| During the Last Glacial Maximum (LGM), global sea level was located at ca.150m below present position. The river valleys were strongly incised by ca.70-50m at large river mouth area. Sea level started to rise as climate turned warm after LGM, filling the incised valley into a large estuary. Rapid sea level rises occurred during the early Holocene till ca.7,000cal yr BP. Sea-level rise decelerated in the middle Holocene, and the marine invasion weakened as well. Therefore, this enhanced freshwater discharge carried a large amount of terrestrial materials to the river mouth to form a delta plain, including coastal wetlands. The Holocene delta formation had attracted our ancestors to move to the delta coast for agricultural activities, starting to settle down for sedentary livings. The Yangtze River delta of Eastern China coast had witnessed prosperous Neolithic cultures, represented by Majiabang, Songze and Liangzhu Cultures, among which the Liangzhu Culture was recognized as the most developed cultural stage, in terms of site numbers and culture connotations. Although the sea level stabilized and approached nearly to the present mean sea level in the middle Holocene, the delta plain was still very sensitive to subtle sea level fluctuations, storm surges and tidal currents, which affected and restrained the development of the agricultural-based cultures. As the Liangzhu culture progressed to a supreme level of civilization represented by the ancient Liangzhu city and an earth dyke (herein, the Liangzhu city complex) on the Southwest of the Yangtze delta plain, there are key questions to be paid attention to in this study:What was environmental changes on the Yangtze River delta that affected the Liangzhu city complex? How did the Neolithic people adapt to the environmental changes? How was the rise and fall of the Liangzhu city complex linked to these environmental changes?This study focuses on the Liangzhu city complex, which was located at the inner Hangzhou Bay. On the basis of previous study, two sediment cores, LZ and XL, were drilled near the ancient city, as well as three sediment profiles at the earth dyke, i.e. BHN-A, BHN-B and BHN-C. AMS14C dating, together with environmental and climate proxies was applied to sediment samples from both cores. The environment evolution processes of the inner Hangzhou Bay area since the middle Holocene was reconstructed, including sedimentary facies, climate and sea level fluctuations. Then, the human-environment interactions of the Liangzhu cultural stage were discussed, and conclusions were drawn as below:1. Sea level stabilization since the mid-Holocene triggers delta formationBased on our14C datings and the altitudes of the lowest habitant bases, the Holocene sea-level curve of the study area was reconstructed. The sea-level curve revealed a rapid rise from-25m to-5m during9,000-7,000cal yr BP, before approaching to the present sea-level at ca.7,000-6,000cal yr BP. The shoreline could have reached the foothill of Mt. Tianmu during the maximum marine invasion, making the study area a shallow bay environmental setting. This was featured by the fine grain sediment strata with abundant foraminifera. The increasing rainfall enhanced the river discharge to bring not only sediments from the Yangtze River to deposit at the study area, but also sediments from the Qiantang River, turning the former shallow bay into delta plain, as witnessed by transformation from grayish bay sediment to brownish coastal silt of delta plain origin. This provided a habitable base for building the ancient Liangzhu city complex.2. Shoreline and paleo-topography in favor of the Liangzhu city constructionAfter the maximum marine invasion, the shoreline of the study area prograded seawards. The densely distributed river networks of the delta plain to the east of the ancient city recorded a fast progradation of the shoreline after ca.5,000cal yr BP. The extensive tidal flat and salt marsh prevailed during7,000-5,000cal yr BP in the delta plain proper. Several huge tidal openings were linked to the open sea, allowing tidal currents free to invade, making the delta plain extremely vulnerable for the long-term occupation and impossible for large constructions like the ancient Liangzhu city complex. The coastal setting formed at ca.5,000cal yr BP at the LZ core site was earlier than that of XL core, ca.4,300cal yr BP, only5km in distance. The burial depth of coastal sediment between the two cores is>3m (1.1m vs4.3m below the ground surface), showing the LZ site was much suitable for the ancient city construction, in stead of a steep gradient of paleo-topography in the former bay area (Core XL). Moreover, the Liangzhu area located at inner Hangzhou Bay was protected by surrounding mountains and hills from storm surges and spring tides. These together would have given an easy access to freshwater, forests, hunting, fishing, plant gathering, and for defensive protection.3. Warm and humid climate and desalinization of salt marsh catalyzed rice cultivationDuring the mid-Holocene, the climate stayed warm as the Holocene Mega-thermal continued. The vegetation of the study area was dominated by evergreen and deciduous broad-leaved arboreal, reflecting a warm climate condition. Pinus became dominant during the Liangzhu