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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: 7)
J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 23)
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: 3, 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: 167, 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]
  • Inherited hypoxia: A new challenge for reoligotrophicated lakes under
           global warming
    • Authors: Jean‐Philippe Jenny; Fabien Arnaud, Benjamin Alric, Jean‐Marcel Dorioz, Pierre Sabatier, Michel Meybeck, Marie‐Elodie Perga
      Abstract: The Anthropocene is characterised by a worldwide spread of hypoxia, among others manifestations, which threatens aquatic ecosystem functions, services and biodiversity. The primary cause of hypoxia onset in recent decades is human‐triggered eutrophication. Global warming has also been demonstrated to contribute to the increase of hypoxic conditions. However, the precise role of both environmental forcings on hypoxia dynamics over the long term remains mainly unknown due to a lack of historical monitoring. In this study, we used an innovative paleolimnological approach on three large European lakes to quantify past hypoxia dynamics and to hierarchies the contributions of climate and nutrients. Even for lake ecosystems that have been well‐oxygenated over a millennia‐long period, and regardless of past climatic fluctuations, a shift to hypoxic conditions occurred in the 1950s in response to an unprecedented rise in total phosphorus concentrations above 10 ± 5 µg P L−1. Following this shift, hypoxia never disappeared despite the fact that environmental policies succeeded in drastically reducing lake phosphorus concentrations. During that period, decadal fluctuations in hypoxic volume were great, ranging between 0.5 and 8% of the total lake volumes. We demonstrate, through statistical modelling, that these fluctuations were essentially driven by climatic factors, such as river discharge and air temperature. In Lakes Geneva and Bourget, which are fed by large river systems, fluctuations in hypoxic volume were negatively correlated with river discharge. In contrast, the expansion of hypoxia has been related only to warmer air temperatures at Annecy, which is fed by small river systems. Hence, we outline a theoretical framework assuming that restored lake ecosystems have inherited hypoxia from the eutrophication period and have shifted to a new stable state with new key controls of water and ecosystem quality. We suggest that controlling river discharge may be a complementary strategy for local management of lakes fed by large river systems.
      PubDate: 2014-11-14T00:14:55.396392-05:
      DOI: 10.1002/2014GB004932
  • Sources and Fate of Polycyclic Aromatic Hydrocarbons in the Antarctic and
           Southern Ocean Atmosphere
    • Authors: Ana Cabrerizo; Cristóbal Galbán‐Malagón, Sabino Del Vento, Jordi Dachs
      Abstract: Polycyclic aromatic hydrocarbons (PAHs) are a geochemically relevant family of semivolatile compounds originating from fossil fuels, biomass burning and their incomplete combustion, as well as biogenic sources. Even though PAHs are ubiquitous in the environment, there are no previous studies of their occurrence in the Southern Ocean and Antarctic atmosphere. Here we show the gas and aerosol phase PAHs concentrations obtained from three sampling cruises in the Southern Ocean (Weddell, Bellingshausen and South Scotia Seas) and two sampling campaigns at Livingston Island (Southern Shetlands). This study shows an important variability of the atmospheric concentrations with higher concentrations in the South Scotia and northern Weddell Seas than in the Bellingshausen Sea. The assessment of the gas‐particle partitioning of PAHs suggests that aerosol elemental carbon contribution is modest due to its low concentrations. Over the ocean, the atmospheric concentrations do not show a temperature dependence which is consistent with an important role of long range atmospheric transport of PAHs. Conversely, over land at Livingston Island, the PAHs gas phase concentrations increase when the temperature increases, consistently with the presence of local diffusive sources. The use of fugacity samplers allowed the determination of the air‐soil and air‐snow fugacity ratios of PAHs showing that there is a significant volatilization of lighter molecular weight PAHs from soil and snow during the austral summer. The higher volatilization observed in correspondence of sites where the organic matter content in soil is higher suggests that there may be a biogenic source of some PAHs. The volatilization of PAHs from soil and snow is sufficient to support the atmospheric occurrence of PAHs over land, but may have a modest regional influence on the atmospheric occurrence of PAHs over the Southern Ocean.
      PubDate: 2014-11-13T07:13:10.289003-05:
      DOI: 10.1002/2014GB004910
  • Issue Information
    • Abstract: No abstract is available for this article.
