CONTROLS ON MODERN STREAMWATER STABLE ISOTOPES IN THE NORTHERN ROCKY MOUNTAINS: IMPLICATIONS FOR 1-D PALEOELEVATION MODELING

Mountain belts influence global climate, control patterns of sedimentation, and serve as a surficial expression of past and present geodynamic processes. Since the geodynamic and climatic importance of mountains depends directly on their height, researchers have made substantial contributions to quantitative modeling of paleorelief, commonly comparing coeval spatial differences (e.g., Δδ¹⁸O) in the isotopic values of paleoprecipitation using 1-D models. While these models form powerful, relatively simple tools to interpret isotopic data, they assume idealized single air mass Rayleigh distillation, ignore non-topographic factors influencing precipitation patterns, and are highly sensitive to initial climatic inputs. Thus, additional work is necessary to understand the uncertainty of model outputs and constrain non-topographic controls on the isotopic patterns of precipitation.

To evaluate the precision, accuracy, and spatial resolution of 1-D paleoelevation models, we present new δD, δ¹⁸O, and d-excess values from modern streamwater and δD values of volcanic glass hydration water from the 7.7 ka Mazama ash, all collected in the northern Rocky Mountains and Cascades. We input stable isotope values into an established 1-D Rayleigh distillation model and compare model-predicted elevations to local catchment hypsometry and regional topographic patterns. We also perform regression analyses to determine other potential climatic or topographic variables influencing the spatial pattern of modern streamwater δD and δ¹⁸O values. Results show that mean maximum elevation along an eastward moisture trajectory functions as the best predictor of local δD_stream values west of the continental divide, though 1-D model-derived paleorelief estimates within continental interiors may under-predict elevation by up to 0.6 km if topography >1 km exists upstream. 1-D modeling can also be applied to more complex geometries in which a moisture source intercepts multiple mountain belts. Local catchment parameters, such as mean aspect or slope, play a secondary role in explaining spatial variations in δD_stream values. Broadly, these results demonstrate that 1-D modeling in the northern Rocky Mountains can accurately reconstruct paleoelevation from isotopic archives of Cenozoic paleoprecipitation.