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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: 48, SJR: 2.668, h-index: 142)
Global Biogeochemical Cycles     Full-text available via subscription   (Followers: 5, SJR: 2.4, h-index: 109)
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J. of Geophysical Research : Earth Surface     Partially Free   (Followers: 24)
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: 16)
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)
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Water Resources Research     Full-text available via subscription   (Followers: 176, 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]
  • Effect of temperature on rates of ammonium uptake and nitrification in the
           western coastal Arctic during winter, spring, and summer
    • Authors: Steven E. Baer; Tara L. Connelly, Rachel E. Sipler, Patricia L. Yager, Deborah A. Bronk
      Pages: n/a - n/a
      Abstract: Biogeochemical rate processes in the Arctic are not currently well constrained and there is very limited information on how rates may change as the region warms. Here we present data on the sensitivity of ammonium (NH4+) uptake and nitrification rates to short‐term warming. Samples were collected from the Chukchi Sea off the coast of Barrow, Alaska during winter, spring, and summer and incubated for 24 hours in the dark with additions of 15NH4+ at ‐1.5, 6, 13, and 20 °C. Rates of NH4+ uptake and nitrification were measured in conjunction with bacterial production. In all seasons, NH4+ uptake rates were highest at temperatures similar to current summertime conditions, but dropped off with increased warming, indicative of psychrophilic (i.e. cold‐loving) microbial communities. In contrast, nitrification rates were less sensitive to temperature and were higher in winter and spring compared to summer. These findings suggest that as the Arctic coastal ecosystem continues to warm, NH4+ assimilation may become increasingly important, relative to nitrification, although the magnitude of NH4+ assimilation would be still be lower than nitrification.
      PubDate: 2014-12-02T16:28:41.469698-05:
      DOI: 10.1002/2013GB004765
  • Benchmarking the seasonal cycle of CO2 fluxes simulated by terrestrial
           ecosystem models
    • Authors: Shushi Peng; Philippe Ciais, Frédéric Chevallier, Philippe Peylin, Patricia Cadule, Stephen Sitch, Shilong Piao, Anders Ahlström, Chris Huntingford, Peter Levy, Xiran Li, Yongwen Liu, Mark Lomas, Benjamin Poulter, Nicolas Viovy, Tao Wang, Xuhui Wang, Sönke Zaehle, Ning Zeng, Fang Zhao, Hongfang Zhao
      Pages: n/a - n/a
      Abstract: We evaluated the seasonality of CO2 fluxes simulated by nine terrestrial ecosystem models of the TRENDY project against 1) the seasonal cycle of gross primary production (GPP) and net ecosystem exchange (NEE) measured at flux tower sites over different biomes, 2) gridded monthly Model Tree Ensembles‐estimated GPP (MTE‐GPP) and MTE‐NEE obtained by interpolating many flux towers measurements with a machine‐learning algorithm, 3) atmospheric CO2 mole fraction measurements at surface sites, and 4) CO2 total columns (XCO2) measurements from the Total Carbon Column Observing Network (TCCON). For comparison with atmospheric CO2 measurements, the LMDZ4 transport model was run with time‐varying CO2 fluxes of each model as surface boundary conditions. Seven out of the nine models overestimate the seasonal amplitude of GPP, and produce a too early start in spring at most flux sites. Despite their positive bias for GPP, the nine models underestimate NEE at most flux sites, and in the Northern Hemisphere compared with MTE‐GPP. Comparison with surface atmospheric CO2 measurements confirms that most models underestimate the seasonal amplitude of NEE in the Northern Hemisphere (except CLM4C and SDGVM). Comparison with TCCON data also shows that the seasonal amplitude of XCO2 is underestimated by more than 10% for seven out of the nine models (except for CLM4C and SDGVM) and that the MTE‐NEE product is closer to the TCCON data using LMDZ4. From CO2 columns measured routinely at 10 TCCON sites, the constrained amplitude of NEE over the Northern Hemisphere is of 1.6 ± 0.4 gC m‐2 day‐1, which translates into a net CO2 uptake during the carbon uptake period in the Northern Hemisphere of 7.9 ± 2.0 PgC yr‐1.
      PubDate: 2014-11-28T05:07:56.325727-05:
      DOI: 10.1002/2014GB004931
  • CO2 and CH4 isotope compositions and production pathways in a tropical
    • Authors: M.E. Holmes; J. Chanton, M. Tfaily, A. Ogram
      Pages: n/a - n/a
      Abstract: While it is widely recognized that peatlands are important in the global carbon cycle, there is limited information on belowground gas production in tropical peatlands. We measured porewater methane (CH4) and carbon dioxide (CO2) concentrations and δ13C isotopic composition and CH4 and CO2 production rates in peat incubations from the Changuinola wetland in Panama. Our most striking finding was that CH4 was depleted in 13C (‐94‰ in porewater and produced at ‐107‰ in incubated peat) relative to CH4 found in most temperate and northern wetlands, potentially impacting the accuracy of approaches that use carbon isotopes to constrain global mass balance estimates. Fractionation factors between CH4 and CO2 showed that hydrogenotrophic methanogenesis was the dominant CH4 production pathway, with up to 100% of the CH4 produced via this route. Far more CO2 than CH4 (7 to 100 X) was measured in porewater, due in part to loss of CH4 through ebullition or oxidation and to the production of CO2 from pathways other than methanogenesis. We analyzed data on 58 wetlands from the literature to determine the dominant factors influencing the relative proportions of CH4 produced by hydrogenotrophic and acetoclastic methanogenesis and found that a combination of environmental parameters including pH, vegetation type, nutrient status and latitude are correlated to the dominant methanogenic pathway. Methane production pathways in tropical peatlands do not correlate with these variables in the same way as their more northerly counterparts and thus may be differently affected by climate change.
