Using rock magnetic parameters to determine different weathering profiles and provenance changes reflecting the mid- Pleistocene Transition in the Pleistocene Blackwater Draw Formation, Southern High Plains, Texas.

The mid-Pleistocene Transition (MPT), represents a shift from 41 ka glacial-interglacial cycles to 100 ka cycles. In the Blackwater Draw Formation (BDF), which is an extensive loess dominated sequence on the southern high plains (SHP), pre, syn, and post-MPT intervals record variations in weathering intensity, magnetic grain sizes, and potentially different sources for the BDF. Bulk magnetic susceptibility (χ, median value:1.57*10-4), intensity of anhysteretic remanent magnetization (ARM, median value:0.16 A/m), intensity of isothermal remanent magnetization different DC fields (IRM_100mT, IRM_300mT, SIRM at 2.9 T, and backfield IRM_-300mT, with median values of:2.53 A/m, 3.36 A/m, 3.92 A/m , and -2.70 A/m respectively) along with environmental magnetic ratios (ARM/χ, ARM/SIRM, SIRM/χ, S-ratio, and HIRM with medians of 1050.86 A/m, 0.0438, 25129.34 A/m, 0.8637 and 0.54) all characterize these three different weathering profiles. The rock magnetic data show that the pre-MPT interval is dominated by magnetically soft coarse-grained magnetite/maghemite, implying a weakly developed soil in an arid environment controlled by 41-ka climate cycles. The syn-MPT interval is inferred to be controlled by magnetite/maghemite that both fines upward and steadily gives way to increasing hematite concentration higher in the section, representing increasing weathering intensity as 100-ka climate cycles begin to dominate. The post-MPT interval displays larger amplitude cycles for rock magnetic parameters when compared to depth interpreted to be representative of 100-Ka climate cycles that dominate after the MPT is completed. The post-MPT section is dominated by fine-grained magnetite/maghemite.

Rock magnetic data from the BDF are also possibly consistent with a shift from a southern and coarser sediment source derived from the Pecos River drainage prior to the MPT, to a mixture and later domination by a more northern source and this far traveled and finer grained input from glacial loess deposits, as shown by the syn- and post-MPT intervals of the BWD respectively. The inferred change in provenance is potentially represented by a decreasing sediment grain size from pre- to post-MPT deposits and cross-plots of the rock magnetic parameters that bin the pre- and post-MPT paleosols into different groups.