A 7000‐year record of environmental change: Evolution of Holocene environment and human activities in the Hangjiahu Plain, the lower Yangtze, China

The Hangjiahu Plain in the lower Yangtze is one of the core areas that sustained the flourishment of the Liangzhu Civilization. This study reconstructed Holocene environmental change on the Hangjiahu Plain based on a sediment core collected from the Tangqi ZK‐3 location situated on the low‐lying Hangzhou‐Taihu region of the Yangtze Delta. We applied OSL dating, grain size analysis, pollen analysis, and magnetic susceptibility to reconstruct Holocene environmental change and compared our data with other published results. Our results showed that (i) before ~7.0 ka B.P., the ZK‐3 core recorded a strong hydrodynamic force, resulting in the widespread deposition of light grayish silt clay or clayey silt in the region. The climate was warm and humid, and the vegetation was mixed evergreen deciduous coniferous forest. (ii) Between 7.0 and 6.0 ka B.P., the hydrodynamic condition in ZK‐3 core became weaker, and the climate remained warm and humid. Although most of the Hangjiahu Plain were still covered by the light grayish silt clay or clayey silt, some higher grounds began to emerge as sea‐level rise slowed, which coincided with the development of the Majiabang culture. (iii) Between 6.0 and 4.5 ka B.P., the deposition of yellowish silty clay indicates a shallow‐water hydrological environment at ZK‐3, as the regional water level was dropping while more land was emerging, which provided a favorable physical environment for the prosperity of the Songze and Liangzhu cultures. The period experienced a drier and cooler climate, with evidence of deforestation. (iv) Between 4.5 and 3.0 ka B.P., the sediments in the ZK‐3 core were dominated by light grayish clay, indicative of a return to a deep‐water environment with a prolonged waterlogging condition. The climate remained dry and cool with further deforestation. However, the widely distributed yellowish silt clay suggests frequent floods in the region, resulting in a sharp reduction of settlement sites and the eventual decline of the Liangzhu Civilization.

frequent floods in the region, resulting in a sharp reduction of settlement sites and the eventual decline of the Liangzhu Civilization.

K E Y W O R D S
geoarchaeology, geomorphic evolution, Holocene, human activities, lower Yangtze River

| INTRODUCTION
The Liangzhu Civilization (5300-4300 B.P.) is now being considered one of the earliest urban centers in prehistoric East Asia. It enjoyed an unprecedented level of agricultural intensification and craft development, which sustained the construction and operation of its enormous urban center and hydraulic system (Liu et al., 2014;Z. H. Zhang, 2004). As one of the core distribution areas of the Liangzhu culture, the southern Taihu Lake plain, or to be more specifically, the Hangjiahu ("Hang" refers to "Hangzhou City," "Jia" refers to "Jiaxing City," and "Hu" refers to "Huzhou City") Plain in the lower reaches of the Yangtze River is highly susceptible to Holocene climate and environmental vagaries caused by changes in East Asian monsoon intensity (An, 2000;Sun & Chen, 1991;Wenxiang & Tungsheng, 2004) and sea level fluctuations (Z. Y. Chen & Stanley, 1998;Song et al., 2013;Xie & Yun, 2012;Zhao et al., 1979;Zong, 2004).
Recently, there has been a growing interest in the scientific quest for the environmental factors that might be linked to the rise and fall of the Liangzhu Civilization. Previous research has demonstrated the close interaction between cultural evolution and environmental change in the lower Yangtze. Some scholars have started to elaborate how humans responded to Holocene relative sea-level change and environment fluctuations (Bird et al., 2010;Innes et al., 2014;Z. Wang et al., 2012Z. Wang et al., , 2013Wu, 1988;Zhuang & Du, 2021). Several studies have found that human activities increased when the relative sea level was stable or dropping (He et al., 2018;Shi et al., 2011;Z. Wang et al., 2013), while such activities became weaker when the relative sea level was rising in the Yangtze Delta region (Z. Chen et al., 2008;Shi et al., 2011;Stanley & Chen, 1996;Wu et al., 2014;Zong et al., 2011a). Since rice farming played a predominant role in the subsistence economies of societies in the lower reaches of the Yangtze River, it has been suggested that changes in the hydrological environment would affect productivity of rice farming and thus have a profound impact on cultural development and succession in the region (He et al., 2018;Innes et al., 2014;Jin et al., 2018;Patalano et al., 2015;Zheng et al., 2011;Zong et al., 2007Zong et al., , 2011b.
Although the causes of the rise and fall of the Liangzhu culture are still under heated scholarly debate (Shi, 1993;Shi et al., 2011;Yu et al., 2000;Z. K. Zhang et al., 1998Z. K. Zhang et al., , 2005Zhu et al., 1996), there is a growing consensus that hydrology and geomorphology are some of the key factors that were directly related to the developmental discourse of the culture (He et al., 2021;Ling et al., 2021;Shi et al., 2011;Stanley et al., 1999;Wu et al., 2014; and thus merit more 2 | REGIONAL SETTING

