Luminescence dating of delta sediments: Novel approaches explored for the Ganges-Brahmaputra-Meghna Delta

Abstract

Deltas where luminescence dating is most essential due to organic-poor geologic records are also those where it is often most challenging due to unsuitable luminescence properties of quartz grains, associated with rapid production of young clastic sediment. One example is the Ganges-Brahmaputra-Meghna Delta (GBMD), where Himalaya uplift drives erosion, production, and delivery to the delta plain of poorly sensitized quartz sand. Luminescence dating of fluvial deposits may be further complicated by partial bleaching prior to deposition. Here, we use GBMD quartz and polymineral sediment, including sand and silt fractions, with constrained depositional ages between a few years and a few centuries to test novel approaches to luminescence dating of fluvial deposits in an otherwise challenging setting. This produces the first delta-wide assessment of GBMD sediment luminescence dateability. We use a new multiple-signal SAR (MS-SAR) bleaching index (BI) to explore zeroing of the luminescence signals of sediment prior to deposition and to quantify the IR, pIRIR, and TL residual doses of GBMD polymineral silt with well-reset BSL signals. This test establishes BI values that can be used to identify sufficient bleaching of Holocene sediment with unknown depositional ages, thereby improving confidence in quartz silt dating. We find that GBMD quartz sand is unsuitable for luminescence dating in most localities. By contrast, GBMD silt is sufficiently bleached and has universally suitable luminescence characteristics, enabling dating of GBMD deposits up to the Last Glacial Maximum. Our findings in the GBMD establish methodology for obtaining and validating luminescence ages for fluvial deposits in challenging settings with unsuitable quartz sand.