SCIENCE CHINA Earth Sciences SPECIAL TOPIC: Advances in organic proxies for research in climate • REVIEW •

doi: 10.1007/s11430-015-5213-4 doi: 10.1007/s11430-015-5213-4

A 12-ka record of microbial branched and isoprenoid tetraether index in Lake Qinghai, northeastern Qinghai-Tibet Plateau: Implications for paleoclimate reconstruction WANG HuanYe1, 2, DONG HaiLiang3, ZHANG ChuanLun4, JIANG HongChen5 & LIU WeiGuo1* 1

State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China; 2 College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China; 3 State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China; 4 State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China; 5 State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China Received July 20, 2015; accepted September 14, 2015

Abstract Branched and Isoprenoid Tetraether (BIT) index was considered as a proxy for terrestrial organic matter input in lake sediments, based on the assumption that branched glycerol dialkyl glycerol tetraethers (bGDGTs) are mainly derived from terrestrial soils. However, mounting evidences have showed that the in situ production of bGDGTs is widespread in lakes, challenging BIT as a reliable terrestrial input proxy. Recently, BIT has been proven to be a reliable proxy for paleohydrology in a small crater lake (Lake Challa) in accordance with a different mechanism. However, the response of BIT to paleohydrology variation may differ for different lakes. In this study, we investigate the variations in the BIT index and the concentrations of its related GDGTs in a 12-ka sediment core from Lake Qinghai, in combination with our previous results for surface sediments. We find that variations in BIT strongly depend on the concentration of crenarchaeol in both surface and ancient sediments of this lake, whereas bGDGT concentration varies much less remarkably. Considering that crenarchaeol production is positively correlated with water depth in Lake Qinghai, water depth may exert negative control on the BIT index in this lake. This case is inconsistent with the positive relationship between BIT and lake levels or rainfall intensity reported for Lake Challa, suggesting that the response of BIT to local paleohydrology is site specific in lacustrine systems. Hence, the application of sedimentary BIT as a paleohydrological proxy in a specific lake requires caution before confirming the environmental controls on BIT in that lake. Keywords Citation:

Lake Qinghai, BIT, lake water depth, precipitation, paleohydrology Wang H Y, Dong H L, Zhang C L, Jiang H C, Liu W G. A 12-ka record of microbial branched and isoprenoid tetraether index in Lake Qinghai, northeastern Qinghai-Tibet Plateau: Implications for paleoclimate reconstruction. Science China: Earth Sciences, doi: 10.1007/s11430-015-5213-4

1. Introduction Glycerol dialkyl glycerol tetraether lipids (GDGTs) are in-

*Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2015

creasingly popular and versatile tools in paleoclimate studies. These lipids are easy to analyze, and the distributions of different GDGT groups are sensitive to various environmental parameters (Schouten et al., 2007a; Castañeda and Schouten, 2011; Yang et al., 2012; Schouten et al., 2013). One group of GDGTs, namely, isoprenoid GDGTs earth.scichina.com

