DONG, LI, CHEN, and WEI: Total organic carbon and its environmental significance for the surface sediments in groundwater recharged lakes from the Badain Jaran Desert, northwest China

Total organic carbon and its environmental significance for the surface sediments in groundwater recharged lakes from the Badain Jaran Desert, northwest China

Abstract

Total organic carbon (TOC) content in lake sediments is typically used for the reconstruction of paleoenvironments. it remains uncertain, however, whether these sediment variables in lakes supplied by groundwater in the hinterland of the Badain Jaran Desert are applicable. Moreover, it is still uncertain whether the TOC content in these lakes can be used as a proxy to identify past climatic change and environmental evolution studies. In this study, the spatial distributions of the TOC contents and C/N ratios were analyzed for 109 surface sediment samples collected from five lakes without runoff recharge. The results revealed that the TOC content of the lake surface sediments was extremely low (0.03% - 1.01%) and consisted of both allochthonous organic matter carried by wind, as well as autochthonous organic matter generated in the lakes. Within a lake, spatial differences in the amount of TOC found in surface sediments may be caused by several processes including bathymetry topography and wind-induced wave activity. In addition, wind-induced wave activity produces a higher TOC content, which is more pronounced in larger lakes (>0.21 km2) with longer fetches. By contrast, in smaller lakes, organic matter accumulates in the deeper waters, but can be affected by many factors. It is therefore necessary to consider lake area when applying the TOC content of lake sediments for the reconstruction of a paleolake evolution. Furthermore, because the TOC content of lake sediments in hyper-arid regions is extremely low, and the organic matter may have multiple and varied sources, a single proxy (TOC) cannot be used to reconstruct lake evolution.




INTRODUCTION

Lake sediments in arid and semiarid regions have been widely used to reconstruct regional and global climate changes (Members, 1988; Zhang et al., 2004; Chen et al., 2008; Liu et al., 2016; hobbs et al., 2017). Total organic carbon (TOC) content in lake sediments is one of the proxies used for the analysis of lake basin ecological environments (Mackay et al., 2012), including the effective humidity (An et al., 1993; Long et al., 2010; Li et al., 2016b), precipitation (Xu et al., 2006; An et al., 2012), runoff (hartmann and Wunnemann, 2009), and has been widely used in paleoenvironmental studies (Ji et al., 2005; Xu et al., 2007; Liu et al., 2013; Kołaczek et al., 2014). in previous studies, TOC contents in surface sediments have been interpreted mainly to reflect climate factors that influence terrestrial plant growth or the input processes of terrestrial organic matter to lakes and rivers (Ji et al., 2005; Xu et al., 2007; Kołaczek et al., 2014). However, the environmental significance of the TOC content may vary in different study areas. Xu et al. (2006) suggested that high values of TOC in the Qinghai Lake sediments were indicative of high precipitation. Hartmann and Wunnemann (2009) reported that the TOC contents in sediments from the Juyanze paleolake were influenced by lake salinity and runoff. Moreover, TOC contents were interpreted as proxies for effective humidity (An et al., 1993; Long et al., 2010; Wang et al., 2013; Li et al., 2016b). Furthermore, some non-climate factors can also affect the TOC content (Sifeddine et al., 2011; Woszczyk et al., 2011). Hence, understanding the environmental significance of the TOC in arid regions is crucial in order to evaluate the reliability of paleoenvironmental reconstructions.

The Badain Jaran Desert is located in a hyper-arid region of northwestern China. It is a key region for understanding past climatic change and environmental evolution studies because it is located in the transition zone between the Asian summer monsoon and the westerly winds in China (Wang et al., 2005; Yang et al., 2011; Li et al., 2015). Thus, it is an ideal region for studying changes in climate at different time scales (Yang and Scuderi, 2010; Li et al., 2016c; Wang et al., 2016). However, for lakes in the hinterland of deserts, which are not recharged by river runoff, sediment is only transported by the wind (Li et al., 2018). Thus, allochthonous organic matter (OM) transport and deposition processes differ from those in lakes with river discharge. Moreover, OM content is lower in hinterland desert lakes than other regions, and surface sediments do not receive a substantial terrigenous OM contribution. Slight changes in the TOC content could be caused by numerous factors including climate and non-climate factors. Therefore, the TOC spatial distribution of the lake sediments and their environmental significance remains unclear. Moreover, it is still uncertain whether the TOC content can be used as a proxy for paleoenvironmental studies in this area.

