Biogeochemical cycling in an organic-rich coastal marine basin 4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis

In situ carbon flux measurements and calculated burial rates are utilized to construct an organic carbon budget for the upper meter of sediment at a single station in Cape Lookout Bight, a small marine basin located on the Outer Banks of North Carolina, U.S.A. (34°37′N, 76°33′W). Of 149 ± 20 mole · m −2 · yr −1 of total organic carbon deposited, 35.6 ± 5.2 mole · m −2 · yr −1 is recycled to overlying waters, 84 ± 18% as ∑CO 2 and 16 ± 8% as CH 4. Approximately 68 ± 20% of the upward carbon flux is supported by sulfate reduction while 32 ± 16% takes place as the result of underlying methanogenesis. Measured ∑CO 2 and CH 4 sediment-water fluxes range seasonally from 1900–6300 and 50–2500 μmole · m −2 · hr −1 respectively. The mean residence time of metabolizable organic carbon in the upper 80 cm of sediment is approximately four months with greater than 98% of the calculated total remineralization taking place within three years. In spite of large upward fluxes of methane, larger molecules derived from metabolizable sedimentary organic carbon appear to be the dominant reductants for dissolved sulfate.