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  • 1
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    Ventilation Pathways for the North Pacific Oxygen Deficient Zone (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: 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.
    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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    Factors Regulating Nitrification in the Arctic Ocean: Potential Impact of Sea Ice Reduction and Ocean Acidification (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: 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.
    Print ISSN: 0886-6236
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 3
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    The evolution of a near‐surface ground thermal regime and modeled active‐layer thickness on James Ross Island, Eastern Antarctic Peninsula, in 2006–2016 (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Thermal regime and thickness of the active layer respond rapidly to climate variations, and thus they are important measures of cryosphere changes in polar environments. We monitored air temperature and ground temperature at a depth of 5 cm and modeled active‐layer thickness using the Stefan and Kudryavtsev models at the Abernethy Flats site, James Ross Island, Eastern Antarctic Peninsula, in the period March 2006 to February 2016. The decadal average of air and ground temperature was −7.3 and −6.1°C, respectively, and the average modeled active‐layer thickness reached 60 cm. Mean annual air temperature increased by 0.10°C y−1 over the study period, while mean annual ground temperature showed the opposite tendency of −0.05°C y−1. The cooling took place mainly in summer and caused thawing season shortening and active‐layer thinning of 1.6 cm y−1. However, these trends need to be taken carefully because all were non‐significant at p 〈 0.05. The Stefan and Kudryavtsev models reproduced the active‐layer thickness with mean absolute errors of 2.6 cm (5.0%) and 3.4 cm (5.9%), respectively, which is better than in most previous studies, making them promising tools for active‐layer modeling over Antarctica.
    Print ISSN: 1045-6740
    Digitale ISSN: 1099-1530
    Thema: Geographie , Geologie und Paläontologie
    Publiziert von Wiley
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  • 4
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    Modeling permafrost changes on the Qinghai–Tibetan plateau from 1966 to 2100: A case study from two boreholes along the Qinghai–Tibet engineering corridor (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Warming permafrost on a global scale is projected to have significant impacts on engineering, hydrology and environmental quality. Greater warming trends are predicted on the Qinghai–Tibetan Plateau (QTP), but most models for mountain permafrost have not considered the effects of water phase change and the state of deep permafrost due to a lack of detailed information. To better understand historical and future permafrost change based on in situ monitoring and field investigations, a numerical heat conduction permafrost model was introduced which differentiated the frozen and thawed state of soil, and considered unfrozen water content in frozen soil, distribution of ground ice and geothermal heat flow. Simulations were conducted at two sites with validation by long‐term monitoring of ground temperature data. After forcing with reconstructed historical ground surface temperature series starting from 1966, the model predicted permafrost changes until 2100 under different RCP scenarios. The results indicate a slow thermal response of permafrost to climate warming at the two investigated sites. Even under the most radical warming scenario (RCP8.5), deepening of the permafrost table is not obvious before 2040. At both sites, the model indicates that shallow permafrost may disappear but deep permafrost may persist by 2100. Moreover, the simulation shows that the degradation modes may differ between zones of discontinuous and continuous permafrost. The main degradation mode of the site in the discontinuous zone appears to be upward thawing from the permafrost base, while that of the site in the continuous zone is downward thawing at the permafrost table with little change at the permafrost base.
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    Thema: Geographie , Geologie und Paläontologie
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  • 5
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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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    Publikationsdatum: 2019
    Beschreibung: 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.
    Print ISSN: 0886-6236
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    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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    Reconciling Observation and Model Trends in North Atlantic Surface CO2 (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: 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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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 7
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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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    Publikationsdatum: 2019
    Beschreibung: 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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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 8
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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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    Publikationsdatum: 2019
    Beschreibung: 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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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 9
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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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    Publikationsdatum: 2019
    Beschreibung: 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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    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 10
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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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    Publikationsdatum: 2019
    Beschreibung: 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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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 11
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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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    Publikationsdatum: 2019
    Beschreibung: 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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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 12
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    Effects of Eddy‐Driven Subduction on Ocean Biological Carbon Pump (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: 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.
    Print ISSN: 0886-6236
    Digitale ISSN: 1944-9224
    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 13
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    Internal Tides Drive Nutrient Fluxes Into the Deep Chlorophyll Maximum Over Mid‐ocean Ridges (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: 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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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 14
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    Issue Information (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: No abstract is available for this article.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 15
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    Underestimation of global photosynthesis in Earth System Models due to representation of vegetation structure (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: 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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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 16
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    Issue Information (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: No abstract is available for this article.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 17
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    Modeling Photoprotection at Global Scale: the Relative Role of Non‐Photosynthetic Pigments, Physiological State and Species Composition (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Microalgae are capable of acclimating to dynamic light environments as they have developed mechanisms to optimize light harvesting and photosynthetic electron transport. When absorption of light exceeds photosynthetic capacity, various physiological protective mechanisms prevent damage of the photosynthetic apparatus. Xanthophyll pigments provide one of the most important photoprotective mechanisms to dissipate the excess light energy and prevent photoinhibition. In this study, we coupled a mechanistic model for phytoplankton photoinhibition with the global biogeochemical model Regulated Ecosystem Model version 2 (REcoM2). The assumption that photoinhibition is small in phytoplankton communities acclimated to ambient light allowed us to predict the photoprotective needs of phytoplankton. When comparing the predicted photoprotective needs to observations of pigment content determined by high‐performance liquid chromatography, our results showed that photoprotective response seems to be mediated in most parts of the ocean by a variable ratio of xanthophyll pigments to chlorophyll. The variability in the ratio appeared to be mainly driven by changes in phytoplankton community composition. Exceptions appeared at high latitudes where other energy dissipating mechanisms seem to play a role in photoprotection and both taxonomic changes and physiological acclimation determine community pigment signature. Understanding the variability of community pigment signature is crucial for modeling the coupling of light absorption to carbon fixation in the ocean. Insights about how much of this variability is attributable to changes in community composition may allow us to improve the match between remotely‐sensed optical data and the underlying phytoplankton community.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 18
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    New insights into processes controlling the δ30Si of sinking diatoms: a seasonally resolved box‐model approach (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract In the Southern Ocean, the silicon (Si) biogeochemical cycle is dominated by processes such as the supply of Si into the surface waters, Si uptake into diatom frustules, and their subsequent dissolution and export. Due to the incomplete assimilation of the silicic acid pool (DSi) and isotopic fractionation during silicification, the Si isotopic composition (δ30Si) of biogenic silica (BSi) is closely linked to the degree of Si utilization in the mixed layer (ML). In this study, we combined modelling approaches and seasonal sediment trap records of δ30Si of exported BSi to investigate the magnitude, timing and isotopic composition of the flux of siliceous particles transferred from the surface to the deep ocean. We implemented a box‐model to describe the temporal evolution of DSi and BSi concentrations and δ30Si in the ML and at depth. The model allows us to quantify fluxes of Si in and out of the ML associated with export, dissolution, and mixing. It highlights that the time‐integrated δ30Si of exported BSi measured in the sediments reflects the extent of DSi consumption at the time when net BSi production and diatom accumulation are maximal in the ML, and confirms that the δ30Si of diatoms is a reliable proxy for past Si utilization.
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    Assessment of rock glaciers and permafrost distribution in Uttarakhand, India (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract We have compiled an inventory of 1004 rock glaciers for Uttarakhand State, India, using high‐resolution satellite data freely available on Google Earth. The inventory is used to analyze the origin, spatial distribution, geometry and dynamics of rock glaciers using a combination of optical remote sensing techniques with a geographic information system (GIS). Results show that development of rock glaciers in this region depends strongly on high elevation (〉 4000 m a.s.l.) and slope aspect. Rock glaciers are more dominant towards the southern quadrant (S, SE, SW) than the northern quadrant (N, NE, NW). A large number (n = 608) of small (〈0.5 km2) rock glaciers originating from glacial moraine indicates glacial retreat in this region as one of the major causes for the formation of such a large number of rock glaciers. Median elevation of intact rock glaciers indicates that climatic conditions above 4600 m a.s.l. are suitable for the existence of permafrost in this region and that the lower limit of discontinuous permafrost gradually increases from west to east. Despite mean annual air temperatures below 0°C, increasing mean temperatures during warmest quarter of the year could be a strong controlling factor for permafrost thawing in the region. Logistic regression modeling using WorldClim version 2 climate data sets and Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) data show that these models can produce fairly reliable estimates of permafrost probability in the studied area. MODIS LST climate data sets can be crucial for mapping and monitoring permafrost in the region.
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    Multi‐temporal 3D point cloud‐based quantification and analysis of geomorphological activity at an alpine rock glacier using airborne and terrestrial LiDAR (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Change analysis of rock glaciers is crucial to analyzing the adaptation of surface and subsurface processes to changing environmental conditions at different timescales because rock glaciers are considered as potentially unstable slopes and solid water reservoirs. To quantify surface change in complex surface topographies with varying surface orientation and roughness, a full three‐dimensional (3D) change analysis is required. This study therefore proposes a novel approach for accurate 3D point cloud‐based quantification and analysis of geomorphological activity on rock glaciers. It is applied to the lower tongue area of the Äußeres Hochebenkar rock glacier, Ötztal Alps, Austria. Multi‐temporal and multi‐source topographic LiDAR data are used to quantify surface changes and to reveal their spatial and temporal characteristics at different timescales within the period 2006–2018. LiDAR‐based examinations are complemented with subsurface characteristics obtained from electrical resistivity tomography. This combined approach reveals active and variable spatial and temporal surface dynamics in the investigated area, with minimum detectable change between 0.09 and 0.65 m at 95% confidence. Given that this approach overcomes current uncertainties in established methods of differentiating complex rock glacier surfaces, we consider it a valuable addition that can be applied to objects of similar properties such as landslides or glaciers.
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    Drivers of Carbon Export Efficiency in the Global Ocean (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract The export of organic carbon from the surface ocean forms the basis of the biological carbon pump, an important planetary carbon flux. Typically, only a small fraction of primary productivity (PP) is exported (quantified as the export efficiency: export/PP). Here we assemble a global data synthesis to reveal that very high export efficiency occasionally occurs. These events drive an apparent inverse relationship between PP and export efficiency, which is opposite to that typically used in empirical or mechanistic models. At the global scale, we find that low PP, high export efficiency regimes tend to occur when macrozooplankton and bacterial abundance are low. This implies that a decoupling between PP and upper ocean remineralization processes can result in a large fraction of PP being exported, likely as intact cells or phytoplankton‐based aggregates. As the proportion of PP being exported declines, macrozooplankton and bacterial abundances rise. High export efficiency, high PP regimes also occur infrequently, possibly associated with nonbiologically mediated export of particles. A similar analysis at a biome scale reveals that the factors affecting export efficiency may be different at regional and global scales. Our results imply that the whole ecosystem structure, rather than just the phytoplankton community, is important in setting export efficiency. Further, the existence of low PP, high export efficiency regimes imply that biogeochemical models that parameterize export efficiency as increasing with PP may underestimate export flux during decoupled periods, such as at the start of the spring bloom.
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    Global fire forecasts using both large‐scale climate indices and local meteorological parameters (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Fire forecasts that predict dry‐season fire activities several months in advance are beneficial for fire management. On a global scale, however, the predictability of fires is limited because fires depend on multiple factors and lack a single dominant predictor to describe diverse fire characteristics across regions. Here, based on 33 local meteorological parameters (MPs) and 37 large‐scale climate indices (CIs), we establish four empirical model clusters to predict global interannual fire variability. We show that across various geographic locations, the models provide reliable fire forecasts at least three months prior to the peak fire months. Compared to MPs, CIs such as the Oceanic Niño Index are comparable or even superior predictors. Globally, as well as in most continents, the El Niño Southern Oscillation (ENSO) is the major driving force, explaining 17% of interannual fire variability, with strong implications for fire carbon emissions and the global carbon cycle. Other important predictors include the Northern Atlantic sea surface temperature (SST) (9%), the Southern Atlantic SST (5%), and the Pacific/North American Pattern (3%). The predictive models reveal a strong interaction between MPs and CIs, indicating potential climate‐induced modification of fire responses to meteorological conditions. We show that the newly developed predictive models can benefit future fire management in response to climate change.
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  • 23
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    Net additions of recalcitrant dissolved organic carbon in the deep Atlantic Ocean (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Most dissolved organic carbon (DOC) sequestered in the deep ocean has residence times of decades to thousands of years, with clear implications for climate regulation, though some net removal is typically observed with increasing water mass age. Here, a high quality–high resolution dataset has allowed us to identify net additions of recalcitrant DOC in specific water masses of the deep South Atlantic. Overall, the South Atlantic is a net source of recalcitrant DOC, adding 0.027 ± 0.019 Pg C y–1, while the North Atlantic is a net sink that removes 0.298 ± 0.141 Pg C y–1. We find that the balance of addition/removal of recalcitrant DOC depends not only on the origin, but also on the temperature, age and depth of the water masses that circulate and mix in the Atlantic Ocean. Future changes in the water mass composition and circulation patterns due to climate change would eventually affect that balance, altering the carbon cycle.
