Refined dating using palaeomagnetic secular variations on a lake sediment core from Guozha Co, northwestern Tibetan Plateau
Introduction
The Tibetan Plateau (TP) is called the “Asian's water tower” as it provides freshwater resources to about 1/3 of the world's population in the East and Southeast Asia (Immerzeel et al., 2010). Climate change on the TP potentially affects freshwater resources, regional social stability and economic development. In order to better predict the future changes in climate on the TP, it is necessary to understand the past spatio-temporal variations in palaeoclimate and its mechanisms. Lakes are widely distributed on the TP and there are over 1000 lakes larger than 1 km2 in this area (Ma et al., 2011). Lake sediments can provide continuous, long-term records of climate change, but access to these records depends on obtaining secure chronological control (Chen et al., 2016; Gasse et al., 1991; Wünnemann et al., 2018).
Radiocarbon dating is one of the most important methods to construct chronological controls for lake sediment core, which could date up to 50 ka. However, radiocarbon reservoir effects are prevalent in lake sediments on the TP due to the input of old carbon, which is then absorbed by aquatic plants or preserved in authigenic carbonate (Lockot et al., 2015; Mischke et al., 2013; Wu et al., 2010). Radiocarbon reservoir ages exhibit marked spatio-temporal variations among different lakes on the TP (Hou et al., 2012). Subtraction of reservoir ages from radiocarbon measurements is routinely carried out to mitigate against age inaccuracy, but cannot omit effects of past reservoir ages resulting from changes in both the climate and lacustrine system (Wang et al., 2017). A solution would therefore be to use another independent dating technique in such cases.
Earth's geomagnetic field (GF) varies over time due to fluid movements in the inner core. The GF is a dipole field on the whole and is the non-dipole field at regional scope, therefore, regional variations are generally consistent. Palaeomagnetic secular variations (PSV) are one of the main features of the GF during the Holocene. Such changes have been recorded by lava, sediments (lacustrine, eolian and marine) and archeological materials (Cai et al., 2016, 2017; Vigliotti et al., 2014). Comparing PSV records to constrain radiocarbon based age-depth relationships has been applied to various sediment cores (e,g, Gjerde et al., 2017; Haberzettl et al., 2019; Olafsdottir et al., 2013). A few PSV records from lake sediments have been established on the TP (Ahlborn et al., 2015; Haberzettl et al., 2015; Henkel et al., 2016; Kasper et al., 2012). They reveal similar variations, suggesting the potential of PSV-stratigraphy to construct PSV-derived age models. In this paper, we analyzed specific rock magnetic proxies (hysteresis measurements and χ−T) on selected samples to identify variations in the magnetic mineral concentration, composition and grain size for a sediment core at Guozha Co, a glacial lake in the northwestern TP. Then, we constructed age controls for the sediment core by matching its PSV-depth model with the PSV stack of East Asia (SEA PSV). The quality and accuracy of this age framework is evaluated by adjacent PSV and palaeoclimatic records.
Section snippets
Site description
Guozha Co (34.96° ~ 35.09° N, 80.91° ~ 81.24° E; 5086 m above sea level) is an open freshwater lake located in the West Kunlun Mountains, the northwestern TP (Fig. 1). The mean annual temperature is −8 ~ −6 °C and annual precipitation ranges from 75 to 100 mm (Zheng et al., 2008). The catchment of Guozha Co mainly consists of valleys and delta plains with gravelly and sandy sediments deposited by glacial meltwater from the Western Kunlun Mountains. Vegetation is scarce with only a small number
Materials and methods
A sediment core, GZHC2014-1 (304.5 cm in length), was retrieved on ice using a UWITEC platform and piston corer at a water depth of 80 m from the western part of the lake in June 2014 (Fig. 1). The core was continuous in the stratigraphy by collecting overlapping cores during core extraction. Based on visual description, the sediments in core GZHC2014-1 consist of light-brown fine silt and clay for the bottom 220 cm. The sediments becomes gradually darker upward, to dark-brown fine silt at the
210Pb, 137Cs and radiocarbon results
The 137Cs measurements yield valid data for the top 8 cm. The maximum 137Cs values occurred at 4 cm (3.16 ± 0.40 Bq. kg−1) and 5 cm (3.06 ± 0.40 Bq. kg−1). Therefore, the depths of the first valid measurements (8 cm) and peak value (4.5 cm) were correlated with the ages of initial production (1950 AD) and peak production (1963 AD) for of 137Cs during the nuclear tests, respectively (Fig. 2a). Unfortunately, the 210Pb measurements did not yield valid data for the same period.
Radiocarbon ages for
Radiocarbon age uncertainties
We used the 137Cs data to date the uppermost sediments. The average sedimentation rate was 0.125 cm/yr for the top 8 cm section (A.D. 1950 to A.D. 2014). According to the radiocarbon age model, the average sedimentation rate was about 0.019 cm/yr for the top 50 cm, which was nearly an order of magnitude less than the sedimentation rate determined for the uppermost 8 cm by 137Cs (0.125 cm/yr).
We applied the Inc and Dec records with the temporary radiocarbon age model (Fig. 6a and e). In order to
Conclusion
We established a PSV-derived age framework for the lake sediments from Northwestern TP by using PSV-stratigraphy correlations, which showed that it is possible to develop a chronological framework using the corresponding points between different PSV records. These results open up the possibility of using the PSV records from TP and adjacent regions, such as SEA and Tangra Yumco PSV records, as a chronostratigraphic tool for lake sediments more widely across the TP.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
The work was financially supported by the National Natural Science Foundation of China (41772178, 41601205); the Second Tibetan Plateau Scientific Expedition and Research Program (STEP), Grant No. 2019QZKK0601; Chinese Academy of Sciences Key Project (XDA20090000). We thank Prof. Zhaoyan Gu, Institute of Geology and Geophysics, Chinese Academy of Sciences, for guidance in 210Pb and 137Cs dating. Thanks are addressed to Dr. Yue He, Hebei GEO University, who joined us during the field work. We
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