The chronology of a sediment core from incised valley of the Yangtze River delta: Comparative OSL and AMS 14C dating

Optically stimulated luminescence (OSL) dating has gained increased use in dating deltatic deposits, however, its application can be hindered by the problem of incomplete bleaching. To address this limitation, we test the single-grain OSL method for the first time in the Yangtze River delta. A total of eight OSL and 14 AMS C-14 samples were obtained from a 50.8 m long sediment core from the incised valley of the delta. Quartz extracted from eight OSL samples from the core was first measured with small multi-grain aliquots using medium (45-63 mu m) and coarse-grained (90-125 mu m or 150-180 mu m) fractions to test the internal consistency of their respective ages. The results showed that four of five medium-grained quartz samples appeared to be well bleached. In contrast, the coarse-grained quartz samples revealed poor bleaching, except for two samples from the delta front facies. Five coarse-grained quartz OSL samples were further analyzed using a single-grain OSL technique. Only 0.5-0.7% of the grains passed the rejection criteria. Single-grain OSL dating is appropriate for age determination of coarse-grained sediments which have been affected by incomplete bleaching. However, it consumes a lot of instrument time. Central and minimum age models (CAM and MAM) were used to calculate the burial age of the samples; MAM ages fit best in the stratigraphic sequence. Small aliquots are recommended as effective in identifying incomplete bleaching for medium- or coarse-grained quartz. A comparison of AMS C-14 dates and OSL ages shows that C-14 values are 1-3 ka older than OSL ages. According to the OSL ages, core SD experienced rapid accumulation 10-8 ka and in the last 2 ka. The former is linked to rapid sea-level rise in early Holocene, while the latter is primarily due to the migration of depo-center towards the core site, rather than entirely increased sediment delivery caused by human activities.