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Keywords:

  • Asian monsoon;
  • bipolar seesaw;
  • millennial-scale climate change;
  • MIS 3;
  • Wulu Cave

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

A speleothem record from south-western China characterizes in detail the millennial-scale changes in Asian Monsoon (AM) intensity from 39.3 to 28.7 ka. The calcite δ18O profile, with an average resolution of ∼8 years, shows several strong monsoon events concurrent with Greenland Interstadials (GIS) 8–4. To gain a systematic perspective of AM millennial-scale variability, the new and previously reported data from the same cave are combined, showing that AM variation exhibits a broad similarity with Greenland ice δ18O records and with Antarctica but in an opposite sense. For the interval that encompasses GIS 5 and GIS 4.1, however, our stalagmite δ18O record depicts a sustained strong monsoon with no distinctive oscillation between these interstadials. Another prominent characteristic in our record is a gradual transition into Chinese Interstadial (CIS) 8, which is well constrained by an annually laminated sequence. We find that an initial rise in monsoon intensity, lasting a few centuries, significantly precedes the abrupt onset of CIS 8 in the AM realm. This suggests that atmospheric moisture and heat transport are probably capable of inducing abrupt climate change when a rapid reorganization of ocean/atmosphere circulations passes a tipping point.


Introduction

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Marine Isotope Stage 3 (MIS 3), with intermediate boundary conditions between typical glacial and interglacial states (McManus et al., 1999), was characterized by numerous abrupt climate changes known as Dansgaard-Oeschger oscillations (e.g. Dansgaard et al., 1993; Grootes et al., 1993) and Heinrich events (HEs) (Heinrich, 1988; Bond et al., 1993). In Greenland, the Dansgaard-Oeschger cycles comprised an abrupt initial warming, then a gradual cooling trend terminated by a rapid temperature drop back to stadial conditions. In contrast, Antarctic warming and cooling on the same millennial scale were more gradual and symmetrical, and out of phase with their northern counterparts (Blunier and Brook, 2001; EPICA Community Members, 2006). This anti-phased relationship was attributed to a bipolar seesaw mechanism (Broecker, 1998; Stocker and Johnsen, 2003). By contrast, the Asian Monsoon (AM), as an integrated component in the global climatic system, may bridge northern and southern hemispheric climates (Cai et al., 2006; Barker and Knorr, 2007; Rohling et al., 2009; An et al., 2011), via a combined influence of northern hemisphere pull and southern hemisphere push (Rohling et al., 2009).

Speleothems are among the best geological archives for recognizing rapid climate fluctuations at the millennial scale (e.g. Wang et al., 2001). However, it remains unclear whether the shifts in cave records are as rapid as those in Greenland records. For instance, the onset of Chinese Interstadial (CIS) 12 (nomenclature from Cheng et al., 2006) is gradual in one Indian monsoon record (Cai et al., 2006), but appears abrupt in a record from another cave locality (Burns et al., 2003). It is not yet clear whether such a discrepancy was introduced by different climate sensitivities, sampling resolution or age constraints between records. The Hulu record from East China (Wang et al., 2001), extremely similar to Greenland temperature variations, was suggested to incorporate signals possibly linked to Antarctic climate fluctuations (Barker and Knorr, 2007; Rohling et al., 2009). Barker and Knorr (2007) contended that the Antarctic-style signal could be transported globally in extent through oceanic/atmospheric circulation, and had the potential to trigger the rapid climate shifts in the North Atlantic. Thus, a clear picture of the events in AM records may facilitate an evaluation of the underlying dynamics that cause abrupt climate changes.

Previously published stalagmite records from Wulu Cave, south-western China, were characterized by high-resolution and high-precision 230Th ages, highlighting a feature of millennial-scale AM changes during the last glacial period. The high growth rates and low detrital content of the stalagmite samples enabled the refinement of the Hulu Cave chronology, which helped to clarify the timing of CIS 4 (Zhao et al., 2010) and a series of multi-decadal oscillations during the MIS 4/3 transition (Liu et al., 2010). Here we provide a new high-resolution and precisely dated δ18O record from the same cave, covering a period from 39.3 to 28.7 ka. We examine the pattern, structure and timing of AM variability inferred from this record in the context of global climate change.