      PubDate: 2014-11-28T00:23:18.927988-05:
      DOI: 10.1002/2014GB004951
  • 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
  • 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
  • Predicting the long‐term fate of buried organic carbon in colluvial
    • Abstract: A significant part of the soil organic carbon that is eroded in uplands is deposited and buried in colluvial settings. Understanding the fate of this deposited soil organic carbon (SOC) is of key importance for the understanding of the role of (accelerated) erosion in the global C cycle: the residence time of the deposited carbon will determine if, and for how long, accelerated erosion due to human disturbance will induce sequestration of SOC from the atmosphere to the soil. Experimental studies may provide useful information, but, given the time scale under consideration, the response of the colluvial SOC can only be simulated using numerical models which need careful calibration using field data. In this study, we present a depth explicit SOC model (ICBM‐DE) including soil profile evolution due to sedimentation to simulate the long‐term C dynamics in colluvial soils. The SOC profile predicted by our model is in good agreement with field observations. The C burial efficiency (the ratio of current C content of the buried sediments to the original C content at the time of sedimentation) of deposited sediments exponentially decreases with time and gradually reached an equilibrium value. This equilibrium C burial efficiency is positively correlated with the sedimentation rate. The sedimentation rate is crucial for the long‐term dynamics of the deposited SOC as it controls the time that buried sediments spend at a given soil depth, thereby determining its temporal evolution of C input and decomposition rate during the burial process: C input and decomposition rate vary with depth due to the vertical variation of root distribution and soil environmental factors such as (but not limited to) humidity, temperature and aeration. The model demonstrates that, for the profiles studied, it takes ca. 300 yr for the buried SOC to lose half of its C load. It would also take centuries for the SOC accumulated in colluvial soils over the past decades due to soil redistribution under mechanized agriculture to be released to the atmosphere after the application of soil conservation measures such as conservation tillage.
  • Issue Information
    • Abstract: No abstract is available for this article.
  • Variability in efficiency of particulate organic carbon export: A model
    • Abstract: The flux of organic carbon from the surface ocean to mesopelagic depths is a key component of the global carbon cycle and is ultimately derived from primary production (PP) by phytoplankton. Only a small fraction of organic carbon produced by PP is exported from the upper ocean, referred to as the export efficiency (herein e‐ratio). Limited observations of the e‐ratio are available and there is thus considerable interest in using remotely‐sensed parameters such as sea surface temperature to extrapolate local estimates to global annual export flux. Currently, there are large discrepancies between export estimates derived in this way; one possible explanation is spatial or temporal sampling bias in the observations. Here we examine global patterns in the spatial and seasonal variability in e‐ratio and the subsequent effect on export estimates using a high resolution global biogeochemical model. NEMO‐MEDUSA represents export as separate slow and fast sinking detrital material whose remineralisation is respectively temperature dependent and a function of ballasting minerals. We find that both temperature and the fraction of export carried by slow sinking particles are factors in determining e‐ratio, suggesting that current empirical algorithms for e‐ratio that only consider temperature are overly simple. We quantify the temporal lag between PP and export, which is greatest in regions of strong variability in PP where seasonal decoupling can result in large e‐ratio variability. Extrapolating global export estimates from instantaneous measurements of e‐ratio is strongly affected by seasonal variability, and can result in errors in estimated export of up to ±60%.
  • Dimethyl Sulfide in the Amazon Rain Forest
    • Abstract: Surface‐to‐atmosphere emissions of dimethyl sulfide (DMS) may impact global climate through the formation of gaseous sulfuric acid, which can yield secondary sulfate aerosols and contribute to new particle formation. While oceans are generally considered the dominant source of DMS, a shortage of ecosystem observations prevents an accurate analysis of terrestrial DMS sources. Using mass spectrometry, we quantified ambient DMS mixing ratios within and above a primary rainforest ecosystem in the central Amazon Basin in real‐time (2010–2011) and at high vertical resolution (2013–2014). Elevated but highly variable DMS mixing ratios were observed within the canopy, showing clear evidence of a net ecosystem source to the atmosphere during both day and night in both the dry and wet seasons. Periods of high DMS mixing ratios lasting up to 8 hours (up to 160 ppt) often occurred within the canopy and near the surface during many evenings and nights. Daytime gradients showed mixing ratios (up to 80 ppt) peaking near the top of the canopy as well as near the ground following a rain event. The spatial and temporal distribution of DMS suggests that ambient levels and their potential climatic impacts are dominated by local soil and plant emissions. A soil source was confirmed by measurements of DMS emission fluxes from Amazon soils as a function of temperature and soil moisture. Furthermore, light and temperature dependent DMS emissions were measured from seven tropical tree species. Our study has important implications for understanding terrestrial DMS sources and their role in coupled land‐atmosphere climate feedbacks.
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