HOL0010.1177/0959683619831433The HoloceneYang et al. 831433research-article2019 Research Paper The Holocene 2019, Vol. 29(6) 1059 –1067 Early-Holocene monsoon instability and © The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions climatic optimum recorded by Chinese DOI:https://doi.org/10.1177/0959683619831433 10.1177/0959683619831433 stalagmites journals.sagepub.com/home/hol Xunlin Yang,1 Hong Yang,1,2 Baoyan Wang,1 Li-Jung Huang,3,4 Chuan-Chou Shen,3,4 R Lawrence Edwards5 and Hai Cheng5,6 Abstract The timing and duration of the Holocene East Asian summer monsoon (EASM) maximum and the interpretation of Chinese stalagmite δ18O records have long been disputed. Notably, interpretations of Holocene EASM variations are frequently based on a single record or study area and are often contradictory. In this study, we conducted stable isotope analyses of four Holocene stalagmites from Chongqing, southwest China. The results reveal differences in the timing of the Holocene EASM maximum and to try to resolve the inconsistency we analyzed and statistically integrated a total of 16 18 18 Holocene stalagmite records from 14 caves in the EASM region. The resulting synthesized Holocene stalagmite δ O (δ Osyn) record is in agreement with other EASM records and confirms that stalagmite δ18O records are a valid indicator of EASM intensity, rather of local precipitation amount. The 18 δ Osyn record shows that the EASM intensified rapidly from the onset of the early Holocene; notably, however, there were distinct EASM oscillations in the early Holocene, consisting of three abrupt millennial-scale events. This indicates that, contrary to several previous interpretations, the early Holocene EASM was unstable. Subsequently, during 8–6 kyr BP, the EASM was relatively stable and strong, with the strongest monsoon occurring during 8–7 kyr BP. This evidence of a stable and strong mid-Holocene EASM in eastern China is in accord with the classical view of a mid-Holocene Optimum in China. Keywords climatic optimum, early Holocene, East Asia, millennial-scale events, Stalagmite δ18O, summer monsoon Received 24 September 2018; revised manuscript accepted 3 January 2019 Introduction for China, which indicates that stalagmite δ18O records in China reflect changes in summer monsoon intensity/monsoon precipita- The East Asian summer monsoon (EASM) is an important compo- tion. However, Chen et al. (2016b) and Yang et al. (2014) suggested nent of atmospheric circulation and plays a major role in global that the stalagmite record supported an early-Holocene Optimum/ hydrological and energy cycles (An et al., 2000; Wang et al., 2005). EASM maximum – in contrast to traditional EASM or EASM pre- It significantly affects the climate, environment and socioeconomic cipitation records from northern China (Lu et al., 2013; Xiao et al., development of East Asia and other regions (Yang et al., 2013). 2008) – and they questioned the reliability of stalagmite records as a However, the evolution of the EASM during the Holocene, espe- proxy for changes in EASM intensity/precipitation (Chen et al., cially the timing of the Holocene Optimum/EASM precipitation maximum, remains controversial (An et al., 2000; Chen et al., 2015, 2016a; Goldsmith et al., 2017; Lu et al., 2013; Shi et al., 1994; Wang 1 et al., 2005). Shi et al. (1994) summarized pollen, lake level, and Chongqing Key Laboratory of Karst Environment, School of Geographical Sciences, Southwest University, China paleosol records from the EASM region and proposed that the cli- 2Department of Geography and Environmental Science, University of mate of China during 7.2–6.0 kyr BP was warm, wet, and stable– Reading, UK that is, the Holocene Megathermal Maximum. A recent quantitative 3High-Precision Mass Spectrometry and Environment Change precipitation reconstruction from Gonghai Lake, in North China, Laboratory (HISPEC), Department of Geosciences, National Taiwan showed that the EASM maximum occurred during the middle Holo- University, Taiwan, ROC cene (7.8–5.3 kyr BP) (Chen et al., 2015), supporting the concept of 4Research Center for Future Earth, National Taiwan University, Taiwan, a mid-Holocene Optimum in China. However, a sequence of eutro- ROC phic peat/mud sediments from Dahu Lake, in southern China, 5Department of Earth Sciences, University of Minnesota, USA revealed a warm and humid interval during 10.0–6.0 kyr BP which 6Institute of Global Environmental Change, Xi’an Jiaotong University, supported the interpretation of an early-Holocene maximum (Zhou China et al., 2004). In addition, a lake-level reconstruction from Lake Dali Corresponding author: (Goldsmith et al., 2017), at the edge of the monsoon region in North Xunlin Yang, Chongqing Key Laboratory of Karst Environment, School China, indicated the occurrence of very high lake levels in the early of Geographical Sciences, Southwest University, Chongqing 400715, and middle