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This paper presents the results of a study of benthic organic matter decomposition on the continental margin in the northwest Atlantic Ocean at 70°W, the same region that was studied as part of the SEEP-I project (Cont. Shelf Res. 8(5–7) (1988) 925). We collected all of our samples via submersible during July, 1996, and September, 1997. An extensive set of in situ microelectrode O2 profiles indicate that sedimentary O2 consumption was essentially constant at 50–55 μmol/cm2/yr from 460 to 1467 m water depth, with a lower rate in the coarser-grained sediments at 260 m. The O2 penetration depth increased steadily over the depth transect, from 1.0±0.1 cm at 260 m to 2.7±0.3 cm at 1467 m. The organic matter oxidation rate increased from ∼30 μmol/cm2/yr at 260 m to ∼42 μmol/cm2/yr over the 460–1035 m depth range, and decreased gradually below that to 32 μmol/cm2/yr at 2500 m. O2 was quantitatively the most important electron acceptor, accounting for 75–91% of organic matter oxidation, while denitrification accounted for 2–5% and iron and sulfate reduction 8–20%. A comparison with organic matter oxidation rates measured at the site of the mid-slope depocenter in the southern Mid-Atlantic Bight showed a north/south difference in benthic organic matter oxidation rates similar to that found in particle fluxes to the sea floor. Benthic organic carbon oxidation rates measured in the southern depocenter were 3–6 times the rates we measured at 70°W, while fluxes of total mass, organic carbon, and excess 210Pb were 3.4–4.6 times larger in the southern depocenter (Cont. Shelf Res. 8(5–7) (1988) 855; Deep-Sea Res. II 41 (1994) 459). Measurements of the enhancement of solute transport between sediments and bottom water, using Br− and the 222Rn deficit as tracers, indicated that sediment irrigation may enhance O2 exchange by 50–100% at 460 and 260 m, but is likely to be of limited significance at deeper water depths. In addition, enhanced solute exchange by irrigation may reduce net bottom water/sediment exchange of NO3− by as much as 50%, and may increase the denitrification rate by a similar amount at our 460 m site. |
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