Material and Method

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Wulu Cave (26°03′N, 105°05′E, elevation 1440 m) is situated in a transitional zone between the East Asian and Indian summer monsoons (Wang and Lin, 2002), on the south slope of the Yun-Gui Plateau, south-western China (Fig. 1), 290 km north-west of Dongge Cave (Yuan et al., 2004). Current mean annual air temperature near the site is 14 °C, with the highest mean monthly temperature in July (∼21 °C) and the lowest in January (∼4 °C). The annual precipitation ranges between 1100 and 1700 mm, with 65% of the rainfall occurring during summer (June–September). More detailed climate conditions near the cave can be found in supplementary figure 1 of Liu et al. (2010).

image

Figure 1. Geography of the Asian monsoon and study location. Star and circles denote Wulu Cave (26°03′N, 105°02′E) in south-western China and other caves mentioned in the context. The dashed grey line designates the northernmost extent of the Asian summer monsoon.

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A stalagmite sample (Wu3) with a total length of 302 cm was collected 300 m from the narrow cave entrance. Its diameter varies from 8 to 12 cm. The upper part of the sample (0–197 cm long) is used in this study (Supporting information Fig. S1), because the lower section, below a distinct change from light yellow to dark brown calcite at 197 cm depth, is interspersed with frequent white bandings, and the growth directions shift laterally and episodically, indicative of deposition hiatuses. After being halved and polished, the specimen shows visual laminations of white-porous and translucent-dense layers only at the two intervals between 2.6–68.6 and 175.2–193.8 cm depth. These laminations are parallel and composed of compact calcite with no sign of diagenetic alteration. The other part of the sample also consists of pure calcite with very low detrital contents.

Eight sub-samples were dated with the U-series methods (Table 1). About 0.3–0.5 g of calcite powder was taken from the polished surface using a 0.9-mm-diameter carbide dental drill. Procedures for chemical separation and purification of uranium and thorium are similar to those described in Edwards et al. (1986/87/87) and Cheng et al. (2000). U and Th isotope measurements were made on a Finnigan ELEMENT inductively coupled plasma mass spectrometer (ICP-MS) equipped with a double-focusing sector magnet and energy filter in reversed Nier–Johnson geometry and a single MasCom multiplier, following procedures modified from Shen et al. (2002) and Cheng et al. (2009). The dating work was performed at the Minnesota Isotope Laboratory, University of Minnesota. All dates are in a stratigraphic order with typical analytical errors (2σ) ranging between 70 and 240 years.

Table 1. 230Th dating results for stalagmite Wu3, Wulu Cave, south-western China
Sample ID no. (mm)Weight (g)238U (p.p.b.)232Th (p.p.t.)δ234U measured[230Th/238U] activityAge (a) uncorrectedAge (a) correctedδ234Uinitial (corrected)
  1. Corrected 230Th ages are indicated in bold. Errors are 2σ analytical errors. Decay constant values are λ230 =9.1577 × 10−6 a−1, λ234 =2.8263 × 10−6 a−1, λ238 =1.55125 × 10−10 a−1. Corrected 230Th ages assume an initial 230Th/232Th atomic ratio of (4.4 ± 2.2) ×10−6. All the ages are calibrated to AD 1950.