| Environmental and geomorphological settings
The Hangjiahu Plain in the northern part of Zhejiang Province is located on the southern edge of the Yangtze River Delta, south of Taihu Lake, and north of the Qiantang River in the Hangzhou Bay, with an area of about 6400 km 2 and abundant water resources ( Figure 1).
The regional terrain is generally low and flat, between 2 and 4 m (above the sea level, same below), with a dense network of rivers and lakes on the alluvial plain. The lowest region in the plain is the northeast, 2-3 m. The Qiantang River and Hangzhou Bay in the southwest are on higher positions, 3.5-5.5 m. The landform types mainly include low hills, mounds, and plains. As the outliers of the Tianmu Mountains, the low hills (100-200 m) are distributed in the eastern part of the Hangjiahu Plain. Mounds with an elevation of 10-100 m are scattered in the southwestern part of the plain. Lowlying plains, less than 10 m, are, however, the predominant landform of the region (Figure 1).
The dense water network in the region includes a complex canal system, the East Tiaoxi River and the West Tiaoxi River, and other natural and artificial waterbodies. The canal system generally flows from southwest to northeast. The East Tiaoxi River originates from the southern piedmont of the Tianmu Mountain and flows east from Lin'an County to merge with the South Tiaoxi River, the North Tiaoxi River, and the Middle Tiaoxi River before flowing east to join the East Tiaoxi River at Pingyao, which then enters Taihu Lake via Deqing and Wuxing to the north. The West Tiaoxi River joins the waters in the northern part of the Tianmu Mountain and meets the East Tiaoxi River via Xiaofeng, Anji, and Huzhou (Xu, 2012;C. M. Yan et al., 1959).
The Hangjiahu Plain traditionally belonged to the so-called Jiangnan (South of the Yangtze River) region, which has experienced several marine transgression and regression since the Pleistocene.
The sediments in the region are dominated by alluvium, mainly of silty clay, clayey silt, and silt, from the Yangtze River, Qiantang River, and lacustrine deposits. Below the Holocene strata are the Pleistocene hard and dense yellowish brown and brownish yellow clay layers rich in ferromanganese nodule and grayish iron-depletion white bands.
There is an evident sedimentation gap between the two layers at the interface between Pleistocene and Holocene (X. L. Chen, 1991;Q. S. Yan & Huang, 1987).
The Holocene climate change in the lower Yangtze River region has been well studied, synchronous with major global climate events.
The mid-Holocene climate was humid and wet as consistently shown by pollen and geochemical studies (Atahan et al., 2008;Yi et al., 2003;Yu et al., 2000). These records also show evidence of possible drierand-cooler climate events at 5.5 and 4.0 ka B.P., respectively (Y. Li et al., 2010;Yu et al., 2000).
The environment of the Hangjiahu Plain is profoundly influenced by sea-level changes. Despite the persisting scholarly disagreement on Holocene sea-level fluctuations, especially on if there was a mid-Holocene sea-level highstand in current studies (Bird et al., 2007(Bird et al., , 2010Z. Y. Chen et al., 1997;Y. Wang, 1989;Zhu et al., 2003), it is generally agreed that the early-Holocene sea level rose rapidly after the last glacial period, and the rate of sea-level rise slowed down until about 7.5-7.0 ka B.P., with the sea level close to the present level ( Figure 2) (Z. Y. Chen & Stanley, 1998;Lambeck et al., 2014;Song et al., 2013;Xie & Yun, 2012;J. Zhang et al., 1982;Zhao et al., 1979;Zong, 2004). This marked a large-scale transition from the marine to terrestrial sedimentation environment. After 7.0 ka B.P., the sea level continued to fluctuate, but with a smaller magnitude. There is no a universally agreed sea-level fluctuation curve for the late Holocene, but according to some studies, the sea level dropped around 6-4.5 ka B.P. (He et al., 2018;Z. Wang et al., 2012Z. Wang et al., , 2013 before rising again around 4.5 ka B.P. (Stanley et al., 1999;Wang et al., 2018;Xie & Yun, 2012;H. R. Yang & Xie, 1984;Zheng et al., 2011). The establishment of coastal barrier ridges (Z. Y. Chen & Stanley, 1998;Q. S. Yan & Huang, 1987) and a rapid sedimentation rate (Q. S. Yan & Huang, 1987) suggest that during the late Holocene, the region was affected by rising groundwater as a result of sea level rise (Zong et al., 2011b) (Figure 2). The Majiabang culture (7.0-6.0 ka B.P.) was mainly distributed in the area around Taihu Lake, but its influential zone reached as far south as the Hangzhou Bay, as far north as the Jianghuai region, and as far west as the Ningzhen region. Rice farming was already an important economic strategy, with the cultivated rice remains being found at sites such as Caoxieshan, Songze, and Luojiajiao (Z. H. Zhang, 2004).  of stone and jade tools, and textile production were all developed to a high degree in the Liangzhu culture. Rice farming became overwhelmingly predominant in the subsistence economy as rice remains have been found at many sites . The rice grains were very close to the present-day japonica rice with stable morphological characteristics . The late Liangzhu period rice paddies were found at the Maoshan site in Yuhang, with irrigation canals, river channels, roads, water outlets, and an east-west ditch separating the paddies from the settlement (Zhuang et al., 2014).

