HOLOCENE ALLUVIAL FLOODPLAIN CATENA RELATED TO DIFFERENCES IN SOIL DRAINAGE: POSSIBLE ALTITHERMAL (MID-HOLOCENE) CLIMATE RECORD IN THE SOUTHEASTERN US

Studies of Quaternary floodplain soils have provided valuable insights for interpreting paleoclimate records. Two soil profiles were examined on a Late Holocene floodplain in southeastern Tennessee. Both soils classify as Alfisols; one is in the somewhat poorly drained class, whereas the other is poorly drained. Data collected include detailed field descriptions, micromorphology, particle size, high-resolution bulk inorganic geochemistry, and stable carbon isotopes of soil organic matter (SOM). The two soils are separated by 150 m distance, with only 10 cm of elevation difference between the somewhat poorly drained (SPD) pedon and the poorly drained (PD) pedon. Three AMS radiocarbon dates on SOM at depths of 130, 170 and 240 cm constrain the ages of the soils as mid-Holocene (5-6 ka). The d¹³C values of SOM for the SPD pedon range from –25.8 to -21.1 ‰ PDB (heaviest values occur at a soil depth of 210 cm); the d¹³C values of SOM for the PD pedon range from –25.8 to -23.2 ‰ (heaviest values occur at a soil depth of 140 cm). These heaviest carbon isotope values identify a soil ecosystem that was 65% C3 during mid-Holocene drier climate conditions, tentatively correlated with the Altithermal event (5000-6000 yrs BP) documented in Texas and the U.S. High Plains; however, the Altithermal in Tennessee was characterized by more rapid onset and shorter duration (300 yrs). Average rates of floodplain aggradation are estimated as between 0.5-3 mm/yr. Although both soils have well-developed Bt (argillic) horizons containing illuviated clay and redoximorphic features, the PD pedon has less advanced pedogenic development than the SPD pedon, and more gley (Fe reduction and stripping) features due to poor soil drainage, which is a direct consequence of higher seasonal water table. Concentrations of Ba, Fe, and Mn are useful for defining the position of the highest seasonal water table. Particle size data and Zr and TiO₂ depth plots establish that the basal portions of the two soil profiles formed from similar fining-upward alluvial parent material, and that additional deposition comprises the upper 70 cm of the SPD pedon that is not present in the PD pedon. This study suggests that floodplain sediments in the inland southeastern US can be useful archives of paleoclimate information that have not been previously recognized.