Role of Benthic Biogeochemical Processes in Primary Production and Ocean Acidification
Passive continental margin sediments receiving outflow from large rivers are well-known deposition centers for organic carbon, but may also be hot spots for metal-reducing microbial activity considering the simultaneous high deposition rates of unconsolidated metal oxides of terrigenous origin.
A combination of in situ depth profiles, benthic flux measurements, and ex situ measurements in the Rhône River Delta (< 80 m), Cape Hatteras slope (< 3,000 m), Louisiana slope (<1,800 m), and Congo River fan (~5,000 m) sediments are compared to assess the main redox species involved in early diagenesis. Metal reduction dominates carbon remineralization processes in the top ~20 cm of sediment subject to high deposition, while sulfate reduction is lacking.
In these environments, the flux of iron across the sediment-water interface may be enhanced, thus providing a source of a significant nutrient for primary production in surface waters. On continental shelves exposed to large riverine inputs, however, sulfate reduction promotes iron and sulfur burial, thus allowing alkalinity to diffuse across the sediment-water interface.
These findings suggest that river-dominated continental margin sediments may simultaneously promote carbon fixation and prevent acidification of the overlying waters.
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