      PubDate: 2014-11-12T09:37:52.299773-05:
      DOI: 10.1002/gbc.20143
  • Sensitivity of tropical forest aboveground productivity to climate
           anomalies in SW Costa Rica
    • Authors: Florian Hofhansl; Johannes Kobler, Joachim Ofner, Sigrid Drage, Eva‐Maria Pölz, Wolfgang Wanek
      Pages: n/a - n/a
      Abstract: The productivity of tropical forests is driven by climate (precipitation, temperature, light) and soil fertility (geology, topography). While large‐scale drivers of tropical productivity are well established, knowledge on the sensitivity of tropical lowland net primary production to climate anomalies remains scarce. We here analyze seven consecutive years of monthly‐recorded tropical forest aboveground net primary production (ANPP) in response to a recent El Niño Southern Oscillation (ENSO) anomaly. The ENSO transition period resulted in increased temperatures and decreased precipitation during the El Niño dry‐period, causing a decrease in ANPP. However, the subsequent La Niña wet‐period caused strong increases in ANPP such that drought‐induced reductions were overcompensated. Most strikingly, the climatic controls differed between canopy production (CP) and wood production (WP). Whereas CP showed strong seasonal variation but was not affected by ENSO, WP decreased significantly in response to a 3 °C increase in annual maximum temperatures during the El Niño period but subsequently recovered to above pre‐drought levels during the La Niña period. Moreover the climate sensitivity of tropical forest ANPP components was affected by local topography (water availability) and disturbance history (species composition). Our results suggest that projected increases in temperature and dry season length could impact tropical carbon sequestration by shifting ANPP partitioning towards decreased WP, thus decreasing the carbon storage of highly productive lowland forests. We conclude that the impact of climate anomalies on tropical forest productivity is strongly related to local site characteristics and will therefore likely prevent uniform responses of tropical lowland forests to projected global changes.
      PubDate: 2014-11-09T22:19:58.54634-05:0
      DOI: 10.1002/2014GB004934
  • External total alkalinity loads vs. internal generation ‐ the
           influence of non‐riverine alkalinity sources in the Baltic Sea
    • Authors: Erik Gustafsson; Teresia Wällstedt, Christoph Humborg, Carl‐Magnus Mörth, Bo G. Gustafsson
      Abstract: In this study we first present updated riverine total alkalinity (TA) loads to the various Baltic Sea sub‐basins, based on monthly measurements in 82 of the major rivers that represent 85% of the total runoff. Simulations in the coupled physical‐biogeochemical BALTSEM model show that these river loads together with North Sea water inflows are not sufficient to reproduce observed TA concentrations in the system, demonstrating the large influence from internal sources. Budget calculations indicate that the required internal TA generation must be similar to river loads in magnitude. The non‐riverine source in the system amounts to about 2.4 mmol m‐2 d‐1 on average. We argue here that the majority of this source is related to denitrification together with unresolved sediment processes such as burial of reduced sulfur and/or silicate weathering. This hypothesis is supported by studies on sediment processes on a global scale, and also by data from sediment cores in the Baltic Sea. In a model simulation with all internal TA sources and sinks switched on, the net absorption of atmospheric CO2 increased by 0.78 mol C m‐2 y‐1 compared to a simulation where TA was treated as a passive tracer. Our results clearly illustrate how pelagic TA sources together with anaerobic mineralization in coastal sediments generate a significant carbon sink along the aquatic continuum, mitigating CO2 evasions from coastal and estuarine systems.
      PubDate: 2014-11-06T01:25:57.976962-05:
      DOI: 10.1002/2014GB004888
  • Interactions between reactive nitrogen and the Canadian landscape: A
           budget approach
    • Authors: Thomas A. Clair; Nathan Pelletier, Shabtai Bittman, Adrian Leip, Paul Arp, Michael D. Moran, Ian Dennis, David Niemi, Shannon Sterling, Craig F. Drury, Jingyi Yang
      Abstract: The movement of excess reactive nitrogen (Nr) from anthropogenic activities to natural ecosystems has been described as one of the most serious environmental threats facing modern society [Rockstrom et al., 2009]. One of the approaches for tracking this movement is the use of budgets that quantify fluxes [Leip et al., 2011a]. We constructed an Nr budget for Canada using measured and modeled values from the scientific literature, government databases, and data from new agri‐environmental indicators, in order to produce information for policy makers and scientists to understand the major flows of nitrogen to allow a better assessment of risks to the Canadian environment. We divided the Canadian territory south of 60oN into areas dominated by natural ecosystems, as well as by agricultural and urban/industrial activities to evaluate Nr flows within, between and out of these units. We show that Canada is a major exporter of Nr due to the availability of inexpensive commercial fertilizers. The large land area suitable for agriculture makes Canada a significant agricultural Nr exporter of both grain crops and livestock. Finally, Canada exports petroleum Nr mainly to the United States. Because of its location and prevailing atmospheric transport patterns, Canada is a net receptor of Nr air pollution from the United States, receiving approximately 20% of the Nr leaving the US airshed. We found that overall, terrestrial natural ecosystems as well as the atmosphere are in balance between Nr inputs and outputs when all N reactive and non‐reactive fluxes are included. However, when only reactive forms are considered, almost 50% of N entering the Canadian atmosphere cannot be accounted for and is assumed to be lost to the Atlantic and Arctic oceans or to unmeasured dry deposition. However, agricultural and freshwater landscapes are showing large differences between measured inputs and outputs of Nr as our data suggest that denitrification in soils and aquatic systems is larger than what models predict. Our work also shows that Canada is a major contributor to the global flow of nitrogen through commercial exports.