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(iGDGTs), are membrane lipids that are synthesized by Archaea (De Rosa and Gambacorta, 1988; Schouten et al., 2013), the third domain of life (Woese et al., 1990). Among iGDGTs, crenarchaeol is the characteristic membrane lipid of Thaumarchaeota (Sinninghe Damsté et al., 2002b; Brochier- Armanet et al., 2008; Pitcher et al., 2010), and is particularly abundant in marine and large lakes (e.g. Schouten et al., 2002; Sinninghe Damsté et al., 2002a; Turich et al., 2007; Kim et al., 2008; Powers et al., 2010; Tierney et al., 2010). By contrast, another group of GDGTs, branched GDGTs (bGDGTs), are presumed to be membrane- spanning lipids of heterotrophic (Pancost et al., 2003; Oppermann et al., 2010; Weijers et al., 2010) bacteria (Weijers et al., 2006; Sinninghe Damsté et al., 2011). Preliminary analysis showed that bGDGTs are predominant in terrestrial soil and peat (Hopmans et al., 2004). Therefore, Hopmans et al. (2004) defined the Branched and Isoprenoid Tetraether (BIT) index, which represents the relative abundance of the three dominant bGDGTs to crenarchaeol, as a proxy for estimating the relative amounts of terrestrial soil organic matter (OM) and aquatic OM in aquatic settings. To date, the BIT index has been widely applied to both ancient and surface (modern) sediments to trace the input of soil-derived OM in lakes and oceans (e.g. Herfort et al., 2006; Ménot et al., 2006; Schouten et al., 2007b; Seki et al., 2009; Weijers et al., 2009; Verschuren et al., 2009; Sun D et al., 2011). In lakes, despite the potential of using BIT for tracing soil OM input (Hopmans et al., 2004; Blaga et al., 2009; Verschuren et al., 2009) or constraining the relative proportion of iGDGTs contributed by terrestrial soils (Powers et al., 2010), increasing evidences have recently demonstrated significant amount of autochthonous bGDGTs in lake water and/or sediments. First, a mismatch of the concentrations and distribution of bGDGTs between lake sediments and surrounding soils for numerous lakes at different regions evidently exists, indicating a different origin of bGDGTs in lake systems (e.g. Sinninghe Damsté et al., 2009; Tierney and Russell, 2009; Tierney et al., 2010; Sun Q et al., 2011; Loomis et al., 2011, 2014; Peterse et al., 2014; Buckles et al., 2014a, 2014b; Naeher et al., 2014). In addition, the relative abundance of bGDGTs that are present as intact polar lipids (IPLs) is significantly higher in lake sediments than in (surrounding) soils (e.g., Tierney et al., 2012; Wang et al., 2012; Buckles et al., 2014b, Huguet et al., 2015). This finding could provide another strong evidence for in situ production of bGDGTs within lakes. Moreover, in the sediments of a Swiss mountain lake, Weber et al. (2015) recently observed a novel hexamethylated bGDGT, which is absent in soil from the lake’s catchment, and found that the stable carbon isotope composition of bGDGT-derived alkanes is relatively lower than those of soils. These findings provide circumstantial evidence of an in situ bGDGT source. In situ production of bGDGTs also occurred in other aquatic systems, such as rivers and estuaries (Zhang et al., 2011; Zhu et al., 2011; Yang et al., 2013; Zell et al., 2013;

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De Jonge et al., 2014; Zhou et al., 2014), as well as coastal and open marine environments (Peterse et al., 2009; Fietz et al., 2012; Hu et al., 2012; Liu et al., 2014; Weijers et al., 2014; Dong et al., 2015). These autochthonous bGDGTs would complicate the interpretation of BIT as a tracer for relative soil OM input in aquatic systems (Schouten et al., 2013). Therefore, the geochemical significance of the sedimentary BIT index should be explored further. The BIT index has been systematically investigated in Lake Challa, a small crater African lake. Initially, Sinninghe Damsté et al. (2009) observed that a major downward flux of bGDGTs in Lake Challa occurs during the short, intense rain season and suggested that sedimentary bGDGTs in this lake are likely derived predominantly from eroded catchment soils mobilized by surface run-off, despite that in-situ production of bGDGTs in the lake or groundwater cannot be fully excluded. According to this observation, Verschuren et al. (2009) used the BIT index in a 25-ka Lake Challa sediment record to infer past regional precipitation intensity. The team found that the BIT record is in good agreement with the lake-level fluctuation inferred from seismic-reflection stratigraphy (Moernaut et al., 2010). However, re-examination of the distribution of GDGTs in this sediment record (Sinninghe Damsté et al., 2012) revealed that the variation in the BIT index are mainly determined by the varying production rate of crenarchaeol instead of the delivery of bGDGTs from soil as proposed earlier. Therefore, Sinninghe Damsté et al. (2012) hypothesized that the general match between precipitation (or lake level) and the BIT index is probably due to the influence of several precipitation-related parameters (lake level, wind strength, and soil nutrient input) on nutrient cycling in the lake. Subsequently, on the basis of multiple lines of modern evidences, Buckles et al. (2014b) clearly showed that the majority of bGDGTs in Lake Challa is produced in situ. In an effort to explore the precise mechanism (s) of the apparent relationship between precipitation and the BIT index, Buckles et al. (2014b, 2015) then investigated the distribution of GDGTs and the BIT index in profundal surface sediments, the time series of monthly sediment-trap samples, and a 2200-year sediment record. Their results suggested that episodic nutrient input to the lake, which results from intense precipitation, may lead to the suppression of Thaumarchaeota and, in consequence, high BIT values via changes in planktonic and microbial communities within the lake (Buckles et al., 2015). Overall, these studies demonstrate that the temporally integrated BIT is a reliable precipitation proxy for the Lake Challa system in (multi-) decadal and longer time scales. However, Buckles et al. (2015) also noted that the reconstruction of precipitation by using BIT elsewhere requires cautions, as the response of BIT to precipitation may depend on site-specific conditions. Hence, the mechanism of BIT index response to hydrological changes in other lakes should be further examined. In this study, we investigated the variations in the BIT

