3.2 Sedimentary evolution characteristics of organic matter in Hulun Lake
3.2.1 TOC
The TOC contents in the HLH16 and HLH26 cores were in the 27.9–43.1 and 30.4–49.0 g/kg ranges, with mean values of 32.6 and 36.7 g/kg, respectively. The average OCBR values in the HLH16 and HLH26 cores were 1.60 and 1.83 g/(m2·a), respectively. The TOC contents in the sediment cores in Hulun Lake have shown a fluctuant and increasing trend in recent years, and the annual TOC variation can be roughly divided into four stages (Fig. 5).
Before 1920, the TOC contents in the sediment cores were relatively stable and the OCBC values were low, with mean values of 0.85 and 0.37 g/(m2·a) in the HLH16 and HLH26 cores, respectively. Years 1920 to 1979 can be considered as the transition period of organic matter sedimentary evolution in Hulun Lake. The TOC contents in the HLH16 and HLH26 cores varied within 27.9–37.6 and 30.4–41.4 g/kg, and the average OCBR values were 2.37 and 2.77, respectively. Since 1980, the TOC content in the sediment cores has shown an obviously increasing trend. From 1980 to 1999, the TOC contents in HLH16 and HLH26 ranged from 28.5 g/kg to 35.7 g/kg and 36.3 g/kg to 40.1 g/kg, with TOC averages of 32.8 and 37.8 g/kg and OCBR averages of 1.59 and 2.68 g/(m2·a), respectively. Since 2000, the TOC contents have increased further and remained at a high level in recent years. The TOC contents in HLH16 and HLH26 varied from 32.1–43.1 and 35.4–49.0 g/kg, and the average values increased to 36.5 and 41.6 g/kg, respectively. The OCBR values increased to 1.74 and 3.30 g/(m2·a), respectively.
The variation trend of the TOC contents in the sediment cores in Hulun Lake over the years indicates that the TOC and the OCBR have an overall increasing trend. The TOC increased significantly after 1980, especially since 2000, and the OCBR reached its maximum value, indicating that the climate, water and hydrological conditions of Hulun Lake may have changed significantly during this period.
3.2.2 C/N
The C/N values in the HLH16 and HLH26 cores in Hulun Lake fluctuated within 11.9–19.7 and 8.6–16.1, with mean values of 15.6 and 12.7, respectively. The C/N in the sediment cores in Hulun Lake has shown a fluctuant and decreasing trend in recent years (Fig. 6).
Similar to the variation characteristics of the TOC content over the years, the variation of C/N could be also roughly divided into four stages. Before 1920, the C/N in the sediment cores was relatively stable, and the C/N values of HLH16 and HLH26 ranged from 14.8 to 18.8 and 12.8 to 14.3, with averages of 16.7 and 13.6, respectively. From 1920 to 1979, the C/N values in the sediment cores fluctuated greatly, and the C/N values of the HLH16 and HLH26 cores varied from 11.9 to 19.7 and 11.8 to 16.1, with mean values of 15.6 and 13.6, respectively. Since 1980, C/N has shown an obvious decreasing trend. In 1980–1999, the C/N values of the HLH16 and HLH26 cores ranged from 13.5 to 16.7 and 11.0 to 13.2, with the averages of 15.6 and 12.4, respectively. From 2000 to 2019, the C/N value decreased further, and the C/N values of the HLH16 and HLH26 cores ranged from 12.9 to 14.7 and 8.6 to 10.7, with averages of 13.8 and 10.2, respectively, reaching the lowest average values in recent years.
3.2.3 δ13C
The δ13C in the sediment cores of Hulun Lake has shown an overall increasing trend in recent years, and experienced four stages, namely, relative stability, fluctuation, gradual increase, and rapid increase successively (Fig. 7). Before 1920, δ13C was relatively stable, and the values in the HLH16 and HLH26 cores ranged from -27.63‰ to -27.24‰ and -27.42‰ to -26.92‰, with average values of -27.40‰ and -27.20‰, respectively. In 1920–979, the δ13C in the HLH16 and HLH26 cores fluctuated from -27.64‰ to -27.15‰ and -27.56‰ to -26.92‰, with mean values of 27.46‰ and -27.23‰, respectively. From 1980 to 1999, δ13C showed an obvious and stable increasing trend, and the mean values for the HLH16 and HLH26 cores reached to -27.39‰ and -26.99‰, respectively. From 2000 to 2019, the δ13C values of HLH16 and HLH26 ranged from -26.94‰ to -26.40‰ and -26.60‰ to -26.17‰, with average values of -26.62‰ and -26.34‰, respectively, reaching the highest level in recent years.
