Resolving the influences of climatology and topography on water isotopes
Abstract:Paleotopography records are critical to understanding geodynamic processes in ancient mountain ranges. The imprint of topography on stable isotopes of precipitation (“water isotopes”) has become the most widely used method for reconstructing topography. Current approximations of how orographic lifting drives fractionation of water isotopes use 1D topography and a model based on either empirical observations or 1D Rayleigh fractionation. However, atmospheric physics tells us that the pattern and magnitude of lifting varies due to the shape of topography. We also know that the source of water isotopes and the local climatology (e.g., wind speed and direction, moist stability of the air) vary on both short and long time scales. Current approaches fail to separate the signal of topography from that of potentially large, short-term (<100 kyr) climate variations in water isotope records.
We present an isotope-enabled version of the linear theory of orographic precipitation (LTOP) of Smith and Barstad (2004), which describes the response of water isotopes to topography. This model provides a first-order approximation of the water-isotope field produced by moist air flowing over complex 3D topography, which allows us to explore how the water isotope field is influenced by variations in climatology and topography. The LTOP model is attractive because of its simplicity: just 7 variables are used calculate the water isotope field, but the model still accounts for lateral flow and upwind phase tilting due to topographic barriers (to the non-linear limit).
We tested this model in modern Patagonia, where there is a simple westerly pattern of atmospheric flow across a north-south mountain range. The model reproduces the strong observed fractionation and matches 166 field measurements of water isotopes in precipitation fairly well (R2 = 0.61). Sensitivity tests indicate the water isotope field is most sensitive to variations in the composition of source water (ocean), wind speed, and wind direction. Short-term climate variations are on the order of 16‰ δD; isotope records showing less variability should be treated as unreliable. The capacity of the LTOP model to describe the observed complexity of atmospheric behavior in response to 3D topography is an important step towards future advances in paleotopography work.