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index and the concentrations of its related GDGTs in a 12-ka sediment core (QH-2011) of Lake Qinghai, the largest interior plateau lake in Central Asia (Fu et al., 2013). By comparing the BIT record with previously inferred lake level history in the same core (Wang et al., 2014) and combining previously published surface sediment data (Wang et al., 2012), we aimed to explore the mechanism of BIT index response to moisture balance for lake sediments in this region.

2. Materials and methods 2.1

Study site and sampling

Lake Qinghai is an alkaline and brackish lake with a salinity range of 14 to 16 g/L (Liu W et al., 2011) and pH of 8.8 to 9.3 (Xu et al., 2010). Surrounded by mountains such as Datongshan, Riyueshan, and Nanshan (Liu W et al., 2011), the lake is hydrologically closed at present, with a surface area of ca. 4260 km2, a catchment of >29660 km2, and a water volume of 7.16×1010 m3 (Jin et al., 2010). The surface water elevation is 3194 m above sea level, and the maximum water depth is 27 m (Xiao et al., 2012). Located on the northeastern margin of the Qinghai-Tibet Plateau, the Lake Qinghai area has a cold and semi-arid regional climate. The regional annual mean temperature is ca. 1.2°C (1951 to 2005, Jin et al., 2010). The mean annual precipitation is 373 mm, with >65% falling during summer (June, July, and August), which indicates the clear seasonality of monsoonal precipitation (An et al., 2012). However, potential evaporation considerably exceeds precipitation (Sun et al., 1992). The lake is covered by ice for ca. 3 months to 4 months each year (December to March), with a maximum thickness of ca. 0.8 m (Colman et al., 2007). In summer, the lake becomes thermally stratified, with the thermocline at 10 to 15 m. During this period, the epilimnion is 12°C to 15°C, while the hypolimnion is ca. 6°C (Lister et al., 1991; Williams, 1991). Five major rivers draining to the lake include the Buha, Shaliu, Quanji, Hargai, and Heima rivers, which lie mainly on the north and northwest (Jin et al., 2010). In August 2011, a 5.8-m core (QH-2011) was recovered at a water depth of ca. 24 m in the southeastern sub-basin of Lake Qinghai. The chronology for this core was a piecewise linear age model (Wang et al., 2014) based on 6 converted calendar years (IntCal13, Reimer et al., 2013) of reservoir effect-corrected AMS14C ages of this core and 6 calendar ages of another core drilled at a similar site in the southeastern sub-basin (Wang et al., 2011). The upper 4.35 m of core QH-2011 covering the whole Holocene and spanning the past 12 ka was reported in this study. We analyzed GDGTs in 145 samples from this section. Considering the significant interference of co-eluting peaks of unknown compounds for bGDGTs, 12 samples were not reported for BIT values. The average sampling resolution is ca. 100 years.

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2.2

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Lipid analysis

The extraction and quantification of GDGTs for core QH2011 were described in detail in the work of Wang et al. (2014). In brief, 1 to 5 g freeze-dried samples were subjected to ultrasonic extraction using MeOH, MeOH/ di: chloromethane (DCM) (1 1, v/v), DCM, MeOH/DCM (1:1, v/v), and MeOH, respectively. After blending a known amount of C46 GDGT internal standard (Huguet et al., 2006), the total lipid extract was dried under N2, re-dissolved in hexane/isopropanol (99:1 v/v), and filtered through a 0.45 μm PTFE syringe filter. High-performance liquid chromatography–atmospheric pressure chemical ionization-mass spectrometry (HPLC-APCI-MS) analysis of GDGTs followed the method of Zhang et al. (2012), using an Agilent 1200 HPLC instrument connected to a QQQ 6460 MS. Scanning was performed in selected ion monitoring (SIM) mode. The concentration of individual GDGTs was quantified under the assumption that their ionization efficiency is the same as that of the C46 internal standard. The BIT index was calculated according to Hopmans et al. (2004): BIT 

I  II  III , I  II  III  crenarchaeol

(1)

