NEW EVIDENCE FOR PRE-CIE WARMING DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

Extreme global warming occurred during the Paleocene-Eocene Thermal Maximum (PETM), which is marked by a negative carbon isotope excursion (CIE). The most widely cited cause for the CIE is the release of CH⁴ from marine methane clathrates, which would have been highly depleted in ¹³C. According to this hypothesis, warming of ca. 4°C of deep ocean waters was required for methane release. Previous marine studies have suggested pre-CIE warming of 2 to 3°C, but others have attributed this to background variability. We present evidence from the continental record that a major pulse of warming preceded the CIE, based on stable oxygen isotope ratios (d¹⁸O) in mammalian tooth enamel. d¹⁸O in tooth enamel reflects the composition of surface water, which is strongly influenced by that of precipitation. d¹⁸O in modern precipitation is strongly correlated with mean annual temperature (MAT), but this relationship is dependent on the latitudinal temperature gradient, which was probably lower in the early Paleogene. We calibrated d¹⁸O/MAT slopes specific to the Bighorn Basin using published MAT estimates from leaf-margin analyses, published d¹⁸O values from hematite, and new d¹⁸O values from early Eocene mammals. We reduced interspecific variability by sampling teeth from a single genus (Phenacodus) and reduced intertooth variability, resulting from tooth formation in different seasons, by sampling a single tooth position. Our results suggest an early Paleogene d¹⁸O/MAT slope that was lower than the modern slope. This approach yields an estimated total increase in temperature of 9 to 10°C during the PETM, which is greater than previous continental estimates. Warming occurred in two pulses. The first occurred in the 150 to 15 kyr interval directly before the CIE. The second was probably coincident with the release of green house gases associated with the CIE. These results corroborate the hypothesis that warming preceded the CIE, resulting in the release of ¹³C-depleted greenhouse gases that further amplified warming.