Paleoclimate Simulations and Nitrate Accumulation in Sonoran Desert Soil.

Soil morphological features, including desert pavement, vesicular (Av) horizon and subsoil horizonation, are known to effect the hydrologic redistribution and sequestering of mobile ions such as (NO₃^-, Cl^-, and SO₄^2-), associated with eolian inputs. The goal of this study was to use modern soil analogs from a chronosequence (i.e., the substitution of space for time) to benchmark numerical simulations of water and solute transport over 40 ka. We used field measurements of soil profile properties, continuous soil moisture monitoring, and numerical modeling to evaluate the soil moisture regime across a hyper-arid chronosequence near Yuma, Arizona ranging in age from 3 ka to >100 ka. A stochastic paleoclimate simulations based upon the Macrophysical Climate Model was used to generate boundary conditions for HYDRUS-1D unsaturated flow model. Results from field data indicate positive correlations to surface age and total dust (<62.5 μm), Cl^-, and SO₄^2-. However, NO₃^-accumulations were 217 kg ha^-1 on younger Holocene-aged surfaces, and approached 2500 kg ha^-1 on all (Pleistocene) surfaces >18 ka. Assuming modern input rates of dust and solute, the oldest surfaces are only ~40 to 80 ka, suggesting that eolian were variable throughout the Holocene.. During the 9 years of field monitoring, no moisture flux was recorded below 50-cm, and numerical results indicate negligible moisture flux below 100-cm throughout the last 40 ka. Although a work-in-progress, results to date illustrate the significance of paleoclimate proxy data obtained from soil pedologic development.