Middle to Late Holocene lake level variations recorded by shoreline tufa of Longmu Co Lake,northwestern Tibet Plateau

Abstract

Tufa deposits have significant potential for reconstructing past lake-level fluctuations. Here, we conducted U-Th dating of tufa deposits exposed along the shoreline of Longmu Co Lake in the northwestern Tibetan Plateau to reconstruct its water level history during the middle-late Holocene. Our data demonstrate that Longmu Co Lake reached its highest level (61 m above the present level) at around 7.4 ka in response to the strengthened Indian Summer Monsoon and increased glacial meltwater; its lake level subsequently decreased due to solar insolation-induced decline in summer monsoon and glacial meltwater. Moreover, we find that the lake level experienced an abrupt decline at ~2.2 ka with a maximum amplitude of 13 m, probably owing to the rapid cooling of the local climate. The δ¹⁸O and δ¹³C of Longmu Co tufa also show a covariance trend, which supports the regional climate change reflected by the lake level fluctuation of Longmu Co. Additionally, we observed an inverse correlation between initial δ²³⁴U content in tufa and lake level variation, suggesting that initial calcium δ²³⁴U can serve as a proxy for reconstructing environmental changes. Our study therefore implies that lake tufa as a reliable archive for accurately reconstructing lake level changes.

Keywords

Holocene lake level Longmu Co Tibetan Plateau Tufa

Introduction

The Tibetan Plateau, known as the “Asian water tower,” is the vital source of numerous rivers providing freshwater resources to billions of people living downstream (Immerzeel et al., 2010; Yao et al., 2022). The hydroclimate dynamics of the Tibetan Plateau are influenced by the Asian monsoon and the westerlies, making it one of the most vulnerable regions to global climate change (Chen et al., 2008; Immerzeel et al., 2020; Liu and Chen, 2000; Yao et al., 2012, 2019; An et al., 2001). The plateau is marked by thousands of lakes with nearly 1400 of them larger than 1 km² encompassing a total area of almost 50,000 km². These lakes account for over half of China’s overall lake area (Zhang et al., 2019). Many of these are closed-basin lakes and exhibit exposed shorelines, with some displaying shore terraces. Such features indicate significant fluctuations in water levels throughout the evolutionary history of these lakes.

The Longmu Co Lake is situated in the northwest region of the Tibetan Plateau. Due to its remote location, instrumental observations of lake levels are limited. Previous studies have employed various proxy indicators, such as lake sediments, ice cores, and moraines, to reconstruct climatic and environmental variations in this region. For instance, the Sino-French joint scientific expedition survey (Avouac et al., 1996; Fontes et al., 1993; Gasse et al., 1991; Li et al., 1991; Van Campo and Gasse, 1993) utilized lake sediments to reconstruct the history of vegetation assemblages since the last deglaciation. Kong et al. (2007) used cosmogenic radionuclide ¹⁰Be dating to establish the age of the bedrock terrace of Sumxi Co, which indicates that the highest water level occurred between 12.8 and 11 ka, coinciding with the peak in solar insolation in the Northern Hemisphere during the Holocene. Luminescence dating of Longmu Co shorelines sediment also demonstrated that the water level of Longmu Co Lake reached its highest point around 12.8–11.3 ka, approximately 145–160 m higher than its present level (Liu et al., 2016).

Avouac et al. (1996) calculated the rate of decline in the lake level based on the analysis of shoreline topography and lake evapotranspiration of Longmu Co Lake, suggesting cyclic variations in water level with cycles lasting 9–12 years. Additionally, Thompson et al. (1997) reconstructed the δ¹⁸O variation in the Guliya ice core from the western Tibetan Plateau spanning the last 125 ka. Furthermore, five Holocene glacial advances in the northwestern Himalayas were identified through the ¹⁰Be dating of moraines (Saha et al., 2018).

The Longmu Co Lake level reconstruction is currently available for the last deglaciation, early and middle Holocene with limited information for the late Holocene (Kong et al., 2007; Liu et al., 2016). Exposed tufa deposits in the region present a promising avenue of research to fill this temporal gap. Tufa deposits offer several advantages including their widespread distribution, excellent preservation, accurate reflection of water level changes, ease of sample collection, and the availability of precise U-series dating methods. In this study, we use the U-series dating method to determine the ages of tufa deposits found at different elevations along the Longmu Co Lake terrace to reconstruct the history of changes in the lake level during the middle and late Holocene.

Tufa is a porous carbonate deposit that forms in underground springs, rivers, and lakes (Yuan, 1988). When the partial pressure of CO₂ in the water is higher than that of the surrounding atmosphere, CO₂ escapes from the water and leads to the supersaturation of calcium carbonate in the water and the formation of tufa (Figure 1). The chemical reactions involved are as follows:

C a^{2 +} + 2 H C {O_{3}}^{-} \leftrightarrow C a C O_{3} ↓ + C O_{2} ↑ + H_{2} O

Figure 1.

