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  • Articles  (922)
  • Wiley  (922)
  • American Physical Society (APS)
  • Public Library of Science
  • Global Biogeochemical Cycles  (922)
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  • Geosciences  (922)
  • 1
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    Is atmospheric phosphorus pollution altering global alpine lake stoichiometry? (2015)
    Wiley
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    Publication Date: 2015-08-11
    Description: Anthropogenic activities have significantly altered atmospheric chemistry and changed the global mobility of key macronutrients. Here, we show that contemporary global patterns in nitrogen (N) and phosphorus (P) emissions drive large hemispheric variation in precipitation chemistry. These global patterns of nutrient emission and deposition (N:P) are in turn closely reflected in the water chemistry of naturally oligotrophic lakes ( r 2 =0.81, p 〈0.0001). Observed increases in anthropogenic N deposition play a role in nutrient concentrations ( r 2 =0.20, p 〈0.05); however, atmospheric deposition of P appears to be major contributor to this pattern ( r 2 =0.65, p 〈0.0001). Atmospheric simulations indicate a global increase in P deposition by 1.4 times the preindustrial rate largely due to increased dust and biomass burning emissions. Although changes in the mass flux of global P deposition are smaller than for N, the impacts on primary productivity may be greater because, on average, one unit of increased P deposition has 16X the influence of one unit of N deposition. These stoichiometric considerations, combined with the evidence presented here, suggest that increases in P deposition may be a major driver of alpine lake trophic status, particularly in the Southern Hemisphere. These results underscore the need for the broader scientific community to consider the impact of atmospheric phosphorus deposition on the water quality of naturally oligotrophic lakes.
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  • 2
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    The imbalance of new and export production in the Western Antarctic Peninsula, a potentially “leaky” ecosystem (2015)
    Wiley
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    Publication Date: 2015-08-13
    Description: To quantify the balance between new production and vertical nitrogen export of sinking particles, we measured nitrate uptake, net nitrate drawdown, ΔO 2 /Ar-based net community production, sediment trap flux, and 234 Th export at a coastal site near Palmer Station, Antarctica during the phytoplankton growing season from October 2012 to March 2013. We also measured nitrate uptake and 234 Th export throughout the northern western Antarctic Peninsula (WAP) region on a cruise in January 2013. We used a non-steady state 234 Th equation with temporally-varying upwelling rates and an irradiance-based phytoplankton production model to correct our export and new production estimates in the complex coastal site near Palmer Station. Results unequivocally showed that nitrate uptake and net community production were significantly greater than the sinking particle export on region-wide spatial scales and season-long temporal scales. At our coastal site, new production (105±17.4 mg N m −2 d −1 , mean±st.err.) was 5.3 times greater than vertical nitrogen export (20.4±2.4 mg N m −2 d -1 ). On the January cruise in the northern WAP, new production (47.9±14.4 mg N m −2 d -1 ) was 2.4 times greater than export (19.9±1.4 mg N m −2 d −1 ). Much of this imbalance can be attributed to diffusive losses of particulate nitrogen from the surface ocean due to diapycnal mixing, indicative of a “leaky” WAP ecosystem. If these diffusive losses are common in other systems where new production exceeds export, it may be necessary to revise current estimates of the ocean's biological pump.
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  • 3
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    Issue Information (2015)
    Wiley
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    Publication Date: 2015-08-16
    Description: Cover: In Somes and Oschlies [doi 10.1002/2014GB005050 ], comparison of surface (0–50m) (a) map and (b) zonally averaged DON observations [Letscher et al., 2013] with annual semirecalcitrant DON from the model experiments (c) Redfield DOM (RedDOM), (d) preferential DOP remineralization (pref_DOP_remin), (e) preferential DOP recycling and phytoplankton DOP uptake (nonRedDOP), (f ) non-Redfield DOP with low DOM production (low_nonRedDOP), (g) non-Redfield DOP with high DOM production (high_nonRedDOP), and (h) fast recycling non-Redfield DOP (fast_nonRedDOP). Note that the zonally averaged model results in (b) are taken only from locations where observations exist. See pp. 973–993.
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  • 4
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    The multiple fates of sinking particles in the North Atlantic Ocean (2015)
    Wiley
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    Publication Date: 2015-08-05
    Description: The direct respiration of sinking organic matter by attached bacteria is often invoked as the dominant sink for settling particles in the mesopelagic ocean. However, other processes, such as enzymatic solubilization and mechanical disaggregation, also contribute to particle flux attenuation by transferring organic matter to the water column. Here, we use observations from the North Atlantic Ocean, coupled to sensitivity analyses of a simple model, to assess the relative importance of particle-attached microbial respiration compared to the other processes that can degrade sinking particles. The observed carbon fluxes, bacterial production rates, and respiration by water column and particle-attached microbial communities each spanned more than an order of magnitude. Rates of substrate-specific respiration on sinking particle material ranged from 0.007 ± 0.003 to 0.173 ± 0.105 d -1 . A comparison of these substrate-specific respiration rates with model results suggested sinking particle material was transferred to the water column by various biological and mechanical processes nearly 3.5 times as fast as it was directly respired. This finding, coupled with strong metabolic demand imposed by measurements of water column respiration (729.3 ± 266.0 mg C m -2 d -1 , on average, over the 50 to 150 m depth interval), suggested a large fraction of the organic matter evolved from sinking particles ultimately met its fate through subsequent remineralization in the water column. At three sites, we also measured very low bacterial growth efficiencies and large discrepancies between depth-integrated mesopelagic respiration and carbon inputs.
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  • 5
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    Climate extremes dominate seasonal and interannual variations in carbon export from the Mississippi River Basin (2015)
    Wiley
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    Publication Date: 2015-08-07
    Description: Knowledge about the annual and seasonal patterns of organic and inorganic carbon (C) exports from the major rivers of the world to the coastal ocean are essential for our understanding and potential management of the global C budget so as to limit anthropogenic modification of global climate. Unfortunately our predictive understanding of what controls the timing, magnitude and quality of carbon export is still rudimentary. Here we use a process-based coupled hydrologic/ecosystem biogeochemistry model (the Dynamic Land Ecosystem Model, DLEM) to examine how climate variability and extreme events, changing land use, and atmospheric chemistry have affected the annual and seasonal patterns of C exports from the Mississippi River basin to the Gulf of Mexico. Our process-based simulations estimate that the average annual exports of dissolved organic C (DOC), particulate organic C (POC), and dissolved inorganic C (DIC) in the 2000s was 2.6 ± 0.4 Tg C yr −1 , 3.4 ± 0.3 Tg C yr −1 and 18.8 ± 3.4 Tg C yr −1 , respectively. Although land-use change was the most important agent of change in C export over the past century, climate variability and extreme events (such as flooding and drought) were primarily responsible for seasonal and interannual variations in C export from the basin. The maximum seasonal export of DIC occurred in summer while for maximum DOC and POC occurred in winter. Relative to the 10-year average (2001–2010), our modeling analysis indicates that the years of maximal and minimal C export co-occurred with wet and dry years (2008: 32% above average and 2006: 32% below average). Given IPCC-predicted changes in climate variability and the severity of rain events and droughts of wet and dry years for the remainder of the 21 st Century, our modeling results suggest major changes in the riverine link between the terrestrial and oceanic realms, which are likely to have a major impact on carbon delivery to the coastal ocean.
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  • 6
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    An observational assessment of the influence of mesoscale and submesoscale heterogeneity on ocean biogeochemical reactions (2015)
    Wiley
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    Publication Date: 2015-08-14
    Description: Numerous observations demonstrate that considerable spatial variability exists in components of the marine planktonic ecosystem at the mesoscale and submesoscale (100 km -1 km). The causes and consequences of physical processes at these scales (‘eddy advection’) influencing biogeochemistry have received much attention. Less studied, the non-linear nature of most ecological and biogeochemical interactions means that such spatial variability has consequences for regional estimates of processes including primary production and grazing, independent of the physical processes. This effect has been termed ‘eddy reactions’. Models remain our most powerful tools for extrapolating hypotheses for biogeochemistry to global scales and to permit future projections. The spatial resolution of most climate and global biogeochemical models means that processes at the mesoscale and submesoscale are poorly resolved. Modelling work has previously suggested that the neglected ‘eddy reactions’ may be almost as large as the mean field estimates in some cases. This study seeks to quantify the relative size of eddy and mean reactions observationally, using in situ and satellite data. For primary production, grazing and zooplankton mortality the eddy reactions are between 7% and 15% of the mean reactions. These should be regarded as preliminary estimates to encourage further observational estimates, and not taken as a justification for ignoring eddy reactions. Compared to modelling estimates, there are inconsistencies in the relative magnitude of eddy reactions and in correlations which are a major control on their magnitude. One possibility is that models exhibit much stronger spatial correlations than are found in reality, effectively amplifying the magnitude of eddy reactions.
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  • 7
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    Seasonal variations, origin and fate of settling diatoms in the Southern Ocean tracked by silicon isotope records in deep sediment traps (2015)
    Wiley
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    Publication Date: 2015-08-14
    Description: The Southern Ocean plays a pivotal role in the control of atmospheric CO 2 levels, via both physical and biological sequestration processes. The biological carbon transfer to the ocean interior is tightly coupled to the availability of other elements, especially iron as a trace limiting nutrient and dissolved silicon (DSi) as the mineral substrate that allows diatoms to dominate primary production. Importantly, variations in the silicon cycling are large but not well understood. Here, we use δ 30 Si measurements to track seasonal flows of silica to the deep sea, as captured by sediment trap time series, for the three major zones (Antarctic, AZ; Polar Frontal, PFZ and Subantarctic, SAZ) of the open Southern Ocean. Variations in the exported flux of biogenic silica (BSi) and its δ 30 Si composition reveal a range of insights, including that i) the sinking rate of BSi exceeds 200 m d −1 in summer in the AZ, yet decreases to very low values in winter that allow particles to remain in the water column through to the following spring, ii) occasional vertical mixing events affect the δ 30 Si composition of exported BSi in both the SAZ and AZ, iii) the δ 30 Si signature of diatoms is well conserved through the water column despite strong BSi and POC attenuation at depth, and is closely linked to the Si consumption in surface waters. With the strong coupling observed between BSi and POC fluxes in PFZ and AZ, these data provide new constraints for application to biogeochemical models of seasonal controls on production and export.
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  • 8
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    Strong dependence of CO2 emissions from anthropogenic land cover change on initial land cover and soil carbon parametrization (2015)
    Wiley
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    Publication Date: 2015-08-13
    Description: The quantification of sources and sinks of carbon from land use and land cover changes (LULCC) is uncertain. We investigated how the parametrization of LULCC and of organic matter decomposition, as well as initial land cover affect the historical and future carbon fluxes in an Earth System Model (ESM). Using the land component of the Max-Planck-Institute ESM, we found that the historical (1750–2010) LULCC flux varied up to 25% depending on the fraction of biomass which enters the atmosphere directly due to burning or is used in short-lived products. We found an uncertainty in the decadal LULCC fluxes of the recent past due to the parametrization of decomposition and direct emissions of 0.6 Pg C yr −1 , which is three times larger than the un-certainty previously attributed to model and method in general. Pre-industrial natural land cover had a larger effect on decadal LULCC fluxes than the aforementioned parameter sensitivity (1.0 Pg C yr −1 ). Re-gional differences between reconstructed and dynamically-computed land cover, in particular at low-latitudes, led to differences in historical LULCC emissions of 84–114 Pg C, globally. This effect is larger than the effects of forest regrowth, shifting cultivation or climate feedbacks and comparable to the effect of differences among studies in the terminology of LULCC. In general, we find that the practice of calibrating the net land carbon balance to provide realistic boundary conditions for the climate component of an ESM hampers the applicability of the land component outside its primary field of application.
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  • 9
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    Small phytoplankton drive high summertime carbon and nutrient export in the Gulf of California and Eastern Tropical North Pacific (2015)
    Wiley
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    Publication Date: 2015-08-04
    Description: Summertime carbon, nitrogen and biogenic silica export was examined using 234 Th: 238 U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, PC and PN concentrations were more rapidly attenuated relative to 234 Th activities in small particles compared to large particles, converging to 1–5 µmol·dpm −1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/ 234 Th ratio, regional trends were consistent across measurement techniques. Highest C fixation rates were associated with diatom dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton and diazotroph dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234 Th fluxes into carbon or other elemental fluxes, including 〈50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method.
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  • 10
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    Climatological distribution of aragonite saturation state in the global oceans (2015)
    Wiley
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    Publication Date: 2015-09-19
    Description: Aragonite saturation state (Ω arag ) in surface and subsurface waters of the global oceans was calculated from up-to-date (through the year of 2012) ocean station dissolved inorganic carbon (DIC) and total alkalinity (TA) data. Surface Ω arag in the open ocean was always supersaturated (Ω〉1), ranging between 1.1 and 4.2. It was above 2.0 (2.0-4.2) between 40°N and 40°S, but decreased towards higher latitude to below 1.5 in polar areas. The influences of water temperature on the TA/DIC ratio, combined with the temperature effects on inorganic carbon equilibrium and apparent solubility product ( K ’ sp ), explain the latitudinal differences in surface Ω arag . Vertically, Ω arag was highest in the surface mixed layer (SML). Higher hydrostatic pressure, lower water temperature, and more CO 2 buildup from biological activity in the absence of air-sea gas exchange helped maintain lower Ω arag in the deep ocean. Below the thermocline, aerobic decomposition of organic matter along the pathway of global thermohaline circulation played an important role in controlling Ω arag distributions. Seasonally, surface Ω arag above 30° latitudes was about 0.06 to 0.55 higher during warmer months than during colder months in the open-ocean waters of both hemispheres. Decadal changes of Ω arag in the Atlantic and Pacific Oceans showed that Ω arag in waters shallower than 100 m depth decreased by 0.10±0.09 (−0.40±0.37% yr −1 ) on average from the decade spanning 1989–1998 to the decade spanning 1998–2010.
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  • 11
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    Dissolved organic carbon pools and export from the coastal ocean (2015)
    Wiley
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    Publication Date: 2015-09-23
    Description: The distribution of dissolved organic carbon (DOC) concentration across coastal waters was characterised based on the compilation of 3510 individual estimates of DOC in coastal waters worldwide. We estimated the DOC concentration in the coastal waters that directly exchange with open ocean waters in two different ways, as the DOC concentration at the edge of the shelf break and as the DOC concentration in coastal waters with salinity close to the average salinity in the open ocean. Using these estimates of DOC concentration in the coastal waters that directly exchange with open ocean waters, the mean DOC concentration in the open ocean and the estimated volume of water annually exchanged between coastal and open ocean, we estimated a median ± SE (and average ± SE) global DOC export from coastal to open ocean waters ranging from 4.4 ± 1.0 Pg C yr −1 to 27.0 ± 1.8 Pg C yr −1 (7.0 ± 5.8 Pg C yr −1 to 29.0 ± 8.0 Pg C yr −1 ) depending on the global hydrological exchange. These values correspond to a median and mean median (and average) range between 14.7 ± 3.3 to 90.0 ± 3.3 (23.3 ± 19.3 to 96.7± 26.7) Gg C yr −1 per km of shelf break, which is consistent with the range between 1.4 to 66.1 Gg C yr −1 per km of shelf break of available regional estimates of DOC export. The estimated global DOC export from coastal to open ocean waters is also consistent with independent estimates of the net metabolic balance of the coastal ocean. The DOC export from the coastal to the open ocean is likely to be a sizeable flux and is likely to be an important term in the carbon budget of the open ocean, potentially providing an important subsidy to support heterotrophic activity in the open ocean.
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  • 12
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    Silicon pools in human impacted soils of temperate zones (2015)
    Wiley
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    Publication Date: 2015-08-27
    Description: Besides well-known effects of climate and parent material on silicate weathering the role of land use change as a driver in the global silicon cycle is not well known. Changes in vegetation cover have altered reservoirs of silicon and carbon in plants and soils. This has potential consequences for plant-Si availability, agricultural yields and coastal eutrophication, as Si is a beneficial element for many crop plants and an essential nutrient for diatom growth. We here examined the role of sustained and intensive land use and human disturbance on silicon (Si) pool distribution in soils with similar climatological and bulk mineralogical characteristics. We show that land use impacts both biogenic and non-biogenic Si pools. While biogenic Si strongly decreases along the land use change gradient (from forest to croplands), pedogenic silica fractions ( e.g. pedogenic clays) increase in top soils with a long duration of cultivation and soil disturbance. Our results suggest that non-biogenic Si pools might compensate for the loss of reactive biogenic silicon in temperate zones.
