CARBON ISOTOPE RECORD OF HOLOCENE BIOCLIMATIC CHANGE IN ALLUVIAL FANS OF THE UPPER MIDWEST, USA

The carbon isotope record of soil organic matter (SOM) provides a proxy for local vegetation conditions. On well to moderately well drained alluvial fans in the Midwest, C₃ vegetation is favored during cool and/or moist climatic episodes while C₄ vegetation is dominant during dry and/or warm periods. Vegetation shifts indicated by changes in δ¹³C can be related to a regional sequence of climatically-driven Holocene vegetation change and can thus provide information on local landscape response to bioclimatic change. Carbon isotope data from well-dated alluvial fan sequences show a consistent pattern of Holocene vegetation shifts from the semiarid plains of eastern Nebraska, across the prairies of Iowa and into the prairie-forest ecotone of southern Illinois. General trends include: 1) a C₄ peak between 10,200 and 10,500 yrs. B.P., suggesting cool and dry conditions during the Younger Dryas, 2) C₃ vegetation dominance during the early Holocene in the eastern part of the Upper Midwest; a subsequent stepwise increase in C₄ vegetation indicating warmer and drier conditions that culminates with the highest δ¹³C values recorded as a broad peak centered around 6000 yrs. B.P., 3) increased abundance of C₃ vegetation in the late Holocene with a pronounced C₄ peaks around 3100 yrs. B.P., and 4) δ¹³C values in surface soils that are consistent with GLO records of early Historic vegetation. These trends are compared to depositional records from alluvial fans across the region to examine relationships among bioclimatic change, tributary valley behavior and fan sedimentation.