EOCENE-OLIGOCENE CLIMATE TRANSITION IN THE SUBTROPIC NORTH AMERICA

The Eocene-Oligocene (E-O) climate transition (ca. 34 Ma) marks the shift from “greenhouse” to “icehouse” climate conditions, which is characterized by global cooling and expansion of the Antarctic ice sheet. The paleoclimatic studies regarding the E-O transition (EOT) have triggered a lot of interests because this global cooling event was related to the first development of ice cap on the Antarctic and global mass extinction. Marine records indicate 3-5 ºC ocean cooling with ~1.5 ‰ positive shift of benthic foraminifera δ¹⁸O during the EOT (~400 kyr), while terrestrial records from mid-high latitude regions captured 7-9 ºC cooling in air temperature in continental interior across E-O boundary. However, due to the paucity of paleoclimatic records and incomplete preservation of sedimentary successions, hydroclimatic and vegetational changes responding to this greenhouse-icehouse transition in the low-latitude terrestrial areas have not been explored. Here, we reconstruct the temperature and hydroclimatic change in the southern United States across the E-O boundary by studying compound-specific carbon and hydrogen isotopes (δ¹³C_n-alk and δD_n-alk) of long-chain n-alkanes and TEX₈₆ sea surface temperature (SST) of a well-dated core in the southern Mississippi. Long-chain n-alkanes (n-C27 and n-C29) with an odd-over-even predominance typically derive from terrestrial higher plants. δ¹³C_n-alk values record a negative excursion of ~ 2 ‰ during the EOT which is synchronous with Oi-1 event indicated by benthic foraminifera δ¹⁸O and ~5 °C cooling in SST recorded by TEX₈₆ SST. δD_n-alk values during the EOT are more negative than today’s δD_n-alk values in the southern US. More depleted precipitation in the southern US during EOT could have resulted from enlarged meridional temperature gradient under cooler climate, which increases isotopic distillation in the low latitudes.