Schematic diagram of lake terrace tufa formation, before (a) and after (b) the lake level declines.

The equation is reversible, and the positive reaction is endothermic, resulting in the precipitation of calcium carbonate from the lake to form tufa.

In the lake environment, tufa occurs in both the deep and shallow depths, and its external morphology varies with the sedimentary environment (Li et al., 2006). The formation of tufa in deep water is generally clastic and mixed with some algae residue, mud and sand, while the formation of lamellar tufa occurs only in the shallowest depth near the land-water boundary. There is also a part of the tufa in the form of a coating on the surface in the lake shore rocks (Ford and Pedley, 1996; Pentecost and Viles, 2007). CO₂ degassing is the fundamental cause of lake tufa formation, and solar insolation is the key factor leading to the degassing of CO₂ in lake water. On the one hand, the water in the lakeshore area has good light transmission, aquatic plants and algae grow luxuriantly, and their continuous photosynthesis reduces the partial pressure of CO₂ (pCO₂₎ in the water; On the other hand, the lakeshore area is shallow, and the temperature of water can increase quickly under the irradiation of sunlight. Strong evaporation will also cause the CO₂ and water vapor in the water to escape. The reaction to generate calcium carbonate is endothermic, and the escape of CO₂, water vapor, and the warming of water will drive the reaction in a positive direction (Figure 1a) (Pedley, 1990; Pentecost and Viles, 2007). The tufa that forms in the boundary zone adheres to a hard substrate (such as lake bedrock terraces), and once formed, grows into overhanging outcrops toward the water’s surface (Ford and Pedley, 1996; Golubić, 1969; Pedley, 1990). As the lake level decline, lake tufa attached to the substrate is exposed to the surface along the shoreline (Figure 1b). The absolute dating of lamellar tufa, thus, can reveal the age of the ancient shorelines. This approach has been successfully used in the reconstruction of lake water level history in the Tibetan Plateau and South America (Blard et al., 2011; Fontes et al., 1993; Hudson et al., 2015; Hudson and Quade, 2013; Placzek et al., 2006b).

Study area

Longmu Co and Sumxi Co constitute a closed lake basin system (34° 35′ 42″ N, 80° 20' 11″ E), which is located in Ritu County in the northwestern Tibetan Plateau (Figure 2b). During the Bølling-Allerød (BA) warm period (14.7–12.7 ka), the two lakes reached the highest water level and merged to form a single lake (Liu et al., 2016). In the early Holocene, the lakes were still connected and the water level was ~50 m higher than the present day (Fontes et al., 1993). Since the early Holocene, the lake levels have declined forming the present-day pattern of two lakes separated by a watershed at an elevation of about 5100 m (Gasse et al., 1991; Liu et al., 2016).

Figure 2.

Study area and sample. (a) Map showing the location of the study area in the northwest Tibetan Plateau (1: Longmu Co; 2: Guozha Co; 3: Bangong Co; 4: Chongce glacier 5: Xiada Co). (b) The location of the sampling points. (c) Geographic location and sampling area map of Longmu Co.

The Longmu Co Lake area is about 97 km² and the lake level is 5004 m above the sea level. The climate of the region is cold and dry with a mean annual temperature of −1.48℃. Mean annual precipitation is 75–100 mm, and the potential evaporation is 1600 ± 300 mm per year. Longmu Co Lake is a hypersaline lake with low meltwater contribution from glaciers. It is mainly fed by groundwater, runoff, and precipitation (Gasse et al., 1991; Liu et al., 2016). Due to high evaporation, its salinity is high (172 g/L). The Sumxi Co lake, which is located to the west of Longmu Co, is a freshwater lake with an elevation of 5058 m and a low salinity (0.5 g/L) due to its recharge from meltwater from Mawang Kangri glacial (Fontes et al., 1993).

Materials and methods

Google Earth images show that Longmu Co is surrounded by concentric circles of regressive lake shorelines. In 2019 and 2020, we conducted a detailed field investigation of the Longmu Co-Sumxi Co Lake basin. The elevation of the shoreline was measured by portable GPS, and the measured elevation was systematically corrected by Google Earth and a 30 m digital elevations model, which could control the deviation within 1 m. The west bank of Longmu Co is the lakeshore terrace with exposed bedrock at an elevation of 5100 m. There are some large rocks formed by the consolidation of gravel along the lake shore, and some tufa deposits are attached to these rock surfaces under an elevation of 5100 m. The tufa deposits are mainly lamellar or oncoid, and some of the tufa deposits are coated on the gravel surface (Figure S1). In addition, some calcium carbonate deposits are consolidated with mud and sand to form calcareous cement, which may indicate they form in a deeper environment than lamellar tufa. Within the range of 1000–1500 m from the west bank of Longmu Co, we collected eight groups of relatively clean tufa of LMC1-LMC8 attached to reefs in the order of altitude from low to high (Figure 3d), and the elevation of the samples was concentrated between 5000 and 5040 m. A group of calcareous cement LMC9 attached to gravel was collected below a high terrace about 2000 m from the lake shore at an altitude of 5065 m (Figure 2b).