| Archaeological background
These findings suggest that rice production was significantly intensified at this time. Stone plows were also commonly found at Liangzhu culture sites. Although their exact functions remains to be further outside of which was an enormous hydraulic system (Liu et al., 2014).
There were few remains excavated from the early Qianshanyang culture. The characteristics of these remains still showed a continuation of the late Liangzhu culture, but some distinctive features of the Qianshanyang culture had already appeared. However, while the late Qianshanyang culture pottery was rich in variety and significantly influenced by cultures from other regions, the main features remain the same (Guo, 2018).
The Guangfulin culture (4.2-4.0 ka B.P.) was distributed in the area around Taihu Lake. Archaeological studies show that the Guangfulin culture was distinctively different from the Liangzhu culture and was instead strongly influenced by cultures outside theTaihu Lake region. This phenomenon is described by some scholars a "cultural replacement" (J. Chen, 2006;Song et al., 2008).
Contrary to this, the Maqiao culture (3.9-3.2 ka B.P., contemporary with the Xia-Shang period in the Central Plains) displayed both the Liangzhu culture elements and influences from the Yellow River basin cultures far in the north (Gao, 2005;Song, 1999;Song et al., 2002). The Maqiao culture sites are mainly distributed in the area surrounding Taihu Lake and in the northeastern of Zhejiang Province, south of the Hangzhou Bay.

| Materials
To obtain continuous Holocene sedimentary sequences in the west central Hangjiahu Plain for the reconstruction of regional environment, we conducted a drilling survey in Tangqi

| Methods
In total, 10 samples were collected from different layers of the ZK-3 borehole for OSL dating. The OSL dating was measured in the OSL were obtained from all the samples following the conventional pretreatment method (Murray & Olley, 2002). The equivalent dose (De) was measured by the single-aliquot regenerative-dose method (SAR) (Murray & Olley, 2002). The contents of U, Th, and K were measured by the neutron activation method (NAA) in the Analysis and Testing Research Center of Beijing Institute of Nuclear Industry and Geology. The water content was with a standard deviation of 5%.
The age-depth model for the cores was created using the Bacon agedepth model.
Grain size composition and distribution can directly reflect sedimentation environments. The particle size analysis was performed at the Laboratory of Surface Process Analysis and Simulation, College of Urban and Environmental Sciences, Peking University. The instrument used to measure grain size was a Mastersizer-2000 laser particle size meter manufactured by Malver, UK, with a resolution of 0.15φ and a measurement range of 0.02-2000 μm and a relative error of <3%. Particle size analysis was calculated by the Folk and Ward formula (Shepard, 1954).
Sixty-five samples were processed for pollen identification at the Laboratory of Environmental Evolution and Ecological Construction, Hebei Normal University. Pollen was extracted by adopting the conventional method using HCl-NaOH-HF treatment (Faegri et al., 1989). Pollen was counted under a Carl Zeiss AX10 optical biological microscope at 400 times magnification. The number of pollen counted for each sample reached more than 300. For a few samples with lower pollen content, at least 150 pollen grains were counted. The species were identified based on Vos and de Wolf (1993) and Zong and Sawai (2015).
The magnetic susceptibility indexes chosen for the analysis include low-frequency magnetic susceptibility (χ lf ), high-frequency magnetic susceptibility (χ hf ), and frequency-dependent magnetic susceptibility (χ fd% ). Low-frequency magnetic susceptibility (χ lf ) and high-frequency magnetic susceptibility (χ hf ) were determined by subtracting the average of the pre and post background values from the magnetic susceptibility measured by the Laboratory for Earth Surface Processes of Peking University. To reduce the effect of measurement errors, each sample was tested six times for highfrequency magnetic susceptibility and low-frequency magnetic susceptibility, and the average value was taken as the final measurement value. The value of the frequency-dependent magnetic susceptibility (χ fd% ) is calculated as follows:

