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, 17 O-excess (often denoted as D 17 O), which describes the deviation from a reference relationship between d 18 O and d 17 O. This tracer, like deuterium excess (d-excess), is affected by kinetic fractionation (diffusion) during phase changes within the hydrologic cycle. However, unlike d-excess, 17 O-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 d 18 O, d 17 O, and d 2 H data from stream waters along two east-west transects in the Pacific Northwest to explore the sensitivity of 17 O-excess to topography, climate, and moisture source. We find that discrepancies in d-excess and 17 O-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 17 O-excess vapor is controlled by relative humidity at its oceanic source. Like dexcess, 17 O-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 d 2 H data are unavailable. We use a raindrop evaporation model and local meteorology to investigate the effects of subcloud evaporation on dexcess and 17 O-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 17 O-excess variation in the Olympic Mountains and Coast Range of Oregon. KEY POINTS 1. 17 O-excess correlates spatially with relative humidity across the Pacific Northwest, supporting its use as an aridity indicator in paleoclimate studies. 2. Discrepancies in d-excess and 17 O-excess between the Olympic Mountains and Oregon Coast Range suggest that their moisture source is different. 3. Subcloud evaporation explains most of observed increases in d-excess with elevation, and a minor amount of 17 O-excess variation in the Olympic Mountains and Oregon Coast Range.

An Investigation and Comparison of Stable Isotopes in Meteoric Waters and Groundwaters From Southern Washington

“…The windward lapse rate for δ 18 O at low to mid elevations is -6.4 ‰ /km (R 2 = 0.72) and is comparable to previously reported lapse rate of -5.1 ‰/km (R 2 = 0.89) for the Olympic Peninsula (Sharp et al, 1960). The high-elevation windward lapse rate for δ 18 O of -2.5 ‰ /km (R 2 = 0.88) compares very well with the combined Coast Range and High Cascades lapse rate of -3.1 ‰/km (R 2 = 0.70) (Bershaw et al, 2020) and the global average for mountain ranges, which is -2.8 ‰/km (Poage & Chamberlin, 2001). The leeward lapse rate, 12.9 ‰/km (R 2 = 0.72), provides new insight on the possibility that minimal atmospheric mixing occurs over the High Cascades and that the vapor cloud travels as an unsplit mass over the topography (Moran et al, 2007;Smith, 1989).…”

“…The calculated lapse rate values compare well to the lapse rates previously reported for Washington and global mountain ranges. The mid-to low-elevation lapse rate of -6.4 ‰/km compares well to the Olympic Peninsula lapse rate of -5.1 ‰/km (R 2 = 0.89) (Sharp et al, 1960), and the high elevation samples compare very well to the combined lapse rate for the Coast Range and High Cascades at latitudes of 45.0° and 48.0° respectively, of -3.1 ‰/km (R 2 = 0.70) (Bershaw, et al, 2020). Additionally, the high elevation lapse rate of -2.5 ‰/km agrees with the global lapse rate for mountain ranges of -2.8 ‰/km (Poage & Chamberlin, 2001).…”

“…The lapse rates are calculated for the 47° N latitude for both the windward and leeward sides of the High Cascades. This latitude was chosen both because it bisects the study area for which groundwater data was obtained ( Figure 1) and helps fill in the data gap for windward lapse rates in the Pacific Northwest region calculated by Bershaw et al, (2020) for the 48° and 45° latitudes and Sharp et al, (1960) for the Olympic Peninsula. Figure 1.…”

“…One important method is the determination and modeling of isotopic lapse rates, the relationship between elevation and isotopic composition of meteoric waters. Currently, there are two lapse rates presented for the state of Washington, one for the Olympic Peninsula (Sharp et al, 1960) and one that combines data along both the 48° and 45° north latitudes (Bershaw et al, 2020). This leaves a rather wide gap (3° of latitude) in the data across the state of Washington and limits our understanding of water isotope chemistry in this dynamic region.…”

Tracing Water Sources and Fluxes in a Dynamic Tropical Environment: From Observations to Modeling

“…In temperate and maritimeinfluenced environments such as the British Isles (Tyler et al, 2016), the overall correlation between daily δ 18 O and precipitation explained obly 17% of the variance, ranging from 5 up to 45% on spatial basis. Similarly, other continental temperate sites have reported weak precipitation amount correlations on daily basis (Bedaso and Wu, 2020;Adhikari et al, 2020;Bershaw et al, 2020). Overall, the amount effect is not a fully valid approximation over the tropics and is commonly less pronounced on a daily basis compared to monthly time scales as indicated by Araguás-Araguás et al 2000(Supplementary Figure S2B).…”