Wednesday, 8 October 2008: 9:15 AM
George R. Brown Convention Center, 352DEF
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.