Elsevier

Quaternary Science Reviews

Volume 250, 15 December 2020, 106699
Quaternary Science Reviews

East Asian monsoon changes early in the last deglaciation and insights into the interpretation of oxygen isotope changes in the Chinese stalagmite record

https://doi.org/10.1016/j.quascirev.2020.106699Get rights and content

Highlights

  • Two mechanisms affect Chinese stalagmite δ18O: the Wang-Cheng mechanism and the Yuan mechanism.

  • A two-phased structure of East Asian Monsoon during Heinrich Stadial 1, an early weakening and a subsequent weaker condition.

  • A remarkable relationship between ice-rafting events and weak monsoon events on the multi-centennial to sub-decadal scale.

Abstract

Stalagmite oxygen isotope18O) records have enhanced our understanding of the history of the East Asian monsoon. However, abrupt changes in the monsoon are not constrained well enough to address certain issues and there are still unknowns in the interpretation of cave δ18O records. Here we present a new high-resolution stalagmite record from Shima Cave, central China. Anchored with 24 230Th/U dates, our sample grew from 19.7 to 17.8 ka and from 16.3 to 13.3 ka, covering much of the early portion of the last deglaciation with a temporal resolution of 7 years. Using this record and other available Asian δ18O records, we test model predictions for shifts in δ18O in Asian caves and further investigate monsoon variations on centennial to decadal timescales during the early portion of the last termination. We conclude the following regarding the interpretation of δ18O in Chinese caves. Two mechanisms affect δ18O: changes in the fraction of monsoon rainfall in annual totals (the Wang-Cheng mechanism) and changes in the amount of rainout between tropical sources and cave sites (the Yuan mechanism). The former is caused by changes in the seasonal migration of the sub-tropical jet and likely has a smaller effect on cave δ18O than the latter. The latter involves changes in rainout from both the Pacific and the Indian Ocean sources. Precisely how that change in rainout is partitioned between sources and cave sites is not fully understood; however, it is clear that some of the change takes place in China; i.e. it is not restricted to upstream sites. Rough calculations for our site suggest that mean annual rainfall may have been two-thirds of modern values at the time of the highest δ18O values during Heinrich Stadial 1 (HS1) (16.1 ka) and one fifth again higher than modern values during the Bølling (14.3 ka). Within HS1, consistent with observations from other localities, Chinese cave records exhibit a twofold structure. Within the phases, we observe a truly remarkable relationship between the North Atlantic ice rafted debris record and the Hulu-Shima Cave record, where events ranging from multi-centennial to sub-decadal timescale can be related across Eurasia. The origin of this two-phased structure may relate to different sources and extent of ice-rafted debris in the North Atlantic as well as the extent of sea ice and the Atlantic Meridional Overturning Circulation.

Introduction

The last deglaciation (21–11 ka) was punctuated by many climate perturbations (Clark et al., 2012; Ng et al., 2018). Changes in the ice sheets and sea level (Denton et al., 2010; Lambeck et al., 2014), ocean circulation (McManus et al., 2004; Ng et al., 2018), the atmospheric-oceanic carbon cycle (Marcott et al., 2014; Bauska et al., 2016) and low-latitude hydroclimate (Wang et al., 2001; Weijers et al., 2007; Deplazes et al., 2013) are closely associated with millennial-scale events (e.g. Heinrich Stadial 1 (HS1) and Bølling-Allerød (BA)). HS1, one of the most prominent Heinrich stadials of the last ice age, features massive freshwater and iceberg discharge from the Northern Hemisphere ice sheets into the North Atlantic (Heinrich, 1988). Following HS1, the BA period is characterized by warm temperatures in the Northern Hemisphere (Marcott et al., 2014; WAIS Divide Project Members, 2015).

Whereas there have been considerable efforts on providing perspective on the orbital- and millennial-scale climate changes during the last deglacial period (Denton et al., 2010), higher resolution paleoclimate records extend investigations to the centennial-scale and can potentially provide a more precise understanding of processes during the deglaciation sequence (Marcott et al., 2014; Chen et al., 2015; Bauska et al., 2016). For example, three abrupt centennial-scale increases in atmospheric CO2 (each of 10–15 ppmv) superimposed on the millennial-scale changes in the last deglaciation, and have been closely linked to CH4 changes and Northern Hemisphere climate (Marcott et al., 2014). Two of these increases at around 14.8 ka and 11.7 ka were synchronous with the rapid intensification of the Asian monsoon and the abrupt recovery of the Atlantic Meridional Overturning Circulation (AMOC) (Chen et al., 2015). Heinrich events are believed to be associated with strong perturbations on the AMOC and global climate; however, with high-resolution records emerging, HS1 is found not to be one monotonous event and exhibit a two-phase structure (Bard et al., 2000; Broecker et al., 2009; Broecker and Putman., 2012; Huang et al., 2019), along with many other high frequency features (Brendryen et al., 2020). Chinese cave records have also documented a generally weak Asian Monsoon during HS1 (e.g. Wang et al., 2001), but some of the detailed characteristics and forcing mechanisms within this event are still not well understood. With our new record and the previously published Hulu record (Wang et al., 2001; Wu et al., 2009), we now have an opportunity to assess ideas with regard to submillennial-scale changes during the last deglaciation.

