Orbital- and suborbital-scale changes in the East Asian summer monsoon since the last deglaciation

Abstract

The variability of the East Asian summer monsoon (EASM) has far-reaching effects on the global climate system and the environment, and full understanding of the variability and dynamics of the EASM contributes to predictions of its future behavior. Here, we present a well-dated mineralogical and total organic carbon record from a saline inland lake in northern China which provides a robust archive of the EASM evolution since 16.0 cal. ka BP. Our record reveals a series of rapid and frequent millennial-scale climatic fluctuations during the last deglaciation; these fluctuations are documented by changes in the abundances of mirabilite, bloedite, and gypsum, which appear to record the Oldest Dryas, the Bølling-Allerød warm period, and the Younger Dryas. The peak EASM moisture occurred in the early and middle Holocene, which was punctuated by a prominent and abrupt weak monsoon interval that occurred synchronously with the 8.2 cal. ka BP cold event. This moisture maximum was terminated at 6.9–5.9 cal. ka BP by a warm-dry event marked by the deposition of gaylussite. Subsequently, the EASM gradually weakened over the late Holocene. The EASM moisture patterns reconstructed from Anguli-nuur Lake display good consistency with records from northern China, as revealed by a regional comparison; moreover, the recorded changes are synchronous with those of the Indian summer monsoon moisture patterns, as revealed by a comparison with the stalagmite records of southern China. Our reconstruction shows that the EASM has responded broadly to Northern Hemisphere summer insolation forcing on orbital time scales since the last deglaciation; thus, insolation is the primary factor that controls regional hydrological variations in the Asian monsoonal domain. The suborbital-scale events are related to the Atlantic meridional overturning circulation, and a slowdown of this circulation would lead to a southward shift of the intertropical convergence zone and a weakening of the EASM.

Keywords

8.2 ka cold event Anguli-nuur Lake East Asian summer monsoon Holocene optimum last deglaciation millennial-scale climatic events

Introduction

The East Asian summer monsoon (EASM), an integral part of the Asian summer monsoon system, pumps moisture from the tropical Pacific and produces rains over East Asia. Its anomalous precipitation behavior results in severe floods or droughts, affecting economic activity and human livelihoods within its area of influence (Clift and Plumb, 2008; Wang, 2006). In recent decades, diverse natural archives have been used to improve our understanding of the patterns of variability of the EASM and its dynamic mechanisms since the last deglaciation and to predict the long-term trends of regional and global climate conditions (An et al., 2000; Chen et al., 2008; Herzschuh, 2006; Shi et al., 1993; Wang et al., 2010b).

Despite the great advances that have been made in paleoclimate studies since the last deglaciation, the pattern of evolution of the EASM on orbital time scales remains controversial. A summary of many records, including lake sediments and paleosols in arid China, suggests that the Holocene megathermal occurred from 8.5 to 3.0 ¹⁴C ka BP (Shi et al., 1993, 1994). In a review covering the arid and semi-arid areas of China, Feng et al. (2006) indicated that the Holocene climatic optimum occurred nearly synchronously (between 7.5 and 5 cal. ka BP) at all sites on both the Inner Mongolian Plateau and the northwestern part of the Loess Plateau, which lie close to the northern limit of the modern EASM. The mean effective moisture inferred from 75 paleoclimatic records in the Asian monsoonal domain showed that the Holocene climatic optimum, which was associated with high precipitation in the EASM domain, may have occurred during the middle Holocene (Herzschuh, 2006). A numerical meta-analysis in monsoonal China has demonstrated that the EASM domain exhibited maximum moisture levels during the middle Holocene, especially in north-central China (Wang et al., 2010b). As discussed above, the pattern of the Holocene optimum proposed by An et al. (2000) in the EASM domain seems to be well-established. However, these interpretations have been challenged by a contrary view, which states that the EASM has evolved similarly to the Indian summer monsoon (ISM) on orbital time scales since the last deglaciation; the Holocene optimum occurred in the early and middle Holocene within this context. For example, the past lake levels covering the past 16 cal. ka BP has been reconstructed from Dali Lake (Figure 1a), and this record demonstrates that the lake levels were overall high during the early and middle Holocene, suggesting that the Holocene optimum occurred during this period (Goldsmith et al., 2017). The history of EASM precipitation during the past 14.5 cal. ka BP has been reconstructed from δ¹⁸O measurements of speleothems found in Lianhua Cave on the Loess Plateau of China (Figure 1a); in this record, a prolonged period of strong EASM precipitation is noted at 11.6–6 cal. ka BP (Dong et al., 2015). A pollen-based annual precipitation record since the last deglaciation has been reconstructed from Gonghai Lake (Figure 1a), and this record demonstrates that the variability in EASM precipitation gradually intensified in the early and middle Holocene (Chen et al., 2015). To understand these discrepancies, additional robust paleoclimatic records from locations in northern China are needed.

