RELATIVE PRECIPITATION VARIATIONS ARCHIVED IN QUATERNARY PALEOSOLS ON THE SOUTHERN HIGH PLAINS

The Southern High Plains (SHP) is a large plateau (130,000 km²) , encompassing southeastern New Mexico to northern Texas. The SHP was largely a site of sediment aggradation since the Pecos River cut off the Rocky Mountain highlands to the west ~1.4 Ma, and forming the Blackwater Draw Formation (BDF), an eolian mantle up to 27 m thick composed of loess deposits with loess tops forming individual paleosols. Loess-paleosol couplets record high resolution climate cyclicity on the order of 10² ky’s with loess deposits representing periods of deposition and paleosols defining periods of landscape stability accompanied by pedogenisis. Here we present data from the BDF that bears on climatic conditions of the past ~1.4 Ma.

Petrographical and geochemical evidence indicates five buried soils (BS5: oldest; BS1:youngest) are present in the Type Section of the BDF. Whole-rock geochemical profiles illuminate distinct chemical weathering profiles that assess specific pedogenic processes such as hydrolysis, salinization, and leaching. BS5 exhibits moderate hydrolysis and leaching, while BS4 shows strong horizination and illuviation. BS3 shows weak pedogenesis, BS2 exhibits an increase in hydrolysis, while the BS1 exhibits a greater increase in weathering intensity. We infer that precipitation variances during each soil forming event account for the different weathering profiles. However, we do note that time and temperature also likely played affect. Nevertheless, these data suggest that from the BS5-to-BS4 precipitation increased, BS4-to-BS3 precipitation decreased, BS3-to-BS1 precipitation continued to increase. Thus, the SHP appears to show relatively arid events at the time of BS5 and BS3 pedogenic events. The current age model of the BDF is crude, but suggests that the BS5 and BS3 formed around 1.4 and 1.0 Ma (Jaramillo subchron), respectively. These rough age constraints indicate pre-Illinoian time, but do not merit correlation to specific events at this time.