The bulk organic matter in sediments from the upper Pleistocene Mississippi Fan and two intraslope basins is primarily terrestrial in origin. Carbon to nitrogen elemental ratios, δ 13 C of the organic matter, the presence of plant biowaxes, and lithologic associations form the basis of this interpretation. The molecular-level compositions of the extractable organic matter suggest significant levels of non-indigenous thermogenic hydrocarbons at all sites. The alkane hydrocarbons are composed of both biogenic (mainly plant biowaxes) and thermogenic hydrocarbons. The thermogenic hydrocarbons are primarily present as an unresolved complex mixture. Increasing amounts of hydrocarbons with depth at the Mississippi Fan sites suggest that the thermogenic hydrocarbons have migrated upward from deeper sediments. Vertical gradients in the intraslope basins were less dramatic, although an upward migration source is inferred. Upward movement of thermogenic hydrocarbons, from deeper sediments where sufficient temperatures are present to produce these hydrocarbons, appears to be a regional phenomenon given the widespread occurrence at these sites.
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