Changes in sediment accumulation rate in an oxbow lake following late 19th century clearing of land for agricultural use: A Pb-210, Cs-137, and C-14 study in Mississippi, USA

Sediment cores were collected from 2 sites in the forested fringe of an oxbow lake surrounded by land that was converted from forest to agricultural use in the late 19th century. The 2 sampling areas were selected to represent areas of high (West site) and low (East site) current sediment accumulation rates, based on distance from a perennially discharging stream. Modem (post settlement and land clearing) sediment accumulation rates were calculated using Pb-210 and Cs-137 on bulk sediment samples from 2 cores from each site. Two additional cores were collected from each site for radiocarbon analysis of twig cellulose with the assumption that most twigs in the sediment within the forested fringe fell from overhead and are contemporaneous with the sediment. Only the West site, however, yielded sufficient identifiable twig material for analysis. Modem sediment accumulation rates based on Pb-210 and Cs-137 fall between 0.2-0.4 cm/yr at the East site, and 0.7-1.3 cm/yr at the West site (nearest the stream inlet), with approximate agreement between the Pb-210 and Cs-137 methods. Modem sediment accumulation rate based on bomb-pulse C-14 activity of twigs from cores from the West site is approximately 1.0 cm/yr, in agreement with the Pb-210 and Cs-137 results. Historic sediment accumulation rates were estimated at the West site using twigs from deeper intervals with pre-bomb C-14 activity. Sediment covering approximately 1000 yr of pre-settlement, sediment accumulation exhibited evidence of minor bioturbation or in-washing of reworked material, but with a clearly lower accumulation rate of less than 0.1 cm/yr.