Inter-comparison of optically stimulated luminescence (OSL) ages between different fractions of Holocene deposits from the Yangtze delta and its environmental implications
Introduction
Optically stimulated luminescence (OSL) dating (Huntley et al., 1985) has become a promising technique for dating fluvial and deltaic sediments (e.g. Jacobs, 2008; Madsen and Murray, 2009; Bateman, 2015; Lamothe, 2016; Nian and Zhang, 2018). However, incomplete bleaching affects the accuracy of OSL ages, which is especially challenging for the age determination of young Holocene sediments (e.g. Madsen and Murray, 2009). One approach to address this problem is to compare OSL ages obtained on different grain-size fractions (e.g. Shen and Mauz, 2012; Wang et al., 2015; Gao et al., 2017, Gao et al., 2019; Nian et al., 2018a, Nian et al., 2018b; Cheng et al., 2020). If the ages of different grain-size fractions are similar, the confidence in the accuracy of the ages will be increased. However, OSL age discrepancies between different grain-size fractions have been commonly observed in different Holocene sedimentary environments, including in the case of fluvial (e.g. Olley et al., 1998), lacustrine (e.g. Zheng et al., 2010), colluvial (e.g. Fuchs and Lang, 2009), glacial (e.g. Hu et al., 2015) and coastal (e.g. Shen and Mauz, 2012; Wang et al., 2015; Nian et al., 2018a, Nian et al., 2018b; Cheng et al., 2020) settings. The cause of such discrepancies is ascribed to differences in the degree of bleaching and the transport history of the grain-size fractions, caused by factors such as sediment source, transport mode (i.e. suspended load vs. bedload), transport distance, and sediment reworking along the pathway from source to sink (e.g. Murray, 1996; Frings, 2008). Therefore, the OSL ages obtained for different grain-size fractions potentially contain information about sediment transport and depositional processes in the environmental setting (e.g. Gray et al., 2019). For example, in fluvial deposits, the OSL signal of fine-grained particles within the suspended load is commonly believed to have a greater chance of being zeroed by sunlight compared with coarse-grained particles within the bedload (Murray, 1996). However, this is not always the case; for example, significant OSL age overestimation of fine-grained quartz was observed in the sediments of a Yellow River terrace (Zhang et al., 2010) and in the North China Plain (Zhao et al., 2019), which was interpreted as being caused by highly turbid water and/or short transport distances due to paleofloods, which reduces the possibility of the complete bleaching of the sediments.
Deltaic deposits are extremely heterogeneous in terms of grain-size composition and one can rarely use the consistent grain-size fraction to date sediments from different depositional settings. It is a common practice to select a single grain-size fraction according to sediment type, and therefore the comparability of OSL ages derived from different grain-size fractions is critical for constructing a reliable age framework. Although there have been several studies of OSL dating using multiple grain-size fractions in deltaic environments, most of them only compared two fractions. For example, the fine silt and sand fractions were compared in the Mississippi River delta (Shen et al., 2015), the Nakdong delta (Kim et al., 2015), the Ganges-Brahmaputra-Meghna delta (Chamberlain et al., 2017) and the Mekong River (Ishii et al., 2020). In these studies, paired OSL ages from the two fractions were generally in agreement, indicating the occurrence of adequate bleaching of the sediments in that environment. In the Yangtze River delta, several studies have reported quartz OSL ages obtained using two grain-size fractions, and the level of agreement between the fractions varied between studies. For example, Wang et al. (2015) compared the 4–11 μm and 100–200 μm fractions for sediment core ECS-DZ1 and found consistent ages. In contrast, the ages of 4–11 μm and 63–100/100–200 μm fractions for cores YZ07 and EGQD14 were inconsistent, due to feldspar contamination of the coarse-grained quartz (Gao et al., 2017, Gao et al., 2019). Our comparison of the ages of the 45–63 μm and 90–125 μm fractions of sediment cores SD, TZ and NT also revealed age inconsistencies for a few samples (Nian et al., 2018a, Nian et al., 2018b). This reflects the complex range of physical processes responsible for quartz signal resetting, which are spatially and temporally heterogeneous in a deltaic setting (Cheng et al., 2020).
Since deltaic deposits vary from clayey-silt salt marsh facies to sandy mouth bar deposits, they are highly suitable for comparing the OSL ages of different grain-size fractions. We conducted OSL dating of Holocene deposits of the Yangtze River delta, using four different quartz grain-size fractions (4–11 μm, 45–63 μm, 63–90 μm, and 90–125 μm). According to our knowledge, this is the first study of this type to be performed. By integrating OSL ages with the analysis of luminescence signal characteristics, we discuss the consistency and discrepancy of OSL ages among the different grain-size fractions and their possible causes. Such a study may also be applied to other deltaic environments for the purpose of proper OSL dating as well as providing insights into deltaic depositional processes.
Section snippets
Study area and samples
The Yangtze River is the world's third largest river, and the evolution of the Holocene Yangtze River delta has been studied extensively (e.g. Chen et al., 1979; Hori et al., 2001; Z. Wang et al., 2018b). During the last glacial maximum, the river incised a deep valley with a maximum depth of ~80 m in the present Yangtze River delta. Post-glacial transgression and regression resulted in a sequence comprising river, estuary, shallow sea, and delta facies, from bottom to top (Delta Research
Methods
Sample preparation and luminescence measurements were undertaken at the Luminescence Dating Laboratory of East China Normal University under subdued red light. In the OSL laboratory, core HM was split into two parts; one half was used for OSL sampling, with the inner and outer parts of the samples used for equivalent dose (De) and dose rate estimations, respectively. The other half was used for lithological description and sampling, in daylight, for other analyses such as grain-size
OSL ages of core HM
Both the central age model (CAM) and minimum age model (MAM) (Galbraith et al., 1999) were used to calculate the De values and burial ages of the samples of fractions B-D, and the OSL data for fraction A were analyzed with the CAM. The overdispersion (OD) for a well-bleached sample was determined to be 10% according to our previous study in the area (Nian et al., 2018a, Nian et al., 2018b); thus, the sigma b of 0.1 was used in the MAM. Age model selection (CAM or MAM) for the quartz of
A comparison of OSL ages obtained from difference grain-size fractions
For samples HM1–3, the weighted mean OSL ages calculated by fractions B-D are respectively 0.44 ± 0.03 ka, 0.45 ± 0.06 ka and 0.51 ± 0.12 ka (Table 1), which are consistent with the ages of neighboring cores WB, MQ, BX (Wang et al., 2019), core EGQD14 (Gao et al., 2019) and historical shoreline changes in this area (Fig. 1). However, the quartz of fraction A clearly overestimated the OSL ages by ~0.5–2 ka (Fig. 4; Table 1). Due to the signal averaging effect for fraction A, the corresponding De
Conclusions
We present quartz OSL ages and luminescence sensitivity results for a Holocene sediment core from the Yangtze River delta using four and three different grain-size fractions, respectively. The three samples from the upper part of the core (~0.4–0.6 ka) exhibit an OSL age discrepancy between the quartz of fraction A (4–11 μm) and the three coarser fractions (B-D). The fraction A quartz samples have significantly overestimated ages for the interval of ~0.4–0.6 ka. The adopted OSL ages from the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (41771009; 41271223), the China Postdoctoral Special Science Foundation (2017T100284), the China Postdoctoral Science Foundation (2015M571521), and the Open Research Fund of State Key Laboratory of Estuarine and Coastal Research (SKLEC-PGKF201906). We wish to thank Mr. Fengyue Qiu (East China Normal University) for helpful discussions and help with drawing figures. We also thank two anonymous reviewers and the editor
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