A Molecular Organic Carbon Isotope Record of Miocene Climate Changes

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The difference in carbon-13 (13) C) contents of hopane and sterane biomarkers in the Monterey formation (Naples Beach, California) parallels the Miocene inorganic record of the change in (18) O (delta18 O), reflecting the Miocene evolution from a well-mixed to a highly stratified photic zone (upper 100 meters) in the Pacific. Steranes (delta13 C equals 25.4 plus minus 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 decreasing13 C concentrations in Middle and Late Miocene samples (delta13 C equals minus 29.5 to minus 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.

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