Studies about the interpretation of δ18O in Asian caves have often stated that this is a controversial topic (e.g. Chiang et al., 2020). A clear reading of the literature shows that this is not now and has not historically been the case. Indeed, the literature shows that the essence of the interpretations put forth in the original papers on the subject (Wang et al., 2001; Yuan et al., 2004; Johnson and Ingram, 2004; Cheng et al., 2009) have been verified by subsequent empirical and theoretical studies (Liu et al., 2014; Orland et al., 2015; Cheng et al., 2012, 2016). This is not to say that there were not significant unknowns at the time of the early publications. A number of those unknowns have since been elucidated and others still remain. The literature, thus shows that the original publications, laid a solid foundation upon which subsequent publications built, with more work yet to come, in this ongoing, vibrant area of research. Here we use our new data along with published data to test some of the ideas regarding the interpretation of oxygen isotopes in Chinese caves. We then present our conclusions and assessment of the current state of this field.

Here we report a new 230Th/U-dated, high-resolution stalagmite δ18O record spanning much of the early portion of the last deglaciation, from Shima Cave, Hunan Province, central China. This record covers about 4900 years, with an average oxygen isotope resolution of better than 7 years. The stalagmite grew continuously for 1900 years at the end of the last glacial maximum (LGM, 19.7--17.8 ka), then after a 1500-year hiatus, for another 3000 years, covering the last half of HS1, on into the BA (16.3–13.3 ka). We analyze this record in the context of other Asian cave records and other correlative records from around the world. Within this context, we examine the suite of records with the notion that HS1 is characterized by two stages as well as high-frequency features. In addition, we use our record and the now large number of published cave records across Asia to test ideas about the interpretation of the δ18O of Chinese cave deposits. Since these records span a large geographic range, we can now use them to test models that hindcast shifts in δ18O as a function of location.

Section snippets

Materials and methods

Stalagmite sample SM7 was collected from Shima Cave (Fig. 1, 29°35′N, 109°31′E), in Hunan Province of central China, ∼930 km southwest of Hulu Cave (32°30′N, 119°10′E). The cave is at an elevation of ∼650 m and is developed in Permian limestone, with a narrow entrance facing to the southwest. The cave site is strongly influenced by the East Asian Monsoon climate. Modern observational data from the nearest Enshi station (∼80 km north of Shima Cave) shows that the mean annual temperature in this

Chronology

The U and Th isotopic compositions are presented in Table 1. Measured 238U concentrations are low, ranging from 131.4 to 281.3 ppb. However, most of the samples have 232Th concentrations less than 1000 ppt and relatively high 230Th/232Th activity ratios so that corrections for initial 230Th are small. The length of time represented by the hiatus was determined to be ∼1.5 ka, based upon extrapolation to 464 mm of the bounding ages drilled at 460 mm and 470 mm. Three dates at the lower part of

Controlling factors of Chinese stalagmite δ18O

The publication of the original Hulu Cave record (Wang et al., 2001) stimulated a long, fruitful, and ongoing debate on the interpretation of δ18O values recorded in South and East Asian caves. With our new record and the now many records from across Asia, we have an opportunity to assess these ideas with regard to millennial-scale changes. Two mechanisms were proposed in the early studies, the Wang-Cheng Mechanism (Wang et al., 2001; Cheng et al., 2009) and the Yuan Mechanism (Yuan et al., 2004

Conclusions

A new 230Th/U dated, ∼7-year-resolution stalagmite δ18O record from Shima Cave, Hunan Province, provides us with new evidence into the interpretation of speleothem δ18O and the millennial-to sub-decadal-scale monsoon variations during the early portion of the last deglaciation. Our sample SM7 grew in time periods from 19.7 to 13.3 ka, with a ∼1500-year hiatus (from 17.8 to 16.3 ka). The record resembles other Chinese cave records and features the abrupt shift to the highest δ18O values in the

Author statement

Yijia Liang: Conceptualization, Formal analysis, Funding acquisition, Investigation, Writing - original draft. Kan Zhao: Writing- Reviewing and Editing, Project administration, Funding acquisition. R. Lawrence Edwards: Conceptualization, Funding acquisition, Writing- Reviewing and Editing. Yongjin Wang: Writing- Reviewing and Editing, Funding acquisition. Qingfeng Shao: Formal analysis, Investigation. Zhenqiu Zhang: Investigation, Resources. Bin Zhao: Investigation, Resources. Quan Wang: Formal

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

We thank Editor Dr. Miryam Bar-Matthews and two anonymous reviewers for their constructive comments. This work was supported by National Natural Science Foundation of China (Grants 42071105, 41931178, 41572151, 41571102, 41672164), the U.S. National Science Foundation (Grant 1702816), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant KYCX19_0789), the 111 Program of China (Grant D19002), the Science and Technology Research Program of Chongqing Municipal

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