The carbon cycle in an anoxic marine sediment: Concentrations, rates, isotope ratios, and diagenetic models

Abstract

The carbon cycle in the anoxic sediments of Skan Bay, Alaska, was investigated in order to better understand the processes that control biogeochemical transformations in an organic-rich sediment environment. Depth distributions of concentration and $\delta\sp{13}$C were determined for five major carbon reservoirs: methane (CH$\sb4$), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), particulate inorganic carbon (PIC), and particulate organic carbon (POC). In addition, methane oxidation and sulfate reduction rates were measured under quasi-in situ conditions using radio-tracer techniques. Diagenetic models were applied to concentration, reaction rate, and isotope ratio depth distributions and the results were integrated into a comprehensive, depth-dependent model of the Skan Bay carbon cycle that considered advective, diffusive, and biological and chemical reactive fluxes for the five major carbon reservoirs. The Skan Bay carbon cycle is fuelled by POC, which is deposited at the sediment surface at a rate of 2290 $\pm$ 480 umol $\cdot$ cm$\sp{-2}$ $\cdot$ yr$\sp{-1}$. Isotope mass-balance calculations indicate that about 60% of this material is derived from kelp while the remainder originates as phytoplankton. About 60% of the organic matter is consumed in the upper 40 cm of the sediment column. The $\delta\sp{13}$C-POC and $\delta\sp{13}$C-DOC depth distributions suggest that the material derived from kelp is more labile, accounting for greater than 60% of the total POC consumption. The products of anaerobic metabolism of POC accumulate in the DOC reservoir creating a large DOC concentration gradient at the sediment-water interface. Flux and stable carbon isotope mass-balance calculations suggest that a sizable portion (30 to 80%) of the DOC produced by degradation of POC diffuses from the sediment prior to oxidation to dissolved inorganic carbon. Methane production appears to occur primarily at depths greater than 40 cm. The CH$\sb4$ diffuses upward and is almost quantitatively oxidized to DIC in a narrow subsurface zone. Methane oxidation accounts for only 20% of the DIC production, but exerts a profound influence on the $\delta\sp{13}$C-DIC profile, contributing to the distinct mid-depth minimum. Pore waters are supersaturated with respect to calcite at depths greater than 10 cm, but isotope mass-balance considerations indicate that carbonate mineral formation is not occurring in these sediments.