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  • 13
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    Increased influence of nitrogen limitation on CO2 emissions from future land use and land-use change (2015)
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    Publication Date: 2015-08-27
    Description: In the latest projections of future greenhouse gas emissions for the Intergovernmental Panel on Climate Change (IPCC), few Earth System Models included the effect of nitrogen limitation, a key process limiting forest regrowth. Few included forest management (wood harvest). We estimate the impacts of nitrogen limitation on the CO 2 emissions from land use and land-use change (LULUC), including wood harvest, for the period 1900-2100. We use a land-surface model that includes a fully coupled carbon and nitrogen cycle, and accounts for forest regrowth processes following agricultural abandonment and wood harvest. Future projections are based on the four Representation Concentration Pathways used in the IPCC Fifth Assessment Report, and we account for uncertainty in future climate for each scenario based on ensembles of climate model outputs. Results show that excluding nitrogen limitation will underestimate global LULUC emissions by 34-52 PgC (20-30%) during the 20 th century (range across three different historical LULUC reconstructions) and by 128-187 PgC (90-150%) during the 21 st century (range across the four IPCC scenarios). The full range for estimated LULUC emissions during the 21 st century including climate model uncertainty is 91 to 227 PgC (with nitrogen limitation included). The underestimation increases with time because: (1) Projected annual wood harvest rates from forests summed over the 21 st century are 380-1080% higher compared to those of the 20 th century, resulting in more regrowing secondary forests, (2) Nitrogen limitation reduces the CO 2 fertilization effect on net primary production of regrowing secondary forests following wood harvest and agricultural abandonment, and (3) Nitrogen limitation effect is aggravated by the gradual loss of soil nitrogen from LULUC disturbance. Our study implies that: (1) Nitrogen limitation of CO 2 uptake is substantial and sensitive to nitrogen inputs, (2) If LULUC emissions are larger than previously estimated in studies without nitrogen limitation, then meeting the same climate mitigation target would require an equivalent additional reduction of fossil fuel emissions, (3) The effectiveness of land-based mitigation strategies will critically depend on the interactions between nutrient limitations and secondary forests resulting from LULUC, and (4) It is important for terrestrial biosphere models to consider nitrogen constraint in estimates of the strength of future land carbon uptake.
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  • 14
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    Environmental controls on the biogeography of diazotrophy and Trichodesmium in the Atlantic Ocean (2015)
    Wiley
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    Publication Date: 2015-05-24
    Description: The cyanobacterium Trichodesmium is responsible for a significant proportion of the annual 'new' nitrogen introduced into the global ocean. Despite being arguably the best studied marine diazotroph, the factors controlling the distribution and growth of Trichodesmium remain a subject of debate, with sea surface temperature, the partial pressure of CO 2 and nutrients including iron (Fe) and phosphorus (P), all suggested to be important. Synthesising data from 7 cruises collectively spanning large temporal and spatial scales across the Atlantic Ocean, including 2 previously unreported studies crossing the largely under-sampled South Atlantic gyre, we assessed the relationship between proposed environmental drivers and both community N 2 fixation rates and the distribution of Trichodesmium . Simple linear regression analysis would suggest no relationship between any of the sampled environmental variables and N 2 fixation rates. However, considering the concentrations of iron and phosphorus together within a simplified resource-ratio framework, illustrated using an idealised numerical model, indicates the combined effects these nutrients have on Trichodesmium and broader diazotroph biogeography, alongside the reciprocal maintenance of different biogeographic provinces of the (sub)-tropical Atlantic in states of Fe or P oligotrophy by diazotrophy. The qualitative principles of the resource-ratio framework are argued to be consistent with both the previously described North–south Atlantic contrast in Trichodesmium abundance and the presence and consequence of a substantial non- Trichodesmium diazotrophic community in the western South Atlantic subtropical gyre. A comprehensive, observation-based explanation of the interactions between Trichodesmium and the wider diazotrophic community with iron and phosphorus in the Atlantic Ocean is thus revealed.
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  • 15
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    On the influence of “non-Redfield” dissolved organic nutrient dynamics on the spatial distribution of N2 fixation and the size of the marine fixed nitrogen inventory (2015)
    Wiley
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    Publication Date: 2015-05-26
    Description: Dissolved organic nitrogen (DON) and phosphorus (DOP) represent the most abundant form of their respective nutrient pool in the surface layer of the oligotrophic oceans and play an important role in nutrient cycling and productivity. Since DOP is generally more labile than DON, it provides additional P that may stimulate growth of N 2 -fixing diazotrophs that supply fixed nitrogen to balance denitrification in the ocean. In this study, we introduce semi-recalcitrant components of DON and DOP as state variables in an existing global ocean–atmosphere-sea ice-biogeochemistry model of intermediate complexity to assess their impact on the spatial distribution of N 2 -fixation and the size of the marine fixed nitrogen inventory. Large-scale surface datasets of global DON and Atlantic Ocean DOP are used to constrain the model. Our simulations suggest that both preferential DOP remineralization and phytoplankton DOP uptake are important “non-Redfield” processes (i.e., deviate from molar N:P=16) that need to be accounted for to explain the observed patterns of DOP. Additional non-Redfield DOP sensitivity experiments testing DOM production rate uncertainties that best reproduce the observed spatial patterns of DON and DOP stimulate additional N 2 -fixation that increases the size of the global marine fixed nitrogen inventory by 4.7±1.7% compared to the simulation assuming Redfield DOM stoichiometry that underestimates the observed nitrogen inventory. The extra 8 Tg yr −1 of N 2 -fixation stimulated in the Atlantic Ocean is mainly responsible for this increase due to its large spatial separation from water column denitrification, which buffers any potential nitrogen surplus in the Pacific Ocean. Our study suggests that the marine fixed nitrogen budget is sensitive to non-Redfield DOP dynamics because access to the relatively labile DOP pool expands the ecological niche for N 2 -fixing diazotrophs.
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  • 16
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    Quantifying the relative importance of land cover change from climate and land-use in the representative concentration pathways (2015)
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    Publication Date: 2015-05-29
    Description: Climate change is projected to cause substantial alterations in vegetation distribution, but these have been given little attention in comparison to land-use in the Representative Concentration Pathway (RCP) scenarios. Here we assess the climate-induced land cover changes (CILCC) in the RCPs, and compare them to land-use land cover change (LULCC). To do this, we use an ensemble of simulations with and without LULCC in earth system model HadGEM2-ES for RCP2.6, RCP4.5 and RCP8.5. We find that climate change causes an expansion poleward of vegetation that affects more land area than LULCC in all of the RCPs considered here. The terrestrial carbon changes from CILCC are also larger than for LULCC. When considering only forest, the LULCC is larger, but the CILCC is highly variable with the overall radiative forcing of the scenario. The CILCC forest increase compensates 90% of the global anthropogenic deforestation by 2100 in RCP8.5, but just 3% in RCP2.6. Overall, bigger land cover changes tend to originate from LULCC in the shorter term or lower radiative forcing scenarios, and from CILCC in the longer term and higher radiative forcing scenarios. The extent to which CILCC could compensate for LULCC raises difficult questions regarding global forest and biodiversity offsetting, especially at different timescales. This research shows the importance of considering the relative size of CILCC to LULCC, especially with regard to the ecological effects of the different RCPs.
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  • 17
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    Oceanic teleconnection for carbon dioxide (2016)
    Wiley
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    Publication Date: 2016-07-23
    Description: Biogeochemical teleconnection links seemingly unrelated chemical/biological anomalies that are geographically separated by large distances. Bronselaer et al propose a new mechanism for an interhemispheric teleconnection of air-sea carbon dioxide fluxes in which the upwelling of the Southern Ocean triggers a series of perturbations leading to the alteration of the carbon uptake in the North Atlantic. The westerly wind over the Southern Ocean has a unique role in the climate system. It energizes the strongest ocean current, Antarctic Circumpolar Current, and it lifts up the carbon- and nutrient-rich deep waters all the way to the surface. It is an end point of the ocean's deep overturning circulation and associated biological carbon storage, where the excess carbon from accumulated decomposition of organic material is finally released back into the atmosphere. It is well established that the Southern Ocean upwelling regionally modulates the de-gassing of carbon dioxide there. However, its global-scale implication is not yet fully understood. What happens to the carbon uptake in the other parts of the oceans? In this volume of Global Biogeochemical Cycles, Bronselaer et al describes the chain of events that link the increased Southern Ocean wind to the ocean carbon uptake in the northern high latitudes. The authors conducted a set of computational experiments, showing that the Southern Ocean is a starting point of the oceanic teleconnection, where the excess nutrient is transported equatorward through the shallow overturning circulation. The stream of macro-nutrient then fertilizes the low-latitude productivity that eventually shifts the carbonate chemistry of the high latitude surface waters. This is an intriguing case of oceanic teleconnection, linking seemingly unrelated biogeochemical anomalies that are geographically separated by large distances. The surprising conclusion is that a stronger Southern Ocean wind increases the de-gassing of carbon dioxide in both northern and southern high latitudes. This happens because more carbon is upwelling into the northern high latitudes due to the increased low-latitude biological pump, approximately doubling the de-gassing intensity relative to the Southern Ocean response alone. There may be more surprises from the Southern Ocean.
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    Oxygen utilization rate (OUR) underestimates ocean respiration – a model study (2016)
    Wiley
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    Publication Date: 2016-07-19
    Description: We use a simple 1D model representing an isolated density surface in the ocean and 3D global ocean biogeochemical models to evaluate the concept of computing the subsurface oceanic oxygen utilization rate (OUR) from the changes of apparent oxygen utilization (AOU) and water age. The distribution of AOU in the ocean is not only the imprint of respiration in the ocean's interior, but is strongly influenced by transport processes and eventually loss at the ocean surface. Since AOU and water age are subject to advection and diffusive mixing, it is only when they are affected both in the same way that OUR represents the correct rate of oxygen consumption. This is the case only when advection prevails or with uniform respiration rates, when the proportions of AOU and age are not changed by transport. In experiments with the 1D-tube model, OUR underestimates respiration when maximum respiration rates occur near the outcrops of isopycnals, and overestimates when maxima occur far from the outcrops. Given the distribution of respiration in the ocean, i.e. elevated rates near high latitude outcrops of isopycnals and low rates below the oligotrophic gyres, underestimates are the rule. Integrating these effects globally in three coupled ocean biogeochemical and circulation models we find that AOU-over-age based calculations underestimate true model respiration by a factor of three. Most of this difference is observed in the upper 1000 m of the ocean with the discrepancies increasing towards the surface where OUR underestimates respiration by as much as factor of four.
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    Nitrate uptake across biomes and the influence of elemental stoichiometry: A new look at LINX II (2016)
    Wiley
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    Publication Date: 2016-07-26
    Description: Considering recent increases in anthropogenic N loading it is essential to identify the controls on N removal and retention in aquatic ecosystems because the fate of N has consequences for water quality in streams and downstream ecosystems. Biological uptake of nitrate (NO 3 - ) is a major pathway by which N is removed from these ecosystems. Here we used data from the second Lotic Intersite Nitrogen eXperiment (LINX II) in a multivariate analysis to identify the primary drivers of variation in NO 3 - uptake velocity among biomes. Across 69 study watersheds in North America, DOC:NO 3 - ratios and photosynthetically active radiation were identified as the two most important predictor variables in explaining NO 3 - uptake velocity. However, within a specific biome the predictor variables of NO 3 - uptake velocity varied, and included various physical, chemical and biological attributes. . Our analysis demonstrates the broad control of elemental stoichiometry on NO 3 - uptake velocity as well as the importance of biome-specific predictors. Understanding this spatial variation has important implications for biome-specific watershed management and the downstream export of NO 3 - , as well as for development of spatially explicit global models that describe N dynamics in streams and rivers.
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    Basin-wide N2 fixation in the deep waters of the Mediterranean Sea (2016)
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    Publication Date: 2016-06-21
    Description: Recent findings indicate that N 2 fixation is significant in aphotic waters, presumably due to heterotrophic diazotrophs depending on organic matter for their nutrition. However, the relationship between organic matter and heterotrophic N 2 fixation remains unknown. Here we explore N 2 fixation in the deep chlorophyll maximum (DCM) and underneath deep waters across the whole Mediterranean Sea and relate it to organic matter composition, characterized by optical and molecular methods. Our N 2 fixation rates were in the range of those previously reported for the euphotic zone of the Mediterranean Sea (up to 0.43 nmol N L -1 d -1 ), and were significantly correlated to the presence of relatively labile organic matter with fluorescence and molecular formula properties representative for peptides and unsaturated aliphatics, and associated with the presence of more oxygenated ventilated water masses. Finally, and despite aphotic N 2 fixation contributes largely to total water column diazotrophic activity (〉50%), its contribution to overall nitrogen inputs to the basin is negligible (〈0.5%).
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    Enzyme-level Interconversion of Nitrate and Nitrite in the Fall Mixed Layer of the Antarctic Ocean (2016)
    Wiley
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    Publication Date: 2016-06-21
    Description: In the Southern Ocean, the nitrogen (N) isotopes of organic matter and the N and oxygen (O) isotopes of nitrate (NO 3 - ) have been used to investigate NO 3 - assimilation and N cycling in the summertime period of phytoplankton growth, both today and in the past. However, recent studies indicate the significance of processes in other seasons for producing the annual cycle of N isotope changes. This study explores the impact of fall conditions on the 15 N/ 14 N (δ 15 N) and 18 O/ 16 O (δ 18 O) of NO 3 - and nitrite (NO 2 - ) in the Pacific Antarctic Zone using depth profiles from late summer/fall of 2014. In the mixed layer, the δ 15 N and δ 18 O of NO 3 -  + NO 2 - increase roughly equally, as expected for NO 3 - assimilation; however, the δ 15 N of NO 3 - -only (measured after NO 2 - removal) increases more than NO 3 - -only δ 18 O. Differencing indicates that NO 2 - has an extremely low δ 15 N, often 〈 -70‰ vs. air. These observations are consistent with the expression of an equilibrium N isotope effect between NO 3 - and NO 2 - , likely due to enzymatic NO 3 - -NO 2 - interconversion. Specifically, we propose reversibility of the nitrite oxidoreductase (NXR) enzyme of nitrite oxidizers that, having been entrained from the subsurface during late summer mixed layer deepening, are inhibited by light. Our interpretation suggests a role for NO 3 - -NO 2 - interconversion where nitrifiers are transported into environments that discourage NO 2 - oxidation. This may apply to surface regions with upwelling, such as the summertime Antarctic. It may also apply to oxygen-deficient zones, where NXR-catalyzed interconversion may explain previously-reported evidence of NO 2 - oxidation.
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    Satellite estimates of net community production based on O2/Ar observations and comparison to other estimates. (2016)
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    Publication Date: 2016-05-09
    Description: We present two statistical algorithms for predicting global oceanic net community production (NCP) from satellite observations. To calibrate these two algorithms, we compiled a large dataset of in situ O 2 /Ar-NCP and remotely sensed observations, including sea surface temperature (SST), net primary production (NPP), phytoplankton size composition, and inherent optical properties. The first algorithm is based on genetic programming (GP) which simultaneously searches for the optimal form and coefficients of NCP equations. We find that several GP solutions are consistent with NPP and SST being strong predictors of NCP. The second algorithm uses support vector regression (SVR) to optimize a numerical relationship between O 2 /Ar-NCP measurements and satellite observations. Both statistical algorithms can predict NCP relatively well, with a coefficient of determination ( R 2 ) of 0.68 for GP and 0.72 for SVR, which is comparable to other algorithms in the literature. However, our new algorithms predict more spatially uniform annual NCP distribution for the world's oceans and higher annual NCP values in the Southern Ocean and the five oligotrophic gyres.
