HIGH-RESOLUTION RAINFALL RECORDS FOR MIDDLE AND LATE HOLOCENE BASED UPON SPELEOTHEM ANNUAL UV FLUORESCENT LAYERS, RACCOON MOUNTAIN CAVE, TN

Speleothems are valuable archives of climate change but resolution of annual records has been elusive. We analyzed 4796 counts of annual UV fluorescent (UVf) laminae observed in polished thin sections of a speleothem, as well as 305 stable carbon and oxygen isotope measurements, using both time-series and spectral analysis, to refine interpretations of mid- and late Holocene paleoclimate records in the southeastern US. The 15 cm-long speleothem RM0710, collected from Raccoon Mountain Cave near Chattanooga, TN with 11 U/Th dates spanning the past 15 ka, has exceptional preservation of mid- and late Holocene annual records that permits semi-continuous time-series of UVf laminae thickness. Speleothem UVF laminae average15 μm/yr, identical to the average determined for the middle and late Holocene portions of the speleothem (7600-796 yr BP) based on the U/Th ages and interval thicknesses. UVf laminae counts between paired U/Th ages are also consistent with determined ages and their uncertainties. In contrast to the incomplete records of the very latest Pleistocene to early Holocene attributed to wet conditions and intermittent speleothem submergence, the mid-Holocene Thermal Maximum is manifested by long periods (100-400 yr) of drought with thin deposits (3-10 μm/yr) punctuated by shorter periods (5-20 yr) of higher rainfall with thicker deposits (30-80 μm/yr), and with occasional high-rainfall “extreme” events (annual deposits as thick as 150-170 μm). The Late Holocene, in comparison, is characterized by overall wetter conditions and more regular (sinusoidal curve) behavior suggesting 50-100 yr cycles of higher and lower rainfall, with deposits ranging from 5-30 μm/yr. Possible millennial-scale (1500 yr) Bond Cycle events are also recorded (Little Ice Age, B1-B5) manifested by major dissolution surfaces abruptly overlain by thick annual deposits. Statistical analyses will test these interpretations of variable periodicities for rainfall. Rainfall records established for the speleothem are in agreement with recent carbon isotope records from floodplain deposits and buried soils from both the Great Plains and eastern US. This method should be used to examine other speleothems in order to resolve detailed paleo-rainfall records, provided preservation of UVf laminae exists.