Stepwise evolution of the Asian summer monsoon during the Holocene revealed by a stalagmite record from Heifeng Cave,Southwest China

Abstract

The abrupt changes of the Asian summer monsoon (ASM) during the Holocene have long been of interest to environmental scientists. Here, RAMPFIT and Bayesian Change Point (BCP) analyses are applied to analyze the stalagmite δ¹⁸O record from Heifeng Cave in southern China, and the results show that it is characterized by a distinct stepwise pattern of variation which can be divided into six stages (Stages, S) and five transitional phases (Transitions, T). In the early Holocene, when Northern Hemisphere summer insolation (NHSI) was at its maximum, the ASM underwent step-like increases in strength, comprising two stages. With the decrease in NHSI during the middle to late-Holocene, the weakening of ASM intensity was characterized by further step-like changes comprising three stages. The transitional phases of the monsoon are broadly correlative with millennial-scale monsoon weakening events associated with Bond events in the North Atlantic region, suggesting that the stepwise evolution of the ASM during the Holocene was caused by the combined effects of changes in NHSI and Bond events. We propose that Bond events may have acted as triggers that caused the ASM to cross a critical threshold, prompting a shift from one regime to another.

Keywords

ASM Bond events Holocene stalagmite step-like pattern threshold effect

Introduction

The rainfall delivered by the Asian summer monsoon (ASM) affects the livelihood and wellbeing of billions of people (Levermann et al., 2009). The variability of Holocene ASM was closely related to the development of human society and has attracted much research attention, with the instability of ASM being a focus. The DA stalagmite oxygen isotope record from Dongge Cave in China shows that the evolution of the ASM during the past 9000 years was interrupted by eight decadal- to centennial-scale weak monsoon events, six of which were related to well-documented ice-rafted detritus (IRD) events in the North Atlantic (Wang et al., 2005). Dykoski et al. (2005) noted that the intensity of the ASM decreased in a stepwise manner during the middle to late-Holocene, and proposed a threshold effect for monsoon intensity as an explanation. Donges et al. (2015) identified four non-linear regime shifts (NRS) in the Holocene evolution of the Asian monsoon, based on Recurrence Network analysis of several Asian speleothem records, which corresponded to episodes of rapid climate change (RCC) (Mayewski et al., 2004) and Bond events (Bond et al., 2001). A speleothem record from northeast China also showed that the ASM weakened in a step-like manner during the transition between the middle and late-Holocene, which affected cultural development within its region of influence (Zhao et al., 2021).

Previous studies of various geological archives have shown that gradual and low-amplitude climate changes, or non-linear feedbacks, may lead to abrupt climate change when a certain threshold is reached (e.g. Claussen et al., 1999; Liu et al., 2006; Zhao et al., 2017). The gradual reduction of insolation (Levermann et al., 2009; Schewe et al., 2012; Zickfeld et al., 2005) and the resulting non-linear vegetation-atmosphere feedback processes (Claussen et al., 1999) are likely to have created a threshold effect in climate and ecosystem change. Zhao et al. (2017) demonstrated the occurrence of two significant abrupt changes in the vegetation of Central Asia during the Holocene, which may be attributed to the threshold effect of vegetation in response to orbitally-driven gradual climatic changes. Zickfeld et al. (2005) and Levermann et al. (2009) used a theoretical model of the monsoon to identify two regimes of monsoon stability during the Holocene. Pre-industrial variations in insolation and atmospheric greenhouse gas concentration may have driven the monsoon system to exceed certain thresholds or tipping points (Zickfeld et al., 2005), which triggered abrupt transitions between monsoon regimes (Levermann et al., 2009; Zickfeld et al., 2005). During these transitions the monsoon shifted abruptly and discontinuously from wet to dry stable states as insolation forcing caused a critical threshold to be exceeded (Boos and Storelvmo, 2016). Although the threshold effect has been discussed based on theoretical models of monsoon dynamics (Levermann et al., 2009; Schewe et al., 2012; Zickfeld et al., 2005), there is a lack of speleothem records of abrupt changes and threshold effects of the ASM during the Holocene.

Against this background, we use RAMPFIT to analyze the stalagmite HF01 record from Heifeng Cave in the middle to upper reaches of the Yangtze River in China to explore the abrupt changes of the Holocene Asian monsoon. Together with records from other sites in China we use the results to assess whether a step-like pattern of variation of the ASM during the Holocene is widely recorded, together with the possible driving mechanism.