The isotopic signal of organic matter in sediments may be affected by the preferential degradation of protein-like organic matter after being buried in sediments. In general, the δ13C values in sediment cores should show a decreasing trend from the bottom to the top if they are influenced by the early degradation of organic matter (Sun et al. 2016). However, the vertical distribution of δ13C in the Hulun Lake sediment cores investigated in this study does not show this trend, so the influence of the early degradation of organic matter on δ13C can be ignored. The influence of selective diagenesis on stable isotopes is generally less than 2‰ (Meyers 1997) and can also be ignored in this study. Therefore, it can be concluded that the δ13C in the sediment cores of Hulun Lake was little affected by early degradation and diagenesis, and can effectively indicate the changes of source and environment.
3.2.4 Protein-like component in WEOM
The SOM components mainly include WEOM, humic acid (HA), fulvic acid (FA), and humin (HM) (Zhang et al. 2017). WEOM is the most easily degraded by microorganisms and has the highest bioactivity among these components (Hur et al. 2014). The WEOM in the sediment cores of Hulun Lake contained four fluorescence components, including three humic-like components and one protein-like component. The protein-like components reflect the tryptophan components generated by microbial and phytoplankton degradation and mainly come from autochthonous sources (Bai et al. 2017, Chari et al. 2013, Rochelle-Newall &Fisher 2002). Therefore, the sedimentary evolution characteristics of protein-like components in the WEOM of sediment cores can reflect the historical changes of the source of organic matter and the water ecological environment in the lake.
The fluorescence intensity (FI) of the protein-like component in WEOM in the HLH16 and HLH26 cores varied in the ranges of 0.35–1.05 and 0.39–1.06 R.U., respectively, and has shown an overall increasing trend in recent years (Fig. 8). Before 1920, the FI values of the protein-like component in the HLH 16 and HLH26 cores were basically stable, with mean values of 0.39 and 0.41 R.U., respectively. In 1920–1999, the FI values of the protein-like components increased insignificantly and varied in ranges of 0.37–0.84 and 0.39–0.66 R.U., with mean values of 0.55 and 0.48 R.U., respectively. Since 2000, the FI values of the protein-like components in the HLH16 and HLH26 cores have shown an obviously increasing trend, with mean values of 0.97 and 0.86 R.U., respectively, which are nearly one time higher than those of before 1920. The protein-like components in the WEOM mainly came from autochthonous sources, and their FI values changed significantly around 2000, indicating that the source construction of the SOM in Hulun Lake may have changed greatly. Therefore, the historical change of the source construction of the SOM in Hulun Lake must be analyzed further and verified.
3.3 Historical variation of the source construction of SOM in Hulun Lake
The main components of land plants are cellulose and lignin, so land plants have a high C/N content. The main component of aquatic phytoplankton is protein, so the C/N of aquatic plants is lower than that of land plants (Meyers 1994). The C/N ratio has been widely used to determine the organic matter sources in lake sediments (Bouton et al. 2020, Carrizo et al. 2019, Pu et al. 2020). The results calculated by the binary model based on C/N indicated that the PT and PA varied within 70.5–98.2% and 1.8–29.5% for the HLH16 core, and within 43.3–89.6% and 10.4–56.7% for the HLH26 core, respectively. δ13C can also be used to analysis the source of SOM. The δ13C values in the sediment cores in Hulun Lake ranged from -27.56‰ to -26.17‰, with a mean value of -27.0‰. The δ13C values of the sediments were near those of the C3 plants, aquatic plants, and terrestrial materials of Hulun Lake (Zhang et al. 2018b), indicating that the SOM in Hulun Lake was influenced by both terrestrial and autochthonous sources. The PT and PA values calculated by δ13C were within 58.5–83.4% and 16.6–41.5% for the HLH16 core, and within 53.9–81.5% and 18.5–46.1% for the HLH26 core, respectively.
The SOM in Hulun Lake generated from both terrestrial and autochthonous sources, but mainly came from terrestrial source. However, the relative contribution of the terrestrial source has shown a decreasing trend in recent years. Similar to the historical variation characteristics of other organic matter indicators, the historical changes of organic matter sources could also be roughly divided into four stages (Fig. 9). Before 1920, the source construction of SOM in Hulun Lake was relatively stable, and the terrestrial source was dominant. The mean values of PT and PA were 82.4% and 17.6%, respectively. In 1920-1979, PT and PA fluctuated with mean values of 80.7% and 19.3%, respectively. In 1980–1999, PT showed an obviously decreasing trend, with a mean value of 76.3%. In 2000–2019, PT decreased further and has remained ina relatively low level in recent years, and the mean value was 63.1%. However, the mean value of PA increased to 36.9%, which is 19.2% more than that before 1920.
