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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: 5, 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: 21)
J. of Geophysical Research : Biogeosciences     Full-text available via subscription   (Followers: 6)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 22)
J. of Geophysical Research : Oceans     Partially Free   (Followers: 16)
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: 156, 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]
  • Estimating the soil carbon sequestration potential of China's Grain for
           Green Project
    • Authors: Shengwei Shi; Pengfei Han
      Pages: n/a - n/a
      Abstract: The largest area of planted forest in the world has been established in China through implementation of key forestry projects in recent years. These projects have played an important role in sequestering CO2 from the atmosphere, which is considered to be a potential mitigation strategy for the effects of global climate change. However, carbon sequestration in soil (soil organic carbon, SOC) after afforestation or reforestation, is not well understood, particularly for specific key forestry projects. In this study, the SOC change in the top 20 cm of soil for each type of restoration implemented under China's Grain for Green Project (GGP) was quantified by a meta analysis of data from published literature and direct field measurements. Soil carbon sequestration due to the GGP during 1999~2012 was estimated using data on the annual restoration area at provincial level and functions that relate SOC stock change to controlling factors (e.g. plantation age, forest zone and type of forestation). Soil carbon sequestration of the GGP was estimated to be 156±108 Tg C (95% confidence interval) for current restoration areas prior to 2013, with a mean accumulation rate of 12±8 Tg C yr−1. The soil carbon sequestration potential of existing plantation zones is predicted to increase from 156±108 Tg C in 2013 to 383±188 Tg C in 2050 under the assumption that all plantation areas are well‐preserved. Plantations in northwestern, southern and southwestern zones contributed nearly 80% of total soil carbon sequestration, while soil C sequestration in northeastern China was much more variable. Improved data sources, measurements of SOC in the organic layer, greater sampling depth, and better distribution of sampling sites among GPP regions will reduce the uncertainty of the estimates made by this study.
      PubDate: 2014-10-17T00:40:40.737353-05:
      DOI: 10.1002/2014GB004924
       
  • Six centuries of changing oceanic mercury
    • Authors: Yanxu Zhang; Lyatt Jaeglé, LuAnne Thompson, David Streets
      Pages: n/a - n/a
      Abstract: Mercury (Hg) is a global and persistent contaminant, affecting human health primarily via marine fish consumption. Large anthropogenic releases of Hg to the atmosphere by mining and coal combustion have resulted in a significant perturbation to the biogeochemical cycling of Hg. The magnitude of this perturbation and the relative roles of the ocean and land as sinks for anthropogenic Hg remain unclear. Here we use a 3D global ocean biogeochemical model to show that surface ocean Hg concentrations have increased four‐fold over the last 600 years. We find that anthropogenic Hg enters the ocean's interior predominantly by absorption onto sinking organic matter particulates, which decompose and release Hg at a depth of 500‐800 m, implying that the human perturbation is largest in subsurface waters of biologically productive regions. Our model simulation predicts that over the last 6 centuries half of emitted anthropogenic Hg has accumulated in the oceans and marine sediments.
      PubDate: 2014-10-15T23:58:06.508052-05:
      DOI: 10.1002/2014GB004939
       
  • The impact of changing surface ocean conditions on the dissolution of
           aerosol iron
    • Authors: Matthew P. Fishwick; Peter N. Sedwick, Maeve C. Lohan, Paul J. Worsfold, Kristen N. Buck, Thomas M. Church, Simon J. Ussher
      Pages: n/a - n/a
      Abstract: The proportion of aerosol iron (Fe) that dissolves in seawater varies greatly and is dependent on aerosol composition and the physicochemical conditions of seawater, which may change depending on location or be altered by global environmental change. Aerosol and surface seawater samples were collected in the Sargasso Sea and used to investigate the impact of these changing conditions on aerosol Fe dissolution in seawater. Our data show that seawater temperature, pH and oxygen concentration, within the range of current and projected future values, had no significant effect on the dissolution of aerosol Fe. However, the source and composition of aerosols had the most significant effect on the aerosol Fe solubility, with the most anthropogenically influenced samples having the highest fractional solubility (up to 3.2%). The impact of ocean warming and acidification on aerosol Fe dissolution is therefore unlikely to be as important as changes in land usage and fossil fuel combustion. Our experimental results also reveal important changes in the size distribution of soluble aerosol Fe in solution, depending on the chemical conditions of seawater. Under typical conditions, the majority (77 – 100%) of Fe released from aerosols into ambient seawater existed in the colloidal (0.02 – 0.4 μm) size fraction. However, in the presence of a sufficient concentration of strong Fe‐binding organic ligands (10 nM) most of the aerosol‐derived colloidal Fe was converted to soluble Fe (
      PubDate: 2014-10-15T16:51:45.239916-05:
      DOI: 10.1002/2014GB004921
       
