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.
In the Campeche Knolls, in the southern Gulf of Mexico, lava-like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level. Chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt, which chilled and contracted after discharge. The site also includes oil seeps, gas hydrate deposits, locally anoxic sediments, and slabs of authigenic carbonate. Asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the Gulf of Mexico.
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