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  • 1
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    External total alkalinity loads vs. internal generation - the influence of non-riverine alkalinity sources in the Baltic Sea (2014)
    Wiley
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    Publication Date: 2014-11-07
    Description: 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 CO 2 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 CO 2 evasions from coastal and estuarine systems.
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  • 2
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    Predicting the long-term fate of buried organic carbon in colluvial soils (2014)
    Wiley
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    Publication Date: 2014-12-16
    Description: 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.
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  • 3
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    Dimethyl Sulfide in the Amazon Rain Forest (2014)
    Wiley
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    Publication Date: 2014-12-09
    Description: 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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  • 4
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    CO2 and CH4 isotope compositions and production pathways in a tropical peatland (2014)
    Wiley
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    Publication Date: 2014-11-29
    Description: 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 (CH 4 ) and carbon dioxide (CO 2 ) concentrations and δ 13 C isotopic composition and CH 4 and CO 2 production rates in peat incubations from the Changuinola wetland in Panama. Our most striking finding was that CH 4 was depleted in 13 C (-94‰ in porewater and produced at -107‰ in incubated peat) relative to CH 4 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 CH 4 and CO 2 showed that hydrogenotrophic methanogenesis was the dominant CH 4 production pathway, with up to 100% of the CH 4 produced via this route. Far more CO 2 than CH 4 (7 to 100 X) was measured in porewater, due in part to loss of CH 4 through ebullition or oxidation and to the production of CO 2 from pathways other than methanogenesis. We analyzed data on 58 wetlands from the literature to determine the dominant factors influencing the relative proportions of CH 4 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.
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  • 5
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    Effect of temperature on rates of ammonium uptake and nitrification in the western coastal Arctic during winter, spring, and summer (2014)
    Wiley
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    Publication Date: 2014-12-03
    Description: 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 (NH 4 + ) 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 15 NH 4 + at -1.5, 6, 13, and 20 °C. Rates of NH 4 + uptake and nitrification were measured in conjunction with bacterial production. In all seasons, NH 4 + 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, NH 4 + assimilation may become increasingly important, relative to nitrification, although the magnitude of NH 4 + assimilation would be still be lower than nitrification.
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  • 6
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    Natural variability and anthropogenic change in equatorial Pacific surface ocean pCO2 and pH (2014)
    Wiley
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    Publication Date: 2014-01-23
    Description: [1]  The equatorial Pacific is a dynamic region that plays an important role in the global carbon cycle. This region is the largest oceanic source of carbon dioxide (CO 2 ) to the atmosphere, which varies interannually dependent on the El Niño-Southern Oscillation (ENSO) and other climatic and oceanic drivers. We present high-resolution observations of surface ocean CO 2 partial pressure ( p CO 2 ) at four fixed locations in the Niño 3.4 area with datasets encompassing 10 ENSO warm and cold events from 1997 to 2011. The mooring observations confirm that ENSO controls much of the interannual variability in surface seawater p CO 2 with values ranging from 315 to 578 µatm. The mooring time series also capture the temporal variability necessary to make the first estimates of long-term pH trends in the equatorial Pacific, which suggest the combination of ocean acidification and decadal variability create conditions for high rates of pH change since the beginning of the mooring record. Anthropogenic CO 2 increases play a dominant role in significant observed seawater p CO 2 trends of +2.3 to +3.3 µatm yr -1 and pH trends of -0.0018 to -0.0026 yr -1 across the full time series in this region. However, increased upwelling driven by increased trade winds, a shallower thermocline, and increased frequency of La Niña events also contribute an average of 40% of the observed trends since 1998. These trends are higher than previous estimates based on underway observations and suggest that the equatorial Pacific is contributing a greater amount of CO 2 to the atmospheric CO 2 inventory over the last decade.
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  • 7
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    Constraining surface carbon fluxes using in situ measurements of carbonyl sulfide and carbon dioxide (2014)
    Wiley
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    Publication Date: 2014-01-23
    Description: [1]  Understanding the processes that control the terrestrial exchange of carbon is critical for assessing atmospheric CO 2 budgets. Carbonyl sulfide (COS) is taken up by vegetation during photosynthesis following a pathway that mirrors CO 2 , but has a small or non-existent emission component. This tracer could thus provide a means to separate photosynthetic and respiration fluxes. We present field measurements of COS and CO 2 mixing ratios made during the summer of 2012 in a forest, senescent grassland and riparian ecosystem using a laser absorption spectrometer installed in a mobile trailer. Measurements of leaf fluxes with a branch-bag gas-exchange system were made across species from 10 genera of trees, and soil fluxes were measured with a flow-through chamber across a number of soil types. These data provide an extensive characterization of surface COS fluxes and show: (1) the existence of a narrow normalized daytime uptake ratio of COS to CO 2 across vascular plant species of 1.7, providing critical information for the application of COS to estimate photosynthetic CO 2 fluxes and (2) a temperature-dependent normalized uptake ratio of COS to CO 2 from soils. Significant nighttime uptake of COS was observed in broad-leafed species and revealed active stomatal opening hours prior to sunrise. Joint measurements of ambient near-surface COS and CO 2 concentrations are used here alongside the flux measurements to partition the relative influence that leaf and soil fluxes and entrainment of air from above have on the surface carbon budget. The time-dependent co-variation of COS and CO 2 in the ambient surface air sheds light on the dominant processes influencing the surface carbon budget in ways that cannot be done with measurements of only CO 2 . The results provide a number of critical constraints on the processes that control surface COS exchange, which can be used to diagnose the robustness of global models that are beginningto use COS to constrain terrestrial carbon exchange.
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  • 8
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    Observational constraints on the distribution, seasonality, and environmental predictors of North American boreal methane emissions (2014)
    Wiley
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    Publication Date: 2014-01-24
    Description: [1]  Wetlands comprise the single largest global source of atmospheric methane, but current flux estimates disagree in both magnitude and distribution at the continental scale. This study uses atmospheric methane observations over North America from 2007–2008 and a geostatistical inversion to improve understanding of Canadian methane fluxes and associated biogeochemical models. The results bridge an existing gap between traditional top-down, inversion studies, which typically emphasize total emission budgets, and biogeochemical models, which usually emphasize environmental processes. The conclusions of this study are threefold. First, the most complete process-based methane models do not always describe available atmospheric methane observations better than simple models. In this study, a relatively simple model of wetland distribution, soil moisture, and soil temperature outperformed more complex model formulations. Second, we find that wetland methane fluxes have a broader spatial distribution across western Canada and into the northern US than represented in existing flux models. Finally, we calculate total methane budgets for Canada and for the Hudson Bay Lowlands, a large wetland region (50–60°N, 75–96°W). Over these lowlands, we find total methane fluxes of 1.8 ± 0.24 TgC yr − 1 , a number in the mid-range of previous estimates. Our total Canadian methane budget of 16.0 ± 1.2 TgC yr − 1 is larger than existing inventories, primarily due to high anthropogenic emissions in Alberta. However, methane observations are sparse in western Canada, and additional measurements over Alberta will constrain anthropogenic sources in that province with greater confidence.
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  • 9
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    The counterintuitive effect of summer-to-fall mixed layer deepening on eukaryotic new production in the Sargasso Sea (2014)
    Wiley
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    Publication Date: 2014-01-11
    Description: [1]  The Sargasso Sea is characterized by strong summertime stratification that is thought to drive oligotrophy, but export production is surprisingly similar to that of high-latitude regions with ample major nutrient supply. Here, we use the summer-to-fall progression in the northwestern Sargasso Sea to investigate the relationship between upper ocean stratification and phytoplankton nitrogen (N) uptake. Euphotic zone particles collected in July, October, and December were sorted by flow cytometry, and the 15  N/ 14  N of separated prokaryotic and eukaryotic phytoplankton was analyzed. The 15  N/ 14  N of Prochlorococcus and Synechococcus was always low, indicating uniform reliance on recycled N. In July and in two fall profiles, the 15  N/ 14  N of eukaryotic phytoplankton was high, reflecting consumption of subsurface nitrate. In three other fall profiles, eukaryotic 15  N/ 14  N was similar to prokaryote 15  N/ 14  N, suggesting a shift toward more complete reliance on recycled N. The progressive deepening of the mixed layer from summer to fall, although reducing the surface-to-deep density contrast, increases the density difference of the euphotic zone as a whole from underlying nutrient-rich waters, which may play a role in the observed decline in euphotic zone nitrate supply into the fall. The apparent summertime nitrate supply to the euphotic zone, when the mixed layer is shallowest, may help to explain the surprisingly high export production of the subtropical and tropical ocean.
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  • 10
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    The viscosity effect on marine particle flux – a climate relevant feedback mechanism (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: Oceanic uptake and long-term storage of atmospheric carbon dioxide (CO 2 ) are strongly driven by the marine ‘biological pump’, i.e. sinking of biotically fixed inorganic carbon and nutrients from the surface into the deep ocean [ Sarmiento and Bender , 1994; Volk and Hoffert , 1985]. Sinking velocity of marine particles depends on seawater viscosity, which is strongly controlled by temperature [ Sharqawy et al. , 2010]. Consequently, marine particle flux is accelerated as ocean temperatures increase under global warming [ Bach et al. , 2012]. Here we show that this previously overlooked 'viscosity effect' could have profound impacts on marine biogeochemical cycling and carbon uptake over the next centuries to millennia. In our global-warming simulation, the viscosity effect accelerates particle sinking by up to 25%, thereby effectively reducing the portion of organic matter that is respired in the surface ocean. Accordingly, the biological carbon pump's efficiency increases, enhancing the sequestration of atmospheric CO 2 into the ocean. This effect becomes particularly important on longer timescales when warming reaches the ocean interior. At the end of our simulation (4000 AD) oceanic carbon uptake is 17% higher, atmospheric CO 2 concentration is 180 ppm lower, and the increase in global average surface temperature is 8% weaker when considering the viscosity effect. Consequently, the viscosity effect could act as a long-term negative feedback mechanism in the global climate system.
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  • 11
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    Calcium carbonate dissolution in the upper 1000 m of the eastern North Atlantic (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: Recent analyses suggest that considerable CaCO 3 dissolution may occur in the upper water column of the ocean (〈 1500 m). This study uses the distribution of particulate calcium from high-resolution suspended matter sampling along the CLIVAR/CO 2 Repeat Hydrography A16N transect in 2003 to estimate CaCO 3 dissolution in the top 1000 m of the North Atlantic. Dissolution rates were also approximated using changes in total alkalinity measurements along isopycnal surfaces. Water masses were found to be undersaturated with respect to aragonite at intermediate depths (400–1000 m) in the eastern tropical North Atlantic. The CaCO 3 dissolution rate in this region is estimated to be 0.9 mmol CaCO 3 m -2 d -1 , indicating this region is a hotspot for upper water column CaCO 3 dissolution compared to the Atlantic basin as a whole. Dissolution rates calculated from particulate calcium distributions outside of this region were significantly lower (0.2 mmol CaCO 3 m -2 d -1 ) and are comparable to previous estimates of CaCO 3 dissolution flux for the Atlantic Ocean. The magnitude of upper water column dissolution rates compared to measured surface-ocean CaCO 3 standing stocks suggests that biologically-mediated CaCO 3 dissolution may be occurring in the top 1000 m of the Atlantic.
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  • 12
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    Air-sea CO2 fluxes in the California Current: Impacts of model resolution and coastal topography (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: The present study uses a suite of coupled physical-biogeochemical model simulations at 1/3°, 1/10°, and 1/30° to assess the impact of horizontal resolution on air-sea CO 2 fluxes in the California Current System (CCS), a relevant issue for downscaling between coarser resolution global climate models and higher resolution regional models. The results demonstrate that horizontal resolution is important to reproduce the sharp transition between near-shore outgassing and offshore absorption, as well as to resolve the regions of enhanced near-shore outgassing in the lee of capes. The width of the outgassing region is overestimated when horizontal resolution is not eddy-resolving (i.e., 1/3°), but becomes more dependent on shelf topography for eddy-resolving simulations (i.e., 1/10° and 1/30°). Enhanced near-shore outgassing is associated with local increases in wind-driven upwelling in the lee of capes (i.e., expansion fans), meaning that sufficient horizontal resolution is needed both in the ocean circulation model and in the wind field forcing the model. From a global carbon budget perspective, the model indicates that biological production generates sufficient absorption within a few hundred kilometers of the coast to offset near-shore outgassing, which is consistent with the notion that mid-latitude eastern boundary current upwelling systems act both as a sink and source for atmospheric CO 2 . Based on the 1/30° solution, the CCS between 35-45N and out to 600 km offshore is as a net carbon sink of ca. 6 TgC yr −1 , with the 1/10° solution underestimating this value by less than 10% and the 1/3° solution by a factor of three.