Site, materials, and methods

Stalagmite HF01 was collected from Heifeng Cave (29°02′N, 107°11′E) in Jinfo Mountain, Chongqing City, at an altitude of 2130 m (Figure 1). The study area is located in the eastern Sichuan Basin, north of the Yunnan-Guizhou Plateau. Climatically, the region is controlled by the ASM (Zhang et al., 2021), and it has a humid subtropical monsoon climate with annual average temperature of 8.5°C and annual precipitation of ~1400 mm (Zhang et al., 2017). The ²³⁰Th ages of the 30 samples from stalagmite HF01 are listed in Supplemental Table S1. No age inversion is evident in the age sequence. The dating errors are small, mostly within 70 a, and therefore the chronology is highly reliable. The age model for stalagmite HF01 was established using MOD-AGE software (Hercman and Pawlak, 2012) (Supplemental Figure S1), and it indicates that the stalagmite grew uniformly, with the average rate of 1.13 mm/l00 a.

Figure 1.

Topography and monsoon circulation in East Asia. The orange arrows show the trajectories of the East Asian summer monsoon (EASM) and the Indian summer monsoon (ISM). The red star shows the location of Heifeng Cave (Yang et al., 2019); the purple dots show the locations of Lianhua Cave (Zhang et al., 2016) and Dongge Cave (Dykoski et al., 2005); and the light blue dots show the locations of Dali Lake (Goldsmith et al., 2017), Qinghai Lake (An et al., 2012) and Ngangla Ring Tso Lake (Hudson et al., 2015).

The δ¹⁸O record from stalagmite HF01 from Heifeng Cave spans the interval from 1.5 ka to 0.48 ka B.P. (thousands of years before 1950 CE), with an average resolution of 34 a. The δ¹⁸O values of stalagmite HF01 range from −6.37‰ to −10.34‰, with the mean of −8.81‰, and the amplitude of variation is relatively large, reaching −3.71‰. The most depleted part of the record is in the early and middle Holocene, when the δ¹⁸O values vary greatly, with the mean value reaching the most depleted level (Figure 2). Yang et al. (2019) constructed an integrated Holocene record based on the δ¹⁸O records of 16 stalagmites from the Asian monsoon domain, and concluded that the principal modes of the Holocene stalagmite δ¹⁸O records and the instability of climate in the early Holocene. However, the monsoon characteristics reflected by HF01 δ¹⁸O record that was one of the δ¹⁸O records of 16 stalagmites have not been thoroughly analyzed (Yang et al., 2019). Therefore, based on the HF01 δ¹⁸O record, RAMPFIT and Bayesian Change Point (BCP) analyses are used to discuss further the evolution of ASM in the Holocene in this paper (Mudelsee, 2000; Ruggieri, 2013).

Figure 2.

The δ¹⁸O record of stalagmite HF01. The blue curve represents the HF01 δ¹⁸O record (Yang et al., 2019), with the error bars indicating the dating error (±2σ); the brown curve represents summer insolation at 30°N (Laskar et al., 2004); and the orange curve represents the results of RAMPFIT analysis (Mudelsee, 2000). The yellow vertical bars indicate transitional phases (T1–T5) of the monsoon regime.

RAMPFIT is a statistical regression approach (Mudelsee, 2000) which has been widely used in paleoclimate reconstruction (e.g. Fleitmann et al., 2003; Jiang et al., 2012; Li et al., 2021; Steffensen et al., 2008; Zhang et al., 2019), and we used it to determine change points in the stalagmite HF01 δ¹⁸O sequence. RAMPFIT involves fitting a “ramp” to a specific part of a record using a weighted least squares method, and it can be used to determine the exact age of climate change points and the average state before and after a change (Mudelsee, 2000) (Figure 2, Supplemental Table S2). According to the trend of variation of the HF01 δ¹⁸O record, we divided the record into several intervals and piecewise fitted the data; no changes were made to the location of change points in the fitting curve. Finally, we integrated the results of multi-segment fitting into a complete and continuous record. In order to verify whether the division of segments in the δ¹⁸O record was objective and reasonable, Acycle software was used to perform Bayesian Change Point analysis (Li et al., 2019; Ruggieri, 2013) (Supplemental Figure S2 and Table S3). Based on a Bayesian mathematical algorithm, the method can be used to estimate the number of change points in a time series and the uncertainty probability of the change point locations (Ruggieri, 2013). PAST software was then used to perform wavelet analysis of the δ¹⁸O record in order to identify the dominant cycles in the ASM during the Holocene (Hammer et al., 2001).

