Climate and provenance variation across the mid-Pleistocene transition revealed through sedimentology, geochemistry, and rock magnetism of the Blackwater Draw Formation, Southern High Plains, Texas, USA

Abstract

The mid-Pleistocene transition is a time interval between ca. 1.2 and 0.7 Ma during which a shift occurred from ~41 k.y. glacial-interglacial cycles to ~100 k.y. cycles. Although the mid-Pleistocene transition has been well documented in global marine records, its effects in continental environments, including North America, are incompletely understood owing to the paucity of terrestrial sediment records spanning the entire Quaternary. A notable exception is the ca. 1.4 Ma and younger Blackwater Draw Formation, an extensive eolian sequence on the Southern High Plains of the United States. Intervals of the Blackwater Draw Formation section that are inferred to span the mid-Pleistocene transition can be divided into pre–, syn–, and post–mid-Pleistocene transition parts. Weathering profiles in the pre–mid-Pleistocene transition section are dominated by weakly developed soils formed in arid environments, as evidenced by well-expressed pedogenic carbonate horizons, lack of clay formation during hydrolysis, and magnetically soft, coarse-grained magnetite/ maghemite populations. Conversely, the syn– and post–mid-Pleistocene transition intervals demonstrate an increase in weathering intensity by an abrupt increase in clay content formed in part by hydrolysis of feldspars, soil profiles that demonstrate leaching and illuviation, and a fining-upward grain size of the magnetite/maghemite population. Sedimentologic, geochemical, and rock-magnetic data are consistent with a southern and coarser sediment source derived from the Pecos River drainage prior to the mid-Pleistocene transition, followed by a mixture of northern and southern sources during and after the mid-Pleistocene transition. Overall, our results indicate that pre–mid-Pleistocene transition conditions on the Southern High Plains were arid with wind energy sufficient to mobilize sand sheets out of the Pecos River and deposit them on the plateau. The syn– and post–mid-Pleistocene transition environments reflect somewhat wetter conditions and potentially an influx of silt from the north, in addition to continued sand derived from the Pecos River valley. The wetter conditions and silt influx may have resulted from longer-lived and more robust glacial activity in the Northern Hemisphere that characterized the post–mid-Pleistocene transition Earth system.

The Effect of Differential Weathering on The Magnetic Properties of Paleosols: A Case Study of Magnetic Enhancement vs. Magnetic Depletion in the Pleistocene Blackwater Draw Formation, Texas

“…The BDF varies in thickness across the SHP, commonly ranging from <1 to about 9 m thick, but locally is up to 27 m thick in the vicinity of Amarillo, Texas (Hovorka, 1995). Internally, the sediments are punctuated by distinct Bt and Bk pedogenic horizons, the number of which varies from 4 to 14 different units depending on location (Holliday, 1989;Hovorka, 1995;Stine et al, 2020). Notably, thicker intervals of the BDF tend to exhibit a greater number of paleosol profiles (Hovorka, 1995).…”

“…The type locality of the BDF is ∼9 m in height and consists of five paleosols punctuated by a surficial soil horizon (Stine et al, 2020) (Figure 2). The topmost and youngest soil horizon is termed the "Surficial Soil"; the physical characteristics of which were all documented by Holliday (1989).…”

“…Dashed line in center of the Southern High Plains defines the inferred boundary between loess and cover sands (after (Reeves and Mahaney, 1976)). Figure modified from (Stine et al, 2020).…”

“…Similar to the A horizon, the Bt horizon of the Surficial Soil has a sandy clay loam texture (Holliday, 1989). The Bk horizon was described by Stine et al (2020) as a Stage III calcic horizon that has 7.5 YR 5/8 hues, is ∼1 m thick, and has a loamy texture.…”

“…In this paper, we contribute to ongoing research on the processes that might favor magnetic enhancement over depletion through studies of the magnetic mineralogy of the type section of the Blackwater Draw Formation (Reeves and Mahaney, 1976;Holliday, 1989). This formation consists of a series of Pleistocene eolian sedimentpaleosol couplets preserved on the Southern High Plains (SHP) region of the United States (Figure 1), which display both magnetically depleted and magnetically enhanced eolian sediment-paleosol layers (Holliday, 1989;Gustavson and Holliday, 1999;Stine et al, 2020).…”