This article clarifies the date when paddy field rice cultivation began at the archaeological site located within the Okayama University campus. The analysis used tree ring oxygen isotope dating and radiocarbon dating. When the oxygen isotope ratios of wood used in paddy weirs detected in the 23rd excavation season were examined, peaks could be seen in two places: in the 10th century BC and the 6th century BC. Since the weir was accompanied by mid-I stage Yayoi pottery, which was dated to the 6th century BC using radiocarbon dating, tree ring dating determined that the weir was made from wood cut down soon after 540 BC. Therefore, it was reaffirmed that one of the points of the mid-I stage can be dated to the middle of the 6th century BC.
Next is the age of the early-I stage, when paddy field rice cultivation began on the Okayama Plain. In the Tsuruba area of the Nishikawazu archaeological site in Shimane Prefecture, where paddy field rice cultivation is thought to have begun at the same time as on the Okayama Plain, tree ring oxygen isotope dating of wood accompanied by early-I stage pottery has been reported. The date of 649 BC date means that one of the earliest stages of the early-I period dates to the middle of the 7th century BC. Therefore, we reaffirmed the view that paddy rice cultivation in the Chugoku region, such as Okayama and Shimane, began in the 7th century BC.
It also became clear that the beginning of paddy field rice cultivation in the Chugoku region occurred at a time when the climate that had been the base of cold in the 10th century BC gradually warmed and the relatively humid climate began to turn to arid.