FACIES CHANGE VS. CLIMATE CHANGE: DIGGING THROUGH FLOODPLAIN SEDIMENTS FOR A RECORD OF BIOCLIMATIC CHANGE

We present a δ¹³C chemofacies model for isolating geomorphic effects from climate change effects on the δ¹³C record of midwestern alluvial records. Channel chemofacies values are erratic in profiles due to pulses of allochthonous organic material, leaf litter, and weakly expressed, ephemeral soils; the average value of the profile represents a watershed-scale vegetation community. Proximal channel chemofacies values are not significantly affected by allochthonous organic carbon, but tend to reflect the dominance of a C₃ riparian community. Distal channel chemofacies values are often uniform in profiles; vegetation changes are time averaged due to slow sedimentation rates and bioturbation; often this facies overlies channel and proximal-channel chemofacies, and underlies or intertongues with colluvial chemofacies. Colluvial chemofacies consistently reflect vegetation changes along the valley margin, but differences in slope aspect, steepness, and sedimentation rate, may produce shifts of δ¹³C values, or compress, stretch, and truncate portions of the record; this facies is also most responsive to climatic conditions. Alluvial δ¹³C chemofacies, when combined with an alluvial chronology, yield a local record of paleoclimate and its influence on the abundance and distribution of C₃ and C₄ vegetation on the valley landscape.