A 200-YEAR RECORD OF CAVE SEDIMENTATION AND FLOODING, CAPSHAW CAVE, TENNESSEE, USA

A 2-meter thick sequence of flood-deposited clastic sediments was investigated from Capshaw Cave, on the East Highland Rim in Middle Tennessee, USA. These sediments were analyzed for particle size and heavy metal concentrations. In addition, Pb-210 and Cs-137 dating methods were utilized to estimate the age of the deposits. The sequence shows approximately 40 alternating couplets of silt/clay (dark color) and fine sand (light color). Silt/clay layers range in thickness from 2 to 10 cm, while fine sand layers were all less than 2 cm in thickness. We interpret these alternating layers to be the result of flooding in the cave, with the silt/clay layers probably being deposited at peak flood stage of large floods, when flooding to the roof of the cave creates sump conditions and low water velocities. The thickness of the silt/clay layers may be related to the length of time the cave was flooded. The fine sand layers were likely deposited in the waning stages of each flood, as the cave drains and water velocities increase. Chronology of this sedimentary sequence was determined using Pb-210 dating and by finding the depth to the atomic testing Cs-137 peak (1963) in cave sediments. Dating results indicate that the upper 40 cm of deposition occurred after 1963 and that the entire 2-m sequence was deposited within the last 200 years. Thus over the last 200 years, the sedimentation rate was approximately 1.1 cm per year up until 1963, and 0.8 cm per year since 1963. The difference in sedimentation rates between these two time periods may reflect a change in the watershed, such as increased soil erosion from hillslopes during land clearing and human settlement during the earlier period. However, the dating methods are not thought to be precise enough to draw such a conclusion with confidence. The 40 flood couplets deposited over the previous 200 years suggest a return interval of ~5 years for major, cave-filling floods. Lead concentrations in the sediments ranged from 80 to 130 ppm, significantly higher than background soil lead concentrations measured in nearby rural areas outside the cave. Chromium concentrations in the cave sediments were also higher than samples from outside the cave. Clastic deposits preserved in Capshaw Cave provide a record of flooding as well as a record of ‘legacy’ sedimentation that includes the effects of urbanization on cave sediments.