Figure 3.

Field sampling and sample photos. (a) Longmu Co west bank shoreline and lake terrace. (b) A worker is collecting tufa sampling from the surface of rock along the shoreline. (c) Rocks along the lake shoreline. (d) Tufa deposits samples (LMC9 is calcareous cement).

The tufa samples LMC1-8 are 1–3 cm in diameter, white and yellowish in color, and exhibit platy and sheet-like morphology (Figure 3d). Some samples have annular growth layers, without any apparent erosion and disturbance traces. LMC9 was formed by the cementation of calcium carbonate with sand and gravel and is light brown in color.

In each group of samples, a sample with a clean surface and relatively white color is selected for experimental use (Figure 3d). The chosen samples were pre-treated with an ultrasonic cleaning machine to remove impurities in the pores of the samples at the Institute of Earth Environment, Chinese Academy of Sciences. A dental drill was used to scrape the surface layer of tufa samples to avoid the error caused by surface contamination. A total of nine 30mg powder subsamples were obtained. The chemical procedure described in Edwards et al. (1987) was used to separate uranium and thorium. The samples were dated with Neptune’s multi-collector inductively coupled plasma-mass spectrometer (MC-ICP-MS) at the Isotope Laboratory of Xi’an Jiaotong University using the U-Th dating method (Cheng et al., 2013; Edwards et al., 1987; Shen et al., 2002).

In order to ensure the consistency of the dating subsample and the isotopic subsample, the isotopic subsample was selected to be sampled close to the dating point. Each isotope subsample was collected about 50 μg. The carbon and oxygen isotopes were measured at Shaanxi Experimental Center of Geological Survey, Shaanxi Institute of Geological Survey using ISOprime PrecisION. The analytical error of δ¹⁸O was less than 0.1‰. The δ¹⁸O values reported here are relative to the Vienna PeeDee Belemnite (VPDB) standard.

Results

U-series dating

Table 1 shows U contents, age estimates, and corrected initial δ²³⁴U for each of the eight samples. All ages are ranked from low to high according to the elevation of the sampling points of tufa deposits. The tufa samples (LMC1 to LMC8) collected from the low terrace are mainly concentrated in the 1–2.5 ka age range. The age errors of LMC7, LMC8, and LMC9 are large exceeding 1000 years. The U content of Longmu Co tufa samples is relatively higher than that of calcite samples, within the 3000–13,000 ppb range, with an average concentration of 7978 ppb. The impurity content is reflected in the ²³⁰Th/²³²Th ratio, and a lower ²³⁰Th/²³²Th ratio indicates a higher level of impurity in a sample (Schwarcz, 1989). The ²³²Th content of LMC7, LMC8, and LMC9 is significantly higher than that of other samples indicating that these samples have relatively high impurity content and ²³⁰Th/²³²Th values are significantly lower, which is one of the important reasons for their large age error. The calcareous binding sample LMC9 collected from the high terrace is older than 7 ka, with a large error, which may be because part of the sediment was mixed in the formation process of the sample, resulting in dirty samples and thus affecting the dating accuracy. According to the previous research using tufa U-Th dating, when the ²³⁰Th/²³²Th ratio exceed 10, indicating they have manageable detrital thorium concentrations for accurate dating (Hudson et al., 2015; Placzek et al., 2006a, 2006b; Wang et al., 2023). The ²³⁰Th/²³²Th ratio of tufa at Longmu Co is >11, indicating that although the ²³²Th content of the sample is high, the higher ²³⁰Th content puts the ²³⁰Th/²³²Th ratio within a reasonable range. Therefore, even if the error is relatively large, the age within the error range is still reliable.

Table 1.

Results of U-series dating of tufa in Longmu Co.

