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Oxygen and carbon isotope stratigraphies are presented for planktonic and benthic foraminifera from Miocene sediments of the tropical Indian Ocean (DSDP Sites 214, 216, 237, 238) and placed into a biostratigraphic framework. Approximately 1000 foraminiferal analyses were performed to determine both oxygen and carbon isotopic ratios. The species analyzed were Globigerinoides sacculifer and Dentoglobigerina altispira (shallow-dwelling planktonic), Globoquadrina venezuelana (deep-dwelling planktonic), and Oridorsalis umbonatus (benthic). We identified a number of isotopic signals which appear to be synchronous with previously recognized signals in the Pacific Ocean, and thus provide useful tools for chronostratigraphic correlations.

The oxygen isotope record is dominated by a permanent increase in benthic foraminiferal δ18O values of 10/00 or more in the middle Miocene, between approximately 16.5 and 12.6 Ma. The 0–18 enrichment in planktonic foraminifera is less distinct and temporary. Thus these data do not unequivocally support the notion of a large ice buildup at this time. A large post-shift separation of benthic and planktonic δ18O values indicates a more stably stratified ocean, presumably with a stronger thermocline, and hence increased regional upwelling.

The carbon isotopic record is dominated by a broad early-to-middle Miocene positive excursion seen in both planktonic and benthic records. The excursion begins with a shift toward greater δ18C values by about 10/00 (the Chron-16 Carbon Shift) in the latest early Miocene between approximately 17.5 and 16.5 Ma and ends with a rather gradual decline toward initial (early Miocene) values. The excursion terminates at approximately 13.5 Ma. The next large disturbance is the Chron-6 event, with a δ13C shift to low values, by about l0/00 in all three foraminiferal records.

We propose that the Chron-16 shift, and subsequent “heavy carbon” state of the ocean, was caused by the extraction of isotopically light carbon into the organic-rich Monterey Formation and its equivalent sediment bodies rimming the Pacific. A decrease in the rate of carbon extraction (due to exhaustion of nutrient phosphorus) would have allowed relaxation toward initial values. We further propose that the Chron-6 carbon shift, toward low δ-values, was due to regression and contribution of organic matter to the ocean from erosion. This would be the reverse process from the Monterey buildup.

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