Figure 1.

(a) Spatial distribution of paleoclimatic records used in this study. The pink dashed line represents the modern limit of the Asian summer monsoon. (b) Map showing the catchment of Anguli-nuur Lake in the DEM. The coring site is indicated by the asterisk.

Furthermore, although the major suborbital-scale events that have occurred since the last deglaciation (e.g. the Oldest Dryas, the Bølling-Allerød, the Younger Dryas, and the 8.2 cal. ka BP cold event) are documented in numerous high-resolution archives in monsoonal China (Chen et al., 2015; Dong et al., 2010, 2015; Dykoski et al., 2005; Goldsmith et al., 2017), the archives display differing responses to these suborbital-scale climatic events. For example, the 8.2 cal. ka BP cold event is clearly recorded in Dongge Cave (Wang et al., 2005) and Lianhua Cave (Dong et al., 2015); however, this event is not clearly recorded in Sanbao Cave (Dong et al., 2010), Gonghai Lake (Chen et al., 2015), or Dali Lake (Goldsmith et al., 2017). The reason for these discrepant responses has not been confirmed. Thus, additional records from the monsoon region of China are still needed to help us fully understand these suborbital-scale events.

Anguli-nuur Lake, which is located in the EASM marginal area, is highly sensitive to climate changes. In this study, we present the orbital- and suborbital-scale changes of the EASM since the last deglaciation, inferred from well-dated total organic carbon (TOC), total nitrogen (TN), and mineralogical records from the central core of Anguli-nuur Lake. We also assess the evolution of the EASM and the underlying dynamic mechanisms.

Study area

Anguli-nuur Lake (41°18′–41°24′N, 114°20′–114°27′E, 1315 m a.s.l.), a saline inland lake of the southern Inner Mongolia Plateau, is located in the northwestern terminal zone of the modern EASM (Figure 1). The bedrock of the drainage basin is basalt, which formed during the late Miocene Epoch (Wang et al., 2010a). The drainage area shrank from 2077.4 to 722.4 km² after the Hailiutu and Huanggai-nuur Reservoirs were built in 1959. The lake had a water surface area of 47.6 km², a maximum depth of approximately 4 m, and a pH of ca. 8.3 recorded in 1986 (Wang and Dou, 1998). The two main streams that feed the lake are the Heishui and Santai Rivers, which enter the lake from the east and south, respectively (Figure 1b). Anguli-nuur Lake lies in a semi-arid region where the mean annual temperature is 3.1°C and the mean annual precipitation is 350 mm, which accounts for only 1/5 to 1/4 of the annual potential evaporation (Liu et al., 2010). During the winter and spring, the north and northwest winds prevail and may transport dust into the lake (Zhai et al., 2006). The lake became totally desiccated in 2004, and the basin surface is presently covered by a white-colored saline efflorescence. Anguli-nuur Lake is highly sensitive to climate changes; thus, it is selected as a site for assessing EASM variability.

Materials and methods

Sampling and lithology

In October 2010, a 38-m-long sediment core (AGL-2010) was recovered from the center of Anguli-nuur Lake (41°20′43.817″N, 114°22′39.852″E) using a modified XY-1B piston corer made in China (Figure 1). The top part (0–60 cm) of core AGL-2010 was subsampled at a 1-cm interval, and the remainder was subsampled at a 2-cm interval in the field. All of the subsamples were transported to the laboratory and stored at 4°C. The TOC, TN, and mineralogy were analyzed in the upper 21.5 m of the core, for which reliable dates were obtained. The upper and basal sediments within the core are composed of grayish-green clay and grayish-yellow clayey silt, respectively. The middle sediments consist of black and grayish clay with stable mirabilite layers at 19.5–18.1 m, coarse salt crystals at 18.1–15.0 m, and fine gaylussite crystals at 10.0–7.0 m (Figure 2).