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    Simulation of soil thermal dynamics using an artificial neural network model for a permafrost alpine meadow on the Qinghai–Tibetan plateau (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract The thermal regime of the active layer temperature (ALT) is a key variable with which to monitor permafrost changes and to improve the precision of simulations and predictions of land surface processes. The dynamics of the active layer thermal regime can differ substantially under various land surface types and climatic conditions. The proper simulation of these different processes is essential for accurately predicting the changes in water cycles and ecosystems under a warming climate scenario. In this paper, an artificial neural network (ANN) forecasting model system was developed using only two accessible parameters, air and ground surface temperatures, to predict and simulate the ALT thermal regime. The model results show that the ANN model has better real‐time prediction capability than other physics‐based models and performs well at simulating and forecasting variations in soil temperature with a step size of 12 days in permafrost regions on the Qinghai–Tibetan Plateau. The influence of an increase in air temperature on the ALT thermal regime was more intense during the thawing process than during the freezing process, and this influence decreased with an increase in soil depth.
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    Issue Information (2019)
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    Publikationsdatum: 2019
    Beschreibung: No abstract is available for this article.
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    Effects of forest fires on the permafrost environment in the northern Da Xing'anling (Hinggan) mountains, Northeast China (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Forest fires have significantly impacted the permafrost environment, and many research programs looking at this have been undertaken at higher latitudes. However, their impacts have not yet been systematically studied and evaluated in the northern part of northeast China at mid‐latitudes. This study simultaneously measured ecological and geocryological changes at various sites in the boreal forest at different stages after forest fires (chronosequence approach) in the northern Da Xing'anling (Hinggan) Mountains, Northeast China. We obtained results through field investigations, monitoring and observations, remote sensing interpretations, and laboratory tests. The results show that forest fires have resulted in a decreased Normalized Difference Vegetation Index (NDVI) and soil moisture contents in the active layer, increased active layer thickness (ALT) and ground temperatures, and the release of a large amount of C and N from the soils in the active layer and at shallow depths of permafrost. NDVI and species biodiversity have gradually increased in the years since forest fires. However, the vegetation has not fully recovered to the climax community structures and functions of the boreal forest ecosystems. For example, ground temperatures, ALT, and soil C and N contents have been slowly recovering in the 30 years after the forest fires, but they have not yet been restored to pre‐fire levels. This study provides important scientific bases for assessment of the impacts of forest fires on the boreal forest ecosystems in permafrost regions, environmental restoration and management, and changes in the carbon stock of soils at shallow (〈3 m) depths in the Da Xingan'ling Mountains in northeast China.
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    Permafrost zonation index map and statistics over the Qinghai–Tibet Plateau based on field evidence (2019)
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    Publikationsdatum: 2019
    Beschreibung: Permafrost is prevalent over the Qinghai–Tibet Plateau (QTP), but mapping its distribution is challenging due to the limited availability of ground‐truth data sets and strong spatial heterogeneity in the region. Based on a recently developed inventory of permafrost presence or absence from 1475 in situ observations, we developed and trained a statistical model and used it to compile a high‐resolution (30 arc‐seconds) permafrost zonation index (PZI) map. The PZI model captures the high spatial variability of permafrost distribution over the QTP because it considers multiple controlling variables, including near‐surface air temperature downscaled from re‐analysis, snow cover days and vegetation cover derived from remote sensing. Our results showed the new PZI map achieved the best performance compared to available existing PZI and traditional categorical maps. Based on more than 1000 in situ measurements, the Cohen's kappa coefficient and overall classification accuracy were 0.62 and 82.5%, respectively. Excluding glaciers and lakes, the area of permafrost regions over the QTP is approximately 1.54 (1.35–1.66) ×106 km2, or 60.7 (54.5–65.2)% of the exposed land, while area underlain by permafrost is about 1.17 (0.95–1.35) ×106 km2, or 46 (37.3–53.0)%.
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    In memoriam: Hugh M. French (2019)
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    Publikationsdatum: 2019
    Beschreibung: Permafrost and Periglacial Processes, Volume 30, Issue 3, Page 129-130, July/September 2019.
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    Ground temperature and deformation analysis for an expressway embankment in warm permafrost regions of the Tibet plateau (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract The physical and mechanical processes relating to infrastructure are strongly intertwined in subgrades of road or railway in warm permafrost regions, where the evolution of these processes may influence the functionality of infrastructure. Thus, this study analyzed the embankment thermal regime and thaw settlement behavior based on monitored data for three sections of the newly constructed Gonghe–Yushu expressway (GYE) in a warm permafrost area of Tibet. The efficiencies of crushed rock, ventilated, and insulated embankments in maintaining permafrost temperature were evaluated in relation to the permafrost table (PT) and the annual warming rate of the permafrost. The deformation characteristics of three embankments for different soil layers are summarized, along with analysis of heat balance. The results show that: (a) the permafrost thaw rate has a positive linear correlation with mean annual ground temperature; (b) the permafrost displays a warming trend regardless of whether PT increases or decreases; (c) the total deformation of a given embankment shows a tendency toward settlement; and (d) subgrade peak thaw settlement occurs later than its maximum seasonal thaw depth.
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    Half a century of discontinuous permafrost persistence and degradation in western Canada (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Long‐term field studies of permafrost change are needed to validate predictive models but few are possible because of a paucity of direct observations prior to the late 1970s. To help fill this knowledge gap, we resurveyed a transect of 68 sites, originally investigated in 1962, to evaluate change in the isolated patches and sporadic discontinuous permafrost zones between Keg River, Alberta (57.8°N) and Hay River, Northwest Territories (60.8°N). The goal was to establish the degree of permafrost degradation due to approximately 2°C of regional climate warming over the intervening 55 years, compounded at some sites by forest fire. By 2017–2018, permafrost had degraded at 36% of the 44 sites which exhibited it in 1962, but had persisted at a minimum of 50% with a further 14% potentially retaining permafrost. This is much less degradation than reported for a 1988–1989 survey of the same transect. Permafrost was maintained under thicker organic layers (86% 〉 40 cm) and at the majority of sites with fine‐grained substrates, while degradation occurred preferentially at sites with coarse soils and thinner organic layers. Forest fire did not enhance the degree of permafrost loss, but greater frost table depths were observed at some burned locations. This study demonstrates that while the trajectory of change is towards permafrost loss, thin permafrost in the discontinuous zone can be persistent, even when disturbed. It also underlines the importance of considering the range of landscape types when projecting the rate of future permafrost thaw.
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    Issue Information (2019)
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    Publikationsdatum: 2019
    Beschreibung: No abstract is available for this article.
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    THE PERIGLACIAL ENVIRONMENT. FOURTH EDITION. By Hugh M. French. 2018. John Wiley & Sons. Chichester, UK, and Hoboken, New Jersey. xxii + 515 pp. £60.00 paperback; £54.99 eBook. ISBN 9781119132783 (2019)
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    Publikationsdatum: 2019
    Beschreibung: Permafrost and Periglacial Processes, EarlyView.
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    Gas‐emission craters of the Yamal and Gydan peninsulas: A proposed mechanism for lake genesis and development of permafrost landscapes (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract This paper describes two gas‐emission craters (GECs) in permafrost regions of the Yamal and Gydan peninsulas. We show that in three consecutive years after GEC formation (2014–2017), both morphometry and hydrochemistry of the inner crater lakes can become indistinguishable from other lakes. Craters GEC‐1 and AntGEC, with initial depths of 50–70 and 15–19 m respectively, have transformed into lakes 3–5 m deep. Crater‐like depressions were mapped in the bottom of 13 out of 22 Yamal lakes. However, we found no evidence that these depressions could have been formed as a result of gas emission. Dissolved methane (dCH4) concentration measured in the water collected from these depressions was at a background level (45 ppm on average). Yet, the concentration of dCH4 from the near‐bottom layer of lake GEC‐1 was significantly higher (824–968 ppm) during initial stages. We established that hydrochemical parameters (dissolved organic carbon, major ions, isotopes) measured in GEC lakes approached values measured in other lakes over time. Therefore, these parameters could not be used to search for Western Siberian lakes that potentially resulted from gas emission. Temperature profiles measured in GEC lakes show that the water column temperatures in GEC‐1 are lower than in Yamal lakes and in AntGEC – close to values of Gydan lakes. Given the initial GEC depth 〉 50 m, we suggest that at least in GEC‐1 possible re‐freezing of sediments from below might take place. However, with the present data we cannot establish the modern thickness of the closed talik under newly formed GEC lakes.
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    Supervised classification of landforms in Arctic mountains (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Erosional and sediment fluxes from Arctic mountains are lower than for temperate mountain ranges due to the influence of permafrost on geomorphic processes. As permafrost extent declines in Arctic mountains, the spatial distribution of geomorphic processes and rates will change. Improved access to high‐quality remotely sensed topographic data in the Arctic provides an opportunity to develop our understanding of the spatial distribution of Arctic geomorphological processes and landforms. Utilizing newly available Arctic digital topography data, we have developed a method for geomorphic mapping using a pixel‐based linear discriminant analysis method that could be applied across Arctic mountains. We trained our classifier using landforms within the Adventdalen catchment in Svalbard and applied it to two adjacent catchments and one in Alaska. Slope gradient, elevation–relief ratio and landscape roughness distinguish landforms to a first order with 〉80% accuracy. Our simple classification system has a similar overall accuracy when compared across our field sites. The simplicity and robustness of our classification suggest that it is possible to use it to understand the distribution of Arctic mountain landforms using extant digital topography data and without specialized classifications. Our preliminary assessments of the distribution of geomorphic processes within these catchments demonstrate the importance of post‐glacial hillslope processes in governing sediment movement in Arctic mountains.
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    Radium isotope fingerprinting of permafrost ‐ applications to thawing and intra‐permafrost processes (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Permafrost in circum‐polar regions has been recently undergoing thawing, with severe environmental consequences, including the release of greenhouse gases and amplification of global warming. Although highly important, direct methods of tracking thawing hardly exist. In a research study conducted at Adventdalen, Svalbard, we identified a permafrost radioisotope fingerprint, and show that it can be used to track thawing. Ratios of long‐ to the shorter‐lived radium isotopes are higher in ground ice than in active layer water, which we attribute to the permafrost closed system and possibly to the long residence time of ground ice in the permafrost. Also, daughter–parent 224Ra/228Ra ratios are lower in permafrost than in the active layer. These fingerprints were also identified in a local stream, confirming the applicability of this tool to tracing thawed permafrost in periglacial watersheds. A combination of radium isotope ratios and 3H allowed the identification of recent intra‐permafrost segregation processes. The permafrost radium fingerprint should be applicable to other permafrost areas, which could assist in regional quantification of the extent of permafrost thawing and carbon emissions to the atmosphere.
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    Groundwater hydrogeochemistry in permafrost regions (2019)
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    Beschreibung: Abstract This review paper provides a summary of the current state of knowledge regarding groundwater hydrogeochemistry in permafrost regions and presents expected impacts of permafrost degradation on groundwater quality. Using published case studies, the most practical monitoring approaches are reviewed, possible monitoring issues are highlighted, and links between groundwater chemistry signatures and associated flow systems in northern climates are identified. Hydrogeochemical characteristics of groundwater in permafrost regions depend on the same reactions as in nonpermafrost regions, but in acting as a confining layer, permafrost can affect groundwater chemistry by restricting recharge and limiting exchange of energy and mass between the ground surface, surface water and groundwater. Rock (mineral)–water interactions can also increase due to longer residence times. The impacts of climate change on groundwater quality in permafrost regions are thought to be linked to the loss of this confining layer. Various studies have reported significant modifications in shallow and deep groundwater contributions to surface water, marked by a decrease in dissolved organic carbon and an increase in total dissolved solids in stream water linked to declining permafrost coverage. Future studies related to hydrogeology in permafrost areas should include better in situ hydrogeochemical characterization of groundwater to assess its potential for future use as the climate warms.
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    Beyond static benchmarking: Using experimental manipulations to evaluate land model assumptions (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Land models are often used to simulate terrestrial responses to future environmental changes, but these models are not commonly evaluated with data from experimental manipulations. Results from experimental manipulations can identify and evaluate model assumptions that are consistent with appropriate ecosystem responses to future environmental change. We conducted simulations using three coupled carbon‐nitrogen versions of the Community Land Model (CLM, versions 4, 4.5 and‐ the newly developed‐ 5), and compared the simulated response to nitrogen (N) and atmospheric carbon dioxide (CO2) enrichment with meta‐analyses of observations from similar experimental manipulations. In control simulations, successive versions of CLM showed a poleward increase in gross primary productivity and an overall bias reduction, compared to FLUXNET‐MTE observations. Simulations with N and CO2 enrichment demonstrate that CLM transitioned from a model that exhibited strong nitrogen limitation of the terrestrial carbon cycle (CLM4) to a model that showed greater responsiveness to elevated concentrations of CO2 in the atmosphere (CLM5). Overall, CLM5 simulations showed better agreement with observed ecosystem responses to experimental N and CO2 enrichment than previous versions of the model. These simulations also exposed shortcomings in structural assumptions and parameterizations. Specifically, no version of CLM captures changes in plant physiology, allocation, and nutrient uptake that are likely important aspects of terrestrial ecosystems’ responses to environmental change. These highlight priority areas that should be addressed in future model developments. Moving forward, incorporating results from experimental manipulations into model benchmarking tools that are used to evaluate model performance will help increase confidence in terrestrial carbon cycle projections.