Wu3-1060.3129215.1 ± 0.4208 ± 2889.5 ± 4.30.4526 ± 0.002129 220 ± 17029 210 ± 170966 ± 5
Wu3-3950.3457248.3 ± 0.7130 ± 2875.8 ± 6.00.4622 ± 0.002430 170 ± 21030 160 ± 210954 ± 7
Wu3-7780.3496222.4 ± 0.5169 ± 2874.7 ± 4.70.4834 ± 0.002031 760 ± 18031 750 ± 180957 ± 5
Wu3-10690.3572364.0 ± 0.4296 ± 6940.0 ± 1.90.5206 ± 0.000833 200 ± 07033 190 ± 701032 ± 2
Wu3-12300.3761241.8 ± 0.2679 ± 14938.8 ± 2.10.5315 ± 0.000834 020 ± 07033 980 ± 801033 ± 2
Wu3-12980.4021232.6 ± 0.2350 ± 7924.4 ± 2.00.5461 ± 0.000935 410 ± 08035 390 ± 801021 ± 2
Wu3-15070.4588328.3 ± 0.9299 ± 8938.1 ± 4.50.5705 ± 0.002936 900 ± 24036 770 ± 2501041 ± 2
Wu3-19620.3861387.0 ± 0.9123 ± 2954.9 ± 4.30.6055 ± 0.002739 170 ± 23039 170 ± 2301067 ± 5

A total of 1371 powder samples (about 50–100 µg) for oxygen isotope measurements were drilled along the axis of the sample with a 0.5-mm-diameter carbide dental burr. Spatial resolution varies from 1 to 2 mm, equivalent to a temporal resolution from 3 to 17 years. The analyses were performed in the Isotope Laboratory of Nanjing Normal University on a Finnigan-MAT-253 mass spectrometer. Precision of δ18O is 0.06‰, at the 1-sigma level.

Results

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Chronology

Eight 230Th dates indicate that the sample grew from 39.3 to 28.7 ka (before AD 1950; Table 1). High U (215–387 p.p.b.) and relatively low Th (120–670 p.p.t.) concentrations yield high-precision ages. Detrital Th is negligible and thus errors from initial 230Th correction are trivial. The age model for stable isotope data is established using the StalAge algorithm (Scholz and Hoffmann, 2011). The results show that the stalagmite grew continuously throughout the upper section, except for a hiatus between 35.2 and 34.0 ka, which coincides with a weak monsoon condition inferred from the Hulu record (Wang et al., 2001) and low temperatures in Greenland (Anderson et al., 2006) (Fig. 2). The growth rate of the sample is exceptionally high, ranging from 357 to 450 mm ka−1 (Fig. 2). The 230Th dates support the contention that the laminations in the two sections are likely to be annual. To test this, we counted 1600–1680 bands in the section from 7.2 to 52.6 cm, which agrees well with an estimation of ∼1630 ± 190 years based on 230Th ages.

image

Figure 2. Age–depth and δ18O plots for stalagmite Wu3. Lower panel: the black line indicates the derived age models and the grey line represents 95% uncertainty envelopes. Age error bars indicate 2σ error. Upper panel: the annually laminated intervals and a hiatus are marked by bars at the top.

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Oxygen isotope sequence

The δ18O values fluctuate dramatically throughout the whole record, ranging from –11.8 to –8.2‰, with an average value of –10.0‰ (Fig. 3). The largest excursion of the δ18O record (3.5‰) occurs between 39.3 and 38.0 ka, indicating a major climatic shift from stadials to interstadials. Amplitudes of other abrupt shifts on the millennial scale are smaller, but still more than 1.8‰ in the Wulu record.

image

Figure 3. Comparison of Wu3 δ18O records with Greenland and Antarctica ice core records. (a) NGRIP record (Anderson et al., 2006); (b) Wu3 stalagmite record; (c) the EPICA Dronning Maud Land (EDML) ice core δ18O record given on the GICC05 time scale (EPICA Community Members, 2006). The chronologies for ice core records are calibrated to AD 1950 for comparison. The vertical bars denote the periods of cold Greenland, weak AM and warm Antarctica, corresponding temporally to HE 3 and 4, respectively. The two dashed lines indicate the inter-Heinrich ice-rafting events f and h discerned in North Atlantic sediments (Bond and Lotti, 1995). Note that the Southern Ocean contribution to the monsoon is recognized by taking the inverse of Antarctic climate variability, so that increased AM parallels with the cold intervals in Antarctica. This figure is available in colour online at wileyonlinelibrary.com.

