HIGH-LATITUDE CONTINENTAL RESPONSE TO CENOZOIC CLIMATE CHANGE IN SOUTHERN ALASKA BASED ON VOLCANIC GLASS HYDROGEN ISOTOPE COMPOSITIONS

The Cenozoic Era is marked by episodes of warming and cooling based on high-resolution marine geochemical records. High-latitude continental responses to these events are poorly documented. In this study, we aim to understand continental paleoclimate response to global events in southern Alaska by studying the hydrogen isotope composition of hydrated volcanic glass. 20 volcanic ash samples were collected from the fluvial Sterling Formation on lowland (<200 m) of the southwestern end of Kenai Peninsula. These ashes are younger than 12 Ma based on previous radiometric dating. Three ash samples were collected from the late Eocene fluvial Hemlock Conglomerate in the Talkeetna and Tordrillos Mountains with a sampling elevation of ~800 m. Because the rain shadow effect of mountain ranges in southern Alaska may have affected the stable isotope composition of paleo-meteoric water, tectonic uplift is also considered when interpreting the stable isotope record. The hydrated volcanic glass δD values range from -181‰ to -97‰. After converting these glass values to environmental water δD values, the reconstructed paleo-meteoric water δD values for < 12 Ma range from -153‰ to -66‰, and for the late Eocene range from -139‰ to -98‰. The most positive values must be influenced by evaporation of surface water. Modern mean annual surface water δD values in the study areas range from -150‰ at ~1000 m to -110‰ on lowland (Johnson, 2014). The most negative late Eocene paleo-meteoric water δD value is only about 10‰ higher than modern water δD value at ~1000 m. Warm late Eocene paleoclimate most likely caused the small isotopic difference, and surface uplift (> 1000 m) of the sampling area was very unlikely during the late Eocene. The late Neogene paleo-meteoric water has lower δD values than modern lowland surface water, and the values have larger magnitude of variation during 12-6.3 Ma than after 6.3 Ma. Given the well-documented late Neogene global cooling, the lower isotope values and larger variation suggest that the paleoclimate in the Kenai lowland may be more seasonal during 12-6.3 Ma than 6.3 Ma.