The 4.2 ka event in East Asian monsoon region, precisely reconstructed by multi-proxies of stalagmite
Abstract. The 4.2 ka event is one of the most salient features of global climate change in the mid-late Holocene and influenced on the evolution of ancient civilizations. Although a lot of paleoclimate reconstructions have focused on it, the detailed structure and driving mechanism of the 4.2 ka event is still unclear. In this study, the variation of Asian summer monsoon (ASM) during 5000–3000 yr BP was reconstructed by using high-precision U-Th dating (average resolution of 7 yr) and multi-proxies (δ13C, δ18O, Ba / Ca, Sr / Ca, Mg / Ca) of stalagmite YK1306 from Yangkou Cave in southwestern China. The results showed that that the ASM weakened and precipitation decreased during 4600–4330 yr BP and 4070–3700 yr BP. During 4330–4070 yr BP, the ASM became strong, and precipitation increased. The multi-proxies variation of YK1306 showed a
weak-strong-weak structure of the ASM during the 4.2 ka event, which reappeared in different geologic records. However, westerlies and Australian-Indian summer monsoon (AISM) both showed the opposite change pattern (strong-weak-strong) with the ASM. This was resulted by the different phases of North Atlantic Oscillation (NAO) on a centennial scale, which regulated by the Atlantic Meridional Overturning Circulation (AMOC). In positive NAO-like, the strength of Azores high and westerly wind restrained the intensity of ASM. Thus, the ASM and the Middle East regions experienced bimodal drought and increased dust flux from the north in both regions during the 4.2 ka event. The strengthened meridional winds in the westerlies-dominated climatic regime (WDCR) lead more water vapor from the Indian Ocean and Arabian Sea transporting to in the WDCR, and subsequently increases precipitation in the WDCR. Meanwhile, the weakening of the AMOC results in the southward migration of the Intertropical Convergence Zone (ITCZ) and strengthens the AISM in the southern Hemisphere, finally results in the opposite change of the AISM contrast to the ASM. In addition, the strong ASM in the era of the Chinese Xia Dynasty maybe produce frequent ancient floods, which led to the decline of Longshan and Liangzhu cultures. The weakening of the ASM after 4070 yr BP contributed to the successful regulation of the ancient floodwaters by Dayu in Chinese history. Therefore, it is maybe credible that the official age for the establishment of the Xia Dynasty in 4070 yr BP. Benefit from the comprehensive comparison and analysis based on the unprecedented high-precise chronology, high-resolution and multi-proxy's stalagmite records, this study not only detailed described the evolution of the ASM during the 4.2 ka event, but also is conducive to verify the age of the first dynasty of China (the Xia Dynasty), and the legend of Dayu.
Chao-Jun Chen et al.
Chao-Jun Chen et al.
Chao-Jun Chen et al.
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We would like to draw the authors attention to our recent submission in discussion at Climate of the Past (manuscript: cp-2020-138) that may help with the interpretation of the trace element signal from YK1306. Both the unidirectional change in trace elements, and the wet-dry-wet/dry-wet-dry pattern of the stable isotope records have both been identified in our submission, which comprises an Indian Ocean wide regional compilation of high-resolution hydroclimate proxies.
In particular these patterns are seen in the first and second principal components of the compilation (figure 3 of cp-2020-138). Our PC1, which we interpret as variability in Indian Ocean summer rainfall dynamics matches the trace element PC1 signal of YK1306. The decrease in rainfall is timed to 3.97 kyr BP (standard error of 94 years) in our study using a rampfit function and taking into account full age model uncertainty of five independent stalagmite records. A shift in tropical hydroclimate at 4.0 kyr BP is well recognised in the literature (examples: Marchant and Hooghiemstra, 2004, de Boer et al., 2014; Denniston et al., 2013; Gagan et al., 2004; Giosan et al., 2018; Li et al., 2018; MacDonald, 2011; Toth et al., 2012), And although it is sometimes interpreted as being caused by the 4.2 kyr event, both the timing and the shape of the anomaly does not match.
Our PC2 (particularly for the third PCA shown in orange which includes additional records from the Arabian Sea) matches the d18O and d13C signals in YK1306. We interpret those signals as likely deriving from winter monsoon variability as dominated by westerly derived moisture and winds through western disturbances. The triple wet-dry-wet (or dry-wet-dry depending on the loading (figure 4 of cp-2020-138)) pattern is evident too. The timings are approximately 4.6-4.25, 4.25-3.9, 3.9-3.6 kyr BP in our study, in good agreement with YK1306. We therefore believe there is significant congruence between the data of our studies and a reasonable agreement on underlying climatic processes.
With this in mind, one could hypothesise that the trace element signal in YK1306 might therefore reflect some kind of tropical forced variability in rainfall in south-west China via the Indian Summer Monsoon component. Meanwhile the stable isotopes respond, as suggested by the authors, to more westerly dominated climatic processes. We believe that a full consideration of our study in the interpretation of YK1306 will help develop the trace element interpretation in the manuscript, and begin to deconvolve the complex multiple drivers of climate variability in south-west China, and the Indian Ocean. Should you wish to use our data prior to final publication of our work, please contact us.
We will leave the full process of peer review to the appointed reviewers, but we have one other minor point: please make the data from this work publicly available in an archive such as NOAA, and also via a submission to the SISAL database.
(on behalf of coauthors)