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Effects of sample mass and type on radiocarbon dating of arctic lake sediments.
Oswald, W Wyatt1, Anderson, Patricia1, Brown, Thomas2, Brubaker, Linda1, Gomez, Patricia1, Hu, Feng Sheng3, Lozhkin, Anatoly4, 1 2 3 4
ABSTRACT- Radiocarbon dating of lacustrine sediments is complicated by the possible contamination of sediments by old organic matter. This problem is acute in arctic regions, where organic materials decompose slowly and may remain in permafrost for hundreds or thousands of years before being eroded into lake basins. Accelerator mass spectrometry (AMS) radiocarbon analysis permits dating of individual macrofossils, thus avoiding some problems associated with radiometric dating of bulk sediments. However, some AMS dates from arctic sediment records are anomalous compared to the majority of dates from a core. These age-reversals occur in many cores from Siberia and northern Alaska. We hypothesize that these unreliable dates result from small sample mass (which might increase risk of laboratory contamination), or from the type of material dated. We conducted experiments to explore the consequence of sample mass and type. Radiocarbon dates were obtained for small (<0.1mg), medium (0.1-0.3mg), and large (>0.3mg) pieces of the same macrofossil, and several types of plant material were dated from the same depth in a sediment core. In these experiments, radiocarbon age did not vary predictably with sample mass. However, lignified macrofossils (wood, sturdy seeds) were generally older than less-robust macrofossils (moss, leaves) from the same depth in the core. These results suggest that interpretations of radiocarbon ages should consider the possibility that plant macrofossils may differ in terms of their taphonomy or susceptibility to contamination.
KEY WORDS: Arctic, paleoecology, radiocarbon dating