CaCO 3 dissolution in California continental margin sediments: The influence of organic matter remineralization

Abstract

In situ benthic flux chamber and oxygen microelectrode and shipboard porewater results have been used to quantify sea floor dissolution of CaCO 3 on the continental rise adjacent to central California, USA. The porewater distributions and benthic fluxes of O 2, NO 3 −, TA, Ca 2+, δ 13C, and TIC are interpreted using a numerical simulation of organic matter remineralization and CaCO 3 dissolution in marine sediments. The processes considered in the simulation include: organic matter oxidation by O 2, NO 3, and SO 4 2−; CaCO 3 dissolution and precipitation; HS − and NH 4 + oxidation; and sediment mixing and sediment accumulation. Calculated benthic fluxes of O 2, NO 3 −, TA, TIC, δ 13C, and Ca 2+; porewater concentrations of O 2, NO 3 −, and NH 4 +; and sediment distributions of organic carbon, CaCO 3, excess 210Pb, and 14C agree well with the measurements. Benthic fluxes of alkalinity and inferred CaCO 3 dissolution rates cannot be explained on the basis of dissolution driven solely by bottom water undersaturation. If the influence of metabolically-produced CO 2 is included, benthic fluxes are fully reconciled, however. This is in agreement with benthic chamber Ca 2+ and δ 13C results that independently imply substantial CaCO 3 dissolution in these sediments. The above observations are in contrast to those reported by Jahnke et al. (1994) for the west African continental rise and the western equatorial Pacific where 1-G diagenetic models predict dissolution fluxes larger than observed with benthic flux chambers. We conclude that the extent of metabolic CaCO 3 dissolution may vary regionally. Numerous factors, such as the depth of metabolic CO 2 production and CaCO 3 dissolution kinetics, are known or predicted to influence metabolic dissolution. Among the factors that should be considered in reconciling these observations are: (l) the extent to which sulfate reduction and reoxidation reactions may influence acid-base properties in surface sediments and (2) the total sedimentary CaCO 3 content of the sites that may influence porewater acidity through surface exchange reactions at the mineral surface.