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    Amazon forest response to repeated droughts (2016)
    T.R. Feldpausch, O.L. Phillips, R.J.W. Brienen, E. Gloor, J. Lloyd, G. Lopez-Gonzalez, A. Monteagudo-Mendoza, Y. Malhi, A. Alarcón, E. Álvarez Dávila, P. Alvarez-Loayza, A. Andrade, L.E.O.C. Aragao, L. Arroyo, G.A. Aymard C., T.R. Baker, C. Baraloto, J. Barroso, D. Bonal, W. Castro, V. Chama, J. Chave, T.F. Domingues, S. Fauset, N. Groot, E. Honorio C., S. Laurance, W.F. Laurance, S.L. Lewis, J.C. Licona, B.S. Marimon, B.H. Marimon-Junior, C. Mendoza Bautista, D.A. Neill, E.A. Oliveira, C. Oliveira dos Santos, N.C. Pallqui Camacho, G. Pardo-Molina, A. Prieto, C.A. Quesada, F. Ramírez, H. Ramírez-Angulo, M. Réjou-Méchain, A. Rudas, G. Saiz, R.P. Salomão, J.E. Silva-Espejo, M. Silveira, H. Steege, J. Stropp, J. Terborgh, R. Thomas-Caesar, G.M.F. Heijden, R. Vásquez Martinez, E. Vilanova, V.A. Vos
    Wiley
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    Publication Date: 2016-05-01
    Description: The Amazon Basin has experienced more variable climate over the last decade, with a severe and widespread drought in 2005 causing large basin-wide losses of biomass. A drought of similar climatological magnitude occurred again in 2010; however, there has been no basin-wide ground-based evaluation of effects on vegetation. We examine to what extent the 2010 drought affected forest dynamics using ground-based observations of mortality and growth utilizing data from an extensive forest plot network. We find that during the 2010 drought interval, forests did not gain biomass (net change: −0.43 Mg ha -1 , CI: −1.11, 0.19, n = 97), regardless of whether forests experienced precipitation deficit anomalies. This loss contrasted with a long-term biomass sink during the baseline pre-2010 drought period (1998 − pre-2010) of 1.33 Mg ha -1 yr -1 (CI: 0.90, 1.74, p  〈 0.01). The resulting net impact of the 2010 drought (i.e., reversal of the baseline net sink) was −1.95 Mg ha -1 yr -1 (CI:−2.77, −1.18; p  〈 0.001). This net biomass impact was driven by an increase in biomass mortality (1.45 Mg ha -1 yr -1 CI: 0.66, 2.25, p  〈 0.001), and a decline in biomass productivity (−0.50 Mg ha -1 yr -1 , CI:−0.78, −0.31; p  〈 0.001). Surprisingly, the magnitude of the losses through tree mortality was unrelated to estimated local precipitation anomalies, and was independent of estimated local pre-2010 drought history. Thus, there was no evidence that pre-2010 droughts compounded the effects of the 2010 drought. We detected a systematic basin-wide impact of drought on tree growth rates across Amazonia, with this suppression of productivity driven by moisture deficits in 2010, an impact which was not apparent during the 2005 event [ Phillips et al. , 2009]. Based on these ground data, both live biomass in trees and corresponding estimates of live biomass in roots, we estimate that intact forests in Amazonia were carbon neutral in 2010 (−0.07 PgC yr -1 CI:−0.42, 0.23), consistent with results from an independent analysis of airborne estimates of land-atmospheric fluxes during 2010 [ Gatti et al. , 2014]. Relative to the long-term mean, the 2010 drought resulted in a reduction in biomass carbon uptake of 1.1 PgC, compared to 1.6 PgC for the 2005 event [ Phillips et al . 2009].
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    The Influence of Southern Ocean Winds on the North Atlantic Carbon Sink (2016)
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    Publication Date: 2016-05-13
    Description: Observed and predicted increases in Southern Ocean winds are thought to upwell deep ocean carbon and increase atmospheric CO 2 . However, Southern Ocean dynamics affect biogeochemistry and circulation pathways on a global scale. Using idealised MITgcm simulations, we demonstrate that an increase in Southern Ocean winds reduces the carbon sink in the North Atlantic sub-polar gyre. The increase in atmospheric CO 2 due to the reduction of the North Atlantic carbon sink is shown to be of the same magnitude as the increase in atmospheric CO 2 due to Southern Ocean outgassing. The mechanism can be described as follows: The increase in Southern Ocean winds leads to an increase in upper ocean northward nutrient transport. Biological productivity is therefore enhanced in the tropics, which alters the chemistry of the sub-thermocline waters that are ultimately upwelled in the subpolar gyre. The results demonstrate the influence of Southern Ocean winds on the North Atlantic carbon sink and show that the effect of Southern Ocean winds on atmospheric CO 2 is likely twice as large as previously thought in past, present, and future climates.
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    Influence of plankton community structure on the sinking velocity of marine aggregates (2016)
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    Publication Date: 2016-07-10
    Description: About 50 gigatons of carbon are fixed photosynthetically by surface ocean phytoplankton communities every year. Part of this organic matter is reprocessed within the plankton community to form aggregates which eventually sink and export carbon into the deep ocean. The fraction of organic matter leaving the surface ocean is partly dependent on aggregate sinking velocity which accelerates with increasing aggregate size and density where the latter is controlled by ballast load and aggregate porosity. In May 2011, we moored nine 25 m deep mesocosms in a Norwegian fjord to assess on a daily basis how plankton community structure affects material properties and sinking velocities of aggregates (Ø 80 – 400 µm) collected in the mesocosms' sediment traps. We noted that sinking velocity was not necessarily accelerated by opal ballast during diatom blooms which could be due to relatively high porosity of these rather fresh aggregates. Furthermore, estimated aggregate porosity ( P estimated ) decreased as the picoautotroph (0.2-2 µm) fraction of the phytoplankton biomass increased. Thus, picoautotroph-dominated communities may be indicative for food-webs promoting a high degree of aggregate repackaging with potential for accelerated sinking. Blooms of the coccolithophore Emiliania huxleyi revealed that cell concentrations of ~1500 cells/mL accelerate sinking by about 35-40% which we estimate (by one-dimensional modelling) to elevate organic matter transfer efficiency through the mesopelagic from 14 to 24%. Our results indicate that sinking velocities are influenced by the complex interplay between the availability of ballast minerals and aggregate packaging, both of which are controlled by plankton community structure.
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    Issue Information (2016)
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    Publication Date: 2016-07-14
    Description: No abstract is available for this article.
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    Quantifying Mesoscale-Driven Nitrate Supply: A Case Study (2016)
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    Publication Date: 2016-07-15
    Description: The supply of nitrate to surface waters plays a crucial role in maintaining marine life. Physical processes at the mesoscale (~10-100 km) and smaller have been advocated to provide a major fraction of the global supply. Whilst observational studies have focussed on well-defined features, such as isolated eddies, the vertical circulation and nutrient supply in a typical 100-200 km square of ocean will involve a turbulent spectrum of interacting, evolving and decaying features. A crucial step in closing the ocean nitrogen budget is to be able to rank the importance of mesoscale fluxes against other sources of nitrate for surface waters for a representative area of open ocean. While this has been done using models, the vital observational equivalent is still lacking. To illustrate the difficulties that prevent us from putting a global estimate on the significance of the mesoscale observationally, we use data from a cruise in the Iceland Basin where vertical velocity and nitrate observations were made simultaneously at the same high spatial resolution. Local mesoscale nitrate flux is found to be an order of magnitude greater than that due to small-scale vertical mixing and exceeds coincident nitrate uptake rates and estimates of nitrate supply due to winter convection. However, a non-zero net vertical velocity for the region introduces a significant bias in regional estimates of the mesoscale vertical nitrate transport. The need for synopticity means that a more accurate estimate can not be simply found by using a larger survey area. It is argued that time-series, rather than spatial surveys, may be the best means to quantify the contribution of mesoscale processes to the nitrate budget of the surface ocean.
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    Physical pathways for carbon transfers between the surface mixed-layer and the ocean interior (2013)
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    Publication Date: 2013-09-15
    Description: [1]  Although they are key components of the surface ocean carbon budget, physical processes inducing carbon fluxes across the mixed-layer base, i.e. subduction and obduction, have received much less attention than biological processes. Using a global model analysis of the pre-industrial ocean, physical carbon fluxes are quantified and compared to the other carbon fluxes in and out of the surface mixed-layer, i.e. air-sea CO 2 gas exchange and sedimentation of biogenic material. Model-based carbon obduction and subduction are evaluated against independent data-based estimates to the extent that was possible. We find that climatological physical fluxes of DIC are two orders of magnitude larger than the other carbon fluxes and vary over the globe at smaller spatial scale. At temperate latitudes, the subduction of DIC and to a much lesser extent (〈10%) the sinking of particles maintain CO 2 undersaturation, whereas DIC is obducted back to the surface in the tropical band (75%) and Southern Ocean (25%).At the global scale, these two large counter-balancing fluxes of DIC amount to +275.5 PgC y -1 for the supply by obduction and -264.5 PgC y -1 for the removal by subduction which is ~ 3 to 5 times larger than previous estimates. Moreover, we find that subduction of organic carbon (dissolved and particulate) represents ~ 20% of the total export of organic carbon: at the global scale, we evaluate that, of the 11 PgC y–1 of organic material lost from the surface every year, 2.1 PgC y -1 are lost through subduction of organic carbon. Our results emphasis the strong sensitivity of the oceanic carbon cycle to changes in mixed-layer depth, ocean currents and wind.
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    Dynamic biogeochemical provinces in the global ocean (2013)
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    Publication Date: 2013-09-21
    Description: [1]  In recent decades, it has been found useful to partition the pelagic environment using the concept of biogeochemical provinces, or BGCPs, within each of which it is assumed that environmental conditions are distinguishable and unique at global scale. The boundaries between provinces respond to features of physical oceanography and, ideally, should follow seasonal and inter-annual changes in ocean dynamics. But this ideal has not been fulfilled except for small regions of the oceans. Moreover, BGCPs have been used only as static entities having boundaries that were originally established to compute global primary production. In the present study, a new statistical methodology based on non-parametric procedures is implemented to capture the environmental characteristics within 56 BGCPs. Four main environmental parameters (bathymetry, chlorophyll-a concentration, surface temperature and salinity) are used to infer the spatial distribution of each BGCP over 1997–2007. The resulting dynamic partition allows us to integrate changes in the distribution of BGCPs at seasonal and inter-annual timescales, and so introduces the possibility of detecting spatial shifts in environmental conditions.
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    Eddy compensation and controls of the enhanced sea-to-air CO2 flux during positive phases of the southern annular mode (2013)
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    Publication Date: 2013-09-13
    Description: [1]  The current positive trend in the Southern Annular Mode (SAM) is thought to reduce the growth rate of the Southern Ocean CO 2 sink because enhanced wind-driven upwelling of dissolved inorganic carbon (DIC) increases outgassing of natural CO 2 . However, no study to date has quantified the potentially large role of mesoscale eddies in compensating intensified upwelling, nor the mixed layer processes in terms of their effects on CO 2 fluxes. Here we report on results from two new simulations in a regional 0.5º eddying model of the Southern Ocean. The first simulation is forced with interannually varying atmospheric reanalysis and coupled to a biogeochemistry model run under constant preindustrial atmospheric CO 2 . The second simulation is like the first except that superimposed on the forcing is a poleward shifted and intensified westerlies wind anomaly consistent with the positive phase of the SAM. In response to the SAM, the Southern Ocean's sea-to-air CO 2 flux is enhanced by 0.1 Pg C yr -1 per standard deviation of the SAM, mostly from the Antarctic Zone (AZ), where enhanced surface DIC is only partly compensated by enhanced surface alkalinity. Increased mixed-layer DIC in the AZ results from a combination of increased upwelling below the mixed layer and increased vertical diffusion at the base of the mixed layer. Previous studies overlooked the latter. Thus upward supply of DIC and alkalinity depends on associated vertical gradients just below the mixed layer, which are affected by interior ocean transport. Our eddying model study suggests that about one-third of the SAM enhancement of the Ekman-induced northward DIC transport is compensated by southward transport from standing and transient eddies.
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    Hysteresis conditions the vertical position of deep chlorophyll maximum in the temperate ocean. (2013)
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    Publication Date: 2013-10-05
    Description: [1]  Deep chlorophyll maxima (DCMs) are widespread features of oceans. In temperate regions, DCMs are commonly associated with isopycnal surfaces that frequently move over a wide vertical range. This general association between DCMs and isopycnals remains unexplained by present theories and we show here that it emerges from the seasonal history of the water column. Analysis of the formation of more than 9,000 seasonal DCMs throughout the world's oceans consistently locates the vertical position of spring/summer DCMs in temperate seas at the density of the previous winter mixed layer, independently of this density value and future depth. These results indicate that DCM formation cannot be understood without hysteresis by solely considering the instantaneous response of phytoplankton to vertical gradients in physical and chemical fields. Presents theories for DCM formation cannot explain why spring and summer DCMs are systematically found at a density equal to that of the previous mixed layer where a bloom has occurred. Rather than reacting to instantaneous physical forcing, the results indicate that DCMs operate as self-preserving biological structures that are associated with particular isopycnals because of their capacity to modify the physico-chemical environment. Combined with remote sensors to measure salinity and temperature in the surface ocean, this new understanding of DCM dynamics have the potential improve the quantification of three-dimensional primary production via satellites. This significant enhancement of the representation of oceanic biological processes can also allow increasingly realistic predictions of future biogeochemical scenarios in a warming ocean.
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    Future arctic ocean primary productivity from CMIP5 simulations: Uncertain outcome, but consistent mechanisms (2013)
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    Publication Date: 2013-06-09
    Description: [1]  Net Arctic Ocean primary production (PP) is expected to increase over this century, due to less perennial sea ice and more available light, but could decrease depending on changes in nitrate (NO 3 ) supply. Here, CMIP5 simulations performed with 11 Earth System Models are analyzed in terms of PP, surface NO 3 and sea ice coverage over 1900-2100. Whereas the mean model simulates reasonably well Arctic-integrated PP (511 TgC/yr, 1998-2005) and projects a mild 58 TgC/yr increase by 2080-2099 for the strongest climate change scenario, models do not agree on the sign of future PP change. However, similar mechanisms operate in all models. The perennial ice loss-driven increase in PP is in most models NO 3 -limited. The Arctic surface NO 3 is decreasing over the 21 st century (-2.3 ± 1 mmol/m 3 ), associated with shoaling mixed layer and with decreasing NO 3 in the nearby North Atlantic and Pacific waters. However, the inter-model spread in the degree of NO 3 limitation is initially high, resulting from 〉1000 yr spin-up simulations. This initial NO 3 spread, combined with the trend, causes a large variation in the timing of oligotrophy onset – which directly controls the sign of future PP change. Virtually all models agree in the open ocean zones on more spatially-integrated PP and less PP per unit area. The source of model uncertainty is located in the sea ice zone, where a subtle balance between light and nutrient limitations determines the PP change. Hence, it is argued that reducing uncertainty on present Arctic NO 3 in the sea ice zone would render Arctic PP projections much more consistent.
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    On the variation of regional CO2 exchange over temperate and boreal North America (2013)
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    Publication Date: 2013-09-17
    Description: [1]  Inverse-estimated net carbon exchange time series spanning two decades for six North American regions are analyzed to examine long-term trends and relationships to temperature and precipitation variations. Results reveal intensification of carbon uptake in eastern boreal North America (0.1 PgC/decade) and the Midwest United States (0.08 PgC/decade). Seasonal cross-correlation analysis shows a significant relationship between net carbon exchange and temperature/precipitation anomalies during the western United States growing season with warmer, dryer conditions leading reduced carbon uptake. This relationship is consistent with “global change-type drought” dynamics which drive increased vegetation mortality, increases in dry woody material, and increased wildfire occurrence. This finding supports the contention that future climate change may increase carbon loss in this region. Similarly, higher temperatures and reduced precipitation are accompanied by decreased net carbon uptake in the Midwestern United States towards the end of the growing season. Additionally, intensified net carbon uptake during the BE growing season is led by increased precipitation anomalies in the previous year, suggesting the influence of “climate memory” carried by regional snowmelt water. The two regions of boreal North America exhibit opposing seasonal carbon-temperature relationships with the eastern half experiencing a net carbon loss with near coincident increases in temperature and the western half showing increased net uptake. The carbon response in the boreal west region lags the temperature anomalies by roughly 6 months. This opposing carbon-temperature relationship in boreal North America may be a combination of different dominant vegetation types, the amount and timing of snowfall, and temperature anomaly differences across boreal North America.
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    Spatial and temporal contrasts in the distribution of crops and pastures across Amazonia: A new agricultural land-use dataset from census data since 1950 (2015)
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    Publication Date: 2015-05-06
    Description: Amazonia holds the largest continuous area of tropical forests with intense land use change dynamics inducing water, carbon and energy feedbacks with regional and global impacts. Much of our knowledge of land-use change in Amazonia comes from studies of the Brazilian Amazon, which accounts for two thirds of the region. Amazonia outside of Brazil has received less attention because of the difficulty of acquiring consistent data across countries. We present here an agricultural statistics database of the entire Amazonia region, with a harmonized description of crops and pastures in geospatial format, based on administrative boundary data at the municipality level. The spatial coverage includes countries within Amazonia and spans censuses and surveys from 1950 to 2012. Harmonized crop and pasture types are explored by grouping annual and perennial cropping systems, C3 and C4 photosynthetic pathways, planted and natural pastures, and main crops. Our analysis examined the spatial pattern of ratios between classes of the groups and their correlation with the agricultural extent of crops and pastures within administrative units of the Amazon, by country and census/survey dates. Significant correlations were found between all ratios and the fraction of agricultural lands of each administrative unit, with the exception of planted to natural pastures ratio and pasture lands extent. Brazil and Peru in most cases have significant correlations for all ratios analyzed even for specific census and survey dates. Results suggested improvements and potential applications of the database for carbon, water, climate and land use change studies are discussed. The database presented here provides an Amazon-wide improved data set on agricultural dynamics with expanded temporal and spatial coverage.
