Multiple Proxy Evidence from Paleosols for Abrupt Late Neogene Warming at the Gray Fossil Site, Northeastern TN, USA

The late-Neogene portion of the Gray Fossil Site (GFS) strata are characterized by >20 m thick succession of lacustrine sediments overlain by ~5 m of fluvial sediments and multiple paleosols that collectively provide an outstanding opportunity to reconstruct paleoclimate. A paleo-Vertisol indicates that the lake base-level decreased substantially and exposed previously deposited lacustrine sediments to pedogenic modification. Additional lacustrine sedimentation was deposited within the basin, capped by a mature paleo-Alfisol. Dominantly fine-grained fluvial overbank sediment and colluvial sediment overlies the paleo-Alfisol and is characterized by relatively immature paleosols (Inceptisols, Entisols) with abundant rooting, illuviated clay, and Fe-oxide nodules. Multiple proxies were applied to reconstruct paleoclimate including: d¹³C from total soil organic carbon (SOC), whole-rock geochemical analysis for estimating mean annual precipitation (MAP) and temperature (MAT), respectively, and micromorphologic and lithostratigraphic analysis of paleosol mineralogy and character. The paleo-Vertisol, paleo-Alfisol, and lower portion of the fluvial paleosol succession are characterized by d¹³C values (SOC) averaging -28.2±1.4‰, MAP estimates of 1319±21 mm/yr, and MAT estimates of 11.5±0.3ºC. Paleosols from the upper sediment facies are characterized by d¹³C values averaging -24.2±0.8‰, MAP estimates of 1363±9 mm/yr, and MAT estimates of 15.8±0.3ºC. The deeper paleosols also include abundant Fe-Mn nodules and tend to be more gleyed, whereas abundant Fe₂O₃ and higher-chroma colors characterize the upper paleosols. Thus the paleosol succession records a paleoclimatic transition that is consistent with an increase in MAT, but nearly constant MAP values. The increase in MAT would have increased evapotranspiration creating drier climatic conditions. Under such conditions d¹³C of SOC would become less negative in response to some combination of plant water-stress, an increase in the abundance of C4 plants, and/ or an increased isotopic fractionation of SOC during burial and mineralization due to the increase in microbial processing associated with elevated temperatures.