OCS Study MMS 95 - Data Center

observed .Extractable hydrocarbons from sediment samples at the site were heavilybiodegraded (MacDonald, unpubl . data) .Surface slicks at the Green Canyon 321 site were evident in all of the remotesensing data sets .We traversed the sea floor below these slicks repeatedly duringmore than 6 hours of submarine exploration that covered depths from 750 to 900 m.Occasional indications of oil seepage, i.e .mats of Beggiatoa bacteria and outcrops ofauthigenic carbonate, were observed, but neither significant aggregations ofchemosynthetic fauna nor other definitive sea floor indications of macro-seepagewere observed .2 .4 .4 Estimates of Total SeepageEstimating the magnitude of seepage required to produce slicks of a givenlength requires knowledge of the detectable thickness of the visible oil slick, the rateat which it advects away from the point at which the oil arrives at the surface, andthe rate of disappearance due to evaporation and dissolution .Direct measurementof slicks over oil seeps suggests slick thicknesses of 0 .1 to 1.0 ~tm (Allen et al . 1970) .Duckworth (personal communication 1993) dispensed a variety of crude oils onseawater and observed the coverage .Although results depended upon oil type,water cleanliness, and evaporation, generally 1 ml of oil will spread to produce astable silver-gray film that covers 10 sq m, which corresponds to a mean thicknessof 0.1 gym .The stable film degraded to an invisible film that suppressed capillarywaves and enhanced reflectivity, and that occupied an area consistent with athickness of approximately 0 .01 gym .Slick thicknesses required to produce therainbow sheen and mousse reported during the sea-truth trials probably exceeded 1gm ;however, these observations were restricted to small areas within the overallslicks, and the slicks themselves contained patches of un-oiled water.A thicknessof 0 .01 gm is therefore a reasonable lower limit for a detectable slick .2-16