Figure 2.

Lithology, TOC/TN, and geochronology of the Anguli-nuur Lake sediment core: (a) Bayesian age–depth model of the Anguli-nuur Lake sediment core (Blaauw and Christen, 2011). (b) Sedimentation-rate histogram. Sedimentation rates are calculated between the AMS ¹⁴C ages that lie on the red dashed line.

Radiocarbon dating

Thirteen AMS radiocarbon dates on sedimentary TOC, charcoal, and plant material were obtained at the Rafter Radiocarbon Laboratory of the Institute of Geological and Nuclear Sciences in New Zealand (Table 1).

Table 1.

AMS ¹⁴C ages and calendar ages obtained for core AGL-2010.

Lab ID	Depth (cm)	Dated material	¹⁴C age(a BP)	δ¹³C (‰)	Calibrated age (cal. a BP) (±2σ)
NZA 37124	49	TOC	2640 ± 20	−24.0	2744–2776
NZA 37144	211	TOC	4363 ± 25	−24.7	4858–4975
NZA 57912	241	Charcoal	3651 ± 21	−25.4	3897–3998
NZA 57913	411	Charcoal	4376 ± 20	−25.5	4866–4975
NZA 36953	611	TOC	5835 ± 25	−25.4	6601–6731
NZA 59224	880	Charcoal	4918 ± 52	−25.3	5582–5748
NZA 36954	1085	TOC	6296 ± 25	−25.6	7170–7266
NZA 50810	1238	TOC	7593 ± 31	−23.6	8357–8429
NZA 59157	1377	Charcoal	8891 ± 33	−25.4	9905–10,171
NZA 59770	1458	Charcoal	8250 ± 31	−24.8	9121–9322
NZA 57914	1627	Charcoal	11,965 ± 44	−19.0	13,712–13,999
NZA 58787	1627	Plant material	11,393 ± 43	−10.2	13,125–13,321
NZA 58788	1837	Plant material	12,314 ± 48	−10.7	14,074–14,624

TOC and TN analyses

TOC and TN analyses of subsamples were treated with 1N HCl to remove inorganic carbonates, rinsed repeatedly with deionized water to remove soluble salts, and dried prior to measurement. The TOC and TN contents were determined using an EA3000 Elemental Analyzer at the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences.

Mineral analysis

All subsamples were air-dried at room temperature and ground to powder in a mortar and pestle prior to measurement. The mineralogy was determined at the Xi’an Institute of Earth Environment of the Chinese Academy of Sciences using a PANalytical X’Pert Pro MPD (Multi-Purpose Diffractometer) x-ray diffractometer set to a scanning rate of 5/min for a 2θ range of 3°–70° with Cu Kα radiation (λ = 1.5406 Å). Mineral identification and quantification were performed by inspecting the intensity of the strongest peak for each bulk mineral diffractogram (Chung, 1974; Last, 2002) using the PANalytical X’Pert HighScore Plus software package.

Results

Chronology

The radiocarbon dating of lacustrine sediments in the arid and semi-arid regions of western China is generally affected by carbon reservoir effects (Hou et al., 2012; Ren, 1998), especially in saline lakes (An et al., 2012; Liu et al., 2008). However, the TOC/TN ratios from Anguli-nuur Lake sediments typically exceed 10 (Figure 2), which indicates that the sedimentary TOC mainly originated from emergent or terrestrial plants. In addition, the dates on TOC lie near or fall on the smooth age–depth line established using the charcoal and plant material, suggesting that the AMS ¹⁴C dating of TOC is not affected by carbon reservoir effects (Figure 2a). The age–depth model is established by 13 AMS ¹⁴C ages, with an age of 16.0 ka at the bottom of the core (Table 1 and Figure 2a). The age of each subsample is set up using the Bacon 2.2 software package (Blaauw and Christen, 2011). The sediment accumulation rate varies from 17.8 to 293.2 cm/ka, with an average of 134.4 cm/ka (Figure 2b). This rate decreased markedly ca. 5.0 ka BP, which is similar to previous conclusions for Anguli-nuur Lake (Liu et al., 2010).

