ASSESSING THE RELIABILITY OF 14C AND OSL DATING OF HOLOCENE FLUVIAL DEPOSITS USING MICROMORPHOLOGY

Two soil and sediment profiles exposed along the floodplain of the Tennessee River provide an excellent opportunity to compare radiocarbon and optically stimulated luminescence (OSL) dating of Holocene fluvial sediments, and to use soil micromorphology as a tool to assess the reliability of these dating methods. The soil profiles occur as interlayered horizons of buried soils and alluvium, with sediments of both profiles indicating an alluvial origin but different degrees of soil development. Clay coatings were found as a common pedological feature in both soil profiles, occurring around mineral grains and in pore spaces. Layered and well-developed clay coatings often take thousands of years to form, because of the time needed for pedogenic clays to weather from parent minerals and for the particles to translocate, accumulate, and orient. Hence, the presence of well-developed clay coatings indicates long periods of surface exposure and pedogenesis. AMS radiocarbon dating results indicate that the profiles at the two sites span the late early Holocene to late Holocene (about 8.5 ka B.P. to 3.3 ka B.P.). In contrast, OSL dating indicates that the point-bar profile is relatively recent (600 to 300 yrs. B.P.) while the cut-bank profile is of upper middle Holocene age (5.7 to 4.7 ka B.P.). Soil micromorphology showed that clay coatings in the point-bar profile are very thin and weakly developed, indicating short times of exposure and pedogenesis to act on the buried soils. In the cut-bank profile, clay coatings are thick and well-developed, indicating longer times of exposure and more extensive pedogenesis. Assessment of the reliability of AMS radiocarbon and OSL ages using soil micromorphology indicates that there has likely been reworking of older datable material used for measuring the AMS radiocarbon ages, because the oldest ages obtained from radiocarbon did not correspond to the thicker and well-developed clay coatings. OSL results show partial bleaching in the quartz grains evidenced in high overdispersion values but a general agreement between the results of soil micromorphology and the ages obtained from OSL. This study shows the utility of using soil micromorphological analysis to evaluate factors that can impact the reliability of depositional age estimates based on radiocarbon and OSL.