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    Issue Information (2015)
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    Publication Date: 2015-05-13
    Description: Cover: In Lovenduski et al. [doi 10.1002/2014GB004933 ], model-estimated trends in the sea-air flux of (first column) natural, (second column) anthropogenic, and (third column) contemporary CO 2 over (first row) the simulated period (1958–2007) and (second row) the observed period (1981–2007). Units are mol C m -2 yr -2 , and positive indicates CO 2 outgassing trend. Only those trends with significance = 95% are shown. Black lines mark the edges of the model biomes used the study. From south to north, these biomes are the Southern Ocean seasonally ice covered biome (SO-ICE) and the Southern Ocean subpolar seasonally stratified biome (SO-SPSS). See pp. 416–426.
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    Global Patterns and controls of soil organic carbon dynamics as simulated by multiple terrestrial biosphere models: current status and future directions (2015)
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    Publication Date: 2015-05-13
    Description: Soil is the largest organic carbon (C) pool of terrestrial ecosystems, and C loss from soil accounts for a large proportion of land-atmosphere C exchange. Therefore, a small change in soil organic C (SOC) can affect atmospheric carbon dioxide (CO 2 ) concentration and climate change. In the past decades, a wide variety of studies have been conducted to quantify global SOC stocks and soil C exchange with the atmosphere through site measurements, inventories, and empirical/process-based modeling. However, these estimates are highly uncertain and identifying major driving forces controlling soil C dynamics remains a key research challenge. This study has compiled century-long (1901–2010) estimates of SOC storage and heterotrophic respiration (Rh) from ten terrestrial biosphere models (TBMs) in the Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) and two observation-based datasets. The ten-TBM ensemble shows that global SOC estimate range from 425 to 2111 Pg C (1 Pg = 10 15 g) with a median value of 1158 Pg C in 2010. The models estimate a broad range of Rh from 35 to 69 Pg C yr −1 with a median value of 51 Pg C yr −1 during 2001–2010. The largest uncertainty in SOC stocks exists in the 40–65°N latitude whereas the largest differences in Rh between models are in the tropics. The modeled SOC change during 1901–2010 ranges from −70 Pg C to 86 Pg C, but in some models the SOC change has a different sign from the change of total C stock, implying very different contribution of vegetation and soil pools in determining the terrestrial C budget among models. The model ensemble-estimated mean residence time of SOC shows a reduction of 3.4 years over the past century, which is primarily caused by the increment in proportion of labile substrate which accelerate C cycling through the land biosphere. All the models agreed that climate and land use changes decreased SOC stocks while elevated atmospheric CO 2 and nitrogen deposition over intact ecosystems increased SOC stocks – even though the responses varied significantly among models. Model representations of temperature and moisture sensitivity, nutrient limitation and land use partially explain the divergent estimates of global SOC stocks and soil C fluxes in this study. In addition, a major source of systematic error in model estimations relates to non-modeled SOC storage in wetlands and peatlands, as well as to old C storage in deep soil layers.
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    Metals and Metalloids in Precipitation Collected during CHINARE Campaign from Shanghai, China to Zhongshan Station, Antarctica: Spatial Variability and Source Identification (2015)
    Wiley
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    Publication Date: 2015-05-15
    Description: Metals and metalloids in continental precipitation have been widely observed, but the data over open oceans are still very limited. Investigation of metals and metalloids in marine precipitation is of great significance to understand global transport of these elements in the atmosphere and their input fluxes to the oceans. So, shipboard sampling of precipitation was conducted during a Chinese National Antarctic Research Expedition (CHINARE) campaign from Shanghai, China to Zhongshan Station, East Antarctica, and 22 samples (including 17 rainfall and 5 snowfall events) were collected and analyzed for concentrations of Pb, Ni, Cr, Cu, Co, Hg, As, Cd, Sb, Se, Zn, Mn and Ti. Results show that concentrations of both metals and metalloids vary considerably along the cruise, with higher concentrations at coastal sites and lower values on the south Indian Ocean. Although only soluble fractions were determined for elements, concentrations in this study are generally comparable to the reported values of marine rain. Enrichment factor analysis shows that most of metals and metalloids are enriched versus crustal sources, even in the samples collected from remote south Indian Ocean. In addition, metals and metalloids in precipitation are also very enriched above sea-salt abundance, indicating that impacts of sea salt aerosols on their concentrations are negligible. Main sources of metals and metalloids were explored with the aid of multivariate statistical analyses. The results show that human emissions have far-reaching distribution, which may exert an important influence on the solubility of elements in precipitation. This investigation provides valuable information on spatial variation and possible sources of trace elements in precipitation over the open oceans corresponding to understudied region.
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    Towards a parameterization of global-scale organic carbon mineralization kinetics in surface marine sediments (2015)
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    Publication Date: 2015-05-15
    Description: An empirical function is derived for predicting the rate-depth profile of particulate organic carbon (POC) degradation in surface marine sediments including the bioturbated layer. The rate takes the form of a power law analogous to the Middelburg function. The functional parameters were optimized by simulating measured benthic O 2 and NO 3 − fluxes at 185 stations worldwide using a diagenetic model. The novelty of this work rests with the finding that the vertically-resolved POC degradation rate in the bioturbated zone can be determined using a simple function where the POC rain rate is the governing variable. Although imperfect, the model is able to fit 71 % of paired O 2 and NO 3 − fluxes to within 50% of measured values. It further provides realistic geochemical concentration-depth profiles, NO 3 − penetration depths and apparent first-order POC mineralization rate constants. The model performs less well on the continental shelf due to the high heterogeneity there. When applied to globally resolved maps of rain rate, the model predicts a global denitrification rate of 182 ± 88 Tg yr −1 of N and a POC burial rate of 107 ± 52 Tg yr −1 of C with a mean carbon burial efficiency of 6.1%. These results are in very good agreement with published values. Our proposed function is conceptually simple, requires less parameterization than multi-G type models and is suitable for non-steady state applications. It provides a basis for more accurately simulating benthic nutrient fluxes and carbonate dissolution rates in Earth system models.
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    Temperature, oxygen, and vegetation controls on decomposition in a James Bay peatland (2015)
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    Publication Date: 2015-05-07
    Description: The biochemical composition of a peat core from James Bay Lowland, Canada was used to assess the extent of peat decomposition and diagenetic alteration. Our goal was to identify environmental controls on peat decomposition, particularly its sensitivity to naturally occurring changes in temperature, oxygen exposure time, and vegetation. All three varied substantially during the last 7000 yr, providing a natural experiment for evaluating their effects on decomposition. The bottom 50 cm of the core formed during the Holocene Climatic Optimum (~7000-4000 yr B.P.), when mean annual air temperature was likely 1-2°C warmer than present. A reconstruction of the water table level using testate amoebae indicated oxygen exposure time was highest in the subsequent upper portion of the core between 150 and 225 cm depth (from ~2560-4210 yr B.P) and the plant community shifted from mostly Sphagnum to vascular plant dominance. Several independent biochemical indices indicated decomposition was greatest in this interval. Hydrolysable amino acid yields, hydroxyproline yields, and acid:aldehyde ratios of syringyl lignin phenols were higher, while hydrolysable neutral sugar yields and carbon:nitrogen ratios were lower in this zone of both vascular plant vegetation and elevated oxygen exposure time. Thus, peat formed during the Holocene Climatic Optimum did not appear to be more extensively decomposed than peat formed during subsequent cooler periods. Comparison with a core from the West Siberian Lowland, Russia, indicates oxygen exposure time and vegetation are both important controls on decomposition, while temperature appears to be of secondary importance. The low apparent sensitivity of decomposition to temperature is consistent with recent observations of a positive correlation between peat accumulation rates and mean annual temperature, suggesting contemporary warming could enhance peatland carbon sequestration, although this could be offset by an increasing contribution of vascular plants to the vegetation.
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    External total alkalinity loads vs. internal generation - the influence of non-riverine alkalinity sources in the Baltic Sea (2014)
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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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    Predicting the long-term fate of buried organic carbon in colluvial soils (2014)
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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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    Multi-century changes in ocean and land contributions to climate-carbon feedbacks (2015)
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    Publication Date: 2015-04-22
    Description: Improved constraints on carbon cycle responses to climate change are needed to inform mitigation policy, yet our understanding of how these responses may evolve after 2100 remains highly uncertain. Using the Community Earth System Model (v1.0), we quantified climate-carbon feedbacks from 1850 to 2300 for the Representative Concentration Pathway 8.5 and its extension. In three simulations, land and ocean biogeochemical processes experienced the same trajectory of increasing atmospheric CO 2 . Each simulation had a different degree of radiative coupling for CO 2 and other greenhouse gases and aerosols, enabling diagnosis of feedbacks. In a fully coupled simulation, global mean surface air temperature increased by 9.3 K from 1850 to 2300, with 4.4 K of this warming occurring after 2100. Excluding CO 2 , warming from other greenhouse gases and aerosols was 1.6 K by 2300, near a 2 K target needed to avoid dangerous anthropogenic interference with the climate system. Ocean contributions to the climate-carbon feedback increased considerably over time, and exceeded contributions from land after 2100. The sensitivity of ocean carbon to climate change was found to be proportional to changes in ocean heat content, as a consequence of this heat modifying transport pathways for anthropogenic CO 2 inflow and solubility of dissolved inorganic carbon. By 2300 climate change reduced cumulative ocean uptake by 330 Pg C, from 1410 Pg C to 1080 Pg C. Land fluxes similarly diverged over time, with climate change reducing stocks by 232 Pg C. Regional influence of climate change on carbon stocks was largest in the North Atlantic Ocean and tropical forests of South America. Our analysis suggests that after 2100, oceans may become as important as terrestrial ecosystems in regulating the magnitude of climate-carbon feedbacks.
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    Dimethyl Sulfide in the Amazon Rain Forest (2014)
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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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    The timescale of the silicate weathering negative feedback on atmospheric CO2 (2015)
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    Publication Date: 2015-04-09
    Description: The ultimate fate of CO 2 added to the ocean–atmosphere system is chemical reaction with silicate minerals and burial as marine carbonates. The timescale of this silicate weathering negative feedback on atmospheric p CO 2 will determine the duration of perturbations to the carbon cycle, be they geological release events or the current anthropogenic perturbation. However, there has been little previous work on quantifying the time-scale of the silicate weathering feedback, with the primary estimate of 300–400 kyr being traceable to an early box model study by Sundquist [1991]. Here we employ a representation of terrestrial rock weathering in conjunction with the ‘GENIE’ Earth System Model to elucidate the different timescales of atmospheric CO 2 regulation whilst including the main climate feedbacks on CO 2 uptake by the ocean. In this coupled model, the main dependencies of weathering – runoff, temperature and biological productivity – were driven from an energy-moisture balance atmosphere model and parameterized plant productivity. Long-term projections (1 Myr) were conducted for idealized scenarios of 1000 and 5000 PgC fossil fuel emissions and their sensitivity to different model parameters was tested. By fitting model output to a series of exponentials we determined the e -folding timescale for atmospheric CO 2 draw-down by silicate weathering to be ~240 kyr (range 170–380 kyr), significantly less than existing quantifications. Although the time-scales for re-equilibration of global surface temperature and surface ocean pH are similar to that for CO 2 , a much greater proportion of the peak temperature anomaly persists on this longest time-scale; ~21% compared to ~10% for CO 2 .
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    Issue Information (2015)
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    Publication Date: 2015-04-16
    Description: Cover: Holzer and Brzezinski [doi: 10.1002/2014GB004967 ] added isotope fractionation to a steady-state data-constrained model of the oceanic silicon cycle to calculate the global distribution of the isotopes of dissolved silicon, plotted here as zonal averages for the global ocean (GLB) and for each basin (ATL, PAC, IND). The δ 30 Si isotope ratio was partitioned into preformed and regenerated contributions traced back to their origin in the euphotic zone. This analysis showed that regenerated silicic acid, and possibly fractionation on opal dissolution, control the deep vertical gradients of δ 30 Si, such as the observed weak vertical gradients in the deep South Pacific that are captured in the PAC panel. See pp. 267–287.
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    Potential use of the N2/Ar ratio as a constraint on the oceanic fixed nitrogen loss (2016)
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    Publication Date: 2016-04-09
    Description: Using a global ocean biogeochemical model, we investigate the suitability of the N 2 /Ar supersaturation ratio (ΔN 2 /Ar) as a tracer of marine nitrogen fixation and denitrification, i.e., the main biological processes that add or remove fixed nitrogen from the ocean. In a series of factorial simulations, we demonstrate that in regions away from the oxygen minimum zones (OMZs) the ΔN 2 /Ar characteristics are mostly determined by benthic denitrification occurring in the deep ocean with minor contributions from benthic and water-column denitrification at shallower depths. In the OMZs, the subsurface maxima of ΔN 2 /Ar are mainly determined by water-column denitrification. In contrast, nitrogen fixation has little impact on ΔN 2 /Ar owing to the rapid loss of the N 2 supersaturation signal through air-sea exchange. We thus conclude that ΔN 2 /Ar can act as a powerful constraint on water column and benthic denitrification occurring in intermediate-to-deep waters, but it cannot be used to estimate nitrogen fixation. The comparison of the currently very limited observations of the ΔN 2 /Ar with our model results shows an acceptable level of agreement, suggesting that the model's prescribed rates and distribution of benthic and water-column denitrification (i.e., 140 and 52 TgN yr -1 , respectively) are reasonable and confirms the results derived from other constraints.
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    How well do global ocean biogeochemistry models simulate dissolved iron distributions? (2015)
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    Publication Date: 2015-12-29
    Description: Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from thirteen global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES programme we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron-fertilisation experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean.
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    A century of human-driven changes in the carbon dioxide concentration of lakes (2016)
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    Publication Date: 2016-01-07
    Description: Now that evasion of carbon dioxide (CO 2 ) from inland waters is accounted for in global carbon models, it is crucial to quantify how these fluxes have changed in the past and forecast how they may alter in the future in response to local and global change. Here, we developed a sediment proxy for the concentration of summer surface dissolved CO 2 concentration and used it to reconstruct changes over the past 150 years for three large lakes that have been affected by climate warming, changes in nutrient load and detrital terrigenous supplies. Initially CO 2 -neutral to the atmosphere, all three lakes subsequently fluctuated between near-equilibrium and supersaturation. Although catchment inputs have supplied CO 2 to the lakes, internal processes and re-allocation have ultimately regulated decadal changes in lake surface CO 2 concentration. Nutrient concentration has been the dominant driver of CO 2 variability for a century although the reproducible, non-monotonic relationship of CO 2 to nutrient concentration suggests an interplay between metabolic and chemical processes. Yet, for two of these lakes, climatic control of CO 2 concentrations has been important over the last 30 years, promoting higher surface CO 2 concentrations, likely by decreasing hypolimnetic carbon storage. This new approach offers the unique opportunity to scale, a posteriori , the long-term impact of human activities on lake CO 2 .
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    Rapid Anthropogenic Changes in CO2 and pH in the Atlantic Ocean: 2003-2014 (2016)
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    Publication Date: 2016-01-07
    Description: The extended multi-linear regression (eMLR) method is used to determine the uptake and storage of anthropogenic carbon in the Atlantic Ocean based on repeat occupations of 4 cruises from 1989–2014 (A16, A20, A22, and A10), with an emphasis on the 2003–2014 period. The results show a significant increase in basin wide anthropogenic carbon storage in the North Atlantic, which absorbed 4.4 ± 0.9 Pg C decade -1 from 2003–2014 compared to 1.9 ± 0.4 Pg C decade -1 for the 1989–2003 period. This decadal variability is attributed to changing ventilation patterns associated with the North Atlantic Oscillation (NAO) and increasing release of anthropogenic carbon into the atmosphere. There are small changes in the uptake rate of CO 2 in the South Atlantic for these time periods (3.7 ± 0.8 Pg C decade -1 versus 3.2 ± 0.7 Pg C decade -1 ). Several eddies are identified containing ~20% more anthropogenic carbon than the surrounding waters in the South Atlantic demonstrating the importance of eddies in transporting anthropogenic carbon. The uptake of carbon results in a decrease in pH of ~0.0021 ± 0.0007 yr -1 for surface waters during the last 10 years, in line with the atmospheric increase in CO 2 .