3.4 Driving factors of the sedimentary evolution of organic matter in Hulun Lake
When organic matter enters the lake water from sources and is deposited at the bottom of the lake, it is affected by human activities, the climatic conditions of the basin, the hydrological conditions of the lake, and other factors. The sedimentary evolution of lake organic matter is also affected by diagenesis (Leonova et al. 2019, Melenevskii et al. 2015). Compared with marine diagenesis, the sulfate content in lacustrine environments is lower because fresh water is low in sulfur, sulphate reduction is very limited, vulcanization is difficult, and early diagenesis is relatively weak. For Hulun Lake, the SOM is mainly generated from terrestrial inputs, such as land plants (pasture). Terrestrial higher plants are more abundant than algae in aromatic compounds, such as lignin, tannin, resin, and suberin, which are very stable and have a strong resistance to bacterial decomposition and are easy to preserve in sediments (Harfmann et al. 2019). Previous results also show that the dominant component of SOM in Hulun Lake was HM, which is difficult to degrade, and the average proportion of HM to TOC was as high as 75.1% (Wang et al. 2021b). The influence of early diagenesis on THE SOM of Hulun Lake may be relatively small and will not be discussed this time. Given its special geographical location and climate conditions, the water environment of Hulun Lake is sensitive to climate change. Some results show no significant correlation between the burial rate of organic carbon and human factors, including population, cultivated area, livestock and fish (Zhang et al. 2018a). Therefore, this study focused on the impact of climate factors on the sedimentary evolution of organic matter in Hulun Lake.
The annual mean changes of temperature, precipitation and evaporation in Hulun Lake Basin from 1951 to 2018 are shown in Fig. S3. The average annual temperature fluctuated between -2.9 ℃ and 2.7 ℃. Overall, the temperature in the Hulun Lake Basin showed an increasing trend (Fig. 10a). The annual average precipitation and evaporation of the basin were within 137.9–590.1 mm and 612.3–1256.3 mm, with mean valuez of 285.9 and 906.6 mm, respectively. The precipitation in the basin showed a gradually decreasing trend (Fig. 10b), while the evaporation showed an increasing trend (Fig. 10c). The climate in the Hulun Lake basin has been warming and drying in recent years.
The correlation relationships between climate factors (temperature, precipitation, evaporation) and SOM related indexes (TOC, C/N, δ13C, FI of protein-like components of WEOM, PA) are shown in Table 1. The correlation between SOM related indexes and temperature is the best, showing significant correlation (P < 0.01), followed by evaporation. The relatively poor correlation between precipitation and the organic matter related indexes may be due to the fact that the evaporation in the Hulun Lake basin is much greater than the precipitation, and the strong evaporation effect is one of the important reasons for the reduction of water volume and level in Hulun Lake in recent years (Li et al. 2019, Liu &Yue 2017). Therefore, the effect of evaporation on the occurrence and sedimentary evolution of organic matter in Hulun Lake may be more obvious than that of precipitation.
Table 1
Correlation relationships between climate factors and SOM related indexes of Hulun Lake
SOM related indexes
|
Temperature
|
Precipitation
|
Evaporation
|
TOC
|
0.420**
|
-0.096
|
0.357*
|
C/N
|
-0.694**
|
0.213
|
-0.528**
|
δ13C
|
0.754**
|
-0.331*
|
0.595**
|
FI of protein-like components of WEOM
|
0.431**
|
-0.261
|
0.597**
|
PA
|
0.707**
|
-0.269
|
0.586**
|
* significantly correlated at 0.05 level, ** significantly correlated at 0.01 level. |
Temperature can directly affect the biomass, microbial quantity, and activity of land and water in lake basins, change the source construction of lake organic matter, and further affect the content and composition of organic matter and the migration and transformation of organic matter at the sediment–water interface, which is of great significance to the sedimentary evolution of organic matter (Dadi et al. 2016, Luff &Moll 2004). Hulun Lake is located in the Hulun Buir steppe, and the vast steppe provides many terrestrial organic matters for Hulun Lake. Meanwhile, due to the low temperature and long ice period in the basin, the growth of aquatic algae is greatly restricted, and the aquatic biomass of the lake is small, resulting in the small contribution of the autochthonous source to the SOM in Hulun Lake. Hence, terrestrial source has an absolute advantage over autochthonous source in the contribution to SOM in Hulun Lake. However, since the 1980s, especially since 2000, the basin has been exhibiting an obvious warming trend, the eutrophication in Hulun Lake has increased, the lake bloom has regularly broken out in summer, and the aquatic biomass has increased. Studies also show that the land around Hulun Lake has seen serious desertification, and the desertification area exceeds 100 km2 due to the significant warming and drying of climate (Zhao et al. 2008). In addition, the warm and dry climate has also led to the decrease in grassland vegetation height and grassland degradation, resulting in the relative decrease of organic matter input from terrestrial sources. Therefore, the source construction of SOM changed significantly as PT gradually decreased and PA gradually increased.
