Organic matter cycling in sediments of the continental margin in the northwest Atlantic Ocean Academic Article uri icon

abstract

  • This paper presents the results of a study of benthic organic matter decomposition on the continental margin in the Northwest Atlantic Ocean at 70 degree 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 O sub(2) profiles indicate that sedimentary O sub(2) consumption was essentially constant at 50-55 mu mol/cm super(2)/yr from 460 to 1467 m water depth, with a lower rate in the coarser-grained sediments at 260 m. The O sub(2) 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 mu mol/cm super(2)/yr at 260 m to ~42 mu mol/cm super(2)/yr over the 460-1035 m depth range, and decreased gradually below that to 32 mu mol/cm super(2)/yr at 2500 m. O sub(2) 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 degree W, while fluxes of total mass, organic carbon, and excess super(210)Pb 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 super(-) and the super(222)Rn deficit as tracers, indicated that sediment irrigation may enhance O sub(2) 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 NO sub(3) super(-) by as much as 50%, and may increase the denitrification rate by a similar amount at our 460 m site.

publication date

  • March 2004