Discussion

Stepwise evolution of the ASM during the Holocene

The RAMPFIT results for stalagmite HF01 indicate that the evolution of the ASM during the Holocene was non-linear and step-like (Mudelsee, 2000) (Figure 2). The evolution can be divided into six stable stages (Stages, S) and five transitional phases (Transitions, T). Each stage is equivalent to a plateau with roughly constant δ¹⁸O values, suggesting a relatively stable monsoon state; and each stage is separated by a transitional phase, characterized by a large shift in δ¹⁸O, indicating an abrupt change in the monsoon regime. There are two stages (S1, S2) and two transitional phases (T1, T2) in the early and middle Holocene, and there are four stages (S3–S6) and three transitional phases (T3–T5) in the middle to late-Holocene. The entire late-Holocene falls within the weak monsoon stage (S6). The results of RAMPFIT analysis are highly consistent with the model output obtained using the BCP algorithm (Mudelsee, 2000; Ruggieri, 2013) (Figures 2 and Supplemental Figure S2). Transition T2 has the highest posterior probability, reaching 71%, while transitions T1 (51%), T4 (59%) and T5 (40%) all have a posterior probability of ~50% (Supplemental Figure S2). Therefore, supported by the BCP analysis, the results of the RAMPFIT analysis of the HF01 δ¹⁸O record are deemed reliable.

Modern cave monitoring studies in the Jinfo Mountain area have shown that δ¹⁸O in cave drip water inherits the signal of the interannual variation of stable isotopes in precipitation, and is sensitive to changes in atmospheric circulation and ENSO mode (Chen and Li, 2018). As a consequence, we argue that the δ¹⁸O of stalagmite HF01 can provide insights into the variation of ASM intensity on a large spatial scale, with more depleted/enriched δ¹⁸O values indicating an increase/decrease in ASM intensity, as has been shown in Yang et al. (2019). The HF01 δ¹⁸O record of the early Holocene can be divided into two stages: (1) In the first stage (S1, 11.41–10.46 ka B.P.) the average δ¹⁸O is −8.90‰, and the values are relatively uniform, indicating that the ASM was stable state with a moderate intensity. (2) In the second stage (S2, 9.97–8.07 ka B.P.), which lasted for almost 2 ka, the average δ¹⁸O value is −9.57‰. Notably, the δ¹⁸O record shows a centennial-scale “W”-shaped oscillation during the transitional period (8.5–8.1 ka B.P.) between the early and middle Holocene (Yang et al., 2020). This oscillation corresponds the well-documented 8.2 ka event (Alley et al., 1997; Cheng et al., 2009), which interrupted the continuous strengthening of the ASM, indicating that the ASM was unstable in the early Holocene (Yang et al., 2019, 2020).

The HF01 δ¹⁸O record for the middle Holocene shows a pronounced enrichment trend, with three stages and three intervening steps: (1) In the first stage of the middle Holocene (S3, 8.04–7.11 ka B.P.), the δ¹⁸O values are the most depleted in the Holocene, with an average of −9.90‰, indicating that the ASM reached a state of maximum intensity (Figure 2). The variation of the ASM intensity during the Holocene was mainly controlled by changes in NHSI (Wang et al., 2005; Zhang et al., 2019), but there was a lag of 2–3 ka between the maximum negative inflection in δ¹⁸O and the maximum in NHSI (Laskar et al., 2004) (Figure 2). This may have been caused by the large global ice volume in the early Holocene (Fleitmann et al., 2003), which delayed the onset of the Holocene climatic optimum (Bakker et al., 2016; Ruddiman, 2006). (2) In the second stage of the middle Holocene (S4, 6.90–5.84 ka B.P.), the δ¹⁸O values are moderately depleted and relatively stable (Yang et al., 2019) (Figure 2), with an average of −9.34‰, suggesting that the ASM remained relatively strong compared to S1. (3) In the third stage of the middle Holocene (S5, 5.56–4.31 ka B.P.), the δ¹⁸O values are substantially more enriched than S4, with an average of −8.43‰, suggesting that the ASM intensity had weakened significantly following the two sharp decreases (Figure 2). The HF01 δ¹⁸O values showed an obvious and rapid enrichment change during the mid-late-Holocene transitional period (T5, 4.3–3.8 ka B.P.), and then emerged a slow enrichment change. The HF01 δ¹⁸O reached the most enrichment in the period of 2–0.48 ka B.P., with an average of −6.95‰, indicating that the ASM was relatively stable and the ASM intensity reached the lowest level in the Holocene (Figure 2).