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  • 13
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    Drivers of pCO2 variability in two contrasting coral reef lagoons: The influence of submarine groundwater discharge (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: The impact of groundwater on p CO 2 variability was assessed in two coral reef lagoons with distinct drivers of submarine groundwater discharge (SGD). Diel variability of p CO 2 in the two ecosystems was explained by a combination of biological drivers and SGD inputs. In Rarotonga, a South Pacific volcanic island, SGD was driven primarily by a steep terrestrial hydraulic gradient, and the water column was influenced by the high p CO 2 (5,501 µatm) of the fresh groundwater. In Heron Island, a Great Barrier Reef coral cay, SGD was dominated by seawater recirculation in permeable sediments (i.e. tidal pumping) and p CO 2 was mainly impacted through the stimulation of biological processes. The Rarotonga water column had a relatively higher average p CO 2 (549 µatm) than Heron Island (471 µatm), however, p CO 2 exhibited a greater diel range in Heron Island (778 µatm) than in Rarotonga (507 µatm). SGD flux rates were quantified using a radon ( 222 Rn) mass balance. The Rarotonga water column received 29.0 ± 8.2 mmol free-CO 2 m −2 d −1 from SGD, while the Heron Island water column received 12.1 ± 4.2 mmol free-CO 2 m −2 d −1 . Both systems were sources of carbon dioxide to the atmosphere (averaging 8.8 ± 3.4 and 2.5 ± 2.1 mmol CO 2 m −2 d −1 in Rarotonga and Heron Island, respectively), with SGD-derived free-CO 2 most likely contributing to the outgassing of CO 2 . Studies measuring the metabolism of coral reefs via changes in carbonate chemistry (e.g. photosynthesis, respiration, calcification, and calcium carbonate (CaCO 3 ) dissolution rates) may need to consider the effects of groundwater seepage on water column carbonate chemistry and greenhouse gas evasion. Local drivers of coral reef carbonate chemistry such as SGD may offer more approachable management solutions to mitigating the effects of ocean acidification (OA) on coral reefs.
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  • 14
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    Ecological processes dominate the 13C land disequilibrium in a Rocky Mountain subalpine forest (2014)
    Wiley
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    Publication Date: 2014-03-14
    Description: Fossil fuel combustion has increased atmospheric CO 2 by ≈ 115 µmol mol -1 since 1750, and decreased its carbon isotope composition (δ 13 C) by 1.7-2 ‰ (the 13 C Suess effect). Because carbon is stored in the terrestrial biosphere for decades and longer, the δ 13 C of CO 2 released by terrestrial ecosystems is expected to differ from the δ 13 C of CO 2 assimilated by land plants during photosynthesis. This isotopic difference between land-atmosphere respiration (δ R ) and photosynthetic assimilation (δ A ) fluxes gives rise to the 13 C land disequilibrium (D). Contemporary understanding suggests that over annual and longer time scales, D is determined primarily by the Suess effect, and thus D is generally positive (δ R  〉 δ A ). A seven-year record of biosphere-atmosphere carbon exchange was used to evaluate the seasonality of δ A and δ R , and the 13 C land disequilibrium, in a subalpine conifer forest. A novel isotopic mixing model was employed to determine the δ 13 C of net land-atmosphere exchange during day and night, and combined with tower-based flux observations to assess δ A and δ R . The disequilibrium varied seasonally, and when flux-weighted was opposite in sign than expected from the Suess effect (D = -0.75 ± 0.21 ‰ or -0.88 ± 0.10 ‰ depending on method). Seasonality in D appeared to be driven by photosynthetic discrimination (Δ canopy ) responding to environmental factors. Possible explanations for negative D include: 1) changes in Δ canopy over decades as CO 2 and temperature have risen, and/or 2) post-photosynthetic fractionation processes leading to sequestration of isotopically-enriched carbon in long-lived pools like wood and soil.
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  • 15
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    Influences of Glacier Melt and Permafrost Thaw on the Age of Dissolved Organic Carbon in the Yukon River Basin (2014)
    Wiley
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    Publication Date: 2014-04-30
    Description: Responses of near-surface permafrost and glacial ice to climate change are of particular significance for understanding long term effects on global carbon cycling and carbon export by high-latitude northern rivers. Here we report Δ 14 C -dissolved organic carbon (DOC) values and dissolved organic matter optical data for the Yukon River, 15 tributaries of the Yukon River, glacial meltwater, and groundwater and soil water endmember sources draining to the Yukon River, with the goal of assessing mobilization of aged DOC within the watershed. Ancient DOC was associated with glacial meltwater and groundwater sources. In contrast, DOC from watersheds dominated by peat soils and underlain by permafrost was typically enriched in Δ 14 C indicating that degradation of ancient carbon stores is currently not occurring at large enough scales to quantitatively influence bulk DOC exports from those landscapes. On an annual basis, DOC exported was predominantly modern during the spring period throughout the Yukon River basin, and became older through summer-fall and winter periods, suggesting that contributions of older DOC from soils, glacial meltwaters and groundwater are significant during these months. Our data indicate that rapidly receding glaciers and increasing groundwater inputs will likely result in greater contributions of older DOC in the Yukon River and its tributaries in coming decades.
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  • 16
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    The triple oxygen isotope tracer of primary productivity in a dynamic ocean model (2014)
    Wiley
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    Publication Date: 2014-04-30
    Description: The triple oxygen isotopic composition of dissolved oxygen ( 17 Δ dis ) was added to the ocean ecosystem and biogeochemistry component of the Community Earth System Model, version 1.1.1. Model simulations were used to investigate the biological and physical dynamics of 17 Δ dis and assess its application as a tracer of gross photosynthetic production ( GOP ) of O 2 in the ocean mixed layer. The model reproduced large-scale patterns of 17 Δ dis found in observational data across diverse biogeographical provinces. Mixed layer model performance was best in the Pacific, had a negative bias in the North Atlantic and a positive bias in the Southern Ocean. Based on model results, the steady-state equation commonly used to calculate GOP from tracer values overestimated the globally averaged model GOP by 29%. Vertical entrainment/mixing and the time rate of change of 17 Δ dis were the two largest sources of bias when applying the steady-state method to calculate GOP . Entrainment/mixing resulted in the largest overestimation in mid latitudes and during summer and fall and almost never caused an underestimation of GOP . The tracer time-rate of change bias resulted both in underestimation of GOP (e.g., during spring blooms at high latitudes) and overestimation (e.g., during the summer following a bloom). Seasonally, bias was highest in the fall (Sept-Oct-Nov in the Northern Hemisphere, Mar-Apr-May in the Southern), overestimating GOP by 62%, globally averaged. Overall, the steady-state method was most accurate in equatorial and low latitude regions where it estimated GOP to within ±10%. Field applicable correction terms are derived for entrainment and mixing that capture 86% of model vertical bias and require only mixed layer depth history and triple oxygen isotope measurements from two depths.
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  • 17
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    Natural biogeochemical cycle of mercury in a global three-dimensional ocean tracer model (2014)
    Wiley
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    Publication Date: 2014-04-29
    Description: We implement mercury (Hg) biogeochemistry in the OFFline global 3-D ocean TRACer model (OFFTRAC) to investigate the natural Hg cycle, prior to any anthropogenic input. The simulation includes three Hg tracers: dissolved elemental (Hg 0 aq ), dissolved divalent (Hg II aq ) and particle-bound mercury (Hg P aq ). Our Hg parameterization takes into account redox chemistry in ocean waters, air-sea exchange of Hg 0 , scavenging of Hg II aq onto sinking particles and resupply of Hg II aq at depth by remineralization of sinking particles. Atmospheric boundary conditions are provided by a global simulation of the natural atmospheric Hg cycle in the GEOS-Chem model. In the surface ocean, the OFFTRAC model predicts global mean concentrations of 0.16 pM for total Hg, partitioned as 80% Hg II aq , 14% Hg 0 aq and 6% Hg P aq . Total Hg concentrations increase to 0.38 pM in the thermocline/intermediate waters (between the mixed layer and 1,000 m depth) and 0.82 pM in deep waters (below 1,000 m), reflecting removal of Hg from the surface to the subsurface ocean by particle sinking followed by remineralization at depth. Our model predicts that Hg concentrations in the deep N. Pacific Ocean (〉2,000 m) are a factor of 2-3 higher than in the deep N. Atlantic Ocean. This is the result of cumulative input of Hg from particle remineralization as deep waters transit from the N. Atlantic to the N. Pacific on their ~2,000-year journey. The model is able to reproduce the relatively uniform concentrations of total Hg observed in the old deep waters of the N. Pacific Ocean (observations: 1.2 ± 0.4 pM; model: 1.1 ± 0.04 pM) and Southern Ocean (observations: 1.1 ± 0.2 pM; model: 0.8 ± 0.02 pM). However, the modeled concentrations are factors of 5-6 too low compared to observed concentrations in the surface ocean and in the young water masses of the deep N. Atlantic Ocean. This large underestimate for these regions implies a factor of 5-6 anthropogenic enhancement in Hg concentrations.
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    New insights into sea ice nitrogen biogeochemical dynamics from the nitrogen isotopes (2014)
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    Publication Date: 2014-01-25
    Description: [1]  We report nitrogen (N) isotopic measurements of nitrate (NO 3 - ), total dissolved nitrogen (TDN), and particulate nitrogen (PN) from Antarctic pack ice in the Bellingshausen Sea during the early spring and in the Weddell Sea during the late spring. The normalized concentrations of total fixed N within the ice are ~2-fold higher than in the underlying seawater, indicating that sea ice continues to exchange fixed N with seawater. Partial nitrate assimilation into immobile organic matter, with low- δ 15 N, leads to the subsequent loss of high- δ 15 N nitrate during brine convection. The combined effect is to lower the δ 15 N of total fixed sea ice N relative to the winter-supplied nitrate. In this regard, the effect of incomplete nitrate consumption in sea ice is similar to that in the summertime surface ocean. However, the degree of nitrate consumption is greater in ice, leading to a higher δ 15 N for the organic N produced (3.9‰) than in the open Antarctic Zone (0.6‰). Relative to previous suggestions of very high- δ 15 N export from sea ice (up to 41‰), this study indicates that it would be more difficult than previously thought for sea ice to explain the high δ 15 N of ice age Antarctic sediments. The partitioning of N isotopes between PN and DON + NH 4 + (dissolved organic nitrogen + ammonium) suggests an imbalance between DON + NH 4 + production and consumption processes in early spring, which was subsequently attenuated by the development of an efficient regeneration loop in late spring. Despite the 15 N enrichment associated with nitrate assimilation, the δ 15 N of sea ice nitrate is frequently lower than that of seawater providing direct evidence of the products of nitrification within sea ice, with the mass and isotopic balances suggesting that nitrification supplies a substantial fraction (up to ~70%) of NO 3 - assimilated within the ice.
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    Global Assessment of Ocean Carbon Export by Combining Satellite Observations and Food-Web Models (2014)
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    Publication Date: 2014-02-02
    Description: [1]  The export of organic carbon from the surface ocean by sinking particles is an important, yet highly uncertain, component of the global carbon cycle. Here, we introduce a mechanistic assessment of the global ocean carbon export using satellite observations, including determinations of net primary production (NPP) and the slope of the particle size spectrum, to drive a food-web model that estimates the production of sinking zooplankton feces and algal aggregates comprising the sinking particle flux at the base of the euphotic zone. The synthesis of observations and models reveals fundamentally different and ecologically consistent regional-scale patterns in export and export efficiency not found in previous global carbon export assessments. The model reproduces regional-scale particle export field observations and predicts a climatological mean global carbon export from the euphotic zone of ~6 Pg C y -1 . Global export estimates show small variation (typically 〈 10%) to factor of two changes in model parameter values. The model is also robust to the choices of the satellite data products used and enables interannual changes to be quantified. The present synthesis of observations and models provides a path for quantifying the ocean's biological pump.
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    Evaluating soil biogeochemistry parameterizations in Earth system models with observations (2014)
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    Publication Date: 2014-02-05
    Description: [1]  Soils contain large reservoirs of terrestrial carbon (C), yet soil C dynamics simulated in Earth systems models show little agreement with each other or with observational datasets. This uncertainty underscores the need to develop a framework to more thoroughly evaluate model parameterizations, structures, and projections. Towards this end we used an analytical solution to calculate approximate equilibrium soil C pools for the Community Land Model version 4 (CLM4cn) and DAYCENT soil biogeochemistry models. Neither model generated sufficient soil C pools when forced with litterfall inputs from CLM4cn; however, global totals and spatial correlations of soil C pools for both models improved when calculated with litterfall inputs derived from observational data. DAYCENT required additional modifications to simulate soil C pools in deeper soils (0-100 cm). Our best simulations produced global soil C pools totaling 746 and 978 Pg C for CLM4cn and DAYCENT parameterizations, respectively; compared to observational estimates of 1259 Pg C. In spite of their differences in complexity and equilibrium soil C pools, predictions of soil C losses with warming temperatures through 2100 were strikingly similar for both models. Ultimately, CLM4cn and DAYCENT come from the same class of models that represent the turnover of soil C as a first-order decay process. While this approach may have utility in calculating steady state soil C pools, the applicability of this model configuration in transient simulations remains poorly evaluated.
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    A Growing Oceanic Carbon Uptake: Results from an inversion study of surface pCO2 data (2014)
    Wiley
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    Publication Date: 2014-02-27
    Description: [1]  Concerted community efforts have been devoted to producing an authoritative climatology of air-sea CO 2 fluxes [ Takahashi et al ., 2009], but identifying decadal trends in CO 2 fluxes has proven to be more challenging. The available surface pCO 2 estimates are too sparse to separate long-term trends from decadal and seasonal variability using simple linear models. We introduce Markov Chain Monte Carlo [MCMC] sampling as a novel technique for estimating the historical pCO 2 at the ocean surface. The result is a plausible history of surface pCO 2 based on available measurements and variability inferred from model simulations. Applying the method to a modern database of pCO 2 data, we find that two thirds of the ocean surface is trending toward increasing uptake of CO 2 , with a mean (year 2000) uptake of 2.3 ± 0.5 PgC yr − 1 of anthropogenic carbon and an increase in the global annual uptake over the 30-year time period of 0.4 ± 0.1 PgC yr − 1 decade − 1 . The results are particularly interesting in the Southern Ocean, where we find increasing uptake of carbon over this time period, in contrast to previous studies. We find evidence for increased ventilation of deep ocean carbon, in response to increased winds, which is more than offset by an associated surface cooling.
