Using Rock Magnetic Proxies to study the Pleistocene paleoclimatic conditions of the Southern High Plains, West Texas

The Southern High Plains (SHP) is a region in the Texas panhandle mantled by the ca.1.4 Ma to latest Quaternary Blackwater Draw Formation (BDF). The BDF is potentially one of North America's best records of latest Cenozoic terrestrial paleoclimate, because it is one of the few continuous Quaternary eolian sequences in North America that extend past the last-glacial maximum (~26 ka). Detailed rock magnetic analyses were carried out on two exposures of the BDF, referred to as the type site and the quarry site. Rock magnetic analysis included bulk magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM) intensity, isothermal remanent magnetization intensity at different DC fields (IRM_100mT, IRM_300mT, SIRM at 2.9 T), and backfield IRM_-300mT) all demonstrate cyclic behavior with depth in the BDF that is more evident in environmental magnetic ratios (ARM/χ, ARM/SIRM, SIRM/χ, and %χ_FD). Moreover, both sites demonstrate a distinct change in signal behavior from a high-frequency low-amplitude signal at lower depths to a low-frequency high-amplitude signal higher in the sequence. This change in behavior is tentatively interpreted to represent the onset of the early-middle Pleistocene transition (MPT), a time interval between about 1.2 and 0.7 Ma, when a shift from ~41 ka glacial-interglacial cycles to ~100 ka cycles occurred. Although the entire scope of the MPT's effects on the SHP still have yet to be determined, the presence of relatively coarse grained maghemite/magnetite in the lower BDF suggest that prior to the onset of the MPT the region was more arid. By contrast the increased presence of hematite and super-paramagnetic magnetite in the upper parts of the BDF imply that the post-MPT was a period of increased precipitation.