Comparison of stalagmite HF01 record with other monsoon records

In order to determine whether the step-like characteristics of the Holocene ASM recorded by stalagmite HF01 are consistent on a large spatial scale, we compared the results with other Holocene climate records from the Asian monsoon region (see Table 1 and Figure 3). The lake-level record of Dali Lake in Inner Mongolia also shows a pronounced step-like pattern, with the period of highest lake level including two stable intervals (Goldsmith et al., 2017), during 6.3–5.8 and 8.2–7.3 ka B.P., in the middle Holocene, indicating that the strongest monsoon occurred at these times (Figure 3a). A record from Ngangla Ring Tso Lake, in the Tibetan Plateau, also shows that the lake level experienced a pronounced stepwise decrease from the early to the late-Holocene (Hudson et al., 2015) (Figure 3b). Moreover, the three major falls in lake level correspond exactly to the shifts in the monsoon regime (Figure 3b and d). The Summer Monsoon Index (SMI) from Qinghai Lake shows that the monsoon was unstable in the early Holocene, while stable stages occurred during 8.5–7.0 and 6.6–5.4 ka B.P., in the middle Holocene (An et al., 2012) (Figure 3c). The δ¹⁸O record of stalagmite LHD5 from Lianhua Cave shows a stepwise pattern of variation with five stages during the Holocene (Zhang et al., 2016) (Figure 3e). The variation of LHD5 δ¹⁸O record shows that the ASM strengthened rapidly at the beginning of the Holocene and subsequently remained strong and stable during 11.2–9.5 ka B.P. (Figure 3e). Maximum ASM intensity occurred during 9.5–7.0 ka B.P., which was followed by a stepwise weakening comprising three stages (Figure 3e). The δ¹⁸O record of stalagmite D4 from Dongge Cave shows a stepwise pattern with five stages (Dykoski et al., 2005) (Figure 3f). The ASM oscillated substantially in the early Holocene and was relatively strong during 9.0–7.1 ka B.P.; and in the middle to late-Holocene, it shows a pronounced stepwise pattern with three stages (Figure 3f). The stalagmite records from Heifeng cave, Dongge cave (Dykoski et al., 2005), and Lianhua cave (Zhang et al., 2016) have obvious stepwise changes in the Holocene, but there are also some differences between them. The records of Heifeng cave have six steps, while Dongge cave and Lianhua cave have only five steps, which may indicate that the stalagmite δ¹⁸O from Heifeng cave at higher altitude are more sensitive to the changes of ASM.

Table 1.

Locations of caves and lakes from monsoonal China used in this study.

No.	Site	Location	Altitude (m a.s.l.)	Reference
1	Dali Lake	43°09ʹN, 116°17ʹE	1220	Goldsmith et al. (2017)
2	Qinghai Lake	36°32ʹ–37°15ʹN, 99°36ʹ–100°47ʹE	3194	An et al. (2012)
3	Ngangla Ring Tso Lake	31°27ʹ–31°39ʹN, 82°48ʹ–83°24ʹE	4727	Hudson et al. (2015)
4	Lianhua Cave	29°29ʹN, 109°32ʹE	455	Zhang et al. (2016)
5	Heifeng Cave	29°01ʹN, 107°11ʹE	2130	Yang et al. (2019)
6	Dongge Cave	25°02ʹN, 108°05ʹE	680	Dykoski et al. (2005)

Figure 3.

