Water depth variation and its impact on carbonate content and oxygen isotopes: A study from a satellite lake near Lake Qinghai over the past 7.6 kyr

Highlights

• 7.6-kyr carbonate records from Lake Gahai, a satellite lake near Lake Qinghai.

• Lake level is the major contributor for the δ¹⁸O_carb variation in Lake Gahai.

• Contrasting responses of carbonate precipitation to water depth in different scales.

• Satellite lakes benefit from their vulnerability and sensitivity to climatic changes.

Abstract

The oxygen isotopic composition of sedimentary carbonate (δ¹⁸O_carb) provides information on the regional water balance between precipitation and evaporation in a lacustrine system. A volume effect was proposed to explain the abnormally low δ¹⁸O_carb values in Lake Qinghai on the northern Tibetan Plateau, under the conditions of high evaporation and low water levels, as observed during the early Holocene. Volume effect theory states that the small volume of the lake and its sensitivity to isotopic signals from freshwater discharge control the δ¹⁸O_carb values. In this study, we present 7.6-kyr records of carbonate content and oxygen isotopes from Lake Gahai, a small and saline satellite lake near Lake Qinghai. The records demonstrate that the water level and lake size of Lake Gahai considerably changed over the past 7.6 kyr, and the volume effect was responsible for the δ¹⁸O_carb variations. The records also suggest contrasting responses of carbonate precipitation to water depth across different timescales. For example, over the past 7.6 kyr, the carbonate content decreased with increasing lake water levels. This can be explained by gradual water freshening under low evaporation conditions and enhanced connection between Lake Qinghai and Lake Gahai. On the centennial scale, however, the carbonate content varied significantly and was positively correlated with Lake Gahai's water level. Such a variation is probably due to the periodically lower lake water levels and a stronger influence of terrestrial clastic material, which contributed to the dilution of the carbonate content under centennial cool and dry periods. Therefore, in order to advance our understanding of past hydroclimatic variations in the climatically sensitive region, information across different timescales should be separated.

Introduction

Knowledge of climate variability under natural conditions helps improve projections for future climate change. The northern Tibetan Plateau in China is important for paleoclimatic reconstruction, as it experiences complex interactions among the mid-latitude westerly circulation and the subtropical Asian monsoonal circulation (Ding and Wang, 2005; Chen et al., 2010; Yang et al., 2011; Cheng et al., 2019; Wang et al., 2020). Lake Qinghai is the largest lake on the arid to semi-arid, high-altitude northeastern Qinghai-Tibet Plateau. Owing to its high sensitivity to the Asian monsoonal precipitation and temperature-induced evaporation, paleoclimatic reconstructions from the Lake Qinghai region are essential for understanding the effect of complex forcing mechanismson the regional climate (Zhang et al., 1989; Lister et al., 1991; Henderson et al., 2003; Xu et al., 2006; An et al., 2012; Wang et al., 2014; Liu et al., 2015, Liu et al., 2018).

The oxygen isotope data from sedimentary carbonate (δ¹⁸O_carb) provide information regarding the regional water balance between precipitation and evaporation in a lacustrine system (Leng and Marshall, 2004; Chen et al., 2016a; Li and Liu, 2017). Knowledge of the variation in the isotopic compositions of lake water is fundamental to understanding the isotopic compositions of lacustrine carbonate (Henderson et al., 2003; Holmes et al., 2007; Henderson and Holmes, 2009; Liu et al., 2013; He et al., 2016; Liu et al., 2018; Sun et al., 2019). Previous studies have suggested that the δ¹⁸O values of water from lakes on the northern Tibetan Plateau should be interpreted as a reflection of effective humidity, i.e., the balance between precipitation and evaporation (Henderson et al., 2003; Leng and Marshall, 2004; Henderson and Holmes, 2009; Chen et al., 2016b; Li and Liu, 2017). Generally, evaporation higher than precipitation results in stronger isotopic enrichment in lake water and thus, higher δ¹⁸O_carb values. As such, several δ¹⁸O_carb studies have been carried out to reconstruct the paleoenvironmental changes in the Lake Qinghai region, where higher δ¹⁸O_carb values corresponded to lower lake levels and drier conditions (e.g., Zhang et al., 1989; Lister et al., 1991; Zhang et al., 1994; Henderson et al., 2003; Xu et al., 2006; Liu et al., 2007; Liu et al., 2018).

This interpretation, however, is not applicable to the Last Glacial Maximum (LGM) and the early Holocene. During the early Holocene, higher evaporation led to lower lake levels, despite the increased East Asian summer monsoon precipitation (Liu et al., 2013; Wang et al., 2014; Liu et al., 2015; Chen et al., 2016b; Liu et al., 2018). Low δ¹⁸O_carb values under such high evaporation tentatively suggest a limited evaporation impact on the δ¹⁸O_carb values (Liu et al., 2018). Inspired by the fact that variation in the surface sedimentary δ¹⁸O_carb of Lake Qinghai correlates well with the water depth of the sampling site (Liu et al., 2009; Liu et al., 2018), Liu et al. (2018) proposed an alternative volume effect to explain the low δ¹⁸O_carb values for the low lake level of Lake Qinghai during the LGM and the early Holocene. The volume effect theory states that lake water isotopes tend to be easily influenced by the inflow of water with a low δ¹⁸O signal when the lake water level is very low (i.e., shallow lake), regardless of the influence of regional evaporation on the isotopic enrichment. As a result, if an individual lake water level or size strongly fluctuates, the δ¹⁸O_carb values correlate positively with the lake water level or size. It remains unknown whether the volume effect prevailed in the Lake Qinghai region across different timescales in the mid- and late Holocene and whether it affected the bulk carbonate content (%CaCO₃).

It is known that Lake Qinghai was larger and deeper during the mid- and late Holocene than during the early Holocene (Lister et al., 1991; Zhang et al., 1994; Wang et al., 2014; Liu et al., 2015; Li et al., 2019). Thus, the lake was insensitive to climatic changes during the mid- and late Holocene, and cannot be used to investigate the volume effect. To the northeast and southeast of Lake Qinghai, a few small satellite lakes with varying salinities can be found (Fig. 1). Among them, there is a small and saline Lake Gahai, distinguished by its limited freshwater supply (Fig. 1, Liu et al., 2009). Such a small lake body is easily influenced by freshwater precipitation and snow meltwater with low δ¹⁸O values from a large catchment basin, where the δ¹⁸O_carb values are potentially impacted by water depth variations, as per the volume effect theory (Liu et al., 2018). Furthermore, small and shallow lakes in this semi-arid region are more sensitive to climatic and hydrological changes, making Lake Gahai suitable for paleoclimatic reconstructions.

Herein, we present the carbonate content and isotopic data for Lake Gahai over the past 7.6 kyr, obtained from a sedimentary core. This study was designed: 1) to confirm whether the volume effect prevailed in the Lake Qinghai region across different timescales during the Holocene; 2) to address Lake Gahai's carbonate-based proxies for paleoclimatic reconstructions; and 3) to further investigate the paleoclimatic variations in the Lake Qinghai region over the past 7.6 kyr.