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

• Published in: Tellus. Series B, Chemical and physical meteorology Vol. 72; no. 1; pp. 1 - 17
• Main Authors: Bershaw, John, Hansen, Dougal D., Schauer, Andrew J.
• Format: Journal Article
• Language: English
• Published: Stockholm Taylor & Francis 01-01-2020; Ubiquity Press; Stockholm University Press
• Subjects: Arid regions; Aridity; atmospheric circulation; climate; Deuterium; Elevation; Evaporation; Fractionation; Humidity; Hydrologic cycle; Hydrological cycle; Hydrology; Meteoric water; Meteorology; Minerals; Moisture; Mountains; Order parameters; Oxygen; Oxygen isotopes; Pacific Northwest; Paleoclimate; Phase changes; Raindrops; Relative humidity; stable isotopes; Temperature; Tracers; Trajectory analysis; Vapors; Water analysis; Oregon; Olympic Mountains; United States > US; Pacific Northwest
• ISSN: 0280-6509; 1600-0889
• DOI: 10.1080/16000889.2020.1773722

High-precision triple oxygen isotope analysis of water has given rise to a novel second-order parameter,
17
O-excess (often denoted as Δ
17
O), which describes the deviation from a reference relationship between δ
18
O and δ
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 δ
18
O, δ
17
O, and δ
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 d-excess,
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 δ
2
H data are unavailable. We use a raindrop evaporation model and local meteorology to investigate the effects of subcloud evaporation on d-excess 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
17
O-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
17
O-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
17
O-excess variation in the Olympic Mountains and Oregon Coast Range.