Text S1. Regional Setting
The study area is located in the broad East China coastal plain, which is composed of Subei, Yangtze Delta, Hangjiahu and Ningshao Plains (Fig. S1a). The Yangtze River Delta Plain stretches from Zhenjiang to the river mouth, and is sub-divided into the north and south wings, with the later joining the Hangjiahu Plain.
Crossing the Qiantangjiang River is the narrow, west-east stretching Ningshao Plain. The East China coastal plain, flat with low elevation and a dense network of waterways, is vulnerable to extreme climatic and environmental events, such as typhoons, storms and flooding, and large tidal bores in the case of Qiantangjiang River and Hangzhou Bay (Fig. S2).
The lower Yangtze River region is believed to be one of the most important birth places of domesticated rice, and therefore bears great importance in the development of the Chinese civilization. The discovery of rice remains in Hemudu site (~7,000–5,000 cal BP) (Fig. 1) marks a milestone in China’s agriculture archaeology. The last few decades have witnessed great achievements in the studies of the origin of rice and paleoenvironmental changes retrieved from the site. The discovery of Kuahuqiao site (about 8,000–7,600 cal BP) (Fig.1), in particular, the identification of domesticated rice at that site, confirmed that rice planting in the lower Yangtze Delta might have started as early as 8,000 cal BP (Liu et al., 2007). Much recent study of Shangshan site even dated rice agriculture back to 10,000–9,000 cal BP (Jiang and Liu, 2006). The lower Yangtze Delta region has therefore become a hot spot for the study of rice origin (Pan, 2008; Fuller et al., 2009).
The East China coastal plain has been through tremendous changes since the Last Glacial period due to post glacial sea level changes (Zheng et al., 2018). A great deal of research on the sea level changes along China’s east coast have been carried out by Chinese scholars since last century (Zhao and Tang, 1994). The competing views regarding the history of sea level changes during the Holocene include: 1) Sea level has been rising persistently during the Holocene (Chen and Stanley, 1998); 2) Sea level rose to a high stand (higher than today) around 6,000 cal BP, and then fell to the present level (Zhao and Tang, 1994). In re-constructing high-resolution regional sea level histories, local effects such as gravity subsidence shall be taken into consideration, because they could have played roles in causing discrepancies between different sites. However, in the context of the current study, we believe that global factors such as changes in ice volumes have played a fundamental role in controlling the sea level history on orbital scale. In this regard, it is critical to consult the global sea level curve, even in the case of understanding the Holocene history of sea level changes in east China coast (Zheng et al., 2018).
Lambeck and others (2014), based on compilation of more than a thousand data points worldwide, together with global ice volume modeling, synthesized a global sea level curve since the LGM, which has been regarded as one of the best and most reliable curves so far (Fig. S1b). It can be observed from the curve that sea level rose rapidly starting from the Last Deglaciation. Sea level rise exhibited a step-wise manner, meaning that there were periods during which the rates were relatively high. Sea level rose by 10–15 m from 21,000 to 18,000 cal BP, and remained almost constant during the next 1,500 a (18,000–16,500 cal BP). During the period of 16,500–7,000 cal BP, sea level rose by about 120 m, with an average rate of 1.2 cm/a. It can also be observed that global sea level reached to -5 to -3 m a.s.l. at 7,000 cal BP, and the rate of sea level rise decelerated remarkably.
The regional geomorphic conditions have undergone profound changes before and after post glacial transgression. In brief, during the Last Glacial Maximum (LGM), the East China coastal area including the shelves of the Yellow Sea and the East China Sea was exposed (Fig. S1). A paleosol, locally known as the First Hard Clay layer with a thickness of several meters, was widely developed in the Yangtze Delta, Hangjiahu and Ningshao plains (Fig. S1) (Li et al., 2002), and thus represents the land surface of the LGM time, with buried depths ranging from a few meters to several tens of meters. The Yangtze River and the Qiantangjiang River were incised to a depth of several tens of meters. Flood plain deposits were distributed along the river valley (Li et al., 2002).
During the Last Deglaciation, sea level rose rapidly, the shelf of the Yellow Sea and the East China Sea was merged (Fig. S1). At about 9,500 cal BP, sea water intruded into the low-lands, and gradually occupied much of the region.
During 9,000–7,500 cal BP, sea level rose at a very high rate, the coastal line advancing westward, and the whole region became an estuary.
Sea level behavior and landscape evolution changed dramatically at about 7,500 cal BP, after which the rate of sea level rise decelerated, and the coastal line advanced eastward. In the meantime, the East Asian summer monsoon strengthened which induced erosion in the drainage (Wang et al., 2005). Vast amount of sediment was delivered to the lower reaches to form flood plain and river delta.