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Four episodes of positive excursion in δ18O values are centred at ∼39.3 (corresponding to the second half of Heinrich Stadial 4), 36.1, 32.7 and 29.8 ka (corresponding to Heinrich Stadial 3), respectively, with a duration ranging from 1.3 to 1.5 ka. Several prominent negative excursions in δ18O values, interpreted below as strong AM events, are also identified, corresponding to Greenland Interstadials (GIS) 8–4. In addition, a series of decadal- to centennial-scale events are superimposed on the millennial-scale variations.

Interpretation of isotope data

An equilibrium test at the time of calcite deposition is crucial to interpret stalagmite δ18O values in terms of a climatic proxy. The correlation between carbon and oxygen isotopes (Fig. S2a) for the whole profile is generally weak (R2 = 0.04), suggesting that the effect of the kinetic fractionation is not significant (Hendy, 1971). Moreover, the ‘Hendy tests’, performed on four single laminae, also indicate that differences in δ13C or δ18O within the same layer are small, with a standard deviation <0.24‰ (Fig. S2b). In addition, Dorale and Liu (2009) suggested that replication was a more rigorous test for isotopic equilibrium. Contemporaneous variations in δ18O values from Wulu Cave and neighbouring Dashibao Cave (Zhao et al., 2010) are essentially similar (Fig. S2c), supporting our interpretation of equilibrium calcite deposition, and hence that the Wulu Cave δ18O data represent a dominant climatic signal.

Previous studies showed that shifts in δ18O of stalagmites in southern China mainly reflected variations in δ18O of meteoric precipitation, generally referred to summer monsoon intensity (Yuan et al., 2004; Wang et al., 2005; Liu et al., 2010; Zhou et al., 2011). This interpretation is strongly supported by the fact that the isotope variation of the calcite is highly correlated with changes in local summer insolation due to the precession of the equinoxes (e.g. Wang et al., 2001, 2005, 2008; Yuan et al., 2004). However, this interpretation of stalagmite δ18O variations has aroused great debate worldwide over the past few years (LeGrande and Schmidt, 2009; Dayem et al., 2010; Pausata et al., 2011; Maher and Thompson, 2012), because it might be unclear whether the data reflect local or remote climatic conditions. Recently, Cheng et al. (2012) clarified that the variations in the δ18O value of AM precipitation as recorded by stalagmites may indicate ‘a mean state of summer monsoon intensity or integrated moisture transport rather than amount of local precipitations’. This idea is consistent with the observation that regional stalagmite δ18O records, although geographically widespread, are well replicated over a large portion of China (Yuan et al., 2004; Zhao et al., 2010), and further supported by a recent simulation result (Liu et al., 2014).

Discussion

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Strong monsoon event CIS 4.1

As illustrated in Fig. 3, the Wu3 δ18O record exhibits several strong monsoon events CIS 8–4, similar in timing and pattern to millennial–centennial climatic changes in the North Atlantic region regarding timing and pacing (Anderson et al., 2006). By contrast, the four weak monsoon anomalies probably correlate with ice-rafting events (HE 4, HE 3, and inter-Heinrich ice-rafting events f and h; nomenclature from Bond and Lotti, 1995) in the North Atlantic. In addition, these millennial-scale AM cycles are broadly of opposite sign to those in Antarctica, with weak (strong) monsoon intensity corresponding to high (low) Antarctica temperature. The four weak AM events can be correlated to the warming events in Antarctica [Antarctic Isotope Maximum (AIM) 8, 7, 5 and 4; EPICA Community Members, 2006). The new Wu3 record combined with the previously published data (Liu et al., 2010; Zhao et al., 2010) from the same cave are presented in Fig. 4 showing that these millennial-scale relationships with Greenland and Antarctic ice core records are persistent at this site.

image

Figure 4. Comparison of AM with Greenland and Antarctica climates. (a) NGRIP δ18O record (Anderson et al., 2006); (b) spliced stalagmite δ18O record from Wulu Cave plotted on their respective time scales (published data are from Liu et al., 2010 and Zhao et al., 2010, respectively); (c) EDML δ18O record given on the GICC05 time scale (EPICA Community Members, 2006). This figure is available in colour online at wileyonlinelibrary.com.

