ABRUPT TEMPERATURE DROP ACROSS THE EOCENE-OLIGOCENE TRANSITION, CENTRAL NORTH AMERICA

We investigated the timing and magnitude of the continental climate change across the Eocene-Oligocene transition (EOT) by analyzing the carbon and oxygen isotope composition of fossil bones and teeth collected from the White River sediments of the northern Great Plains (USA). The analyzed samples include ~200 bone fragments from various mammalian taxa and turtles, and a total of 66 teeth of four different mammalian taxa. Our results suggest an abrupt, moderate mean annual temperature drop and no change in temperature seasonality and aridity across the transition.

Tooth enamel is resistant to isotopic exchange and therefore maintains the in vivo signal. With respect to δ¹⁸O, enamel composition is principally determined by rainwater composition. In contrast, bone mineral recrystallizes on time scales of 20-50 kyr and assumes the composition of the diagenetic environment. In a pure C3 ecosystem, bone δ¹³C mainly tracks changes in aridity. Instead, soil water composition and soil temperature determine bone carbonate δ¹⁸O. Hence, we used bone δ¹³C to investigate changes in relative humidity across the EOT and combined δ¹⁸O values of tooth enamel and bone carbonate to distinguish changes in water composition from changes in temperature.

For all the taxa, late Eocene and early Oligocene enamel average δ¹⁸O values are indistinguishable suggesting no change in rainwater composition across the EOT. In addition, Eocene vs. Oligocene ranges in enamel δ¹⁸O are similar, indicating no significant change in temperature seasonality. The range in isotope composition exhibited by Mesohippus (an ancestral horse) provides an estimate of Eocene and Oligocene MART (26±6°C).

With respect to bones, the indistinguishable Eocene vs. Oligocene average δ¹³C (~ -7.7‰) indicates no change in aridity across the EOT. In contrast, the δ¹⁸O record exhibits a statistically significant, abrupt (£400 kyr), 1.6‰ increase across the transition (23.0‰ vs. 24.6‰; VSMOW). This increase translates into a MAT drop of 8.2±3.1°C. The isotopic shift in our continental record seems to lag the marine transition by 0-400 kyr possibly indicating a decoupling of the two components of the climate system during periods of abrupt global change.