Submitted to: Radiocarbon
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/27/2007
Publication Date: 9/1/2007
Citation: Walker, W.G., Davidson, G.R., Lange, T., Wren, D.G. 2007. Accurate lacustrine and wetland sediment accumulation rates determined from 14c activity of bulk sediment fractions. Radiocarbon. 49(2):983-992. Interpretive Summary: Sediments trapped in reservoirs represent an opportunity to assess historic erosion patterns in watersheds. 14C is a method that has been used in the past to date sediments by dating organic matter such as leaves or twigs from the sediment deposit. In the present paper, bulk sediment with some organic matter was used to determine the rate of deposition of sediment in Sky Lake, an oxbow in the Mississippi Delta region. The 14C method allowed for both recent and ancient (1000’s of years ago) sedimentation rates to be measured. This gives an indication of how much erosion took place prior to settlement of the area. It was found that sedimentation rates of up to two centimeters per year occurred while the oxbow was either still part of the Mississippi River, or during the time immediately after when it would have been subjected to more frequent flooding. Since that time, the sedimentation rate has been steady at approximately 0.17 mm/yr.
Technical Abstract: In the absence of identifiable macrofossils in lacustrine sediments, 14C dating must rely on pollen or bulk sediment fractions. Bulk sediment fractions are not generally preferred because they contain an unknown mixture of organic material of variable age, they may contain dead carbon such as lignite that is difficult to eliminate, and material of aquatic origin may be subject to reservoir effects. However, if the various processes that contribute carbon to the system are relatively constant over time, changes in 14C activity with depth may be used to accurately estimate sediment accumulation rates even if the absolute ages are erroneous. In this study, fine-grained fractions (250-710 µm organic material, humic acid extract from <250 µm size fraction, and <250 µm size fraction combusted at low temperature) were analyzed and compared with terrestrial plant stems (twigs), charcoal, and wood fragments in sediments from an oxbow lake in Mississippi, USA. The 14C activity of the bulk fractions were highly linear with depth, and produced consistent calculated sediment accumulation rates similar to, and perhaps more reliable than, rates determined using twigs or charcoal.