RECONCILING MEASURED AND PREDICTED FLUXES OF OXYGEN ACROSS THE DEEP-SEA SEDIMENT-WATER INTERFACE

Rates of sediment community oxygen consumption determined in situ are compared to fluxes predicted from oxygen microelectrode gradients measured in cores from .apprx. 3,750-m water depth in the eastern North Pacific. Oxygen concentrations decrease exponentially over > 1.5 cm and suggest that organic matter in the sediments is degraded most rapidly immediately below the sediment-water interface. Molecular diffusion of oxygen across the interface is modeled as an "internal regime" and can account for nearly all the directly measured in situ flux, 0.20 .+-. 0.02 .mu.mol cm-2 d-1. This differs from published accounts of nearshore marine environments, where activity of bottom-dwelling macrofauna or bubble ebullition enhances benthic fluxes of dissolved nutrients of gases 2-4 times. Millimeter depth-scale profiles of porosity, organic C, carbonate C, and bacterial abundance are reported to provide additional constraints on interface processes, including the relative effects of organic matter degradation and bioturbation. The highest organic C, bacterial, and porosity values are in the uppermost sediments (0-0.2 cm). Within the stratum 0-1 cm, porewater flux ratios of oxygen to nitrate support a model of exchange by diffusion leading to organic matter oxidation and nitrification according to Redfield stoichiometry. Below the oxygen reduction zone, changes in the porewater concentrations of NO3-, Mn2+, Fe2+, NH4+, and alkalinity indicate that anaerobic respiration reactions using NO3-, MnO2, Fe2O3, and SO42- as electron acceptors complete the early oxidation of organic matter at this site.