Elemental and isotopic analysis of sediments from Oxbow lakes in the Mississippi Delta.

摘要

Sediment cores from seasonal wetland and open water areas from Oxbow lakes [Beasley (BL), Hampton (HL), Washington (WL), Roundaway (RL), Sky (SL) and Wolf (WL)] in the Mississippi Delta, whose chronology was previously determined by conventional Pb-210 and Cs-137 age-dating, were analyzed, for the first time, for total-Hg, Pu isotopes (Pu-239 and Pu-240), and Pb isotopes (Pb-206, Pb-207, Pb-208 and Pb-210). The purpose was to: (1) assess temporal and spatial patterns of metal deposition in the region, and (2) evaluate the feasibility of using ICPMS as an alternative to radiochemical analyses for fallout-Pu and Pb-210.;For Hg, wetland sediments contained higher levels than open water areas. This is likely because the wetland generally contained finer particles with higher levels of total organic carbon (TOC) from natural organic matter which has a strong affinity for mercury. Concentrations were generally similar between lakes suggesting the source of Hg to the sediments is widespread; non-point sources may include atmospheric deposition and geologic sources within the watershed. Mercury levels were uniform with depth, except for BL wetland and SL open water which decreased with depth. However, normalizing the data to loss-on-ignition (TOC) gave a weak but consistent trend for increasing Hg with depth for four of five lakes. Annual loading (flux) of Hg was estimated at 4.3, 19.2, 13.4, and 20.7 ng/cm2/year for BL, HL, SL and WL, respectively. For Pu, the mean Pu-240/Pu-239 atom ratio was ∼0.177 indicating global fallout as the primary and likely source of the Pu. Analyses of an undisturbed sediment core yielded a Pu peak at a depth which is in good agreement with conventional Cs-137 and Pb-210 dating. Analysis of a sediment core which was later found out to have been disturbed (in-situ) yielded a broad and uncharacteristic profile of Pu vs. depth. Because sediment can be mixed by natural events after deposition, ICPMS can thus serve as a tool to simultaneously identify (screen) sediments that have been disturbed (eliminating costly Pb-210 analyses on such cores), and to provide a chronological marker for those that haven't.;For Pb, total concentrations varied from 1.5 ppm to 12 ppm, with a peak level during the 1950's. Isotopic signature plots suggest "natural" Pb (from soil) as the primary source of the lead, with coal and gasoline as slight contributors; however, more study is needed to confirm this. Accurately measuring the Pb-210 isotope by ICPMS was problematic. The levels were low and subject to interference from stable isotopes of Pb, possibly by the tail of the large 208Pb peak and/or from polyatomic interference such as 208Pb 1H1H.