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    Dicarboxylic and oxocarboxylic acids in the Arctic coastal ocean (Beaufort Sea‐Mackenzie margin) (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract The distribution of bi‐functional carboxylic acids (BCAs) is largely reported as primary or secondary organic aerosols. However, sparse studies describe the distribution of these organic compounds in fluvial and marine environments. In the context of a global warming, we present the first results of a study of the distribution of BCAs in a surface Arctic coastal area near the mouth of the Mackenzie River. These results showed that the Beaufort Sea is an area with elevated BCA content among which glyoxylic acid is predominant, in contrast to low concentrations and predominance of oxalic acid in aerosols reported elsewhere. The carbon fraction of BCAs represents 1.8 to 4.5 % of DOC pool in Arctic Ocean. This study reinforces the hypothesis that aquatic biological processes govern the molecular distribution of BCA in marine/river waters, whereas photochemical oxidation reactions regulate their molecular distribution in rain and aerosols. Our results indicate that the Mackenzie River is an important source of BCAs in the Arctic Ocean during July‐October period, with a first estimate of 35 x103 tons of BCAs including 12 x103 tons of diacids and 23 x103 tons of oxoacids.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 39
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    Absence of age‐related trends in stable oxygen isotope ratios from oak tree rings (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract The potential for age‐related trends in the stable oxygen isotope ratios of late‐wood alpha cellulose was investigated in samples of living oak trees and historic building timbers from the UK. When the series are examined individually, it is clear that the strongest trends in individual trees and timbers reflect concurrent trends in climate. Non‐climatic trends are very small and represent random noise that can be removed by averaging. If the same data are analysed using the more conventional approach of aligning the series by ring number and fitting a regression line, so that the magnitude of the age‐trend is based on the slope of the mean and the statistical significance on the correlation coefficient, the results are very different. We demonstrate that this conventional approach regularly produces spurious age‐trends with grossly inflated probabilities, because of offsets in the mean values of series of different length. We conclude that there is no need to de‐trend stable oxygen isotope series from individual trees or timbers of oak from the UK and that to do so would remove important climatic information. Long isotope chronologies can safely be constructed by combining data from multiple individual trees, or by pooling material from trees prior to chemical treatment and isotopic measurement. Age‐related trends may occur in other species or in other regions, but where they have been identified using the conventional ‘slope of the mean’ approach they should be reassessed using the ‘mean of the slopes’ approach.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 40
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    Model Structure and Climate Data Uncertainty in Historical Simulations of the Terrestrial Carbon Cycle (1850–2014) (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The divergence among Earth system models in the terrestrial carbon cycle has prompted interest in how to reduce uncertainty. Previous studies have identified model structural uncertainty arising from process parameterizations and parameter values. The current study highlights the importance of climate forcing in generating carbon cycle uncertainty. We use simulations in which three models (CLM4, CLM4.5, CLM5) with substantially different carbon cycles are forced with two climate reconstructions (CRUNCEPv7, GSWP3v1) to examine the contributions of model structure and climate to uncertainty in the carbon cycle over the period 1850–2014. Climate uncertainty for global annual net biome production exceeds one‐third of total uncertainty (defined as the sum of climate and model structure uncertainty) in the first half of the twentieth century, but declines after the 1950s. Global annual gross primary productivity, net primary productivity, heterotrophic respiration, and vegetation and soil carbon stocks have substantial climate uncertainty (relative to total uncertainty) throughout the simulation period. Climate forcing contributes more than one‐half of total uncertainty for these carbon cycle fluxes and stocks throughout boreal North America and Eurasia, some mid‐latitude regions, and in eastern Amazonia and western equatorial Africa during the decade 2000–2009. Comparison with observationally‐based datasets of the carbon cycle using model benchmarking methods provides insight into strengths and deficiencies among models and climate forcings, but we caution against overreliance on benchmarking to discriminate among models. The conceptualization of uncertainty arising from this study implies embracing multiple feasible model simulations rather than focusing on which model or simulation is best.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 41
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    Simulated Arctic Ocean response to doubling of riverine carbon and nutrient delivery (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The Arctic Ocean, more than any other ocean, is influenced by riverine input of carbon and nutrients. That riverine delivery is likely to change with climate change as runoff increases, permafrost thaws, and tree lines advance. But it is unknown to what extent these changes in riverine delivery will affect Arctic Ocean primary production, air‐to‐sea CO2 fluxes, and acidification. To test their sensitivity to changing riverine delivery, we made sensitivity tests using an ocean circulation model coupled to an ocean biogeochemical model. In separate idealized simulations, riverine inputs of dissolved inorganic carbon (CT), dissolved organic carbon (DOC), and nutrients were increased by 1% per year until doubling. Doubling riverine nutrient delivery increased primary production by 11% on average across the Arctic basin and by up to 34‐35% locally. Doubling riverine DOC delivery resulted in 90% of that added carbon being lost to the atmosphere, partly because it was imposed that once delivered to the ocean, the riverine DOC is instantaneously remineralized to CT. That additional outgassing, when considered alone, reduced the net ingassing of natural CO2 into the Arctic Ocean by 25%, while converting the Siberian shelf seas and the Beaufort Sea from net sinks to net sources of carbon to the atmosphere. The remaining 10% of DOC remained in the Arctic Ocean, but having been converted to CT, it enhanced acidification. Conversely, doubling riverine CT increased the Arctic Ocean's average surface pH by 0.02 because riverine total alkalinity delivery increased at the same rate as riverine CT delivery.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 42
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    Variations in Ocean Deoxygenation Across Earth System Models: Isolating the Role of Parameterized Lateral Mixing (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Modern Earth system models (ESMs) disagree on the impacts of anthropogenic global warming on the distribution of oxygen and associated low‐oxygen waters. A sensitivity study using the GFDL CM2Mc model points to the representation of lateral mesoscale eddy transport as a potentially important factor in such disagreement. Because mesoscale eddies are smaller than the spatial scale of ESM ocean grids, their impact must be parameterized using a lateral mixing coefficient AREDI. The value of AREDI varies across modern ESMs and nonlinearly impacts oxygen distributions. This study shows that an increase in atmospheric CO2 results in a decline in productivity and a decrease in ventilation age in the tropics, increasing oxygen concentrations in the upper thermocline. In high latitudes global warming causes shallowing of deep convection, reducing the supply of oxygen to the deep. The net impact of these processes depends on AREDI, with an increase in hypoxic volume yet smaller total deoxygenation in the low‐mixing models, but a decrease in hypoxic volume yet larger total deoxygenation in the high‐mixing models. All models show decreases in suboxic volume, which are largest in the low‐mixing models. A subset of Coupled Model Intercomparison Project Phase 5 models exhibits a similar range of responses to global warming and similar decoupling between total deoxygenation and change in hypoxic volume. Uncertainty in lateral mixing remains an important contributor to uncertainty in projecting ocean deoxygenation.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 43
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    Fertilization of the Northwest Pacific Ocean by East Asia Air Pollutants (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Haze particles as a key air pollutant contain high level of toxins, which were hypothesized to inhibit phytoplankton growth when deposited to the ocean, and thus indirectly affect the climate. However, field observations have yet to provide conclusive evidence to confirm this hypothesis. Onboard microcosm experiments in the Northwest Pacific Ocean (NWPO) show that haze particles collected at the East Asia continent had an inhibition impact on phytoplankton growth only when at very high particle loading (2 mg/L). In contrast, haze particles at low and medium loadings (0.03–0.6 mg/L) stimulated phytoplankton growth and shifted phytoplankton size structure toward larger cells, primarily due to the supply of inorganic nitrogen nutrients from the particles. Model simulations showed that haze particle loading in NWPO surface seawater was usually more than an order of magnitude lower than 2 mg/L. This indicates that haze particles are unlikely to cause harm but to stimulate phytoplankton growth in the nitrogen‐limited NWPO. Ocean biogeochemical modeling further shows that deposited nitrogen significantly enhanced surface ocean chlorophyll a concentration in the winter and spring of 2014. Overall, these results demonstrate that haze particles stimulate rather than inhibit primary production in the NWPO.
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  • 44
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    Vegetation Functional Properties Determine Uncertainty of Simulated Ecosystem Productivity: A Traceability Analysis in the East Asian Monsoon Region (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Global and regional projections of climate change by Earth system models are limited by their uncertain estimates of terrestrial ecosystem productivity. At the middle to low latitudes, the East Asian monsoon region has higher productivity than forests in Europe‐Africa and North America, but its estimate by current generation of terrestrial biosphere models (TBMs) has seldom been systematically evaluated. Here, we developed a traceability framework to evaluate the simulated gross primary productivity (GPP) by 15 TBMs in the East Asian monsoon region. The framework links GPP to net primary productivity, biomass, leaf area and back to GPP via incorporating multiple vegetation functional properties of carbon‐use efficiency (CUE), vegetation C turnover time (τveg), leaf C fraction (Fleaf), specific leaf area (SLA), and leaf area index (LAI)‐level photosynthesis (PLAI), respectively. We then applied a relative importance algorithm to attribute intermodel variation at each node. The results showed that large intermodel variation in GPP over 1901–2010 were mainly propagated from their different representation of vegetation functional properties. For example, SLA explained 77% of the intermodel difference in leaf area, which contributed 90% to the simulated GPP differences. In addition, the models simulated higher CUE (18.1 ± 21.3%), τveg (18.2 ± 26.9%), and SLA (27.4±36.5%) than observations, leading to the overestimation of simulated GPP across the East Asian monsoon region. These results suggest the large uncertainty of current TBMs in simulating GPP is largely propagated from their poor representation of the vegetation functional properties and call for a better understanding of the covariations between plant functional properties in terrestrial ecosystems.
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  • 45
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    Remote Western Arctic Nutrients Fuel Remineralization in Deep Baffin Bay (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract The oceanic nitrogen cycle is critically important for the partitioning of greenhouse gases between ocean and atmosphere. Baffin Bay connects ocean regions that are major sources (North Atlantic) and sinks (North Pacific and western Arctic) of biologically available nitrogen and further harbors supersaturation of nitrous oxide and a coincident deficit in nitrate in the deep basin. Isotopic tracer profiles of both nitrogen species presented here provide novel insights into the origin and cycling of reactive nitrogen in Baffin Bay, highlighting the connectivity between different Arctic systems and horizontal components of basin‐scale nutrient transport. Baffin Bay bottom water properties are derived from export production in northern Baffin Bay, which is largely fueled by Pacific‐derived nutrients. In situ remineralization at depth gives rise to benthic denitrification, evidenced by a pronounced accumulation of nitrous oxide with a distinctively high site preference (〈44‰) in the deep basin. Nutrients supplied to Baffin Bay are hence stripped from surface waters and trapped at depth over long timescales, where sedimentary denitrification further adds to the N removal capacity of the Arctic Ocean.
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  • 46
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    Isopycnal Processes Allow for Summertime Heterotrophy Despite Net Oxygen Accumulation in the Lower Euphotic Zone of the Western North Atlantic Subtropical Gyre (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract In the oligotrophic subtropical gyre of the North Atlantic, the processes that allow for an imbalance between annual biological productivity and organic carbon export have been sought for decades. We use biogeochemical data from profiling floats and 26‐year bottle samples off Bermuda to provide the first evidence for a mechanism that allows for heterotrophy in the presence of oxygen accumulation in the lower euphotic zone (50–100 m) during the stratified season. After the spring bloom, surface waters that are enriched in oxygen and organic matter, but low in nitrate, are subducted and transported along the seasonal isopycnals that progressively displace downward. Due exclusively to this downward displacement, a positive 50‐ to 100‐m depth‐integrated O2 anomaly appears (1,688 ± 545 mmol O2/m2) from mid‐May to mid‐October. Neglecting this effect of isopycnal displacement would suggest an excess of biological productivity over remineralization at 50–100 m (344 ± 330 mmol O2/m2). Yet, when these changes are differenced, significant along‐isopycnal oxygen consumption (−1,344 ± 537 mmol O2/m2) is identified. After accounting for mixing, net biological‐driven oxygen consumption is still found (−827 ± 509 mmol O2/m2), which indicates heterotrophy. Remineralization of sinking and suspended organic matters at 50–100 m could support 90 ± 67% of the heterotrophic demand. Our analysis also shows that the spread in the biological‐driven oxygen sink is linked to the strength of isopycnal displacement that modulates the supply of nutrients and organic matters. This along‐isopycnal transport and heterotrophy in the lower euphotic zone reduces carbon export at 100 m and helps to resolve previously noted imbalances between surface biological productivity and total organic carbon export.
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    A Satellite Assessment of Environmental Controls of Phytoplankton Community Size Structure (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Phytoplankton play a key role as the base of the marine food web and a crucial component in the Earth's carbon cycle. There have been a few regional studies that have utilized satellite‐estimated phytoplankton functional type products in conjunction with other environmental metrics. Here we expand to a global perspective and ask, what are the physical drivers of phytoplankton composition variability? Using a variety of satellite‐observed ocean color products and physical properties spanning 1997–2015, we characterize spatial and temporal variability in phytoplankton community size structure in relation to satellite‐based physical drivers. We consider the relationships globally and by major thermal regimes (cold and warm), dominant size distribution, and chlorophyll concentration variability. Globally, euphotic depth is the most important parameter driving phytoplankton size variability and also over the majority of the high‐latitude ocean and the central gyres. In all other regions, size variability is driven by a balance of light and mode of nutrient delivery. We investigated the relationship between size composition and chlorophyll concentration and the physical drivers through correlation analysis. Changes in size composition over time are regionally varying and explained by temporal shifts in the varying physical conditions. These changes in phytoplankton size composition and the varying underlying physical drivers will ultimately impact carbon export and food web processes in our changing ocean.
