PAVING THE OCEAN FLOOR - ASPHALT FLOWS, GAS HYDRATE DEPOSITS, AND AUTHIGENIC CARBONATES FROM THE CAMPECHE ESCARPMENT, SOUTHERN GULF OF MEXICO

During October and November 2003, the German ship R/V Sonne visited the Campeche Escarpment in the southern Gulf of Mexico. Guided by data from satellite imagery that showed evidence for persistent oil seeps in this region, we discovered lava-like flows of solidified asphalt that cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level. The chemosynthetic community at this structure, which we named Chapopote, was extensive and diverse, including white microbial films, vestimentiferan tubeworms (cf. Lamellibrachia sp.), large bivalve shells, including the chemosynthetic family Vesicomyidae (cf. Calyptogena sp.), and chemosynthetic mussels (cf. Bathymodiolus sp. and Solemya sp.). The asphalt pieces included small fragments and large, irregular blocks weighing more than 10 kg. This material, which was brittle and had no residual stickiness, shows columnar jointing and chilled margins that indicate molten flow followed by rapid cooling. No H₂S was detected, and the presence of NO₃^– in a gradient from 14 to 4 µM over sediment depths from ~1 to 10 cm below the interface indicated that the surface sediments were oxidized. A sample taken through one of the few bacterial mats immediately adjacent to the asphalt flow contained ~20% viscous, liquid petroleum dispersed in veins and pockets; asphalt was entirely absent. A surface crust comprised slabs of authigenic carbonate with layers of oil pooled beneath. Sediments were entirely anoxic with H₂S concentrations of 8 to 13 mM. Gas hydrate formed thin layers in the surface sediments. A negative chloride anomaly (482 mM) in the upper 4 cm was consistent with gas hydrate layers. An alkalinity profile showed extremely high values from 29 to 35 mM, which indicates the oxidation of hydrocarbons by reduction of seawater sulfate. Molecular and isotopic compositions of the gas hydrate and sediment headspace from the second grab sample indicate moderately mature, thermogenic gas. Oily sediment extracts and asphalt pieces were composed of a degraded, unresolved complex mixture of hydrocarbons with a peak at n-C₃₀ and a few resolved C₂₉ to C₃₂ hopanes.