TOC, TN, and TOC/TN

The TOC content is lowest (<0.6%) at depths of 21.5–18.5 m in core AGL-2010. Rapid and frequent variations in TOC between 0.7% and 11% occur at depths of 18.5–15.1 m. TOC is generally high between 15.1 and 6.9 m, and the highest value appears at 15.1–10.1 m. Moreover, it begins to decline at 6.9 m and declines further at 2.4–0 m (Figure 3). The fluctuations in TN closely resemble those of TOC. The TOC/TN ratios are generally greater than 10 in the sequence, except in the lowermost ca. 2.5 m (Figure 2).

Figure 3.

Lithology, TOC content (three-point running means), and mineralogy in core AGL-2010.

Mineral variations

The mineral constituents of core AGL-2010 at 21.5–19.5 m are detrital minerals, including quartz, feldspars, and clay minerals. The sediments are then dominated by evaporites (ca. 48%) from 19.5 to 15.0 m, where mirabilite (Na₂SO₄·10H₂O), gypsum (CaSO₄·2H₂O), and bloedite (Na₂Mg(SO₄)₂·4H₂O) alternate with one another (Figure 3). The upper sediments are characterized by various carbonates, including calcite, dolomite, aragonite, and gaylussite (Na₂Ca(CO₃)₂·5H₂O). Alternating aragonite and gaylussite occur at depths of 15.0–0 m. The calcite deposition is sequential and stable, and dolomite is relatively abundant from 10.0–3.0 m (Figure 3).

Discussion

Proxy interpretation

TOC and TN

TOC and TN are reliable indicators of primary productivity in lacustrine sediment, especially in arid and semi-arid China (Liu et al., 2009; Xiao et al., 2006). However, the organic matter found in lake sediments has two principal sources, autochthonous aquatic plants living in the lake water and allochthonous terrestrial plants growing in the lake catchment. The TOC/TN ratios of lacustrine organic matter are usually used to distinguish between autochthonous and allochthonous sources. The TOC/TN ratios from aquatic phytoplankton vary between 4 and 10, whereas those from terrestrial vegetation are 20 or more (Meyers, 2003).

The measured TOC/TN ratios in the Anguli-nuur Lake sediments generally exceed 10 and have an average value of 14.8, except for the bottom 2.5 m (Figure 2). This result suggests that the organic matter found in the Anguli-nuur Lake sedimentary record is mainly allochthonous in origin (Meyers, 2003). In general, the organic matter content of sediment is related to mean annual precipitation; that is, more precipitation over a lake catchment leads to increases in the transport capacity of streams, and more land-derived organic matter is carried to the lake (Li et al., 2016a; Xiao et al., 2006). Therefore, we interpret the TOC content of core AGL-2010 as a proxy index for precipitation changes in the lake region. That is, an increase in the TOC content indicates an increase in the precipitation intensity of the summer monsoon circulation over the lake region.

Detrital minerals

The detrital minerals in core AGL-2010 were derived from the drainage basin of the lake through weathering and erosion and were transported into the lake by wind and/or streams (An et al., 2012; Liu et al., 2014). Both eolian origins and river discharges controlled by climatic changes play important roles in controlling sediment inputs to Anguli-nuur Lake; that is, sandstorms carry dust to the lake in the winter or under arid conditions, and river runoff transports detrital materials into the lake in the summer or under humid conditions (Li et al., 2016b).

Carbonates

Based on field investigations and a literature review, no limestone is present within the Anguli-nuur Lake drainage basin. Thus, the carbonates found in core AGL-2010 are primarily authigenic in origin and were produced by chemical or biological processes. Authigenic carbonate deposition is related to many factors, such as temperature, salinity, and primary productivity within lakes (Kelts and Hsü, 1978; Tucker and Wright, 1990); of these factors, temperature is the most important. When temperatures rise, carbonates are deposited through several mechanisms, specifically (1) decreases in the solubility of calcium carbonates; (2) the supersaturation of Ca²⁺ and Mg²⁺ in the lake water via evaporation, which promotes the deposition of carbonates (Xiao et al., 2006); (3) more CO₂ (aq) assimilation by thriving algae photosynthesis (Opitz et al., 2012), leading to supersaturation of CO₃²⁻ and promoting the deposition of carbonates via the reaction 2HCO₃⁻ → CO₂ (aq) + H₂O + CO₃²⁻; and (4) increases in the abundance of cyanobacteria, which serve as nuclei for calcite precipitation (Hodell et al., 1998). Therefore, we interpret the carbonate content of the Anguli-nuur Lake sediments as a proxy index for temperature variations in the lake region. That is, an increase in the carbonate content indicates an increase in temperature over the lake region.