| OSL dating
Ten OSL dates from the ZK-3 borehole profile are shown in Table 1.
The age-depth model for the cores corrected by the Bacon age-depth model is shown in Figure 4.  1.87φ < So < 2.57φ, and a skewness index of −0.32φ < Sk < −0.11φ.
An obvious fluctuation occurs at the depth of about 100 cm (~3 ka B.P.) suggesting a more obvious influence by human activities and a lacustrine shallow-lake environment.

| Magnetic susceptibility analysis
The magnetic susceptibility can reflect the variation of the relative content of magnetic minerals in sediments, especially ferromagnetic minerals (Thompson et al., 1980;Zhisheng et al., 1993). The magnetic susceptibility is widely used in the study of Quaternary loess, palaeosols (Lv et al., 1994;Wang & Dong, 1996), and lake sediments

| Pollen analysis
Of the 65 pollen samples from the ZK-3 borehole profile, 71 pollen species were identified, including 24 arboreal types, 10 shrub types, and 37 herbaceous types. Abundant fern spores were also identified.  Further synthesizing these results with published data allows us to refine the resolution of our reconstruction of the Holocene climate in the region. The climate between 8.0 and 7.0 ka B.P. was warm and humid (Yi et al., 2003), and the vegetation was mixed evergreen deciduous coniferous forest (Yi et al., 2003). Between 7.0 and 4.5 ka B.P., while the climate enjoyed a warmest and most humid period during the Holocene (Ma & Tian, 2010;Qu et al., 2000;, it was also punctuated by a dry and cool event around 5.5 ka B.P. But it should be noted that, despite this climate event, the mixed evergreen deciduous broad-leaf forest was maintained due to the overall optimal water and heat conditions in the region (Shi et al., 2011). During 4.5-3.0 ka B.P., the vegetation was dominated by a mixed evergreen deciduous broadleaf forest. The trend of a dry and cool climate was obvious, and the forest vegetation was disturbed to a large extent by human activities (Ma & Tian, 2010;Shi et al., 2011). From 3.0 ka B.P., the climate saw some evident short-term fluctuations (G. F. Yang et al., 2008). The pollen records from the profile at the Guangfulin site showed a significant decrease in arboreal species and a prominent increase in Chenopodiaceae and Artemisia postdating the late Liangzhu culture, which must have been associated with a gradual deterioration of the regional climate while the intensity of human activities was significantly reduced during this time (C. H. Li et al., 2009).

