A terrestrial record of water isotopes reveals the Eocene-Oligoene transition in southern Argentina

Abstract:

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., Zachos et al., 2001) and the terrestrial realm (e.g., Zanazzi et al., 2007). However, existing terrestrial records from Patagonia show negligible change in the water isotope record (Kohn et al., 2010; Kohn et al., 2015), despite the proximity of South America to Antarctica. 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 allow us to reconstruct 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 marine record, although the magnitude of the isotopic excursion is different. Such a record could potentially be explained by large changes in source (seawater) δD, temperature, paleolatitude, or orographic effect. As the latter two possibilities are geologically improbable, these data suggest large, rapid climatic changes accompanying the EOT in Patagonia. If corroborated by other data, this would suggest that the evolution of hypsodonty in grazers in Patagonia was not driven by or sensitive to the global climate.