ALLUVIAL RECORDS OF HOLOCENE ENVIRONMENTAL CHANGE, NORTHEAST OHIO

Recent invigorated stream downcutting at many gorge sites in Northeast Ohio is revealing complex Holocene alluvial sequences preserved in alluvial fans and terraces. These fans and terraces integrate fluvial responses to past climate and land use changes. Bog bottom radiocarbon ages show that the region was deglaciated by 14,500 yr. BP; this age also provides an estimate for the beginning of fluvial excavation of the post-glacial bedrock gorges. Radiocarbon ages at the base of alluvium suggest that downcutting of bedrock gorges proceeded for the next 8000 years until alluvium began to aggrade. Ages on charcoal and wood in debris flows and alluvium cluster around 6000, 2500,1350 and 200 yr. BP. This earliest period of aggradation and the beginning of our record is consistent with work done in the upper Mississippi Valley that has shown a changing flood regime about this time associated with rapid warming into the mid Holocene. Intervals centered on 2500 and 1350 yr BP are times of climatic transition, as well as being associated with increased human occupation in the region. The most recent alluvium is associated with European settlement and soil erosion. We are now targeting sites where mills were established during settlement as mill pond infills often preserve earlier Holocene sedimentary sequences. Ongoing, cooperative work including dating of human occupation in the region, as well as dating of slackwater sediments to better define flood regimes should improve our understanding of the environmental signal that can be inferred from alluvial sequences in Northeast Ohio.