RECONSTRUCTING PALEOCENE-EOCENE PATAGONIAN PALEOCLIMATE: ORGANIC PROXY DATA FROM THE LIGORIO MARQUEZ FORMATION

The early Eocene is considered to have been a generally warm period punctuated by hyperthermal events; however, little is known about conditions in southern hemisphere high-latitude terrestrial environments, such as Chilean Patagonia, during this time. For this study, 22 organic-rich mudstones were collected from a roughly 50-meter interval of the Ligorio Márquez Formation near Lago Jeinimeni in the central region of Chilean Patagonia (S 46.8°, W 71.8°). Previous work using fossil leaves and regional stratigraphic correlations indicates a middle Paleocene to early Eocene age. This age is confirmed by U/Pb detrital zircon dating performed for this study, which yields a range of minimum ages from 62 Ma to 50 Ma through the section. The variable sedimentology at the site, from red mudstone to increasingly common coal beds interspersed with sandstone channels near the top, suggests a shift in paleoenvironment across the formation. Here we present data from a suite of lipid biomarkers to test for the presence of a hyperthermal event at this site. Leaf wax n-alkane carbon chain lengths show strong odd-over-even preference and long average chain lengths, suggesting minimal thermal degradation and the dominance of terrestrial organic matter. In addition, we report the carbon (δ¹³C) and hydrogen (δD) isotopic compositions of higher land plant n-alkanes (C₂₅, C₂₇, and C₂₉). δ¹³C measurements show the carbon isotope history of the region from the middle Paleocene to the early Eocene, while δD measurements reflect the site’s paleoprecipitation isotopic composition after the fractionating effects of evapotranspiration and biosynthetic pathways. MBT’-CBT mean annual temperature (MAT) estimates based on branched GDGTs formed by soil bacteria indicate warm MATs that are broadly consistent with previous leaf physiognomy-based estimates. Together, these organic proxies offer a window into the poorly characterized paleoclimate history of Early Cenozoic Patagonia, providing constraints on both its mean state and, potentially, hyperthermal intervals.