A MOLECULAR STABLE CARBON-ISOTOPE STUDY OF ORGANIC-MATTER IN IMMATUREMIOCENE MONTEREY SEDIMENTS, PISMO BASIN

The 300 m section of the Miocene Monterey Formation outcropping at She ll Beach (Pismo basin; ca. 15-11 Ma) is composed of calcareous phospha tic (15.1-14.5 Ma) and siliceous facies (14.5-11.0 Ma). An objective o f this paper is to document lateral paleoenvironmental changes in the Miocene Monterey Formation by comparing the Shell Beach (SE) profile w ith the Naples Beach (NB) section in the Santa Barbara-Ventura basin ( Schouten et al.. 1997) which is similar to 80 km to the south. Eight s amples (one sample representing, on average, a time period of ca. 2000 y) from this section were analyzed for variations of extractable biom arkers and their carbon isotopic signatures as indicators for paleoenv ironmental change during the Miocene. Saturated hydrocarbons present i nclude 28,30-dinorhopane, phytane, n-alkanes (C-17-C-31), lycopane, an d 17 beta,21 beta(H)-homohopane. The biomarkers released after desulfu rization of the polar fractions predominantly consist of phytane, 2,6, 10,14-tetramethyl-7-(3-methylpen tyl)pentadecane, C-17-C-31 n-alkanes, regular 5 alpha- and 5 beta-steranes, dinosteranes, and (22R)-17 beta ,21 beta(H)-pentakishomohopane. Steranes have similar carbon isotopic compositions (-25 to -27 parts per thousand) throughout the section an d are isotopically similar at both sites, indicating laterally similar and vertically stable environmental conditions for algae living in th e upper part of the photic zone. Free and S-bound n-alkanes at SE main ly originate from marine organisms and not from terrestrial sources as in the NE section. S-bound pentakishomohopane is ca. 1-4 parts per th ousand depleted compared to the steranes and is thought to be derived from the deeper water dwelling cyanobacteria. These findings are consi stent with stable carbon isotopic data obtained for these compounds Fr om Middle Miocene Monterey sediments at Naples Beach and indicates sim ilar environmental conditions in the depositional environments of the Santa Barbara-Ventura and the Pismo basin. S-bound highly branched iso prenoids have, at both sites, different isotopic compositions indicati ng the presence of different diatom species, special growth conditions , or different bloom periods in the Pismo basin. The carbon isotopic c omposition of 28,30-dinorhopane shifts to more depleted values up sect ion, suggesting that the dinorhopane-synthesizing organism or organism s live on CO2, which is isotopically influenced by methane production and oxidation. The C-31 hopane is enriched by 1-4 parts per thousand i n C-13 compared to other hopanes and steranes. Specific bacteria, poss ibly heterotrophs, may have been the organisms producing this compound . Copyright (C) 1997 Elsevier Science Ltd.