Dating and interpretation of recent clastic sediments in an urban cave

Abstract

Capshaw Cave functions as a major stormwater runoff channel for the city of Cookeville, Tennessee, receiving inputs from several large sinkholes. Sediments deposited in the cave reflect the history of erosion and runoff from the city as it grew over the last century. At various locations in the cave, ~1 m thick sequences of flood-deposited, laminated fine sediments were observed along the modern stream. Alternating laminations observed in the upper ~40 cm of the sediment profile varied between 0.5 cm thick (10Y 7/6 yellow, fine sand) and 2.0 cm thick (10Y 3/2 very dark grayish brown silty sand) layers. Based on measurements of 137Cs activity, the upper 35 cm of sediment was deposited between 1963 (the peak year of 137Cs fallout from nuclear testing) and 2013 (the year samples were collected), at an average rate of 0.7 cm/y. A total of 23 alternating pairs of layers indicate an average flood recurrence interval of ~2.2 years between 1963 and 2013. Total Pb concentrations measured in cave sediments showed a peak at the 45 cm depth, suggesting that sediments above this level were deposited after the decline in Pb emissions in the 1970s, and showing general agreement with the timing of deposition suggested by 137Cs. Below 40 cm, the dark silty sand layers were fewer in number and increased in thickness (up to 10 cm), possibly due to changes in cave hydrology or sediment erosion from the surrounding watershed. These findings suggest that, before the 1960s, sedimentation rates were higher and floods were less frequent. After the 1960s, sedimentation rates decreased and floods became more common, probably as a result of urbanization in the watershed.