VARIABILITY OF DEUTERIUM EXCESS (D-EXCESS) AND 17O-EXCESS (D17O) IN METEORIC WATER THROUGHOUT THE PACIFIC NORTHWEST

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.