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