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  • Articles  (73)
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  • 2015-2019  (73)
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  • 1
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    Biological and climate controls on North Atlantic marine carbon dynamics over the last millennium: Insights from an absolutely-dated shell based record from the North Icelandic Shelf (2017)
    Wiley
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    Publication Date: 2017-11-19
    Description: Given the rapid increase in atmospheric carbon dioxide concentrations ( p CO 2 ) over the industrial era there is a pressing need to construct long term records of natural carbon cycling prior to this perturbation and to develop a more robust understanding of the role the oceans play in the sequestration of atmospheric carbon. Here we reconstruct the historical biological and climatic controls on the carbon isotopic (δ 13 C-shell) composition of the North Icelandic shelf waters over the last millennium derived from the shells of the long-lived marine bivalve mollusc Arctica islandica. Variability in the annually resolved δ 13 C-shell record is dominated by multi-decadal variability with a negative trend (-0.003±0.002‰yr -1 ) over the industrial era (1800-2000 CE). This trend is consistent with the marine Suess effect brought about by the sequestration of isotopically light carbon (δ 13 C of CO 2 ) derived from the burning of fossil fuels. Comparison of the δ 13 C-shell record with contemporary proxy archives, over the last millennium, and instrumental data over the 20 th century, highlight that both biological (primary production) and physical environmental factors such as relative shifts in the proportion of Subpolar Mode Waters and Arctic Intermediate Waters entrained onto the North Icelandic shelf, atmospheric circulation patterns associated with the winter North Atlantic Oscillation, and subpolar gyre sea surface temperatures and salinity, are the likely mechanisms that contribute to natural variations in seawater δ 13 C variability on the North Icelandic shelf. Contrasting δ 13 C fractionation processes associated with these biological and physical mechanisms likely cause the attenuated local marine Suess effect signal at this locality.
    Print ISSN: 0886-6236
    Electronic ISSN: 1944-9224
    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 2
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    Carbon budget of tidal wetlands, estuaries, and shelf waters of Eastern North America (2018)
    Wiley
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    Publication Date: 2018-02-21
    Description: Carbon cycling in the coastal zone affects global carbon budgets and is critical for understanding the urgent issues of hypoxia, acidification, and tidal wetland loss. However, there are no regional carbon budgets spanning the three main ecosystems in coastal waters: tidal wetlands, estuaries, and shelf waters. Here, we construct such a budget for Eastern North America using historical data, empirical models, remote-sensing algorithms, and process-based models. Considering the net fluxes of total carbon at the domain boundaries, 59 ± 12% (± 2 standard errors) of the carbon entering is from rivers and 41 ± 12% is from the atmosphere, while 80 ± 9% of the carbon leaving is exported to the open ocean and 20 ± 9% is buried. Net lateral carbon transfers between the three main ecosystem types are comparable to fluxes at the domain boundaries. Each ecosystem type contributes substantially to exchange with the atmosphere, with CO 2 uptake split evenly between tidal wetlands and shelf waters, and estuarine CO 2 outgassing offsetting half of the uptake. Similarly, burial is about equal in tidal wetlands and shelf waters, while estuaries play a smaller but still substantial role. The importance of tidal wetlands and estuaries in the overall budget is remarkable given that they respectively make up only 2.4 and 8.9% of the study domain area. This study shows that coastal carbon budgets should explicitly include tidal wetlands, estuaries, shelf waters and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 3
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    Pathways and transformations of dissolved methane and dissolved inorganic carbon in Arctic tundra watersheds: Evidence from analysis of stable isotopes (2015)
    Wiley
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    Publication Date: 2015-09-10
    Description: Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO 2 ) and methane (CH 4 ) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem-scale CH 4 and CO 2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO 2 ) and CH 4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH 4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH 4 and DIC balance by estimating mechanisms of CH 4 and DIC production and transport pathways and oxidation of subsurface CH 4 . We observed a shift from acetoclastic (July) towards hydrogenotropic (September) methanogenesis at sites located towards the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape-scale effect on CH 4 production mechanism. The majority of subsurface CH 4 was transported upward by plant-mediated transport and ebullition, predominantly bypassing the potential for CH 4 oxidation. Thus, surprisingly CH 4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH 4 produced at the frost table, contributing to 〈 0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e- acceptors. This work highlights the importance of spatial and temporal variability of CH 4 production at the watershed scale, and suggests broad scale investigations are required to build better regional or pan-Arctic representations of CH 4 and CO 2 production.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 4
