RECORD OF LATE QUATERNARY CLIMATE CHANGE IN CALCIC SOILS: INTERPRETATION OF SOIL GEOMORPHOLOGY AND HYDROLOGIC MODELING

Numerical modeling of soil moisture combined with morphologic and hydrologic analysis of desert soils provides information about late Quaternary climate change. Simulations of soil-water flux under varied conditions are used to determine the impact of episodic periods of wetter climate during the Holocene and latest Pleistocene on calcic soils and ecosystem processes in the in the Mojave and Sonoran Deserts of the southwest U.S. Analysis of atmospheric circulation patterns indicate that historic wet years provide an analog for wetter climates that occurred during the latest Pleistocene and episodically during Holocene periods of pluvial activity.

Daily weather data associated with a wet (~33 cm/yr) and dry (~15 cm/yr) climate was used to simulate the affects of climate change on soil-water balance and the bimodal distribution of carbonate in soils on Pleistocene surfaces in the Mojave Desert. Results indicate that soil-water balance for dry and wet years strongly corresponds with the upper and lower zones of carbonate accumulation respectively. Soil water only reached the lower zone during wet years due to an increase in winter and spring frontal storms. This relation indicates that the upper carbonate zone is due to a decrease in rainfall and not an increase in Holocene temperature or the development of clay-rich soils. Calculation of carbonate solubility and accumulation rates suggests that much of the carbonate in the upper zone accumulated during the late Holocene rather than throughout the entire Holocene.

Simulations of soil water balance provide clues about climate change and the formation of plants scars, features common to areas of extensive desert pavement in the hyper-arid areas of the Sonoran Desert. Plants scars are circular areas 2-5 m across that are devoid of vegetation and often lack pavement. Soil morphology underlying the scars preserves evidence of a biologic origin of the scars. Modeling soil-water flux suggests that a decrease in large storms that produce surface runoff from pavement covered soils marginal to the scars has resulted in a substantial decrease in plant available soil water, enhancing plant mortality. Formation of plant scars from an interaction of biologic and pedologic processes implies changing environmental conditions and may record long-term trends in regional climate.