A molecular organic carbon isotope record of Miocene climate changes

Abstract

The difference in carbon-13 (13C) contents of hopane and sterane biomarkers in the Monterey formation (Naples Beach, California) parallels the Miocene inorganic record of the change in 18O (δ18O), reflecting the Miocene evolution from a well-mixed to a highly stratified photic zone (upper 100 meters) in the Pacific. Steranes (δ13C = 25.4 ± 0.7 per mil versus the Pee Dee belemnite standard) from shallow photic-zone organisms do not change isotopically throughout the Miocene. In contrast, sulfur-bound C35 hopanes (likely derived from bacterial plankton living at the base of the photic zone) have systematically decreasing 13C concentrations in Middle and Late Miocene samples (δ13C = –29.5 to –31.5 per mil), consistent with the Middle Miocene formation of a carbon dioxide—rich cold water mass at the base of the photic zone.