, but some is evaded to the atmosphere and some is exported to the open ocean. We investigate the fate of riverine Hg by developing a new global 3‐D simulation for Hg in the MIT ocean general circulation model (MITgcm). The model includes plankton dynamics and carbon respiration (DARWIN project model) coupled to inorganic Hg chemistry. Results are consistent with observed spatial patterns and magnitudes of surface ocean Hg concentrations. We use observational constraints on seawater Hg concentrations and evasion to infer that most Hg from rivers is sorbed to refractory organic carbon and preferentially buried. Only 6% of Hg discharged by rivers (1.8 Mmol a−1) is transported to the open ocean on a global basis. This fraction varies from a low of 2.6% in East Asia due to the barrier imposed by the Korean Peninsula and Japanese Archipelago, up to 25% in eastern North America facilitated by the Gulf Stream. In the Arctic Ocean, low tributary particle loads and efficient degradation of particulate organic carbon by deltaic microbial communities favors a more labile riverine Hg pool. Evasion of Hg to the Arctic atmosphere is indirectly enhanced by heat transport during spring freshet that accelerates sea‐ice melt and ice rafting. Discharges of 0.23 Mmol Hg a −1 from Arctic rivers can explain the observed summer maximum in the Arctic atmosphere and this magnitude of releases is consistent with recent observations. Our work indicates that rivers are major contributors to Hg loads in the Arctic Ocean.
      PubDate: 2015-06-25T13:54:46.531055-05:
      DOI: 10.1002/2015GB005124
       
  • Environmental controls on the biogeography of diazotrophy and
           Trichodesmium in the Atlantic Ocean
    • Authors: J. T. Snow; C. Schlosser, E.M.S. Woodward, M.M. Mills, E. P. Achterberg, C.A. Mahaffey, T.S. Bibby, C.M. Moore
      First page: 865
      Abstract: The cyanobacterium Trichodesmium is responsible for a significant proportion of the annual 'new' nitrogen introduced into the global ocean. Despite being arguably the best studied marine diazotroph, the factors controlling the distribution and growth of Trichodesmium remain a subject of debate, with sea surface temperature, the partial pressure of CO2 and nutrients including iron (Fe) and phosphorus (P), all suggested to be important. Synthesising data from 7 cruises collectively spanning large temporal and spatial scales across the Atlantic Ocean, including 2 previously unreported studies crossing the largely under‐sampled South Atlantic gyre, we assessed the relationship between proposed environmental drivers and both community N2 fixation rates and the distribution of Trichodesmium. Simple linear regression analysis would suggest no relationship between any of the sampled environmental variables and N2 fixation rates. However, considering the concentrations of iron and phosphorus together within a simplified resource‐ratio framework, illustrated using an idealised numerical model, indicates the combined effects these nutrients have on Trichodesmium and broader diazotroph biogeography, alongside the reciprocal maintenance of different biogeographic provinces of the (sub)‐tropical Atlantic in states of Fe or P oligotrophy by diazotrophy. The qualitative principles of the resource‐ratio framework are argued to be consistent with both the previously described North–south Atlantic contrast in Trichodesmium abundance and the presence and consequence of a substantial non‐Trichodesmium diazotrophic community in the western South Atlantic subtropical gyre. A comprehensive, observation‐based explanation of the interactions between Trichodesmium and the wider diazotrophic community with iron and phosphorus in the Atlantic Ocean is thus revealed.
      PubDate: 2015-05-23T16:25:44.14183-05:0
      DOI: 10.1002/2015GB005090
       
  • Spatial and temporal contrasts in the distribution of crops and pastures
           across Amazonia: A new agricultural land‐use dataset from census
           data since 1950
    • Authors: P. Imbach; M. Manrow, E. Barona, A. Barretto, G. Hyman, P Ciais
      First page: 898
      Abstract: Amazonia holds the largest continuous area of tropical forests with intense land use change dynamics inducing water, carbon and energy feedbacks with regional and global impacts. Much of our knowledge of land‐use change in Amazonia comes from studies of the Brazilian Amazon, which accounts for two thirds of the region. Amazonia outside of Brazil has received less attention because of the difficulty of acquiring consistent data across countries. We present here an agricultural statistics database of the entire Amazonia region, with a harmonized description of crops and pastures in geospatial format, based on administrative boundary data at the municipality level. The spatial coverage includes countries within Amazonia and spans censuses and surveys from 1950 to 2012. Harmonized crop and pasture types are explored by grouping annual and perennial cropping systems, C3 and C4 photosynthetic pathways, planted and natural pastures, and main crops. Our analysis examined the spatial pattern of ratios between classes of the groups and their correlation with the agricultural extent of crops and pastures within administrative units of the Amazon, by country and census/survey dates. Significant correlations were found between all ratios and the fraction of agricultural lands of each administrative unit, with the exception of planted to natural pastures ratio and pasture lands extent. Brazil and Peru in most cases have significant correlations for all ratios analyzed even for specific census and survey dates. Results suggested improvements and potential applications of the database for carbon, water, climate and land use change studies are discussed. The database presented here provides an Amazon‐wide improved data set on agricultural dynamics with expanded temporal and spatial coverage.
      PubDate: 2015-05-05T13:31:30.542301-05:
      DOI: 10.1002/2014GB004999
       
