Radiocarbon dates in China's Holocene Yangtze delta: Record of sediment storage and reworking, not timing of deposition

Evaluation of conventional and AMS radiocarbon dates, obtained for 65 Holoc ene sediment samples in 20 cores collected in the Yangtze delta, China, ind icates that only about one in 4 dates falls within an expected time range. In this Holocene deltaic system, most radiocarbon-dated samples do not beco me progressively younger upcore, and there is no direct relationship betwee n age and depth. About one in 6 dates is too young, recording locally trunc ated stratigraphic sections. Of particular note are the nearly two-thirds o f all radiocarbon dates that record ages that are much too old, by 1000 to 10,000 years (23% are of Pleistocene age). This phenomenon results from the introduction of old carbon during sediment storage and reworking along the dispersal path between headlands and the coast. Thus, rather than recordin g final time of Holocene deposition at deltaic core sites, dates in Yangtze delta sediment appear to indicate a time lapse that occurred during storag e and transport cycles in the fluvial valley and delta plain. There is no s imple, obvious or universal solution to this radiocarbon-dating problem in deltaic sequences. The problem warrants attention since reliable dating of Holocene sequences is essential for measurement of rates of sediment accumu lation and delta margin subsidence relative to sea level, two of the parame ters needed to help implement protection measures along the vulnerable low- lying Yangtze delta coast. At present, a multi-method dating approach (amin o acid racemization, isotopic analyses, archaeological determination, and o thers) would constrain sediment age, and provide a more reliable measure of deposition time in such settings than by radiocarbon dating alone.