RECONSTRUCTING PALEOTEMPERATURE USING CLUMPED ISOTOPE PALEOTHERMOMETRY ACROSS THE K-PG BOUNDARY FROM GARFIELD COUNTY, MT, USA

Since the discovery of a bolide impact at the K-Pg boundary, scientists have debated its importance as a cause of the coincident mass extinction. Alternative or additional causes, such as sea level rise or volcanically induced climate change, have been suggested. Testing the hypothesis of climate change has been complicated by the difficulty or reconstructing paleotemperature in terrestrial settings. While there are well-established geochemical techniques for estimating paleotemperature in marine settings, the necessary assumptions about water chemistry (estimating δ¹⁸O of water) are not valid in terrestrial environments. However, clumped isotope paleothermometry can provide an estimate of temperature independent of water chemistry.

The Hell Creek and Tullock Formations of the Western Interior of the United States consist of fluvial, lacustrine and soil deposits spanning the latest Cretaceous (Maastrichtian) through lower Paleogene (Danian). This interval is exposed in Garfield County, MT in largely flat-lying beds. The lacustrine and fluvial deposits preserve abundant freshwater mussels (Unio). These fossils preserve their original aragonite mineralogy (confirmed by XRD without detectable calcite), suggesting that no material alteration has occurred. Trace element data for these shells are also within acceptable published ranges. Six stratigraphic horizons, four below the K-Pg boundary and two above, were analyzed in an attempt to reconstruct paleotemperature across the K-Pg boundary and compare this reconstruction to estimates made using leaf margin analysis over the same interval.

Preliminary data (39 analyses on 24 samples) suggest that the clumped isotopic technique is effective in recording growth temperatures of aragonitic organisms of Cretaceous age. A consistent temperature (26 ± 4°C) was recorded throughout the interval, despite substantial variations (~8‰) in the δ¹⁸O of water. We attribute the consistency of recorded paleotemperature throughout the studied interval to the tendency of Unio to grow at a preferred temperature, muting any signal due to climatic variability. Late Maastrichtian cooling (~5°C) is observed, though not statistically significant, and we intend to further test this observation.