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    Long-term P weathering and recent N deposition control contemporary plant-soil C, N and P (2016)
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    Publication Date: 2016-01-07
    Description: Models are needed to understand how plant-soil nutrient stores and fluxes have responded to the last two centuries of widespread anthropogenic nutrient pollution and predict future change. These models need to integrate across carbon, nitrogen and phosphorus (C, N, & P) cycles and simulate changes over suitable timescales using available driving data. It is also vital that they are constrainable against observed data to provide confidence in their outputs. To date, no models address all of these requirements. To meet this need, a new model, N14CP, is introduced, which is initially applied to Northern hemisphere temperate and boreal ecosystems over the Holocene. N14CP is parameterized and tested using 88 northern Europe plot-scale studies, providing the most robust test of such a model to date. The model simulates long-term P weathering, based on the assumption of a starting pool of weatherable P ( P weath0 , g m −2 ), which is gradually transformed into organic and sorbed pools. Nitrogen fixation (and consequently primary production) is made dependent on available P. In the absence of knowledge about the spatial variability of P weath0 , N14CP produces good average soil and plant variables, but cannot simulate variations among sites. Allowing P weath0 to vary between sites improves soil C, N and P results greatly, suggesting contemporary soil C, N and P are sensitive to long-term P weathering. Most sites were found to be N limited. Anthropogenic N deposition since 1800 was calculated to have increased plant biomass substantially, in agreement with observations, and consequently increased soil carbon pools.
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    Mercury presence and speciation in the South Atlantic Ocean along the 40°S transect (2016)
    Wiley
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    Publication Date: 2016-01-09
    Description: Mercury (Hg) natural biogeochemical cycle is complex and a significant portion of biological and chemical transformation occurs in the marine environment. To better understand the presence and abundance of Hg species in the remote ocean regions, waters of South Atlantic Ocean along 40°S parallel were investigated during UK-GEOTRACES cruise GA10. Total mercury (THg), methylated mercury (MeHg) and dissolved gaseous mercury (DGM) concentrations were determined. The concentrations were very low in the range of pg/L (femtomolar). All Hg species had higher concentration in western than in eastern basin. THg did not appear to be a useful geotracer. Elevated methylated Hg species were commonly associated with low-oxygen water masses and occasionally with peaks of Chlorophyll a , both involved with carbon (re)cycling. The overall highest MeHg concentrations were observed in the mixed layer (500 m) and in the vicinity of the Gough Island.. Conversely, DGM concentrations showed distinct layering and differed between the water masses in a nutrient-like manner. DGM was lowest at surface, indicating degassing to the atmosphere; and was highest in the Upper Circumpolar Deep Water, where the oxygen concentration was lowest. DGM increased also in Antarctic Bottom Water. At one station, dimethylmercury was determined and showed increase in region with lowest oxygen saturation. Altogether, our data indicate that the South Atlantic Ocean could be a source of Hg to the atmosphere and that its biogeochemical transformations depend primarily upon carbon cycling and are thereby additionally prone to global ocean change.
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    Annual cycles of phytoplankton biomass in the Subarctic Atlantic and Pacific Ocean (2016)
    Wiley
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    Publication Date: 2016-01-09
    Description: High latitude phytoplankton blooms support productive fisheries and play an important role in oceanic uptake of atmospheric carbon dioxide. In the subarctic North Atlantic Ocean, blooms are a recurrent feature each year, while in the eastern subarctic Pacific only small changes in chlorophyll (Chl) are seen over the annual cycle. Here, we show that when evaluated using phytoplankton carbon biomass (C phyto ) rather than Chl, an annual bloom in the North Pacific is evident and can even rival blooms observed in the North Atlantic. The annual increase in subarctic Pacific phytoplankton biomass is not readily observed in the Chl record because it is paralleled by light- and nutrient-driven decreases in cellular pigment levels (C phyto :Chl). Specifically, photoacclimation and iron stress effects on C phyto :Chl oppose the biomass increase, leading to only modest changes in bulk Chl. The magnitude of the photoacclimation effect is quantified using descriptors of the near-surface light environment and a photophysiological model. Iron-stress effects are diagnosed from satellite chlorophyll fluorescence data. Last, we show that biomass accumulation in the Pacific is slower than the Atlantic, but is closely tied to similar levels of seasonal nutrient uptake in both basins. Annual cycles of satellite-derived Chl and C phyto are reproduced by in situ autonomous profiling floats. These results contradict the long-standing paradigm that environmental conditions prevent phytoplankton accumulation in the subarctic Northeast Pacific and suggest a greater seasonal decoupling between phytoplankton growth and losses than traditionally implied. Further, our results highlight the role of physiological processes in shaping bulk properties, such as Chl, and their interpretation in studies of ocean ecosystem dynamics and climate change.
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    Net community production and calcification from seven years of NOAA Station Papa Mooring measurements (2016)
    Wiley
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    Publication Date: 2016-01-09
    Description: Seven years of near-continuous observations from the Ocean Station Papa (OSP) surface mooring were used to evaluate drivers of marine carbon cycling in the eastern subarctic Pacific. Processes contributing to mixed layer carbon inventory changes throughout each deployment year were quantitatively assessed using a time-dependent mass-balance approach in which total alkalinity and dissolved inorganic carbon were used as tracers. By using two mixed layer carbon tracers it was possible to isolate the influences of net community production (NCP) and calcification. Our results indicate that the annual NCP at OSP is 2 ± 1 mol C m -2 yr -1 and the annual calcification is 0.3 ± 0.3 mol C m -2 yr -1 . Piecing together evidence for potentially significant dissolved organic carbon cycling in this region, we estimate a particulate inorganic carbon to particulate organic carbon ratio between 0.15 and 0.25. This is at least double the global average, adding to the growing evidence that calcifying organisms play an important role in carbon export at this location. These results, coupled with significant seasonality in the NCP, suggest that carbon cycling near OSP may be more complex than previously thought, and highlight the importance of continuous observations for robust assessments of biogeochemical cycling.
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    Longitudinal distributions of dicarboxylic acids, ω-oxoacids, pyruvic acid, α-dicarbonyls and fatty acids in the marine aerosols from the central Pacific including equatorial upwelling (2016)
    Wiley
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    Publication Date: 2016-03-10
    Description: Remote marine aerosol samples (total suspended particles, TSP) were collected during a cruise in the central Pacific from Japan to Mexico (1°59’N-35°N and 171°54’E-90°58’W). The aerosol samples were analyzed for dicarboxylic acids (C 2 -C 11 ), ω-oxoacids, pyruvic acid, α-dicarbonyls and fatty acids as well as organic and elemental carbon (OC/EC), water-soluble organic carbon (WSOC) and total nitrogen (WSTN). During the study, diacids were the most abundant compound class followed by fatty acids, ω-oxoacids and α-dicarbonyls. Molecular compositions of diacids showed a predominance of oxalic (C 2 ) acid followed by malonic (C 3 ) and succinic (C 4 ) acids. Oxalic acid comprises 74% of total diacids. This result suggests that photochemical production of oxalic acid is significant over the central Pacific. Spatial distributions of diacids, ω-oxoacids, pyruvic acid, α-dicarbonyls and fatty acids together with TC and WSTN showed higher abundances in the eastern equatorial Pacific where the upwelling of high nutrient waters followed by high biological productivity is common, indicating that their in-situ production is important in the warmer central Pacific through photochemical oxidation from their gaseous and particulate precursors. This study demonstrates that there is a strong linkage in biogeochemical cycles of carbon in the sea-air interface via ocean upwelling, phytoplankton productivity, sea-to-air emissions of organic matter and formation of secondary organic aerosols in the eastern equatorial Pacific.
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    Microzooplankton regulation of surface ocean POC:PON ratios (2016)
    Wiley
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    Publication Date: 2016-01-14
    Description: The elemental composition of particulate organic matter in the surface ocean significantly affects the efficiency of the oceanâĂŹs store of carbon. Though the elemental composition of primary producers is an important factor, recent observations from the western North Atlantic Ocean revealed that carbon-to-nitrogen ratios (C:N) of phytoplankton were significantly higher than the relatively homeostatic ratio of the total particulate pool (Particulate Organic Carbon: Particulate Organic Nitrogen; POC:PON). Here we use an idealized ecosystem model to show how interactions between primary and secondary producers maintain the mean composition of surface particulates, and the difference between primary producers and bulk material. Idealized physiological models of phytoplankton and microzooplankton, constrained by laboratory data, reveal contrasting autotrophic and heterotrophic responses to nitrogen limitation: under nitrogen limitation, phytoplankton accumulate carbon in carbohydrates and lipids while microzooplankton deplete internal C reserves to fuel respiration. Global ecosystem simulations yield hypothetical global distributions of phytoplankton and microzooplankton C:N ratio predicting elevated phytoplankton C:N ratios in the high light, low nutrient regions of the ocean despite a lower, homeostatic POC:PON ratio due to respiration of excess carbon in systems subject to top-down control. The model qualitatively captures, and provides a simple interpretation for, a global compilation of surface ocean POC:PON data.
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    Nitrogen trace gas fluxes from a semi-arid subtropical savanna under woody legume encroachment (2016)
    Wiley
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    Publication Date: 2016-04-01
    Description: Savanna ecosystems are a major source of nitrogen (N) trace gases that influence air quality and climate. These systems are experiencing widespread encroachment by woody plants, frequently associated with large increases in soil N, with no consensus on implications for trace gas emissions. We investigated the impact of encroachment by N-fixing tree Prosopis glandulosa on total reactive N gas flux (N t  = NO + N 2 O + NO y  + NH 3 ) from south Texas savanna soils over two years. Contrary to expectations, upland Prosopis groves did not have greater N t fluxes than adjacent unencroached grasslands. However, abiotic conditions (temperature, rainfall, and topography) were strong drivers. Emissions from moist, low-lying Prosopis playas were up to three times higher than from Prosopis uplands. Though NO dominated emissions, NH 3 and NO y (non-NO oxidized N) comprised 12-16% of the total summer N flux (up to 7.9 µg N m -2 h -1 ). Flux responses to soil wetting were temperature-dependent for NO, NH 3 and NO y : a 15 mm rainfall event increased flux 3-22 fold after 24 hours in summer, but had no effect in winter. Repeated soil wetting reduced N flux responses, indicating substrate depletion as a likely control. Rapid (〈1 min) increases in NO emissions following wetting of dry soils suggested that abiotic chemodenitrification contributes to pulse emissions. We conclude that temperature and wetting dynamics, rather than encroachment, are primary drivers of N flux from these upland savannas, with implications for future emission patterns under altered precipitation regimes.
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    Ventilation Pathways for the North Pacific Oxygen Deficient Zone (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Oxygen deficient zones (ODZs) in the tropical ocean exert a profound influence on global biogeochemical cycles, but the factors that regulate their long‐term structure and sensitivity to oceanic change remain poorly understood. We analyzed hydrographic observations and a high‐resolution physical/biogeochemical model to diagnose the primary pathways that ventilate the tropical Pacific ODZs. Historical and recent autonomous observations reveal pronounced and widespread O2 peaks, termed secondary oxygen maxima (SOMs), within the depths of the broader O2 minimum layer, especially at the equatorward edge of both northern and southern ODZs. In the northern ODZ, Lagrangian particle tracking in an eddy‐permitting numerical model simulation attributes these features to intrusions of the Northern Subsurface Countercurrent along the equatorial edge of the ODZ. Zonal subsurface jets also ventilate the poleward edge of the northern ODZ but induce a smaller O2 flux and do not yield detectable SOMs. Along the ODZ's eastern boundary, oxygenation is achieved by the seasonal cycle of upwelling of low‐O2 water onto the continental shelf, followed by downwelling of O2‐replenished near‐surface waters back into the ODZ. Waters entering the northern Pacific ODZ originate from the extratropics in both hemispheres, but two thirds are from the Southern Hemisphere and arrive later and with a wider range of transit times. These results suggest that predicting future changes in the large Pacific ODZs will require a better understanding of the climate sensitivity of the narrow zonal jets and seasonal dynamics of coastal upwelling that supply their O2.
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    Factors Regulating Nitrification in the Arctic Ocean: Potential Impact of Sea Ice Reduction and Ocean Acidification (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Nitrification is susceptible to changes in light and pH and, thus, could be influenced by recent sea ice reductions and acidification in the Arctic Ocean. We investigated the sensitivity of nitrification to light, pH, and substrate availability in a natural nitrifier community of the Arctic Ocean. Nitrification was active near the bottom of the shelf region (〈60 m) and in the halocline layer (50–200 m) of the Arctic basin, where ammonium was abundant, but was low in the ammonium‐depleted Atlantic layer (〉250 m). In pH control experiments, nitrification rates significantly declined when the pH was manipulated to be 0.22 lower than the controls. However, nitrification was relatively insensitive to changes in pH compared to changes in light. Light control experiments showed that nitrification was inhibited by a light intensity above 0.11 mol photons m−2 day−1, which was presumably the light threshold. A light intensity greater than the light threshold extended to the shelf bottom and upper halocline layer, limiting nitrification in these waters. Satellite data analyses indicated that the area where light levels inhibit nitrification has increased throughout the Arctic Ocean due to the recent sea ice reduction, which may lead to a declining trend in nitrification. Our results suggest that stronger light levels in the future Arctic Ocean could further suppress nitrification and alter the composition of inorganic nitrogen, with implications for the structure of ecosystems.
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    Phaeodaria: an important carrier of particulate organic carbon in the mesopelagic twilight zone of the North Pacific Ocean (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Phaeodaria, which comprise one group of large, single‐celled eukaryotic zooplankton, have been largely ignored by past marine biological studies because Phaeodaria and their delicate skeletons are liable to collapse. As a result, collection and quantification of specimens are difficult, and seasonal changes of phaeodarian abundance have not been thoroughly studied. The transport of biogenic elements by sinking phaeodarians has been estimated for only a few representative species. Sinking particles 〉1 mm in size and swimmers have traditionally been excluded when estimating sinking particle fluxes. The focus of this study is the large number of phaeodarians among the 〉1 mm sinking particles collected in the western North Pacific from June 2014 to July 2015. Careful sorting by microscopic examination and chemical analyses revealed that phaeodarians accounted for up to about 10% of the organic carbon in all sinking particles and accounted for a mean of 33% of the organic carbon in the 〉1 mm sinking particles. The high standing stocks of phaeodarians at depths of 150–1000 m in the mesopelagic twilight zone suggested that particles sinking from the euphotic zone as aggregates and fecal pellets can be efficiently ex to the deep sea by the ballasting effect of large phaeodarian particles rich in organic carbon.
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    Reconciling Observation and Model Trends in North Atlantic Surface CO2 (2019)
    Wiley
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    Publication Date: 2019
    Description: The North Atlantic Ocean is a region of intense uptake of atmospheric CO2. To assess how this CO2 sink has evolved over recent decades, various approaches have been used to estimate basin‐wide uptake from the irregularly sampled in‐situ CO2 observations. Until now, the lack of robust uncertainties associated with observation‐based gap‐filling methods required to produce these estimates has limited the capacity to validate climate model simulated surface ocean CO2 concentrations. After robustly quantifying basin‐wide and annually‐varying interpolation uncertainties using both observational and model data, we show that the North Atlantic surface ocean fugacity of CO2 (fCO2−ocean) increased at a significantly slower rate than that simulated by the latest generation of Earth System Models during the period 1992‐2014. We further show, with initialised model simulations, that the inability of these models to capture the observed trend in surface fCO2−ocean is primarily due to biases in the models' ocean biogeochemistry. Our results imply that current projections may underestimate the contribution of the North Atlantic to mitigating increasing future atmospheric CO2 concentrations.
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    Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract We present results from a global inverse marine nitrogen (N) cycle model that include nitrate (NO3−) and nitrite (NO2−) concentrations and their N isotopic compositions as constraints on N cycle process rates in marine oxygen deficient zones (ODZs). NO2− is an important intermediate in the N cycle, particularly in ODZs where it is a substrate in the N loss processes, denitrification, and anammox. Similar to earlier work, our model yields a total water column N loss rate of 61 ± 10 Tg N/year. However, by including NO2− and its N isotopic composition, we are able to assess the relative contributions of denitrification and anammox to N loss and examine some of the potential drivers of that balance. We find that anammox contributes 60% of global water column N loss, dominating N loss along the edges of ODZs, while denitrification is more important in the anoxic ODZ cores. The decoupling of anammox and denitrification is supported by NO2− oxidation, which co‐occurs with NO3− reduction and anammox in ODZs. High rates of NO2− oxidation (up to 400 nM/day), which are tightly coupled to heterotrophic NO3− reduction, are required to match NO3− and NO2− concentration and isotope observations in marine ODZs. Lowering the rate of NO2− oxidation in ODZs by adjusting O2‐sensitive parameters results in higher rates of water column N loss, highlighting the role of NO2− oxidation in maintaining the marine fixed N inventory.
