BASIN-SCALE VALIDATION OF STABLE ISOTOPE PALEOALTIMETRY IN THE INTERMONTANE WESTERN USA

Stable isotopes have long been used to reconstruct paleoaltitudes, with most published lapse rates derived from precipitation. However, the precipitation of any given point within a watershed does not reflect the waters integrated by most inorganic and biogenic carbonates; rather, these materials are preserved in stream-fed downstream lacustrine basins which integrate upstream precipitation and snowmelt. While previous paleoelevation reconstructions have estimated mean regional elevations from surface water stable isotopes, there’s been minimal validation that average upstream elevations of drainage basins control downstream isotopic composition. Empirical testing of how upstream watershed elevation (e.g. maximum, mean, or point of integration) reflects the isotopic composition may clarify these processes.

To resolve how isotopic composition of surface waters evolves from that of precipitation, we calculate upstream watersheds of nearly 6,000 individual surface water isotopic measurements from across the intermontane west, USA, and evaluate predictiveness of isotopic composition to maximum, mean, and minimum upstream drainage basin elevations. We derive isotope-elevation lapse rates for basin-wide watershed elevations, which we compare to published lapse rates derived from precipitation isotopes. From this, we constrain how isotopes evolve from precipitation to surface waters across elevation gradients – reflecting surface waters’ integration of precipitation, groundwater and snowmelt across both seasonal climates and topographic gradients, and clarifying how hydroclimate may govern regional isotopic lapse rates.

Preliminary findings suggest that linear lapse rate models are limited by their abilities to constrain high-elevation processes. Accounting for latitude, highest elevations within watersheds have extremely narrow surface water isotopic lapse rates (-0.5‰/km), which become wider for mean (-1.9‰/km) and minimum elevations (-1.6‰/km). Moreover, while R2 values between surface water isotopes and elevation are nearly equivalent for lapse rate models based on both precipitation and average upstream elevations (R2 ≈ 0.34), surface waters are poorer at predicting maximum elevations (R2 = 0.26). Paleoaltitude studies estimating high elevations may therefore benefit from recontextualizing high-elevation hydrology.