  • The significance of the episodic nature of atmospheric deposition to Low
           Nutrient Low Chlorophyll regions
    • Authors: C. Guieu; O. Aumont, A. Paytan, L. Bopp, C.S. Law, N. Mahowald, E. P. Achterberg, E. Marañón, B. Salihoglu, A. Crise, T. Wagener, B. Herut, K. Desboeufs, M. Kanakidou, N. Olgun, F. Peters, E. Pulido‐Villena, A. Tovar‐Sanchez, C. Völker
      Pages: n/a - n/a
      Abstract: In the vast Low Nutrient Low‐Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (
      PubDate: 2014-10-15T16:50:38.347567-05:
      DOI: 10.1002/2014GB004852
       
  • Heterotrophic bacteria are major nitrogen fixers in the euphotic zone of
           the Indian Ocean
    • Authors: Takuhei Shiozaki; Minoru Ijichi, Taketoshi Kodama, Shigenobu Takeda, Ken Furuya
      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.
      PubDate: 2014-10-14T11:23:21.854671-05:
      DOI: 10.1002/2014GB004886
       
  • Isotopic evidence for a marine ammonium source in rainwater at Bermuda
    • Authors: K. E. Altieri; M. G. Hastings, A. J. Peters, S. Oleynik, D. M. Sigman
      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.
      PubDate: 2014-10-10T14:15:04.090548-05:
      DOI: 10.1002/2014GB004809
       
  • Issue Information
    • Abstract: No abstract is available for this article.
      PubDate: 2014-10-08T18:21:30.026508-05:
      DOI: 10.1002/gbc.20085
       
  • Spatial and seasonal variability of the air‐sea equilibration
           timescale of carbon dioxide
    • Authors: Daniel C. Jones; Takamitsu Ito, Yohei Takano, Wei‐Ching Hsu
      Abstract: The exchange of carbon dioxide between the ocean and the atmosphere tends to bring waters within the mixed layer toward equilibrium by reducing the partial pressure gradient across the air‐water interface. However, the equilibration process is not instantaneous; in general there is a lag between forcing and response. The timescale of air‐sea equilibration depends on several factors involving the depth of the mixed layer, wind speed, and carbonate chemistry. We use a suite of observational datasets to generate climatological and seasonal composite maps of the air‐sea equilibration timescale. The relaxation timescale exhibits considerable spatial and seasonal variations that are largely set by changes in mixed layer depth and wind speed.The net effect is dominated by the mixed layer depth; the gas exchange velocity and carbonate chemistry parameters only provide partial compensation. Broadly speaking, the adjustment timescale tends to increase with latitude. We compare the observationally‐derived air‐sea gas exchange timescale with a model‐derived surface residence time and a data‐derived horizontal transport timescale, which allows us to define two non‐dimensional metrics of equilibration efficiency. These parameters highlight the tropics, subtropics, and northern North Atlantic as regions of inefficient air‐sea equilibration where carbon anomalies are relatively likely to persist. The efficiency parameters presented here can serve as simple tools for understanding the large‐scale persistence of air‐sea disequilibrium of CO2 in both observations and models.
      PubDate: 2014-10-08T10:40:43.546212-05:
      DOI: 10.1002/2014GB004813
       
  • Tropical wetlands: A missing link in the global carbon cycle'
    • Authors: Sofie Sjögersten; Colin R Black, Stephanie Evers, Jorge Hoyos‐Santillan, Emma L Wright, Benjamin L Turner
      Abstract: Tropical wetlands are not included in Earth system models, despite being an important source of methane (CH4) and contributing a large fraction of carbon dioxide (CO2) emissions from land use, land use change and forestry in the tropics. This review identifies a remarkable lack of data on the carbon balance and gas fluxes from undisturbed tropical wetlands, which limits the ability of global change models to make accurate predictions about future climate. We show that the available data on in situ carbon gas fluxes in undisturbed forested tropical wetlands indicate marked spatial and temporal variability in CO2 and CH4 emissions, with exceptionally large fluxes in Southeast Asia and the Neotropics. By up‐scaling short term measurements, we calculate that c. 90 ± 77 Tg CH4 yr‐1 and 4540 ± 1480 Tg CO2 yr‐1 are released from tropical wetlands globally. CH4 fluxes are greater from mineral than organic soils, whereas CO2 fluxes do not differ between soil types. The high CO2 and CH4 emissions are mirrored by high rates of net primary productivity and litter decay. Net ecosystem productivity was estimated to be greater in peat‐forming wetlands than on mineral soils, but the available data are insufficient to construct reliable carbon balances or estimate gas fluxes at regional scales. We conclude that there is an urgent need for systematic data on carbon dynamics in tropical wetlands to provide a robust understanding of how they differ from well‐studied northern wetlands and allow incorporation of tropical wetlands into global climate change models.
      PubDate: 2014-10-08T00:21:03.164843-05:
      DOI: 10.1002/2014GB004844
       
