Sediment Hydrocarbons

Heavy hydrocarbons are present in sediment at the seeps as well as in surficial shelf sediment obtained by conventional box coring. Aliphatic and aromatic hydrocarbons have been identified in eight sediment samples from the site of the seeps and in seven samples of surface sediment collected at or near the head of Soquel Canyon, one of the subsidiary canyons of the Monterey Canyon complex.

Aliphatic hydrocarbons at the seeps and at Soquel Canyon include acyclic and cyclic compounds. Total concentrations of acyclic hydrocarbons range from about 1 to 3 mg/g. Distributions of these hydrocarbons are similar in samples from both areas and are believed to be mainly autochthonous. The n-alkanes range from n-C15 to n-C36 with n-alkanes of lower molecular weight likely derived from aquatic sources (odd- and even-carbon-number molecules equally abundant) and with n-alkanes of higher molecular weight from terrigenous sources (predominance of odd-carbon-number molecules).

Cyclic terpanes also have similar distributions in all samples; immature biogenic molecules are present, and concentrations are low. Of particular interest, however, is the ubiquitous presence of 28,30-bisnorhopane and oleanane. These are compounds which have been commonly identified in Monterey Formation petroleum and rock extracts. The presence of these terpanes in surficial sediment of Monterey Bay may signal a connection with the Monterey Formation which is present in the subsurface and in some surface outcrops in parts of Monterey Bay.