Sample	Elevation (m)	²³⁸U (ppb)	²³²Th (ppt)	²³⁰Th/²³²Th (atomic × 10⁻⁶)	δ²³⁴U* (measured)	²³⁰Th/²³⁸U (activity)	²³⁰Th age(a) (unconnected)	²³⁰Th age(a) (connected)	δ²³⁴U_Initial** (connected)	²³⁰Th age (a BP)*** (connected)
LMC1	5022	12641.8 ± 75.8	146,590 ± 3084	29 ± 1	742.5 ± 4.2	0.0205 ± 0.0003	1293 ± 18	1099 ± 138	745 ± 4	1030 ± 138
LMC2	5024	2860.6 ± 15.7	85,740 ± 1775	24 ± 1	790.6 ± 5.4	0.0430 ± 0.0007	2647 ± 43	2160 ± 347	795 ± 5	2091 ± 347
LMC4	5027	6940.4 ± 68.4	141,164 ± 3149	29 ± 1	756.4 ± 8.0	0.0358 ± 0.0006	2240 ± 38	1903 ± 241	760 ± 8	1834 ± 241
LMC5	5028	6271.7 ± 28.5	275,034 ± 5655	15 ± 0	748.8 ± 3.7	0.0391 ± 0.0002	2460 ± 16	1729 ± 517	752 ± 4	1660 ± 517
LMC6	5031	11040.0 ± 118.9	350,374 ± 12094	23 ± 2	728.8 ± 7.4	0.0439 ± 0.0026	2802 ± 169	2268 ± 413	733 ± 7	2199 ± 413
LMC7	5033	7561.7 ± 55.7	678,702 ± 14,575	12 ± 0	734.8 ± 5.4	0.0646 ± 0.0006	4125 ± 39	2615 ± 1070	740 ± 6	2546 ± 1070
LMC8	5037	12195.9 ± 125.5	1,073,224 ± 24,771	11 ± 0	719.5 ± 7.3	0.0587 ± 0.0007	3779 ± 51	2285 ± 1059	724 ± 8	2216 ± 1059
LMC9	5065	4316.4 ± 26.1	876,261 ± 18,387	13 ± 0	682.2 ± 4.7	0.1631 ± 0.0018	11,048 ± 131	7517 ± 2507	697 ± 7	7448 ± 2507

U decay constants: λ₂₃₈ = 1.55125 × 10⁻¹⁰ (Jaffey et al., 1971) and λ₂₃₄ = 2.82206 × 10⁻⁶ (Cheng et al., 2013). Th decay constants: λ₂₃₀ = 9.1705 ×10⁻⁶ (Cheng et al., 2013).

δ²³⁴U = ([²³⁴U/²³⁸U] _activity − 1) × 1000.

δ²³⁴_initial was calculated based on ²³⁰Th age(T), that is, δ²³⁴U_initial = δ²³⁴U_measured × e^λ234T. Corrected ²³⁰Th ages assume the initial ²³⁰Th/²³²Thatomic ratio of 4.4 ± 2.2 × 10⁻⁶. Those are the values for a material at secular equilibrium, with the bulk earth ²³²Th/²³⁸U value of 3.8. The errors are arbitrarily assumed to be 50%.

***

B.P. stands for “Before Present” where the “Present” is defined as the year AD1950.

Lake level variation histories of Longmu Co

Changes in the position of the shoreline are the most intuitive evidence of lake-level fluctuation. Therefore, within the closed lake basin system, the advance and retreat of the regressive lake shoreline can be used to indicate the history of lake level change. The tufa U-series dating shows that the ages of Longmu Co tufa ranged from the middle to late Holocene. Among these samples, LMC9 on the high terrace is calcareous binding. Due to the mixing of some sediment and other impurities, it is speculated that the formation environment may be located in deeper water than other samples. Thus, the lake level may be higher than the elevation where LMC9 is located when it was formed. The elevation of the LMC9 point reached 5065 m (~61 m above present lake level) at about 7.4 (±2.5) ka in the middle Holocene. After that, the overall water level showed an abrupt decline at around 2.2 ka, superimposed by a series of fluctuations. The highest water level of LMC8 reached 5037 m, 13 m higher than the lowest water level of LMC2 (5024 m). Then the water level rose to 5028 m at LMC5. The water level of Longmu Co decreased since 1.6 (±0.5) ka.

δ¹⁸O and δ¹³C

The δ¹⁸O values of Longmu Co tufa range from 2.30‰ to 4.14‰ with an average value of 2.92‰, and the δ¹³C ranges values from 6.26‰ to 7.36‰ with an average value of 6.88‰. The δ¹⁸O and δ¹³C show a covariance trend (r = 0.79, p < 0.01; Figure S2) and consistent with the lake level changes of Longmu Co. The overall decline trend, with an abrupt event at ~2.2 ka can be observed in the δ¹⁸O and δ¹³C curves (Figure 4).

Figure 4.

Multi-proxy of tufa in Longmu Co, the blue line is the fluctuation curve of the lake level (the gray line is the error bar of age), and the black number is the sample number.

In closed lakes with long residence time, the δ¹⁸O and δ¹³C compositions of carbonate change synergically when the lake volume change (Drummond et al., 1995), and the correlation coefficient > 0.7. The δ¹⁸O and δ¹³C synergies of Longmu Co tufa were obvious (r = 0.79, p < 0.01), indicating that Longmu Co tufa was formed in a closed lake environment (Lan et al., 2013, 2020).