  • Criteria for rejection of papers without review
    •  
  • A thank you to our GBC Reviewers
    •  
  • Influence of ENSO and the NAO on Terrestrial Carbon Uptake in the
           Texas‐northern Mexico Region
    • Abstract: Climate extremes such as drought and heat waves can cause substantial reductions in terrestrial carbon uptake. Advancing projections of the carbon uptake response to future climate extremes depends on (1) identifying mechanistic links between the carbon cycle and atmospheric drivers, (2) detecting and attributing uptake changes, and (3) evaluating models of land response and atmospheric forcing. Here, we combine model simulations, remote sensing products, and ground observations to investigate the impact of climate variability on carbon uptake in the Texas‐northern Mexico region. Specifically, we (1) examine the relationship between drought, carbon uptake, and variability of ENSO and the NAO using JULES biosphere simulations from 1950–2012, (2) quantify changes in carbon uptake during record drought conditions in 2011, and (3) evaluate JULES carbon uptake and soil moisture in 2011 using observations from remote sensing and a network of flux towers in the region. Long term simulations reveal systematic decreases in regional‐scale carbon uptake during negative phases of ENSO and NAO, including amplified reductions of gross primary production (GPP) (−0.42 ± 0.18 Pg C yr‐1) and net ecosystem production (NEP) (−0.14 ± 0.11 Pg C yr‐1) during strong La Niña years. The 2011 mega‐drought caused some of the largest declines of GPP (−0.50 Pg C yr‐1) and NEP (−0.23 Pg C yr‐1) in our simulations. In 2011, consistent declines were found in observations, including high correlation of GPP and surface soil moisture (r = 0.82 ± 0.23, p = 0.012) in remote sensing based products. These results suggest a large‐scale response of carbon uptake to ENSO and NAO, and highlight a need to improve model predictions of ENSO and NAO in order to improve predictions of future impacts on the carbon cycle and the associated feedbacks to climate change.
       
  • Sources of new nitrogen in the Indian Ocean
    • Abstract: Quantifying the different sources of nitrogen (N) within the N cycle is crucial to gain insights in oceanic phytoplankton production. To understand the controls of primary productivity and the associated capture of CO2 through photosynthesis in the southeastern Indian Ocean we compiled the physical and biogeochemical data from 4 voyages conducted in 2010, 2011, 2012 and 2013. Overall higher NH4+ assimilation rates (~530 μmol m−2 h−1) relative to NO3− assimilation rates (~375 μmol m−2 h−1) suggest that the assimilation dynamics of C are primarily regulated by microbial regeneration in our region. N2 fixation rates did not decline when other source of dissolved inorganic nitrogen (DIN) were available, although the assimilation of N2 is a highly energetic process. Our data showed that the diazotrophic community assimilated ~2 nmol N L−1 h−1 at relative elevated NH4+ assimilation rates ~12 nmol L−1 h−1 and NO3− assimilation rates ~6 nmol L−1 h−1. The small diffusive deep‐water NO3− fluxes could not support the measured NO3− assimilation rates and consequently point towards another source of dissolved inorganic NO3−. Highest NO2− values coincided consistently with shallow lower dissolved O2layers (100–200 m; 100‐180μmol L−1). These results suggest that nitrification above the pycnocline could be a significant component of the N cycle in the eastern Indian Ocean. In our analysis we provide a conceptual understanding how NO3− in the photic zone could be derived from new N through N2‐fixation. We conclude with the hypothesis that N injected through N2 fixation can be recycled within the photic zone as NH4+, and sequentially oxidized to NO2− and NO3− in shallow lower DO layers.
       
