Paper No. 3
Presentation Time: 2:20 PM
SPELEOTHEM MICROMORPHOLOGY IMPROVES INTERPRETATIONS OF FLOODPLAIN PALEOCLIMATE RECORDS AND ENHANCES INTERPRETATIONS OF THE TIMING AND STRUCTURE OF THE MID-HOLOCENE WARM PERIOD
Floodplains are valuable archives of climate change, evaluated using δ13C of soil organic matter, as well as pollen, phytoliths and charcoal. A mid-Holocene interval characterized as generally warmer and drier than either the early or late Holocene, has been identified in buried soils in the eastern US in the upper Delaware and lower Tennessee river valleys. In the best case high sedimentation can improve the resolution of the floodplain archive: e.g., in a small tributary of the Tennessee, 4 short-duration (circa 200-300 year) drought cycles between 4500-6500 yr BP were previously reported. However, precisely dating floodplain profiles is often problematic due to a highly dynamic depositional and soil-forming environment. We use micromorphological features and stable carbon isotopes in a speleothem, collected from Raccoon Mountain Cave near Chattanooga, TN with 11 U/Th dates spanning the past 15 ka, to refine interpretations of floodplain paleoclimate records in this region. Ultra-polished thin sections were examined using standard PPL and XPL, CL, and UV fluorescence (UVf) to characterize disconformity surfaces and microfacies. Disconformity surfaces, identified by calcite crystal corrosion and truncation, associated non-UVf capped by bright UVf, and perched terra rossa (clastic) material, record intermittent wet periods during the latest Pleistocene to mid-Holocene (13,000-7,500, 6890-5959, and 5340-4870 yr BP), which correlate with periods of alluviation identified in nearby floodplains. Coarse, sector-zoned UVf crystals with wide bands may have formed during wetter periods, whereas 10-100 μm thick, micro-banded (bright/dark UVf) intervals, which may represent annual precipitation bands, probably record drier periods (7400~6890, 5950~5340, and 4870~4050 yr BP) reflected by soil formation on floodplains. An interval at 87-83 cm depth in Anderson Pond (>charcoal age at 80-81 cm depth, 6242-5997 yr BP) with evidence of subaerial exposure could record the first dry period identified in the speleothem. This study demonstrates that multiple proxy approaches are robust in reconstructing Quaternary paleoclimatic variations in rainfall and their subsequent influences on alluvial history. Furthermore, speleothems provide a fine-scale resolution unobtainable using floodplain deposits.