The primary controls on the quantity of methane found in sediments are the rates of production and the rates of diffusion. In Santa Barbara basin sediments, the diffusion rate of methane is found to be very slow compared to the rate of bacterial production. The production rate correlates with the amount of organic matter in the sediment. Thus, the quantity of methane at any depth in the sediment is a function of the amount of marine organic matter initially trapped in the sediment.

Sediment cores show the concentration of SO₄ decreasing from 27.6 mM at the surface to zero below 2 m. The methane concentration is < 0.3 mM in the upper 2 m, increases to 12.3 mM at 3.8 m, then decreases and fluctuates. The production rate of HCO₃ decreases from 1.5 × 10^-4 mmol/cu cm/year at the surface to less than 10^-7 mmol/cu cm/year at 9 m. The production rate of HCO₃ fluctuates in direct correlation with organic carbon content. Methane production decreases in a similar manner. The ^dgr¹³C distribution of biogenic methane variesfrom -92.8 to -23.6 ppt. Heavy biogenic methane occurs in the upper sulfate-reducing zone and may result from the preferential anaerobic oxidation of light methane by sulfate-reducin bacteria. The diffusive flux of methane into the sulfate-reducing zone is between 8.6 × 10^-5 mmol/sq

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cm/year and 1.75 × 10^-7 mmol/sq cm/year. The diffusion coefficient in this zone is in the range of 2.8 × 10^-7 to 4.2 × 10^-7 sq cm/sec. Below 4 m the diffusion coefficient was less than 5.4 × 10^-8 sq cm/sec.

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