  • Relevance of methodological choices for accounting of land use change
           carbon fluxes
    • Abstract: Accounting for carbon fluxes from land use and land cover change (LULCC) generally requires choosing from multiple options of how to attribute the fluxes to regions and to LULCC activities. Applying a newly‐developed and spatially‐explicit bookkeeping model BLUE (bookkeeping of land use emissions) we quantify LULCC fluxes and attribute them to land‐use activities and countries by a range of different accounting methods. We present results with respect to a Kyoto Protocol‐like “commitment” accounting period, using land use emissions of 2008‐12 as example scenario. We assess the effect of accounting methods that vary (1) the temporal evolution of carbon stocks, (2) the state of the carbon stocks at the beginning of the period, (3) the temporal attribution of carbon fluxes during the period, and (4) treatment of LULCC fluxes that occurred prior to the beginning of the period. We show that the methodological choices result in grossly different estimates of carbon fluxes for the different attribution definitions.
       
  • Understanding large‐extent controls of soil organic carbon storage
           in relation to soil depth and soil‐landscape systems
    • Abstract: In this work we aimed at developing a conceptual framework in which we improve our understanding of the controlling factors for soil organic carbon (SOC) over vast areas at different depths. We postulated that variability in SOC may be better explained by modelling SOC within soil‐landscape systems (SLSs). The study was performed in mainland France and explanatory SOC models were developed for the sampled topsoil (0–30 cm) and subsoil (>30 cm), using both directed and undirected data mining techniques. With this study we demonstrated that there is a shift in controlling factors both in space and depth which were mainly related to 1) typical SLSs characteristics and 2) human induced changes to SLSs. The controlling factors in relation to depth alter from predominantly biotic to more abiotic with increasing depth. Especially, water availability, soil texture and physical protection control deeper stored SOC. In SLSs with similar SOC levels, different combinations of physical protection, the input of organic matter and climatic conditions largely determined the SOC level. The SLSs approach provided the means to partition the data into datasets that were having homogenous conditions with respect to this combination of controlling factors. This information may provide important information on the carbon storage and sequestration potential of a soil.
       
  • Recent Amazon Climate as background for possible ongoing and future
           changes of Amazon humid forests
    • Abstract: Recent analyses of Amazon runoff and gridded precipitation data suggest an intensification of the hydrological cycle over the past few decades in the following sense: wet‐season precipitation and peak river runoff (since ∼ 1980) as well as annual‐mean precipitation (since ∼ 1990) have increased while dry‐season precipitation and minimum runoff have slightly decreased. There has also been an increase in the frequency of anomalously severe floods and droughts. Here we extend and expand these analyses to characterize recent climate state and change, as a background for possible ongoing and future changes of these forests. The contrasting recent changes in wet and dry season precipitation have continued and are generally consistent with changes in catchment‐level peak and minimum river runoff as well as a positive trend of water vapour inflow into the basin. Consistent with the river records the increased vapour inflow is concentrated to the wet season. Temperature has been rising by 0.7∘C since 1980 with more pronounced warming during dry months. Suggestions for the cause of the observed changes of the hydrological cycle come from patterns in tropical sea surface temperatures (SST's). Tropical and North Atlantic SST's have increased rapidly and steadily since 1990, while Pacific SST's have shifted from a negative Pacific Decadal Oscillation (PDO) phase (approximately pre 1990) with warm eastern Pacific temperatures to a positive phase with cold eastern Pacific temperatures. These SST conditions have been shown to be associated with an increase in precipitation over most of the Amazon except the south and south‐west. If ongoing changes continue we expect these to be generally beneficial for forests in those regions where there is an increase in precipitation with the exception of floodplain forests. An increase in flood‐pulse height and duration could lead to increased mortality at higher levels of the floodplain and, over the long term, to a lateral shift of the zonally stratified floodplain forest communities. Negative effects on forests are mainly expected in the south‐west and south, which have become slightly drier and hotter, consistent with tree mortality trends observed at the RAINFOR forest plot census network.
       
  • The mutual importance of anthropogenically and climate induced changes in
           global vegetation cover for future land carbon emissions in the
           MPI‐ESM CMIP5 simulations
    • 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.
       
  • Ocean nutrient pathways associated with the passage of a storm
    • Abstract: Storms that affect ocean surface layer dynamics and primary production are a frequent occurrence in the open North Atlantic Ocean. In this study we use an interdisciplinary dataset collected in the region to quantify nutrient supply by two pathways associated with a storm event: entrainment of nutrients during a period of high wind forcing and subsequent shear‐spiking at the pycnocline due to interactions of storm generated inertial currents with wind. The post‐storm increase in surface layer nitrate (by ~20 mmol m−2) was predominantly driven by the first pathway: nutrient intrusion during the storm. Alignment of post‐storm inertial currents and surface wind stress caused shear instabilities at the ocean pycnocline, forming the second pathway for nutrient transport into the euphotic zone. During the alignment period, pulses of high turbulent nitrate flux through the pycnocline (up to 1 mmol m−2 day−1; approximately 25 times higher than the background flux) were detected. However, the impact of the post‐storm supply was an order of magnitude lower than during the storm due to the short duration of the pulses. Cumulatively, the storm passage was equivalent to 2.5‐5 % of the nitrate supplied by winter convection and had a significant effect compared to previously reported (sub)‐mesoscale dynamics in the region. As storms occur frequently, they can form an important component in local nutrient budgets.
       
