Formation of a Thick A_Horizon through Aggradational Pedogenesis In the Buried Brady Soil, Central Great Plains

Thick, dark-colored A horizons characterize the buried Brady Soil, formed in loess on the central Great Plains, as well as some of the buried soils in dunefields of the Great Plains and China. The genesis of thick, organic-matter rich A horizons on upland summits in midlatitude grasslands has most commonly been attributed to large belowground organic matter additions coupled with deep pedoturbation. Jacobs and Mason (2007, GSAB 119:462-475) concluded that thick surface soil A horizons in loess of the central Plains also result from aggradational pedogenesis, with ongoing accumulation of Holocene loess as the soil developed. Goble et al. (2004, QSR 23:1173-1182) identified similar upbuilding in a buried dunefield soil. High-resolution OSL and ¹⁴C age profiles through the Brady Soil are consistent with ongoing loess sedimentation as organic matter accumulation and other pedogenic processes developed a prominent A horizon. Dating uncertainty precludes a distinction between a linear age-depth trend (i.e. steady accumulation) and a stepped age-depth model (episodic accumulation). In theory, deep pedoturbation could also yield a similar age-depth trend, however, and trace fossils provide direct evidence for pedoturbation by soil fauna. High-resolution profiles of particle size and isotopic data through the Brady Soil are quite similar at locations up to ~1 km apart, with replication of relatively minor peaks and troughs. This observation appears to rule out thorough mixing of the A horizon by pedoturbation, except within a relatively shallow depth beneath the land surface at any given time. We conclude that the thick A horizon of the Brady Soil was built upward as it developed, while acknowledging the role of pedoturbation in developing this well-structured soil from freshly deposited loess.