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    Incorporating microbial ecology concepts into global soil mineralization models to improve predictions of carbon and nitrogen fluxes (2014)
    Wiley
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    Publication Date: 2014-02-19
    Description: [1]  Global models of soil carbon (C) and nitrogen (N) fluxes become increasingly needed to describe climate change impacts, yet they typically have limited ability to reflect microbial activities that may affect global-scale soil dynamics. Benefiting from recent advances in microbial knowledge, we evaluated critical assumptions on microbial processes to be applied in global models. We conducted a sensitivity analysis of soil respiration rates ( Cmin ) and N mineralization rates ( Nmin ) for different model structures and parameters regarding microbial processes, and validated them with laboratory incubation data of diverse soils. Predicted Cmin was sensitive to microbial biomass, and the model fit to observed Cmin improved when using site-specific microbial biomass. Cmin was less affected by the approach of microbial substrate consumption (i.e. linear, multiplicative, or Michaelis–Menten kinetics). The sensitivity of Cmin to increasing soil N fertility was idiosyncratic and depended on the assumed mechanism of microbial C:N stoichiometry effects: a C overflow mechanism upon N limitation (with decreased microbial growth efficiency) led to the best model fit. Altogether, inclusion of microbial processes reduced prediction errors by 26% (for Cmin ) and 7% (for Nmin ) in our validation dataset. Our study identified two important aspects to incorporate into global models: site-specific microbial biomass and microbial C:N stoichiometry effects. The former requires better understandings of spatial patterns of microbial biomass and its drivers, while the latter urges for further conceptual progress on C–N interactions. With such advancements, we envision improved predictions of global C and N fluxes for a current and projected climate.
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    The contribution of aeolian sand and dust to iron fertilization of phytoplankton blooms in southwestern Ross Sea, Antarctica (2014)
    Wiley
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    Publication Date: 2014-03-28
    Description: Iron (Fe) limitation during the austral summer is a persistent characteristic of primary production in the Ross Sea, Antarctica. Recent observations reveal low dissolved Fe (dFe) concentrations in the Ross Sea polynya after the dFe winter reserve has been consumed in association with high rates of primary production, suggesting significant new sources of dFe are required to sustain the phytoplankton bloom during this period. The accumulation of locally-derived aeolian sand and dust (ASD) on sea ice is a potential source. To constrain aeolian Fe inputs from melting sea ice, we determined ASD mass accumulation rates as well as the total and soluble Fe content on first year sea ice in McMurdo Sound (Latitude 77.5ºS; Longitude 165ºE), southwestern (SW) Ross Sea. The mean ASD mass accumulation rate was ~1.5 g m -2  yr -1 , total Fe content of this ASD was 4 ± 1 wt % and the percentage of soluble Fe was 11 ± 1 %. Assuming our results are representative of the 7400 km 2 McMurdo Sound region, we use our mean estimate of the bulk aeolian dFe flux of 122.1 µmol m -2  yr -1 to calculate that aeolian Fe potentially supports between 9.0 x 10 9 and 4.1 x 10 11  mol C yr -1 (0.1-4.9 Tg C yr -1 ) of new primary production in McMurdo Sound. This equates to ~15 % of new primary production in the SW Ross Sea, suggesting that locally-derived aeolian dFe is a minor component of seasonal Fe supply. Consequently, this study highlights the need to quantify other sources of dFe to the Ross Sea region, which can potentially sustain phytoplankton blooms during the austral summer. In comparison to other estimates in the Antarctic region, McMurdo Sound has very high ASD and represents an upper limit of dFe that can be contributed to the ocean from melting sea ice in the Ross Sea.
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    APPLICATION OF REMOTE SENSING TO UNDERSTANDING FIRE REGIMES AND BIOMASS BURNING EMISSIONS OF THE TROPICAL ANDES (2014)
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    Publication Date: 2014-03-28
    Description: In the tropical Andes, fires play an important cultural and ecological role. However, there have been very few systematic studies aimed at understanding the biomass burning dynamics in the area. This paper seeks to advance on our understanding of burning regimes in this region, with the first detailed and comprehensive assessment of fire occurrence and the derived gross biomass burning emissions of an area of the Peruvian tropical Andes. We selected for analysis an area of 2.8 million hectares at altitudes over 2000 m. We analyzed fire occurrence over a 12-year period with three types of satellite data: active fire pixels from the MODerate Resolution Imaging Spectroradiometer (MODIS) MCD14ML product, burned area scars from the MODIS MCD45 product, and higher resolution Landsat 5 TM imagery. Fire dynamics showed a large intra- and inter-annual variability, with most fires occurring May-October (the period coinciding with the dry season), and year 2005 having the largest number of fires and burned area. Total area burned decreased with increasing rainfall until a given rainfall threshold beyond which no relationship was found. The estimated fire return interval (FRI) for the area is 37 years for grasslands, which is within the range reported for grasslands and 65 years for forests, which is remarkably shorter than other reported FRI in tropical moist forests. The greatest contribution (60-70%, depending of the data source) to biomass burning emissions came from burned montane cloud forests (4.5 million Mg CO 2 over the study period), despite accounting for only 7.4-10% of the total burned area. Gross aboveground biomass emissions (7.55 ± 2.14 Tg CO 2 ; 0.43 ± 0.04 Tg CO; 24,012 ± 2,685 Mg CH 4 for the study area) were larger than previously reported for the Tropical Andes.
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    Natural and anthropogenic variations in atmospheric mercury deposition during the Holocene near Quelccaya Ice Cap, Peru (2014)
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    Publication Date: 2014-04-01
    Description: Mercury (Hg) is a toxic metal that is transported globally through the atmosphere. The emission of Hg from mineral reservoirs and subsequent recycling in surface reservoirs (i.e., soil/biomass, ocean, and atmosphere) are fundamental to the modern global Hg cycle, yet past emissions from anthropogenic and natural sources are not fully constrained. We use a sediment core from Yanacocha, a headwater lake in southeastern Peru, to study the anthropogenic and natural controls on atmospheric Hg deposition during the Holocene. From 12.3 to 3.5 ka, Hg fluxes in the record are relatively constant (mean ± 1σ: 1.4 ± 0.6 µg m -2 a -1 , n = 189) . Past Hg deposition does not correlate with changes in regional temperature and precipitation, inferred from nearby paleoclimate records, or with most large volcanic events that occurred regionally, in the Andean Central Volcanic Zone (~300-400 km from Yanacocha), and globally. In B.C. 1450 (3.4 ka), Hg fluxes abruptly increased and reached the Holocene-maximum flux (6.7 µg m -2 a -1 ) in B.C. 1200, concurrent with a ~100-year peak in Fe and chalcophile metals (As, Ag, Tl) and the presence of framboidal pyrite. Continuously elevated Hg fluxes from B.C. 1200-500 suggest a protracted mining-dust source near Yanacocha that is identical in timing to documented pre-Incan cinnabar mining in central Peru. During Incan and Colonial time (A.D. 1450-1650), Hg deposition remains elevated relative to background levels but lower relative to other Hg records from sediment cores in central Peru, indicating a limited spatial extent of preindustrial Hg emissions. Hg fluxes from A.D. 1980 to 2011 (4.0 ± 1.0 µg m -2 a -1 , n = 5 ) are 3.0 ± 1.5 times greater than pre-anthropogenic fluxes and are similar to modern fluxes documented in remote lakes around the world.
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    Representative regional sampling of carbon dioxide and methane concentrations in hemiboreal headwater streams reveal underestimates in less systematic approaches (2014)
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    Publication Date: 2014-04-01
    Description: Boreal headwater streams have been identified as hotspots for water-air exchange of greenhouse gases (GHG´s). Despite these findings GHG concentrations and fluxes in headwaters are to a great extent unexplored at large (regional/national) scales. This study was the first to systematically determine the concentrations of CO 2 and CH 4 in hemiboreal (southern boreal and boreonemoral) headwater streams. The use of a headspace sampling method focusing on GHG´s in combination with a statistically representative selection of more than 200 streams across two regions in Sweden was the basis for defining the base flow supersaturation level of CO 2 and CH 4 . All streams were supersaturated relative to the atmosphere in CO 2 (median concentration, 1.9 (±1.1) mg C L -1 ) and the majority in CH 4 (median concentration, 7.1 (±54.0) µg C L -1 for the 82% of streams in which CH 4 was detected). The spatial variability in both CO 2 and CH 4 was high but positively related to total organic carbon, mean annual temperature and proportion of peatland in the catchment. There were however regional differences in the spatial controls, which is something that predictive models need to consider. The large and representative data set allowed for comparison between a headspace and an alkalinity-based method for determining CO 2 in these headwaters. More than 50% of the streams contained no alkalinity which made the alkalinity based determination of CO 2 impossible. In addition, half of the streams with alkalinity alkalinities low enough (〈0.07 mEq L -1 ) to make the CO 2 determination very uncertain. The streams with low pH and no alkalinity contained median CO 2 concentrations that were 45% higher than the streams containing alkalinity. Therefore large scale generalizations about the supersaturation of CO 2 from such headwaters will be significantly underestimated if (1) headwaters are underrepresented, and (2) the headwaters are sampled but CO 2 is calculated from their alkalinity.
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    Intra-annual variability of organic carbon concentrations in running waters: drivers along a climatic gradient (2014)
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    Publication Date: 2014-04-01
    Description: Trends in surface water dissolved organic carbon (DOC) concentrations have received considerable scientific interest during recent decades. However, intra-annual DOC variability is often orders of magnitude larger than long-term trends. Unravelling the controls on intra-annual DOC dynamics holds the key to a better understanding of long-term changes and their ecological significance. We quantified and characterized intra-annual DOC variability and compared it with long-term DOC trends in 136 streams and rivers, varying in size and geographical characteristics, across a 1400 km latitudinal gradient during 2000–2010. Discharge, temperature and month of the year were the most significant predictors of intra-annual DOC variability in a majority of the running waters. Relationships between DOC, discharge, and temperature were however different along a mean annual temperature (MAT) gradient. Running waters with low MAT generally displayed positive DOC-discharge correlations whereas the relationships in sites with higher MAT were more variable. This reflected contrasting relationships between temperature and discharge with discharge positively correlated with temperature in cold areas, while it was negatively correlated with temperature in catchments with higher MAT. Sites where flow, temperature and month were poorly related to intra-annual DOC dynamics were large catchments or sites with extensive upstream lake cover. DOC trends were generally much smaller than intra-annual DOC variability and did not show any North–south gradient. Our findings suggest that DOC in running waters could respond to a changing climate in ways not predictable, or even discernible, from extrapolation of recent inter-annual trends.
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    Issue Information (2014)
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    Publication Date: 2014-09-21
    Description: No abstract is available for this article.
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    Tropical wetlands: A missing link in the global carbon cycle? (2014)
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    Publication Date: 2014-10-09
    Description: Tropical wetlands are not included in Earth system models, despite being an important source of methane (CH 4 ) and contributing a large fraction of carbon dioxide (CO 2 ) 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 CO 2 and CH 4 emissions, with exceptionally large fluxes in Southeast Asia and the Neotropics. By up-scaling short term measurements, we calculate that c . 90 ± 77 Tg CH 4 yr -1 and 4540 ± 1480 Tg CO 2 yr -1 are released from tropical wetlands globally. CH 4 fluxes are greater from mineral than organic soils, whereas CO 2 fluxes do not differ between soil types. The high CO 2 and CH 4 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.
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    Separating the influence of temperature, drought and fire on interannual variability in atmospheric CO2 (2014)
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    Publication Date: 2014-10-09
    Description: 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 (CO 2 ) 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 CO 2 anomalies. Here, we examined how the temporal evolution of CO 2 in different latitude bands may be used to separate contributions from temperature stress, drought stress, and fire emissions to CO 2 variability. We developed atmospheric CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 variability prior to using contemporary observations to constrain long-term ESM responses.
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    N2O production in the eastern South Atlantic: analysis of N2O stable isotopic and concentration data (2014)
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    Publication Date: 2014-10-09
    Description: The stable isotopic composition of dissolved nitrous oxide (N 2 O) is a tracer for the production, transport, and consumption of this greenhouse gas in the ocean. Here we present dissolved N 2 O 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 N 2 O production by nitrifier denitrification contributed a flux of isotopically depleted N 2 O to the atmosphere. Along the African coast, N 2 O fluxes to the atmosphere of up to 46 µmol/m 2 /day were calculated using satellite-derived QuikSCAT wind speed data, while fluxes at the offshore stations averaged 0.04 µmol/m 2 /day. Comparison of the isotopic composition of the deeper N 2 O 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 N 2 O to the mesopelagic South Atlantic via the Southern Ocean. This deeper N 2 O was isotopically and isotopomerically enriched (δ 15 N bulk -N 2 O =8.7 ± 0.1‰, δ 18 O-N 2 O =46.5 ± 0.2‰, and Site Preference =18.7 ± 0.6‰) relative to the shallow N 2 O source, indicating that N 2 O consumption by denitrification influenced its isotopic composition. The N 2 O concentration maximum was observed between 200 m and 400 m and reached 49 nM near the Angolan coast. The depths of the N 2 O concentration maximum coincided with those of sedimentary particle resuspension along the coast. The isotopic composition of this N 2 O (δ 15 N bulk -N 2 O =5.8 ± 0.1‰, δ 18 O-N 2 O =39.7 ± 0.1‰, and Site Preference =9.8 ± 1.0‰) was consistent with production by diffusion-limited nitrate (NO 3 − ) reduction to nitrite (NO 2 − ), followed by NO 2 − reduction to N 2 O by denitrification and/or nitrifier denitrification, with additional N 2 O production by NH 2 OH decomposition during NH 3 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 N 2 O.
