Early to middle Holocene rice cultivation in response to coastal environmental transitions along the South Hangzhou Bay of eastern China

摘要

South Hangzhou Bay (SHB) in eastern China is one of the regions where agriculture began in the early Holocene. To better understand how farming-based societies developed in this region, we examined pollen, charcoal, foraminifera and grain size information of a well-dated sediment core (YJ1503) in the Yaojiang Valley (YJV). Pollen assemblages show that before 8600 cal yr BP, Pinus, Quercus, Juglans and Pterocarya woodlands were dominant. During ca.8600-8400 cal yr BP, abundant foraminifera suggest a transient marine incursion. Although the former woodland species re-established during 8400-7600 cal yr BP, freshwater algae and dinoflagellates indicate a transition to brackish environments. Two peaks of charcoal at around 8200 and 7800 cal yr BP, are possible early signs of human occupation that pre-dated the Hemudu Culture. After 7600 cal yr BP, an increase of salt-tolerant herbs including Chenopodiaceae, Cyperaceae and Poaceae ( < 35 mu m) and a decrease of dinoflagellate and foraminifera assemblages, imply a reduction in saline influence. Freshwater wetlands established around 6600 cal yr BP as indicated by increasing Typha and diminishing Chenopodiaceae pollen. This coincided with a sharp decline in Pinus pollen, marked increases of Poaceae pollen and a rise in charcoal suggesting more activity of human communities. Rice cultivation is confirmed by archaeological findings from this interval, with two distinctive periods at ca. 6600-6300 and 5500-5200 cal yr BP. The initial coastal land development started as early as 7800-7600 cal yr BP in the YJV, which was concurrent with that happened at the apex of SHB where the Kuahugiao Culture originated, but rice farming trajectory at these two places differed. In the Kuahugiao area, low salinity coastal settings developed, freshened by the discharge of the Qiantang River,and this area would have been especially suitable for agriculture. In contrast, in the YJV, early rice cultivation was possibly hampered by longer episodes of brackish intrusion during time of lower discharge of the Yaojiang River until a freshwater environment prevailed after ca. 7000 cal yr BP.