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    Changes in the dust‐influenced biological carbon pump in the Canary Current System: implications from a coastal and an offshore sediment trap record off Cape Blanc, Mauritania (2019)
    Wiley
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    Publication Date: 2019
    Description: Long‐term data characterizing the oceans’ biological carbon pump are essential for understanding impacts of climate variability on marine ecosystems. The ‘Bakun upwelling intensification hypothesis’ suggests intensified coastal upwelling due to a greater land‐sea temperature gradient influenced by global warming. We present long time‐series of bathypelagic (ca. 1200‐3600m) particle fluxes from a coastal (CBeu: 2003‐2016] and an offshore (CBmeso: 1988‐2016) sediment trap setting located in the Canary Current upwelling. Organic carbon (Corg) and biogenic opal (BSi, diatoms) fluxes were two‐ to three‐fold higher at the coastal upwelling site compared to the offshore site, respectively, and showed higher seasonality with flux maxima in spring. A relationship between winter and spring BSi fluxes to the North Atlantic Oscillation (NAO) index was best expressed at the offshore site CBmeso. Lithogenic (dust) fluxes regularly peaked in winter when frequent low‐altitude dust storms and deposition occurred, decreasing offshore by about three‐fold. We obtained a high temporal match of short‐term peaks of BSi and dust fluxes in winter‐spring at the inner site CBeu. We found synchronous flux variations at both sites and an anomalous year 2005, characterized by high BSi and Corg fluxes under a low NAO. Corg and BSi fluxes revealed a decreasing trend from 2006 to 2016 at the coastal site CBeu, pointing to coastal upwelling relaxation during the last two decades. The permanent offshore upwelling zone of the deflected Canary Current represented by the flux record of CBmeso showed no signs of increasing upwelling as well which contradicts the Bakun hypothesis.
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    Insights into the Major Processes Driving the Global Distribution of Copper in the Ocean from a Global Model (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Copper (Cu) is an unusual micronutrient as it can limit primary production, but can also become toxic for growth and cellular functioning under high concentrations. Cu also displays an atypical linear profile, which will modulate its availability to marine microbes across the ocean. Multiple chemical forms of Cu coexist in seawater as dissolved species and understanding the main processes shaping the Cu biogeochemical cycling is hampered by key knowledge gaps. For instance, the drivers of its specific linear profile in seawater are unknown and the bioavailable form of Cu for marine phytoplankton is debated. Here, we developed a global 3D biogeochemical model of oceanic Cu within the NEMO/PISCES global model, which represents the global distribution of dissolved copper well. Using our model, we find that reversible scavenging of Cu by organic particles drives the dissolved Cu vertical profile and its distribution in the deep ocean. The low modeled inorganic copper (Cu') in the surface ocean means that Cu' cannot maintain phytoplankton cellular copper requirements within observed ranges. The global budget of oceanic Cu from our model suggests that its residence time may be shorter than previously estimated, and provides a global perspective on Cu cycling and the main drivers of Cu biogeochemistry in different regions. Cu scavenging within particle microenvironments and uptake by denitrifying bacteria could be a significant component of Cu cycling in oxygen minimum zones.
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    The Influence of Plankton Community Structure on Sinking Velocity and Remineralization Rate of Marine Aggregates (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Gravitational sinking of photosynthetically fixed particulate organic carbon (POC) constitutes a key component of the biological carbon pump. The fraction of POC leaving the surface ocean depends on POC sinking velocity (SV) and remineralization rate (Cremin), both of which depend on plankton community structure. However, the key drivers in plankton communities controlling SV and Cremin are poorly constrained. In fall 2014, we conducted a 6‐week mesocosm experiment in the subtropical NE Atlantic Ocean to study the influence of plankton community structure on SV and Cremin. Oligotrophic conditions prevailed for the first 3 weeks, until nutrient‐rich deep water injected into all mesocosms stimulated diatom blooms. SV declined steadily over the course of the experiment due to decreasing CaCO3 ballast and—according to an optical proxy proposed herein—due to increasing aggregate porosity mostly during an aggregation event after the diatom bloom. Furthermore, SV was positively correlated with the contribution of picophytoplankton to the total phytoplankton biomass. Cremin was highest during a Synechococcus bloom under oligotrophic conditions and in some mesocosms during the diatom bloom after the deep water addition, while it was particularly low during harmful algal blooms. The temporal changes were considerably larger in Cremin (max. fifteenfold) than in SV (max. threefold). Accordingly, estimated POC transfer efficiency to 1,000 m was mainly dependent on how the plankton community structure affected Cremin. Our approach revealed key players and interactions in the plankton food web influencing POC export efficiency thereby improving our mechanistic understanding of the biological carbon pump.
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    Nitrogen Fixation Changes Regulated by Upper Water Structure in the South China Sea During the Last Two Glacial Cycles (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Marine nitrogen fixation contributes to the budget of biologically available N and thus fuels phytoplankton productivity and carbon cycle through biological pump. Modern N fixation rates are proved to be constrained by oceanographic condition and nutrient supply to the surface waters. However, the paleoceanographic reconstruction of N fixation and its regulation mechanism remain highly uncertain in many regions. Here we present records of N fixation changes in the South China Sea over the past 250,000 years reconstructed by compound‐specific nitrogen isotopes of individual amino acids. The δ15N of source amino acids (δ15NSrc), reflecting the δ15N of the substrate nitrate originating from the subsurface water, is distinctly lower during interglacial periods, indicating intensified N fixation during interglacials. The δ15NSrc of the South China Sea covaries with the thermal gradient between surface and subsurface waters, implying a tight link between the upper water structure and N fixation. It could be hypothesized that stronger mixing during interglacials enhances the supply of excess phosphorous from the subsurface waters and thus encourages the growth of diazotrophs. Furthermore, records of bulk sediment δ15N with relatively high time resolution show dominant precession cycle, probably related to the nutrient supply from subsurface water driven by summer monsoon and associated upper water structure changes. Similar mechanism controlling N fixation is also effective in regions with enough iron supply and low concentrations of nitrogen and phosphorous, like the North Atlantic, supporting that upper water structure can dominate N fixation rates by regulating nutrient stoichiometry supplied to the surface waters.
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    Effects of Eddy‐Driven Subduction on Ocean Biological Carbon Pump (2019)
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    Publication Date: 2019
    Description: Estimates of the ocean biological carbon pump are limited by uncertainties in the magnitude of the physical injection of particulate and dissolved organic carbon to the ocean interior. A major challenge is to evaluate the contribution of these physical pumps at small spatial and temporal scales (〈100 km and 〈1 month). Here, we use a submesoscale permitting biophysical model covering a large domain representative of a subpolar and a subtropical gyre to quantify the impact of small‐scale physical carbon pumps.The model successfully simulates intense eddy‐driven subduction hot spots with a magnitude comparable to what has been observed in nature (1,000–6,000 mg C·m−2·day−1). These eddy‐driven subduction events are able to transfer carbon below the mixed‐layer, down to 500‐ to 1,000‐m depth. However, they contribute 〈5% to the annual flux at the scale of the basin, due to strong compensation between upward and downward fluxes. The model also simulates hot spots of export associated with small‐scale heterogeneity of the mixed layer, which intermittently export large amounts of suspended particulate and dissolved organic carbon. The mixed‐layer pump contributes ∼20% to the annual flux. High‐resolution measurements of export flux are needed to test models such as this one and to improve our mechanistic understanding of the biological pump and how it will respond to climate change.
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    Internal Tides Drive Nutrient Fluxes Into the Deep Chlorophyll Maximum Over Mid‐ocean Ridges (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Diapycnal mixing of nutrients from the thermocline to the surface sunlit ocean is thought to be relatively weak in the world's subtropical gyres as energy inputs from winds are generally low. The interaction of internal tides with rough topography enhances diapycnal mixing, yet the role of tidally induced diapycnal mixing in sustaining nutrient supply to the surface subtropical ocean remains relatively unexplored. During a field campaign in the North Atlantic subtropical gyre, we tested whether tidal interactions with topography enhance diapycnal nitrate fluxes in the upper ocean. We measured an order of magnitude increase in diapycnal nitrate fluxes to the deep chlorophyll maximum (DCM) over the Mid‐Atlantic Ridge compared to the adjacent deep ocean. Internal tides drive this enhancement, with diapycnal nitrate supply to the DCM increasing by a factor of 8 between neap and spring tides. Using a global tidal dissipation database, we find that this spring‐neap enhancement in diapycnal nitrate fluxes is widespread over ridges and seamounts. Mid‐ocean ridges therefore play an important role in sustaining the nutrient supply to the DCM, and these findings may have important implications in a warming global ocean.
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    Publication Date: 2019
    Description: No abstract is available for this article.
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    Underestimation of global photosynthesis in Earth System Models due to representation of vegetation structure (2019)
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    Publication Date: 2019
    Description: Abstract The impact of vegetation structure on the absorption of shortwave radiation in Earth System Models (ESMs) is potentially important for accurate modelling of the carbon cycle and hence climate projections. A proportion of incident shortwave radiation is used by plants to photosynthesize and canopy structure has a direct impact on the fraction of this radiation which is absorbed. This paper evaluates how modelled carbon assimilation of the terrestrial biosphere is impacted when clumping derived from satellite data is incorporated. We evaluated impacts of clumping on photosynthesis using the Joint UK Land Environment Simulator, the land surface scheme of the UK Earth System Model. At the global level, Gross Primary Productivity (GPP) increased by 5.53 ± 1.02 PgC yr−1 with the strongest absolute increase in the tropics. This is contrary to previous studies that have shown a decrease in photosynthesis when similar clumping data sets have been used to modify light interception in models. In our study additional transmission of light through upper canopy layers leads to enhanced absorption in lower layers in which photosynthesis tends to be light limited. We show that this result is related to the complexity of canopy scheme being used.
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    Publication Date: 2019
    Description: No abstract is available for this article.
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    Biological and physical controls on N2, O2 and CO2 distributions in contrasting Southern Ocean surface waters (2015)
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    Publication Date: 2015-06-17
    Description: We present measurements of p CO 2 , O 2 concentration, biological oxygen saturation (ΔO 2 /Ar) and N 2 saturation (ΔN 2 ) in Southern Ocean surface waters during austral summer, 2010–2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chla) concentrations in regions of frontal mixing and sea-ice melt. p CO 2 and ΔO 2 /Ar exhibited large spatial gradients (range 90 to 450 µatm and −10 to 60%, respectively) and co-varied strongly with Chla. However, the ratio of biological O 2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time-scales of O 2 and CO 2 air-sea equilibration. We measured significant oceanic CO 2 uptake, with a mean air-sea flux (~ −10 mmol m −2 d −1 ) that significantly exceeded regional climatological values. N 2 was mostly supersaturated in surface waters (mean ΔN 2 of +2.5 %), while physical processes resulted in both supersaturation and undersaturation of mixed layer O 2 (mean ΔO 2phys = 2.1 %). Box model calculations were able to reproduce much of the spatial variability of ΔN 2 and ΔO 2phys along the cruise track, demonstrating significant effects of air-sea exchange processes ( e . g . atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment-corrected surface ΔO 2 /Ar data, ranged from ~ −40 to 〉 300 mmol O 2 m −2 d −1 and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and stratified regions of sea-ice melt, reflecting physical controls on surface water light fields and nutrient availability.
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    Why are biotic iron pools uniform across high- and low-iron pelagic ecosystems? (2015)
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    Publication Date: 2015-06-18
    Description: Dissolved iron supply is pivotal in setting global phytoplankton productivity and pelagic ecosystem structure. However, most studies of the role of iron have focussed on carbon biogeochemistry within pelagic ecosystems, with less effort to quantify the iron biogeochemical cycle. Here, we compare mixed-layer biotic iron inventories from a low-iron (~0.06 nmol L −1 ) subantarctic (FeCycle study) and a seasonally high-iron (~0.6 nmol L −1 ) subtropical (FeCycle II study) site. Both studies were quasi-Lagrangian, and had multi-day occupation, common sampling protocols, and indirect estimates of biotic iron (from a limited range of available published biovolume/carbon/iron quotas). Biotic iron pools were comparable (~100± 30 pmol L −1 ) for low- and high-iron waters, despite a ten-fold difference in dissolved iron concentrations. Consistency in biotic iron inventories (~80±24 pmol L −1 , largely estimated using a limited range of available quotas) was also conspicuous for three Southern Ocean polar sites. Insights into the extent to which uniformity in biotic iron inventories was driven by the need to apply common iron quotas obtained from laboratory cultures were provided from FeCycle II. The observed two- to three-fold range of iron quotas during the evolution of FeCycle II subtropical bloom was much less than reported from laboratory monocultures. Furthermore, the iron recycling efficiency varied by fourfold during FeCycle II, increasing as stocks of new iron were depleted, suggesting that quotas and iron recycling efficiencies together set biotic iron pools. Hence, site-specific differences in iron recycling efficiencies (which provide 20-50% and 90% of total iron supply in high and low iron waters, respectively) helps offset the differences in new iron inputs between low- and high-iron sites. Future parameterisation of iron in biogeochemical models must focus on the drivers of biotic iron inventories, including the differing iron requirements of the resident biota, and the subsequent fate (retention/export/recycling) of the biotic iron.
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    Low particulate carbon to nitrogen ratios in arctic surface waters (2015)
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    Publication Date: 2015-10-16
    Description: During the Canada Three Oceans and Joint Ocean Ice Study projects in the summers of 2007 and 2008, we measured particulate organic carbon to nitrogen ratios (POC:PON) throughout the euphotic zone in subarctic and arctic waters. Depth-integrated values averaged 2.65 (±0.19) in the Beaufort Sea and Canada Basin (BS-CB domain), and were much lower than both the Redfield ratio (6.6) and the average ratios (3.9 to 5.6) measured across other arctic-subarctic domains. Average uptake ratios of C and N ( ρ C : ρ N ) were also lower (0.87±0.14) in BS-CB than in the other four domains (2.10 to 3.51). Decreasing POC:PON ratios were associated with low concentrations of phytoplankton C, reduced abundance of biogenic silica (bSiO 2 ), a smaller relative contribution of the 〉5 µm fraction to total chlorophyll a and a larger relative contribution of small flagellates (〈8 µm) to phytoplankton C. In the subsurface chlorophyll a maximum (SCM) within the BS-CB domain, phytoplankton C represented only ~13% of POC, and therefore low POC:PON may be influenced by the presence of heterotrophic microbes. These ratios are supported by data obtained during other arctic programs in 2006, 2008 and 2009. Previous work has suggested a link between freshening of surface waters and increasing dominance of picophytoplankton and bacterioplankton in the Canada Basin, and the low POC:PON ratios measured during this study may be a consequence of this shift. Our results have ramifications for the conversion between C- and N-based estimates of primary productivity, and for biogeochemical modeling of marine arctic waters.
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    Publication Date: 2015-10-21
    Description: Cover: In Brahney et al. [doi: 10.1002/2015GB005137 ], (a) Community Atmospheric Model (CAM) (v4) results of the ratio of current to preindustrial TP deposition. (b) CAM (v4) results of the current to preindustrial N:P ratios in deposition. The hemispheric difference in the mass flux changes of N versus P is clearly shown. (c) Stoichiometric representation of the relative impact of P versus N displayed as a ratio of the current to preindustrial deposition rates based on results from the CAM (v4). Values are calculated as Current/Preindustrial –1 in units of C, such that P is represented as P × 106, and N as N × 6.625. Values are shown as positive (reds) when increases in P deposition dominate, and negative (blues) when increases in N dominate. See pp. 1369–1383.
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    Significant mixed layer nitrification in a natural iron-fertilized bloom of the Southern Ocean (2015)
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    Publication Date: 2015-10-22
    Description: Nitrification, the microbially mediated oxidation of ammonium into nitrate, is generally expected to be low in the Southern Ocean mixed layer. This paradigm assumes that nitrate is mainly provided through vertical mixing and assimilated during the vegetative season, supporting the concept that nitrate uptake is equivalent to the new primary production (i.e., primary production which is potentially available for export). Here we show, that nitrification is significant (~40 to 80% of the seasonal nitrate uptake) in the naturally iron-fertilized bloom over the southeast Kerguelen Plateau. Hence, a large fraction of the nitrate-based primary production is regenerated, instead of being exported. It appears that nitrate assimilation (light-dependent) and nitrification (partly light-inhibited) are spatially separated between the upper and lower parts, respectively, of the deep surface mixed layers. These deep mixed layers, extending well below the euphotic layer, allow nitrifiers to compete with phytoplankton for the assimilation of ammonium. The high contributions of nitrification to nitrate uptake are in agreement with both low export efficiency (i.e., the percentage of primary production that is exported) and low seasonal nitrate drawdown despite high nitrate assimilation.
