Abstract
The 300 m section of the Miocene Monterey Formation outcropping at Shell Beach (Pismo basin; ca. 15-11 Ma) is composed of calcareous phosphatic (15.1–14.5 Ma) and siliceous facies (14.5–11.0 Ma). An objective of this paper is to document lateral paleoenvironmental changes in the Miocene Moneterey Formation by comparing the Shell
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Beach (SB) profile with the Naples Beach (NB) section in the Santa Barbara-Ventura basin (Schouten et al., 1997) which is ~80 km to the south. Eight samples (one sample representing, on average, a time period of ca. 2000 y) from this section were analyzed for variations of extractable biomarkers and their carbon isotopic signatures as indicators for paleoenvironmental change during the Miocene. Saturated hydrocarbons present include 28,30-dinorhopane, phytane, n-alkanes (C17---C31), lycopane, and 17,21(H)-homohopane. The biomarkers released after desulfurization of the polar fractions predominantly consist of phytane, 2,6,10,14-tetramethyl-7-(3-methylpentyl)pentadecane, C17---C31n-alkanes, regular 5- and 5-steranes, dinosteranes, and (22R)-17,21(H)-pentakishomohopane. Steranes have similar carbon isotopic compositions (−25 to −27‰) throughout the section and are isotopically similar at both sites, indicating laterally similar and vertically stable environmental conditions for algae living in the upper part of the photic zone. Free and S-bound n-alkanes at SB mainly originate from marine organisms and not from terrestrial sources as in the NB section. S-bound pentakishomohopane is ca. 1–4‰ depleted compared to the steranes and is thought to be derived from the deeper water dwelling cyanobacteria. These findings are consistent with stable carbon isotopic data obtained for these compounds from Middle Miocene Monterey sediments at Naples Beach and indicates similar environmental conditions in the depositional environments of the Santa Barbara-Ventura and the Pismo basin. S-bound highly branched isoprenoids have, at both sites, different isotopic compositions indicating the presence of different diatom species, special growth conditions, or different bloom periods in the Pismo basin. The carbon isotopic composition of 28,30-dinorhopane shifts to more depleted values up section, suggesting that the dinorhopane-synthesizing organism or organisms live on CO2, which is isotopically influenced by methane production and oxidation. The C31 hopane is enriched by 1–4‰ in 13C compared to other hopanes and steranes. Specific bacteria, possibly heterotrophs, may have been the organisms producing this compound.
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