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However, a closer inspection of Fig. 5 highlights a minor but significant difference between the cave-based AM intensity and Greenland temperature records. In the Greenland record, the long stadial between GIS 5 and GIS 4 is divided into two phases by the GIS 4.1 event, with the first phase slightly warmer than the second (Fig. 5a). However, at Wulu Cave, the magnitude of the CIS 4.1 event implies a relatively strong monsoon (Fig. 5d), comparable to CIS 5. Moreover, the substantial inferred monsoon weakening evident between other interstadials is not observed between CIS 4.1 and CIS 5. A similar pattern is evident in the stalagmite records from south-west China (Fig. 5e; Zhao et al., 2010), mid-China (Fig. 5c; Zhao et al., 2010), eastern China (Fig. 5b; Wang et al., 2001) and from as far as southern Brazil (Fig. 5f) (Wang et al., 2006) in an anti-phased fashion. A combination of the strong CIS 4.1 and CIS 5 events results in an extended relative strong monsoon period with a duration of ca. 2000 years. Interestingly, the Wu3 (Fig. 5d) and DSB3 (Fig. 5e) δ18O records during the period of CIS 5–4.1 appear different from the Hulu (Fig. 5b) and Sanbao (Fig. 5c) records, where the CIS 5 and CIS 4.1 events appear isolated. Whether the discrepancy arises from the cave locations dominated by the Indian and East Asian monsoons, respectively, is an open question. This is because the previously reported records from Wulu Cave have shown a striking resemblance to the Greenland records (Liu et al., 2010; Zhao et al., 2010), like the Hulu record. Contrasting flow-through times and mixing conditions in the epikarst can also produce the minor difference. Future cave monitoring data and additional records might explain the disparity. Overall, the AM structure is broadly similar, but in an anti-phased fashion, to the EPICA Dronning Maud Land (EDML) ice core record from Antarctica during the episode, despite the short-lived pulse AIM 4.1 (Fig. 5g). This anti-phased correspondence may support the idea that the AM is also influenced by latent heat of cross-equatorial moisture (Clemens et al., 2008) and latitudinal shifts in the intertropical convergence zone (ITCZ) (Fleitmann et al., 2007) via the Southern Hemisphere ‘push’.

image

Figure 5. Detailed comparison of AM, Greenland and Antarctica temperatures over Heinrich Stadial 3. (a) NGRIP δ18O record (Anderson et al., 2006); (b), (c), (d) and (e) Chinese speleothem records from Hulu (Wang et al., 2001), Sanbao (Zhao et al., 2010), Wulu (this study) and Dashibao (Zhao et al., 2010) caves, respectively; (f) stalagmite δ18O record from southern Brazil (Wang et al., 2006); (g) Antarctica temperature record plotted on the new GICC05 time scale (EPICA Community Members, 2006). Note that the Hulu time scale, not well constrained (Zhao et al., 2010), is shifted younger by 640 years for an easy comparison. The vertical shaded region represents unusual climatic responses in different climate systems. Grey data points are U/Th ages with 2σ uncertainties. This figure is available in colour online at wileyonlinelibrary.com.

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The CIS 4.1 event is immediately followed by a weak monsoon event centred at ∼29.8 ka, with a 3‰ increase in δ18O values, coincident with HE 3 in the North Atlantic (Fig. 3). Constrained by layer-counting results, the transitions into and out of the event are 392 ± 25 and 298 ± 18 years, respectively, and its duration at minimum monsoon state lasts 716 ± 32 years, while their counterparts in the NGRIP records are 200 ± 13 years, 1540 ± 101 years and 120 ± 4 years (Anderson et al., 2006). HE 3 is preceded by a rebound-type event GIS 4.1, as expected to be induced by the enhanced Atlantic Meridional Overturning Circulation (AMOC) (Capron et al., 2010). The increased AMOC could be implicated by the strong monsoon event CIS 4.1, because of the strong coupling between them (Sun et al., 2012).