Gaylussite, a relatively rare carbonate in nature, has infrequently been reported in the field of paleoclimate reconstruction (Anoop et al., 2013; Eugster and Hardie, 1978; Mees et al., 1998), especially in China (Li et al., 2008). Gaylussite usually succeeds the deposition of calcite or aragonite when saline and alkaline waters are further concentrated by evaporation (Anoop et al., 2013; Jones, 1965; Mees et al., 1998). Thus, gaylussite deposition indicates significant drought in the drainage basin.

Evaporites

The deposition of evaporites is primarily related to the salinity and chemical composition of lake waters (Liu et al., 2008). Mirabilite is regarded as a good indicator of cold/dry environments (Herrero et al., 2015; Zheng et al., 2000) and has proven to be very useful in the study of paleoclimate (Grasby et al., 2013; Herrero et al., 2015; Wang et al., 2012). Bloedite and gypsum have been found in various saline lakes in arid or semi-arid regions around the world and are used as indicators in paleoclimatology and sedimentology (Liu et al., 2008; Mees et al., 2011; Sönmez and Çelik, 2016). These minerals are generally related to intensive evaporation under warm/dry conditions (Escavy et al., 2012; Li et al., 2010; Sönmez and Çelik, 2016; Valero-Garcés et al., 2000). Therefore, bloedite and gypsum are regarded as indicators of warm/dry climate conditions.

Reconstruction of climate changes

Based on the mineralogical composition and TOC content of the sediments of Anguli-nuur Lake (Figure 3), the climate evolution of Anguli-nuur Lake during the past 16.0 ka can be classified into the following six stages.

Stage I (21.5–19.5 m, 16.0–14.9 cal. ka BP)

The lowest section is composed of predominantly detrital minerals (99.5%), including quartz (52.7%), clay minerals (28.2%), and feldspars (18.6%) (Figure 4a). Almost no evaporites are present, and carbonates are absent. Low TOC contents with an average value of 0.39% indicate low primary productivity, implying that the detrital deposition probably occurred through eolian processes. Therefore, the climate was cold during this stage.

Figure 4.

XRD diffractograms of the major minerals in the sediments of the core AGL-2010 at the depths of (a) 2062 cm, (b) 1946 cm, (c) 1796 cm, (d) 1724 cm, (e) 1370 cm, and (f) 914 cm. Thenardite (Na₂SO₄) is converted from mirabilite (Na₂SO₄·10H₂O), which loses crystal water in the freeze-drying step.

Stage II (19.5–14.9 m, 14.9–11.1 cal. ka BP)

The evaporites (44.1%) predominate in this stage, which consists of a ca. 1.4-m-thick layer of pure mirabilite (Figure 4b) at the bottom and alternating abundant bloedite (Figure 4c), gypsum (Figure 4d), and mirabilite crystals. In addition, the carbonate minerals calcite (1.5%) and dolomite (1.8%) occur in trace amounts. The TOC content increases unsteadily and then decreases dramatically, suggesting that the climate was alternating cold and warm, with abrupt and frequent fluctuations.

Stage III (14.9–10.1 m, 11.1–6.9 cal. ka BP)

This stage is characterized by the disappearance of evaporites and increases in carbonates containing aragonite (12.7%) and small amounts of calcite (6.3%) (Figure 4e). The TOC contents show a proportionate dramatic increase and reach almost their highest values within this stage, suggesting that the climate was warm and wet during the early Holocene.

Stage IV (10.1–7.0 m, 6.9–5.9 cal. ka BP)

On the whole, the carbonates maintain high values, among which crystalline gaylussite (15%) is a major constituent (Figure 4f), during this stage. The TOC contents clearly decrease, but still maintain relatively high values. All these observations suggest that the climate was warm and dry in this period.

