SPELEOTHEM MICROMORPHOLOGY ENHANCES INTERPRETATIONS OF THE TIMING AND STRUCTURE OF THE MID-HOLOCENE WARM PERIOD

Speleothems are valuable archives of climate change, typically evaluated using δ¹³C and δ¹⁸O of carbonate, as well as trace elements. We use micromorphological features and stable carbon isotopes in a speleothem, collected from Raccoon Mountain Cave (RMC) near Chattanooga, TN with 11 U/Th dates spanning the past 15 ka, to refine interpretations of mid-Holocene paleoclimate records in the southeastern US region. Ultra-polished thin sections were examined using standard PPL and XPL, CL, and UV fluorescence (UVf) to characterize disconformity surfaces and microfacies. UVf behavior of speleothem calcite is controlled by the presence of organic macromolecules (humic substances, lipids and lignin) transported in cave water as dissolved organic matter (DOM). During the growing season (spring, summer) DOM remains recycled in the soil surface, cave water is low in DOM, and ppt cave calcite is non UVf. During the dormant, non-growing season (fall, winter) DOM enters cave water in higher concentrations and ppt cave calcite is bright UVf. Seasonal dark-bright UVf bands are thus interpreted like tree-ring records. Abrupt onset of wet climate phases are identified by disconformity surfaces with calcite crystal corrosion and truncation, associated non-UVf calcite overlain by bright UVf, and perched terra rossa (clastic) material, which occur at: (13,000-7,500, 6890-5959, and 5340-4870 yr BP). The 2-100 μm thick, micro-banded (bright/dark UVf) intervals record drier periods, which occur at: (7400~6890, 5950~5340, and 4870~4050 yr BP). Measurement of sequential microbands has the potential to yield time-series annual records of paleoclimate in the speleothem. Drought cycles of 200-300 yrs duration between 4500-6500 yr BP, which were previously reported in a nearby floodplain, are correlative with the RMC speleothem micromorphology. However, because precisely dating floodplain profiles is often problematic due to a highly dynamic depositional and soil-forming environment, speleothems examined using UVf provide a fine-scale resolution unobtainable using floodplain deposits.