Stable isotope ratios of benthic and planktonic foraminifera from Late Miocene sediments of DSDP Hole 24-238 on the Central Indian Ocean Ridge (Appendix 1)

Tropical Indian Ocean planktonic and benthic foraminifera have 13C/12C ratios which change abruptly within Magnetic Epoch 6 about 6.2 million years ago. All species analyzed in the Late Miocene section of DSDP Site 238 show a shift towards lighter values of delta13C by about 0.8‰. The oxygen isotope signal indicates that the pre-shift period is climatically quiet while the post-shift period has strong fluctuations. We suggest that the shift reflects a sudden increase in the rate of supply of organic carbon from coastal lowlands and from shelves exposed by regression, as well as a change in deep circulation patterns and ocean fertility. The event marks the transition of the ocean-atmosphere system from a quiet Early and Middle Neogene climatic regime toward a Late Neogene regime characterized by climatic amplifying mechanisms (albedo feedback, bottom water production) located around the northern North Atlantic. The beginning of this regime may have been strongly influenced by the isolation of the Mediterranean basin.