3. Results A wide fluctuation in the composition of GDGTs occurred for the past 12 ka in Lake Qinghai. In a previous study, we have reported the distribution of iGDGTs in core QH-2011 (Wang et al., 2014, 2015). In the present study, we report additional data on the concentration of crenarchaeol and bGDGTs, as well as the BIT index. The concentration of crenarchaeol varied significantly by three orders of magnitude between 1.9 and 535.9 ng g−1 dry weight sediment, with a mean value of 140.2 ng g−1 (Figure 1(a)). During early Holocene, the concentration of crenarchaeol was generally lower than 10 ng g−1, except for a short period at ca. 8.4 ka BP with considerably higher crenarchaeol production. During mid-Holocene, the concentration of crenarchaeol increased with certain centennial-scale to millennial-scale oscillations. During late Holocene, the concentration of crenarchaeol was generally higher than the mean value (140.2 ng g−1). The maximum crenarchaeol production occurred at 4.8 ka BP. The concentration of total bGDGTs ranged between 122.8 and 751.6 ng g−1, with the range between 100.0 and 713.0 ng g−1 for the sum of the three major bGDGTs (Ia+IIa+IIIa) included in the BIT index (Figure 1(b)). The average values of total bGDGTs and (Ia+IIa+IIIa) are 268.6 and 221.2 ng g−1, which are both higher than that of crenarchaeol. However, the concentration of total bGDGTs and (Ia+IIa+IIIa) both varied within only one order of magni-

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Figure 1 Variations in the concentrations of (a) crenarchaeol, (b) bGDGTs, and (c) the BIT index in the Holocene section of core QH-2011. In (b), black dots indicate the total bGDGTs, whereas grey dots represent the sum of the three major bGDGTs (Ia+IIa+IIIa). The horizontal dotted lines indicate the mean values of each parameter in this core.

tude throughout the record. The BIT index fluctuated between 0.22 and 0.99 (with an average of 0.61, Figure 1(c)), exhibiting an opposite trend to that of crenarchaeol concentration. In general, the BIT index is close to 1 during the early Holocene (except for a short period at ca. 8.4 ka BP). The index subsequently decreased to the late-Holocene low BIT stage (with an average of 0.43 for late Holocene), with certain oscillations. The lowest BIT value was also found at 4.8 ka BP.

4. Discussion 4.1

Holocene lake level history of Lake Qinghai

Over the past several decades, the Holocene paleoclimatic changes of the Lake Qinghai region have been extensively investigated (e.g. Zhang et al., 1989; Lister et al., 1991; Yu, 2005; Ji et al., 2005; Shen et al., 2005; Colman et al., 2007; Liu X et al., 2011; An et al., 2012; Li and Liu, 2014; Liu et al., 2015). However, continuous records of lake-level changes covering the Holocene are relatively few. Based on seismic reflection data, carbonate, fossil seeds of aquatic plants, sedimentary structure and characteristics (Kelts et al., 1989; Lister et al., 1991; Yu, 2005), as well as lake salinity inferred from Sr/Ca ratios of ostracod shells (Zhang et al., 1994), previous studies on sediment cores that were re-

trieved from Lake Qinghai in the 1980s have indicated that the lake was relatively shallow during early Holocene and relatively deep during late Holocene. Recent high-resolution organic geochemical results of lake-level proxies in new sediment cores drilled form the southeast (core QH-2011) and southwest (core 1F; drilled in 2005) sub-basins of the lake further provided a relatively clear Holocene lake level fluctuation history for Lake Qinghai: an early-Holocene shallow lake, a mid-Holocene expanding lake with millennial-scale oscillations of lake levels, and a generally stable late-Holocene highstand (Liu et al., 2013; Wang et al., 2014). The overall Holocene lake level variations of Lake Qinghai were consistent with those for several other lakes on the northeastern Qinghai-Tibet Plateau (Mischke et al., 2008; Herzschuh et al., 2009) and in arid/semi-arid China (Herzschuh, 2006; Chen et al., 2008). The relatively lower water depth of Lake Qinghai during the early Holocene was possibly due to the strong impact of summer temperature on regional effective moisture in arid and semi-arid regions via controlling the evapotranspiration loss (Yu, 2005; He et al., 2013; Liu et al., 2013; Wang et al., 2014; Liu et al., 2015). 4.2

In situ production of bGDGTs in Lake Qinghai

In a previous study of the distribution of GDGTs in surface sediments and surrounding soils of Lake Qinghai, we have suggested that certain sedimentary bGDGTs may be au-