Stage V (7.0–2.4 m, 5.9–4.0 cal. ka BP)

The carbonates are composed of aragonite (9.1%), dolomite (6.8%), and calcite (5.9%) (Figure 4e). Given the sequential dolomite deposition, which is controlled by high Mg/Ca molar ratios following high salinity (Müller et al., 1972), the salinity was probably greater than that of Stage III, suggesting that the climate was drier than that of Stage III. In addition, the carbonate and TOC contents show gradual decreasing trends, suggesting that the climate conditions were relatively cool and dry during this period.

Stage VI (2.4–0 m, 4.0–0 cal. ka BP)

The top section is characterized by an abrupt and simultaneous decrease in carbonates and TOC, indicating that the regional temperature and precipitation declined synchronously. Thus, the climate further deteriorated during the late- Holocene.

EASM variability on orbital time scales

To fully understand the EASM variability on orbital time scales, we compare our records of TOC and carbonate contents with other records from regions influenced by the EASM and the ISM (Figure 5).

Figure 5.

Comparison of the Anguli-nuur Lake EASM record with various other regional and global environmental signals. (a) The carbonate composition of core AGL-2010. The dark line represents the three-point running mean of the total carbonate contents. (b) The dark line represents the three-point running mean of the TOC contents for core AGL-2010, and the orange line represents the 65°N summer insolation (Berger and Loutre, 1991). (c) The EASM index synthesized by principal components analysis using published climate indices (Wang et al., 2010b). (d) TOC content of Daihai Lake (Xiao et al., 2006). (e) Lianhua Cave speleothem δ¹⁸O record (Dong et al., 2015). (f) Dali Lake level (Goldsmith et al., 2017). (g) Pollen-based P_ANN (annual precipitation) reconstructed from Gonghai Lake (Chen et al., 2015). (h) Sanbao Cave speleothem δ¹⁸O records (Dong et al., 2010; Wang et al., 2008). (i) Dongge Cave speleothem δ¹⁸O records ((Dykoski et al., 2005; Wang et al., 2005). (j) North Greenland Ice Core Project (NGRIP) δ¹⁸O record (Rasmussen et al., 2006). The dark gray bar represents the peak monsoonal precipitation, and the light gray bar indicates the 8.2 cal. ka BP cold event. The six stages (I–VI) are consistent with those in Figure 3.

The Holocene optimum was initially thought to have occurred during the middle Holocene in the East Asian monsoon domain (Herzschuh, 2006; Shi et al., 1993; Wang et al., 2010b; Xiao et al., 2002, 2004), as shown in Figure 5c and d; however, this interpretation has been challenged by a contrary view, which states that the Holocene optimum occurred during the early and middle Holocene. In the Anguli-nuur Lake region, our records reveal that the warmth and moisture indices increased abruptly at the beginning of Holocene, the wettest and warmest phase occurred in the early and middle Holocene, and cold-dry climate conditions prevailed in the late- Holocene (Figure 5a and b). These patterns are consistent with records from Lianhua Cave (Figure 5e; Dong et al., 2015), Dali Lake (Figure 5f; Goldsmith et al., 2017), Gonghai Lake (Figure 5g; Chen et al., 2015), and other records in northern China that are influenced by the EASM (Chen et al., 2003; Hong et al., 2005; Jiang et al., 2010; Li et al., 2004; Xiao et al., 2008, 2009; Zhang et al., 2011). Our records are also generally in agreement with the speleothem δ¹⁸O records from Saobao Cave and Dongge Cave shown in Figure 5h and i, which are influenced by the ISM (Dong et al., 2010; Dykoski et al., 2005; Wang et al., 2005, 2008); thus, both the EASM and ISM subsystems experienced synchronous hydrological variations in monsoonal China. On orbital time scales, the climate history of Anguli-nuur Lake displays an evolutionary pattern that resembles the 65° N summer insolation curve (Figure 5b). Modern data and model results show that high summer insolation increases the ocean-continent temperature and pressure gradients, further enhancing the summer monsoon (Fleitmann et al., 2007; Wang et al., 2001, 2005), which transports additional moisture far into northern China; similarly, low summer insolation produces the opposite effects. Accordingly, our records support the view that insolation is the primary factor that controls regional hydrological variations in the Asian monsoonal domain (Wang et al., 2005).

