High-resolution (10(4) years) deep-sea foraminiferal stable isotope records of the Eocene-Oligocene climate transition

We have constructed high-resolution (10(4)-10(5) years) benthic foraminiferal delta(13)C and delta(18)O records for the upper Eocene through lower Oligocene of two pelagic sequences, Deep Sea Drilling Project (DSDP) Site 522 in the Angola Basin, South Atlantic Ocean, and Ocean Drilling Program (ODP) Site 744 in the southern Indian Ocean. These records provide improved constraints on both the timing and magnitude of marine oxygen and carbon isotope events from 30 to 35 Ma. The oxygen isotope records indicate that the ubiquitous delta(18)O increase (Oi-l), which marks the rapid expansion of continental ice sheets and a minimum of 3 degrees to 4 degrees C of cooling of bottom waters in the earliest Oligocene (33.6 Ma), occurred in <350 kyr. More than half the transition occurred over the final 40-50 kyr. This period of lower temperatures and widespread continental glaciation persisted for roughly 400 kyr (i.e., the duration of magnetochron C13n). These records also indicate that this interval was characterized by at least two similar to 100-kyr waxing and waning cycles (Oi-la and Oi-lb) and possibly several higher-frequency events. The benthic foraminiferal delta(13)C records show a positive 0.8 parts per thousand excursion that is nearly isochronous with the Oi-1 oxygen isotope increase. Similar magnitude delta(13)C increases at other sites indicate this was a global phenomenon suggestive of an unusually large perturbation to the carbon cycle, This excursion was followed by smaller amplitude delta(13)C oscillations with periods of roughly similar to 400 kyr. We suspect that the ubiquitous Oi-l delta(13)C excursion resulted from a brief but substantial increase in export production and carbon burial.