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    Global oceanic emission of ammonia: constraints from seawater and atmospheric observations (2015)
    Wiley
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    Publication Date: 2015-07-19
    Description: Current global inventories of ammonia emissions identify the ocean as the largest natural source. This source depends on seawater pH, temperature, and the concentration of total seawater ammonia ( NH x ( sw )), which reflects a balance between remineralization of organic matter, uptake by plankton, and nitrification. Here, we compare [ NH x ( sw )] from two global ocean biogeochemical models (BEC and COBALT) against extensive ocean observations. Simulated [ NH x ( sw )] are generally biased high. Improved simulation can be achieved in COBALT by increasing the plankton affinity for NH x within observed ranges. The resulting global ocean emissions is 2.5 TgN a −1 , much lower than current literature values(7–23 TgN a −1 ), including the widely used GEIA inventory (8 TgN a −1 ). Such a weak ocean source implies that continental sources contribute more than half of atmospheric NH x over most of the ocean in the Northern hemisphere. Ammonia emitted from oceanic sources is insufficient to neutralize sulfate aerosol acidity, consistent with observations. There is evidence over the Equatorial Pacific for a missing source of atmospheric ammonia that could be due to photolysis of marine organic nitrogen at the ocean surface or in the atmosphere. Accommodating this possible missing source yields a global ocean emission of ammonia in the range 2–5 TgN a −1 , comparable in magnitude to other natural sources from open fires and soils.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 5
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    Towards More Realistic Projections of Soil Carbon Dynamics by Earth System Models (2015)
    Wiley
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    Publication Date: 2015-12-20
    Description: Soil carbon (C) is a critical component of Earth system models (ESMs) and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the 3 rd to 5 th assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. Firstly, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by 1 st -order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic SOC dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Secondly, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based datasets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Thirdly, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable datasets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.
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  • 6
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    Source and sink carbon dynamics and carbon allocation in the Amazon basin (2015)
    Wiley
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    Publication Date: 2015-04-25
    Description: Changes to the carbon cycle in tropical forests could affect global climate, but predicting such changes has been previously limited by lack of field based data. Here we show seasonal cycles of the complete carbon cycle for fourteen, one hectare intensive carbon cycling plots which we separate into three regions: humid lowland, highlands and dry lowlands. Our data highlight three trends: (1) there is differing seasonality of total NPP with the highlands and dry lowlands peaking in the dry season and the humid lowland sites peaking in the wet season; (2) seasonal reductions in wood NPP are not driven by reductions in total NPP but by carbon during the dry season being preferentially allocated towards either roots or canopy NPP; and (3) there is a temporal decoupling between total photosynthesis and total carbon usage or plant carbon expenditure (PCE). This decoupling indicates the presence of non-structural carbohydrates (NSC) which may allow growth and carbon to be allocated when it is most ecologically beneficial rather when it is most environmentally available.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 7
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    Coupling atmospheric mercury isotope ratios and meteorology to identify sources of mercury impacting a coastal urban-industrial region near Pensacola, Florida, USA (2015)
    Wiley
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    Publication Date: 2015-09-19