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    Spatial and seasonal variability of the air-sea equilibration timescale of carbon dioxide (2014)
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    Publication Date: 2014-10-09
    Description: 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 CO 2 in both observations and models.
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    Regionalized global budget of the CO2 exchange at the air-water interface in continental shelf seas (2014)
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    Publication Date: 2014-10-09
    Description: Over the past decade, estimates of the atmospheric CO 2 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 〈 100). Here, we propose to derive the CO 2 air-sea exchange of the shelf seas by extracting 3 · 10 6 direct surface ocean CO 2 measurements from the global database SOCAT (Surface Ocean CO 2 Atlas), atmospheric CO 2 values from GLOBALVIEW and calculating gas transfer rates using readily available global temperature, salinity and wind speed fields. We then aggregate our results using a global segmentation of the shelf in 45 units and 152 sub-units to establish a consistent regionalized CO 2 exchange budget at the global scale. Within each unit, the data density determines the spatial and temporal resolutions at which the air-sea CO 2 fluxes are calculated and range from a 0.5 degree resolution in the best surveyed regions to a whole unit resolution in areas where data coverage is limited. Our approach also accounts, for the first time, for the partial sea ice cover of polar shelves. Our new regionalized global CO 2 sink estimate of 0.19 ± 0.05 Pg C yr −1 falls in the low end of previous estimates. Reported to an ice-free surface area of 22 · 10 6  km 2 , this value yields a flux density of 0.7 mol C m −2  yr −1 , ~40% more intense than that of the open ocean. Our results also highlight the significant contribution of Arctic shelves to this global CO 2 uptake (0.07 Pg C yr −1 ).
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    Issue Information (2014)
    Wiley
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    Publication Date: 2014-10-10
    Description: No abstract is available for this article.
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    Contrasting biogeochemical cycles of cobalt in the surface Western Atlantic Ocean (2014)
    Wiley
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    Publication Date: 2014-10-30
    Description: 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.
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    Interactions between reactive nitrogen and the Canadian landscape: A budget approach (2014)
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    Publication Date: 2014-11-01
    Description: The movement of excess reactive nitrogen (N r ) 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 N r 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 60 o N into areas dominated by natural ecosystems, as well as by agricultural and urban/industrial activities to evaluate N r flows within, between and out of these units. We show that Canada is a major exporter of N r due to the availability of inexpensive commercial fertilizers. The large land area suitable for agriculture makes Canada a significant agricultural N r exporter of both grain crops and livestock. Finally, Canada exports petroleum N r mainly to the United States. Because of its location and prevailing atmospheric transport patterns, Canada is a net receptor of N r air pollution from the United States, receiving approximately 20% of the N r leaving the US airshed. We found that overall, terrestrial natural ecosystems as well as the atmosphere are in balance between N r 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 N r 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.
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    Observations and modeling of slow sinking particles in the twilight zone (2014)
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    Publication Date: 2014-10-23
    Description: 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 CO 2 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 CO 2 . 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 (〈10 m · d -1 ) sinking particles. Therefore there is an interest in determining sinking particle velocities and their variations with depth, as well as in understanding the interplay between sinking velocity distributions and carbon export. Here, we use profiles of total and particulate concentrations of the naturally occurring radionuclide pair 210 Po- 210 Pb from the Porcupine Abyssal Plain (PAP) site (48°N, 16.5°W) to estimate depth variation in particle sinking speed using a one-box model and inverse techniques. Average sinking speeds increase from 60 ± 30 m · d -1 at 50 m, to 75 ± 25 m · d -1 and 90 ± 20 m · d -1 at 150 and 500 m. Furthermore, a sensitivity analysis suggests that at the PAP site the measured 210 Po profiles are inconsistent with the usually assumed sinking velocities of 200 m d -1 . We hypothesise that a trend of increasing velocity with depth might be caused by a gradual loss of slow sinking material with depth, a factor with significant implications for regional carbon budgets.
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    The impact of Neogene grassland expansion and aridification on the isotopic composition of continental precipitation (2014)
    Wiley
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    Publication Date: 2014-08-28
    Description: The late Cenozoic was a time of global cooling, increased aridity, and expansion of grasslands. In the last two decades numerous records of oxygen isotopes have been collected to assess plant ecological changes, understand terrestrial paleoclimate, and to determine the surface history of mountain belts. The δ 18 O values of these records, in general, increase from the mid-Miocene to the Recent. We suggest that these records record an increase in aridity and expansion of grasslands in mid-latitude continental regions. We use a non-dimensional isotopic vapor transport model coupled with a soil water isotope model to evaluate the role of vapor recycling and transpiration by different plant functional types. This analysis shows that increased vapor recycling associated with grassland expansion along with bio-mechanistic changes in transpiration by grasses themselves conspire to lower the horizontal gradient in the δ 18 O of atmospheric vapor as an air mass moves into continental interiors. The resulting signal at a given inland site is an increase in δ 18 O of precipitation with the expansion of grasslands and increasing aridity, matching the general observed trend in terrestrial Cenozoic δ 18 O records. There are limits to the isotopic effect that are induced by vapor recycling, which we refer to here as a “hydrostat”. In the modern climate, this hydrostatic limit occurs at approximately the boundary between forest and grassland ecosystems.
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    Close coupling of N-cycling processes expressed in stable isotope data at the redoxcline of the Baltic Sea (2014)
    Wiley
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    Publication Date: 2014-08-24
    Description: Over the past decades, the hypoxic state of the central Baltic Sea has deteriorated because of eutrophication, but little is known about the extent to which related factors such as nitrogen removal have been altered. The Baltic Sea is a stratified semi-enclosed basin with a large, anoxic bottom-water mass in its central Gotland Basin and highly active microbial nitrogen transformation processes at the redoxcline, the interface between oxic and anoxic waters. In this study, we identified and quantified the dominant transformation processes of reactive nitrogen by exploiting fine-resolution profiles of δ 15 N NO3 , δ 18 O NO3 , and δ 15 N NH4 through the pelagic redoxcline between 60 and 140 m. Our results showed increasing δ 15 N NO3 and δ 18 O NO3 values with decreasing nitrate concentrations, but the associated low apparent isotope effect (ϵ = ~5 ‰), as inferred from a closed system Rayleigh model, was not consistent with the high ϵ (~25 ‰) characteristic of denitrification in the water column. These findings could be explained by substrate limitation. The observed δ 18 O NO3 :δ 15 N NO3 ratio of 1.38:1 rather than the usual 1:1 ratio typical for denitrification-dominated systems could be explained by the occurrence of both nitrification and denitrification We then developed a numeric reaction–diffusion model, according to which a realistic denitrification rate of 14 nmol N L −1 d −1 was estimated and a nitrification rate of 6.6 nmol N L −1 d −1 confirmed. Our study demonstrates the value of stable isotope data for investigating nitrogen transformation processes but also highlights that care is needed in interpreting systems with closely coupled processes such as those at ocean redoxclines.
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    Late summer net community production in the central Arctic Ocean using multiple approaches (2014)
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    Publication Date: 2014-09-25
    Description: 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 (O 2 /Ar), (ii) underway measurements of surface partial pressure of carbon dioxide (pCO 2 ), (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 (〈1 mol C m −2 ) were found in the sea-ice covered deep basins with a strong spatial variability. Lowest values were found in the Amundsen Basin and moderate values in the Nansen and Makarov Basins with slightly higher estimates over the Mendeleev Ridge. Our findings support a coupling of NCP to sea ice coverage and nutrient supply and thus stress a potential change in spatial and temporal distribution of NCP in a future Arctic Ocean. To follow the evolution of NCP in space and time, it is suggested to apply one or several of these approaches in shipboard investigations with a time interval of three to five years.
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    Nitrous oxide emissions from cultivated black soil: A case study in Northeast China and global estimates using empirical model (2014)
    Wiley
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    Publication Date: 2014-10-22
    Description: Manure application is effective in promoting soil carbon sequestration, but its impact on N 2 O 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 N 2 O 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 N 2 O was related to nitrification at 30–70% water-filled pore space (WFPS), and heterotrophic nitrification contributed 49% and 25% to total N 2 O at 30% and 70% WFPS, respectively. In an incubation, N 2 O 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 N 2 O 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 N 2 O emissions (kg N ha –1 ) to precipitation (P r , m) and fertilizer N application rate (N r , kg N ha –1 ) (N 2 O =1.533P r +0.0238P r N r ), annual N 2 O emission from global cultivated black soil applied with inorganic fertilizer N was estimated as 347 Gg N. Our results suggested that N 2 O 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 N 2 O emission should be taken into consideration as applying manure increases soil organic carbon sequestration.
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    Sensitivity of tropical forest aboveground productivity to climate anomalies in SW Costa Rica (2014)
    Wiley
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    Publication Date: 2014-11-11
    Description: 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.
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    Iron sources and dissolved-particulate interactions in the seawater of the Western Equatorial Pacific, iron isotope perspectives (2014)
    Wiley
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    Publication Date: 2014-09-04
    Description: 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 (δ 56 Fe) 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 δ 56 Fe 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 Δ 56 Fe DFe – 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.
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    Global Dry Deposition of Nitrogen Dioxide and Sulfur Dioxide Inferred from Space-Based Measurements (2014)
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    Publication Date: 2014-09-04
    Description: ABSTRACT A method is developed to estimate global NO 2 and SO 2 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 SO 2 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 NO 2 contributes 1.5 ± 0.5 Tg of nitrogen and as SO 2 contributes 13.7 ± 4.0 Tg of sulfur. Differences between OMI-inferred NO 2 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. SO 2 dry deposition fluxes compare well with in situ CASTNET-network-inferred flux over North America (slope = 0.98, r = 0.71). The most significant NO 2 dry deposition flux to land per area occurs in the Pearl River Delta, China at 13.9 kg N ha −1  yr −1 , while SO 2 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 NO 2 contributes on average 9–36% and as much as 85% of total NO y dry deposition.
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    Climate change reduces the capacity of northern peatlands to absorb the atmospheric carbon dioxide: the different responses of bogs and fens (2014)
    Wiley
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    Publication Date: 2014-09-07
    Description: The carbon (C) storage of northern peatlands is equivalent to ~34-46% of the ~795 T g C currently held in the atmosphere as CO 2 . Most studies report that northern peatlands are a sink of between 20 and 60 g CO 2 -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.
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    Benchmarking the seasonal cycle of CO2 fluxes simulated by terrestrial ecosystem models (2014)
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    Publication Date: 2014-11-29
    Description: We evaluated the seasonality of CO 2 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 CO 2 mole fraction measurements at surface sites, and 4) CO 2 total columns (X CO2 ) measurements from the Total Carbon Column Observing Network (TCCON). For comparison with atmospheric CO 2 measurements, the LMDZ4 transport model was run with time-varying CO 2 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 CO 2 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 X CO2 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 CO 2 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 CO 2 uptake during the carbon uptake period in the Northern Hemisphere of 7.9 ± 2.0 PgC yr -1 .
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    Variability in efficiency of particulate organic carbon export: A model study (2014)
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    Publication Date: 2014-12-12
    Description: 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%.
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    Issue Information (2014)
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    Publication Date: 2014-12-13
    Description: No abstract is available for this article.
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    Atmospheric iron deposition in the Northwestern Pacific Ocean and its adjacent marginal seas: The importance of coal burning (2014)
    Wiley
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    Publication Date: 2014-12-20
    Description: This study applied a regional air-quality model, incorporated with an emission module, to quantitatively differentiate the atmospheric iron sources originating from lithogenic dusts or coal-burning fly ashes deposited in the Northwest Pacific Ocean and its marginal seas. Particular attention was paid to the high iron content of fly ashes emitted from steel and iron plants burning coals. Using the year 2007 as an example, the modeling results exhibit large seasonal variations in iron deposition, with highest deposition fluxes occurred during spring and autumn, which are comparable to the seasonal fluctuation of chlorophyll a concentrations estimated by satellite images in the oceanic regions. Fly ash from coal burning accounted for 7.2% of the total iron deposited over the Northwest Pacific Ocean and 15% of that over the northern South China Sea. After considering the difference of iron solubility in the aerosols, anthropogenic aerosol associated with coal burning would be the major bioavailable iron source in the surface water of the oceanic regions.
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    The impact of changing surface ocean conditions on the dissolution of aerosol iron (2014)
    Wiley
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    Publication Date: 2014-10-17
    Description: 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  μ m) 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 (〈0.02  μ m). This finding highlights the potential importance of organic ligands in retaining aerosol Fe in a biologically available form in the surface ocean.
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    The significance of the episodic nature of atmospheric deposition to Low Nutrient Low Chlorophyll regions (2014)
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    Publication Date: 2014-10-16
    Description: 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 (〈1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple “fertilization effect” as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.
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    Six centuries of changing oceanic mercury (2014)
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    Publication Date: 2014-10-17
    Description: 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.
