Assessing Pleistocene climate on the Southern High Plains through geochemical and physical characteristics of paleosols archived in the Blackwater Draw Formation

The Southern High Plains (SHP) is a large plateau, encompassing an area ~130,000 km2 from 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 between 2 and 1.4 Ma, and forming the Blackwater Draw Formation, an eolian mantle, locally up to 27 m thick, composed of loess (wind-blown silt) deposits with loess tops forming individual paleosols. Loess-paleosol couplets represent periods of deposition followed by periods of landscape stability accompanied by pedogenesis. Here we utilize geochemical and physical properties of loess-paleosol couplets recorded in the Black Water Draw Formation are used to interpret on climatic conditions of the Pleistocene. Petrographical and geochemical evidence indicates five buried soils (buried soil 5 = oldest to buried soil 1=youngest) are present in the Type Section of the Blackwater Draw Formation. Buried soils 4 and 5 are classified as Aridisols due to low clay content and shallow horizons of calcareous nodules. Buried soils 3 through 1 are classified as Mollisols as predominantly evidenced by circumgranular, clayey peds. Physical and geochemical characteristics indicate two distinct facies are present within the type section, with buried soils 3, 4, and 5 representing a coarse, very fine to fine sand facies and buried soils 1 and 2 representing a finer, sandy mud facies. Whole-rock geochemical profiles illuminate distinct chemical weathering profiles that indicate specific pedogenic processes such as hydrolysis, salinization, and leaching. These pedogenic processes are interpreted through analysis of select ratios and trends of relatively reactive elements versus immobile elements. Degree of pedogenesis is then predominantly a function of water availability and soil temperature. These parameters are quantitatively assessed through climo-functions developed from the relationship between geochemical ratios and modern soil forming conditions. Paleosol characteristics suggest a wetting trend to buried soil 2, as evidenced by the transition from Aridisols to Mollisols between buried soils 4 and 3 and increase in weathering intensity from buried soil 3 to 2. This wetting trend is followed by an increase in aridity to modern conditions from buried soil 2 and 1, seen in a decrease in intensity of leaching and hydrolysis. Climofunctions indicate all soils had higher precipitation than modern day which is supported by records from flora, fauna, and pollen data, which indicates cooler and wetter conditions through most of the Pleistocene on the SHP.
This study tentatively posits a mechanism for the cyclicity of deposition and pedogenesis of the buried soils of the type section. The lowest two cycles are silica-rich, well-to-moderate, fine sandstone with weakly developed soil horizonation which is best characterized as eolian sand sheet deposition. However, the upper buried soils 1-2 are coarse siltstone with significant amount of finer material. This grain size switch is tentatively proposed as reflecting different sediment source areas. Specifically, buried soils 1-2 are younger than the middle Pleistocene transition—which records a switch from obliquity (41,000 year) cycles to eccentricity (100,000-year) cycles—when northern hemisphere ice was periodically robust. In the northern Great Plains, during especially robust northern hemisphere ice periods, silt is generated in large enough volumes to reset the pedogenic cycle. Therefore, it is plausible that these periods of increased silt generation may also have contributed to the SHP. The limited age model for the Blackwater Draw Formation permits this hypothesis.