OSL dating of coastal sediments from the southwestern Korean Peninsula: A comparison of different size fractions of quartz

The comparison of optically stimulated luminescence (OSL) ages of different grain-size fractions has given rise to conflicting results over recent years. To test the reliability and validity of OSL dating of fine- and coarse-grained fractions, 25 samples were collected from a 46-m sediment core taken from the southwestern coast of Korea, which contained two typically fine-grained tidal units separated stratigraphically by a coarse-grained sand and gravel layer. A single aliquot regenerative dose (SAR) procedure was applied to chemically separated fine- (4–11 μm, n = 16) and coarse- (90–212 μm, n = 24) grained quartz. The ages of coarse-grained quartz in the upper unit were relatively consistent with those of fine-grained quartz in the same unit of Holocene age (12–0 ka). In contrast, in the lower unit the coarse quartz fraction OSL ages are 30–60% (mean 46%) lower than the fine quartz fraction ages. OSL ages for all fine-grained quartz in the lower unit (139–110 ka) were in good agreement with the last interglacial sea-level highstand [Marine Isotope Stage (MIS) 5e]. However, all OSL ages for coarse-grained quartz in this lower unit (90–45 ka) were younger, corresponding broadly to the last glacial period (MIS 4 and 3). OSL ages obtained for fine-grained quartz were in agreement with indirect age controls provided by lithological and palynological data, implying that the use of the SAR protocol to date fine-grained quartz is appropriate for age control of coastal sediments extending to the last interglacial period. The discrepancies between the ages of different grain-size fractions may have arisen from early saturation of the natural signal and, in some cases, from contamination of the feldspar signal in the coarse quartz. However, more research is required to assess the reliability of OSL ages for fine-grained quartz obtained from the high-dose linear region of the dose response curve.