Reconciling measured and predicted fluxes of oxygen across the deep sea sediment-water interface1

Rates of sediment community oxygen consumption determined in situ are compared to fluxes predicted from oxygen microelectrode gradients measured in cores from -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, 0.2Ot-0.02 pmol cm-2 d-l. This differs from published accounts of nearshore marine environments, where activity of bottom-dwelling macrofauna or bubble ebullition enhances benthic fluxes of dissolved nutrients or 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 (O-O.2 cm). Within the stratum O-l 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 N03-, Mn2+, Fe2+, NH4+, and alkalinity indicate that anaerobic respiration reactions using N03-, MnO,, Fe,O,, and S042as electron acceptors complete the early oxidation of organic matter at this site. Fluxes of oxygen across sediment-seawater boundaries can serve as sensitive measurements for predicting rates of benthic organic matter oxidation. In deep sea environments, oxygen fluxes have been determined directly with in situ chambers (e.g. Smith 1978; Smith et al. 1978, 1979; Hinga et al. 1979) and indirectly with porewater profiles and Fick’s law of diffusion (Murray and Grundmanis 1980; Goloway and Bender 1982; Jahnke et al. 1982; Reimers et al. 1984). Both techniques have been compared at a common location for determinations of the sediment-water exchange of methane, radon, ammonium, phosphate, C02, manganese, and silicate (McCaffrey et al. 1980; Klump and Martens 198 1; Jahnke et al. 1983; Andrews and Hargrave 1984), but not for the exchange of oxygen. When measured and predicted fluxes of exchangeable porewater constituents have not agreed in previous studies, both biochemical and systemic causes have been suspected. For the case of rates of sea floor oxygen utilization, biological or chemical ’ Support for this research was provided by NSF grants OCE 81-17661 and OCE 83-15306. processes which consume oxygen in tubes, burrows, or a very thin surface layer could lead to an excess flux that is not gradient supported. Likewise, if sampling methods disturb the natural system, or assumptions made in flux determinations are incorrect, measured or predicted fluxes may be systematically in error. Our major objective here was to compare in situ measurements of oxygen fluxes with predicted fluxes derived from detailed microelectrode oxygen gradients in the top centimeter of sediments from a deep Pacific Ocean site. The site chosen was in an abyssal plain region at the base of the Patton Escarpment. We thank W. Stockton, D. Wahlberg, M. Laver, N. Brown, J. Edelman, and R. Wilson for assistance at sea with samples and equipment. A. A. Yayanos and J. Gieskes were generous with laboratory space and equipment, and T. Shaw provided advice on techniques for porewater analyses. Reviews of the manuscript were made by R. Jahnke, F. Sayles, D. Craven, S. Emerson, and W. Wakefield. Materials and methods In situ jlux measurements, sediment and water samples-The Patton Escarpment site