Initial δ²³⁴U

The initial δ²³⁴U is generally expressed as δ²³⁴U_i, which is a very important index in the U-Th dating method. Previous studies on δ²³⁴U_i in carbonate sediments focused more on the area of chronology. However, since δ²³⁴U_i represents the ²³⁴U content at the initial formation of sediments, it can preserve the climate and environment information at the formation of carbonate sediments like other geochemical indexes (such as Sr, Mg, and Ba, etc.) (Kaufman et al., 1998). The calculation method is as follows:

δ^{234} U_{i} = δ^{234} U_{m e a s u r e d} \times e^{λ 234 T,}

and:

δ^{234} U_{m e a s u r e d} = ({[U^{234} /^{238} U]}_{a c t i v i t y} - 1) \times 1000,

where δ²³⁴U_measured is the degree of disequilibrium of the sample; λ₂₃₄ represents the decay constant of ²³⁴U, 2.82206 × 10^–6 (Cheng et al., 2013); T is the age to be measured.

The δ²³⁴U_i of tufa ranged from 697‰ to 795‰, with an average of 743‰. It exhibited lower values during the middle Holocene followed by a gradual increase with significant fluctuations in the late Holocene (Figure 4). Generally, the overall pattern of δ²³⁴U_i in Longmu Co’s tufa exhibits synchronous change with lake level.

Discussion

Hydroclimatic implications of δ¹⁸O and δ¹³C

Tufa is one of lake carbonate minerals, in which δ¹⁸O and δ¹³C are mainly controlled by the isotopic composition of lake water (Leng and Marshall, 2004). According to the study of Lake Sayram, which is similar to Longmu Co’s hydrological recharge, when δ¹⁸O and δ¹³C are relatively positive and present a covariance trend (r = 0.79, p < 0.01), δ¹³C of the lake carbonates may reflect the equilibrium of dissolved CO₂ with atmospheric CO₂, meanwhile δ¹⁸O of Longmu Co is predominantly regulated by regional effective moisture (the ratio of precipitation and evaporation, P/E) (Lan et al., 2020; Li and Ku, 1997; Xu et al., 2006). In the closed lake, due to the longer residence time, long-term evaporation conditions will gradually enrich heavy isotopes, which is also the reason for the overall positive δ¹⁸O and δ¹³C of Longmu Co tufa. Owing to long-distance water vapor transport, δ¹⁸O in atmospheric precipitation and glacial meltwater in Longmu Co is relatively negative (Gasse et al., 1991). Under drought conditions, the runoff into the lake decreases and evaporation is relatively high, thus P/E decrease, and δ¹⁸O in the lake is relatively positive. However, in the humid environment, the runoff into the lake increases, and the lake water is diluted by negative δ¹⁸O of precipitation and glacial meltwater, leading to relatively negative δ¹⁸O of the lake water. For δ¹³C, high evaporation of lake water will increase the partial pressure of CO₂ (pCO₂) and will enhance the preferential evaporative loss of the light isotope (¹²CO₂) to the atmosphere, which contribute to an increase in δ¹³C in lake water (Lan et al., 2020; Lei et al., 2012). Therefore, both δ¹⁸O and δ¹³C in Longmu Co reflect the effective moisture of the basin (Leng and Marshall, 2004).

Potential mechanism of lake level variations since the Middle Holocene

The Asian summer monsoon includes the Indian Summer Monsoon (ISM) and the East Asian Summer Monsoon (EASM), which interact with the westerlies on different time scales. The EASM influences the water vapor transport and hydrological cycle over the Tibetan Plateau and the arid regions of northwest China (An et al., 2000; Tian et al., 2001). The northwestern Tibetan Plateau where Longmu Co is located, is currently in the westerlies. Although the EASM affects the precipitation in the northeastern Tibetan Plateau and northern China, it does not reach the hinterland of the Tibetan Plateau. Therefore, the climate in the western Tibetan Plateau is mainly controlled by the westerlies and the ISM (Li et al., 2022). Although some studies have pointed out that it is also difficult for modern ISM to reach the northwestern region of the Tibetan Plateau (Chen F et al., 2008, 2010, 2019; Chen J et al., 2015; Chen S et al., 2022; Huang et al., 2023; Wang et al., 2005), other paleoclimate records show that when solar insolation was strong in the early and middle Holocene, ISM was strong enough to reach northwest Tibet, and even Pamir Plateau and Tianshan Mountains (Cheng et al., 2016; Leroy et al., 2019; Liu et al., 2019). For example, the A/C (Artemisia/Chenopodiaceae) of the Sumxi Co (Figure 5d), which belongs to the same drainage basin as Longmu Co, shows that the early-middle Holocene is significantly higher than theLate-Holocene, indicating that the Longmu Co-Sumxi Co Basin was in a relatively humid environment at that time (Gasse et al., 1991). Hudson and Quade (2013) reconstructed the paleo-rainfall (paleo-monsoon) model of the early to middle Holocene by using the lake shorelines of Nangal Ring Tso and Chabert Taka (Bubye) in Tibet and concluded that the precipitation in the northwest Tibetan Plateau was closely related to ISM. The Bangong lake (150 km away from Longmu Co) showed that δ¹⁸O of carbonate and δD of leaf wax are negative in the early and middle Holocene (Figure 5e), reflecting that the strengthening of the ISM brought more precipitation at that time (Fontes et al., 1996; Hou et al., 2017). The δ¹⁸O of carbonate and ostracod shells of Aweng Co (Figure 5g) from the western Tibetan Plateau also indicate that the Holocene climate change in this region is mainly controlled by the Northern Hemisphere summer solar insolation, and the northern limit of ISM maximum range in the early to middle Holocene can reach 34.5°N (Zhang et al., 2021). In addition, lake records from Tso Kar Basin and Tso Moriri in the western Tibetan Plateau show that ISM was strong in the early and middle Holocene, which dominated the precipitation changes in northwest Tibet (Leipe et al., 2014; Mishra et al., 2015; Wünnemann et al., 2010). The relatively negative δ¹⁸O and δ¹³C of Longmu Co tufa in the middle Holocene also indicate that the effective moisture of the basin was relatively high. Therefore, the middle Holocene high lake level record (5065 m) reconstructed at Longmu Co is supported by numerous hydroclimatic evidence from the western Tibetan Plateau.

