A PROMINENT PEDOGENIC SIGNAL FOR MIS 3 IN THE CENTRAL GREAT PLAINS, USA

Well-expressed soil development during MIS 3 has long been documented in the ubiquitous loess deposits by several investigators working in the Central Great Plains (and beyond). Mid to late MIS 3 was dominated by pedogenesis, although expressed at some localities as two to three distinct episodes. Stratigraphically, the soils occur within and typically dominate the Gilman Canyon Formation, a relatively thin loess unit, deposition of which likely began in MIS 4. The formation rests on the Loveland soil of MIS 5 and is overlain by the Late Wisconsinan Peoria Loess (MIS 2). Substantial chronological data (¹⁴C, TL, OSL) have accumulated, and recent age data indicate pedogenesis began as early 45 ka, and possibly earlier at some sites, and terminated as late as c. 25 ka. Overall, δ¹³C data produce a pattern clearly indicating that the soil forming intervals are C₄ grass-dominated, whereas C₃-plant domination occurs in the zones largely unaffected by pedogenesis. In addition, a systematic east-to-west and south-to-north decrease occurs in the δ¹³C values across the array of documented sites. Recent phytolith and other biogenic opal data are in agreement with the isotopic data, e.g., C₄-level δ¹³C values correspond to increased concentrations of C₄ grass tribes (Panicoids and Chloridoids). Phytolith concentration peaks during soil formation and is in agreement with the generalized age model. Limited grass charcoal counts suggest an increase in fire frequency during the periods of soil development, whereas macro-charcoal (Picea sp.) has been found in the lowermost Peoria Loess, immediately overlying the MIS 3 soils. Though not as pronounced, MIS 3 pedogenesis is also expressed within coeval alluvial and aeolian sand stratigraphy, which suggests widespread landscape stability.