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Part of the oil now present in the world's oceans results from the direct discharge of submarine seeps into the water. Proper assessment of the present "hydrocarbon load" carried by the oceans, as well as the evaluation of the proportion contributed by other sources (both natural and by man), requires a reliable estimate of the yearly worldwide seepage rate from marine seeps. Although only a few marine oil seeps presently are known and located, all evidence suggests that with further investigation, many more will be found. In view of the limited observations, only a few cursory estimates of total worldwide annual-marine seepage have been made; there appears to be no substantiation for any of these estimates. This paper, which represents a first documented approach based o geologic considerations, presents an estimate of yearly oil input into the oceans from naturally occurring marine seepage.
A geologic model based on structural styles, recent earthquake activity, and sediment thicknesses was used to classify the continental margins into areas of high-, moderate-, and low-seepage potential. Flow-rate data for each of the 3 type potential areas were fitted to a log-normal probability distribution and seepage volumes were determined.
The probable range of seepage into the marine environment is 0.2 to 6.0 million metric tons per year. Within this range the best estimate for present marine seepage worldwide is 0.6 million metric tons per year. Based on this estimate, areas of high-seepage potential contribute about 45% of the worldwide seepage. In the Pacific Ocean, high-seep potential areas are by far the major contributors. In the Atlantic and Indian oceans, moderate-seep potential areas are most significant. The circum-Pacific area is the area of greatest seepage; it is estimated to contribute about 40% of the world's total.
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