McClelland James W.

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McClelland
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James W.
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  • Article
    Recent changes in nitrate and dissolved organic carbon export from the upper Kuparuk River, North Slope, Alaska
    (American Geophysical Union, 2007-11-08) McClelland, James W. ; Stieglitz, Marc ; Pan, Feifei ; Holmes, Robert M. ; Peterson, Bruce J.
    Export of nitrate and dissolved organic carbon (DOC) from the upper Kuparuk River between the late 1970s and early 2000s was evaluated using long-term ecological research (LTER) data in combination with solute flux and catchment hydrology models. The USGS Load Estimator (LOADEST) was used to calculate June–August export from 1978 forward. LOADEST was then coupled with a catchment-based land surface model (CLSM) to estimate total annual export from 1991 to 2001. Simulations using the LOADEST/CLSM combination indicate that annual nitrate export from the upper Kuparuk River increased by ~5 fold and annual DOC export decreased by about one half from 1991 to 2001. The decrease in DOC export was focused in May and was primarily attributed to a decrease in river discharge. In contrast, increased nitrate export was evident from May to September and was primarily attributed to increased nitrate concentrations. Increased nitrate concentrations are evident across a wide range of discharge conditions, indicating that higher values do not simply reflect lower discharge in recent years but a significant shift to higher concentration per unit discharge. Nitrate concentrations remained elevated after 2001. However, extraordinarily low discharge during June 2004 and June–August 2005 outweighed the influence of higher concentrations in determining export during these years. The mechanism responsible for the recent increase in nitrate concentrations is uncertain but may relate to changes in soils and vegetation associated with regional warming. While changes in nitrate and DOC export from arctic rivers reflect changes in terrestrial ecosystems, they also have significant implications for Arctic Ocean ecosystems.
  • Article
    Lability of DOC transported by Alaskan rivers to the Arctic Ocean
    (American Geophysical Union, 2008-02-09) Holmes, Robert M. ; McClelland, James W. ; Raymond, Peter A. ; Frazer, Breton B. ; Peterson, Bruce J. ; Stieglitz, Marc
    Arctic rivers transport huge quantities of dissolved organic carbon (DOC) to the Arctic Ocean. The prevailing paradigm is that DOC in arctic rivers is refractory and therefore of little significance for the biogeochemistry of the Arctic Ocean. We show that there is substantial seasonal variability in the lability of DOC transported by Alaskan rivers to the Arctic Ocean: little DOC is lost during incubations of samples collected during summer, but substantial losses (20–40%) occur during incubations of samples collected during the spring freshet when the majority of the annual DOC flux occurs. We speculate that restricting sampling to summer may have biased past studies. If so, then fluvial inputs of DOC to the Arctic Ocean may have a much larger influence on coastal ocean biogeochemistry than previously realized, and reconsideration of the role of terrigenous DOC on carbon, microbial, and food-web dynamics on the arctic shelf will be warranted.
  • Preprint
    Sulfur isotopes in rivers : insights into global weathering budgets, pyrite oxidation, and the modern sulfur cycle
    ( 2018-05) Burke, Andrea ; Present, Theodore M. ; Paris, Guillaume ; Rae, Emily C. M. ; Sandilands, Brodie H. ; Gaillardet, Jerome ; Peucker-Ehrenbrink, Bernhard ; Fischer, Woodward W. ; McClelland, James W. ; Spencer, Robert G. M. ; Voss, Britta M. ; Adkins, Jess F.
    The biogeochemical sulfur cycle is intimately linked to the cycles of carbon, iron, and oxygen, and plays an important role in global climate via weathering reactions and aerosols. However, many aspects of the modern budget of the global sulfur cycle are not fully understood. We present new δ34S measurements on sulfate from more than 160 river samples from different geographical and climatic regions—more than 46% of the world’s freshwater flux to the ocean is accounted for in this estimate of the global riverine sulfur isotope budget. These measurements include major rivers and their tributaries, as well as time series, and are combined with previously published data to estimate the modern flux-weighted global riverine δ34S as 4.4 ± 4.5 ‰ (V-CDT), and 4.8 ± 4.9 ‰ when the most polluted rivers are excluded. Combined with major anion and cation concentrations, the sulfur isotope data allow us to tease apart the relative contributions of different processes to the modern riverine sulfur budget, resulting in new estimates of the flux of riverine sulfate due to the oxidative weathering of pyrites (1.3 ± 0.2 Tmol S/y) and the weathering of sedimentary sulfate minerals (1.5 ± 0.2 Tmol S/y). These data indicate that previous estimates of the global oxidative weathering of pyrite have been too low by a factor of two. As pyrite oxidation coupled to carbonate weathering can act as a source of CO2 to the atmosphere, this global pyrite weathering budget implies that the global CO2 weathering sink is overestimated. Furthermore, the large range of sulfur isotope ratios in modern rivers indicates that secular changes in the lithologies exposed to weathering through time could play a major role in driving past variations in δ34S of seawater.