Figure 5.

History of water level change in Longmu Co and comparison of regional climatic and environmental records. (a) Changes of water level in Longmu Co, the orange dots are tufa deposits in this study, and the green dots are luminescence dating results along the shoreline of Longmu Co Lake (Liu et al., 2016). (b) Initial δ²³⁴U of Longmu Co tufa. (c) 30°N summer solar insolation (Laskar et al., 2004). (d) A/C pollen ratios in the Sumxi Co core (Gasse et al., 1991). (e) Bangong Co δ¹⁸O record (Fontes et al., 1996). (f) The δ¹⁸O of the fine carbonate from Guozha Co (Li et al., 2021). (g) The black line and red line are the δ¹⁸O of carbonate and ostracod shells from Aweng Co (Zhang et al., 2021). (h) Xiada Co GDGTs temperature record (Li et al., 2023). (i) δ¹⁸O values in the Chongce ice core (Pang et al., 2020).

The Guliya ice core record from the northwest Tibetan Plateau shows positive δ¹⁸O values in the early and middle Holocene (Thompson et al., 1997), indicating relatively high temperatures at that time. The annual average temperature reconstructed by GDGTs from Xiada Co in the western Tibetan Plateau also showed that the middle Holocene temperature is significantly higher than the late Holocene (Figure 5h) (Li et al., 2023). Therefore, in this warm and humid environment background, a strong ISM will bring more abundant precipitation. In addition, the meltwater model of the glacier in Guozha Co (50 km to the north of Longmu Co) (Figure 5f), shows that the meltwater volume of the glacier remained at a high level about 5 ka ago (Li et al., 2021). Therefore, we can conclude that when the ISM was strong, the driving factor of the high lake level in the early to middle Holocene might also be related to the high temperature causing a large amount of glacial meltwater recharge. This conclusion is consistent with the luminescence results of Longmu Co (Liu et al., 2016).

The declining trend of Longmu Co Lake’s shoreline in the Holocene, as determined by luminescence ages, suggests a gradual shift in the climate from warm and wet to relatively dry and cold (Figure 5a) (Liu et al., 2016). This change aligns with the decrease in summer solar insolation in the Northern Hemisphere (Figure 5c) (Laskar et al., 2004), the decline of the A/C ratio (Gasse et al., 1991), and the negative δ¹⁸O values observed in the ice core (Thompson et al., 1997). Multiple pieces of evidence, including these, indicate a significant decline in the ISM after the middle Holocene, which is evident in several lakes across the northwest Tibetan Plateau (Fontes et al., 1996; Gasse et al., 1991; Hou et al., 2017; Li et al., 2021). The overall trend of δ¹⁸O and δ¹³C of Longmu Co tufa from middle Holocene to late Holocene is gradually positive, indicating that the runoff recharge of lakes decreased significantly after the middle Holocene. By approximately 2.5 ka, the water level of Longmu Co had dropped to 5033 m, about 30 m higher than its present level, resulting in the separation of Longmu Co and Sumxi Co into two independent lakes (Fontes et al., 1996; Van Campo and Gasse, 1993). During this period, the lower glacial meltwater recharge, and the weakened monsoon intensity, driven by solar insolation, likely played a major role on decreasing water level.

