Abstract:
The Eocene-Oligocene interval represents a transition from global "greenhouse" to "icehouse" conditions with significant cryospheric growth and paleoceanographic reorganization, likely related to drawdown of atmospheric carbon dioxide and tectonic isolation of Antarctica. Benson and co-workers (1975, 1990), using 1-Myr sampling intervals, documented the global deep-sea ostracode response to this transition to be decreased abundance, increased diversity, and decreased provinciality.To better understand the tempo and mode of such changes in the Southern Ocean, an important region for thermohaline circulation, high-resolution ( approximately 22 kyr) faunal analyses were conducted on Eocene-Oligocene (34.8-32.8 Mya) ostracodes from ODP Site 744 on the Kerguelen Plateau. This time interval coincides with a major positive step in oxygen isotopes representing significant cryospheric growth and deep-water cooling of at least 3-4 degrees C (Zachos et al., 1996). Ostracode abundance is greatest prior to the oxygen isotope event, decreases rapidly at its onset, and then increases towards initial abundances during the following approximately 500 kyr. The fauna is initially predominated by "Protoechinocytheris", but progressively shifts to one predominated by "Krithe", a cosmopolitan cryophile. Some taxa (e.g., "Cytheropteron") are transiently abundant during the event and faunal eveness increases slightly afterwards, with the appearance of "Wichmannella". These faunal responses attest to the sensitivity of deep-sea ostracodes to ambient bottom-water cooling and possible surface productivity changes. Ongoing morphometric analyses of selected taxa will examine the effect of these environmental changes on valve formation and the degree to which morphologic trends were transient versus retained.