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tochthonous, or at least partly, on the basis of two lines of evidences (Wang et al., 2012). First, the distributions of bGDGTs (e.g. MBT and CBT) in lake sediments were markedly different from those in surrounding soils. This finding is in agreement with the observations in most lake bGDGT investigations (e.g. Sinninghe Damsté et al., 2009; Tierney and Russell, 2009; Tierney et al., 2010; Sun Q et al., 2011; Loomis et al., 2011, 2014; Peterse et al., 2014; Buckles et al., 2014a, 2014b; Naeher et al., 2014), except for certain exceptions (e.g. Niemann et al., 2012). Second, the relative fractions of IPL bGDGTs in the surface sediments of Lake Qinghai are significantly higher than those in the surrounding soil. A higher proportion of IPL bGDGTs in lake sediments was also observed in several recent studies (Tierney et al., 2012; Buckles et al., 2014b, Huguet et al., 2015). Although IPL GDGTs may not exactly represent living organisms (Schouten et al., 2010), the remarkably higher proportion of IPL bGDGTs in lake sediments seems unlikely if soils were the sole source of bGDGTs (Buckles et al., 2014b). Moreover, when comparing the BIT values of surface sediments and surrounding soils of Lake Qinghai, we note that the BIT values of nearshore sediments with water depth of 40 African lakes (Tierney et al., 2010), as well as in down-core sediments of Lake Challa (Sinninghe

Figure 3 Relationships between the BIT index and the concentrations of GDGTs (crenarchaeol or bGDGTs) in the upper 4.35 m of core QH-2011.

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Damsté et al., 2012; Buckles et al., 2015), Lake Baikal (Fietz et al., 2011), and certain marine settings (Fietz et al., 2011; Smith et al., 2012).

performs an important negative function in controlling the sedimentary BIT values in Lake Qinghai and numerous other lakes.

4.4 Impact of water depth on BIT in Lake Qinghai

4.5 Comparison with Lake Challa and implications for the application of BIT in paleohydrological studies

Considering that the variation in the BIT index is mainly controlled by crenarchaeol production, which is strongly related with water depth in Lake Qinghai (Wang et al., 2014), we can reasonably infer that the variation in BIT of this lake is also affected by changes in water depth. For down-core sediments, the BIT record in core QH-2011 resembles the lake-level history of Lake Qinghai as inferred from the %cren and δ13C records of core QH-2011 and core 1F (Figure 4, Liu et al., 2013; Wang et al., 2014), with generally higher BIT values during early Holocene, during which the lake was shallow, and lower values during late Holocene, during which the lake was relatively deep. For core QH-2011 in particular, the fluctuation of BIT is nearly identical to those for the %cren and δ13C records, especially for early and mid-Holocene episodes (Wang et al., 2014). We also noticed that BIT is correlated significantly with water depth (Figure 2(a)) for the surface sediments of Lake Qinghai (Wang et al., 2012). Moreover, for 41 lakes across a full gradient of African lake environments (Tierney et al., 2010) and for 82 globally distributed lakes (Blaga et al., 2010), the BIT values in both sets of surface sediments are correlated significantly with lake water depths (Figure 5). These results collectively suggest that water depth generally

The correlation between BIT and lake water depth implies that the BIT index can still be (qualitatively) used to reconstruct past hydrological changes. In Lake Qinghai, however, although BIT correlates significantly with water depth, the negative relationship between the two values seems to contradict the case for Lake Challa, in which the sedimentary BIT is correlated positively with lake level (Verschuren et al., 2009) or precipitation amount (Buckles et al., 2015). This difference is possibly due to the different mechanisms controlling the production of crenarchaeol or the producer, Thaumarchaeota, for the two types of lakes. The positive correlation between crenarchaeol concentration and lake water depth in Lake Qinghai possibly reflects that Thaumarchaeota prefer living in the relative deeper zone in lacustrine systems (Wang et al., 2014). This explanation is based on recent observations showing that Thaumarchaeota and/or crenarchaeol showed maximum abundance in the oxycline/thermocline and nitrocline of lake water columns (Pouliot et al., 2009; Llirós et al., 2010; Auguet et al., 2012; Schouten et al., 2012; Woltering et al., 2012; Buckles et al., 2013). An additional potential explanation for the positive relationship between crenarchaeol

Figure 4 Comparison of BIT and lake-level proxies in the Holocene sediments of Lake Qinghai. (a) δ13C records (black: Liu et al., 2013; grey: Wang et al., 2014). (b) %cren record (Wang et al., 2014). (c) BIT record.

