INFERRING LATE-QUATERNARY FLOOD MAGNITUDES FROM BURIED ALLUVIAL PALEOSOLS IN THE CENTRAL GREAT PLAINS, USA (Invited Presentation)

This paper demonstrates how buried alluvial paleosols can be used to reconstruct ancient flood magnitudes. A systematic study of late-Quaternary landscape evolution in the Central Great Plains documented widespread, deeply buried alluvial paleosols dating to the terminal Pleistocene and early Holocene. Although soil development was underway as early as ca. 13,400 ¹⁴C yr B.P. in some alluvial settings and continued until ca. 9000 ¹⁴C yr B.P. at a few localities, 11,000-10,000 ¹⁴C yr B.P. appears to encompass a major episode of quasi landscape stability characterized by cumulative soil development in stream valleys throughout the region. This episode coincides with the Younger Dryas (YD) Chronozone, a period of cooler climate compared to the preceding Allerod interstadial. A reduction in moisture, which also seems to have characterized the YD in the mid-continent, cannot by itself account for the thick, organic-rich alluvial soils that formed during this period. The ubiquitous cumulative soils typically have overthickened A horizons and organic-rich B horizons that are products of either in situ organic-matter accumulation during periods of slow alluviation, gradual deposition of organic-rich alluvium, or a combination of both processes. Regardless of the dominant process of soil melanization, alluviation did not cease during the YD. Instead, the rate of alluviation slowed and allowed thick, organic-rich soil horizons to form. If mean annual precipitation declined but extreme rainfall events periodically occurred during the YD, paleosols with cumulative profiles would not be present in floodplain deposits dating to that period; high-magnitude floods produced by excessive rainfall favor rapid deposition instead of soil cumulization on floodplains. Therefore, based on the paleosol record, the Central Plains did not experience high-magnitude floods immediately before, during, or shortly after the YD Chronozone. This finding has important paleoclimatic implications.