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    Pacific Anthropogenic Carbon Between 1991 and 2017 (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract We estimate anthropogenic carbon (Canth) accumulation rates in the Pacific Ocean between 1991 and 2017 from 14 hydrographic sections that have been occupied two to four times over the past few decades, with most sections having been recently measured as part of the Global Ocean Ship‐based Hydrographic Investigations Program. The rate of change of Canth is estimated using a new method that combines the extended multiple linear regression method with improvements to address the challenges of analyzing multiple occupations of sections spaced irregularly in time. The Canth accumulation rate over the top 1,500 m of the Pacific increased from 8.8 (±1.1, 1σ) Pg of carbon per decade between 1995 and 2005 to 11.7 (±1.1) PgC per decade between 2005 and 2015. For the entire Pacific, about half of this decadal increase in the accumulation rate is attributable to the increase in atmospheric CO2, while in the South Pacific subtropical gyre this fraction is closer to one fifth. This suggests a substantial enhancement of the accumulation of Canth in the South Pacific by circulation variability and implies that a meaningful portion of the reinvigoration of the global CO2 sink that occurred between ~2000 and ~2010 could be driven by enhanced ocean Canth uptake and advection into this gyre. Our assessment suggests that the accuracy of Canth accumulation rate reconstructions along survey lines is limited by the accuracy of the full suite of hydrographic data and that a continuation of repeated surveys is a critical component of future carbon cycle monitoring.
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  • 49
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    Coupling and Decoupling of Soil Carbon and Nutrient Cycles Across an Aridity Gradient in the Drylands of Northern China: Evidence From Ecoenzymatic Stoichiometry (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Drylands are characterized by stressful conditions with the limitation of both carbon (C) and nutrients, particularly nitrogen (N) and phosphorus (P). Biological C, N, and P releases from soil organic matter by enzymes are essential components for biogeochemical cycles and are sensitive to the climate in drylands. However, how the ecoenzymatic C:N:P stoichiometry responds to environmental factors (i.e., climatic and edaphic factors) over broad geographical scales remains largely unclear. We examined the patterns of ecoenzymatic C:N:P ratios across a 3,700‐km aridity gradient (0.43 〈 aridity 〈 0.97) in northern China. In wetter sites (aridity 〈 0.70), the relative C:N:P acquisition ratios via enzymes remained relatively constant with increasing aridity. In contrast, in drier sites (aridity 〉 0.70), the enzymatic C:nutrient (N and P) ratios declined as the aridity increased, while the enzymatic P:N ratios were mostly lower than those in the wetter sites. In drier sites with low C availability, the increasing carbon use efficiency and the increasing proportion of C converted to biomass (than the proportion of respiration) contributed to the declines of the enzymatic C:nutrient ratios as the aridity increased. The overall lower enzymatic P:N ratios were related to the higher soil P availability compared with N availability (higher organic P and lower soil NH4+:available P ratios) in drier sites. Overall, our findings indicate that intrinsic linkages of biological C, N, and P acquisitions and cycles were broken at the aridity threshold of 0.70, with higher acquisition efforts for N and P (particularly for N) with increasing aridity in drier sites with aridity 〉 0.70.
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    High rates of N2 fixation in temperate, western North Atlantic coastal waters expands the realm of marine diazotrophy (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Dinitrogen (N2) fixation can alleviate N limitation of primary productivity by introducing fixed nitrogen (N) to the world's oceans. Although measurements of pelagic marine N2 fixation are predominantly from oligotrophic oceanic regions, where N limitation is thought to favor growth of diazotrophic microbes, here we report high rates of N2 fixation from seven cruises spanning four seasons in temperate, western North Atlantic coastal waters along the North American continental shelf between Cape Hatteras and Nova Scotia, an area representing 6.4% of the North Atlantic continental shelf area. Integrating average areal rates of N2 fixation during each season and for each domain in the study area, the estimated N input from N2 fixation to this temperate shelf system is 0.02 Tmol N y‐1, an amount equivalent to that previously estimated for the entire North Atlantic continental shelf. Unicellular group A cyanobacteria (UCYN‐A) were most often the dominant diazotrophic group expressing nifH, a gene encoding the nitrogenase enzyme, throughout the study area during all seasons. This expands the domain of these diazotrophs to include coastal waters where dissolved N concentrations are not always depleted. Further, the high rates of N2 fixation and diazotroph diversity along the western North Atlantic continental shelf underscore the need to reexamine the biogeography and activity of diazotrophs along continental margins. Accounting for this substantial, but previously overlooked source of new N to marine systems, necessitates revisions to global marine N budgets.
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    Issue Information (2019)
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    Publikationsdatum: 2019
    Beschreibung: No abstract is available for this article.
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    Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The oceans are an important global reservoir for mercury (Hg), and marine fish consumption is the dominant human exposure pathway for its toxic methylated form. A more thorough understanding of the global biogeochemical cycle of Hg requires additional information on the mechanisms that control Hg cycling in pelagic marine waters. In this study, Hg isotope ratios and total Hg concentrations are used to explore Hg biogeochemistry in oligotrophic marine environments north of Hawaii. We present the first measurements of the vertical water column distribution of Hg concentrations and the Hg isotopic composition in precipitation, marine particles, and zooplankton near Station ALOHA (22°45′N, 158°W). Our results reveal production and demethylation of methylmercury in both the euphotic (0–175 m) and mesopelagic zones (200–1,000 m). We document a strong relationship between Hg isotopic composition and depth in particles, zooplankton, and fish in the water column and diurnal variations in Δ199Hg values in zooplankton sampled near the surface (25 m). Based on these observations and stable Hg isotope relationships in the marine food web, we suggest that the Hg found in large pelagic fish at Station ALOHA was originally deposited largely by precipitation, transformed into methyl‐Hg, and bioaccumulated in situ in the water column. Our results highlight how Hg isotopic compositions reflect abiotic and biotic production and degradation of methyl‐Hg throughout the water column and the importance of particles and zooplankton in the vertical transport of Hg.
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    Nitrogen Deposition Maintains a Positive Effect on Terrestrial Carbon Sequestration in the 21st Century Despite Growing Phosphorus Limitation at Regional Scales (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Nitrogen (N) and phosphorus (P) are two dominant nutrients regulating the productivity of most terrestrial ecosystems. The growing imbalance of anthropogenic N and P inputs into the future is estimated to exacerbate P limitation on land and limit the land carbon (C) sink, so that we hypothesized that P limitation will increasingly reduce C sequestered per unit N deposited into the future. Using a global land surface model (CABLE), we simulated the effects of increased N deposition with and without P limitation on land C uptake and the fate of deposited N on land from 1901 to 2100. Contrary to our hypothesis, we found that N deposition continued to induce land C sequestration into the future, contributing to 15% of future C sequestration as opposed to 6% over the historical period. P limitation reduced the future land C uptake per unit N deposited only moderately at the global scale but P limitation increasingly caused N deposition to have net negative effects on the land C balance in the temperate zone. P limitation further increased the fraction of deposited N that is lost via leaching to aquatic ecosystems, globally from 38.5% over the historical period to 53% into the future, and up to 75% in tropical ecosystems. Our results suggest continued N demand for plant productivity but also indicate growing adverse N deposition effects in the future biosphere, not fully accounted for in global models, emphasizing the urgent need to elaborate on model representations of N and P dynamics.
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    Seasonality of Dissolved Organic Carbon in the Upper Northeast Pacific Ocean (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract From seasonal cruises in the NE Pacific Ocean during 2017, we (1) determined dissolved organic carbon concentrations; (2) calculated net community production (NCP) from nitrate drawdown; and (3) established relationships between NCP and seasonal dissolved organic carbon (DOC) accumulation in the upper 75 m. The fraction of NCP that accumulated as DOC, hereafter referred to as the net dissolved production ratio, was calculated for several stations during spring and summer. The net dissolved production ratio was about 0.26 at the oceanic station Ocean Station Papa during different seasons and years. Using nitrate concentration profiles obtained from Bio‐Argo floats during 2009–2018 operating near Ocean Station Papa, we calculated NCP at high temporal resolution and then applied the 0.26 constant in order to (4) estimate DOC variability for the 9‐year period. We found strong seasonality near Ocean Station Papa, with NCP maxima during summers ranging from 0.3 to 2.9 mol C/m2 and surface DOC concentrations estimated from 56 μmol/kg in winters to 73 μmol/kg in summers. There was a 10‐fold interannual variability in the seasonally accumulated inventory of DOC, ranging from 0.078 to 0.75 mol C/m2. This study reinforces the value of deploying floats equipped with chemical sensors in order to better understand marine biogeochemical cycles, especially when high resolution data cannot be obtained otherwise. Given that ~26% of NCP accumulates as DOC in the central Gulf of Alaska, the remaining balance of ~74% is available for export as sinking biogenic particles.
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    Modeling Iron and Light Controls on the Summer Phaeocystis antarctica Bloom in the Amundsen Sea Polynya (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Of all the Antarctic coastal polynyas, the Amundsen Sea Polynya is the most productive per unit area. Observations from the 2010–2011 Amundsen Sea Polynya International Research Expedition (ASPIRE) revealed that both light and iron can limit the growth of phytoplankton (Phaeocystis antarctica), but how these controls manifest over the bloom season is poorly understood, especially with respect to their climate sensitivity. Using a 1‐D biogeochemical model, we examine the influence of light and iron limitation on the phytoplankton bloom and vertical carbon flux at 12 stations representing different bloom stages within the polynya. Model parameters are determined by Bayesian optimization and assimilation of ASPIRE observations. The model‐data fit is most sensitive to phytoplankton physiological parameters, which among all model parameters are best constrained by the optimization. We find that the 1‐D model captures the basic elements of the bloom observed during ASPIRE, despite some discrepancies between modeled and observed dissolved iron distributions. With this model, we explore the way iron availability, in combination with light availability, controlled the rise, peak, and decline of the bloom at the 12 stations. Modeled light limitation by self‐shading is very strong, but iron is drawn down as the bloom rises, becoming limiting in combination with light as the bloom declines. These model results mechanistically confirm the importance of climate‐sensitive controls like stratification and meltwater on phytoplankton bloom development and carbon export in this region.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Exposing the Distributions and Elemental Associations of Scavenged Particulate Phases in the Ocean Using Basin‐Scale Multi‐Element Data Sets (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The GEOTRACES program has greatly increased basin‐scale concentration measurements for a large number of elements in the ocean, both constraining external sources and internal sinks and exposing complex internal cycles of trace elements. Our conceptual frameworks for marine trace element cycling, however, often remain simplified as the production and remineralization of phytoplankton biomass. Despite their complexity, or perhaps because of it, trace element cycles are often predominantly considered as an extension of traditional Redfield macronutrient ratios to C or P. Here we utilize extensive data sets of particulate trace element concentrations from GEOTRACES section cruises in the South Pacific and North Atlantic Oceans to look for evidence of the internal cycles of multiple trace elements without requiring normalization to phytoplankton biomass. Using both traditional and expanded power law regression analyses and multi‐element factor analysis, we expose the internal distributions of six authigenic, biogenic, and lithogenic particulate phases and their multi‐element associations. Critically, no particulate trace element is observed to behave identically to P. Observations include a scavenged Fe phase with a slight surface maximum, which increases linearly with depth below ~ 300 m and which appears to co‐scavenge Cu, V, and La. Particulate Co is found to be associated with phytoplankton, Mn‐biooxides just below the mixed layer, and with a putative heterotrophic phase observed in the surface and at depth. We present an expanded conceptual framework for particulate trace element cycling that has explicit roles for these multiple particulate phases.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Environmental Controls on the Riverine Export of Dissolved Black Carbon (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Each year, tropical rivers export a dissolved organic carbon (DOC) flux to the global oceans that is equivalent to ~4% of the global land sink for atmospheric CO2. Among the most refractory fractions of terrigenous DOC is dissolved black carbon (DBC), which constitutes ~10% of the total flux and derives from the charcoal and soot (aerosol) produced during biomass burning and fossil fuel combustion. Black carbon (BC) has disproportionate storage potential in oceanic pools and thus its export has implications for the fate and residence time of terrigenous organic carbon (OC). In contrast to bulk DOC, there is limited knowledge of the environmental factors that control riverine fluxes of DBC. We thus completed a comprehensive assessment of the factors controlling DBC export in tropical rivers with catchments distributed across environmental gradients of hydrology, topography, climate and soil properties. Generalised linear models explained 70% and 64% of the observed variance in DOC and DBC concentrations, respectively. DOC and DBC concentrations displayed coupled responses to the dominant factors controlling their riverine export (soil moisture; catchment slope, and; catchment stocks of OC or BC, respectively) but varied divergently across gradients of temperature and soil properties. DBC concentrations also varied strongly with aerosol BC deposition rate, indicating further potential for deviation of DBC fluxes from those of DOC due to secondary inputs of DBC from this unmatched source. Overall, this study identifies the specific drivers of BC dynamics in river catchments and fundamentally enhances our understanding of refractory DOC export to the global oceans.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    A Mechanistic Model of Microbially Mediated Soil Biogeochemical Processes: A Reality Check (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Present gaps in the representation of key soil biogeochemical processes such as the partitioning of soil organic carbon among functional components, microbial biomass and diversity, and the coupling of carbon and nutrient cycles present a challenge to improving the reliability of projected soil carbon dynamics. We introduce a new soil biogeochemistry module linked with a well‐tested terrestrial biosphere model T&C. The module explicitly distinguishes functional soil organic carbon components. Extracellular enzymes and microbial pools are differentiated based on the functional roles of bacteria, saprotrophic, and mycorrhizal fungi. Soil macrofauna is also represented. The model resolves the cycles of nitrogen, phosphorus, and potassium. Model simulations for 20 sites compared favorably with global patterns of litter and soil stoichiometry, microbial and macrofaunal biomass relations with soil organic carbon, soil respiration, and nutrient mineralization rates. Long‐term responses to bare fallow and nitrogen addition experiments were also in agreement with observations. Some discrepancies between predictions and observations are appreciable in the response to litter manipulation. Upon successful model reproduction of observed general trends, we assessed patterns associated with the carbon cycle that were challenging to address empirically. Despite large site‐to‐site variability, fine root, fungal, bacteria, and macrofaunal respiration account for 33%, 40%, 24%, and 3% on average of total belowground respiration, respectively. Simulated root exudation and carbon export to mycorrhizal fungi represent on average about 13% of plant net primary productivity. These results offer mechanistic and general estimates of microbial biomass and its contribution to respiration fluxes and to soil organic matter dynamics.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Degradation of Internal Organic Matter is the Main Control on Pteropod Shell Dissolution After Death (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The potential for preservation of thecosome pteropods is thought to be largely governed by the chemical stability of their delicate aragonitic shells in seawater. However, sediment trap studies have found that significant carbonate dissolution can occur above the carbonate saturation horizon. Here we present the results from experiments conducted on two cruises to the Scotia Sea to directly test whether the breakdown of the organic pteropod body influences shell dissolution. We find that on the timescales of 3 to 13 days, the oxidation of organic matter within the shells of dead pteropods is a stronger driver of shell dissolution than the aragonite saturation state of seawater. Three to four days after death, shells became milky white and nano scanning electron microscope images reveal smoothing of internal surface features and increased shell porosity, both indicative of aragonite dissolution. These findings have implications for the interpretation of the condition of pteropod shells from sediment traps and the fossil record, as well as for understanding the processes controlling particulate carbonate export from the surface ocean.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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  • 60
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    Soil organic matter temperature sensitivity cannot be directly inferred from spatial gradients (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Developing and testing decadal‐scale predictions of soil response to climate change is difficult because there are few long‐term warming experiments or other direct observations of temperature response. As a result, spatial variation in temperature is often used to characterize the influence of temperature on soil organic carbon (SOC) stocks under current and warmer temperatures. This approach assumes that the decadal‐scale response of SOC to warming is similar to the relationship between temperature and SOC stocks across sites that are at quasi steady state; however, this assumption is poorly tested. We developed four variants of a Reaction‐network‐based model of soil organic matter and microbes using measured SOC stocks from a 4,000‐km latitudinal transect. Each variant reflects different assumptions about the temperature sensitivities of microbial activity and mineral sorption. All four model variants predicted the same response of SOC to temperature at steady state, but different projections of transient warming responses. The relative importance of Qmax, mean annual temperature, and net primary production, assessed using a machine‐learning algorithm, changed depending on warming duration. When mineral sorption was temperature sensitive, the predicted average change in SOC after 100 years of 5 °C warming was −18% if warming decreased sorption or +9% if warming increased sorption. When microbial activity was temperature sensitive but mineral sorption was not, average site‐level SOC loss was 5%. We conclude that spatial climate gradients of SOC stocks are insufficient to constrain the transient response; measurements that distinguish process controls and/or observations from long‐term warming experiments, especially mineral fractions, are needed.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Issue Information (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: No abstract is available for this article.