| Evolution of geomorphology and hydrology in the Hangjiahu Plain
The sedimentary sequence from the ZK-3 borehole also offers direct evidence for the changing hydrologic and geomorphic environments in the region. Comparing our results with the lithological characteristics and dating results of other published profiles in the region (Figures 1 and 8 higher terrains began to emerge as the sedimentation process stopped, such as at the Liangzhu West section (Shi et al., 2011), the E2 and T1 sections (Zong et al., 2011b), and the LZ1501 section (Ling et al., 2021). Although most of the area was still in the tidalterrestrial environment, such as FQT and ZK-3, the depositional characteristics, smaller grain size, and decreasing magnetic susceptibility also indicate further deepening of the lakes and weaker hydrological conditions than the previous phase. This might be caused by the slow rise of the relative sea level from about 7.0 ka B.P.
The sedimentation environment changed during this stage. The sediments were dominated by yellowish brown clayey silt with horizontal beddings, which indicated a weak hydrological condition and a shallow-water environment as an even larger area emerged (Wang & Liu, 1996). At the Yujiashan site, for instance, the Liangzhu cultural layer (5.3-4.3 ka B.P.) overlaid on the yellow silt, and at the Liangzhu West, Shanlongdi, Hulinmiao, and Maocaodi locations too, the Liangzhu cultural layer sat directly on top of the light grayish silt (Shi et al., 2011). This is also consistent with previous studies that most of the Hangjiahu Plain was above the sea level by this stage (Yu et al., 2000). It seems that the backwater effect of regional rivers was weakened by the slight decrease in the sea level around 6.0-4.5 ka B.P. (Figure 2) (Xie & Yun, 2012;H. R. Yang & Xie, 1984;J. Zhang et al., 1982;Zhao et al., 1979), leading to the drop of the water level of regional rivers and a reduction of waterbodies.
The sediments at the ZK-3 core being deposited during 4.5-3.0 ka B.P. were of light grayish clayey silt, with a few ferromanganese nodules in the upper part and iron-depleted blackish gray stripes in the lower part, pointing to a lacustrine sedimentation environment.
Yellowish clayey silt is commonly found in the Hangjiahu Plain during this period (Figure 8), such as in the LZ1501 and LZ1601 profiles (Ling et al., 2021) and the T4 and ZX-1 sites (Zong et al., 2011b). At some locations, the yellowish clayey silt directly covered the Liangzhu cultural layer, such as at the Yujiashan profile, but also at the Maoshan profile (Jin et al., 2018), the Liangzhu West, Shanlongdi, Hulinmiao, Beishanche, and Yanjiaqiao profiles (Shi et al., 2011). This stratigraphic order is widely became suitable for rice farming, and some sites even saw the construction of ditches and wells, which are recognized as the earliest irrigation systems (Ding, 2010). However, as the range of waters was still large, fishing, hunting, and gathering continued to be important sources of food production in this period (Z. H. Zhang, 2004).
Between the Songze culture (6.0-5.3 ka B.P.) and the Liangzhu culture (5.3-4.3 ka B.P.) period, the water level of rivers and lakes in the Hangjiahu Plain dropped substantially. The sedimentation environment in low-lying areas became a shallow water condition, while a much larger area emerged, which created an unprecedentedly conducive environment for the development of the Songze and Liangzhu cultures. The number of Songze culture sites increased to 91 (Figure 9). The growth was most notable in the northeastern part of Taihu Lake, and interestingly, the number of settlements in the Songze culture period on the Hangjiahu Plain was even less than the previous period, with only 31 sites (Figures 9   and 10b). In the Liangzhu culture period, however, the number of settlements increased dramatically to 312 (Figures 9 and 10c), and they were concentrated in the Hangjiahu Plain, with almost 220 sites being found here. In particular, there was only one settlement in the piedmont area of the western mountains during the Songze culture period. This for the decline of the Liangzhu culture are widely discussed (Shi et al., 2011;Shi, 1993;Yu et al., 2000;Y. F. Zhang et al., 2005; Z. K. Zhang et al., 1998;Zhu et al., 1996). Many studies support the viewpoint that the decline of the Liangzhu culture was at least partly related to sea level change (Bird et al., 2010;He et al., 2018;Innes et al., 2014;Z. Wang et al., 2012Z. Wang et al., , 2013Wu, 1988), but evidence to critically evaluate such claims has not been available.
As shown by Figure  R. Yang & Xie, 1984). In the low-lying deltaic plains like the Hangjiahu Plain, the impact of the persistent and widespread flood on the Neolithic culture was catastrophic (Stanley & Chen, 1996;. The frequent floods made most of the settlements of the Liangzhu culture no longer suitable for human occupation.
Moreover, such large-scale floods also severely affected rice cultivation, without which it was impossible to support the subsistence needs of a large population in large ancient cities as well as normal settlement sites. These eventually led to the decline of Liangzhu culture.

| CONCLUSION
The climate and vegetation history in the Hangjiahu Plain was reconstructed by pollen and magnetic susceptibility results of this study. Between 8.0 and 4.5 ka B.P., the climate was warm and humid, and the vegetation was mixed evergreen deciduous coniferous forest, and the~5.5 ka B.P. event was shown during this period. 4.5-3.0 ka B.P., the trend of climate drying and cooling was obvious, and the forest vegetation was significantly disturbed by human activities.
From 3.0 ka B.P., the climate further deteriorated.
Based on the sedimentary records at the ZK-3 and evidence from other studies in the Hangjiahu Plain, we established the regional environmental process. Before~7.0 ka B.P., the Hangjiahu Plain was a sea-land transition environment. Between 7.0 and 6.0 ka B.P., the hydrodynamic condition became weaker, and some higher grounds in the Hangjiahu Plain began to emerge. Between 6.0 and 4.5 ka B.P., the regional water level was dropping, and more lands were emerging. Between 4.5 and 3.0 ka B.P., the ZK-3 core returned to a deep-water environment, and floods occurred frequently in the Hangjiahu Plain.