Facies change vs. climate change; digging through floodplain sediments for a record of bioclimatic change; Geological Society of America, South-Central Section, 41st annual meeting; Geological Society of America, North-Central Section, 41st annual meeting

We present a delta (super 13) C chemofacies model for isolating geomorphic effects from climate change effects on the delta (super 13) C 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 C (sub 3) 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 delta (super 13) C values, or compress, stretch, and truncate portions of the record; this facies is also most responsive to climatic conditions. Alluvial delta (super 13) C chemofacies, when combined with an alluvial chronology, yield a local record of paleoclimate and its influence on the abundance and distribution of C (sub 3) and C (sub 4) vegetation on the valley landscape.