  • N2O production in the eastern South Atlantic: analysis of N2O stable
           isotopic and concentration data
    • Authors: Caitlin H. Frame; Eric Deal, Cynthia D. Nevison, Karen L. Casciotti
      Abstract: The stable isotopic composition of dissolved nitrous oxide (N2O) is a tracer for the production, transport, and consumption of this greenhouse gas in the ocean. Here we present dissolved N2O concentration and isotope data from the South Atlantic Ocean, spanning from the western side of the mid Atlantic Ridge to the upwelling zone off the southern African coast. In the eastern South Atlantic, shallow N2O production by nitrifier denitrification contributed a flux of isotopically depleted N2O to the atmosphere. Along the African coast, N2O fluxes to the atmosphere of up to 46 µmol/m2/day were calculated using satellite‐derived QuikSCAT wind speed data, while fluxes at the offshore stations averaged 0.04 µmol/m2/day. Comparison of the isotopic composition of the deeper N2O in the South Atlantic (800 m to 1000 m) to measurements made in other regions suggests that water advected from one or more of the major oxygen deficient zones (ODZs) contributed N2O to the mesopelagic South Atlantic via the Southern Ocean. This deeper N2O was isotopically and isotopomerically enriched (δ15Nbulk‐N2O =8.7 ± 0.1‰, δ18O‐N2O =46.5 ± 0.2‰, and Site Preference =18.7 ± 0.6‰) relative to the shallow N2O source, indicating that N2O consumption by denitrification influenced its isotopic composition. The N2O concentration maximum was observed between 200 m and 400 m and reached 49 nM near the Angolan coast. The depths of the N2O concentration maximum coincided with those of sedimentary particle resuspension along the coast. The isotopic composition of this N2O (δ15Nbulk‐N2O =5.8 ± 0.1‰, δ18O‐N2O =39.7 ± 0.1‰, and Site Preference =9.8 ± 1.0‰) was consistent with production by diffusion‐limited nitrate (NO3−) reduction to nitrite (NO2−), followed by NO2− reduction to N2O by denitrification and/or nitrifier denitrification, with additional N2O production by NH2OH decomposition during NH3 oxidation. The sediment surface, benthic boundary layer, or particles resuspended from the sediments are likely to have provided the physical and chemical conditions necessary to produce this N2O.
      PubDate: 2014-10-07T16:54:40.80458-05:0
      DOI: 10.1002/2013GB004790
       
  • Regionalized global budget of the CO2 exchange at the air‐water
           interface in continental shelf seas
    • Authors: Goulven G. Laruelle; Ronny Lauerwald, Benjamin Pfeil, Pierre Regnier
      Abstract: Over the past decade, estimates of the atmospheric CO2 uptake by continental shelf seas were constrained within the 0.18‐0.45 Pg C yr−1 range. However, most of those estimates are based on extrapolations from limited datasets of local flux measurements (n 
      PubDate: 2014-10-07T16:50:48.51821-05:0
      DOI: 10.1002/2014GB004832
       
  • Separating the influence of temperature, drought and fire on interannual
           variability in atmospheric CO2
    • Authors: Gretchen Keppel‐Aleks; Aaron S. Wolf, Mingquan Mu, Scott C. Doney, Douglas C. Morton, Prasad S. Kasibhatla, John B. Miller, Edward J. Dlugokencky, James T. Randerson
      Abstract: The response of the carbon cycle in prognostic Earth system models (ESMs) contributes significant uncertainty to projections of global climate change. Quantifying contributions of known drivers of interannual variability in the growth rate of atmospheric carbon dioxide (CO2) is important for improving the representation of terrestrial ecosystem processes in these ESMs. Several recent studies have identified the temperature dependence of tropical net ecosystem exchange (NEE) as a primary driver of this variability by analyzing a single, globally averaged time series of CO2 anomalies. Here, we examined how the temporal evolution of CO2 in different latitude bands may be used to separate contributions from temperature stress, drought stress, and fire emissions to CO2 variability. We developed atmospheric CO2 patterns from each of these mechanisms during 1997‐2011 using an atmospheric transport model. NEE responses to temperature, NEE responses to drought, and fire emissions all contributed significantly to CO2 variability in each latitude band, suggesting that no single mechanism was the dominant driver. We found that the sum of drought and fire contributions to CO2 variability exceeded direct NEE responses to temperature in both the Northern and Southern Hemispheres. Additional sensitivity tests revealed that these contributions are masked by temporal and spatial smoothing of CO2 observations. Accounting for fires, the sensitivity of tropical NEE to temperature stress decreased by 25% to 2.9 ± 0.4 Pg C y‐1 K‐1. These results underscore the need for accurate attribution of the drivers of CO2 variability prior to using contemporary observations to constrain long‐term ESM responses.
      PubDate: 2014-10-07T00:54:54.663154-05:
      DOI: 10.1002/2014GB004890
       
