Organic Geochemistry of Laminated Sediments from the Gulf of California

Abstract

Both varved (laminated) and homogenous sediments from DSDP/IPOD Sites 479 and 480 have been analyzed to examine any potential differences in the organic matter composition. The interstitial gas is composed mainly of methane and carbon dioxide and it is derived from biogenic sources in both types of sequences. The lipid composition is very similiar for both sequences and indicates a primarily autochthonous microbial origin, with some influx of allochthonous higher plant detritus. The non-laminated sections contain relatively more terrigenous plant wax detritus versus autochthonous residues and a much greater amount of perylene than the laminated sections. The laminated zones consist of rhythmic couplets of diatom ooze and terrigenous clay deposited in an oxygen minimum environment and the homogeneous zones consist of diatomaceous mud to muddy ooze with evidence of extensive burrowing deposited from a less pronounced oxygen minimum environment. The factors which resulted in these different lithologies also determined the distribution of organic facies as reflected in the kerogen, which has also been analyzed. The data indicate the presence of different types of kerogen in the oxic, homogeneous zones (low hydrogen index, high oxygen index, i.e., more oxidized organic matter) than in the anoxic, laminated zones (high hydrogen index, low oxygen index, i.e., more aliphatic and unaltered organic matter).