Small mollusks and big cooling

Abstract

The early Eocene was a relatively warm period during with large numbers of tropic and warm-water biotas. Global surface temperature decreased through the Middle Eocene and into the Late Eocene by 4 to 5 degrees (Prothero et al, 1994). At the end of the Late Eocene a much more rapid cooling took place, the Eocene Oligocene transition (EOT). During the EOT a stepwise increase in δ18Ο occurred (Lear et al, 2008). The buildup to the most positive oxygen isotope values took two steps, each of 40 kyr and with a 200 kyr interval. The first increase in δ18Ο was caused by cooling and the second by an increase in ice volume and a drop in sea level. After the EOT, δ18Ο had increased by 1‰ and the temperature had dropped by 4 degrees (Pearson & Coxall, 2007). The Eocene Oligocene boundary (EOB) itself is defined by the extinction of the foraminifer Hantkeninia, which went extinct during the 200 kyr interval (Berggen et al, 1995). Also, re-organization of continents coincide with changes in ocean circulation and the thermal isolation of Antarctica(Pearson & Coxall, 2007). These changes might have caused the increase in oceanic mixing and higher nutrient availability. During the Eocene several extinction events can be distinguished, one at the end of the Middle Eocene, one during the Late Eocene and one during the EOT (Hansen et al., 1987). This research focuses on the EOT extinction. Some of these EOT extinctions have already been described, for example the extinction of certain planktonic foraminifera (Cotton & Pearson, 2011) and certain nanofossils (Dunkley Jones et al, 2008). The response of mollusks to the EOT however has not been extensively studied.