In the late Holocene, around 2.2(±1) ka, the lake level experienced a significant decrease, with the range of 13 m. This abrupt change can also be observed in Guozha Co glacier meltwater model and the δ¹⁸O record of Bangong Co (Figure 5e and f) (Fontes et al., 1996; Li et al., 2021). Additionally, the high-resolution δ¹⁸O record from the Chongce ice cap (Figure 5i), which is located in the northwestern Tibetan Plateau, shows a rapid decline in temperature at ~1.8 ka (Pang et al., 2020). The temperature record reconstructed by Xiada Co based on brGDGTs also shows that the western Tibetan Plateau decreased by ~4.4℃ at ~2.1 ka and maintained a cooling trend (Li et al., 2023). The δ¹⁸O and δ¹³C of Longmu Co tufa also showed significant fluctuation from negative to positive, indicating that the runoff into the lake decreased significantly at ~2.2 ka. Therefore, it is speculated that the decline in Longmu Co lake level at ~ 2.2 ka may be caused by a decrease in glacial meltwater triggered by a cooling event. The decrease of glacier meltwater may affect the lake level of Longmu Co from two ways: (1) the decrease of glacier meltwater directly entering Longmu Co; (2) the decrease of glacial meltwater may also reduce the recharge of Sumxi Co to Longmu Co through groundwater.

Although there is a brief delay in time, this may be attributed to the accuracy of tufa dating. In fact, based on the errors in our data, it is reasonable that late Holocene water level fluctuations in Longmu Co occurred in the 1–3 ka range. Combined with the reconstructed history of lake level change by luminescence dating (Liu et al., 2016), the lake level at 1–2 ka is stable (The maximum height difference ⩽6 m) with relatively accurate age, so we propose that the water level fluctuation between LMC8 and LMC2 is more likely to be occurred between 1.8 and 3 ka (Figure S3), possible with abrupt change of lake level at ~2.2 ka (LMC2). Indeed, a limited records in the northwestern Tibetan Plateau also show cooling events at ~2 ka (Fontes et al., 1996; Li et al., 2021, 2023; Pang et al., 2020). Therefore, we inferred that the abrupt decline of lake level in Longmu Co occurred at ~2.2 ka is reasonable.

Indicative significance of tufa initial δ²³⁴U

There are several sources of ²³⁴U in the natural environment. In addition to ²³⁴U formed by the normal decay of ²³⁸U, α-recoil can affect the change of ²³⁴U/²³⁸U value. Additionally, the destruction of the mineral lattice can also cause ²³⁴U to break away from the mineral crystal. Physical weathering (i.e. mechanical breakdown of rocks) can both release ²³⁴U in damaged crystallographic sites and increase the surface area from which α-recoil can occur, thus contributing more ²³⁴U (Henderson, 2002). Studies of river U isotopes from South Africa show that the ²³⁴U/²³⁸U activity ratios of rivers in areas with strong physical weathering (2.03 ± 0.42) are much higher than those in areas where chemical weathering occurs continuously (1.20). Under dry climatic conditions, new crystal surfaces are repeatedly exposed to the soil water, so the contribution of physical weathering to ²³⁴U is more important than chemical weathering (Kronfeld and Vogel, 1991). In stalagmite-based paleoclimate research, δ²³⁴U_i (or [²³⁴U/²³⁸U]₀), as a new climate proxy, has been increasingly used to reflect external climate change (Cui et al., 2019; Oster et al., 2023; Plagnes et al., 2002; Wendt et al., 2020; Zhou et al., 2005). However, due to several influencing factors and complex processes, there are many explanations for the mechanism of δ²³⁴U_i reflecting climate change. Some researchers have found that (²³⁴U/²³⁸U)₀ in speleothems is lower in warm, humid, or heavy rainfall periods, and higher in glacial climate conditions (Hellstrom and McCulloch, 2000; Kaufman et al., 1998; Zhao et al., 2001). The possible explanation for this phenomenon is that compared with ²³⁸U, ²³⁴U is more easily oxidized into uranyl ion [UO₂]²⁺ which is easily soluble in water, so surface water has the characteristics of preferential leaching to ²³⁴U. In the relatively cold and dry climate, the physical weathering of the land surface is strengthened, and the destruction of the mineral lattice can cause ²³⁴U to break away from the mineral crystal, resulting in the increase of ²³⁴U in soil, and the infiltration of soil water further leads to the increase of δ²³⁴U_i in speleothems (Yang et al., 2008).

