Inter-comparison of optically stimulated luminescence (OSL) ages between different fractions of Holocene deposits from the Yangtze delta and its environmental implications

Abstract

In fluvial and deltaic environments, different grain-size fractions experience varying degrees of bleaching of OSL signals due to variations in hydrodynamics and depositional processes. This has led to ambiguity for selection of grain-size fraction suitable for OSL dating. Comparison of OSL results from multiple grain-size fractions is one approach to ensure the reliability of OSL ages. We determined the quartz OSL ages of four grain-size fractions from fine silt to sand, together with the single grain OSL sensitivity in a Holocene sediment core from the Yangtze delta. For samples from the lower part of the sediment succession (including tidal river, tidal trough fill, estuarine front, tidal sand and delta front facies), OSL ages determined for different grain-size fractions were generally mutually consistent within the age range of ~2–9 ka, implying minor portion of incompletely bleached grains in the sediment at the time of burial. In contrast, significant partial bleaching was observed for three samples from younger delta front facies deposited during 0.4–0.6 ka. In these three samples, fine-silt quartz OSL ages were overestimated by ~0.5–2.0 ka compared with ages of coarser fractions. Based on the investigation of quartz luminescence sensitivity, we suggest that the southward shift of the Yellow River and increasing erosion in the Yangtze River catchment have led to enhanced water turbidity, resulting in partial bleaching of the sediment and substantial overestimation of the quartz OSL ages of fine silt during 0.4–0.6 ka. Alternatively, the erosion of older deposits and their redeposition elsewhere could retain large residual OSL ages. Notably, fine-silt quartz provides reliable ages for the period of 9–2 ka, suggesting that there was a lower degree of water turbidity compared with that since 2 ka. Therefore, the degree of OSL age overestimation may provide an indication of changes in water turbidity and sedimentary environment. Overall, our findings highlight the complexity of OSL dating in delta environments, and that an understanding of sediment transport and depositional processes is critical for reliable OSL dating.