Detailed CIS 8 onset

Previous studies (e.g. EPICA Community Members, 2006; Zhao et al., 2010) have indicated a coherent climatic correspondence between China and North Atlantic millennial-scale climate changes in anti-phase with comparable-scale climate events in Antarctica. Figure 6 presents a snapshot of our record with a 5-year resolution and highlights a monsoonal transition at the onset of CIS 8 with comparable ice core records from Greenland and Antarctica. Three phases are discerned during the transition based on the rate of AM increase. Phase 1 presents a gradual increase in the monsoon intensity, as indicated by a decreasing trend in δ18O from –8.2 to –9.8‰. Phase 2 displays an abrupt decrease in calcite δ18O from –9.3 to –10.0‰ within several decades, suggesting a dramatic jump in monsoon intensity. Phase 3 is another stage of gradual increase in monsoon intensity. A generally gradual transition, relative to abrupt Greenland warmings, was also observed during the transition into the Bølling–Allerød period in the Hulu record (Wang et al., 2001).

image

Figure 6. Detailed comparison of AM, Greenland and Antarctica temperatures (on the GICC05 age scale) across CIS 8. Three phases can be detected at the CIS 8 onset. If the sharp change of CIS 8 onset is concurrent to abrupt GIS 8 warming and a reversal point of Antarctica temperature variation, the strong/weak AM episodes of CIS 8 would correlate to warm/cold phases of GIS 8, and to cold/warm intervals at Antarctica, strongly supporting the bipolar seesaw mechanism. The vertical bars indicate the boundaries between stadials and interstadials. Grey data points are U/Th ages with 2σ uncertainties. This figure is available in colour online at wileyonlinelibrary.com.

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Because of the close relationship between the Greenland temperature and AM intensity (Wang et al., 2001), it is likely that the sharp AM jump, which characterizes the major onset of CIS 8, is synchronous with the rapid temperature rise in Greenland at ∼38.2 ka. This rapid AM change is preceded by the apparently gradual increase (aforementioned Phase 1), which lasts approximately 941 ± 36 years based on our band counting result, equivalent to a half duration of the cold Heinrich Stadial 4. If the sharp changes in the AM and North Atlantic regions are concurrent with a reversal point of Antarctica temperature variations, the initial AM increase would correlate with the late part of AIM 8, which shows a slightly decreasing trend (Fig. 6). This correlation seems to be consistent with an overall anti-phased inter-hemispheric relationship and hence with the atmospheric bipolar seesaw mechanism (Cvijanovic et al., 2013).

This phased transition into CIS 8 supports the idea that abrupt changes happen when the climate systems reach a critical tipping point, which commonly refers to a critical threshold at which a small perturbation can qualitatively alter the state or development of the system (Lenton et al., 2008). Levermann et al. (2009) and Schewe et al. (2012) suggest positive moisture–advection feedback (Webster et al., 1998) as a candidate for the main cause of abrupt changes in monsoon dynamics. The feedback dominates the continent–ocean heat balance (governed by latent heat) of the AM system and could lead to abrupt changes as long as atmospheric humidity over an ocean passes a threshold, which could critically alter the moisture transport to the AM region due to increased latent heat release in the continent. The anti-phased correlation between our stalagmite and the EDML ice core record suggests that the moisture transport associated with Antarctica/Southern Ocean temperature conditions might be an important trigger for abrupt switches of AM (Fig. 3). As illustrated in Fig. 6, the decrease in Antarctica/Southern Ocean temperature, at the turning point to a rapid Greenland warming, might approach a critical threshold and contribute to the subsequent abrupt increases in AM and Greenland temperature. Several modelling (Ganopolski and Rahmstorf, 2001; Schmittner et al., 2002; Alley et al., 2003 for a review) and statistical (Cimatoribus et al., 2012) studies have indicated that abrupt Greenland warmings were also connected to a crossing of a tipping point in heat balance between the North and South Atlantic associated with the AMOC. The simultaneous abrupt climate changes in the AM and North Atlantic regions indicate that the rapid reorganization of oceanic/atmospheric circulations has the potential to push the Earth system across a critical state into another qualitatively different mode of operation. In addition, the phased pattern is also seen at the start of CIS 12 preceded by HE 5 despite low resolution (Wang et al., 2001) and at the end of the Younger Dryas (Liu et al., 2013).

