Paleopedology of soils in thick Holocene loess, Nebraska, USA

Abstract

Bignell Loess is a Holocene loess unit that has accumulated in the central Great Plains since 9,000 ¹⁴C yr BP. The Brady Soil separates Bignell Loess from last glacial Peoria Loess and thick sections, which can reach 6 m thickness, may contain four buried soils beneath the modern surface soil. The Brady Soil has A/B/C horizonation, including a thick dark A and a blocky or prismatic structured B horizon. The Brady soil is intensely burrowed, probably by nymphs of cicada species, which require woody vegetation. Secondary carbonate is present in all genetic horizons and was emplaced post burial. The Holocene soils are dominated by A horizon characteristics, namely dark colors, but blocky or prismatic structured B horizons occur in several sections. Secondary carbonate is present in most, but not all Holocene soils. Three phases of loess deposition and soil formation are evident from the thick proximal stratigraphic sections presented here. Initial rates of Bignell Loess deposition were low relative to the rate of soil formation, producing a basal aggraded A horizon above the Brady Soil, that is capped by a increment of loess with minimal pedogenic features. The middle portion of the Bignell Loess is finer textured and contains multiple dark buried soils. The final increment of loess is coarser and contains the weakly expressed modern surface soil. Climatically, termination of Brady Soil formation marks a shift to drier Holocene climates with a decrease in woody species, episodes of new loess deposition and soil formation, and secondary carbonate accumulation.