ORIENTATION OF CARBONATE LAMINATIONS IN GRAVELLY SOILS ALONG A WINTER/SUMMER PRECIPITATION GRADIENT IN BAJA CALIFORNIA, MEXICO

Abstract

We observed, along a north-to-south transect in central Baja California, that the positioning(top vs. bottom) of pedogenic carbonate on gravel in Holocene soils was related to the proportion of winter versus summer/early fall precipitation. Carbonate coatings were found on gravel bottoms at sites where precipitation occurs predominantly in winter but on gravel tops at sites where summer/fall precipitation is dominant. The positioning of carbonate coatings co-varied with other ecosystem properties such as floral distribution, oxygen isotope composition of carbonate, and depth to the top of the Bk horizon in response to precipitation patterns. The processes responsible for the orientation of carbonate on gravels are unknown. We proposed that the nature of soil thermal gradients during major precipitation events or periods may influence carbonate solubility and, therefore, the loci of deposition. While the deposition of carbonate on gravel tops is apparently unique and restricted to areas of summer/fall precipitation in Baja California, the positioning of carbonate coatings in gravelly soils in this region may also provide some insight into long-term climate patterns in the perarid regions of the Sonoran Desert.