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The fate of terrigenous dissolved organic carbon in a river‐influenced ocean margin
Author(s) -
Fichot Cédric G.,
Benner Ronald
Publication year - 2014
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1002/2013gb004670
Subject(s) - mineralization (soil science) , terrigenous sediment , biogeochemical cycle , oceanography , environmental chemistry , total organic carbon , sink (geography) , dissolved organic carbon , mixed layer , environmental science , carbon cycle , geology , ecosystem , sedimentary rock , chemistry , geochemistry , soil science , ecology , soil water , cartography , biology , geography
The mineralization of terrigenous dissolved organic carbon (tDOC) discharged by rivers can impact nutrient and trace metal cycling, biological productivity, net ecosystem metabolism, and air‐sea CO 2 exchange in ocean margins. However, the extreme heterogeneity of river‐influenced ocean margins represents a major challenge for quantitative assessments of tDOC transformations and thereby obscures the role of tDOC in biogeochemical cycles. Here a lignin‐based optical proxy for tDOC and a shelf‐wide mass balance approach were used to quantitatively assess the fate of tDOC discharged from the Mississippi‐Atchafalaya River System (M‐ARS) to the Louisiana shelf. The mass balance revealed that ~40% of the tDOC discharged by the M‐ARS during March 2009–2010 was mineralized to CO 2 on the Louisiana shelf, with two thirds of the mineralization taking place in the mixed layer. A strong seasonality in tDOC mineralization was observed, with mineralization rates severalfold higher during summer than during winter. Independent assessments of specific mineralization processes indicated biomineralization accounted for ~94% of the tDOC mineralization on an annual basis and suggest that photochemical transformations of tDOC enhanced biomineralization by ~50% in the mixed layer. Direct photomineralization accounted for a relatively small fraction (~6%) of the tDOC mineralization on an annual basis. This quantitative assessment directly confirms that ocean margins are major sinks of the tDOC discharged by rivers and indicates that tDOC mineralization rates in the shelf mixed layer are sufficiently large to influence whether the Louisiana shelf is a net sink or source of atmospheric CO 2 .