Changing Paleoclimatic Conditions in the Southern High Plains during the Quaternary and Their Effects on the Development of Different Magnetic Phases: A Study of Magnetic Enhancement and Magnetic Depletion within the Blackwater Draw Formation

Abstract:

The Blackwater Draw formation (BDF) consists of a series of Quaternary-aged loess-paleosol couplets that serve as the primary surficial sediment covering the Southern High Plains (SHP) of the continental United States. Rock-magnetic properties (e.g. χ, ARM, IRM) vs depth data from the type-section of the BDF display both magnetically enhanced and magnetically depleted signals. In order to ascertain the magnetic mineralogic cause of these differing responses, detailed high-, low-, and room-temperature rock-magnetic experiments, as well as scanning electron microscopy, were conducted on representative samples from the six soil units that comprise the formation at the type-section. Our results show that low coercivity (Hc=~35 mT or less) magnetite/maghemitized-magnetite, largely in the pseudo-single domain state (Mr/Ms=~0.14 +/- 0.03588, Hcr/Hc=~2.68 +/- 0.298789), dominates the magnetic signal. However, superparamagnetic hematite (and also a minor contribution from single-domain sized hematite (Hc=~500 mT)) dominates all the soils in terms of weight percent concentration. These results suggest that within the type-section of the BDF, enhancement and depletion of the magnetic signal are competing phenomena. Furthermore, the dominance of one process over another appears to correlate with weathering intensity, such that higher intensity weathering results in magnetic depletion and moderate intensity weathering results in magnetic enhancement. The ultimate cause of varying degree of weathering intensity is tentatively interpreted to correlate to regional changes in precipitation due to the onset of the mid-Pleistocene transition (MPT). The MPT a time interval (~1.2-~0.7 Ma) when the temporal frequency of Earth’s glacial-interglacial cycles shifted from ~41 k.y. cycles to ~100 k.y. cycles.