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.
Highlights
Different geological carriers (Ice cores, stalagmites, loess, lake sediments, marine sediments, etc.) in the world have indicated that the 8.2 ka and 4.2 ka events affected global climate change in Holocene (Alley et al, 1997; Kobashi et al, 2007; Cheng et al, 2009; Berkelhammer et al, 2012; Kathayat et al, 2018; Railsback et al, 2018)
The results showed that the stalagmites deposited in 5040–2920 yr BP from 70 to 0 mm of distance from top
We used the multi-proxies of stalagmite YK1306 of YK cave in southwestern China to reconstruct the variation of ASM during 5000–3000 yr BP
Summary
Different geological carriers (Ice cores, stalagmites, loess, lake sediments, marine sediments, etc.) in the world have indicated that the 8.2 ka and 4.2 ka events affected global climate change in Holocene (Alley et al, 1997; Kobashi et al, 2007; Cheng et al, 2009; Berkelhammer et al, 2012; Kathayat et al, 2018; Railsback et al, 2018). Agassiz and Ojibway lake (AOL) at 8470 ± 300 yr BP triggered the 8.2 ka event (Alley et al, 1997; Barber et al, 1999; Kobashi et al, 2007; Cheng et al, 2009; Daley et al., 2011). The ASM showed a double weak structure during the 8.2 ka event (Cheng et al., 2009; Tan et al, 2020). What drove this double weak structure is still unclear
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