The Jōmon period/culture corresponds to the Neolithic period/culture in Japanese prehistory. The Sannai-Maruyama site (5.9–4.2 cal. kyr BP), the most famous, the largest, and the well-studied mid-Holocene (mid-Jōmon) archeological site inhabited by hunter-gatherers with sedentary lifestyle in northern Japan, started at early Bond event 4 and collapsed at late Bond event 3 (4.2 cal. kyr BP at the boundary between mid-Holocene, Northgrippian, and late-Holocene, Meghalayan), synchronous with the decline of the north Mesopotamian civilization and the Yangtze River civilization in China. Alkenone sea surface temperatures (SSTs), a proxy for early-midsummer SSTs, generally suggest that the early-midsummer SSTs (and atmospheric temperatures (ATs)) at 41° 00′ N, 140° 46′ E, about 20 km north to the Sannai-Maruyama site, located in Aomori Prefecture, peaked around 4.8–4.3 cal. kyr BP and showed minima at 5.9 and 4.1 cal. kyr BP. In spite of some discrepancy in short periods, this feature is consistent with that estimated from the assemblages of Ostracodas. δ18O value of benthic foraminifera of Nonionellina labradorica and Nonionella stella, alkenone production flux, and pollen assemblages could reflect annual-based temperature, which generally suggests that the climate was warmer at 6.0–4.2 cal. kyr BP, which could show the warmer environments at 6.0–5.0 cal. than expected from alkenone SST in early-midsummer. Overall, northward shift of the westerly jet, in association with a strengthened East Asian Summer Monsoon, could cause a relatively warm climate around 6.0–4.3 cal. kyr BP, when the Sannai-Maruyama site flourished. High food production density, by effective hansaibai (selective preservation or growth) in Castanea- and Aesculus-dominated forests, up to one sixth of the rice production density, could have supported high population density, resulting large community at the Sannai-Maruyama site. Cooling episode at 4.2 cal. kyr BP could have resulted in the decline of chestnut hansaibai, leading to the collapse of the site. Recent results from a compiled archeological site map 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, in contrast to those from the immigrants from Far East Asia to the Japanese Archipelago with paddy rice cultivation technology after 2.9 cal. kyr BP, based on modern Japanese molecular sequences.
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