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    Reconciling modeled and observed atmospheric deposition of soluble organic nitrogen at coastal locations (2014)
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    Publication Date: 2014-05-22
    Description: Atmospheric deposition of reactive nitrogen (N) species from air pollutants is a significant source of exogenous nitrogen in marine ecosystems. Here we use an atmospheric chemical transport model to investigate the supply of soluble organic nitrogen (ON) from anthropogenic sources to the ocean. Comparisons of modeled deposition with observations at coastal and marine locations show good overall agreement for inorganic nitrogen and total soluble nitrogen. However, previous modeling approaches result in significant underestimates of the soluble ON deposition if the model only includes the primary soluble ON and the secondary oxidized ON in gases and aerosols. Our model results suggest that including the secondary reduced ON in aerosols as a source of soluble ON contributes to an improved prediction of the deposition rates (g N m −2  yr −1 ). The model results show a clear distinction in the vertical distribution of soluble ON in aerosols between different processes from the primary sources and the secondary formation. The model results (excluding the biomass burning and natural emission changes) suggest an increase in soluble ON outflow from atmospheric pollution, in particular from East Asia, to the oceans in the twentieth century. These results highlight the necessity of improving the process-based quantitative understanding of the chemical reactions of inorganic nitrogen species with organics in aerosol and cloud water.
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    Sunlight-induced carbon dioxide emissions from inland waters (2014)
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    Publication Date: 2014-06-21
    Description: The emissions of carbon dioxide (CO 2 ) from inland waters are substantial on a global scale. Yet, the fundamental question remains open which proportion of these CO 2 emissions is induced by sunlight via photochemical mineralization of dissolved organic carbon (DOC), rather than by microbial respiration during DOC decomposition. Also, it is unknown on larger spatial and temporal scales how photochemical mineralization compares to other C fluxes in the inland water C cycle. We combined field and laboratory data with atmospheric radiative transfer modeling to parameterize a photochemical rate model for each day of the year 2009, for 1086 lakes situated between latitudes from 55 to 69°N in Sweden. The sunlight-induced production of dissolved inorganic carbon (DIC) averaged 3.8 ± 0.04 g C m -2  yr -1 , which is a flux comparable in size to the organic carbon burial in the lake sediments. Countrywide, 151 ± 1 kt C yr -1 was produced by photochemical mineralization, corresponding to about 12% of total annual mean CO 2 emissions from Swedish lakes. With a median depth of 3.2 m, the lakes were generally deep enough that incoming, photochemically active photons were absorbed in the water column. This resulted in a linear positive relationship between DIC photoproduction and the incoming photon flux, which correspond to the absorbed photons. Therefore, the slope of the regression line represents the wavelength- and depth-integrated apparent quantum yield of DIC photoproduction. We used this relationship to obtain a first estimate of DIC photoproduction in lakes and reservoirs worldwide. Global DIC photoproduction amounted to 13 and 35 Mt C yr -1 under overcast and clear sky, respectively. Consequently, these directly sunlight-induced CO 2 emissions contribute up to about one tenth to the global CO 2 emissions from lakes and reservoirs, corroborating that microbial respiration contributes a substantially larger share than formerly thought, and generate annual C fluxes similar in magnitude to the C burial in natural lake sediments worldwide.
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    Variability of alkalinity and the alkalinity-salinity relationship in the tropical and subtropical surface ocean (2014)
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    Publication Date: 2014-06-22
    Description: The variability of total alkalinity (TA) and its relationship with salinity in the tropical and subtropical surface ocean were examined using data collected in various marine environments from a ship of opportunity. In the open ocean regions of the Atlantic, Pacific and Indian Ocean, sea surface TA variability was observed to be mainly controlled by the simple dilution or concentration (SDC) effect of precipitation and evaporation, and the measured concentrations of TA agreed well with those predicted from salinity and temperature. Non-SDC changes in alkalinity in ocean margins and inland seas were examined by comparing the salinity-normalized alkalinity (NTA) with that of the open ocean end member. Non-SDC alkalinity additions to the western North Atlantic margin, eastern North Pacific margin and Mediterranean Sea were identified, which mainly resulted from river inputs and shelf currents. In contrast, removal of TA through formation and sedimentation of calcium carbonate was observed to be an important control in the Red Sea. The concentration of the river end member can only be reliably derived from the y-intercept of TA-S regression (TA S0 ) in river-dominated systems such as estuaries and river plumes. In coastal regions where other processes (evaporation, shelf currents, upwelling, calcification etc.) are more influential, TA S0 can significantly deviate from the river water concentration and hence be an unreliable indicator of it. Negative values of TA S0 can result from non-SDC TA removal at the low salinity end (relative to the salinity of the oceanic end member) and/or non-SDC TA addition at high salinities (as occurs in the Mediterranean Sea).
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    Publication Date: 2014-07-13
    Description: No abstract is available for this article.
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    The role of soil processes in δ18O terrestrial climate proxies (2014)
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    Publication Date: 2014-02-19
    Description: [1]  A paleoclimate interpretation of a terrestrial hydrologic proxy such as the δ 18 O of tree cellulose or speleothem calcite may be biased or mis-interpreted if the isotopic composition of the soil water from which the proxy originated undergoes isotopic exhange or fractionation. In this study, we use a global isotope-enabled land surface model (IsoLSM) to investigate how the δ 18 O of precipitation may be altered in a soil column due to evaporation and vertical moisture transport. In order to assess how precipitation and evaporation contribute to the soil water isotopic variability, we compare seasonal and interannual changes in simulated xylem water δ 18 O within a control simulation and in a suite of sensitivity experiments where the effect of precipitation δ 18 O, water vapor δ 18 O, and soil water evaporation are independently removed. The simulations, carried out for the period 1979 to 2004, reveal that in semi-arid regions, such as the southwest United States, the seasonal cycle in xylem water δ 18 O is strongly affected by evaporative loss during the dry season and evaporation can also constitute as much as 50% of the interannual δ 18 O variance. Additional simulations, including soil water tagging experiments, indicate that upward fluxes of soil water occur during drier periods. For soil water δ 18 O profiles that are isotopically more depleted in 18 O at depth, this imparts a low isotopic signature to xylem water δ 18 O during such dry intervals. Hence, without taking into account vertical moisture transport in the soils, low δ 18 O years could be misinterpreted as wet conditions (due to decreased evaporative enrichment) when instead drier conditions are equally as likely.
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    Potential future dynamics of carbon fluxes and pools in New England forests and their climatic sensitivities: a model-based study (2014)
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    Publication Date: 2014-02-19
    Description: [1]  Projections of terrestrial carbon (C) dynamics must account for interannual variation in ecosystem C exchange associated with climate change, increasing atmospheric CO 2 concentration, and species dynamics. We used the dynamic ecosystem model LPJ-GUESS to (i) project the potential dynamics of C in New England forests under nine climate change scenarios (CCSs) for the 21 st century, and (ii) examine the sensitivity of potential C dynamics to changes in climate and atmospheric CO 2 concentration. Our results indicated that forest net primary productivity (NPP) and soil heterotrophic respiration (RH) averaged 428 and 279 gC/m 2 /yr and New England forests sequestered CO 2 by 149 gC/m 2 /yr in the baseline period (1971–2000). Under nine future CCSs, NPP and RH were modeled to increase by an average rate of 0.85 and 0.56 gC/m 2 /yr 2 during 1971–2099. The asymmetric increase in modeled NPP and RH resulted in New England forests sequestering atmospheric CO 2 at a net rate of 0.29 gC/m 2 /yr 2 with increases in both vegetation and soil C. In addition, changes in C fluxes and pools varied spatially with generally larger changes (except for NEE) occurring in southern compared to northern New England. Model simulations also indicated that climate warming alone decreases NPP, resulting in a net efflux of C from forests. In contrast, increasing precipitation by itself stimulates CO 2 sequestration by forests. At the individual cell level, however, changes in temperature or precipitation can either positively or negatively affect consequent C dynamics. Elevation of CO 2 levels was found to be the biggest driver for modeled future enhancement of C sequestration. Without elevation of CO 2 levels, climate warming has the potential to change New England forests from C sinks to sources in the late 21 st century.
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    Factors influencing export of dissolved inorganic nitrogen by major rivers: a new, seasonal, spatially explicit, global model (2014)
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    Publication Date: 2014-02-06
    Description: ABSTRACT [1]  Substantial effort has focused on understanding spatial variation in dissolved inorganic nitrogen (DIN) export to the coastal zone and specific basins have been studied in depth. Much less is known, however, about seasonal patterns and controls of coastal DIN delivery across large spatial scales. Understanding seasonal patterns of DIN export is critical to efforts to predict impacts of coastal eutrophication, such as algal blooms and hypoxic areas, which are often seasonal phenomena. Here we describe, test, and apply a global model that predicts seasonal DIN export to coastal regions for 〉6,000 rivers using the Nutrient Export from Watersheds (NEWS2) model. NEWS2-DIN-S used spatially explicit, seasonal N inputs and was calibrated with measured DIN yield (kg N km -2 season -1 ) for 77 rivers, distributed globally. Of the characteristics considered, DIN-transport efficiency was positively related to runoff and negatively related to temperature (r 2  = 0.34-0.60, depending on season p 〈 0.0001), likely due to flushing effects and increased retention by plants and soils, respectively. NEWS2-DIN-S incorporated these insights and performed well in predicting DIN yield (Nash-Sutcliffe Efficiency = 0.54-0.65, depending on season). Catchments were effective in retaining DIN and average export rates were lower during the growing season (3-5% of TN inputs) compared to other seasons (6-10%) for major latitude bands. Model output was insensitive to changes in the magnitude of N inputs, suggesting that refinement of seasonal N input budgets will not substantially improve model performance. Rather, better representation of land-to-river N transfers could improve future models because of strong landscape N attenuation.
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    CO2 and CH4 emissions from streams in a lake-rich landscape: Patterns, controls and regional significance (2014)
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    Publication Date: 2014-02-07
    Description: [1]  Aquatic ecosystems are important components of landscape carbon budgets. In lake-rich landscapes, both lakes and streams may be important sources of carbon gases (CO 2 and CH 4 ) to the atmosphere, but the processes that control gas concentrations and emissions in these interconnected landscapes have not been adequately addressed. We use multiple datasets that vary in their spatial and temporal extent during 2001-2012 to investigate the carbon gas source strength of streams in a lake-rich landscape and to determine the contribution of lakes, metabolism and groundwater to stream CO 2 and CH 4 . We show that streams emit roughly the same mass of CO 2 (23.4 Gg C yr -1 ; 0.49 mol CO 2 m -2  day -1 ) as lakes at a regional scale (27 Gg C yr -1 ), and that stream CH 4 emissions (189 Mg C yr -1 ; 8.46 mmol CH 4 m -2  day -1 ) are an important component of the regional greenhouse gas balance. Gas transfer velocity variability (range = 0.34 to 13.5 m day -1 ) contributed to the variability of gas flux in this landscape. Groundwater inputs and in-stream metabolism control stream gas supersaturation at the landscape scale, while carbon cycling in lakes and deep groundwaters do not control downstream gas emissions. Our results indicate the need to consider connectivity of all aquatic ecosystems (lakes, streams, wetlands and groundwater) in lake-rich landscapes and their connections with the terrestrial environment in order to understand the full nature of the carbon cycle.
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    Contribution of Anthropogenic Phosphorus to Agricultural Soil Fertility and Food Production (2014)
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    Publication Date: 2014-06-25
    Description: Agricultural intensification over the last few decades has been accompanied by the extensive use of anthropogenic phosphorus (P) derived from mined phosphate rock. Given the increasing scarcity of P resources, accurate estimates of the reliance of agriculture on anthropogenic P are required. Here, we propose a modeling approach for assessing the contribution of anthropogenic P to agricultural soil fertility and food production. We performed computations at country level and France was chosen as a typical Western European country with intensive agriculture. Four soil P pools were identified based on their bioavailability (labile vs. stable) and origin (anthropogenic vs. natural). Pool evolution between 1948 and 2009 was estimated by combining international databases and a simple biogeochemical model. An optimization procedure demonstrated the necessity of representing a stable P pool capable of replenishing the labile pool within 14 to 33 years in order to match country-scale observations. Mean simulated P pool sizes for 2009 (0–35 cm soil horizon) were 146, 616, 31 and 156 kgP/ha for natural stable, anthropogenic stable, natural labile and anthropogenic labile pools, respectively. We found that, on average 82% (min-max: 68–91%) of soil P (sum of labile and above defined stable) in that year was anthropogenic. The temporal evolution of this contribution is directly related to the integral of chemical fertilizer use over time, starting from 1948. The contribution of anthropogenic P to food production was similar at 84% (min-max: 72–91%), which is greater than budget-based estimates (~50–60%) commonly reported in the literature. By focusing on soil fertility and food production, this study provides a quantitative estimation of human perturbations of the P cycle in agroecosystems.
