Deuterium excess and 17O-excess variability in meteoric water across the Pacific Northwest, USA

Abstract

High-precision triple oxygen isotope analysis of water has given rise to a novel second-order parameter, 17O-excess (often denoted as Δ17O), which describes the deviation from a reference relationship between δ18O and δ17O. This tracer, like deuterium excess (d-excess), is affected by kinetic fractionation (diffusion) 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 relatively arid continental regions where traditional approaches have produced equivocal results. We present new δ18O, δ17O, and δ2H 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. 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. We suggest that vapor d-excess is affected by relative humidity and temperature at its oceanic source, whereas 17O-excess vapor is controlled by relative humidity at its oceanic source. Like d-excess, 17O-excess is significantly affected by evaporation in the rain shadow of the Cascade Mountains, supporting its utility as an aridity indicator in paleoclimate studies where δ2H data are unavailable. We use a raindrop evaporation model and local meteorology to investigate the effects of subcloud evaporation on d-excess and 17O-excess along altitudinal transects. We find that subcloud evaporation explains much, but not all of observed increases in d-excess with elevation and a minor amount of 17O-excess variation in the Olympic Mountains and Coast Range of Oregon.Key Points 17O-excess correlates spatially with relative humidity across the Pacific Northwest, supporting its use as an aridity indicator in paleoclimate studies. Discrepancies in d-excess and 17O-excess between the Olympic Mountains and Oregon Coast Range suggest that their moisture source is different. Subcloud evaporation explains most of observed increases in d-excess with elevation, and a minor amount of 17O-excess variation in the Olympic Mountains and Oregon Coast Range.