    Print ISSN: 1045-6740
    Digitale ISSN: 1099-1530
    Thema: Geographie , Geologie und Paläontologie
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    Report from the International Permafrost Association: Increasing regional activities on a global scale (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Permafrost and Periglacial Processes, Volume 30, Issue 2, Page 121-125, April/June 2019.
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    Thema: Geographie , Geologie und Paläontologie
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  • 63
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    Application of distributed temperature sensing for mountain permafrost mapping (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Permafrost distribution in mountains is typically more heterogeneous relative to low‐relief environments due to greater variability in the factors controlling the ground thermal regime, such as topography, snow depth, and sediment grain size (e.g., coarse blocks). Measuring and understanding the geothermal variability in high mountains remains challenging due to logistical constraints. This study presents one of the first applications of distributed temperature sensing (DTS) in periglacial environments to measure ground surface temperatures in a mountain permafrost area at much higher spatial resolution than possible with conventional methods using discrete temperature sensors. DTS measures temperature along a fibre‐optic cable at high spatial resolution (i.e., ≤ 1 m). Its use can be limited by power supply and calibration requirements, although recent methodological developments have relaxed some of these restrictions. Spatially continuous DTS measurements at a studied rock glacier provided greater resolution of geothermal variability and facilitated the interpretation of bottom temperature of snowpack data to map patchy permafrost distribution. This research highlights the potential for DTS to be a useful tool for permafrost mapping, ground thermal regime interpretation, conceptual geothermal model development, and numerical model evaluation in areas of heterogeneous mountain permafrost.
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    Thema: Geographie , Geologie und Paläontologie
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  • 64
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    Large contribution of pteropods to shallow CaCO3 export. (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The literature on the relative contributions of pelagic calcifying taxa to the global ocean export of CaCO3 is divided. Studies based on deep sediment trap data tend to argue that either foraminifers or coccolithophores, both calcite producers, dominate export. However, the compilations of biomass observations for pteropods, coccolithophores and foraminifers instead show that pteropods dominate the global ocean calcifier biomass, and therefore likely also carbonate export. Here we present a new global ocean biogeochemical model that explicitly represents these three groups of pelagic calcifiers. We synthesize databases of the physiology of the three groups to parameterise the model, and then tune the unconstrained parameters to reproduce the observations of calcifier biomass and CaCO3 export. The model can reproduce both these observational databases, however, substantial dissolution of aragonite above the aragonite saturation horizon is required to do so. We estimate a contribution of pteropods to shallow (100 m) export of CaCO3 of at least 33% and to pelagic calcification of up to 89%. The high production ‐ high dissolution configuration that shows closest agreement with all the observations has a CaCO3 production of 4.7 Pg C y‐1, but CaCO3 export at 100 m of only 0.6 Pg C y‐1.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Global Nitrous Oxide Emissions From Pasturelands and Rangelands: Magnitude, Spatiotemporal Patterns, and Attribution (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The application of manure and mineral nitrogen (N) fertilizer, and livestock excreta deposition are the main drivers of nitrous oxide (N2O) emissions in agricultural systems. However, the magnitude and spatiotemporal variations of N2O emissions due to different management practices (excreta deposition and manure/fertilizer application) from grassland ecosystems remain unclear. In this study, we used the Dynamic Land Ecosystem Model to simulate the spatiotemporal variation in global N2O emissions and their attribution to different sources from both intensively managed (pasturelands) and extensively managed (rangelands) grasslands during 1961–2014. Over the study period, pasturelands and rangelands experienced a significant increase in N2O emissions from 1.74 Tg N2O‐N in 1961 to 3.11 Tg N2O‐N in 2014 (p 〈 0.05). Globally, pasturelands and rangelands were responsible for 54% (2.2 Tg N2O‐N) of the total agricultural N2O emissions (4.1 Tg N2O‐N) in 2006. Natural and anthropogenic sources contributed 26% (0.64 Tg N2O‐N/year) and 74% (1.78 Tg N2O‐N/year) of the net emissions, respectively. Across different biomes, pasturelands (i.e., C3 and C4) were the single largest contributor to N2O fluxes, accounting for 86% of the net global emissions from grasslands. Among different sources, livestock excreta deposition contributed 54% of the net emissions, followed by manure N (13%) and mineral N (7%) application. Regionally, southern Asia contributed 38% of the total emissions, followed by Europe (29%) and North America (16%). Our modeling study demonstrates that livestock excreta deposition and manure/fertilizer application have dramatically altered the N cycle in pasturelands, with a substantial impact on the climate system.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Biogeochemical cycling of dissolved zinc in the Western Arctic (Arctic GEOTRACES GN01) (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The biogeochemical cycling of dissolved zinc (dZn) was investigated in the Western Arctic along the U.S. GEOTRACES GN01 section. Vertical profiles of dZn in the Arctic are strikingly different than the classic “nutrient‐type” profile commonly seen in the Atlantic and Pacific Oceans, instead exhibiting higher surface concentrations (~1.1 nmol/kg), a shallow subsurface absolute maximum (~4–6 nmol/kg) at 200 m coincident with a macronutrient maximum, and low deep water concentrations (~1.3 nmol/kg) that are homogenous with depth. In contrast to other ocean basins, typical inputs such as rivers, atmospheric inputs, and especially deep remineralization are insignificant in the Arctic. Instead, we demonstrate that dZn distributions in the Arctic are controlled primarily by: 1) shelf fluxes following the sediment remineralization of high Zn:C and Zn:Si cells and the seaward advection of those fluxes, and 2) mixing of dZn from source waters such as the Atlantic and Pacific Oceans, rather than vertical biological regeneration of dZn. This results in both the unique profile shapes and the largely decoupled relationship between dZn and Si found in the Arctic. We found a weak dZn:Si regression in the full water column (0.077 nmol/μmol, r2 = 0.58) that is higher than the global slope (0.059 nmol/μmol, r2 = 0.94) because of the shelf‐derived halocline dZn enrichments. We hypothesize that the decoupling of Zn:Si in Western Arctic deep waters results primarily from a past ventilation event with unique pre‐formed Zn:Si stoichiometries.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Modeling the Impact of Zooplankton Diel Vertical Migration on the Carbon Export Flux of the Biological Pump (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: One pathway of the biological pump that remains largely unquantified in many export models is the active transport of carbon from the surface ocean to the mesopelagic by zooplankton diel vertical migration (DVM). Here, we develop a simple representation of zooplankton DVM and implement it in a global export model as a thought experiment to illustrate the effects of DVM on carbon export and mesopelagic biogeochemistry. The model is driven by diagnostic satellite measurements of net primary production, algal biomass, and phytoplankton size structure. Due to constraints on available satellite data, the results are restricted to the latitude range from 60°N to 60°S. The modeled global export flux from the base of the euphotic zone was 6.5 PgC/year, which represents a 14% increase over the export flux in model runs without DVM. The mean (± standard deviation, SD) proportional contribution of the DVM‐mediated export flux to total carbon export, averaged over the global domain and the climatological seasonal cycle, was 0.16 ± 0.04 and the proportional contribution of DVM activity to total respiration within the twilight zone was 0.16 ± 0.06. Adding DVM activity to the model also resulted in a deep local maximum in the oxygen utilization profile. The model results were most sensitive to the assumptions for the fraction of individuals participating in DVM, the fraction of fecal pellets produced in the euphotic zone, and the fraction of grazed carbon that is metabolized.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    A Global Perspective on Iron and Plankton through the Tara Oceans Lens (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The Tara Oceans program has delivered major advances in our knowledge of ocean plankton diversity and complexity, shedding light on key interactions that explain their success on a planetary scale. In this issue, Caputi et al. (2019, https://doi.org/10.1029/2018GB006022) further contribute to this knowledge through combining comprehensive bio‐oceanographic genomic and transcriptomic Tara Oceans datasets with iron distributions derived from two global‐scale biogeochemical models. Their findings reveal the prevalence of iron as a limiting nutrient in pelagic ecosystems at both local and global scales, exerting a considerable force that drives plankton evolution and shapes community structure. Integration of omics data (i.e., genomics, transcriptomics, proteomics and metabolomics) with oceanographic properties and biogeochemical models will transform our view of the ocean ecosystem and its role on a changing planet.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Direct observations of biological carbon export from profiling floats in the subtropical North Atlantic (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The complex interplay of biological and physical mechanisms comprising the ocean's biological carbon pump has not been well‐characterized to date, due to the difficulty of observing these mechanisms in situ at adequate spatial and temporal resolution. An annual timeseries is presented of direct measurements of export production and particle properties collected using optical sediment trap‐equipped profiling floats cycling every 1.5‐2.5 days. The observations indicate strong variability in particle export and bio‐optical properties, influenced by the spring bloom, mesoscale eddy activity, and the mixed layer pump. Temporal and vertical decoupling of fluxes at depths ranging from 150‐1000 m was also observed, and remineralization length scales were more variable than predicted by temperature‐ and oxygen‐based models. Net primary production was computed from float observations using a modification of the Carbon‐based Productivity Model (Behrenfeld et al., 2005; Westberry et al., 2008) and used to estimate export and export (e‐) ratios, which were compared to predictions of literature export models. Mechanistic models explicitly incorporating ecosystem processes and their depth dependence may perform better at reproducing regional observations collected at high temporal resolution.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Evaluations of climate and land management effects on lake carbon cycling need to account for temporal variability in CO2 concentrations (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Carbon dioxide (CO2) concentrations in lakes vary strongly over time. This variability is rarely captured by environmental monitoring, but is crucial for accurately assessing the magnitude of lake CO2 emissions. However, it is unknown to what extent temporal variability needs to be captured to understand important drivers of lake carbon cycling such as climate and land management. We used environmental monitoring data of Swedish forest lakes collected in autumn (n=439) and throughout the whole open‐water season (n=22) from a wet and a dry year to assess temporal variability in effects of climate and forestry on CO2 concentrations across lakes. Effects differed depending on the season and year sampled. According to cross‐lake comparisons based on autumn data, CO2 concentrations increased with annual mean air temperature (dry year) or catchment forest productivity (wet year) but were not related to colored dissolved organic matter (CDOM) concentrations. In contrast, open‐water season averaged CO2 concentrations were similar across temperature and productivity gradients but increased with CDOM. These contradictions resulted from scale mismatches in input data, lead to weak explanatory power (R2=9‐32%) and were consistent across published data from 79 temperate, boreal and arctic lakes. In a global survey of 144 published studies, we identified a trade‐off between temporal and spatial coverage of CO2 sampling. This trade‐off clearly determines which conclusions are drawn from landscape‐scale CO2 assessments. Accurate evaluations of the effects of climate and land management require spatially and temporally representative data that can be provided by emerging sensor technologies and forms of collaborative sampling.