Holocene, and in addition there was a significant nega- China. tive correlation between lake level and the stalagmite δ18O record Email: [email protected] 1060 The Holocene 29(6) Figure 1. Sites with Holocene stalagmite records from the Asian summer monsoon region and locations of moisture or precipitation records from northern China mentioned in the text. The modern Asian summer monsoon limit is shown by the green dashed line. 1. Nuanhe Cave (Wu et al., 2011); 2. Lianhua Cave (Dong et al., 2015); 3. Buddha Cave (Li et al., 2000); 4. Jiuxian Cave (Cai et al., 2010); 5. Sanbao Cave (Dong et al., 2010); 6. Heshang Cave (Hu et al., 2008); 7. Lianhua Cave (Zhang et al., 2013); 8.Jinfo Cave (this study); 9.Heifeng Cave (this study); 10. Shizi Cave (this study); 11. Shigao Cave (Jiang et al., 2012); 12. Dongge Cave (Dykoski et al., 2005; Wang et al., 2005); 13. Tianmen Cave (Cai et al., 2012); 14. Dali Lake (Goldsmith et al., 2017); 15. Gonghai Lake (Chen et al., 2015); 16. Qinghai Lake (An et al., 2012); 17. Hongyuan peat bog (Hong et al., 2003); 18. Dongshiya Cave (Zhang et al., 2018); 19. Dahu Lake (Zhou et al., 2004). 2016b; Dayem et al., 2010; Pausata et al., 2011; Maher, 2008; Tan, 106°17′17″ E, altitude 401 m) located in Qingmuguan Town, 2009). Based on the assumption that stalagmite δ18O records directly northwest of Chongqing City. The Qingmuguan karst system reflect EASM intensity, several researchers have reconstructed the consists of carbonate of the lower Triassic Jialingjiang Forma- EASM evolution on various timescales and proposed causal mecha- tion with a thickness exceeding 600 m. Exposed at the anticlinal nisms (e.g. Cheng et al., 2012, 2016; Wang et al., 2005, 2008). More axis, it is the oldest stratum at the site and is the main body of the recently, however, it was proposed that stalagmite records in China mountain. Lithologically, the formation is mainly composed of are indicators of water vapor source rather than EASM intensity gray-colored thick massive limestone, dolomite limestone and (Chen et al., 2016b; Pausata et al., 2011; Maher, 2008; Tan, 2009). brecciaous limestone. Shizi Cave is a small underground river The forgoing summary highlights that the controversy regard- cave system. Stalagmite QM09 was collected from the rock wall ing the pattern of Holocene climate change in China focuses on 50 m from the entrance of the cave; a total of 24 230Th dates and the early to middle Holocene, and that the contrasting interpreta- 315 stable isotope measurements were obtained, with an average tions of the stalagmite δ18O record in China are so far unresolved. resolution of 30 years. Stalagmites J12 (length 395 mm) and J13 Given the possibility of regional differences in the processes of (length 210 mm) were collected from Jinfo Cave (29°01′00″N, carbonate deposition, it is important to analyze stalagmite δ18O 107°10′45″E, altitude 2114 m), in southeast Chongqing, specifi- records from multiple sites. Accordingly, in this study, four Holo- cally in Mt. Jinfo of Nanchuan. The study site is located on the cene stalagmite records from three caves (Jinfo, Heifeng, and southeastern margin of Sichuan Basin, along the northern margin Shizi) in Chongqing, southwest China, were collected and ana- of the Yun-Gui Plateau and at the northern end of the Dalou lyzed. We then combined the results from these caves with an Mountains. The uppermost rock unit comprising Mt. Jinfo is additional 12 stalagmite records from the EASM region of China Permian limestone, and a huge and complex underground cave and use them to discuss the pattern of early- to mid-Holocene system has developed within this unit. The cave exhibits a corri- climate change in eastern China. In addition, by comparing the dor planar form and is 2800 m long, 8–25 m wide, and generally stalagmite records with other proxy records, we further assess the 8–12 m high. Stalagmites J13 and J12 were collected at locations climatic significance of stalagmite δ18O records in East Asia. 100 and 1200 m from the entrance of the cave, respectively. Twenty-eight 230Th ages and 432 stable isotope measurements were obtained from stalagmite J13, with an average resolution of Materials and methods 27 years. In this study, only Holocene data are considered, and Four Holocene stalagmites were collected from three caves in the length of the Holocene interval of stalagmite J12 is about 70 Chongqing (Figure 1), in the upper reaches of the Yangtze River, mm. Eight 230Th dates and 161 stable isotope measurements were in southwest China. The region has a typical EASM climate, with obtained from stalagmite J12, with an average resolution of 50 an average annual precipitation of 1125 mm, which occurs years. Stalagmite HF01 (length 130 mm) was collected 10 m mainly from May to September; summer precipitation comprises from the entrance of Heifeng Cave (altitude 2132 m). Heifeng up to about 70% of the total annual precipitation.