  • Production of dissolved organic carbon enriched in deoxy‐sugars
           represents an additional sink for biological C drawdown in the Amazon
           River plume
    • Authors: Travis B. Meador; Lihini I. Aluwihare
      Abstract: In N. Atlantic waters impacted by discharges from the Amazon and Orinoco Rivers, where planktonic diatom‐diazotroph associations (DDA) were active, we observed that an average (±standard deviation) of 61 ± 12% of the biological drawdown of dissolved inorganic carbon (DIC) was partitioned into the accumulating total organic carbon (TOC) pool, representing a flux of up to 9 ± 4 Tg C y‐1. This drawdown corresponded with chemical alteration of ultrafiltered dissolved organic matter (UDOM), including increases in stable C isotopic composition (δ13C) and C:N. The dissolved carbohydrate component of UDOM also increased with biological DIC drawdown and diatom‐associated diazotroph (i.e., Richelia) abundance. New carbohydrates could be distinguished by distinctively high relative abundances of deoxy‐sugars (up to 55% of monosaccharides), which may promote aggregate formation and enhance vertical carbon export. The identified production of non‐Redfieldian, C‐enriched UDOM thus suggests a mechanism to explain enhanced C‐sequestration associated with DDA N2 fixation, which may be widespread in mesohaline environments.
      PubDate: 2014-09-29T08:05:05.326179-05:
      DOI: 10.1002/2013GB004778
       
  • Contrasting vulnerability of drained tropical and high‐latitude
           peatlands to fluvial loss of stored carbon
    • Authors: Chris D. Evans; Susan E. Page, Tim Jones, Sam Moore, Vincent Gauci, Raija Laiho, Jakub Hruška, Tim E.H. Allott, Michael F. Billett, Ed Tipping, Chris Freeman, Mark H. Garnett
      Abstract: Carbon sequestration and storage in peatlands rely on consistently high water tables. Anthropogenic pressures including drainage, burning, land conversion for agriculture, timber and biofuel production, cause loss of peat‐forming vegetation and exposure of previously anaerobic peat to aerobic decomposition. This can shift peatlands from net CO2 sinks to large CO2 sources, releasing carbon held for millennia. Peatlands also export significant quantities of carbon via fluvial pathways, mainly as dissolved organic carbon (DOC). We analysed radiocarbon (14C) levels of DOC in drainage water from multiple peatlands in Europe and Southeast Asia, to infer differences in the age of carbon lost from intact and drained systems. In most cases, drainage led to increased release of older carbon from the peat profile, but with marked differences related to peat type. Very low DOC‐14C levels in runoff from drained tropical peatlands indicate loss of very old (centuries to millennia) stored peat carbon. High latitude peatlands appear more resilient to drainage; 14C measurements from UK blanket bogs suggest that exported DOC remains young (500 year) carbon in high‐latitude systems. Re‐wetting at least partially offsets drainage effects on DOC age.
      PubDate: 2014-09-29T08:04:55.447074-05:
      DOI: 10.1002/2013GB004782
       
  • Late summer net community production in the central Arctic Ocean using
           multiple approaches
    • Authors: Adam Ulfsbo; Nicolas Cassar, Meri Korhonen, Steven Heuven, Mario Hoppema, Gerhard Kattner, Leif G. Anderson
      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 (
      PubDate: 2014-09-24T08:52:56.11678-05:0
      DOI: 10.1002/2014GB004833
       
  • The biogeochemical cycling of zinc and zinc isotopes in the North Atlantic
           Ocean
    • Authors: Tim M. Conway; Seth G. John
      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.
      PubDate: 2014-09-23T00:05:50.273756-05:
      DOI: 10.1002/2014GB004862
       
  • 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
       
  • 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
       
 
 
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