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    The Relationships Between Dissolved Zinc and Major Nutrients Phosphate and Silicate Along the GEOTRACES GA02 Transect in the West Atlantic Ocean (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Dissolved zinc (Zn) has a nutrient‐type distribution in the ocean that more closely resembles the distribution of silicate (Si) than phosphate (PO4). However, Zn is a trace‐nutrient and mostly present in the organic fraction of phytoplankton rather than the siliceous frustule. It has been suggested the coupling of Zn and Si is caused by the strong depletion of nutrients in the Southern Ocean, possibly combined with scavenging of Zn. Here we assess the distribution of Zn and nutrients along the conduit of southward traveling waters of northern origin and northward traveling waters of Antarctic origin to unravel the influence of various water masses and local biogeochemical processes in the Atlantic Ocean. The distribution of Zn and Si is governed by mixing such that influence of remineralization is barely distinguishable, whereas the distribution of PO4 is influenced by both processes. The subsurface water masses that supply nutrients to the surface ocean are depleted in Zn. Indeed, the Southern Ocean water masses play a driving role, but remarkably, also subsurface water masses from northern high‐latitude origin are depleted in Zn. Both northern and southern high‐latitude waters have a relatively high Zn:PO4 uptake and remineralization ratio, implying it is Zn availability and not only chronic iron limitation that leads to increased Zn uptake in the high‐latitude regions. The limited supply of Zn to the surface Atlantic Ocean can explain the lack of an Atlantic Zn remineralization signal and indicates Zn might play a role in phytoplankton community composition and productivity.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Remobilization of Old Permafrost Carbon to Chukchi Sea Sediments During the End of the Last Deglaciation (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Climate warming is expected to destabilize permafrost carbon (PF‐C) by thaw‐erosion and deepening of the seasonally thawed active layer and thereby promote PF‐C mineralization to CO2 and CH4. A similar PF‐C remobilization might have contributed to the increase in atmospheric CO2 during deglacial warming after the last glacial maximum. Using carbon isotopes and terrestrial biomarkers (Δ14C, δ13C, and lignin phenols), this study quantifies deposition of terrestrial carbon originating from permafrost in sediments from the Chukchi Sea (core SWERUS‐L2‐4‐PC1). The sediment core reconstructs remobilization of permafrost carbon during the late Allerød warm period starting at 13,000 cal years before present (BP), the Younger Dryas, and the early Holocene warming until 11,000 cal years BP and compares this period with the late Holocene, from 3,650 years BP until present. Dual‐carbon‐isotope‐based source apportionment demonstrates that Ice Complex Deposit—ice‐ and carbon‐rich permafrost from the late Pleistocene (also referred to as Yedoma)—was the dominant source of organic carbon (66 ± 8%; mean ± standard deviation) to sediments during the end of the deglaciation, with fluxes more than twice as high (8.0 ± 4.6 g·m−2·year−1) as in the late Holocene (3.1 ± 1.0 g·m−2·year−1). These results are consistent with late deglacial PF‐C remobilization observed in a Laptev Sea record, yet in contrast with PF‐C sources, which at that location were dominated by active layer material from the Lena River watershed. Release of dormant PF‐C from erosion of coastal permafrost during the end of the last deglaciation indicates vulnerability of Ice Complex Deposit in response to future warming and sea level changes.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    The intra‐seasonal dynamics of the mixed layer pump in the subpolar North Atlantic Ocean: a BGC‐Argo float approach. (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract The detrainment of organic matter from the mixed layer, a process known as the mixed layer pump (ML pump), has long been overlooked in carbon export budgets. Recently, the ML pump has been investigated at seasonal scale and appeared to contribute significantly to particulate organic carbon export to the mesopelagic zone, especially at high latitudes where seasonal variations of the mixed layer depth are large. However, the dynamics of the ML pump at intra‐seasonal scales remains poorly known, mainly because the lack of observational tools suited to studying such dynamics. In the present study, using a dense network of autonomous profiling floats equipped with bio‐optical sensors, we captured widespread episodic ML pump‐driven export events, during the winter and early spring period, in a large part of the subpolar North Atlantic Ocean. The intra‐seasonal dynamic of the ML pump exports fresh organic material to depth (basin‐scale average up to 55 mg C m‐2 d‐1), providing a significant source of energy to the mesopelagic food web before the spring bloom period. This mechanism may sustain the seasonal development of overwintering organisms such as copepods with potential impact on the characteristics of the forthcoming spring phytoplankton bloom through predator‐prey interactions.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
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    Dissolved Organic Matter Chemistry and Transport Along an Arctic Tundra Hillslope (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Permafrost thaw is projected to restructure the connectivity of surface and subsurface flow paths, influencing export dynamics of dissolved organic matter (DOM) through Arctic watersheds. Resulting shifts in flow path exchange between both soil horizons (organic‐mineral) and landscape positions (hillslope‐riparian) could alter DOM mobility and molecular‐level patterns in chemical composition. Using conservative tracers, we found relatively rapid lateral flows occurred across a headwater Arctic tundra hillslope, as well as along the mineral‐permafrost interface. While pore waters collected from the organic horizon were associated with plant‐derived molecules, those collected from permafrost‐influenced mineral horizons had a microbial origin, as determined by fluorescence spectroscopy. Using high‐resolution nuclear magnetic resonance spectroscopy, we found that riparian DOM had greater structural diversity than hillslope DOM, suggesting riparian soils could supply a diverse array of compounds to surface waters if terrestrial‐aquatic connectivity increases with warming. In combination, these results suggest that integrating DOM mobilization with its chemical and spatial heterogeneity can help predict how permafrost loss will structure ecosystem metabolism and carbon‐climate feedbacks in Arctic catchments with similar topographic features.
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    Very strong atmospheric methane growth in the four years 2014‐2017: Implications for the Paris Agreement (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Atmospheric methane grew very rapidly in 2014 (12.7±0.5 ppb/yr), 2015 (10.1±0.7 ppb/yr), 2016 (7.0± 0.7 ppb/yr) and 2017 (7.7±0.7 ppb/yr), at rates not observed since the 1980s. The increase in the methane burden began in 2007, with the mean global mole fraction in remote surface background air rising from about 1775 ppb in 2006 to 1850 ppb in 2017. Simultaneously the 13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, in a new trend to more negative values that have been observed worldwide for over a decade. The causes of methane's recent mole fraction increase are therefore either a change in the relative proportions (and totals) of emissions from biogenic and thermogenic and pyrogenic sources, especially in the tropics and sub‐tropics, or a decline in the atmospheric sink of methane, or both. Unfortunately, with limited measurement data sets, it is not currently possible to be more definitive. The climate warming impact of the observed methane increase over the past decade, if continued at 〉5 ppb/yr in the coming decades, is sufficient to challenge the Paris Agreement, which requires sharp cuts in the atmospheric methane burden. However, anthropogenic methane emissions are relatively very large and thus offer attractive targets for rapid reduction, which are essential if the Paris Agreement aims are to be attained.
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    Mechanisms of Low‐Frequency Oxygen Variability in the North Pacific (2019)
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    Publikationsdatum: 2019
    Beschreibung: This study investigates the mechanisms of interannual and decadal variability of dissolved oxygen (O2) in the North Pacific using historical observations and a hindcast simulation using the Community Earth System Model. The simulated variability of upper ocean (200 m) O2 is moderately correlated with observations where sampling density is relatively high. The dominant mode of O2 variability explains 24.8% of the variance and is significantly correlated with the Pacific Decadal Oscillation (PDO) index (r = 0.68). Two primary mechanisms are hypothesized by which the PDO controls upper ocean O2 variability. Vertical movement of isopycnals (“heave”) drives O2 variations in the deep tropics; isopycnal surfaces are depressed in the eastern tropics under the positive (El Niño‐like) phase of PDO, leading to O2 increases in the upper water column. In contrast to the tropics, changes in subduction are the primary control on extratropical O2 variability. These hypotheses are tested by contrasting O2 anomalies with the heave‐induced component of variability calculated from potential density anomalies. Isopycnal heave is the leading control on O2 variability in the tropics, but heave alone cannot fully explain the amplitude of tropical O2 variability, likely indicating reinforcing changes from the biological O2 consumption. Midlatitude O2 variability indeed reflects ocean ventilation downstream of the subduction region where O2 anomalies are correlated with the depth of winter mixed layer. These mechanisms, synchronized with the PDO, yield a basin‐scale pattern of O2 variability that are comparable in magnitude to the projected rates of ocean deoxygenation in this century under “unchecked” emission scenario.
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    Estimating Air‐Sea Carbon Flux Uncertainty over the Tropical Pacific: Importance of Winds and Wind Analysis Uncertainty (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract The tropical Pacific is a major natural source of CO2 to the atmosphere and contributor to global air‐sea carbon flux variability. High time‐resolution wind and CO2 measurements from equatorial Pacific moorings reveal the primary factor controlling mooring‐observed flux variability to be near‐surface wind variability, above CO2 variability, in this region over the last ten years. The analysis‐product winds used most widely in previous calculations of basin‐scale carbon flux are compared with mooring winds and found to exhibit significant differences in mean, variability, and trend. Earth system model calculations are in basic agreement with the mooring results and used to estimate effects of wind uncertainty on our knowledge of regional air‐sea carbon exchange. Results show NCEP1 and NCEP2 winds contain biases large enough to obscure the interannual variability of CO2 flux (RMSE ≈σ) and cause spurious 25‐year (1992‐2016) trend components in equatorial Pacific carbon flux of 0.038‐0.039 and 0.016‐0.021 PgC yr‐1 decade‐1, respectively. These spurious trends act to reduce by up to 50% the 25‐year trend in equatorial Pacific carbon flux simulated by the Earth system sodel under increasing atmospheric CO2 concentration. The Cross‐Calibrated‐Multi‐Platform (CCMP) wind product tracks observed variability of equatorial Pacific wind better (interannual RMSE ≈ 0.4σ) than the NCEP reanalyses when site‐sampled at mooring locations, yet still causes a spurious regional trend (0.03 PgC yr‐1 decade‐1) that masks 40% of the simulated 25‐year trend in carbon flux. The mooring observations are fundamental to identifying the limitations of current wind products to characterizing long‐term trends and understanding air‐sea carbon exchange.
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    Community‐Level Responses to Iron Availability in Open Ocean Planktonic Ecosystems (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Predicting responses of plankton to variations in essential nutrients is hampered by limited in situ measurements, a poor understanding of community composition, and the lack of reference gene catalogs for key taxa. Iron is a key driver of plankton dynamics and, therefore, of global biogeochemical cycles and climate. To assess the impact of iron availability on plankton communities we explored the comprehensive bio‐oceanographic and ‐omics datasets from Tara Oceans in the context of the iron products from two state‐of‐the‐art global scale biogeochemical models. We obtained novel information about adaptation and acclimation towards iron in a range of phytoplankton, including picocyanobacteria and diatoms, and identified whole sub‐communities co‐varying with iron. Many of the observed global patterns were recapitulated in the Marquesas archipelago, where frequent plankton blooms are believed to be caused by natural iron fertilization, although they are not captured in large scale biogeochemical models. This work provides a proof‐of‐concept that integrative analyses, spanning from genes to ecosystems and viruses to zooplankton, can disentangle the complexity of plankton communities and can lead to more accurate formulations of resource bioavailability in biogeochemical models, thus improving our understanding of plankton resilience in a changing environment.
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    Shallow ground temperature measurements on the highest volcano on Earth, Mt. Ojos del Salado, Arid Andes, Chile (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Mt. Ojos del Salado (6893 m a.s.l.) lies within the Andean Arid Diagonal, on the Chilean–Argentinean border. Due to the extremely arid climate, surface ice is not widespread on Mt. Ojos del Salado and at similar high‐altitude massifs in the region, although ice‐bearing permafrost might be present. However, the thermal regime of the ground is relatively unknown in the region, especially outside of rock glaciers at high elevations north of 30°S. To study ground thermal regimes, in‐situ shallow ground temperature and snow coverage from satellite imagery have been surveyed for four years (2012–2016) at six sites between the elevations of 4200–6893 m a.s.l. on Mt. Ojos del Salado (27°07′S, 68°32′W). According to the ground temperature and snow coverage data at the six monitoring sites, the presence of permafrost is unlikely below 4550 m a.s.l. but likely above 5250 m a.s.l. on Mt. Ojos del Salado. In addition, the active layer becomes extremely thin around 6750 m a.s.l.
