Role of the oxygen‐deficient zone in transfer of organic carbon to the deep ocean

Abstract

Benthic carbon oxidation rates and carbon burial rates have been determined for two contrasting continental margins and their sum has been used as an estimate of carbon rain rate to the sediments. On the Washington State margin, where the water column is oxic, rain rates at 100 m were about 15 to 20 mmoles C m Ϫ2 d Ϫ1 and they decreased with increasing water depth to values of near 3 mmoles C m Ϫ2 d Ϫ1 at 1,000 m. The rain rate estimates, C R , were described by a power function, C R ϭ C R 100m (z/100) Ϫ␣ , with an attenuation coefficient, ␣, of 0.93 (C R 100m ϭ 16.2, r 2 ϭ 0.89). This attenuation rate is similar to numerous others previously reported for various oceanic areas. In contrast, off northwest Mexico, where the water column is oxygen deficient between 180 and 700 m, rain rates at 100 m were considerably less, about 7.5 mmoles C m Ϫ2 d Ϫ1 , but rain rates at 1,000 m were similar to those off Washington. Thus, the attenuation coefficient for the Mexican margin was significantly lower, ␣ ϭ 0.36 (C R 100m ϭ 7.4, r 2 ϭ 0.77). Off Mexico, the rain rates estimated from sedimentary parameters were corroborated by values determined directly from sediment-trap deployments. The generally smaller rain rates off Mexico are probably due to the lower primary production, hence lower initial supply. The lower attenuation rate, however, is hypothesized to result from a decreased oxidation rate of the sinking flux within the oxygen-deficient zone relative to a more typical oxic water column.The transition from the last glacial to the present interglacial was accompanied by an 85 ppm increase in the CO 2 content of the atmosphere. It is currently thought that much of this change was due to movement of CO 2 between the deep ocean and the atmosphere, although the exact mechanism is still under debate (see Broecker and Henderson 1998 for discussion). Three important carbon cycle processes that determine CO 2 uptake from the atmosphere and subsequent transfer and storage in the ocean interior are (1) photosynthetic carbon fixation in the euphotic zone, (2) transfer of some fraction of this fixed carbon out of the euphotic zone as export production, and (3) permanent carbon burial in marine sediments. As the export flux falls through the oceans, it is biologically oxidized such that the sinking flux of organic matter decreases progressively with increasing depth leaving only a small fraction available for burial. The manner in which the export flux, or carbon rain rate, decreases with depth is important for two reasons. First, it determines the quantity and time scale at which carbon is sequestered as CO 2 in the deep ocean, i.e., the shallower the regeneration depth the shorter, in general, the sequestration time. Second, greater attenuation rates result in proportionally less of the initial carbon flux reaching the sediments, which has attendant implications for benthic ecology and 1 Current address: Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901. Acknow...

Cryptic oxygen cycling in anoxic marine zones

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Metaproteomics reveals differential modes of metabolic coupling among ubiquitous oxygen minimum zone microbes

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Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

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