MOLLUSCAN RESPONSE ACROSS THE EOCENE-OLIGOCENE TRANSITION: TANZANIA DRILLING PROJECT SITES 11, 12 AND 17

The Eocene-Oligocene transition (EOT) was an extended period of climatic and oceanographic change from 34-35.5 Ma, associated with widespread biological turnover in both marine and terrestrial environments. It has been possible to study this biotic response at relatively high resolution within groups such as the planktonic foraminifera, calcareous nannofossils and the larger benthic foraminifera (LBF) with an increasing number of complete or near complete sections being found. However, records for larger, multicellular organisms tend to be patchier. Previous studies have shown that molluscs are affected by the EOT, with the disappearance of many long-ranging forms in the Gulf coast and Paris basin and a large influx of European taxa to North America, potentially linked to changes in ocean circulation within the North Atlantic. An abrupt turnover is also described from the western United States, with warm water taxa declining and cold water forms expanding their ranges. However, these studies have a relatively low temporal resolution, and whilst they show the long term response and general trends, they lack detailed correlation with the global stable isotope curve and specific climatic events.

Complete sections through the EOT containing molluscs are rare, due to associated sea-level fall across the boundary and a lower preservation potential of molluscs compared to microfossils such as foraminfera. However, three Tanzania Drilling Project sites, 11, 12 and 17 contain a record of small, but exceptionally well preserved, gastropods (including pteropods) and bivalves within hemi-pelagic clays which continuously span the EOT. Extensive studies of the nannofossils, planktonic foraminifera, LBF and stable isotopes have been carried out allowing the sites to be well correlated to global bio and chemo-stratigraphy. This unique record enables the response of the mollusc assemblage to be compared with the exact timing of the events of the transition, allowing further insight into ecological changes across this major climatic event. In contrast to other records, molluscan abundance increases across the EOT, indicating a possible increase in nutrients through the transition and further suggesting that de-stratification of the water column may have occurred during this interval.