      PubDate: 2014-10-30T16:57:03.742235-05:
      DOI: 10.1002/2014GB004880
  • Contrasting biogeochemical cycles of cobalt in the surface Western
           Atlantic Ocean
    • Authors: Gabriel Dulaquais; Marie Boye, Rob Middag, Stéphanie Owens, Viena Puigcorbe, Ken Buesseler, Pere Masqué, Hein Baar, Xavier Carton
      Abstract: Dissolved cobalt (DCo; < 0.2μm; 14 to 93 pM) and the apparent particulate cobalt (PCo; > 0.2μm; < 1 to 15 pM) were determined in the upper water‐column (< 1000 m) of the Western Atlantic Ocean along the GEOTRACES‐A02 section (64°N to 50°S). The lowest DCo concentrations, typical of a nutrient‐type distribution were observed in surface waters of the subtropical domains. Strong linear relationships between DCo and phosphate (P) as well as meridional gradients of decreasing DCo from high latitudes were characterized and both linked to the Co biological requirement. External sources such as the Amazon and the atmospheric deposition were found to contribute significantly (> 10%) to the DCo stock of the mixed layer in the equatorial and north subtropical domains. Biotic and abiotic processes as well as the physical terms involved in the biogeochemical cycle of Co were defined and estimated. This allowed establishing the first global budget of DCo for the upper 100 m in the Western Atlantic. The biological DCo uptake flux was the dominant sink along the section, as reflected by the overall nutrient‐type behavior of DCo. The regeneration varied widely vary within the different biogeochemical domains, accounting for 10% of the DCo‐uptake rate in the subarctic gyre and for up to 85% in southern subtropical domain. These findings demonstrated that the regeneration is likely the prevailing source of DCo in the surface waters of the Western Atlantic, except in the subpolar domains where physically‐driven sources can sustain the DCo biological requirement.
      PubDate: 2014-10-29T16:53:09.004432-05:
      DOI: 10.1002/2014GB004903
  • Observations and modeling of slow sinking particles in the twilight zone
    • Authors: M. Villa‐Alfageme; F. Soto, F. A. C. Le Moigne, S. L. C. Giering, R. Sanders, R. García‐Tenorio
      Abstract: The biological carbon pump (BCP) transfers carbon from the surface ocean into the oceans’ interior, mainly in the form of sinking particles with an organic component, and thereby keeps atmospheric CO2 at significantly lower levels than if the oceans were abiotic. The depth at which these sinking particles are remineralised is a key control over atmospheric CO2. Particle sinking speed is likely to be a critical parameter over remineralisation depth. Carbon export is usually controlled by large, rapidly sinking particles (>150 m · d‐1), however under some circumstances sinking velocity distributions are strongly bimodal with a significant fraction of total flux being carried by slowly (
      PubDate: 2014-10-22T16:21:17.455328-05:
      DOI: 10.1002/2014GB004981
  • Nitrous oxide emissions from cultivated black soil: A case study in
           Northeast China and global estimates using empirical model
    • Authors: Zengming Chen; Weixin Ding, Yiqi Luo, Hongyan Yu, Yehong Xu, Christoph Müller, Xia Xu, Tongbin Zhu
      Abstract: Manure application is effective in promoting soil carbon sequestration, but its impact on N2O emission is not well understood. A field experiment was conducted in a maize‐cultivated black soil in Northeast China with six treatments: inorganic fertilizer (NPK), 75% inorganic fertilizer N plus 25% pig (PM1) or chicken (CM1) manure N, 50% inorganic fertilizer N plus 50% pig (PM2) or chicken (CM2) manure N, and no N fertilizer (CK). Annual N2O emission significantly increased from 0.34 kg N ha–1 for CK to 0.86 kg N ha–1 for NPK and further to 1.65, 1.02, 1.17 and 0.93 kg N ha–1 for PM1, CM1, PM2 and CM2, respectively. A 15 N tracing study showed that 71–79% of total N2O was related to nitrification at 30–70% water‐filled pore space (WFPS), and heterotrophic nitrification contributed 49% and 25% to total N2O at 30% and 70% WFPS, respectively. In an incubation, N2O emission was only stimulated when nitrate and glucose were applied together at 60% WFPS, indicating that denitrification was carbon limited. PM had a stronger effect on denitrification than CM due to higher decomposability and the lower N2O emission at higher manure application rate was associated with decreased mineral N supply. After compiling a worldwide database and establishing an empirical model that related N2O emissions (kg N ha–1) to precipitation (Pr, m) and fertilizer N application rate (Nr, kg N ha–1) (N2O =1.533Pr +0.0238PrNr), annual N2O emission from global cultivated black soil applied with inorganic fertilizer N was estimated as 347 Gg N. Our results suggested that N2O emission from cultivated black soils in China was low primarily due to low precipitation and labile organic carbon availability, and would be stimulated by manure application; thus increased N2O emission should be taken into consideration as applying manure increases soil organic carbon sequestration.
      PubDate: 2014-10-21T08:21:49.067032-05:
      DOI: 10.1002/2014GB004871
  • 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
  • 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
  • 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
  • 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
  • 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
      First page: 1066
      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
  • 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
      First page: 1096
      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
  • The biogeochemical cycling of zinc and zinc isotopes in the North Atlantic
    • Authors: Tim M. Conway; Seth G. John
      First page: 1111
      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
  • 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
      First page: 1129
      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
  • 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
      First page: 1149
      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
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