Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 12-1
Presentation Time: 1:35 PM


BERSHAW, John, Department of Geology, Portland State University, 1721 SW Broadway Ave, Portland, OR 97201 and HANSEN, Dougal, Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St., Madison, WI 53706

High-precision triple oxygen isotope analysis of water has given rise to a new second-order parameter, 17O-excess (often denoted as Δ17O or D17O), which describes the deviation from a reference relationship between δ18O and δ17O. This tracer, like deuterium excess (d-excess), reflects relative humidity during phase changes within the hydrologic cycle. However, unlike d-excess, 17O-excess is present in paleowater proxy minerals and is not thought to vary significantly with temperature. This makes it a promising tool in paleoclimate research, particularly in continental regions where traditional approaches using isotopes have produced equivocal results. We present new δ18O, δ17O, and δD data from stream waters along two east-west transects in the Pacific Northwest to explore the sensitivity of 17O-excess to topography, climate, and moisture source. Isotopic results are paired with a raindrop evaporation model to show that subcloud evaporation is a primary driver of changes in d-excess and 17O-excess along altitudinal transects in the Olympic Mountains (Washington) and Coast Range (Oregon). That said, many stream samples have higher 17O-excess values than subcloud evaporation predicts. We attribute this to stratospheric oxygen, which may be entrained through tropopause folding events. We find that discrepancies in d-excess and 17O-excess between the Olympic Mountains and Coast Range are consistent with distinct moisture source meteorology, inferred from air-mass back trajectory analysis. Our results confirm that vapor d-excess is affected by temperature at its oceanic source whereas 17O-excess vapor is not. Both are significantly influenced by relative humidity. Lastly, an inverse relationship is observed between 17O-excess and relative humidity in the rain shadow of the Cascade Mountains, supporting its use as an aridity indicator in paleoclimate studies where δD data is unavailable.