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growth of Thaumarchaeota in Lake Challa, given its large water depth. This finding is different from the situation in Lake Qinghai. For a specific site in Lake Qinghai, the potential suppression on Thaumarchaeota by enhanced terrestrial OM and nutrient input during periods under enhanced moisture condition can be offset and overwhelmed by the stimulation of crenarchaeol production by lake expansion, which corresponds to an increase in water depth and distance offshore. Hence, in ancient sediments, BIT is generally correlated positively with moisture availability (lake levels) in Lake Challa but negatively with moisture availability (lake levels) in Lake Qinghai. Figure 5 Relationships between the BIT index and water depth for 82 globally distributed lakes (grey: Blaga et al., 2010) and 41 lakes across a full gradient of African lake environments (black: Tierney et al., 2010). The two datasets were not integrated, because differences in analytical conditions may result in different BIT values for the same sample.

concentration and lake water depth in Lake Qinghai is also likely. In a specific lake such as Lake Qinghai, water depth generally correlates strongly with the distance to shoreline which affects nutrient and terrestrial OM input derived from surface runoff, and nutrient availability would control the primary productivity within the lake. In consequence, the amount of ammonium derived from decomposed OM (both allochthonous and autochthonous) in shallow water areas may be high. Since ammonia-oxidizing Thaumarchaeota are generally considered to be autotrophs (Pester et al., 2011) that are adept in oligotrophic nitrification (MartensHabbena et al., 2009; Pester et al., 2011) but may be at a disadvantage when competing with ammonia-oxidizing bacteria at higher ammonium levels (Di et al., 2009; Buckles et al., 2015), Thaumarchaeota would be suppressed in shallow water areas suffering from high terrestrial OM and nutrient input. Hence, for a specific site in the large and oligotrophic Lake Qinghai, the shrinking of the lake would both reduce the water depth and enhance nutrient availability, hindering the growth of Thaumarchaeota. Lake Challa is a relatively small (4.2 km2) but deep (maximum depth ~ 95 m) volcanic crater lake in equatorial East Africa, with a typical crater basin morphology of steep underwater slopes to ~ 60 to 70 m depth (Moernaut et al., 2010; Sinninghe Damsté et al., 2011). The lake-level variation of Lake Challa is influenced by local rainfall and run-off at first approximation (Moernaut et al., 2010). Higher lake levels generally correspond to higher precipitation or rainstorm frequency. In consequence, more terrestrial nutrient input results, suppressing Thaumarchaeota and leading to reduced production of crenarchaeol (Buckles et al., 2015). On the other hand, for a specific site in small but deep lakes with steep underwater slopes, the distance to shoreline would remain approximately unchanged with the deepening of the lake, and therefore, terrestrial nutrient and OM availability is less affected if only lake depth changed. Moreover, water depth may not be the limiting factor for the

5

Conclusion

Variation in sedimentary BIT of Lake Qinghai is dominated by the production of crenarchaeol instead of bGDGTs. Considering that the production of crenarchaeol is positively related with water depth, water depth may exert a negative effect on the BIT index in this lake. This case seems contrary to that for Lake Challa, a small crater lake in equatorial East Africa, in which sedimentary BIT is correlated positively with lake level and precipitation (Verschuren et al., 2009; Buckles et al., 2015). The difference in relationships between BIT and hydrological changes for the two lakes is possibly due to different local lake conditions, which determine the variables affecting Thaumarchaeota productivity. Hence, the response of BIT to local paleohydrology may be site specific in lacustrine systems. Considering that the BIT values of surface sediments are negatively correlated with lake water depths for both regional and global lake groups across a wide gradient of environments (Blaga et al., 2010; Tierney et al., 2010), sedimentary BIT is probably correlated negatively with lake level variation for most lakes. However, further investigations for more lakes are still needed to test how BIT responds to hydrological change in a specific lake in greater detail. Acknowledgements

The authors would like to thank Dr. Hongxuan Lu (Institute of Earth Environment, Chinese Academy of Sciences) and groups from CUGB and Nanjing Institute of Geography and Limnology for their help during the field work. Two anonymous reviewers and the editors are thanked for providing useful comments and suggestions. This research was supported by the Major State Basic Research Development Program of China (Grant No. 2013CB955900), and the National Natural Science Foundation of China (Grant Nos. 41373022, 41573005).

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