    Description: Identifying the anthropogenic and natural sources of mercury (Hg) emissions contributing to atmospheric mercury on local, regional, and global scales continues to be a grand challenge. The relative importance of various direct anthropogenic emissions of mercury, in addition to natural geologic sources and re-emission of previously released and deposited mercury, differs regionally and temporally. In this study, we used local, mesoscale, and synoptic scale meteorological analysis to couple the isotopic composition of ambient atmospheric mercury with potential sources of mercury contributing to a coastal urban-industrial setting near a coal-fired power plant in Pensacola, Florida, USA. We were able to broadly discern four influences on the isotopic composition of ambient atmospheric mercury impacting this coastal urban-industrial region: (1) local to regional urban-industrial anthropogenic emissions (mean δ 202 Hg = 0.44 ± 0.05‰, 1SD, n = 3); (2) marine-influenced sources derived from the Gulf of Mexico (mean δ 202 Hg = 0.77 ± 0.15‰, 1SD, n = 4); (3) continental sources associated with north-northwesterly flows from within the planetary boundary layer (mean δ 202 Hg = 0.65 ± 0.04‰, 1SD, n = 3); and (4) continental sources associated with north-northeasterly flows at higher altitudes (i.e., 2000 m above ground level; mean δ 202 Hg = 1.10 ± 0.21‰, 1SD, n = 8). Overall, these data, in conjunction with previous studies, suggest that the background global atmospheric mercury pool is characterized by moderately positive δ 202 Hg values; that urban-industrial emissions drive the isotopic composition of ambient atmospheric mercury toward lower δ 202 Hg values; and that air-surface exchange dynamics across vegetation and soils of terrestrial ecosystems drive the isotopic composition of ambient atmospheric mercury toward higher positive δ 202 Hg values. The data further suggest that mass independent fractionation (MIF) of both even-mass- and odd-mass-number isotopes, likely generated by photochemical reactions in the atmosphere or during re-emission from terrestrial and aquatic ecosystems, can be obscured by mixing with anthropogenic emissions having different MIF signatures.
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  • 8
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    Dissolved Organic Matter Chemistry and Transport Along an Arctic Tundra Hillslope (2019)
    Wiley
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    Publication Date: 2019
    Description: 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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  • 9
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    Strong biotic influences on regional patterns of climate regulation services (2017)
    Wiley
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    Publication Date: 2017-04-19
    Description: Climate regulation services from forests are an important leverage in global-change mitigation treaties. Like most ecosystem services, climate regulation is the product of various ecological phenomena with unique spatial features. Elucidating which abiotic and biotic factors relate to spatial patterns of climate regulation services advances our understanding of what underlies climate-mitigation potential, and its variation within and across ecosystems. Here we quantify and contrast the statistical relations between climate regulation services (albedo and evapotranspiration, primary productivity and soil carbon) and abiotic and biotic factors. We focus on 16,955 forest plots in a regional extent across the eastern United States. We find the statistical effects of forest litter and understory carbon on climate regulation services to be as strong as those of temperature-precipitation interactions. These biotic factors likely influence climate regulation through changes in vegetation and canopy density, radiance scattering, and decomposition rates. We also find a moderate relation between leaf nitrogen traits and primary productivity at this regional scale. The statistical relation between climate regulation and temperature-precipitation ranges, seasonality, and climatic thresholds highlights a strong feedback with global climate change. Our assessment suggests the expression of strong biotic influences on climate regulation services at a regional, temperate extent. Biotic homogenization and management practices manipulating forests structure and succession will likely strongly impact climate-mitigation potential. The identity, strength, and direction of primary influences differed for each process involved in climate regulation. Hence, different abiotic and biotic factors are needed to monitor and quantify the full climate-mitigation potential of temperate forest ecosystems.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 10
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    Rising atmospheric methane: 2007-14 growth and isotopic shift. (2016)
    Wiley
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    Publication Date: 2016-09-27
    Description: From 2007 to 2013, the globally-averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr -1 . Simultaneously, δ 13 C CH4 (a measure of the 13 C/ 12 C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics: for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies, or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13 C-depleted values together with its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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