PALEOHYDROLOGY DURING THE MIDDLE MIOCENE WARM PERIOD: INSIGHTS FROM MOLECULAR δD VALUES OF IN SITU PLANT LIPIDS

Molecular hydrogen isotopic compositions (δD) of leaf wax recovered from fossils and sediments have become a powerful tool to probe Cenozoic paleohydology by inferring δD for ancient precipitation. We obtained δD values from n-alkanes extracted from exceptionally-preserved fossil Larix, Glyptostrobus, Taxodium and Metasequoia from two middle Miocene sites: Ballast Brook, on Banks Island in Canada (paleolatitude: 75⁰N) and the Clarkia deposit in Idaho of the United States (paleolatitude: 45⁰N). Using apparent hydrogen isotope fractionations between source water and individual lipids (ε_{lipid -water}) calculated based upon data obtained from their modern equivalents under greenhouse controlled conditions, we inferred middle Miocene precipitation δD of -165‰ and -109‰ for Banks Island and Clarkia respectively during the middle Miocene warm period. δD values from two fossil taxa at each site yielded self-consistent results. The inferred middle Miocene precipitation δD values for both sites are lower than that of modern mean annual precipitation in these areas, but the hydrogen isotope offsets between the two locations (ΔδD_Miocene) remain similar as those in modern day (ΔδD_Modern). Our data suggest that latitudinal gradient of precipitation δD values during middle Miocene is similar to that of the present day, although the global temperature gradient was much lower in middle Miocene. These hydrogen isotope results compare well to inferred paleoclimate using traditional paleobotanical data. If additional data confirm this observation, such paleohydrological conditions in middle and high-latitudes of the Northern Hemisphere would offer new insights into lateral moisture delivery during the unique Cenozoic warming period when high global temperature developed under moderate atmospheric CO₂ concentration.