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    Combined Impact of Catchment Size, Land Cover and Precipitation on Streamflow and Total Dissolved Nitrogen: A Global Comparative Analysis (2015)
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    Publication Date: 2015-07-03
    Description: Nitrogen (N) loading is a global stressor to fresh and salt water systems with cascading effects on ecosystem processes [ Galloway et al. , 2003; Galloway et al. , 2004]. However, it is unclear if generalized global response patterns exist between discharge and N sourcing and retention with respect to land cover and precipitation. Using data compiled from 78 catchments from across the world, we identified how discharge and total dissolved nitrogen (TDN) vary with precipitation and land cover; and how TDN yields deviate from a generalized global response pattern. Area-weighted discharge regressions indicate that catchment size and the absence of vegetation largely control hydrologic responses. TDN concentrations and yields varied significantly (p〈0.05) with some land cover types, but these were overall poor TDN predictors (r 2 〈0.26). In 42 of 78 catchments, TDN concentrations varied independently (p〉0.05) of discharge suggesting that these sites are less sensitive to shifts in discharge associated with global climate change; but are more sensitive to shifts in hydrologic partitioning in response to land cover change. Clustering based on precipitation and stepwise multiple linear regression analyses show a shift in TDN responses from physical transport controls on TDN sourcing at the most arid and water limited sites, to climate and biologically mediated controls on TDN retention at the wetter sites. Combined, these results indicate that terrestrial systems may have differential response to changes in precipitation based on existing land use and that the impact of land use change on N fate and transport occurs within the context of climate conditions.
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    Sea-air CO2 exchange in the western Arctic coastal ocean (2015)
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    Publication Date: 2015-07-17
    Description: The biogeochemical seascape of the western Arctic coastal ocean is in rapid transition. Changes in sea ice cover will be accompanied by alterations in sea-air carbon dioxide (CO 2 ) exchange, of which the latter has been difficult to constrain owing to sparse temporal and spatial datasets. Previous assessments of sea-air CO 2 flux have targeted specific sub-regional areas of the western Arctic coastal ocean. Here a holistic approach is taken to determine the net sea-air CO 2 flux over this broad region. We compiled and analyzed an extensive dataset of nearly 600,000 surface seawater CO 2 partial pressure (pCO 2 ) measurements spanning 2003 through 2014. Using space-time co-located, reconstructed atmospheric pCO 2 values coupled with the seawater pCO 2 dataset, monthly climatologies of sea-air pCO 2 differences (∆pCO 2 ) were created on a 0.2° latitude x 0.5° longitude grid. Sea-air CO 2 fluxes were computed using the ∆pCO 2 grid and gas transfer rates calculated from a climatology of wind speed second moments. Fluxes were calculated with and without the presence of sea ice, treating sea ice as an imperfect barrier to gas exchange. This allowed for carbon uptake by the western Arctic coastal ocean to be assessed under existing and reduced sea ice cover conditions, in which carbon uptake increased 30% over the current 10.9 ± 5.7 Tg C (1 Tg = 10 12 g) yr −1 of sea ice adjusted exchange in the region. This assessment extends beyond previous sub-regional estimates in the region in an all-inclusive manner, and points to key unresolved aspects that must be targeted by future research.
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    Decoupling of net community and export production on submesoscales in the Sargasso Sea (2015)
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    Publication Date: 2015-07-19
    Description: Determinations of the net community production (NCP) in the upper ocean and the particle export production (EP) should balance over long time and large spatial scales. However, recent modeling studies suggest that a horizontal decoupling of flux-regulating processes on submesoscales (≤10 km) could lead to imbalances between individual determinations of NCP and EP. Here we sampled mixed-layer biogeochemical parameters and proxies for NCP and EP during ten, high spatial-resolution (~2 km) surface transects across strong physical gradients in the Sargasso Sea. We observed strong biogeochemical and carbon flux variability in nearly all transects. Spatial coherence among measured biogeochemical parameters within transects was common, but rarely did the same parameters co-vary consistently across transects. Spatial variability was greater in parameters associated with higher trophic levels, such as chlorophyll in 〉 5.0 µm particles, and variability in EP exceeded that of NCP in nearly all cases. Within sampling transects, coincident EP and NCP determinations were uncorrelated. However when averaged over each transect (30 to 40 km in length), we found NCP and EP to be significantly and positively correlated (R = 0.72, p = 0.04). Transect-averaged EP determinations were slightly smaller than similar NCP values (Type-II regression slope of 0.93, std = 0.32); but not significantly different from a 1:1 relationship. The results show the importance of appropriate sampling scales when deriving carbon flux budgets from upper ocean observations.
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    Global oceanic emission of ammonia: constraints from seawater and atmospheric observations (2015)
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    Publication Date: 2015-07-19
    Description: Current global inventories of ammonia emissions identify the ocean as the largest natural source. This source depends on seawater pH, temperature, and the concentration of total seawater ammonia ( NH x ( sw )), which reflects a balance between remineralization of organic matter, uptake by plankton, and nitrification. Here, we compare [ NH x ( sw )] from two global ocean biogeochemical models (BEC and COBALT) against extensive ocean observations. Simulated [ NH x ( sw )] are generally biased high. Improved simulation can be achieved in COBALT by increasing the plankton affinity for NH x within observed ranges. The resulting global ocean emissions is 2.5 TgN a −1 , much lower than current literature values(7–23 TgN a −1 ), including the widely used GEIA inventory (8 TgN a −1 ). Such a weak ocean source implies that continental sources contribute more than half of atmospheric NH x over most of the ocean in the Northern hemisphere. Ammonia emitted from oceanic sources is insufficient to neutralize sulfate aerosol acidity, consistent with observations. There is evidence over the Equatorial Pacific for a missing source of atmospheric ammonia that could be due to photolysis of marine organic nitrogen at the ocean surface or in the atmosphere. Accommodating this possible missing source yields a global ocean emission of ammonia in the range 2–5 TgN a −1 , comparable in magnitude to other natural sources from open fires and soils.
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    Origins, Seasonality and Fluxes of Organic Matter in the Congo River (2016)
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    Publication Date: 2016-07-06
    Description: The Congo River in central Africa represents a major source of organic matter (OM) to the Atlantic Ocean. This study examined elemental (%OC, %N, C:N), stable isotopic (δ 13 C, δ 15 N) and biomarker composition (lignin phenols) of particulate OM (POM) and dissolved OM (DOM) across the seasonal hydrograph. Even though the Congo exhibits an extremely stable intraannual discharge regime, seasonal variability in OM composition was evident. DOM appears predominantly derived from vascular plant inputs with greater relative contribution during the rising limb and peak in discharge associated with the major November-December discharge maximum. Generally, POM appears to be sourced from soil-derived mineral-associated OM (low C:N, low Λ 8 , higher (Ad:Al) v ) but the relative proportion of fresh vascular plant material (higher C:N, higher Λ 8 , lower (Ad:Al) v ) increases with higher discharge. During the study period (September 2009-November 2010) the Congo exported 29.21 Tg yr -1 of total suspended sediment (TSS), 1.96 Tg yr -1 of particulate organic carbon (POC) and 12.48 Tg yr -1 of dissolved organic carbon (DOC). The Congo exports an order of magnitude lower TSS load in comparison to other major riverine sources of TSS (e.g. Ganges, Brahmaputra), but due to its OM rich character actually exports a comparable amount of POC. The Congo is also 2.5 times more efficient at exporting dissolved lignin per unit volume compared to the Amazon. Including Congo dissolved lignin data in residence time calculations for lignin in the Atlantic Ocean results in an approximately 10% reduction from the existing estimate, suggesting this material is more reactive than previously thought.
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    Factors regulating the Great Calcite Belt in the Southern Ocean and its biogeochemical significance (2016)
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    Publication Date: 2016-08-07
    Description: The Great Calcite Belt (GCB) is a region of elevated surface reflectance in the Southern Ocean (SO) covering ~16% of the global ocean and is thought to result from elevated, seasonal concentrations of coccolithophores. Here, we describe field observations and experiments from two cruises that crossed the GCB in the Atlantic and Indian sectors of the SO. We confirm the presence of coccolithophores, their coccoliths, and associated optical scattering, located primarily in the region of the sub-tropical, Agulhas, and sub-Antarctic frontal regions. Coccolithophore-rich regions were typically associated with high-velocity frontal regions with higher seawater partial pressures of CO 2 ( p CO 2 ) than the atmosphere, sufficient to reverse the direction of gas exchange to a CO 2 source. There was no calcium carbonate (CaCO 3 ) enhancement of particulate organic carbon (POC) export, but there were increased POC transfer efficiencies in high-flux particulate inorganic carbon (PIC) regions. Contemporaneous observations are synthesized with results of trace-metal incubation experiments, 234 Th-based flux estimates, and remotely-sensed observations to generate a mandala that summarizes our understanding about the factors that regulate the location of the GCB.
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    Sources of uncertainties in 21st century projections of potential ocean ecosystem stressors (2016)
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    Publication Date: 2016-08-16
    Description: Future projections of potential ocean ecosystem stressors, such as acidification, warming, deoxygenation and changes in ocean productivity, are uncertain due to incomplete understanding of fundamental processes, internal climate variability, and divergent carbon emissions scenarios. This complicates climate change impact assessments. We evaluate the relative importance of these uncertainty sources in projections of potential stressors as a function of projection lead-time and spatial scale. Internally generated climate variability is the dominant source of uncertainty in mid-to-low latitudes and in most coastal Large Marine Ecosystems over the next few decades, suggesting irreducible uncertainty inherent in these short projections. Uncertainty in projections of century-scale global sea surface temperature (SST), global thermocline oxygen, and regional surface pH is dominated by scenario uncertainty, highlighting the critical importance of policy decisions on carbon emissions. In contrast, uncertainty in century-scale projections of net primary productivity (NPP), low oxygen waters, and Southern Ocean SST is dominated by model uncertainty, underscoring the importance of overcoming deficiencies in scientific understanding and improved process representation in Earth system models are critical for making more robust projections of these potential stressors. We also show that changes in the combined potential stressors emerge from the noise in 39% (34 – 44%) of the ocean by 2016-2035 relative to the 1986-2005 reference period and in 54% (50 – 60%) of the ocean by 2076-2095 following a high carbon emissions scenario. Projected large changes in surface pH and SST can be reduced substantially and rapidly with aggressive carbon emission mitigation, but only marginally for oxygen. The regional importance of model uncertainty and internal variability underscores the need for expanded and improved multi-model and large initial condition ensemble projections with Earth system models for evaluating regional marine resource impacts.
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    Century-long increasing trend and variability of dissolved organic carbon export from the Mississippi River basin driven by natural and anthropogenic forcing (2016)
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    Publication Date: 2016-08-16
    Description: There has been considerable debate as to how natural forcing and anthropogenic activities alter the timing and magnitude of the delivery of dissolved organic carbon (DOC) to the coastal ocean, which has ramifications for the ocean carbon budget, land-ocean interactions, and coastal life. Here, we present an analysis of DOC export from the Mississippi River to the Gulf of Mexico during 1901-2010 as influenced by changes in climate, land use and management practices, atmospheric CO 2 , and nitrogen deposition, through the integration of observational data with a coupled hydrologic-biogeochemical land model. Model simulations show that DOC export in the 2000s increased more than 40% since the 1900s. For the recent three decades (1981-2010), however, our simulated DOC export did not show a significant increasing trend, which is consistent with observations by USGS. Our factorial analyses suggest that land use and land cover change, including land management practices (LMPs: i.e., fertilization, irrigation, tillage, etc.), were the dominant contributors to the century-scale trend of rising total riverine DOC export, followed by changes in atmospheric carbon dioxide, nitrogen deposition, and climate. Decadal and inter-annual variations of DOC export were largely attributed to year-to-year climatic variability and extreme flooding events, which have been exacerbated by human activity. LMPs show incremental contributions to DOC increase since the 1960s, indicating the importance of sustainable agricultural practices in coping with future environmental changes such as extreme flooding events. Compared to the observational-based estimate, the modeled DOC export was 20% higher, while DOC concentrations were slightly lower. Further refinements in model structure and input datasets should enable reductions in uncertainties in our prediction of century-long trends in DOC.
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    The anthropogenic perturbation of the marine nitrogen cycle by atmospheric deposition: Nitrogen cycle feedbacks and the 15N Haber-Bosch effect (2016)
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    Publication Date: 2016-08-26
    Description: Over the last 100 years, anthropogenic emissions have led to a strong increase of atmospheric nitrogen deposition over the ocean, yet the resulting impacts and feedbacks are neither well understood nor quantified. To this end, we run a suite of simulations with the ocean component of the Community Earth System Model v1.2 forced with five scenarios of nitrogen deposition over the period from 1850 through 2100, while keeping all other forcings unchanged. Even though global oceanic net primary production increases little in response to this fertilization, the higher export and the resulting expansion of the oxygen minimum zones cause an increase in pelagic and benthic denitrification and burial by about 5%. In addition, the enhanced availability of fixed nitrogen in the surface ocean reduces global ocean N 2 -fixation by more than 10%. Despite the compensating effects through these negative feedbacks that eliminate by the year 2000 about 60% of the deposited nitrogen, the anthropogenic nitrogen input forced the upper ocean N-budget into an imbalance of between 9 to 22 Tg N yr −1 depending on the deposition scenario. The excess nitrogen accumulates to highly detectable levels and causes in most areas a distinct negative trend in the δ 15 N of the oceanic fixed nitrogen pools - a trend we refer to as the 15 N Haber-Bosch effect. Changes in surface nitrate utilization and the nitrogen feedbacks induce further changes in the δ 15 N of NO 3 , making it a good, but complex recorder of the overall impact of the changes in atmospheric deposition.
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    Carbon fate in a large temperate human-impacted river system: focus on benthic dynamics (2016)
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    Publication Date: 2016-06-30
    Description: Fluvial networks play an important role in regional and global carbon (C) budgets. The Seine River, from the Paris urban area to the entrance of its estuary (220 km), is studied here as an example of a large human-impacted river system subject to temperate climatic conditions. We assess organic C (OC) budgets up- and downstream from one of the world's largest waste water treatment plants and for different hydrological conditions using a hydro-biogeochemical model. The fine representation of sediment accumulation on the river bed allows for the quantification of pelagic and benthic effects on OC export towards the estuary and on river metabolism ( i.e. net CO 2 production). OC export is significantly affected by benthic dynamics during the driest periods, when 25 % of the inputs to the system is transformed or stored in the sediment layer. Benthic processes also substantially affect river metabolism under any hydrological condition. On average, benthic respiration accounts for one third of the total river respiration along the studied stretch (0.27 out of 0.86 gC·m −2 ·d −1 ). Even though the importance of benthic processes was already acknowledged by the scientific community for headwater streams, these results stress the major influence of benthic dynamics, and thus of physical processes such as sedimentation and re-suspension, on C cycling in downstream river systems. It opens the door to new developments in the quantification of C emissions by global models, whereby biogeochemical processing and benthic dynamics should be taken into account.
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    Methane Emissions from global rice fields: Magnitude, spatio-temporal patterns and environmental controls (2016)
    Wiley
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    Publication Date: 2016-08-09
    Description: Given the importance of the potential positive feedback between methane (CH 4 ) emissions and climate change, it is critical to accurately estimate the magnitude and spatio-temporal patterns of CH 4 emissions from global rice fields and better understand the underlying determinants governing the emissions. Here, we used a coupled biogeochemical model in combination with satellite-derived contemporary inundation area to quantify the magnitude and spatio-temporal variation of CH 4 emissions from global rice fields and attribute the environmental controls of CH 4 emissions during 1901-2010. Our study estimated that CH 4 emissions from global rice fields varied from 18.3 ± 0.1 Tg CH 4 /yr (Avg. ± 1 std. dev.) under intermittent irrigation to 38.8 ± 1.0 Tg CH 4 /yr under continuous flooding in the 2000s, indicating that the magnitude of CH 4 emissions from global rice fields was largely dependent on different water schemes. Over the past 110 years, our simulated results showed that global CH 4 emissions from rice cultivation increased 85%. The expansion of rice fields was the dominant factor for the increasing trends of CH 4 emissions, followed by elevated CO 2 concentration, and nitrogen fertilizer use. On the contrary, climate had the negative effect on the cumulative CH 4 emissions for most of the years over the study period. Our results imply that CH 4 emissions from global rice fields could be reduced through implementing optimized irrigation practices. Therefore, the future magnitude of CH 4 emissions from rice fields will be determined by the human demand for rice production as well as the implementation of optimized water management practices.
