Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz

Abstract

This paper presents an improved method for the optical dating of Holocene sediments from a variety of geomorphic settings. We have measured the equivalent dose (D-e) in individual grains of quartz, using green laser light for optical stimulation, and have simulated the D-e distributions for multiple-grain 'synthetic' aliquots using the single-grain data. For 12 samples of known (independent) age, we show that application of a 'minimum age model' to the single-grain and 'small' (10-grain) aliquot D-e data provides the most accurate estimate of the burial dose for nine of the samples examined (3 aeolian, 5 fluvial, and I marine). The weighted mean D-e (as obtained using the 'central age model') gives rise to burial age overestimates of up to a factor of 10 for these nine samples, whether single grains, small aliquots, or 'large' (100-grain) aliquots are used. For the other three samples (two aeolian and one fluvial), application of either the minimum age model or the central age model to the single-grain, small aliquot, and large aliquot D-e data yields burial ages in accord with the independent age control. We infer that these three samples were well bleached at the time of deposition. These results show that heterogeneous bleaching of the optical dating signal is commonplace in nature, and that aeolian transport offers no guarantee that the sample will be well bleached at the time of deposition. We also show that grains sensitive to infrared (IR) stimulation can give rise to low D-e values, which will result in significant underestimation of the burial dose and, hence, of the age of deposition. We demonstrate that use of a modified single-aliquot regenerative-dose protocol incorporating IR stimulation prior to green light stimulation deals effectively with contamination by IR-sensitive grains. We conclude that application of the modified protocol to single grains or small aliquots of quartz, using the lowest D-e population to estimate the burial dose, is the best means of obtaining reliable ages for Holocene sediments from a wide range of depositional environments.