Changes in Sediment Accumulation Rate in An Oxbow Lake Following Late 19Th Century Clearing of Land for Agricultural Use: A ²¹⁰Pb, ¹³⁷Cs, and ¹⁴C Study in Mississippi, USA

Abstract

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. Modern (post settlement and land clearing) sediment accumulation rates were calculated using 210 Pb and 137 Cs 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. Modern sediment accumulation rates based on 210 Pb and 137 Cs 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 210 Pb and 137 Cs methods. Modern sediment accumulation rate based on bomb-pulse 14 C activity of twigs from cores from the West site is approximately 1.0 cm/yr, in agreement with the 210 Pb and 137 Cs results. Historic sediment accumulation rates were estimated at the West site using twigs from deeper intervals with pre-bomb 14 C 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.