New Mexico Geological Society Annual Spring Meeting — Abstracts

A small first order drainage basin developed in early Pleistocene fan gravels shows a marked contrast in vegetation between north and south facing slopes. The south-facing slope is characterized by a creosote-black gramma grassland, whereas the north facing slope has black gramma-juniper woodland vegetation. There is a concomitant change in the depth and amount of calcium carbonate in the soils on the opposing slopes. Depth to calcic horizon and the average profile mass of calcium carbonate in south-facing slope soils is 5 cm and 10g/cm² respectively, compared to 30 cms and 15 g/cm² for soils on the north-facing slope. Catenary relations are not strongly developed for the soils on either slope. However, a very strong catenary relationship exists in the soils of the east facing headslope of the drainage basin. A decrease in profile mass of calcium carbonate and an increase in depth to calcic horizon occurs with lower slope soils. The initial driving force for these contrasts is the variation in solar radiation, with the north-facing slope receiving less winter sunlight than south-facing slope, however, the greatest differences in PET occur in the spring and fall. These differences in PET determine the nature of the vegetation communities on the opposing slopes which in tum influences the available soil moisture. Northfacing soils also have greater amounts of organic carbon and silt than south-facing soils, and these features increase the available moisture for vegetation on the north-facing slopes. CO₂ measurements throughout the year indicate that north-facing soils have a greater CO₂ flux which may be the main factor controlling the difference in profile mass of calcium carbonate between the two slopes.