Accurate Lacustrine and Wetland Sediment Accumulation Rates Determined from¹⁴C Activity of Bulk Sediment Fractions

Abstract

In the absence of identifiable macrofossils in lacustrine sediments, radiocarbon 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. If the various processes that contribute carbon to the system are relatively constant over time, however, 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 acids extracted from <250-μm fraction, and untreated <250-μm 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 activities 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.

The evolution of an oxbow lake in the Mississippi alluvial floodplain

“…Removal of large particulate matter was done to ensure that a single large organic fragment would not disproportionately bias the 14 C activity. Samples were not pretreated because previous work at Sky Lake demonstrated that 14 C activities of bulk sediment fractions in this system are not significantly different with and without pretreatment (Walker et al 2007). Sieved samples were combusted in the presence of O 2 at 400°C (750°F).…”

“…This age reflects a probable bias of uncertain magnitude due to incorporation of reworked material (Walker et al 2007), making it useful only as an upper limit on the possible age rather than the actual age of abandonment. A cultural resources survey by Thorne and Curry (1983) included several radiocarbon dates from Sky Lake sediment, but the reported ages varied widely without meaningful correlation with depth.…”

“…Where macrofossils are difficult to find, Walker et al (2007) demonstrated that the 14 C activity of bulk sediments can be used to determine sediment accumulation rates from which ages of specific layers can be inferred. Bulk sediments are not generally used for 14 C dating because they include organic material of a variety of ages and sources, and often yield erroneously old apparent ages (Olsson1991; Björck et al 1994Björck et al , 1998Pessenda et al 2001).…”

“…The methods previously described were tested and employed to decipher a partial history of sediment accumulation in Sky Lake, an oxbow lake in northwestern Mississippi (Davidson et al 2004;Walker et al 2007). In the current study, improved precision from incorporating 14 C activities from bulk sediment samples and collection of deeper cores from the open water region of the lake (figure 1) have allowed a complete mapping of the history of sedimentation in the lake from the time it was abandoned as an active meander bend to the present.…”

Dating Human Occupation on Diatom-Phytolith-Rich Sediment: Case Studies of Mustang Spring and Lubbock Lake, Texas, USA

“…A common difficulty in 14 C dating sediment is the presence of carbon from different sources that would give very different and misleading 14 C signatures (e.g. McGeehin et al 2001;Walker et al 2007). Sources of carbon include residual geological material (e.g.…”