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    Soil organic carbon sequestration in upland soils of northern China under variable fertilizer management and climate change scenarios (2014)
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    Publication Date: 2014-03-04
    Description: [1]  We determined the historical change in soil organic carbon (SOC) stocks from long-term inorganic fertilizer and/or organic manure trials (maize and wheat dominated rotations) that represent major soil types and climatic conditions of northern China. Soil carbon (RothC, Rothamsted, UK) and general circulation models (BCCR, Bjerknes Centre for Climate Research, Norway, and IPSL, Institute Pierre Simon Laplace, France) were validated using these field trial data sets. We then applied these models to predict future change in SOC stocks to 2100 using two net primary production (NPP) carbon input scenarios (i.e., current NPP or 1% yr -1 NPP increase). Here we show that the conversion rate of plant residues to SOC was higher in single-cropping sites than in double-cropping sites. The prediction of future SOC sequestration potential indicated that these soils will be a net source of carbon dioxide (CO 2 ) under no fertilizer inputs. Even when inorganic nutrients were applied the additional carbon input from increased plant residues could not meet the depletion of SOC in parts of northern China. Manure or straw application could however improve the SOC sequestration potential at all study sites. The SOC sequestration potential in northern China was estimated to be −4.3 to 18.2 t C ha -1 by 2100. The effect of projected climate change on the annual rate of SOC change did not differ significantly between climate scenarios. The average annual rate of SOC change under current and increased NPP scenarios (when using the IPSL and BCCR models at 850 ppm CO 2 ) was ca . 0.136 t C ha -1  yr -1 in northern China. These findings highlight the need to maintain, and where possible increase, organic carbon inputs into these farming systems which are rapidly becoming inorganic fertilizer intensive.
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    The fate of terrigenous dissolved organic carbon in a river-influenced ocean margin (2014)
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    Publication Date: 2014-03-04
    Description: [1]  The mineralization of terrigenous dissolved organic carbon (tDOC) discharged by rivers can impact nutrient and trace metal cycling, biological productivity, net ecosystem metabolism, and air-sea CO2 exchange in ocean margins. However, the extreme heterogeneity of river-influenced ocean margins represents a major challenge for quantitative assessments of tDOC transformations, and thereby obscures the role of tDOC in biogeochemical cycles. Here, a lignin based optical proxy for tDOC and a shelf-wide mass balance approach were used to quantitatively assess the fate of tDOC discharged from the Mississippi-Atchafalaya River System (M-ARS) to the Louisiana shelf. The mass balance revealed that ~40% of the tDOC discharged by the M-ARS during March 2009-2010 was mineralized to CO2 on the Louisiana shelf, with 2/3 of the mineralization taking place in the mixed layer. A strong seasonality in tDOC mineralization was observed, with mineralization rates several-fold higher during summer than during winter. Independent assessments of specific mineralization processes indicated biomineralization accounted for ~94% of the tDOC mineralization on an annual basis, and suggest that photochemical transformations of tDOC enhanced biomineralization by ~50% in the mixed layer. Direct photomineralization accounted for a relatively small fraction (~6%) of the tDOC mineralization on an annual basis. This quantitative assessment directly confirms ocean margins are major sinks of the tDOC discharged by rivers, and indicate tDOC mineralization rates in the shelf mixed layer are sufficiently large to influence whether the Louisiana shelf is a net sink or source of atmospheric CO2.
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    Publication Date: 2014-04-16
    Description: No abstract is available for this article.
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    Publication Date: 2014-03-21
    Description: No abstract is available for this article.
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    Linking variability in soil solution dissolved organic carbon to climate, soil type and vegetation type (2014)
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    Publication Date: 2014-04-12
    Description: Lateral transport of carbon plays an important role in linking the carbon cycles of terrestrial and aquatic ecosystems. There is, however, a lack of information on the factors controlling one of the main C sources of this lateral flux i.e. the concentration of dissolved organic carbon (DOC) in soil solution across large spatial scales and under different soil, vegetation and climate conditions. We compiled a database on DOC in soil solution down to 80 cm and analyzed it with the aim, firstly, to quantify the differences in DOC concentrations among terrestrial ecosystems, climate zones, soil and vegetation types at global scale and, secondly, to identify potential determinants of the site-to-site variability of DOC concentration in soil solution across European broadleaved and coniferous forests. We found that DOC concentrations were 75% lower in mineral than in organic soil and temperate sites showed higher DOC concentrations than boreal and tropical sites. The majority of the variation (R 2  = 0.67-0.99) in DOC concentrations in mineral European forest soils correlates with NH 4 + , C/N, Al and Fe as the most important predictors. Overall, our results show that the magnitude (23% lower in broadleaved than in coniferous forests) and the controlling factors of DOC in soil solution differ between forest types, with site productivity being more important in broadleaved forests and water balance in coniferous stands.
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    Inherited hypoxia: A new challenge for reoligotrophicated lakes under global warming (2014)
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    Publication Date: 2014-11-15
    Description: 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.
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    Publication Date: 2014-11-13
    Description: No abstract is available for this article.
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    Sources and Fate of Polycyclic Aromatic Hydrocarbons in the Antarctic and Southern Ocean Atmosphere (2014)
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    Publication Date: 2014-11-14
    Description: 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.
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    Net ecosystem production and organic carbon balance of U.S. east coast estuaries: A synthesis approach (2014)
    Wiley
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    Publication Date: 2014-12-30
    Description: Net ecosystem production (NEP) and the overall organic carbon budget for the estuaries along the east coast of the United States are estimated. We focus on the open estuarine waters, excluding the fringing wetlands. We developed empirical models relating NEP to loading ratios of dissolved inorganic nitrogen to total organic carbon, and carbon burial in the sediment to estuarine water residence time and total nitrogen input across the landward boundary. Output from a data-constrained water quality model was used to estimate inputs of total nitrogen and organic carbon to the estuaries across the landward boundary, including fluvial and tidal-wetland sources. Organic carbon export from the estuaries to the continental shelf was computed by difference, assuming steady state. Uncertainties in the budget were estimated by allowing uncertainties in the supporting model relations. Collectively, U.S. east coast estuaries are net heterotrophic, with the area-integrated NEP of −1.5 (−2.8, −1.0) Tg C yr −1 (best estimate and 95% confidence interval) and area-normalized NEP of −3.2 (−6.1, −2.3) mol C m −2  yr −1 . East coast estuaries serve as a source of organic carbon to the shelf, exporting 3.4 (2.0, 4.3) Tg C yr −1 or 7.6 (4.4, 9.5) mol C m −2  yr −1 . Organic carbon inputs from fluvial and tidal-wetland sources for the region are estimated at 5.4 (4.6, 6.5) Tg C yr −1 or 12 (10, 14) mol C m −2  yr −1 and carbon burial in the open estuarine waters at 0.50 (0.33, 0.78) Tg C yr −1 or 1.1 (0.73, 1.7) mol C m −2  yr −1 . Our results highlight the importance of estuarine systems in the overall coastal budget of organic carbon, suggesting that in the aggregate, U.S. east coast estuaries assimilate (via respiration and burial) ~40% of organic carbon inputs from fluvial and tidal-wetland sources and allow ~60% to be exported to the shelf.
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    Estimating the soil carbon sequestration potential of China's Grain for Green Project (2014)
    Wiley
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    Publication Date: 2014-10-18
    Description: 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 CO 2 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.
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    Decadal (1994–2008) change in the carbon isotope ratio in the eastern South Pacific Ocean (2014)
    Wiley
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    Publication Date: 2014-07-12
    Description: We determined the 14-year change in the anthropogenic CO 2 inventory in the eastern South Pacific Ocean along the 110˚W meridian from 67˚S to 21˚N, using seawater δ 13 C datasets collected in 1994 and 2008. The vertical integral of the 14-year δ 13 C change was assessed in five latitude bands, and found to be greatest (–14.7‰ m yr –1 ) in the subpolar band (38˚S–55˚S) and smallest (–3.0‰ m yr –1 ) in the tropical band (21˚N–18˚S). The δ 13 C change in each of the latitudinal bands was primarily caused by inputs of anthropogenic CO 2 via air-sea exchange and transport. More than 50% of the total anthropogenic CO 2 was added to the subpolar band via the northward movement of Antarctic Intermediate Water (AAIW) from the south, and the remaining 50% was added via air-sea exchange. We also calculated the ratio of the temporal change in δ 13 C to the change in dissolved inorganic carbon, which is a measure of the efficiency of oceanic uptake of anthropogenic CO 2 . The ratio for AAIW in 1994 (–0.017‰ (µmol kg –1 ) –1 ) was greater than that in 2008 (–0.010‰ (µmol kg –1 ) –1 ) based on the change in preformed δ 13 C and DIC, indicating reduced efficiency of CO 2 uptake by the Southern Ocean in 2008 relative to that in 1994. AAIW remained at the surface for a shorter period in 2008 relative to 1994, and thus would have taken up less atmospheric CO 2 prior to subduction. The projected reduction in this ratio indicates a weakening of CO 2 uptake by the Southern Ocean in the future.
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    Contrasting vulnerability of drained tropical and high-latitude peatlands to fluvial loss of stored carbon (2014)
    Wiley
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    Publication Date: 2014-09-30
    Description: 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 CO 2 sinks to large CO 2 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 ( 14 C) 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- 14 C 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; 14 C measurements from UK blanket bogs suggest that exported DOC remains young (〈50 years) despite drainage. Boreal and temperate fens and raised bogs in Finland and the Czech Republic showed intermediate sensitivity. We attribute observed differences to physical and climatic differences between peatlands, in particular hydraulic conductivity and temperature, as well as the extent of disturbance associated with drainage, notably land-use changes in the tropics. Data from the UK Peak District, an area where air pollution and intensive land-management have triggered Sphagnum loss and peat erosion, suggest that additional anthropogenic pressures may trigger fluvial loss of much older (〉500 year) carbon in high-latitude systems. Re-wetting at least partially offsets drainage effects on DOC age.
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    Production of dissolved organic carbon enriched in deoxy-sugars represents an additional sink for biological C drawdown in the Amazon River plume (2014)
    Wiley
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    Publication Date: 2014-09-30
    Description: 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 (δ 13 C) 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 N 2 fixation, which may be widespread in mesohaline environments.
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    Non-growing-season soil respiration is controlled by freezing and thawing processes in the summer-monsoon-dominated Tibetan alpine grassland (2014)
    Wiley
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    Publication Date: 2014-09-14
    Description: 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 (R s ), but detailed measurements to assess the patterns, drivers of and potential feedbacks on R s have not been made yet. We conducted a 4-year study on R s using a unique R s -measuring system, composed of an automated soil CO 2 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 R s was 82–89 g C m −2 , accounting for 11.8-13.2% of the annual total R s ; 2) surface soil freezing controlled the diurnal pattern of R s and bulk soil freezing induced lower reference respiration rate (R 0 ) and temperature sensitivity (Q 10 ) than those in the growing season (0.40-0.53 vs. 0.84-1.32 µmol CO 2 m −2  s −1 for R 0 and 2.5-2.9 vs. 2.9-5.6 for Q 10 ); and 3) the intra-annual variation in cumulative R s 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 R s , respectively. Our results suggest that warmer winters can trigger carbon loss from this ecosystem because of higher Q 10 of thawed than frozen soils.
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    Isotopic evidence for a marine ammonium source in rainwater at Bermuda (2014)
    Wiley
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    Publication Date: 2014-09-18
    Description: 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 δ 15 N 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 δ 15  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 δ 15 N data appear to require that most of the ammonium/a flux to the ocean is by dissolution in surface waters rather than atmospheric deposition. This suggests that the atmosphere and surface ocean are near equilibrium with respect to air/sea gas exchange, implying that anthropogenic ammonia will equilibrate near the coast and not reach the open marine atmosphere. Whereas ~90% of the ammonium deposition to the global ocean has previously been attributed to anthropogenic sources, the evidence at Bermuda suggests that the anthropogenic contribution could be much smaller.
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    Heterotrophic bacteria are major nitrogen fixers in the euphotic zone of the Indian Ocean (2014)
    Wiley
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    Publication Date: 2014-09-24
    Description: 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.
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    The biogeochemical cycling of zinc and zinc isotopes in the North Atlantic Ocean (2014)
    Wiley
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    Publication Date: 2014-09-24
    Description: Zinc (Zn) is a marine micronutrient, with an overall oceanic distribution mirroring the major macronutrients, especially silicate. Seawater Zn isotope ratios (δ 66 Zn) 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 δ 66 Zn 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 δ 66 Zn, 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 δ 66 Zn. In the ocean below 1000 m, δ 66 Zn 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 δ 66 Zn 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 δ 66 Zn signature of marine sources and the globally homogenous deep oceans (δ 66 Zn of +0.5‰) on long timescales, with overall patterns pointing to sediments as an important sink for isotopically light Zn throughout the oceans.
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    Recent variability of the global ocean carbon sink (2014)
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    Publication Date: 2014-08-14
    Description: . We present a new observation-based estimate of the global oceanic carbon dioxide (CO 2 ) sink and its temporal variation on a monthly basis from 1998 through 2011 and at a spatial resolution of 1° × 1°. This sink estimate rests upon a neural network-based mapping of global surface ocean observations of the partial pressure of CO 2 (pCO 2 ) from the Surface Ocean CO 2 Atlas (SOCAT) database. The resulting pCO 2 has small biases when evaluated against independent observations in the different ocean basins, but larger randomly distributed differences exist particularly in the high latitudes. The seasonal climatology of our neural network-based product agrees overall well with the Takahashi et al. [2009] climatology, although our product produces a stronger seasonal cycle at high latitudes. From our global pCO 2 product, we compute a mean net global ocean (excluding the Arctic Ocean and coastal regions) CO 2 uptake flux of -1.42 ± 0.53 Pg C yr −1 , which is in good agreement with ocean inversion based estimates. Our data indicate a moderate level of interannual variability in the ocean carbon sink (±0.12 Pg C yr −1 , 1 σ ) from from 1998 through 2011, mostly originating from the equatorial Pacific Ocean, and associated with the El Niño Southern Oscillation (ENSO). Accounting for steady-state riverine and Arctic Ocean carbon fluxes our estimate further implies a mean anthropogenic CO 2 uptake of -1.99 ± 0.59 Pg C yr −1 over the analysis period. From this estimate plus the most recent estimates for fossil fuel emissions and atmospheric CO 2 accumulation, we infer a mean global land sink of -2.82 ± 0.85 Pg C yr −1 over the 1998 through 2011 period with strong interannual variation.
