Pleistocene paleosols associated with megafauna in Northwestern Mexico: Paleoecological inferences

Abstract

The pedological cover of the state of Sonora, in northern Mexico, is predominantly composed of moderately developed red soils that evidence processes of weathering, humification, rubification, neoformation of clay, and carbonation, classified as Cambisols according to the WRB. These soils constitute a pedological unit denominated the San Rafael Paleosol (SRP). In contrast, gray soils are recorded in some sites located in semi-closed basins and are characterized by processes of weathering, neoformation of clay, reductomorphism and carbonation. These soils developed during the late Pleistocene under a semi-arid and cold climate, slightly more humid than the present one, with winter dominant rains, and marked seasonal changes. These paleosols are associated with remnants of Pleistocene Rancholabrean fauna of diverse composition, associated with arid and humid climates, demonstrating local climatic variations much more complex than at present. This paper evaluates the physical, chemical and micromorphological attributes of paleosols located in the San Francisco and El Arenoso ranches in the north of Sonora. The analyses are undertaken in order to identify the main pedogenetic processes and to establish the predominant environmental conditions during their formation, specifically the particular characteristics associated with semi-enclosed basins that allowed the accumulation of water and the formation of ponds. At both sites remains of Pleistocene megafauna have been found associated with paleosols. These results are contrasted with previous paleopedological studies and the paleontological record, permitting a broader discussion of regional paleoclimatic trends.