The difference in carbon-13 (C-13) contents of hopane and sterane biom
arkers in the Monterey formation (Naples Beach, California) parallels
the Miocene inorganic record of the change in O-18 (deltaO-18), reflec
ting the Miocene evolution from a well-mixed to a highly stratified ph
otic 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 Miocen
e. In contrast, sulfur-bound C35 hopanes (likely derived from bacteria
l plankton living at the base of the photic zone) have systematically
decreasing C-13 concentrations in Middle and Late Miocene samples (del
taC-13 = -29.5 to -31.5 per mil), consistent with the Middle Miocene f
ormation of a carbon dioxide rich cold water mass at the base of the p
hotic zone.