MICROELECTRODE STUDIES OF ORGANIC-CARBON DEGRADATION AND CALCITE DISSOLUTION AT A CALIFORNIA CONTINENTAL RISE SITE

Fine scale porewater profiles of resistivity, O-2, pH, and pCO(2) were measured in situ with microelectrodes at a site on the California continental rise. They are reported here with more traditional measurements of shipboard porewater NO3-, alkalinity and TCO2, and sediment organic C and CaCO3 profiles. A numerical model encompassing diffusion, advection, and multiple reaction terms is applied to these data to characterize organic carbon degradation and CaCO3 dissolution processes near the sediment-water interface. A benthic O-2 flux of 58 mu mol cm(-2) yr(-1) over the top 1 mm sediment is estimated based on one measured sub-mm depth scale O-2 profile. The numerical model indicates that this high O-2 consumption rate is related to a very labile fraction of organic C which has a degradation rate constant of approximately 4.5 x 10(-8) s(-1) (1.4 yr(-1)), and which is not mixed downward by biological mixing. Seasonality in the benthic O-2 consumption rate of this site may be attributed to variations in the input rate of this highly labile organic C. Degradation rate constants of the bulk organic C by oxygen reduction and by nitrate reduction are estimated to be about the same (1 x 10(-9) s(-1)). The organic C rain rate responsible for the bulk organic yr(-1). This estimate exceeds the average rain rate determined by sediment C is estimated to be 53 mu mol cm(-2) yr(-1). trap collections at this site. Saturation state profiles, calculated from the in situ pH and pCO(2) data and from modelling the in situ O-2 and pH data, both indicate that the porewater is undersaturated with respect to calcite to a depth of at least 8 cm. The dissolution rate constant of calcite in this CaCO3-poor station is determined to be 5-10% day(-1) (assuming the reaction order is 4.5), which is close to the minimum estimates for environments richer in calcite. A uniform calcite dissolution rate constant, therefore, may be adequate for representing carbonate dissolution in global-scale models of ocean chemistry.