Based on the notion that climate changes might recur in a similar fashion at various temporal scales (Wolff et al., 2009; Liu et al., 2010), the three-phase features discerned at the onset of CIS 8 may be comparable to glacial termination (Terminations I, II and IV) processes that have been linked to AM records (Wang et al., 2001; Cheng et al., 2006, 2006, 2009). These observations confirm that the AM is indeed a potential tipping element of the Earth system, where a critical tipping point can induce abrupt changes whether on the millennial or orbital scales.

Conclusions

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Based on 1371 oxygen isotope measurements and eight precise 230Th dates of one stalagmite from Wulu Cave in south-west China, located in the transitional zone between the Indian and East Asian monsoons, we have reconstructed a detailed history of the AM from 39.3 to 28.7 ka. The high-resolution δ18O sequence registers four major wetting/drying cycles from CIS 8 to CIS 4. The strong monsoon events broadly resemble the Greenland warming events GIS 8–4 and Antarctica cooling events regarding timing and frequency.

Three main conclusions can be drawn from detailed comparison of the timing, structure and transitioning of monsoon variability at Wulu Cave with Greenland and Antarctica temperature variations. First, over the interval of GIS 5–4.1, our stalagmite δ18O record depicts a sustained and relatively strong period of AM with a duration that encompasses both interstadials, unlike distinctly isolated peaks for GIS 5 and GIS 4.1 in Greenland, with GIS 4.1 weaker. Second, the transitions of the millennial-scale AM events are more gradual than those in Greenland. For example, the onset and termination of HE 3 last about 392 ± 25 and 298 ± 25 years, respectively. Also, the beginning of strengthened AM at CIS 8 is characterized by two phases of gradual increase separated by an abrupt increase. Third, the initial slow AM increasing significantly pre-dates the abrupt onset of CIS 8, suggesting that the dynamic AM circulation might have the potential to transport enough moisture and heat to induce abrupt climate change at times when the AM system reaches a critical point.

Acknowledgements

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

We thank Professor Rewi Newnham and Dr Dan Sinclair for their thorough reviews, constructive suggestions and comments, and editorial corrections. Thanks are also given to an anonymous reviewer for his/her critical and instructive comments. This work was supported by grants of the National Natural Science Foundation of China (Nos. 41130210, 41172314, 41072126), and Research and Innovation Project for College Graduates of Jiangsu Province (No. CXZZ12_0399).

Abbreviations
AIM

Antarctic Isotope Maximum

AM

Asian Monsoon

AMOC

Atlantic Meridional Overturning Circulation

CIS

Chinese Interstadial

EDML

EPICA Dronning Maud Land

GIS

Greenland Interstadial

HE

Heinrich event

MIS

marine isotope stage

References

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Supporting Information

  1. Top of page
  2. ABSTRACT
  3. Introduction
  4. Material and Method
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Additional supporting information can be found in the online version of this article:

FilenameFormatSizeDescription
jqs2681-sm-0001-SuppFig-S1.pdf120KFigure S1. Photograph of stalagmite Wu3 from Wulu Cave, south-west China.
jqs2681-sm-0002-SuppFig-S2.pdf102KFigure S2. Isotope equilibrium test, including ‘Hendy Test’ (Hendy, 1971) and replication test.

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