The distributions of the concentrations of methane, ethene, ethane, propene, and propane in twelve 1-to 2-m-long gravity cores for two transects from nearshore to midslope off the southwest Texas Gulf Coast are reported. Methane profiles exhibit maxima in the top 40 cm of sediment on the shelf, in contrast to downward increasing gradients in the slope region. Nearshore surface methane concentrations ranging from 50 to 400 μl (normal temperature and pressure) per liter pore water are apparently due to microbial production in sulfate-free microenvironments such as fecal pellets in a near-seawater sulfate environment. A decrease in sediment methane levels to less than 5 μl/l pore water in downslope sediments is attributed to reduced microbial activity due to lower organic contents and temperatures. Profiles of the saturated and unsaturated C2 and C3 hydrocarbons suggest that these gases are also microbially produced.
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