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    Partitioning N2O Emissions within the US Corn Belt using an Inverse Modeling Approach (2016)
    Wiley
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    Publication Date: 2016-08-09
    Description: Nitrous oxide (N 2 O) emissions within the US Corn Belt have been previously estimated to be 200-900% larger than predictions from emission inventories, implying that one or more source categories in bottom-up approaches are underestimated. Here we interpret hourly N 2 O concentrations measured during 2010 and 2011 at a tall tower using a time-inverted transport model and a scale factor Bayesian inverse method to simultaneously constrain direct and indirect agricultural emissions. The optimization revealed that both agricultural source categories were underestimated by the Intergovernmental Panel on Climate Change (IPCC) inventory approach. However, the magnitude of the discrepancies differed substantially, ranging from 42–58% and 200–525% for direct and indirect components, respectively. Optimized agricultural N 2 O budgets for the Corn Belt were 319 ± 184 (total), 188 ± 66 (direct), and 131 ± 118 Gg-N yr -1 (indirect) in 2010, versus 471 ± 326, 198 ± 80, and 273 ± 246 Gg-N yr -1 in 2011. We attribute the inter-annual differences to varying moisture conditions, with increased precipitation in 2011 amplifying emissions. We found that indirect emissions represented 41–58% of the total agricultural budget, a considerably larger portion than the 25–30% predicted in bottom-up inventories, further highlighting the need for improved constraints on this source category. These findings further support the hypothesis that indirect emissions are presently underestimated in bottom-up inventories. Based on our results, we suggest an indirect emission factor for runoff and leaching ranging from 0.014–0.035 for the Corn Belt, which represents an upward adjustment of 1.9–4.6 times relative to the IPCC and is in agreement with recent bottom-up field studies.
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    Phosphorus transformations along a large-scale climosequence in arid and semi-arid grasslands of northern China (2016)
    Wiley
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    Publication Date: 2016-08-10
    Description: The Walker and Syers model of phosphorus (P) transformations during long-term soil development has been verified along many chronosequences, but has rarely been examined along climosequences, particularly in arid regions. We hypothesized that decreasing aridity would have similar effects on soil P transformations as time by increasing the rate of pedogenesis. To assess this, we examined P fractions in arid and semi-arid grassland soils along a 3,700 km aridity gradient in northern China (aridity between 0.43 and 0.97, calculated as 1–[mean annual precipitation / potential evapotranspiration]). Primary mineral P declined as aridity decreased, although it still accounted for about 30% of the total P in the wettest sites. In contrast, the proportions of organic and occluded P increased as aridity decreased. These changes in soil P composition occurred in parallel with marked shifts in soil nutrient stoichiometry, with organic carbon:organic P and nitrogen:organic P ratios increasing with decreasing aridity. These results indicate increasing P demand relative to carbon or nitrogen along the climosequence. Overall, our results indicate a broad shift from abiotic to biotic control on P cycling at an aridity threshold of approximately 0.7 (corresponding to about 250 mm mean annual rainfall). We conclude that the Walker and Syers model can be extended to climosequences in arid and semi-arid ecosystems, and that the apparent decoupling of nutrient cycles in arid soils is a consequence of their pedogenic immaturity.
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    Issue Information (2016)
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    Publication Date: 2016-08-11
    Description: No abstract is available for this article.
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    Partitioning uncertainty in ocean carbon uptake projections: Internal variability, emission scenario, and model structure (2016)
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    Publication Date: 2016-08-12
    Description: We quantify and isolate the sources of projection uncertainty in annual-mean sea-air CO 2 flux over the period 2006-2080 on global and regional scales using output from two sets of ensembles with the Community Earth System Model (CESM) and models participating in the 5 t h Coupled Model Intercomparison Project (CMIP5). For annual-mean, globally-integrated sea-air CO 2 flux, uncertainty grows with prediction lead time and is primarily attributed to uncertainty in emission scenario. At the regional scale of the California Current System, we observe relatively high uncertainty that is nearly constant for all prediction lead times, and is dominated by internal climate variability and model structure, respectively in the CESM and CMIP5 model suites. Analysis of CO 2 flux projections over 17 biogeographical biomes reveals a spatially heterogenous pattern of projection uncertainty. On the biome scale, uncertainty is driven by a combination of internal climate variability and model structure, with emission scenario emerging as the dominant source for long projection lead times in both modeling suites.
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    A structural equation model analysis of phosphorus transformations in global unfertilized and uncultivated soils (2016)
    Wiley
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    Publication Date: 2016-08-27
    Description: Understanding the soil phosphorus (P) cycle is a prerequisite for predicting how environmental changes may influence the dynamics and availability of P in soil. We compiled a database of P fractions sequentially extracted by the Hedley procedure and its modification in 626 unfertilized and uncultivated soils worldwide. With this database, we applied structural equation modeling to test hypothetical soil P transformation models and to quantify the importance of different soil P pools and P transformation pathways in shaping soil P availability at a global scale. Our models revealed that soluble inorganic P (Pi, a readily available P pool) was positively and directly influenced by labile Pi, labile organic P (Po), and primary mineral P, and negatively and directly influenced by secondary mineral P; soluble Pi was not directly influenced by moderately labile Po or occluded P. The overall effect on soluble Pi was greatest for labile Pi followed by the organic P pools, occluded P, and then primary mineral P; the overall influence from secondary mineral P was small. Labile Pi was directly linked to all other soil P pools and was more strongly linked than soluble Pi to labile Po and primary mineral P. Our study highlights the important roles of labile Pi in mediating P transformations and in determining overall P availability in soils throughout the world.
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    Linking temperature sensitivity of soil CO2 release to substrate, environmental and microbial properties across alpine ecosystems (2016)
    Wiley
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    Publication Date: 2016-08-27
    Description: Our knowledge of fundamental drivers of the temperature sensitivity (Q 10 ) of soil carbon dioxide (CO 2 ) release is crucial for improving the predictability of soil carbon dynamics in Earth System Models. However, patterns and determinants of Q 10 over a broad geographic scale are not fully understood, especially in alpine ecosystems. Here, we address this issue by incubating surface soils (0-10 cm) obtained from 156 sites across Tibetan alpine grasslands. Q 10 was estimated from the dynamics of the soil CO 2 release rate under varying temperatures of 5-25 o C. Structure equation modeling was performed to evaluate the relative importance of substrate, environmental and microbial properties in regulating the soil CO 2 release rate and Q 10 . Our results indicated that steppe soils had significantly lower CO 2 release rates but higher Q 10 than meadow soils. The combination of substrate properties and environmental variables could predict 52% of the variation in soil CO 2 release rate across all grassland sites, and explained 37% and 58% of the variation in Q 10 across the steppe and meadow sites, respectively. Of these, precipitation was the best predictor of soil CO 2 release rate. Basal microbial respiration rate ( B ) was the most important predictor of Q 10 in steppe soils, whereas soil pH outweighed B as the major regulator in meadow soils. These results demonstrate that carbon quality and environmental variables co-regulate Q 10 across alpine ecosystems, implying that modelers can rely on the ‘carbon-quality temperature’ hypothesis for estimating apparent temperature sensitivities, but relevant environmental factors, especially soil pH, should be considered in higher-productivity alpine regions.
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    Multi-molecular tracers of terrestrial carbon transfer across the pan-Arctic: 14C characteristics of sedimentary carbon components and their environmental controls (2015)
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    Publication Date: 2015-09-30
    Description: Distinguishing the sources, ages and fate of various terrestrial organic carbon (OC) pools mobilized from heterogeneous arctic landscapes is key to assessing climatic impacts on the fluvial release of carbon from permafrost. Through molecular 14 C measurements, including novel analyses of suberin- and/or cutin-derived diacids (DAs) and hydroxy fatty acids (FAs), we compared the radiocarbon characteristics of a comprehensive suite of terrestrial markers (including plant wax lipids, cutin, suberin, lignin and hydroxy phenols) in the sedimentary particles from nine major arctic and sub-arctic rivers in order to establish a benchmark assessment of the mobilization patterns of terrestrial OC pools across the pan-Arctic. Terrestrial lipids, including suberin-derived longer-chain DAs (C 24,26,28 ), plant wax FAs (C 24,26,28 ) and n -alkanes (C 27,29,31 ), incorporated significant inputs of aged carbon, presumably from deeper soil horizons. Mobilization and translocation of these “old” terrestrial carbon components was dependent on non-linear processes associated with permafrost distributions. By contrast, shorter-chain (C 16,18 ) DAs and lignin phenols (as well as hydroxy phenols in rivers outside eastern Eurasian Arctic) were much more enriched in 14 C, suggesting incorporation of relatively young carbon supplied by runoff processes from recent vegetation debris and surface layers. Furthermore, the radiocarbon content of terrestrial markers is heavily influenced by specific OC sources and degradation status. Overall, multi-tracer molecular 14 C analysis sheds new light on the mobilization of terrestrial OC from arctic watersheds. Our findings of distinct ages for various terrestrial carbon components may aid in elucidating fate of different terrestrial OC pools in the face of increasing arctic permafrost thaw.
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    Terrestrial pyrogenic carbon export to fluvial ecosystems: Lessons learned from the White Nile watershed of East Africa (2015)
    Wiley
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    Publication Date: 2015-10-20
    Description: Pyrogenic carbon (PyC) is important because of its role in the global organic C (OC) cycle and in modifying soil properties. However, our understanding of PyC movement from terrestrial to fluvial ecosystems is not robust. This study examined (i) whether erosion or subsurface transport was more important for PyC export from headwaters, (ii) whether PyC was exported preferentially to total OC (TOC), and (iii) whether the movement of PyC from terrestrial to aquatic ecosystems provides an explanation for the coupling of PyC and non-PyC observed in rivers at a global scale. In the Guineo-Congolian highland forest region of western Kenya, duplicate catchments with sizes of 1–12 ha were equipped with stream gauges in primary forest and adjacent mixed agricultural landscapes that were cleared by fire 10, 16 or 62 years before. Stream water samples were taken weekly throughout one year and compared with runoff to assess PyC movement. Additional stream samples were taken from all major tributaries of the White Nile watershed of Lake Victoria. PyC was not preferentially eroded relative to TOC or non-PyC, as topsoil (0–0.15 m) PyC concentrations (6.3±0.3% of TOC; means and standard errors) were greater than runoff sediment (1.9±0.4%) and dissolved PyC concentrations (2.0±0.4%, n =252). In addition, PyC proportions in eroded sediment were lower than and uncorrelated (r 2 =0.04; P =0.14) with topsoil PyC. An enrichment of PyC was found with depth in the soil, from 6.3±0.3% of TOC in the topsoil (0–0.15 m) to 12.3±0.3% of TOC at 1–2 m. Base-flow PyC proportions of TOC correlated well with subsoil PyC (r 2 =0.57; P 〈0.05) but not with topsoil PyC (r 2 =0.18; P 〉0.05). Similar PyC proportions were found in the studied headwater streams (2.7±0.2%), their downstream inflow into Lake Victoria (3.7%), the other nine major rivers into Lake Victoria (4.9±0.8%) and its outflow into the White Nile (1.1%). A strong positive correlation between dissolved PyC and non-PyC (r 2 =0.91; P 〈0.0001) in the headwater streams reflect relationships previously seen for a range of globally important rivers, and contrasts with a negative relationship for suspended sediments (r 2 =−0.5; P 〈0.0001). The estimated PyC export from the Lake Victoria watershed of 11 Gg yr −1 may therefore originate to a large extent from subsoil pathways in dissolved form that appeared to be an important source of PyC in aquatic environments, and may explain the coupling of PyC and non-PyC at a global scale.
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    Towards More Realistic Projections of Soil Carbon Dynamics by Earth System Models (2015)
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    Publication Date: 2015-12-20
    Description: Soil carbon (C) is a critical component of Earth system models (ESMs) and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the 3 rd to 5 th assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. Firstly, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by 1 st -order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic SOC dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Secondly, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based datasets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Thirdly, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable datasets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.
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    Publication Date: 2015-12-16
    Description: No abstract is available for this article.
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    Profiling float-based observations of net respiration beneath the mixed layer (2016)
    Wiley
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    Publication Date: 2016-06-09
    Description: We employ profiling floats with dissolved oxygen sensors to observe in situ temporal oxygen evolution below the mixed layer, allowing us to characterize net respiration of organic carbon in eight distinct regions over the globe. Export and export efficiency are generally high in locations with strong seasonal variability, and low in locations of weak seasonality. Vertically integrated respiration is weakly, yet significantly, correlated with remote observations of chlorophyll, net primary production, and planktonic community size structure. These correlations suggest that regimes of high net primary production and large phytoplankton fuel elevated respiration at depth. Several regions of float-based observations intersect with sites of other detailed observations (e.g. Hawaii and Sargasso Sea), which allows us to compare our results to independent studies. We find that there is good agreement among export production estimates at highly seasonal locations, and that float-based observations may be biased low at weakly seasonal locations. We posit that the reason for the low-latitude discrepancy is the relative steady-state of oxygen concentration caused by weak seasonality and shallow wintertime mixed layer depths.
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    Issue Information (2016)
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    Publication Date: 2016-06-12
    Description: No abstract is available for this article.
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    Changes in anthropogenic nitrogen and phosphorus inputs to the St. Lawrence sub-basin over 110 years and impacts on riverine export (2016)
    Wiley
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    Publication Date: 2016-06-16
    Description: Human activities have increased the flow of nitrogen (N) and phosphorus (P) over much of the Earth, leading to increased agricultural production, but also the degradation of air, soil, and water quality. Here, we quantify the sources of anthropogenic N and P inputs to 76 watersheds of the St. Lawrence Basin (SLB) throughout the 20th century using NANI/NAPI (net anthropogenic N/P input to watersheds), a mass balance modeling approach, and estimate the fraction of these inputs exported to adjacent rivers. Our results show that since 1901, NANI and NAPI increased 4.5- and 3.8-fold respectively with a peak in 1991 mainly due to high atmospheric N deposition and P fertilizer application. However the relative increase over the course of the last century was much higher in certain watersheds, particularly those where there was greater urbanization. Ranges in NANI and NAPI vary greatly among watersheds (110 to 9,351 kg N km-2 yr-1 and 0.16 to 1,938 kg P km-2 yr-1, respectively in 2011) and are strongly related to riverine fluxes (R2 = 0.87 and 0.71 for N and P, respectively). Our results suggest that 22% of NANI (ranging from 11% to 68% across watersheds) and 17% of NAPI (ranging from 3% to 173%) are exported to rivers. Predominant sources of inputs vary spatially and through time largely due to changes in farming practices. By tracking the main sources of inputs to specific watersheds and through time, our work provides insights for N and P management. Reduction strategies will likely need to be watershed specific, although through time, our results clearly show the large-scale impact of targeted legislation.
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    Quantifying the drivers of ocean-atmosphere CO2 fluxes (2016)
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    Publication Date: 2016-06-12
    Description: A mechanistic framework for quantitatively mapping the regional drivers of air–sea CO 2 fluxes at a global scale is developed. The framework evaluates the interplay between: (1) surface heat and freshwater fluxes that influence the potential saturated carbon concentration, which depends on changes in sea surface temperature, salinity and alkalinity, (2) a residual, disequilibrium flux influenced by upwelling and entrainment of remineralized carbon- and nutrient-rich waters from the ocean interior, as well as rapid subduction of surface waters, (3) carbon uptake and export by biological activity as both soft tissue and carbonate, and (4) the effect on surface carbon concentrations due to freshwater precipitation or evaporation. In a steady state simulation of a coarse resolution ocean circulation and biogeochemistry model, the sum of the individually determined components is close to the known total flux of the simulation. The leading order balance, identified in different dynamical regimes, is between the CO 2 fluxes driven by surface heat fluxes and a combination of biologically-driven carbon uptake and disequilibrium-driven carbon outgassing. The framework is still able to reconstruct simulated fluxes when evaluated using monthly-averaged data and takes a form that can be applied consistently in models of different complexity and observations of the ocean. In this way, the framework may reveal differences in the balance of drivers acting across an ensemble of climate model simulations or be applied to an analysis and interpretation of the observed, real-world air–sea flux of CO 2 .
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