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    Thema: Geographie , Geologie und Paläontologie
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    Greenhouse gas formation in ice wedges at Cyuie, central Yakutia (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Greenhouse gases (GHGs) trapped in ice wedges may provide useful information on biogeochemical environments in ground ice. Previous studies have reported highly elevated CO2 and CH4 mixing ratios in ice wedges. However, N2O mixing ratios in ice wedges remain unknown. Here, we present CO2, CH4 and N2O mixing ratios in bubbles and plausible mechanisms of GHG formation for two lakeside ice wedges at Cyuie village near Yakutsk. The CO2 gas age corresponds to the Last Glacial Maximum (18–19 ka). The δ(N2/Ar) values and bubble shapes indicate that the ice wedges formed by dry snow compaction rather than snowmelt water refreezing, while the δ18O and δD values of the ice indicate changes in the source area location and/or the climate during the Last Glacial Maximum. Using a dry extraction method, we obtained gas mixing ratios of 7–13% CO2, 5–130 ppm CH4 and 100–5000 ppb N2O. The δ(O2/Ar) values imply that most of the O2 was consumed by biological respiration. The CH4 is negatively correlated with N2O and CO2. The N2O might have inhibited CH4 production.
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    Issue Information (2019)
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    Publikationsdatum: 2019
    Beschreibung: No abstract is available for this article.
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    Water and energy transfer modeling in a permafrost‐dominated, forested catchment of Central Siberia: The key role of rooting depth (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract To quantify the impact of evapotranspiration phenomena on active layer dynamics in a permafrost‐dominated forested watershed in Central Siberia, we performed a numerical cryohydrological study of water and energy transfer using a new open source cryohydrogeology simulator, with two innovative features: spatially distributed, mechanistic handling of evapotranspiration and inclusion of a numerical tool in a high‐ performance computing toolbox for numerical simulation of fluid dynamics, OpenFOAM. In this region, the heterogeneity of solar exposure leads to strong contrasts in vegetation cover, which constitutes the main source of variability in hydrological conditions at the landscape scale. The uncalibrated numerical results reproduce reasonably well the measured soil temperature profiles and the dynamics of infiltrated waters revealed by previous biogeochemical studies. The impacts of thermo‐hydrological processes on water fluxes from the soils to the stream are discussed through a comparison between numerical results and field data. The impact of evapotranspiration on water fluxes is studied numerically, and highlights a strong sensitivity to variability in rooting depth and corresponding evapotranspiration at slopes of different aspect in the catchment.
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    Using stable isotopes to illuminate thermokarst lake hydrology in permafrost regions on the Qinghai‐Tibet plateau, China (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Thermokarst lakes are widespread and developing on the Qinghai‐Tibet Plateau (QTP), and they have modified hydrological processes and water balance in the region. Despite increasing attention, our understanding of the hydrological behaviors of thermokarst lakes under permafrost degradation remains limited on the QTP. In this study, water stable isotope tracers were used to characterize the spatial and seasonal hydrological changes of thermokarst lakes on the central QTP. Significant seasonal variations and factors influencing the isotopic hydrology of lakes were revealed through the ice‐free season. Substantial differences in lake‐specific input water isotope compositions (δI) among the lakes indicate two lake‐recharge regimes: rain‐dominated, and snowmelt/permafrost thaw‐dominated. We suggest that precipitation and active‐layer hydrology controlled these hydrological changes in the ice‐free season. However, under ice cover, melting of the surrounding permafrost (including ground ice) dominated the hydrology of thermokarst lakes. Importantly, a conceptual model delineates the hydrological evolution of thermokarst lakes under permafrost degradation. This study serves as a baseline for future investigations of hydrological processes and water balance relating to thawing permafrost on the QTP.
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    Cliff retreat of permafrost coast in south‐west Baydaratskaya Bay, Kara Sea, during 2005–2016 (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Recent years of increasing air temperature in the Arctic have led to a significant increase in the rate of retreat of permafrost coast, which has threatened livelihoods and infrastructure in these areas. The Kara Sea hosts more than 25% of the total Arctic coastline. However, little is known about how coastal erosion in the Kara Sea may have changed through time, and the climatic and environmental drivers remain unclear. Here we study coastal dynamics along a 4‐km stretch of permafrost and sea‐ice‐affected coastline in south‐west Baydaratskaya Bay of the Kara Sea, western Siberia, between 2005 and 2016, by using handheld differential GPS mapping and satellite imagery. We identified temporal and spatial variations in the retreat rates, ranging between 1.0 (+0.1/−0.6) and 1.9 (+0.7/−1.3) m/yr over the studied coastline during 2005–2016. We also made ground temperature measurements, subsurface resistivity measurements and estimates of wave energy flux of wind‐driven ocean waves, to investigate the dominant climatic factors influencing the observed retreat rates through time. We found that wind‐driven wave activity during sea‐ice‐free days influences the magnitude of coastal retreat in the study area, while recent temperature rise has contributed less to enhancing coastal retreat during the study period. This suggests that the amount of eroded sediment and the associated release of nutrient to the nearshore zone are controlled by the magnitude of wave activity, which may influence infrastructure along the permafrost coast and marine ecosystems in the proximal ocean.
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    Quantifying Degradative Loss of Terrigenous Organic Carbon in Surface Sediments Across the Laptev and East Siberian Sea (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Ongoing permafrost thaw in the Arctic may remobilize large amounts of old organic matter. Upon transport to the Siberian shelf seas, this material may be degraded and released to the atmosphere, exported off‐shelf, or buried in the sediments. While our understanding of the fate of permafrost‐derived organic matter in shelf waters is improving, poor constraints remain regarding degradation in sediments. Here we use an extensive data set of organic carbon concentrations and isotopes (n = 109) to inventory terrigenous organic carbon (terrOC) in surficial sediments of the Laptev and East Siberian Seas (LS + ESS). Of these ~2.7 Tg terrOC about 55% appear resistant to degradation on a millennial timescale. A first‐order degradation rate constant of 1.5 kyr−1 is derived by combining a previously established relationship between water depth and cross‐shelf sediment‐terrOC transport time with mineral‐associated terrOC loadings. This yields a terrOC degradation flux of ~1.7 Gg/year from surficial sediments during cross‐shelf transport, which is orders of magnitude lower than earlier estimates for degradation fluxes of dissolved and particulate terrOC in the water column of the LS + ESS. The difference is mainly due to the low degradation rate constant of sedimentary terrOC, likely caused by a combination of factors: (i) the lower availability of oxygen in the sediments compared to fully oxygenated waters, (ii) the stabilizing role of terrOC‐mineral associations, and (iii) the higher proportion of material that is intrinsically recalcitrant due to its chemical/molecular structure in sediments. Sequestration of permafrost‐released terrOC in shelf sediments may thereby attenuate the otherwise expected permafrost carbon‐climate feedback.
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    Global Estimates of Inorganic Nitrogen Deposition Across Four Decades (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Atmospheric deposition of inorganic nitrogen is critical to the function of ecosystems and elemental cycles. During the industrial period, humans have doubled the amount of inorganic nitrogen in the biosphere and radically altered rates of atmospheric nitrogen deposition. Despite this rapid change, estimates of global nitrogen deposition patterns generally have low, centennial‐scale temporal resolution. Lack of information on annual‐ to decadal‐scale changes in global nitrogen deposition makes it difficult for scientists researching questions on these finer timescales to contextualize their work within the global nitrogen cycle. Here we use the GEOS‐Chem Chemical Transport Model to estimate wet and dry deposition of inorganic nitrogen globally at a spatial resolution of 2° × 2.5° for 12 individual years in the period from 1984 to 2016. During this time, we found an 8% increase in global inorganic nitrogen deposition from 86.6 to 93.6 TgN/year, a trend that comprised a balance of variable regional patterns. For example, inorganic nitrogen deposition increased in areas including East Asia and Southern Brazil, while inorganic nitrogen deposition declined in areas including Europe. Further, we found a global increase in the percentage of inorganic nitrogen deposited in chemically reduced forms from 30% to 35%, and this trend was largely driven by strong regional increases in the proportion of chemically reduced nitrogen deposited over the United States. This study provides spatially explicit estimates of inorganic nitrogen deposition over the last four decades and improves our understanding of short‐term human impacts on the global nitrogen cycle.
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    A global 3‐D ocean model for polychlorinated biphenyls (PCBs): Benchmark compounds for understanding the impacts of global change on neutral persistent organic pollutants (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract Human activities have released large quantities of neutral persistent organic pollutants (POPs) that may be biomagnified in food webs and pose health risks to wildlife, particularly top predators. Here we develop a global 3‐D ocean simulation for four polychlorinated biphenyls (PCBs) spanning a range of molecular weights and volatilities to better understand effects of climate‐driven changes in ocean biogeochemistry on the lifetime and distribution of POPs. Observations are most abundant in the Arctic Ocean. There, model results reproduce spatial patterns and magnitudes of measured PCB concentrations. Sorption of PCBs to suspended particles and subsequent burial in benthic marine sediment is the dominant oceanic loss process globally. Results suggest benthic sediment burial has removed 75% of cumulative PCB releases since the onset of production in 1930. Wind speed, light penetration and ocean circulation exert a stronger and more variable influence on volatile PCB congeners with lower particle affinity such as CB‐28 and CB‐101. In the Arctic Ocean between 1992 and 2015, modeled evasion (losses) of the more volatile PCB congeners from the surface ocean increased due to declines in sea ice and changes in ocean circulation. By contrast, net deposition increased slightly for higher molecular weight congeners with stronger partitioning to particles. Our results suggest future climate changes will have the greatest impacts on the chemical lifetimes and distributions of volatile POPs with lower molecular weights.
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    Issue Information (2019)
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    Publikationsdatum: 2019
    Beschreibung: No abstract is available for this article.
