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  • 1
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    WATER MASS AGE AND AGEING DRIVING CHROMOPHORIC DISSOLVED ORGANIC MATTER IN THE DARK GLOBAL OCEAN (2015)
    Wiley
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    Publikationsdatum: 2015-06-04
    Beschreibung: The omnipresence of chromophoric dissolved organic matter (CDOM) in the open ocean enables its use as a tracer for biochemical processes throughout the global overturning circulation. We made an inventory of CDOM optical properties, ideal water age (τ) and apparent oxygen utilization (AOU) along the Atlantic, Indian and Pacific Ocean waters sampled during the Malaspina 2010 expedition. A water mass analysis was applied to obtain intrinsic, hereinafter archetypal, values of τ, AOU, oxygen utilisation rate (OUR), and CDOM absorption coefficients, spectral slopes and quantum yield for each one of the 22 water types intercepted during this circumnavigation. Archetypal values of AOU and OUR have been used to trace the differential influence of water mass ageing and ageing rates, respectively, on CDOM variables. Whereas the absorption coefficient at 325nm ( a 325 ) and the fluorescence quantum yield at 340 nm ( Φ 340 ) increased, the spectral slope over the wavelength range 275–295 nm ( S 275–295 ) and the ratio of spectral slopes over the ranges 275 –295 nm and 350–400 nm ( S R ) decreased significantly with water mass ageing (AOU). Combination of the slope of the linear regression between archetypal AOU and a 325 with the estimated global OUR allowed us to obtain a CDOM turnover time of 634 ± 120 years, which exceeds the flushing time of the dark ocean (〉200 m) by 46%. This positive relationship supports the assumption of in situ production and accumulation of CDOM as a by-product of microbial metabolism as water masses turn older. Furthermore, our data evidence that global-scale CDOM quantity ( a 325 ) is more dependant on ageing (AOU), whereas CDOM quality ( S 275–295 , S R , Φ 340 ) is more dependent on ageing rate (OUR).
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 2
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    Black carbon aerosol dynamics and isotopic composition in Alaska linked with boreal fire emissions and depth of burn in organic soils (2015)
    Wiley
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    Publikationsdatum: 2015-10-09
    Beschreibung: Black carbon (BC) aerosol emitted by boreal fires has the potential to accelerate losses of snow and ice in many areas of the Arctic, yet the importance of this source relative to fossil fuel BC emissions from lower latitudes remains uncertain. Here we present measurements of the isotopic composition of BC and organic carbon (OC) aerosols collected at two locations in interior Alaska during the summer of 2013, as part of NASA's Carbon in Arctic Reservoirs Vulnerability Experiment. We isolated BC from fine air particulate matter (PM 2.5 ) and measured its radiocarbon (Δ 14 C) content with accelerator mass spectrometry (AMS). We show that fires were the dominant contributor to variability in carbonaceous aerosol mass in interior Alaska during the summer by comparing our measurements with satellite data, measurements from an aerosol network, and predicted concentrations from a fire inventory coupled to an atmospheric transport model. The Δ 14 C of BC from boreal fires was 131 ± 52‰ in year 2013 when the Δ 14 C of atmospheric CO 2 was 23 ± 3‰, corresponding to a mean fuel age of 20 years. Fire-emitted OC had a similar Δ 14 C (99 ± 21‰) as BC, but during background (low fire) periods OC (45 to 51‰) was more positive than BC (−354 to −57‰). We also analyzed the carbon and nitrogen elemental and stable isotopic composition of the PM 2.5 . Fire-emitted aerosol had an elevated carbon to nitrogen (C/N) ratio (29 ± 2) and δ 15 N (16 ± 4‰). Aerosol Δ 14 C and δ 13 C measurements were consistent with a mean depth of burning in organic soil horizons of 20 cm (and a range of 8 to 47 cm). Our measurements of fire-emitted BC and PM 2.5 composition constrain the endmember of boreal forest fire contributions to aerosol deposition in the Arctic and may ultimately reduce uncertainties related to the impact of a changing boreal fire regime on the climate system.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 3
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    Pathways and transformations of dissolved methane and dissolved inorganic carbon in Arctic tundra watersheds: Evidence from analysis of stable isotopes (2015)
    Wiley
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    Publikationsdatum: 2015-09-10
    Beschreibung: Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO 2 ) and methane (CH 4 ) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem-scale CH 4 and CO 2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO 2 ) and CH 4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH 4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH 4 and DIC balance by estimating mechanisms of CH 4 and DIC production and transport pathways and oxidation of subsurface CH 4 . We observed a shift from acetoclastic (July) towards hydrogenotropic (September) methanogenesis at sites located towards the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape-scale effect on CH 4 production mechanism. The majority of subsurface CH 4 was transported upward by plant-mediated transport and ebullition, predominantly bypassing the potential for CH 4 oxidation. Thus, surprisingly CH 4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH 4 produced at the frost table, contributing to 〈 0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e- acceptors. This work highlights the importance of spatial and temporal variability of CH 4 production at the watershed scale, and suggests broad scale investigations are required to build better regional or pan-Arctic representations of CH 4 and CO 2 production.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 4
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    Is atmospheric phosphorus pollution altering global alpine lake stoichiometry? (2015)
    Wiley
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    Publikationsdatum: 2015-08-11
    Beschreibung: 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.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 5
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    The multiple fates of sinking particles in the North Atlantic Ocean (2015)
    Wiley
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    Publikationsdatum: 2015-08-05
    Beschreibung: 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.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 6
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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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    Publikationsdatum: 2015-08-07
    Beschreibung: 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.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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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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    Publikationsdatum: 2015-08-14
    Beschreibung: 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.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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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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    Publikationsdatum: 2015-08-13
    Beschreibung: 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.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 9
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    Multi-century changes in ocean and land contributions to climate-carbon feedbacks (2015)
    Wiley
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    Publikationsdatum: 2015-04-22
    Beschreibung: 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.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 10
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    Significant mixed layer nitrification in a natural iron-fertilized bloom of the Southern Ocean (2015)
    Wiley
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    Publikationsdatum: 2015-10-22
    Beschreibung: 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.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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