Comparison of the stalagmite HF01 δ¹⁸O record with other climate records from the Asian monsoon region. (a) Lake level record of Dali Lake (Goldsmith et al., 2017). (b) Lake level record of Ngangla Ring Tso Lake (Hudson et al., 2015). (c) Summer Monsoon Index (SMI) from Qinghai Lake (An et al., 2012). (d) δ¹⁸O record of stalagmite HF01 from Heifeng Cave (Yang et al., 2019). (e) δ¹⁸O record of stalagmite LHD5 from Lianhua Cave (Zhang et al., 2016). (f) δ¹⁸O record of stalagmite D4 from Dongge Cave (Dykoski et al., 2005); Red solid lines represent RAMPFIT results, and the yellow vertical bars represent abrupt monsoon transitional phases.

Lake records (An et al., 2012; Goldsmith et al., 2017) and speleothem records (Dykoski et al., 2005; Zhang et al., 2016) from Chinese monsoon region reveal differences in the stepwise enhancement of the ASM in the early Holocene (Figure 3), suggesting that the ASM variability in the early Holocene was complex and unstable (Yang et al., 2019). Even though NHSI was at a maximum in the early Holocene (Laskar et al., 2004), the ice volume in the Northern Hemisphere was still large (Fleitmann et al., 2003), and the enhancement of ASM was probably constrained by glacial background conditions (Liu et al., 2018b). What’s more, the disturbances of fresh water discharge resulted in an unstable ASM (Bond et al., 2001; Yang et al., 2019). In addition, the principal modes of the ASM in the Asian monsoon region were similar during the middle to late-Holocene, showing a typical stepwise pattern with three stages of weakening, which is consistent with the gradual weakening of NHSI from a maximum to a minimum (Laskar et al., 2004; Yang et al., 2019). Previous studies have shown the cold and dry climatic events in the middle to late-Holocene were transmitted across middle to low latitudes of the Northern Hemisphere and the resulting monsoon-weakening events were important causes of a worldwide decline of prehistoric civilizations and ethnic migrations at this time (Arz et al., 2006; Booth et al., 2005; Gasse, 2000; Shao et al., 2006). For example, the demise of Neolithic civilizations in Central China (Wu and Liu, 2004; Yang et al., 2021), the Mesopotamian civilization (deMenocal, 2001), and civilizations in India (Dalfes et al., 1997) may have been linked to these events.

Mechanism of the stepwise evolution of the Holocene ASM

The long-term trend of the δ¹⁸O record from stalagmite HF01 essentially tracks the variation of NHSI during the Holocene (Laskar et al., 2004) (Figure 2), indicating that the NHSI is the dominant driver of the ASM on the orbital scale (Dykoski et al., 2005; Wang et al., 2005; Zhang et al., 2019). However, a series of centennial- and millennial-scale abrupt events are superimposed on this main trend. These abrupt climatic events essentially correspond to the NRS defined by Donges et al. (2015) (Supplemental Figure S3). During the intervals of 10.9–10.5, 9.9–9.7, 9.4–9.1, 8.4–8.1, 7.4–7.1, 5.9–5.5, and 4.3–3.9 ka B.P., the HF01 δ¹⁸O record shows varying degrees of positive shifts, corresponding to 7 weak monsoon events. Five of these events mainly occurred during transitional phases (T1–T5) of the ASM. These events are not only consistent with episodes of rapid climate change (RCC) during the Holocene (Mayewski et al., 2004), but they also coincide with Bond events (Bond et al., 2001) (Figure 4d and e, Supplemental Figure S3), which suggests that the Bond events may have acted as a trigger that prompted the ASM to switch from one regime to another.

Figure 4.

Comparison of Holocene weak monsoon events with Bond events in the North Atlantic. (a) Summer insolation at 30°N (Laskar et al., 2004). (b) Atmospheric ¹⁴C production rate record (Reimer et al., 2013). (c) Greenland ice core δ¹⁸O record (Johnsen et al., 2001). (d) Ice-rafted detritus (IRD) record from the North Atlantic (Bond et al., 2001); the red numbers correspond to Bond events. (e) δ¹⁸O record of stalagmite HF01 (Yang et al., 2019); the red solid line represents RAMPFIT results. (f) Synthesis Holocene stalagmite δ¹⁸O_syn record from China (Yang et al., 2019). Light blue vertical bars represent weak monsoon events that correspond to Bond events.