In this study, five lakes in the hinterland of the Badain Jaran Desert without runoff recharge were selected, and a total of 109 lake surface samples collected from the five lakes were used to analyze the spatial distribution of surface- sediment TOC and C/N ratios. Moreover, we also evaluated the environmental significance of the sediment TOC and C/N ratio records in the study area.

Study site

The Badain Jaran Desert (39°04′15″−42°12′23″ n, 99°23′18″−104°34′02″ e) is located in northwest China (Fig. 1a). With an area of approximately 52,100 km2, it is the second largest desert in China. The elevation of the area varies between 1,500 m above sea level in the southeast, and 900 m above sea level in the northwest. The desert is bordered to the south and east by the Beida-Shan and Yabulai-Shan mountain ranges, respectively (Fig. 1b).

The Badain Jaran Desert is located in the mid-latitude arid region (Zhu et al., 1980). The average annual temperature is approximately 8°C, and the mean summer and winter temperatures are 25 and -9°C, respectively (Ma et al., 2014). The desert is strongly continental, and the annual average precipitation decreases gradually from 120 to 40 mm from the southeast to the northwest of the desert. Annual precipitation in the hinterland of the Badain Jaran Desert is ~100 mm (Li et al., 2013; Ma et al., 2014). In contrast to the distribution of precipitation, evaporation increases gradually from the southeast to the northwest. Mean annual water surface evaporation is more than 1000 mm (Yang et al., 2010; Li et al., 2016a).

In the southeast of the Badain Jaran Desert, sand hills have average heights between 200 – 300 m, and reach as high as 460 m. A total of 110 lakes are found in this region (Wang et al., 2016), most of which are saline soda lakes with high Total Dissolved Solids (TDS). The lakes are recharged by groundwater, not surface runoff, and due to the extremely low precipitation and high potential evaporation rates (Ma et al., 2014), the lakes have variable concentrations of TDS (Wu et al., 2014; Dong et al., 2016). However, attributed to groundwater reductions in the drainage areas, the number of lakes in the region had fallen to 68 in 2006 (Zhang et al., 2013). Most plants that grow in the region around the lakes, are hygrophytes and halophytes, which are distributed concentrically around the lakes (Fig. 2).

Five lakes in the hinterland of the Badain Jaran Desert of varying area dimensions and TDS concentrations were selected (Fig. 1c). The general characteristics of the lakes are listed in Tab. 1, and include the locations, lake areas dimensions, and the TDS concentrations.

METHODS

Surface sediment sampling

In May 2015, 109 surface sediment samples were collected from five lakes in the hinterland of the Badain Jaran Desert. of these, 22 samples were collected from Sumubarunjaran Lake (Fig. 3a), 25 samples from Zhunsangenjaran Lake (Fig. 3b), 23 samples from Taosenjaran Lake (Fig. 3c), 25 samples from Yindeer Lake (Fig. 3c), and 14 samples were collected from Baoritaolegai Lake (Fig. 3e). The distribution of the sampling points in the lakes was established based on latitude and longitude, and the exact geographical positions of the sampling points were tracked using global Positioning System (gPS). Fig. 3 shows the distribution of the specific sampling points.

Methods

All samples were air-dried naturally and ground to pass through a size 100 mesh. Samples of approximately 0.5 g each were weighed, and the measured values, accurate to 0.0001 g, were recorded. Each sample was then placed in a centrifuge tube with 10 mL of 10 mol L–1 hCl to remove carbonates and was left to stand for 24 h to ensure complete removal. Subsequently, the ph of each sample was brought to neutral using 5 mL of deionized water and centrifuged 4-5 times. Samples were then dried in a drum wind drying oven and placed in a room-temperature environment for 2 h to attain moisture absorption equilibrium with the air. Finally, the samples were weighed, and the values accurate to 0.0001 g, were recorded. TOC and Total n contents of each treated sample were measured using a Vario-iii element analyzer (elementar Analysensysteme gmbh, germany), the estimated error was <0.001%. To calculate the actual organic carbon and nitrogen contents of the untreated, dry sediment samples, the following formula was applied:

M0 = M*G/G0

Where M0 is percentage of the element in the actual sample (%), M is the percentage of the element in the treated sample (%), G0 is the actual sample weight (g), and G is the treated sample weight (g).

The spatial distributions of the TOC contents and C/N ratio in the surface sediments were analyzed using Topo to Raster in ArcgiS 10.2.

RESULTS

Contents of TOC and C/N

The TOC contents of the surface sedime