EPISODIC LANDSCAPE INSTABILITY RECORDED IN LATE QUATERNARY LOESS-PALEOSOL SEQUENCES

Loess-paleosol sequences in the mid-continent of North America provide excellent records of episodic landscape instability. Landscape instability is evident at different temporal scales, demonstrating a hierarchy of records. The broadest record is at the scale of interglacial-glacial cycles. The last-interglacial Sangamon Soil represents a long period (at least ~130 ka to ~70 ka BP) of landscape stability, at loess sections from Colorado to Indiana. This soil shows significant pedogenic development and chemical weathering. Modern soils at the same sections show somewhat weaker profile development and less chemical weathering, but represent landscape stability since ~12 ka. In contrast, a period of landscape instability in source areas is represented by last-glacial-age Peoria Loess (~25 ka to ~12 ka) at downwind localities. At this time, eolian erosion of both glacial and non-glacial sediments dominated the North American mid-continent south of the Laurentide ice sheet. At a finer temporal scale, the loess record also shows episodic landscape instability within glacial and interglacial periods, particularly in climatically sensitive regions, such as the semiarid Great Plains. Within Peoria Loess, accumulation rates increase upsection, indicating increasing landscape instability in the source areas. In addition, some parts of the present interglacial in the Great Plains are not recorded solely as a period of pedogenesis. Multiple paleosols within Holocene loess in the Great Plains show that periods of landscape stability within an interglacial can also be episodic. At Moran Canyon, Nebraska, periods of landscape instability and loess deposition, based on radiocarbon ages, are recorded between ~11-6 ka, ~5-1 ka, and since 1 ka. Loess sequences contain valuable landscape instability records, but must be viewed at different timescales.