During the early to middle Holocene, the weak ASM events corresponding to Bond events 8–5 were considered as the results of the combined effect of the Atlantic Meridional Overturning Circulation (AMOC) and solar output (Cheng et al., 2009; Liu et al., 2018b, 2020; Zhang et al., 2018). The transitional phases T1 and T2 in the HF01 δ¹⁸O record coincide with Bond events 8 and 5, respectively (Figure 4d and e). These two events interrupted the strengthening process of the ASM (Yang et al., 2019), and each transitional phase was rapidly followed by a stage of rapid monsoon strengthening, respectively reaching a peak and sub-peak in ASM intensity. This may be attributed to the fact that NHSI was strong in the early Holocene (Figure 4a), and the consequent Northern Hemisphere warming caused a northward shift of the Intertropical Convergence Zone (ITCZ) (Haug et al., 2001), resulting in the continuous strengthening of ASM. However, the Bond events disrupted the relatively stable monsoon states (Liu et al., 2018a), promoting a non-linear shift in ASM intensity after the end of each weak monsoon event. The ASM before and during the Bond event 8 was probably constrained by glacial background conditions (Liu et al., 2018b). The HF01 δ¹⁸O record shows that when the weak ASM event corresponding to Bond event 7 came to the end, ASM shifted rapidly from S1 to S2 (Figure 4d and e), which suggests that the constraining effect of averaged climate state on the ASM was alleviated (Liu et al., 2018b). However, the weak monsoon event, corresponding to the Bond event 6, did not promote a monsoon shift from one regime to a stronger one, which may be due to the fact that ASM had just gone through T1 at this point and its intensity had not yet reached the next critical threshold.

In the middle to late-Holocene, the three transitional phases of the monsoon regime (T2, T4, and T5) correspond precisely to Bond events 5, 4, and 3, respectively (Figure 4d and e). Due to the relatively long duration of Bond event 5 (Bond et al., 2001), it is likely that the 7.2 ka weak monsoon event, corresponding to T3, was also the result of the weakening of the AMOC (Feng et al., 2020). Different from the early Holocene, during the middle to late-Holocene, the ASM weakened substantially in a stepwise manner with no recovery. Accompanied by the gradual decrease of NHSI during the middle to late-Holocene (Laskar et al., 2004) (Figure 4a), the so-called positive feedback of moisture-advection dominated the continent-ocean heat balance (governed by latent heat) of the AM system (Levermann et al., 2009; Schewe et al., 2012). When the positive feedback of moisture-advection strengthened continuously, the monsoon intensity would enhance to gradually approach the threshold of abrupt change (Schewe et al., 2012). After the end of weak monsoon events corresponding to triggered by Bond events, the ASM dropped to a weaker but stable state (Figure 4e). However, the intensity of the ASM did not recover to its original level, which may have been directly related to the weakening of NHSI. With the gradual decrease of NHSI, the cooling of the Northern Hemisphere resulted in a southward shift of the ITCZ (Haug et al., 2001), and therefore the intensity of the ASM failed to regain its previous level. The foregoing scenario suggests a close and complex relationship between insolation, the threshold effect of abrupt monsoon change, and Bond events, in the course of the overall process of monsoon climate change during the Holocene.

As the precursors of transitional phases, these pronounced weak monsoon events differ from Bond events in terms of structure (Figure 4d and e). Bond events occurred repeatedly during the Holocene, and the temperature recovered after the end of each event, resulting in a symmetrical pattern of cooling and subsequent warming (Bond et al., 2001). However, the weakening and strengthening phases of the ASM, as revealed in the δ¹⁸O record from stalagmite HF01, are asymmetrical, which is confirmed by the synthesis of stalagmite records from 14 caves in the Asian monsoon region (Yang et al., 2019) (Figure 4f). There were three pronounced weak monsoon events in the early Holocene, but there is no evidence of similar weak monsoon events in the middle to late-Holocene (Yang et al., 2019). This may be due to the fact that the weakening of ASM was monotonic in the middle to late-Holocene, with no recovery process, which resulted in these events being smoothed out in the stacked record comprising multiple curves.

