2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 51
Presentation Time: 1:30 PM-5:30 PM


HAJ, Adel1, BETTIS III, E. Arthur1, CARPENTER, Scott2 and DORALE, Jeffrey A.2, (1)Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242, (2)Geoscience, Univ of Iowa, 121 Trowbridge Hall, Iowa City, IA 52242, adel-haj@uiowa.edu

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.