Natural supply of oil-derived hydrocarbons into marine sediments along the California continental margin during the late Quaternary

Sediment samples from three Ocean Drilling Program (ODP) Leg 167 sites, representing a south–north transect along the California continental margin and covering the last 160 ky—with selected samples extending in age to 950 ka—were analyzed for their bulk and molecular biomarker compositions. The aliphatic/alicyclic hydrocarbon fractions revealed clear evidence of a largely uniform natural supply of petroleum hydrocarbons during the entire time period. The mature fossil hydrocarbons in the sediments from the southern-central and central California continental margin can be related to the oil-prone rocks of the Monterey Formation in that area. The petroleum hydrocarbon signal on the northern California continental margin differs significantly from that of the two southern sites indicating another, presumably more northern source of the oil-type hydrocarbons. Their distribution pattern matches those of Eel River basin oils and, thus, suggests these oils or their source rocks to be the most likely origin of the hydrocarbons.