Changes in sediment accumulation rate in an oxbow lake following late 19th century clearing of land for agricultural use; a (super 210) Pb, (super 137) Cs, and (super 14) C 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 (super 210) Pb and (super 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 (super 210) P and (super 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 (super 210) Pb and (super 137) Cs methods. Modern sediment accumulation rate based on bomb-pulse (super 14) C activity of twigs from cores from the West site is approximately 1.0 cm/yr, in agreement with the (super 210) Pb and (super 137) Cs results results. Historic sediment accumulation rates were estimated at the West site using twigs from deeper intervals with pre-bomb 14C 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.