K.D. Nelson Lecture Series – Dr. Daniel Ibarra

The Department of Earth & Environmental Sciences in the College of Arts and Sciences presents the K.D. Nelson Lecture Series featuring speaker Daniel Ibarra. His talk is titled: Triple oxygen isotope geochemistry and paleoaltimetry.

Triple oxygen isotopes of minerals preserved in lacustrine sediments and hydrothermally altered minerals from crystalline rocks can be used to determine past elevations of mountain ranges as they may preserve an isotopic record of ancient precipitation. Here we use the measurement of all three stable isotopes of oxygen (16O, 17O, and 18O) to create data trends in 17O/16O vs 18O/16O that are extrapolated back to the meteoric water line using water/rock mixing constraints and models for lake water evaporation. We summarize recent advances in this approach and focus on examples from Cenozoic western North America and Tibet. Three key findings emerge from our analysis of hydrothermally-altered crystalline rocks and lacustrine systems. First, in combination with carbonate clumped isotope measurements we demonstrate that Eocene lacustrine chert forms during early burial diagenesis at temperatures <60 °C. Importantly, these Eocene lacustrine cherts still preserve signals of ancient evaporative lake systems demonstrating δ’17O-δ’18O mass laws shallower than Rayleigh-dominated meteoric processes. Second, we compare this approach to prior work that jointly used oxygen and hydrogen isotopes to determine the isotopic composition of meteoric waters. Our analysis suggests that hydrogen isotopes may exchange with ambient fluids in some systems but not others and is dependent on mineralogy and cooling rate. Third, hydrothermal system and core-complex based paleoaltimetry constraints agree well with carbonate and clay-based estimates from nearby age-equivalent sedimentary basins. This new tool is nascent but provides complementary information to be used in conjunction with more traditional isotope-based paleoaltimetry approaches.