OSTRACODE BODY SIZE CHANGES ACROSS AN EOCENE HYPERTHERMAL EVENT

Understanding the many ways that rapid climatic changes can impact biotic systems is increasingly important for evaluating what changes we can expect as we face changing climates today. Climatic conditions in the late Paleocene and early Eocene were marked by multiple global hyperthermal events that were characterized by negative carbon isotope excursions and rapidly rising temperatures. The second of these events, Eocene Thermal Maximum 2 (ETM2, ~54 Ma) occurred about 2 million years after the Paleocene-Eocene Thermal Maximum (PETM) and lasted about half as long. ETM2 has been identified in records around the world and is distinguished by carbon and oxygen isotope excursions roughly half the magnitude of the PETM.

Ostracodes (small, bivalved crustaceans) represent an opportunity to examine the response of multicellular organisms to environmental changes associated with this smaller hyperthermal event. This study investigates changes in the body sizes of four ostracode species from two different genera (Neonesidea and Krithe) at two deep marine sites (ODP Site 1258 in the equatorial Atlantic and IODP Site U1409 in the north Atlantic) from before, during, and after ETM2. At both sites, all four species show a decrease in average body size during ETM2 relative to before the event. Following ETM2, ostracodes from three of the four species at each site show an increase in average body size relative to body size during ETM2, although one species at each site continues to decrease in size. While these changes in body size correlate with the temperature changes associated with the ETM2 hyperthermal event as determined by oxygen isotopes, they may also be related to other factors associated with the event. Regardless, the changes observed as a result of this relatively smaller event indicate that ostracodes may be adversely impacted by even small changes to their environments.