New insights on Chinese cave δ18O records and their paleoclimatic significance
Introduction
Owing to their advantages of high precision uranium-series dating and high temporal resolution, Chinese cave stalagmite records have emerged as a major paleoclimatic archive over the past two decades (e.g. Wang et al., 2001, Wang et al., 2005; Wang et al., 2008a; Yuan et al., 2004; Zhang et al., 2008; Cheng et al., 2009, Cheng et al., 2016; Cai et al., 2015; Zhang et al., 2018c). Chinese stalagmite oxygen isotopic (δ18O) records have been used to investigate the history of the East Asian summer monsoon (EASM) from millennial to orbital timescales which has been extended to cover the full age range of uranium/thorium dating (the last ~640 kyr) (Cheng et al., 2016). Climatologists normally use either “wind” or “rainfall” to indicate the intensity of the EASM. On a long timescale, stalagmite δ18O records from eastern monsoonal China have been directly regarded as a proxy of EASM intensity (e.g. Wang et al., 2001; Yuan et al., 2004; Cheng et al., 2016) and/or the related EASM rainfall amount (e.g. Wang et al., 2005; Cosford et al., 2008; Cheng et al., 2006, Cheng et al., 2009; Cai et al., 2010; Zhao et al., 2015), with negative δ18O values corresponding to increased summer monsoon rainfall, and vice versa. Based on these pioneering studies, the relationships between the EASM, Northern Hemisphere summer insolation (NHSI), and North Atlantic climate have been discussed (Wang et al., 2005; Dykoski et al., 2005; Cheng et al., 2016).
Nevertheless, the paleoclimatic significance of cave δ18O records from the EASM region has been strongly debated. This reflects the complex range of factors controlling the precipitation oxygen isotopic composition (the source signal of cave δ18O). As a consequence, various hypotheses have been proposed to explain the variations of Chinese cave δ18O records on a wide range of timescales (Maher, 2008; LeGrande and Schmidt, 2009; Clemens et al., 2010; Dayem et al., 2010; Pausata et al., 2011; Maher and Thompson, 2012; Caley et al., 2014; Nan et al., 2014; Tan, 2014; Liu et al., 2015; Rao et al., 2015; Baker et al., 2015, Baker et al., 2019), challenging the prevailing viewpoint of cave δ18O as an EASM rainfall indicator (Wang et al., 2005; Cheng et al., 2009). For example, Maher (2008) argued that the Chinese cave δ18O record does not reflect changes in rainfall amount during the Holocene, but rather changes in rainfall source. Isotope-enabled simulation results indicated that the Chinese speleothem δ18O record reflects the δ18O of precipitation delivered from water vapor sources (Battisti et al., 2014), or annual variations in hydrologic processes and the circulation regime (Caley et al., 2014), rather than Asian summer monsoon intensity and precipitation amount, on an orbital timescale. In addition, Chinese cave δ18O records do not match other independent EASM rainfall records, including those from the loess/paleosol sequences of the Loess Plateau (e.g. Xiao et al., 2002; Maher and Hu, 2006; Lu et al., 2005, Lu et al., 2013) and marine sediments in the East China Sea (e.g. Kubota et al., 2010, Kubota et al., 2015). These contrasting results emphasize the need to reinterpret the Chinese cave δ18O records and to reevaluate the forcing role of NHSI in EASM variations. Furthermore, these differences in the interpretation of Chinese cave δ18O records have led to different conclusions regarding changes in EASM rainfall intensity during the Holocene in densely populated China (e.g. Chen et al., 2015b; Goldsmith et al., 2017; Liu et al., 2017). In China, monsoon rainfall has played a significant role in cultural development and ecosystem maintenance in the past (Zhang et al., 2008; Chen et al., 2015b; Chen et al., 2020), and is important for sustainable development in the future. Therefore, more effort is needed to determine the precise paleoclimatic significance of cave δ18O records from eastern monsoonal China.
