δ 13C, TCO 2, and the metabolism of organic carbon in deep sea sediments

Abstract

The stoichiometry and rates of benthic sediment metabolism at five stations have been determined on the basis of the carbon released to the pore waters as TCO 2. The stations cover a range of redox conditions permitting an evaluation of stoichiometry in oxic environments and under conditions where both denitrification and Mn reduction occur. At four of the five stations the flux of TCO 2 from the sediment is significantly enriched in 13C relative to predictions from traditional Redfield ratio based stoichiometry. The Ca 2+:TCO 2 stoichiometry under oxic conditions also appears to depart systematically from predicted values, being consistently lower than model calculations. The heavy isotopic composition of the flux can be explained in two ways. The TCO 2 derived from organic matter may be fractionated relative to the organic matter intercepted in sediment traps set in the general areas studied. Alternatively, the heavy isotopic composition of the flux may result from the neutralization of as much as one-third of the metabolic acid produced in diagenesis by CO 2− 3 diffusing into the sediment from the overlying water. Rates of sub-oxic diagenesis, determined from flux estimates across redox boundaries, are usually small compared to oxic rates. However, at the most reduced stations studied sub-oxic rates are quite substantial, accounting for up to 30 to 45% of the total TCO 2 flux.