Previous studies have shown that there is a teleconnection between the weak ASM events and the North Atlantic Bond events (Cheng et al., 2009; Liu et al., 2013; Liu et al., 2020; Wang et al., 2005). The global coupled ocean-atmosphere model (CM2.0) shows that the freshwater forcing substantially weakens the Atlantic Thermohaline Circulation (THC), resulting in a southward shift of the ITCZ over the Atlantic and Pacific, an El Niño-like pattern in the southeastern tropical Pacific, and weakened Asian summer monsoons through air-sea interactions (Zhang and Delworth, 2005).

The stalagmite HF01 δ¹⁸O record further shows that stepwise evolution of Holocene ASM was probably driven jointly by NHSI and Bond events (Figure 4), and not every Bond event can trigger a change in the monsoon regime. The comparison between the atmospheric ¹⁴C production rate record and HF01 record revealed that solar activity also played a role in the transitions of ASM regimes such as T1, T3, and T4 (Reimer et al., 2013) (Figure 4b and e). On the orbital scale, the gradual change in NHSI may have caused the monsoon system to approach a critical state (deMenocal et al., 2000; Dykoski et al., 2005). It is likely that Bond events acted as a trigger of the threshold effect and was a key driving force for the transition of the ASM from one regime to another; moreover, the internal feedback of the monsoon system may have amplified this external forcing (Levermann et al., 2009). At present, the threshold of NHSI and trigger mechanism are still unclear, and further research is needed. Schewe et al. (2012) speculated that orbital-timescale variations in NHSI and the associated surface temperature changes might have affected evaporation at the ocean’s surface, such that average humidity over the ocean persistently crossed the threshold, thus critically altering the moisture supply for the adjacent monsoon region and triggering a transition between the two regimes.

The results of wavelet analysis reveal a 1.45 ka cycle in the HF01 stalagmite δ¹⁸O (Figure 5), which is close to the 1.5 ka cycle of North Atlantic climate change (Bianchi and McCave, 1999; Bond et al., 1997). This common periodicity in records from the North Atlantic and the ASM region points to a teleconnection between climate change in the two climatic regions (Gupta et al., 2005), and further confirms a relationship between the stepwise pattern of variation of the ASM during the Holocene and the centennial- to millennial-scale cold events in the North Atlantic, suggesting that Bond events were important factors that triggered major changes in the ASM.

Figure 5.

Wavelet analysis results for the δ¹⁸O record of stalagmite HF01.

Conclusions

Based on a high-resolution δ¹⁸O record from stalagmite HF01 from Heifeng Cave in southern China and other paleoclimate records, we have reconstructed the evolution of the ASM during the Holocene (~11.5–0.48 ka B.P.). The following conclusions are drawn.

(1) The results of RAMPFIT and BCP analysis of the δ¹⁸O record show that the evolution of the ASM during the Holocene was characterized by a stepwise pattern, which can be divided into six stages separated by five transitional phases.

(2) The Holocene variation of the ASM is consistent with the weakening of NHSI on the orbital scale, indicating that NHSI was a major driver of the ASM. Due to the strong NHSI in the early Holocene, the ASM intensity underwent two stages of enhancement in the early Holocene, followed by three stages of weakening in the middle to late-Holocene, as NHSI gradually weakened.

(3) The HF01 δ¹⁸O record shows that the transitional phases of the ASM regime are consistent with Bond events in the North Atlantic. Additionally, wavelet analysis reveals the presence of a 1.45 ka cycle in the HF01 δ¹⁸O record, which is close to the well-documented 1.5 ka cycle of North Atlantic climate change, suggesting that Bond events may have acted as a trigger for the transition of one ASM state to another.

Supplemental Material

sj-docx-1-hol-10.1177_09596836221074032 – Supplemental material for Stepwise evolution of the Asian summer monsoon during the Holocene revealed by a stalagmite record from Heifeng Cave, Southwest China

Supplemental material, sj-docx-1-hol-10.1177_09596836221074032 for Stepwise evolution of the Asian summer monsoon during the Holocene revealed by a stalagmite record from Heifeng Cave, Southwest China by Yingran Yan, Xunlin Yang, Rui Zhang, Riping Zhang, Saisi Zuli and Yong Wang in The Holocene

Footnotes

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 grants from the National Natural Science Foundation of China (41971109 and 41572158), National Key R&D Program of China (2016YFC0502301).

ORCID iDs

Yingran Yan

Xunlin Yang

Riping Zhang

Saisi Zuli

Supplemental material

Supplemental material for this article is available online.

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