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    Global patterns of ecosystem carbon flux in forests: A biometric data-based synthesis (2014)
    Wiley
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    Publication Date: 2014-08-21
    Description: Forest ecosystems function as a significant carbon sink for atmospheric carbon dioxide. However, our understanding of global patterns of forest carbon fluxes remains controversial. Here we examined global patterns and environmental controls of forest carbon balance using biometric measurements derived from 243 sites and synthesized from 81 publications around the world. Our results showed that both production and respiration increased with mean annual temperature and exhibited unimodal patterns along a gradient of precipitation. However, net ecosystem production (NEP) initially increased and subsequently declined along gradients of both temperature and precipitation. Our results also indicated that ecosystem production increased during stand development but eventually leveled off, whereas respiration was significantly higher in mature and old forests than in young forests. The residual variation of carbon flux along climatic and age gradients might be explained by other factors such as atmospheric CO 2 elevation and disturbances ( e . g ., forest fire, storm damage and selective harvest). Heterotrophic respiration (R h ) was positively associated with net primary production (NPP), but the R h -NPP relationship differed between natural and planted forests: R h increased exponentially with NPP in natural forests but tended toward saturation with increased NPP in planted forests. Comparison of biometric measurements with eddy-covariance observations revealed that ecosystem carbon balance derived from the latter generated higher overall NEP estimates. These results suggest that the eddy-covariance observations may overestimate the strength of carbon sinks, and thus biometric measurements need to be incorporated into global assessments of the forest carbon balance.
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    Biogeochemical control of marine productivity in the Mediterranean Sea during the last 50 years (2014)
    Wiley
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    Publication Date: 2014-08-07
    Description: The temporal dynamics of biogeochemical variables derived from a coupled 3D model of the Mediterranean Sea are evaluated for the last 50 years (1960–2010) against independent data on fisheries catches per unit effort (CPUE) for the same time period. Concordant patterns are found in the time-series of all of the biological variables (from the model and from fisheries statistics), with low values at the beginning of the series, a later increase, with maximum levels reached at the end of the 1990's, and a posterior stabilization. Spectral analysis of the annual biological time series reveals coincident low-frequency signals in all of them. The first, more energetic signal peaking around the year 2000, while the second, less energetic signal peaks near 1982. Almost identical low-frequency signals are found in the nutrient loads of the rivers and in the integrated nutrient levels in the surface marine ecosystem. Nitrate concentration shows a maximum level in 1998, with a later stabilization to present day values, coincident with the first low-frequency signal found in the biological series. Phosphate shows maximum concentrations around 1982 and a posterior sharp decline, in concordance with the second low-frequency signal observed in the biological series. That result seems to indicate that the control of marine productivity (plankton to fish) in the Mediterranean is principally mediated through bottom-up processes that could be traced back to the characteristics of riverine discharges. The high sensitivity of CPUE time series to environmental conditions might be another indicator of the overexploitation of this marine ecosystem.
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    Influence of water depth on the carbon sequestration capacity of seagrasses (2014)
    Wiley
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    Publication Date: 2014-08-17
    Description: The actual estimates of carbon stocks beneath seagrass meadows worldwide are derived from few data, resulting in a tendency to generalize global carbon stocks from a very limited number of seagrass habitats. We surveyed Posidonia oceanica and Posidonia sinuosa meadows along depth-induced gradients of light availability to assess the variability in their sedimentary organic carbon (C org ) stocks and accretion rates. This study showed a 4-fold decrease in C org stocks from 2-4 m to 6-8 m depth P. sinuosa meadows (averaging 7.0 and 1.8 kg m -2 , respectively; top meter of sediment) and a 14- to 16-fold decrease from shallow (2 m) to deep (32 m) P. oceanica meadows (200 and 19 kg m -2 average, respectively; top 2.7 m of sediment). The average C org accretion rates in shallow P. sinuosa meadows were higher (10.5 g m -2 y -1 ) than in deeper meadows (2.1 g m -2 y -1 ). The reduction of sedimentary C org stocks and accretion rates along depth-related gradients of light reduction suggest that irradiance, controlling plant productivity, meadow density and sediment accretion rates, is a key environmental factor affecting C org storage potential of seagrasses. The results obtained highlighted the exceptional carbon storage capacity of P. oceanica meadows at Balearic Islands (Spain), containing the highest areal C org stocks of all seagrasses (estimated in up to 691-770 kg m -2 in 8-13 m-thick deposits). Seagrass communities are experiencing worldwide decline, and reduced irradiance (following e.g. eutrophication or sediment regime alterations) will lead to photo-acclimation responses (i.e. reduced plant productivity and shoot density), which may impact the carbon sequestration capacity of seagrasses.
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    Issue Information (2014)
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    Publication Date: 2014-08-13
    Description: No abstract is available for this article.
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    Radium isotopes as a tracer of sediment-water column exchange in the North Sea (2014)
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    Publication Date: 2014-07-19
    Description: Sediment-water column exchange plays an important role in coastal biogeochemistry. We utilize short-lived radium isotopes ( 224 Ra and 223 Ra) to understand and quantify the dominant processes governing sediment-water column exchange throughout the North Sea. Our comprehensive survey, conducted in September 2011, represents the first of its kind conducted in the North Sea. We find that two main sources regulate surface Ra distributions: minor coastal input from rivers and shallow mudflats, and North Sea sediments as the dominant source. Porewaters show 100-fold larger activities than the water column. North Sea sediment characteristics such as porosity and mean grain size, as well as turbulence at the sediment-water interface, are the dominant factors contributing to variability of Ra efflux. Ra inventory and mass-balance approaches consistently yield high benthic Ra effluxes in the southern North Sea, driven by strong tidal and wind mixing, which in turn cause high sediment irrigation rates. These results exceed incubation-based Ra flux estimates, and the majority of previously reported Ra flux estimates for other regions. Ra-based estimates of benthic alkalinity fluxes compare well to observed values and the high rates of Ra efflux imply a potentially significant exchange of other products of sedimentary reactions, including carbon and nutrient species. Passive tracer simulations lend strong support to the Ra source attribution and imply seasonal variation in the surface water Ra distribution depending on stratification conditions.
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    Impacts and effects of mesoscale ocean eddies on ocean carbon storage and atmospheric pCO2 (2014)
    Wiley
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    Publication Date: 2014-08-03
    Description: An idealised numerical ocean model is used to investigate the sensitivity of the partial pressure of atmospheric carbon dioxide ( pCO 2 ) to changes in surface wind stress when mesoscale eddies are permitted in the flow. When wind stress increases, pCO 2 increases, and vice versa. The introduction of mesoscale eddies reduces the overall sensitivity of pCO 2 by changing the sensitivity of ocean carbon storage due to the saturation state of carbon dioxide, the net air-sea disequilibrium, soft tissue carbon, and the carbonate pump. However, a full carbon pump decomposition shows different responses for different ocean carbon storage terms. For example, air-sea disequilibrium is actually more sensitive to increased winds at eddy-permitting resolution, whereas soft tissue carbon is much less sensitive to wind changes in an eddy-permitting ocean. Changes in pycnocline depth and the strength of both upper and lower cells of the meridional overturning circulation affect this sensitivity.
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    Effects of soot deposition on particle dynamics and microbial processes in marine surface waters (2014)
    Wiley
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    Publication Date: 2014-06-22
    Description: Large amounts of soot are continuously deposited on the global ocean. Even though significant concentrations of soot particles are found in marine waters, the effects of these aerosols on ocean ecosystems are currently unknown. Using a combination of in situ and experimental data, and results from an atmospheric transport model, we show that the deposition of soot particles from an oil-fired power plant impacted biogeochemical properties and the functioning of the pelagic ecosystem in tropical oligotrophic oceanic waters off New Caledonia. Deposition was followed by a major increase in the volume concentration of suspended particles, a change in the particle size spectra that resulted from a stimulation of aggregation processes, a 5% decrease in the concentration of dissolved organic carbon (DOC), a decreases of 33 and 23% in viral and free bacterial abundances, respectively, and a factor ~2 increase in the activity of particle-attached bacteria suggesting that soot introduced in the system favoured bacterial growth. These patterns were confirmed by experiments with natural seawater conducted with both soot aerosols collected in the study area and standard diesel-soot. The data suggest a strong impact of soot deposition on ocean-surface particles, DOC, and microbial processes, at least near emission hotspots.
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  • 87
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    Characterizing the annual cycle of African dust transport to the Caribbean Basin and South America and its impact on the environment and air quality. (2014)
    Wiley
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    Publication Date: 2014-06-14
    Description: Decades of aerosol measurements on Barbados have yielded a detailed picture of African mineral dust transport to the Caribbean Basin that shows a strong seasonal cycle with a maximum in boreal summer and a minimum in winter. Satellite aerosol products suggest that in spring there is a comparable transport to northeastern South America. Here we characterize the complete annual cycle of dust transport to the western Atlantic by linking the Barbados record to multi-year records of airborne particulate matter less than 10 um diameter (PM 10 ) measured in air quality programs at Cayenne (French Guiana) and Guadeloupe. Comparisons of PM 10 at these sites with concurrent dust measurements at Barbados demonstrate that high PM 10 levels are almost entirely due to dust. Cayenne PM 10 peaks in spring in a cycle which is consistent with satellite aerosol optical depth and suggests the Sahel is the dominant source. The persistent transport of dust during much of the year could impact a wide range of environmental processes over a broad region that extends from the southern United States to the Amazon Basin. Finally, the average 24-hour PM 10 concentrations at Cayenne and Guadeloupe frequently exceed the World Health Organization air quality guideline which suggests that soil-dust PM 10 could be a significant, but generally unrecognized, health factor at western Atlantic sites and also in other relatively remote regions affected by long-range dust from Africa. Because dust emissions and transport are highly sensitive to climate variability, climate change in coming decades could greatly affect a wide range of biogeochemical processes and human health in this region.
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    The oceanic anthropogenic CO2 sink: Storage, air-sea fluxes, and transports over the industrial era (2014)
    Wiley
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    Publication Date: 2014-06-15
    Description: This study presents a new estimate of the oceanic anthropogenic CO 2 (C ant ) sink over the industrial era (1780-present), from assimilation of potential temperature, salinity, radiocarbon, and CFC-11 observations in a global steady-state Ocean Circulation Inverse Model (OCIM). This study differs from previous data-based estimates of the oceanic C ant sink in that dynamical constraints on ocean circulation are accounted for and the ocean circulation is explicitly modeled, allowing the calculation of oceanic C ant storage, air-sea fluxes, and transports in a consistent manner. The resulting uncertainty of the OCIM-estimated C ant uptake, transport, and storage is significantly smaller than the comparable uncertainty from purely data-based or model-based estimates. The OCIM-estimated oceanic C ant storage is 160–166 PgC in 2012, and the oceanic C ant uptake rate averaged over the period 2000–2010 is 2.6 PgC yr −1 , or about 30% of current anthropogenic CO 2 emissions. This result implies a residual (primarily terrestrial) C ant sink of about 1.6 PgC yr −1 for the same period. The Southern Ocean is the primary conduit for C ant entering the ocean, taking up about 1.1 PgC yr −1 in 2012, which represents about 40% of the contemporary oceanic C ant uptake. It is suggested that the most significant source of remaining uncertainty is due to potential variability in the ocean circulation over the industrial era.
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    Global patterns of phytoplankton nutrient and light colimitation inferred from an optimality-based model (2014)
    Wiley
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    Publication Date: 2014-06-15
    Description: The widely used concept of constant “Redfield” phytoplankton stoichiometry is often applied for estimating which nutrient limits phytoplankton growth in the surface ocean. Culture experiments, in contrast, show strong relations between growth conditions and cellular stoichiometry with often substantial deviations from Redfield stoichiometry. Here we investigate to what extent both views agree by analyzing remote-sensing and in-situ data with an optimality-based model of non-diazotrophic phytoplankton growth in order to infer seasonally varying patterns of colimitation by light, nitrogen (N), and phosphorus (P) in the global ocean. Our combined model-data analysis suggests strong N and N-P colimitation in the tropical ocean, seasonal light and N-P colimitation in the northern hemisphere, and strong light limitation only during winter in the Southern Ocean. The eastern equatorial Pacific appears as the only ocean area that is essentially not limited by N, P or light. Even though our optimality-based approach specifically accounts for flexible stoichiometry, inferred patterns of N and P limitation are to some extent consistent with those obtained from an analysis of surface inorganic nutrients with respect to the Redfield N:P ratio. Iron is not part of our analysis, implying that we cannot accurately predict N cell-quotas in HNLC regions. Elsewhere, we do not expect a major effect of iron on the relative distribution of N, P, and light colimitation areas. The relative importance of N, P, and light in limiting phytoplankton growth diagnosed here by combining observations and an optimal-growth model, provides a useful constraint for models used to predict future marine biological production under changing environmental conditions.
