Keywords:4.2 ka event (Northgrippian/ Meghalayan boundary), Mitochondrial DNA haplogroup, Asian summer monsoon, Jōmon, Yayoi
The change in climate known as the 4.2 ka event has received much attention and has been cited as a plausible explanation for the collapse of ancient major civilizations/societies in Egypt, the Indus Valley, Mesopotamia, China and Japan. The climatic/environmental fluctuation between Bond events 4 and 3 around the Sannai-Maruyama site (5.9-4.2 cal. kyr BP), the largest and well-studied mid-Holocene (mid- Jōmon ) archeological site was investigated. The stable isotopic compositions of benthic foraminifera and the relative abundance of coccoliths, as well as by re-evaluation of Ostracoda and pollen data and alkenone accumulation rate suggested that the Jōmon people living at the Sannai-Maruyama site would generally have enjoyed a warmer climate, that led to improved living conditions between Bond events 4 and 3. Hansaibai (selective preservation or growth) of Castaneacould have supported high population density, resulting in large community at the Sannai-Maruyama site. Cooling episode at 4.2 cal. kyr BP could have caused the decline of chestnut hansaibai, leading to the collapse of the site. A broad northward shift of the westerly jet, in association with strengthened East Asian Summer Monsoon, could have fostered a relatively warm climate at 6.0-4.2 cal. kyr BP, when the Sannai-Maruyama site flourished. Recent archeological results around the Sannnai-Maruyama site suggested no large decline of the population but, instead, a dispersal to the surrounding area at 4.2 cal. kyr BP. It is consistent with ancestral population dynamics for the descendent from Jōmon people based on modern Japanese molecular sequences. In contrast, the immigrants from Far East Asia to the Japanese Archipelago with paddy rice cultivation technology after 2.9 cal, kyr BP experienced severe cold events in eastern China around 4.2 ka event, which were confirmed by ancestral population dynamics. The contrast in response to different climatic events suggests that the mitochondrial DNA haplogroups could be a good archive for paleoclimate and/or paleoenvironments (Kawahata, 2019, Progress in Earth and Planetary Science 6:63, https://doi.org/10.1186/s40645-019-0308-8).