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DOC-dynamics in a small headwater catchment as driven by redox fluctuations and hydrological flow paths – are DOC exports mediated by iron reduction/oxidation cycles?

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Author(s): K.-H. Knorr

Journal: Biogeosciences Discussions
ISSN 1810-6277

Volume: 9;
Issue: 9;
Start page: 12951;
Date: 2012;
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ABSTRACT
Dissolved organic carbon (DOC) exports from many catchments in Europe and North-America are steadily increasing. Several studies have sought to explain this observation. As possible causes, a decrease in acid rain or sulfate deposition, concomitant reductions in ionic strength and increasing temperatures were identified. DOC often originates from riparian wetlands; but here, despite higher DOC concentrations, ionic strength in pore waters usually exceeds that in surface waters. In the catchment under study, DOC concentrations were synchronous with dissolved iron concentrations in pore and stream water. This study aims at testing the hypothesis that DOC exports are mediated by iron reduction/oxidation cycles. Following the observed hydrographs, δ18O of water, and DOC fluorescence, the wetlands were identified as main source of DOC. Antecedent biogeochemical conditions, i.e. water table levels in the wetlands, influenced the discharge patterns of nitrate, iron, and DOC during an event. The correlation of DOC with pH was positive in pore waters but negative in surface waters; it was negative for DOC with sulfate in pore waters but only weak in surface waters. The positive correlation of DOC with iron was universal for pore and surface water, though. The decline of DOC and iron concentrations in transition from anoxic wetland pore water to oxic stream water suggests a flocculation of DOC with oxidizing iron, leading to a drop in pH in the stream during high DOC fluxes. The pore water did not per se differ in pH. There is thus a need to more thoroughly consider processes of DOC mobilization in wetlands when interpreting DOC exports from catchments. The coupling of DOC with iron fluxes suggested that increased DOC exports could at least in part be caused by increasing activities in iron reduction, possibly due to increases in temperature or wetness of riparian wetlands.
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