cultural period (5,200-4,000cal yr BP), followed by Quercus, Juglans and Liquidambar in great portions, reflecting a moderate and humid climate condition, which was favorable for rice cultivation and human activities. At5,200-5,000cal yr BP, the extremely high pollen concentration in core LZ, the occurrence of salt-tolerant Chenopodiaceae and foraminifera illustrated a tidal flat environment, which was not the best environment for rice cultivation (as the Poaceae (>40μm) was low). Later, during5,000-4,000cal yr BP, Chenopodiaceae was replaced by Ephedra and Artemisia, freshwater herbs (Typha and Polygonum hydropiper) and Polypodiaceae, as the salt marsh turned into freshwater wetland, making rice cultivation and human activities possible. The pollen record of core LZ showed Poaceae (>40μm) has started to rise since5,000cal yr BP and then roared up after4,800cal yr BP, which fit the growing trend of macro charcoal (>125μm), and peaked at mid-late Liangzhu cultural period, implying the highlight of rice cultivation and human activities at and near the Liangzhu city complex. The content of Poaceae and macro charcoal dropped dramatically in the late Liangzhu cultural period, showing a weakened human occupation which would be linked to the fall of the city complex.4. The belated construction of earth dyke reflected the need for further developmentThe transformation of sediment in core LZ from greyish clay to brownish silt at ca.5,000-4,800cal yr BP shows the coastal formation in the study area. The gray sediment at the base of the Liangzhu city wall was dated at4,800cal yr BP, suggesting the city was built no earlier than4,800cal yr BP, which was close to the age of the city reported by the archaeologists (4,500-4,300cal yr BP). The Optically Stimulated Luminescence (OSL) results of sediment profile BHN-C, which was obtained under the dyke construction, showed the age of the dyke construction at ca.4.11kyr BP, late Liangzhu cultural period. The OSL dating results from profile BHN-A were vertically disordered, showing the feature of artificial dyke construction at this section. Similarly, the anisotropy magnetic susceptibility of profile BHN-A was showing chaotic directions of magnetic declination, reflecting the man-made nature of the dyke. In our study, we suspected the soil used for dyke construction was taken from the adjacent flood plain, which would be over3million m3at least. The results from profile BHN-C was showing a main direction, S-SE, paralleled to the river flow direction from the north, reflecting the characteristics of fluvial plain.On the basis of the OSL dating and the anisotropy magnetic susceptibility measurement at the BHN profiles, we deducted that this part of dyke was constructed by human during the late Liangzhu cultural period, which was later than the city construction. We consider this time difference to be the process of learning from the nature. The study area was warm and humid, and the annual rainfall was concentrated during monsoon season, which was easy to trigger debris flood hazard to threat the city and rice field nearby. After hundreds of years living in that area, the ancient people came to realize the importance of building an earth dyke:not only to defend the flood hazard, but also to impound freshwater for irrigation and city needs.5. The rise and fall of the city complex and the adaptation to environmental changeOur study revealed that the coastal formation started at ca.6,000cal yr BP as the sea level stabilized in the study area. The former tidal flat transformed into freshwater marshland after5,000cal yr BP, which attracted ancient foragers to settle down to develop the Liangzhu city complex, on the basis of the previous2,000years of agriculture-based civilization. The freshwater marshland was fundamental to rice cultivation, which was also crucial to the development of Liangzhu culture. Both macro charcoal and Poaceae data showed the human activity around the city complex peaked at mid-late Liangzhu cultural period before its deterioration. Human activity did not occurred in the study area until East Han Dynasty, as evidenced by macro charcoal and Poaceae data.The earth dyke, built in the late Liangzhu cultural period, was considered as a multi-functional system. It was not only a great effort in fighting against floods, but also used for water restore and irrigation. To build the earth dyke was a positive move in dealing with natural hazard, showing an important leap from adaptation toward modification to the environment. The city complex was abandoned mysteriously at ca.4,000cal yr BP, and the reason was still debatable. In view of the present study, the flourish of ferns and algae after ca.4,000cal yr BP were indicative of expanded water body around the city area, depressing human activities severely. Also, it seems that the cultural abandonment is linked to the expansion of brackish wetlands following marine invasion in the mid-late Holocene. The salt marsh area was twice as large as it is today, making significant habitation impossible on the entire southern flank of Yangtze River delta. This would have limited population growth and decreased human demands, thus implying that environmental drivers were responsible for the demise of the city. |
PDF Full Text Request