AN INDEPENDENT CONSTRAINT ON THE AGE OF THE GRAY FOSSIL SITE, TN USING COSMOGENIC NUCLIDE BURIAL DATING ON A 35-M-DEEP CORE

The Gray Fossil Site is located in northeastern Tennessee and consists of multiple coalesced sinkholes infilled with fossil-bearing lacustrine sediments (Shunk et al., 2006, 2008; Whitelaw et al., 2008; Zobaa et al., 2011). Initial age estimates (4.5-7 Ma) were broad and based on limited biostratigraphy (Wallace and Wang, 2004). Subsequent palynological work on core samples interpreted the site as a Paleocene-Eocene filled basin with a secondary Late Miocene veneer (Zobaa et al., 2011). More recently, Samuels et al. (2018) refined the age of the uppermost material to 4.5-4.9 Ma on the basis of micromammal biostratigraphy. Precisely constraining the age of the deposit has significant implications for interpreting the occurrence of mammal species and their movement through time, as well as regional erosional trends. To date, no radiometric constraints exist on the sinkhole’s age.

Cosmogenic nuclide burial dating provides an opportunity to independently estimate the age of the site and the local erosion rate. By measuring cosmogenic ²⁶Al and ¹⁰Be at a range of depths and modeling their production and decay, it is possible to date deposits through the Pliocene. We measured ²⁶Al and ¹⁰Be concentrations at eight intervals throughout the 35-meter-deep core previously described by Zobaa et al. (2011). The ¹⁰Be concentrations showed an exponential decrease with depth, reflecting the transition from neutron-dominated postburial production of cosmogenic nuclides at shallow depths to muon-dominated postburial production at greater depths. At the core’s base, ²⁶Al and ¹⁰Be concentrations were 3.48 ± 0.60 ·10⁴ at/g and 4.54 ± 0.29 ·10⁴ at/g, respectively. These concentrations correspond to a minimum age of 4.51 ± 0.42 Ma, consistent with biostratigraphic age estimates. Additional ²⁶Al measurements will provide a more precise estimate of the site’s age. We will present additional measurements and a refined age of the fossil site, as well as an overview of its implications for regional geologic history.