It is widely accepted that changes in stalagmite δ18O bear the imprint of variations in the oxygen isotopic composition of precipitation (δ18Op) on multiple timescales (Cheng et al., 2012; Lee et al., 2012; Caley et al., 2014; Zhao et al., 2014). Therefore, based on uniformitarian principles, it is important to compare the trends of the variation of stalagmite δ18O records and monitored rainfall amount during the instrumental period in order to clarify its relationship with precipitation, as well to understand its paleoclimatic significance. Although not annually calibrated by the observed regional precipitation record (Tan, 2016), several case studies have shown that cave δ18O is negatively correlated with local precipitation amount on the decadal timescale during the instrumental period. For example, the Wanxiang cave δ18O record from central China is negatively correlated with 5-yr average local precipitation (r = −0.64, n = 48) at the 99% confidence level during 1953–2000 (Zhang et al., 2008; Liu et al., 2008). Notably, only a few cave δ18O records (e.g. Wanxiang, Dayu, and Xianglong caves) are highly correlated with local rainfall amount, and most of the cave δ18O records from eastern monsoonal China are not significantly correlated with local rainfall during the instrumental period. For example, during the last 100 years the relationship between the stalagmite δ18O record of Heshang cave in Hubei, South China, and local rainfall amount is weak (r = 0.08, n = 95) and not statistically significant (He et al., 2009). Similarly, a poor correlation is also evident in the relationship between local rainfall amount and the stalagmite δ18O records from Dongge cave (Zhao et al., 2015) and Xiaobailong cave (Tan et al., 2017; Yang et al., 2019) in southwest China. These contrasting results raise the following questions: Why are some cave δ18O records highly correlated with local precipitation while others are not? Is there a spatial similarity between caves where stalagmite δ18O is highly correlated with local rainfall on decadal and longer timescales during the instrumental period? If Chinese cave δ18O is a signal of large-scale atmospheric circulation (e.g. integrated monsoon rainfall (Cheng et al., 2016)), what is the relationship between cave δ18O and rainfall amount for the whole of eastern monsoonal China? None of these questions have been comprehensively addressed so far, although addressing them is likely to help clarify the paleoclimatic significance of stalagmite δ18O records from eastern monsoonal China.
In this study, we revisit the issue of the origin of variations in cave δ18O in eastern China and we produce a representative cave δ18O proxy record (δ18Osyn) for the whole of eastern monsoonal China and three separate δ18Osyn records for North China, the Yangtze River Valley (YRV), and South China, which capture the trend of variation of δ18O during the instrumental period. The δ18Osyn records are compared with time series of summer (JJA) rainfall for the whole of eastern monsoonal China, North China, the YRV, and South China, respectively, in order to determine the relationships between stalagmite δ18O and summer rainfall in different regions over the past ~60 years. In addition, based on recent progress in paleoclimatic research, we review the relationship between Chinese cave δ18O records and other independent EASM rainfall records on a broad range of timescales. In order to determine the reason for the abovementioned inconsistency between Chinese cave δ18O records and precipitation in eastern monsoonal China, we explore the correlations and linkages between Chinese cave δ18O records and ISM rainfall (ISM intensity), which elucidate the potential dynamic mechanism(s) that controlled changes in Chinese cave δ18O during the instrumental period and the Holocene. Finally, we use the findings to provide new insights into the relationship between stalagmite δ18O and monsoon rainfall on various timescales, and thus help clarify the paleoclimatic significance of Chinese cave δ18O records. The spatial differences discussed herein refer to temporal variability rather than to the mean climate state.
Section snippets
Data and methods
Many cave δ18O records from eastern monsoonal China spanning the instrumental period have been published (Wang et al., 2005; Zhang et al., 2008; Hu et al., 2008; Wan et al., 2011; Jiang et al., 2012; Tan et al., 2011, Tan et al., 2014, Tan et al., 2015a, Tan et al., 2015b, Tan et al., 2017, Tan et al., 2018; Zhao et al., 2015; Li et al., 2017; Yin et al., 2014, Yin et al., 2017; Zhang et al., 2018b; Zhao et al., 2019a). Most of these records are of high resolution (better than 1 year), spanning
Inconsistency between the trend of Chinese cave δ18O records and that of summer rainfall in eastern China during the instrumental period
In order to characterize the trend of variation of Chinese cave δ18O records, a δ18Osyn record was compiled from 13 high-resolution (better than 2 yr) cave records (Fig. 1a; Table 1). As shown in Fig. 1c, the normalized δ18Osyn record exhibits lowest values during the 1950s and highest values during the 2000s. This indicates a strong decadal to longer variation trend of increasing cave δ18O values over the past 66 years (r = 0.85, p < 0.001, n = 66). Although different patterns of variability
Conclusions
- (1)
During the instrumental period, the prominent increasing trend of Chinese cave δ18O records (represented by the δ18Osyn record for the whole of eastern monsoonal China) is substantially different to the relatively invariant summer rainfall records throughout eastern monsoonal China. In addition, the similarity of the variation of Chinese cave δ18O records conflicts with observed spatial differences in summer rainfall in eastern monsoonal China over the last ~60 yr. This inconsistency between
Acknowledgements
We sincerely thank the two anonymous reviewers for their constructive comments on the manuscript. We are grateful to Drs. Liangcheng Tan, Xiuyang Jiang, and Jianjun Yin for providing the raw data of previously published cave records. We thank Dr. Jan Bloemendal for improving the English. This work was supported by the National Natural Science Foundation of China (grant No. 41722105 to J.L.).
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.
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