NATURAL OIL-SLICKS IN THE GULF-OF-MEXICO VISIBLE FROM SPACE

Natural oil seepage in the Gulf of Mexico causes persistent surface sl icks that are visible from space in predictable locations. A photograp h of the sun glint pattern offshore from Louisiana taken from the spac e shuttle Atlantis on May 5, 1989, shows at least 124 slicks in an are a of about 15,000 km2; a thematic mapper (TM) image collected by the L andsat orbiter on July 31, 1991, shows at least 66 slicks in a cloud-f ree area of 8200 km2 that overlaps the area of the photograph. Samples and descriptions made from a surface ship, from aircraft, and from a submarine confirmed the presence of crude oil in floating sticks. The imagery data show surface slicks near eight locations where chemosynth etic communities dependent upon seeping hydrocarbons are known to occu r on the seafloor. Additionally, a large surface slick above the locat ion of an active mud volcano was evident in the TM image. In one locat ion the combined set of observations confirmed the presence of a flour ishing chemosynthetic community, active seafloor oil and gas seepage, crude oil on the sea surface, and slick features that were visible in both images. We derived an analytical expression for the formation of floating slicks based on a parameterization of seafloor flow rate, dow nstream movement on the surface, half-life of floating oil, and thresh old thickness for detection. Applying this equation to the lengths of observed slicks suggested that the slicks in the Atlantis photograph a nd in the TM image represent seepage rates of 2.2-30 m3 1000 km-2 d-1 and 1.4-18 m3 1000 km-2 d-1, respectively. Generalizing to an annual r ate suggests that total natural seepage in this region is of the order of at least 20,000 m3 yr-1 (120,000 barrels yr-1).