A GLOBAL TERRESTRIAL CLIMATE SHIFT AT THE EOCENE-OLIGOCENE TRANSITION: EVIDENCE FROM MID-LATITUDE SOUTH AMERICA

The Eocene-Oligocene transition (EOT) from a greenhouse to icehouse climate state and the onset of large-scale Antarctic glaciation has been widely documented geochemically in both the marine realm (e.g., Lear et al., 2008; Liu et al., 2009) and the terrestrial realm (e.g., Zanazzi et al., 2007). Nearly all of the terrestrial climate records of the EOT come from the northern hemisphere, particularly North America. These data paint a picture of rapid cooling and faunal change in many taxa, excepting mammals. The few existing terrestrial records from the southern hemisphere, however, show negligible change in the water isotope record (Kohn et al., 2010; Kohn et al., 2015), despite the proximity of South America to Antarctica. Records from Patagonia also show negligible changes in mammal and floral diversity (Strömberg et al., 2013).

Here we present the first evidence for significant climate change at a terrestrial site in southern hemisphere during the EOT. Analyses of volcanic glasses from the Vera Member of the Sarmiento Formation in the well-dated (Dunn et al., 2013) sedimentary section at Gran Barranca (~45°S) allow us to reconstruct the water isotope record in central Patagonia during the EOT. These data show a drop in precipitation δD of ~20‰ over ~100-200 kyr followed by a recovery of ~15‰ over the next 0.5-1.0 Myr. This pattern of a rapid shift and a more gradual recovery fits the shape and time scale of the northern hemisphere and marine records, although the magnitude of the isotopic excursion varies. Such a record could potentially be explained by large, rapid changes in source (seawater) δD, temperature, paleolatitude, or orographic effect. As the latter two possibilities are geologically improbable, these data suggest the EOT saw substantial cooling and/or a shift to more negative seawater δD in Patagonia. The similarity of this record to northern hemisphere data suggests that records from North America and Europe document the expression of a globally synchronous or near-synchronous climatic shift that broadly altered the terrestrial climate state.