  • Sea‐air CO2 exchange in the western Arctic coastal ocean
    • Abstract: The biogeochemical seascape of the western Arctic coastal ocean is in rapid transition. Changes in sea ice cover will be accompanied by alterations in sea‐air carbon dioxide (CO2) exchange, of which the latter has been difficult to constrain owing to sparse temporal and spatial datasets. Previous assessments of sea‐air CO2 flux have targeted specific sub‐regional areas of the western Arctic coastal ocean. Here a holistic approach is taken to determine the net sea‐air CO2 flux over this broad region. We compiled and analyzed an extensive dataset of nearly 600,000 surface seawater CO2 partial pressure (pCO2) measurements spanning 2003 through 2014. Using space‐time co‐located, reconstructed atmospheric pCO2 values coupled with the seawater pCO2 dataset, monthly climatologies of sea‐air pCO2 differences (∆pCO2) were created on a 0.2° latitude x 0.5° longitude grid. Sea‐air CO2 fluxes were computed using the ∆pCO2 grid and gas transfer rates calculated from a climatology of wind speed second moments. Fluxes were calculated with and without the presence of sea ice, treating sea ice as an imperfect barrier to gas exchange. This allowed for carbon uptake by the western Arctic coastal ocean to be assessed under existing and reduced sea ice cover conditions, in which carbon uptake increased 30% over the current 10.9 ± 5.7 Tg C (1 Tg = 1012 g) yr−1 of sea ice adjusted exchange in the region. This assessment extends beyond previous sub‐regional estimates in the region in an all‐inclusive manner, and points to key unresolved aspects that must be targeted by future research.
       
  • Global oceanic emission of ammonia: constraints from seawater and
           atmospheric observations
    • Abstract: Current global inventories of ammonia emissions identify the ocean as the largest natural source. This source depends on seawater pH, temperature, and the concentration of total seawater ammonia (NHx(sw)), which reflects a balance between remineralization of organic matter, uptake by plankton, and nitrification. Here, we compare [NHx(sw)] from two global ocean biogeochemical models (BEC and COBALT) against extensive ocean observations. Simulated [NHx(sw)] are generally biased high. Improved simulation can be achieved in COBALT by increasing the plankton affinity for NHx within observed ranges. The resulting global ocean emissions is 2.5 TgN a−1, much lower than current literature values(7–23 TgN a−1), including the widely used GEIA inventory (8 TgN a−1). Such a weak ocean source implies that continental sources contribute more than half of atmospheric NHx over most of the ocean in the Northern hemisphere. Ammonia emitted from oceanic sources is insufficient to neutralize sulfate aerosol acidity, consistent with observations. There is evidence over the Equatorial Pacific for a missing source of atmospheric ammonia that could be due to photolysis of marine organic nitrogen at the ocean surface or in the atmosphere. Accommodating this possible missing source yields a global ocean emission of ammonia in the range 2–5 TgN a−1, comparable in magnitude to other natural sources from open fires and soils.
       
  • Decoupling of net community and export production on submesoscales in the
           Sargasso Sea
    • Abstract: Determinations of the net community production (NCP) in the upper ocean and the particle export production (EP) should balance over long time and large spatial scales. However, recent modeling studies suggest that a horizontal decoupling of flux‐regulating processes on submesoscales (≤10 km) could lead to imbalances between individual determinations of NCP and EP. Here we sampled mixed‐layer biogeochemical parameters and proxies for NCP and EP during ten, high spatial‐resolution (~2 km) surface transects across strong physical gradients in the Sargasso Sea. We observed strong biogeochemical and carbon flux variability in nearly all transects. Spatial coherence among measured biogeochemical parameters within transects was common, but rarely did the same parameters co‐vary consistently across transects. Spatial variability was greater in parameters associated with higher trophic levels, such as chlorophyll in > 5.0 µm particles, and variability in EP exceeded that of NCP in nearly all cases. Within sampling transects, coincident EP and NCP determinations were uncorrelated. However when averaged over each transect (30 to 40 km in length), we found NCP and EP to be significantly and positively correlated (R = 0.72, p = 0.04). Transect‐averaged EP determinations were slightly smaller than similar NCP values (Type‐II regression slope of 0.93, std = 0.32); but not significantly different from a 1:1 relationship. The results show the importance of appropriate sampling scales when deriving carbon flux budgets from upper ocean observations.
       
 
 
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