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 p CO 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 NO 3 − , alkalinity and TCO 2 , and sediment organic C and CaCO 3 profiles. A numerical model encompassing diffusion, advection, and multiple reaction terms is applied to these data to characterize organic carbon degradation and CaCO 3 dissolution processes near the sediment-water interface. A benthic O 2 flux of 58 μmol 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 × 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 × 10 −9 s −1 ). The organic C rain rate responsible for the bulk organic C is estimated to be 53 μmol cm −2 yr −1 . This estimate exceeds the average rain rate determined by sediment trap collections at this site. Saturation state profiles, calculated from the in situ pH and p CO 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 CaCO 3 -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.