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  • 90
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    Experimental assessment of diazotroph responses to elevated seawater pCO2 in the North Pacific Subtropical Gyre (2014)
    Wiley
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    Publication Date: 2014-05-22
    Description: We examined short-term (24-72 hours) responses of naturally occurring marine N 2 fixing microorganisms (termed diazotrophs) to abrupt increases in the partial pressure of carbon dioxide ( p CO 2 ) in seawater during 9 incubation experiments conducted between May 2010 and September 2012 at Station ALOHA (22°45’N, 158˚W) in the North Pacific Subtropical Gyre (NPSG). Rates of N 2 fixation, nitrogenase (nifH) gene abundances and transcripts of six major groups of cyanobacterial diazotrophs (including both unicellular and filamentous phylotypes), and rates of primary productivity (as measured by 14 C-bicarbonate assimilation into plankton biomass) were determined under contemporary (~390 ppm) and elevated p CO 2 conditions (~1100 ppm). Quantitative polymerase chain reaction (QPCR) amplification of planktonic nifH genes revealed that unicellular cyanobacteria phylotypes dominated gene abundances during these experiments. In the majority of experiments (7 out of 9), elevated p CO 2 did not significantly influence rates of dinitrogen (N 2 ) fixation or primary productivity (two-way ANOVA, P 〉 0.05). During two experiments, rates of N 2 fixation rates and primary productivity were significantly lower (by 79 to 82% and 52 to 72%, respectively) in the elevated p CO 2 treatments relative to the ambient controls (two-way ANOVA, P 〈 0.05). QPCR amplification of nifH genes and gene transcripts revealed that diazotroph abundances and nifH gene expression were largely unchanged by the perturbation of the seawater p CO 2 . Our results suggest that naturally occurring N 2 fixing plankton assemblages in the NPSG are relatively resilient to large, short-term increases in p CO 2 .
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    Satellite-detected fluorescence: decoupling non-photochemical quenching from iron stress signals in the South Atlantic and Southern Ocean (2014)
    Wiley
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    Publication Date: 2014-05-07
    Description: Satellite detected sunlight induced chlorophyll fluorescence could offer valuable information about the physiological status of phytoplankton on a global scale. Realization of this potential is confounded by the considerable uncertainty that exists in deconvolving the multiple ecophysiological processes that can influence the satellite signal. A dominant source of current uncertainty arises from the extent of reductions in chlorophyll fluorescence caused by the high light intensities phytoplankton are typically exposed to when satellite images are captured. In this study, results from over two hundred non-photochemical quenching (NPQ) experiments conducted on cruises spanning from subtropical gyre to Southern Ocean waters have confirmed that satellite fluorescence quantum yields have the potential to reveal broad regions of iron (Fe) stress. However, our results suggest significant variability in phytoplankton NPQ behaviour between oceanic regimes. Dynamic NPQ must therefore be considered to achieve a reliable interpretation of satellite fluorescence in terms of Fe stress. Specifically, significantly lower NPQ was found in stratified subtropical gyre-type waters than in well-mixed Southern Ocean waters. Such variability is suggested to result from differences in incident irradiance fluctuation experienced by phytoplankton, with highly variable irradiance conditions likely driving phytoplankton to acclimate or adapt towards a higher dynamic NPQ capacity. Sea surface temperature empirically demonstrated the strongest correlation with NPQ parameters and is presented as a means of correcting the chlorophyll fluorescence signature for the region studied. With these corrections, a decadal composite of satellite austral summer observations is presented for the Southern Ocean, potentially reflecting spatial variability in the distribution and extent of Fe stress.
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    Variability in under-ice export fluxes of biogenic matter in the Arctic Ocean (2014)
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    Publication Date: 2014-05-08
    Description: A critical question regarding the organic carbon cycle in the Arctic Ocean is whether the decline in ice extent and thickness and the associated increase in solar irradiance in the upper ocean will result in increased primary production and particulate organic carbon (POC) export. To assess spatial and temporal variability in POC export, under-ice export fluxes were measured with short-term sediment traps in the northern Laptev Sea in July-August-September 1995, north of the Fram Strait in July 1997 and in the Central Arctic in August-September 2012. Sediment traps were deployed at 2-5 m and 20-25 m under ice for periods ranging from 8.5 to 71 hours. In addition to POC fluxes, total particulate matter, chlorophyll a , biogenic particulate silica, phytoplankton, and zooplankton fecal pellet fluxes were measured to evaluate the amount and composition of the material exported in the upper Arctic Ocean. Whereas elevated export fluxes observed on and near the Laptev Sea shelf were likely the combined result of high primary production, resuspension, and release of particulate matter from melting ice, low export fluxes above the central basins despite increased light availability during the record minimum ice extent of 2012 suggest that POC export was limited by nutrient supply during summer. These results suggest that the ongoing decline in ice cover affects export fluxes differently on Arctic shelves and over the deep Arctic Ocean, and that POC export is likely to remain low above the central basins unless additional nutrients are supplied to surface waters.
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    Issue Information (2014)
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    Publication Date: 2014-05-15
    Description: No abstract is available for this article.
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    Impact of Large-Scale Climate Extremes on Biospheric Carbon Fluxes: An Intercomparison Based on MsTMIP Data (2014)
    Wiley
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    Publication Date: 2014-05-15
    Description: Understanding the role of climate extremes and their impact on the carbon (C) cycle is increasingly a focus of Earth system science. Climate extremes such as droughts, heat waves, or heavy precipitation events can cause substantial changes in terrestrial C fluxes. On the other hand, extreme changes in C fluxes are often, but not always, driven by extreme climate conditions. Here we present an analysis of how extremes in temperature and precipitation, and extreme changes in terrestrial C fluxes are related to each other in 10 state-of-the-art terrestrial carbon models, all driven by the same climate forcing. We use model outputs from the North American Carbon Program (NACP) Multi-scale synthesis and Terrestrial Model Intercomparison Project (MsTMIP). A global scale analysis shows that both droughts and heatwaves translate into anomalous net releases of CO 2 from the land surface via different mechanisms: Droughts largely decrease gross primary production (GPP) and to a lower extent total respiration (TR), while heat waves slightly decrease GPP but increase TR. Cold and wet periods have a smaller opposite effect. Analyzing extremes in C fluxes reveals that extreme changes in GPP and TR are often caused by strong shifts in water availability, but for extremes in TR shifts in temperature are also important. Extremes in net CO 2 exchange are equally strongly driven by deviations in temperature and precipitation. Models mostly agree on the sign of the C flux response to climate extremes, but model spread is large. In tropical forests, C cycle extremes are driven by water availability, whereas in boreal forests temperature plays a more important role. Models are particularly uncertain about the C flux response to extreme heat in boreal forests.
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    Impact of Changes in Barometric Pressure on Landfill Methane Emission (2014)
    Wiley
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    Publication Date: 2014-06-14
    Description: Landfill methane emissions were measured continuously using the eddy covariance method from June to December 2010. The study site was located at the Bluff Road Landfill in Lincoln, Nebraska USA. Our results show that landfill methane emissions strongly depended on changes in barometric pressure; rising barometric pressure suppressed the emission, while falling barometric pressure enhanced the emission, a phenomenon called barometric pumping. There was up to a 35-fold variation in day-to-day methane emissions due to changes in barometric pressure. Wavelet coherence analysis revealed a strong spectral coherency between variations of barometric pressure and methane emission at periodicities ranging from 1 day to 8 days. Power spectrum and ogive analysis showed that at least 10 days of continuous measurements were needed in order to capture 90% of the total variance in the methane emission time series at our landfill site. From our results, it is clear that point-in-time measurements taken at monthly or longer time intervals using techniques such as the trace plume method, the mass balance method, or the closed-chamber method will be subject to large variations in measured emission rates because of the barometric pumping phenomenon. Estimates of long-term integrated methane emissions from landfills based on such measurements could yield uncertainties, ranging from 28.8% underestimation to 32.3% overestimation. Our results demonstrate a need for continuous measurements to quantify annual total landfill emissions. This conclusion may apply to the study of methane emissions from wetlands, peatlands, lakes, and other environmental contexts where emissions are from porous media or ebullition. Other implications from the present study for hazard gas monitoring programs are also discussed.
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  • 96
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    Publication Date: 2014-06-12
    Description: No abstract is available for this article.
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  • 97
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    Western Pacific atmospheric nutrient deposition fluxes, their impact on surface ocean productivity (2014)
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    Publication Date: 2014-06-14
    Description: The atmospheric deposition of both macro- and micronutrients plays an important role in driving primary productivity, particularly in the low latitude ocean. We report aerosol major ion measurements for five ship-based sampling campaigns in the western Pacific from ~25 o N to 20 o S and compare the results with those from Atlantic meridional transects (~50 o N to 50 o S) with aerosols collected and analysed in the same laboratory, allowing full incomparability. We discuss sources of the main nutrient species (nitrogen (N), phosphorus (P) and iron (Fe)) in the aerosols and their stoichiometry. Striking north-south gradients are evident over both basins with the northern hemisphere more impacted by terrestrial dust sources and anthropogenic emissions and the North Atlantic apparently more impacted than the North Pacific. We estimate the atmospheric supply rates of these nutrients and the potential impact of the atmospheric deposition on the tropical western Pacific. Our results suggest that the atmospheric deposition is P-deficient relative to the needs of the resident phytoplankton. These findings suggest that atmospheric supply of N, Fe and P increases primary productivity utilising some of the residual excess phosphorus (P*) in the surface waters to compensate for aerosol P-deficiency. Regional primary productivity is further enhanced via the stimulation of nitrogen fixation fuelled by the residual atmospheric iron and P*. Our stoichiometric calculations reveal that a P* of 0.1 µmol l -1 can offset the P-deficiency in atmospheric supply for many months. This study suggests that atmospheric deposition may sustain ~10% of primary production in both the western tropical Pacific.
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    Long-term variability of phytoplankton carbon biomass in the Sargasso Sea (2014)
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    Publication Date: 2014-07-27
    Description: Time series of phytoplankton carbon biomass are scarce yet may provide important insights into ocean productivity and carbon export to depth via the oceanic biological pump. We combine recent flow-cytometric measurements with pigment concentrations and other standard measurements to reconstruct taxon-specific phytoplankton carbon biomass in the Sargasso Sea over 22 years, using a multiple regression approach. The reconstructed series reveal an increasing trend (~3% per year) in total phytoplankton carbon, apparently driven by increasing nutrient supply by vertical mixing associated with a shift to a negative phase in the winter North Atlantic Oscillation index. Also, the reconstructed eukaryote biomass fraction shows a multiannual shift from ~45% in the early 1990s/late 2000s to ~70% in the late 1990s/early 2000s. We hypothesize that a multiannual shift in the seasonal pattern of mixing may have stimulated and restructured the eukaryote community while suppressing prokaryote populations by increasing photodamage and grazing mortality.
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  • 99
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    Evidence for production and lateral transport of dissolved organic phosphorus in the eastern subtropical North Atlantic (2014)
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    Publication Date: 2014-07-24
    Description: The concentration of phosphate and dissolved organic phosphorus (DOP) is chronically low and limits phytoplankton growth in the subtropical North Atlantic relative to other ocean basins. Transport of phosphate and DOP from the productive flanks of the gyre to its interior has been hypothesized as an important phosphorus supply pathway. During a cruise in the eastern Atlantic in spring 2011, the rates of phosphate uptake, alkaline phosphatase (APA) and DOP production were measured in the northwest African shelf region, subtropics and tropics. Rates of DOP production were 6-fold higher in the shelf region (43 ± 41 nM d −1 ) relative to the subtropics (6.9 ± 4.4 nM d −1 ). In contrast, APA production was 3-fold higher in the subtropics (8.0 ± 7.3 nM d −1 ), indicative of enhanced DOP utilization, relative to the shelf region (2.6 ± 2.1 nM d −1 ). Hence, observations suggest net production of DOP in the shelf region and either net consumption of DOP or a near balance in DOP production and consumption in the gyre interior. Eddy-permitting model experiments demonstrate that (i) DOP accounts for over half the total phosphorus in surface waters, (ii) DOP is transported westward from the shelf region by a combination of gyre and eddy circulations, and (iii) advected DOP supports up to 70% of the particle export over much of the subtropical gyre. Our combined observational and modeling study supports the view that the horizontal transport of DOP from the shelf region is an important mechanism supplying phosphorus to the surface subtropical North Atlantic.
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    Worldwide retention of nutrient silicon by river damming: From sparse data set to global estimate (2014)
    Wiley
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    Publication Date: 2014-07-26
    Description: Damming of rivers represents a major anthropogenic perturbation of the hydrological cycle, with the potential to profoundly modify the availability of nutrient silicon (Si) in streams, lakes and coastal areas. A global assessment of the impact of dams on river Si fluxes, however, is limited by the sparse data set on Si budgets for reservoirs. To alleviate this limitation, we use existing data on dissolved Si (DSi) retention by dams to calibrate a mechanistic model for the biogeochemical cycling of DSi and reactive particulate Si (PSi) in reservoir systems. The model calibration yields a relationship between the annual in-reservoir siliceous primary productivity and the external DSi supply. With this relationship and an estimate of catchment Si loading, the model calculates the total reactive Si (RSi = DSi + PSi) retention for any given reservoir. A Monte Carlo analysis accounts for the effects of variations in reservoir characteristics, and generates a global relationship that predicts the average reactive Si retention in reservoirs as a function of the water residence time. This relationship is applied to the GRanD database to estimate Si retention by damming worldwide. According to the results, dams retain 163 Gmol yr −1 (9.8 Tg SiO 2 yr −1 ) of DSi and 372 Gmol yr −1 (22.3 Tg SiO 2 yr −1 ) of RSi, or 5.3% of the global RSi loading to rivers.
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