USE OF AN ALLUVIAL SEDIMENT ALLOCHTHONOUS CARBON ISOTOPE CLIMATE PROXY FOR RECONSTRUCTING LATE HOLOCENE MISSISSIPPI RIVER BASIN CLIMATE

Although stable isotopes of C in soils and lake sediments preserve information on changing C3/C4 vegetation and have proven to be effective tools for elucidating climate change, there are limitations to both. C-isotopes in alluvial sediments offer an additional tool for study of vegetation changes and climate that overcomes certain shortcomings of soils and lake sediment isotope approaches. Soil C-isotope studies are subject to localized aberrations in conditions (fire, disease, local climate effects) that may affect vegetation and the isotopic signature of resulting autochthonous C and introduce error when expanding climate interpretations to broader spatial and temporal scales. Isotopes of allochthonous C in alluvial sediments provide information integrating input from across a watershed. Alluvial sediments are abundant and sampling locations can be taken from a point within a watershed chosen to yield information at a specific scale. Alluvial sediments are subject to varying sediment contribution amounts from different parts of a watershed, which may result in overrepresentation and affect vegetation and climate interpretation for watersheds large enough to span vegetation/climate zones. Stable C isotopes of an abandoned meander deposit in the Mississippi River meander belt were used to interpret vegetation and climate trends at both basinal and local scales. Allochthonous C was deposited with alluvial sediment, probably sometime between 2.5 and 2.0 kya. Sediment d¹³C values indicate that basin vegetation shifted from a dominantly forested to a dominantly grassland environment. Based on the C3/C4 ratios calculated-as low as 40%, grasslands appear to be over represented in the d¹³C record when taken in the context of the entire Mississippi River watershed; this is probably due to greater erosion rates in the grasslands than in the forest. The shift to grasslands is correlated with a relatively short arid period in the Great Plains, dated between 3.8 and 2.7 kya. If this correlation is correct, the lag between climate change and the isotopic signature preserved in the sedimentary record is 0.2-1.8 ky. In contrast to the allochthonous C, the autochthonous C record from surface soil and a buried soil at the site exhibit d¹³C values that indicate local vegetation remained forested between 2.5 and 2.0 kya.