In comparison to speleothems, the formation of lake tufa occurs in a relatively simpler environment and involves fewer reaction processes. However, the distribution of tufa along the shoreline limits its use as a proxy for reflecting paleoclimate information. As tufa primarily forms at the land-water interface, changes in the ²³⁴U content must be controlled by the moisture sources of the lake. Since the middle Holocene, the replenishment of water in Longmu Co primarily originated from precipitation and glacial meltwater (Fontes et al., 1993), suggesting that ²³⁴U in the lake water may be leached from the soil. It is hypothesized that under the warm and humid climate conditions of the early to middle Holocene, land physical surface weathering weakened, resulting in a decrease in the total ²³⁴U released from soil water due to reduced lattice destruction in the soil (Figure 5b). This, in turn, led to the low δ²³⁴U_i values observed in the lake water. Conversely, in the late Holocene, decreased precipitation resulted in increased land surface physical weathering, causing more precipitation and glacial meltwater to leach ²³⁴U from the soil (Henderson, 2002). In addition, Sumxi Co and Longmu Co exist in the same lake basin, and the physical weathering intensity of the basin should be the same. Even if there is a certain groundwater recharge from Sumxi Co to Longmu Co in the late Holocene, its ²³⁴U also indirectly reflects the physical weathering intensity of the lake basin. Consequently, this runoff flows into the lake, increasing the water content of ²³⁴U. Notably, the δ²³⁴U_i values, lake level, δ¹⁸O and δ¹³C changes exhibit synchronous fluctuations, indicating that δ²³⁴U_i in tufa is highly sensitive to environmental changes and serves as a reliable climate proxy.

Conclusion

We used U series dating of tufa deposits along the shoreline of Longmu Co Lake from the northwest Tibetan Plateau to reconstruct its lake level variations since the middle Holocene. Our data indicate that during the middle Holocene, the water level of Longmu Co Lake was about 5065 m, which is 61 m higher than the present day likely caused by the strong Indian summer monsoon and increased glacial meltwater. After that, the lake level declined driven by the weakened summer monsoon and reduced glacial meltwater in response to the decrease in solar insolation. The lake level fluctuated several times around 2.2 ka, with a maximum amplitude of 13 m, which may be related to the rapid cooling of local climate in the northwest Tibetan Plateau. The δ¹⁸O and δ¹³C of Longmu Co tufa show a covariance trend, both of them reflect the effective moisture of the basin, which supports the regional climate change reflected by the lake level fluctuation of Longmu Co. We find an inverse correlation between Longmu Co tufa δ²³⁴U_i and lake level, which may be related to the variation of δ²³⁴U_i content caused by land surface physical weathering. Tufa δ²³⁴U_i is sensitive to environmental information and is expected to be a reliable indicator of climate change.

Supplemental Material

sj-docx-1-hol-10.1177_09596836241254474 – Supplemental material for Middle to Late Holocene lake level variations recorded by shoreline tufa of Longmu Co Lake, northwestern Tibet Plateau

Supplemental material, sj-docx-1-hol-10.1177_09596836241254474 for Middle to Late Holocene lake level variations recorded by shoreline tufa of Longmu Co Lake, northwestern Tibet Plateau by Xu Lin, Jianghu Lan, Youbin Sun, Ashish Sinha, Xing Cheng, Fangyuan Lin, Jin Zhang, Yanzhen Li, Peng Cheng, Hong Chang, Syed Asim Hussain, Le Wang and Liangcheng Tan in The Holocene

Footnotes

Author contributions

Xu Lin: Data curation; Formal analysis; Investigation; Methodology; Visualization; Writing – original draft; Writing – review & editing.

Jianghu Lan: Funding acquisition; Investigation; Methodology; Writing – review & editing.

Youbin Sun: Investigation; Writing – review & editing.

Ashish Sinha: Formal analysis; Methodology; Writing – review & editing.

Xing Cheng: Formal analysis; Methodology; Writing – review & editing.

Fangyuan Lin: Software; Writing – review & editing.

Jin Zhang: Formal analysis; Investigation; Writing – review & editing.

Yanzhen Li: Investigation.

Peng Cheng: Investigation.

Hong Chang: Investigation.

Syed Asim Hussain: Writing – review & editing.

Le Wang: Investigation.

Liangcheng Tan: Formal analysis; Funding acquisition; Investigation; Methodology; Project administration; Resources; Writing – review & editing.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Second Tibetan Plateau Scientific Expedition and Research (2019QZKK0101), and the Chinese Academy of Sciences (xbzg-zdsys-202217, 2021411).

ORCID iD

Xu Lin

Supplemental material

Supplemental material for this article is available online.

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Supplementary Material

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Middle to Late Holocene lake level variations recorded by shoreline tufa of Longmu Co Lake,northwestern Tibet Plateau

Abstract

Keywords

Introduction

Study area

Materials and methods

Results

U-series dating

Lake level variation histories of Longmu Co

δ18O and δ13C

Initial δ234U

Discussion

Hydroclimatic implications of δ18O and δ13C

Potential mechanism of lake level variations since the Middle Holocene

Indicative significance of tufa initial δ234U

Conclusion

Supplemental Material

sj-docx-1-hol-10.1177_09596836241254474 – Supplemental material for Middle to Late Holocene lake level variations recorded by shoreline tufa of Longmu Co Lake, northwestern Tibet Plateau

Footnotes

Author contributions

Funding

ORCID iD

Supplemental material

References

Supplementary Material

δ¹⁸O and δ¹³C

Initial δ²³⁴U

Hydroclimatic implications of δ¹⁸O and δ¹³C

Indicative significance of tufa initial δ²³⁴U