Climate events on suborbital time scales

Our reconstruction also documents several events that occurred at suborbital time scales since the last deglaciation (Figures 5 and 6).

Figure 6.

Variations in evaporites in core AGL-2010 during the last deglaciation.

The cold-dry conditions indicated by the low TOC contents and the deposition of abundant mirabilite from 14.9 to 14.2 cal. ka BP in Anguli-nuur Lake (Figures 5b and 6) are consistent with the cold phase of the speleothem δ¹⁸O records from monsoonal China (Wang et al., 2001, 2008) and the Oldest Dryas cold period recognized in high-latitude ice core records (Rasmussen et al., 2006).

The termination of the last glacial period was marked by a warm period, as inferred from increases in TOC and the abrupt deposition of bloedite and gypsum at 14.2–13.2 cal. ka BP in Anguli-nuur Lake (Figures 5b and 6). This warm period is synchronous with the Bølling-Allerød warm period recognized in European and North Atlantic climate records (Severinghaus and Brook, 1999; Weaver et al., 2003). However, several layers of mirabilite are nevertheless identified within this warm period (Figure 6), which indicates that the climate was unstable (Stuiver et al., 1995). Our record reflects a non-linear response to the summer insolation at the beginning of the Bølling–Allerød warm period (Figure 5b), which suggests that insolation was not the only forcing mechanism. Instead, the changing glacial boundary conditions may have led to increased warming over the Tibetan Plateau in the summer, thus producing the pressure gradient required for a strong Asian summer monsoon (Overpeck et al., 1996) and in turn generating the Bølling–Allerød warm period in monsoonal China.

Following the warm Bølling–Allerød interstadial, a cold reversal immediately occurred, as documented by the low TOC contents and renewed mirabilite deposition prior to the Holocene in Anguli-nuur Lake (Figures 5b and 6); this change is coeval with the Younger Dryas cold event recorded in the Greenland ice cores (Dansgaard et al., 1989; Rasmussen et al., 2006). During the Holocene, a centennial-scale cold event indicated by low TOC and carbonate contents occurred at approximately 8.6–8.1 cal. ka BP (Figure 5a and b), which is synchronous with the notable 8.2 cal. ka BP global cold event (Barber et al., 1999; Thomas et al., 2007). These two cold events have a similar forcing mechanism; the northward retreat of the southern margin of the Laurentide Ice Sheet released massive amounts of freshwater into the North Atlantic, leading to slowdown of the Atlantic meridional overturning circulation (Alley et al., 1997; Teller et al., 2002). This chain of events subsequently produced a southward shift of the intertropical convergence zone, as well as a weakening of the Asian summer monsoon (Cheng et al., 2009; Liu et al., 2013).

Conclusion

The TOC, TN, and mineralogical proxies, together with 13 AMS ¹⁴C ages, obtained from a continuous, 21.5-m-long sequence of sediment from Anguli-nuur Lake reveal the detailed history of the EASM in northern China since the last deglaciation. Our main conclusions are as follows:

On orbital time scales, our record represents a fluctuating climate amelioration during the last deglaciation, maximum monsoon moisture in the early and middle Holocene, and a gradual climate deterioration in the late Holocene. Comparison with stalagmite records influenced by the ISM system indicates that the EASM and ISM show similar and consistent changes on orbital time scales since the last deglaciation. Our record supports the view that insolation is the primary factor that controls regional hydrological variations in the Asian monsoonal domain.

The major suborbital-scale global climate events that have occurred since the last deglaciation have been documented in Anguli-nuur Lake. The millennial-scale climatic events, such as the Oldest Dryas cold event, the Bølling–Allerød warm period, and the Younger Dryas cold event, appear to be recorded in the lake sediments by variations in the abundances of mirabilite, bloedite, and gypsum. The centennial-scale 8.2 cal. ka BP cold event is also prominently represented in core AGL-2010. This series of suborbital-scale climate events indicates that a teleconnection exists between the EASM and the North Atlantic.

Footnotes

Acknowledgements

We thank Associate Professor Yongbo Wang for the constructive advice. We acknowledge American Journal Experts (AJE) for English language editing.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 41372176).

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