Foraminiferal evidence for Holocene environmental transitions in the Yaojiang Valley, south Hangzhou Bay, eastern China, and its significance for Neolithic occupations

The transition from a marine-influenced environment to a coastal plain setting during the Early to Middle Holocene was crucial for early human occupation along the eastern China coast. Here, detailed foraminiferal analyses were completed for two sediment cores (YJ1505 and YJ1508) retrieved from the Yaojiang Valley (YJV) along the southern Hangzhou Bay coast of eastern China. Brief environmental changes in the YJV during the Early-Middle Holocene were recovered on the basis of radiocarbon chronology. The assemblages and Detrended Correspondence Analysis (DCA) results of the foraminifera of both cores indicated that the YJV was evidently under a marine influence from 9200 to 7900 cal yr BP; prior to that, it was a fluvial incised valley. The increasing trends in shallow marine foraminifera and the planktonic/benthic foraminifera (P/B) ratio showed that a transgressive period possibly occurred in the valley ca. 9200-7900 cal yr BP, and their following decreases suggested that a rapid regression process occurred thereafter. The disappearance of foraminifera and formation of a peaty layer in core YJ1505 ca. 7600-6500 cal yr BP implied a brief environmental transition to a limnetic wetland setting in the valley centre, which would have been attractive to early human settlers. The disappearance of foraminifera in core YJ1508 at the valley's eastern entrance suggested that most areas of the valley were located far from marine influence after ca. 6250 cal yr BP. Then, the limnetic wetland setting transitioned into a coastal plain environment, attracting more people to the valley. Such a transition in the YJV would have provided more opportunities for ancient people and possibly catalysed the development of the Hemudu culture (HC) for approximately two millennia.