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    Detection of Transient Denitrification During a High Organic Matter Event in the Black Sea (2019)
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    Publikationsdatum: 2019
    Beschreibung: Abstract N2 production by denitrification can occur in anoxic water or potentially inside organic particles. Here we compare data from the Black Sea, a permanently anoxic basin, during two organic matter regimes: suspended particulate organic matter concentrations were high in the oxycline after the spring bloom in March 2005 compared to lower organic matter concentrations in June 2005, May and October 2007, July 2008, and May 2001. For all cruises, N2 gas had a maximum in the suboxic zone (O2 〈 10 μmol/L). During the high organic matter event (March 2005), an additional shallower N2 gas and δ15N‐N2 maxima occurred above the suboxic zone in the oxycline where oxygen concentrations were 30–50 μmol/L. Examination of 16S rRNA indicated that anammox bacteria were not present in the oxycline. The δ15N of biologically produced N2 in the oxycline in March 2005 was significantly enriched (+7‰ to +38‰), not depleted, as would be expected from water column fractionation. A simple diffusion calculation indicated that ammonium produced from remineralization inside particles could be oxidized to nitrate and then completely consumed by denitrification inside the particle. In this calculation, half of denitrified N atoms originated from organic N [δ15N = 11‰] and half of N atoms originated from ambient nitrate [δ15N = 5‰–7‰], producing enriched δ15N‐N2 values. We suggest that denitrifiers were active in microzones inside particulates in hypoxic waters above the suboxic zone of the Black Sea. Denitrification in particles may also explain previous data from the oxycline above ocean oxygen deficient zones.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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    Deep‐sea oxygen depletion and ocean carbon sequestration during the last ice age (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Enhanced ocean carbon storage during the Pleistocene ice ages lowered atmospheric CO2 concentrations by 80 to 100 ppm relative to interglacial levels. Leading hypotheses to explain this phenomenon invoke a greater efficiency of the ocean's biological pump, in which case carbon storage in the deep sea would have been accompanied by a corresponding reduction in dissolved oxygen. We exploit the sensitivity of organic matter preservation in marine sediments to bottom water oxygen concentration to constrain the level of dissolved oxygen in the deep central equatorial Pacific Ocean during the last glacial period (18,000 – 28,000 years BP) to have been within the range of 20‐50 μmol/kg, much less than modern value of ca. 168 μmol/kg. We further demonstrate that reduced oxygen levels characterized the water column below a depth of ~1000 m. Converting the ice‐age oxygen level to an equivalent concentration of respiratory CO2, and extrapolating globally, we estimate that deep‐sea CO2 storage during the last ice age exceeded modern values by as much as 850 PgC, sufficient to balance the loss of carbon from the atmosphere (ca. 200 PgC) and from the terrestrial biosphere (ca. 300‐600 PgC). In addition, recognizing the enhanced preservation of organic matter in ice‐age sediments of the deep Pacific Ocean helps reconcile previously unexplained inconsistencies among different geochemical and micropaleontological proxy records used to assess past changes in biological productivity of the 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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    Modeling the recent changes in the Arctic Ocean CO2 sink (2006–2013) (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: The Arctic Ocean (AO) and its associated marginal seas have recently experienced rapid climate and environmental changes, most notably sea‐ice area (SIA) loss and warming potentially impacting its uptake of carbon dioxide (CO2). We used the state‐of‐the‐art ECCO2‐Darwin coupled ocean‐biogeochemistry model to simulate the 2006–2013 period and investigate the impact of changing SIA on the CO2 uptake of the AO. We find that the mean annual CO2 sink of the AO is 153 ± 14 TgC and the CO2 sink decreased at a rate of 3.6 TgC a−1 even though SIA decreased by 8×104 km2 a−1 over the same period. First extreme SIA loss in 2007 resulted in a 185.4 TgC CO2 sink, an increase ∼20% over the 2006–2013 mean. In contrast, second SIA loss of 2012 resulted in a CO2 sink of the AO of only 146.3 TgC due to two main factors: (1) increased both wind speed and stratifcation in the Eastern Siberian Sea absorbing less CO2, (2) decreased primary production and area of air‐sea gas exchange in the Chukchi and Nordic Seas. Our model captures a trend of decreasing CO2 sink in most of the Chukchi Sea during fall but does not show the changes in winter CO2 sink in the Nordic and Barents Seas as previous independent studies have suggested. Our results indicate that future Arctic ocean‐atmosphere CO2 exchange will be determined by complex interplay of SIA and other environmental drivers.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 92
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    A Machine Learning (kNN) Approach to Predicting Global Seafloor Total Organic Carbon (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Seafloor properties, including total organic carbon (TOC), are sparsely measured on a global scale, and interpolation (prediction) techniques are often used as a proxy for observation. Previous geospatial interpolations of seafloor TOC exhibit gaps where little to no observed data exists. In contrast, recent machine learning techniques, relying on geophysical and geochemical properties (e.g., seafloor biomass, porosity, and distance from coast), show promise in making comprehensive, statistically optimal predictions. Here we apply a nonparametric (i.e., data‐driven) machine learning algorithm, specifically k‐nearest neighbors (kNN), to estimate the global distribution of seafloor TOC. Our results include predictor (feature) selection specifically designed to mitigate bias and produce a statistically optimal estimation of seafloor TOC, with uncertainty, at 5 × 5‐arc minute resolution. Analysis of parameter space sample density provides a guide for future sampling. One use for this prediction is to constrain a global inventory, indicating that just the upper 5 cm of the seafloor contains about 87 ± 43 gigatons of carbon (Gt C) in organic form.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 93
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    Have Synergies Between Nitrogen Deposition and Atmospheric CO2 Driven the Recent Enhancement of the Terrestrial Carbon Sink? (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract The terrestrial carbon sink has increased since the turn of this century at a time of increased fossil fuel burning, yet the mechanisms enhancing this sink are not fully understood. Here we assess the hypothesis that regional increases in nitrogen deposition since the early 2000s has alleviated nitrogen limitation and worked in tandem with enhanced CO2 fertilization to increase ecosystem productivity and carbon sequestration, providing a causal link between the parallel increases in emissions and the global land carbon sink. We use the Community Land Model (CLM4.5‐BGC) to estimate the influence of changes in atmospheric CO2, nitrogen deposition, climate, and their interactions to changes in net primary production and net biome production. We focus on two periods, 1901–2016 and 1990–2016, to estimate changes in land carbon fluxes relative to historical and contemporary baselines, respectively. We find that over the historical period, nitrogen deposition (14%) and carbon‐nitrogen synergy (14%) were significant contributors to the current terrestrial carbon sink, suggesting that long‐term increases in nitrogen deposition led to a substantial increase in CO2 fertilization. However, relative to the contemporary baseline, changes in nitrogen deposition and carbon‐nitrogen synergy had no substantial contribution to the 21st century increase in global carbon uptake. Nonetheless, we find that increased nitrogen deposition in East Asia since the early 1990s contributed 50% to the overall increase in net biome production over this region, highlighting the importance of carbon‐nitrogen interactions. Therefore, potential large‐scale changes in nitrogen deposition could have a significant impact on terrestrial carbon cycling and future climate.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 94
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    Insights From the 238U‐234Th Method Into the Coupling of Biological Export and the Cycling of Cadmium, Cobalt, and Manganese in the Southeast Pacific Ocean (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Better constraints on the magnitude of particulate export and the residence times of trace elements are required to understand marine food web dynamics, track the transport of anthropogenic trace metals in the ocean, and improve global climate models. While prior studies have been successful in constructing basin‐scale budgets of elements like carbon in the upper ocean, the cycling of particulate trace metals is poorly understood. The 238U‐234Th method is used here with data from the GP‐16 GEOTRACES transect to investigate the upper ocean processes controlling the particulate export of cadmium, cobalt, and manganese in the southeastern Pacific. Patterns in the flux data indicated that particulate cadmium and cobalt behave similarly to particulate phosphorus and organic carbon, with the highest export in the productive coastal region and decreasing flux with depth due to remineralization. The export of manganese was influenced by redox conditions at the low oxygen coastal stations and by precipitation and/or scavenging elsewhere. Residence times with respect to export (total inventory divided by particulate flux) for phosphorus, cadmium, cobalt, and manganese in the upper 100 and 200 m were determined to be on the order of months to years. These GEOTRACES‐based synthesis efforts, combining a host of concentration and tracer data with unprecedented resolution, will help to close the oceanic budgets of trace metals.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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  • 95
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    Increased global land carbon sink due to aerosol‐induced cooling (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Anthropogenic aerosols have contributed to historical climate change through their interactions with radiation and clouds. In turn, climate change due to aerosols has impacted the C cycle. Here we use a set of offline simulations made with the ORCHIDEE Land Surface Model (LSM) driven by bias‐corrected climate fields from simulations of three CMIP5 Earth System Models (ESM) (IPSL‐CM5A‐LR, CSIRO‐Mk3.6.0 and GISS‐E2‐R) to quantify the climate‐related impacts of aerosols on land carbon fluxes during 1860‐2005. We found that Climate Change from Anthropogenic Aerosols (CCAA) globally cooled the climate, and increased land carbon storage, or cumulative Net Biome Production (NBP), by 11.6‐41.8 PgC between 1860 and 2005. The increase in NBP from CCAA mainly occurs in the tropics and northern mid latitudes, primarily due to aerosol‐induced cooling. At high latitudes, cooling caused stronger decrease in gross primary production (GPP) than total ecosystem respiration (TER), leading to a lower NBP. At mid latitudes, cooling‐induced decrease in TER is stronger than that of GPP, resulting in NBP increase. At low latitudes, NBP was also enhanced due to the cooling‐induced GPP increase, but precipitation decline from CCAA may negate the effect of temperature. The three ESMs show large divergence in low‐latitude CCAA precipitation response to aerosols, which results in considerable uncertainties in regional estimations of CCAA effects on carbon fluxes. Our results suggest that better understanding and simulation of how anthropogenic aerosols affect precipitation in ESMs is required for a more accurate attribution of aerosol effects on the terrestrial carbon cycle.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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    Significance of Acetate as a Microbial Carbon and Energy Source in the Water Column of Gulf of Mexico: Implications for Marine Carbon Cycling (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Acetate is a key intermediate of organic matter mineralization, but its metabolism remains largely unconstrained in the pelagic ocean. We conducted an integrated biogeochemical study to investigate microbial acetate cycling in the northern Gulf of Mexico with the goal of elucidating the importance of acetate as a carbon and energy source. Acetate was used primarily as an energy source, as evidenced by observed oxidation rates (rate constant k: 0.06–0.22 day−1) that varied between 42% and 96% of total biological acetate uptake (i.e., assimilation + oxidation; k: 0.06–0.34 day−1). The assimilation of acetate into biomass (k: 0.01–0.20 day−1) illustrated the potential significance of acetate as a biomass carbon source, particularly in nutrient‐rich coastal waters. No relationship between acetate assimilation or oxidation and environmental factors, such as chlorophyll and nutrients, was observed. However, elevated acetate uptake in reduced oxygen waters characterized by particulate organic carbon mineralization suggests that acetate metabolism may be a good proxy for particulate organic carbon breakdown. Molecular genetic analysis revealed that SAR11 Alphaproteobacteria were the most abundant heterotrophic bacteria and suggest that they may utilize acetate. At some sites, acetate carbon may have accounted for up to 50.4% of the bacterial carbon production. These results suggest that acetate may serve as an important carbon and energy source for heterotrophic bacteria thus revealing a potentially significant role of acetate for dissolved organic carbon cycling in the ocean.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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    Large‐Scale Landscape Drivers of CO2, CH4, DOC, and DIC in Boreal River Networks (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: The boreal biome is characterized by extremely dense and complex fluvial networks that are closely coupled to land. Reconstructing the role that these fluvial networks play in regional carbon (C) budgets requires identifying landscape and environmental drivers of riverine C that operate at the whole network scale and that can be applied across landscapes. Here we explore drivers of CO2, CH4, and dissolved organic (DOC) and inorganic C (DIC) across 190 streams and rivers spanning 8 Strahler orders over an area of 500,000 km2 of heterogeneous boreal landscape in Québec, focusing on those drivers that can be readily obtained from remote sensing data. Each C species (except DIC) could be modeled as a function of a proximal network‐scale property, such as flow distance or elevation, but adding regional structure to these models greatly improved predictions. These modeled regional effects were similar for DOC, CO2, and CH4 and were strongly related to average regional soil organic content and especially to NPP, the latter integrating regional differences in climate and other environmental factors. These results suggest that there may be regional C baselines determined by a combination of landscape and climate features, which simultaneously influence the average CO2, DOC, and CH4 within fluvial networks, albeit through different underlying mechanisms and with varying degrees of influence on each C species. The latter two C species appear to be more sensitive to regional differences in soil, NPP, and climate than CO2 or DIC, and therefore more likely to shift under future scenarios of change in northern landscapes.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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    Geomorphology of Gullies at Thomas Lee Inlet, Devon Island, Canadian High Arctic (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Slopes in and around Thomas Lee Inlet (Devon Island, Nunavut, Canada) are eroded by means of gullying, yet the driving factors, such as the nature of the substrate and availability of different sources for water, influencing gully morphology remain poorly understood. Here we investigate the factors that contribute to gully formation using a combination of satellite mapping, field observations and statistical analysis of morphological data. In total, 161 gullies were mapped within the 126 km2 study area. Factors linked to gullies, such as the nature of its substrate and the presence of glaciers, were integrated into a spatial geodatabase. A Factor Analysis of Mixed Data performed on the geodatabase was used to discriminate which factors may influence gully geometry. Our results show that the type of geological formation has a strong impact on gully slope. In addition, supplemental sources of water are often found near alcoves of the steep, longer and mature gullies, and levees often form in their aprons. Immature debris flow‐like gullies were dryer and found on short and variable (from steep to gentle) slopes. This detailed study of the rocky and arid Thomas Lee Inlet plateaus and slopes provides the first insight into gullied slopes as a hydrological component connecting upland units to downslope in this area.
    Print ISSN: 1045-6740
    Digitale ISSN: 1099-1530
    Thema: Geographie , Geologie und Paläontologie
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  • 99
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    Nutrient controls on export production in the Southern Ocean (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract We use observations from novel biogeochemical profiling floats deployed by the SOCCOM program to estimate annual net community production (ANCP) (associated with carbon export) from the seasonal drawdown of mesopelagic oxygen and surface nitrate in the Southern Ocean. Our estimates agree with previous observations in showing an increase in ANCP in the vicinity of the polar front (~3 mol C m−2 y−1), compared to lower rates in the subtropical zone (≤1 mol C m−2 y−1) and the seasonal ice zone (〈2 mol C m−2 y−1). Paradoxically, the increase in ANCP south of the subtropical front is associated with elevated surface nitrate and silicate concentrations, but decreasing surface iron. We hypothesize that iron limitation promotes silicification in diatoms, which is evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic polar front. High diatom silicification increases the ballasting effect of POC and overall ANCP in this region. A model‐based assessment of our methods shows a good agreement between ANCP estimates based on oxygen and nitrate drawdown and the modeled downward organic carbon flux at 100 m. This agreement supports the presumption that net biological consumption is the dominant process affecting the drawdown of these chemical tracers, and that, given sufficient data, ANCP can be inferred from observations of oxygen and/or nitrate drawdown in the Southern Ocean.
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    Thema: Biologie , Chemie und Pharmazie , Geographie , Geologie und Paläontologie , Physik
    Publiziert von Wiley im Namen von American Geophysical Union (AGU).
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    Relict periglacial soils on Quaternary terraces in the Central Ebro Basin (NE Spain) (2019)
    Wiley
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    Publikationsdatum: 2019
    Beschreibung: Abstract Pedofeatures associated with ancient cold climatic conditions have been recognized in soils on terraces in the Monegros area (central Ebro Basin, Spain), at a latitude of 41°49′N and an altitude of 300 m a.s.l. Eleven soil profiles were described on fluvial deposits corresponding to the most extensive terrace (T5) of the Alcanadre River, Middle Pleistocene in age (MIS8–MIS7). Each soil horizon was sampled for physical, chemical, mineralogical and micromorphological analyses. Macromorphological features related to pedocryogenic processes were described: involutions, jacked stones, shattered stones, detached and vertically oriented carbonatic pendents, fragmented carbonatic crusts, laminar microstructures, succitic fabric, silt cappings on rock fragments and aggregates, and irregular, broken, discontinuous and deformed gravel and sandy pockets. Accumulations of Fe–Mn oxides, dissolution features on the surface of carbonatic stones, and calcitic accumulations were identified related to vadose–phreatic conditions. The observed periglacial features developed under cold environmental conditions in exceptional geomorphic and hydrological conditions. This soil information may have potential implications in studies of paleoclimate in the Ebro Valley as well as in other Mediterranean areas.
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    Thema: Geographie , Geologie und Paläontologie
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