The change in source construction can also cause the change in SOM composition. Organic matters from terrestrial sources mainly consist of hard-degraded humic-like substances, and autochthonous organic matters mainly consist of protein-like components (Hu et al. 2019, Lu et al. 2019). WEOM is a highly active component of SOM and is closely related to microbial activities. Protein-like components mainly come from the metabolic activities or degradation of plankton and microorganisms (Chen et al. 2017, Li et al. 2020). Hence, the FI values of the protein-like component in the WEOM of the SOM in Hulun Lake show an increasing trend with the increase of PA.
The carbon isotope composition of SOM is also affected by temperature, and the change in carbon isotope composition can reflect the level of the lake’s primary productivity. The δ13C values of land plants are more negative than that of planktonic algae. The SOM in Hulun Lake mainly came from terrestrial sources during the lower temperature period, so the δ13C was low. The PA and the water primary productivity increased gradually, increasing the δ13C. The temperature and evaporation in the basin have increased, the precipitation and lake inflow have decreased, and the surface area of Hulun Lake has shrunk rapidly since 2000. The increase in temperature and evaporation leads to the decrease in CO2 concentration and the increase of the Ca2+, CO32-, and HCO3- concentrations in water. The δ13C value is inversely proportional to the water CO2 supply during the synthesis of organic matter by phytoplankton, indicating that the lower the δ13C value is, the higher the dissolved CO2 concentration is, and vice versa (Wang et al. 2003). Hence, the CO2 concentration in the lake decreased with the increase in temperature, and the algae preferentially used HCO3- to increase the δ13C value. The evaporation concentration also resulted in enhanced isotopic fractionation in Hulun Lake, and δ13C was more easily enriched in the water, maintaining the δ13C values in the sediments remaining at a high level.
The sedimentary evolution of TOC in Hulun Lake had a good response and indicator effect on the changes in temperature, evaporation, and lake surface area. From 1951 to 1979, the climate of the basin was dry and cold, which was not conducive to the vegetation growth of the basin, and the inputs of terrestrial and autochthonous organic matters were small. Moreover, the water surface area of Hulun Lake was relatively large, and the evaporation and concentration effects were relatively weak, so the TOC content that accumulated in the sediments of this period was small. From 1980 to 1999, evaporation and water surface area fluctuated with an unobvious trend, but the TOC content also presented a fluctuating and increasing trend due to the rising air temperature, primary productivity, and organic matter inputs. The TOC contents increased significantly from 2000 to 2008. On the one hand, the increase of the TOC content in this stage was due to the increase of organic matter inputs caused by the temperature increase. On the other hand, the increase of the TOC content from 2000 to 2008 was due to the strong evaporation concentration effect. During this period, the evaporation increased significantly, the water level dropped sharply, and the lake area shrank seriously. Hulun Lake became a closed lake that could only enter and not exit. The strong evaporation effect led to the enrichment and concentration of organic matters in the water body, and the concentration remained at a high level, which was conducive to the settlement and accumulation of organic matters from the water body to sediments, resulting in the increase of the TOC contents in the sediments.
From 2009 to 2013, the evaporation of the basin decreased, and the water inflow and surface area of Hulun Lake increased with the operation of the river diversion project to the lake. The reduction of evaporation and the increase of water volume led to the weakening of the concentration effect, and the increase of the water level also led to the increase of the settlement distance of organic matters from the water body to the sediments and the increase of the decomposition consumption of organic matters. Therefore, the TOC contents in the sediments showed a decreasing trend in 2009 and 2013. From 2014 to 2018, the evaporation increased, the climate was warm and dry, the primary productivity increased, and concentration effect was strong, resulting in the increasing trend of the TOC contents in the sediments.
The result of this study is consistent with previous results, that is, temperature increase will lead to the increase in SOM content. However, the temperature increase will also enhance the SOM mineralization effect. Therefore, further studies on the effects of temperature rise on the stability of the SOM of Hulun Lake and other similar lakes located in cold and arid areas should be conducted in the future, and the two-way feedback mechanism between climate change and lake carbonaceous organic matters should be further explored.