FLOODPLAINS AS REPOSITORIES OF INFORMATION ON CARBON ADDITION RATES IN DEPOSITIONAL SOIL PROFILES

River valley bottoms accumulate and store autochthonous and allochthonous soil C. Improving C dynamic models of riverine setting require a detailed knowledge of sedimentation rates and organic C composition for sedimentary sequences spanning the late Holocene. This study examines the effect that fluvial landform type has on C addition rates by comparing two depositional sequences—one a 1.5 m deep, coarse-grained profile from a relict natural levee and the other a 1.8 m, fine-grained profile from a terrace located in the humid subtropical southeastern U.S. and within the Tennessee River drainage of Alabama. Eighteen optically stimulated luminescence (OSL) ages from quartz grains provided age estimates for the profiles. OSL ages from the relict levee range from 3180 to 40 years before present (BP=AD 2010) and a mean sedimentation rate of 0.09 cm yr^-1. In contrast, OSL ages from the terrace are between 1380 and 100 years BP and a mean sedimentation rate of 0.13 cm yr^-1. These sequences contain numerous weakly-developed paleosols, indicating quasi-equilibrium for decadal periods separated by a sequence of flood events in the late Holocene. The mean C addition rate for the surface soil and buried soils in the relict levee, 3.55 ± 1.85 g m^-2 yr^-1, is significantly lower than the mean C addition rate of the terrace profile, 20.84 ± 13.42 g m^-2 yr^-1. Grain size and micromorphology showed that the terrace had more well-developed microstructure from increased silt and clay content, protecting C from oxidation. Subtle differences in landform position along this humid subtropical river valley, and texture and drainage properties resulting from hydropedologic spatial complexity, have a significant effect on the history of C addition and storage.