Introduction

Understanding the variability of the East Asian summer monsoon (EASM) is vital for predicting global climate change because this fluctuation is a significant part of the global climate system (Wang 2006; IPCC 2014). The variability of the Holocene climate has attracted significant attention because its most recent phase resembles the modern climate. Oxygen isotope records from ice cores indicate a generally stable climate during the Holocene (Dansgaard et al. 1993); however, studies such as those from Greenland ice cores and the North Atlantic ice-raft detritus records have identified abrupt Holocene climate changes (O'Brien et al. 1995; Bond et al. 1997). Sudden Holocene climate events have been extensively studied because of their rapid changes, large fluctuations and long durations (Alley et al. 2003; Chen et al. 2015). These events include those dated to ca. 8.2 ka (Park et al. 2019; Waltgenbach et al. 2020), ca. 5.5 ka (Wu et al. 2018), ca. 4.2 ka (Park et al. 2019; Pleskot et al. 2020) and ca. 2.8 ka cal bp (Park et al. 2019). The dates of these changes to the EASM and the areas affected by them have been studied (for example, Wang et al. 2005; Chen et al. 2015; Porter et al. 2019); however, such events, particularly those occurring at ca. 7.5–7.0 ka cal bp and their effects on the vegetation are yet to be clearly understood. Therefore, reliable palaeoclimatic records are needed with clear proxy evidence of the monsoon and with a good chronology.

The subtropical regions of southern China have numerous areas of particular vegetation diversity (Huang et al. 2016), which are particularly suitable for Holocene vegetation studies (Yue et al. 2012; Ma et al. 2016; Zhao et al. 2016, 2017). Southeastern subtropical China is a critical area for studying the variability of the EASM during the late Quaternary period because it lies in the path of the summer monsoon circulation (Wang et al. 2005). Major peat-based Holocene palaeoclimate research up to now in Zhejiang province has mainly studied sediment particle (grain) size, organic matter content and magnetic susceptibility (Li et al. 2019). Few studies have analysed the intensity of the abrupt Holocene climate events in southeast China, their mechanisms and the resulting responses of the ecosystem to them.

Pollen analyses of peat sediment in southeast China can help reconstruct Holocene EASM variability and determine its impact on the vegetation. A previous pollen study in this region together with analyses of sediment particle sizes and geochemicals was done on cores from Wangdongyang (WDY) peat bog, Zhejiang province, but it was not well dated or of high resolution (Gu et al. 2016). Our age model, which is supported by 13 accelerator mass spectrometry (AMS) 14C dates, suggests that the combined records from this site cover the period from ca. 8.3/8.1 ka cal bp to the present day. The period ca. 7.5–7.2 ka cal bp shows significant reductions in the content of Si, Al, Ti and Fe, along with a reduction in the average sediment particle size and percentages of evergreen broadleaved trees, which suggest a cooler, drier climate. This study aims to explore Holocene climate change, particularly the abrupt climatic events that occurred at ca. 7.5–7.0 ka cal bp and the natural variability of the EASM. Our study provides new understanding of the vegetation dynamics and their response to Holocene climate change in the East Asian monsoon region.

Materials and methods

Regional setting

The Wangdongyang (WDY) peat bog (cores WDY1: 27° 40′ 48″ N, 119° 38′ 15″ E, 1,303 m a.s.l.; WDY2: 27° 40′ 58″ N, 119° 38′ 15″ E, 1,300 m) is located in the Shangbiao forest in Jingning county, which lies in the southern part of Zhejiang province in southeastern China (Fig. 1b, c). It is the most developed inland high altitude swamp in eastern China and the climate there is warm and moist, and it is controlled by the monsoon circulation (Domrös and Peng 1988). The mean annual temperature is 12 °C and the mean temperatures of January and July are 1.8 °C and 21.8 °C respectively (Wen et al. 2018). There is significant rainfall, with a mean annual precipitation of 2,067 mm (Wen et al. 2018) and the abundant rains occurring from June to September contribute to the moist regional climate. Most rainfall occurs in June in response to the start of the fully developed EASM. The regional geology shown by the main exposed strata in the region is of pre-Sinian metamorphic, Jurassic volcanic and Cretaceous sedimentary rocks, along with overlying loose Quaternary sediments (Gu et al. 2016). The study region lies within the subtropical climate zone (Domrös and Peng 1988) and has a diverse and rich forest vegetation (Gu et al. 2016). The majority of our study area is covered by forests, open grasslands and cultivated land. It can be described as a mountainous terrain, with a favourable climate and a relatively low population density. The vegetation in this region includes evergreen and deciduous broadleaved mixed forests with some conifers (Fig. 1c). The main trees found in these forests are Pinus massoniana Lambert, Cunninghamia lanceolata (Lamb.) Hook (Taxodiaceae-type) and the bamboo Phyllostachys edulis (Carrière) Houzeau. Evergreen broadleaved species include Cyclobalanopsis glauca (Thunb.) Oerst., Castanopsis eyrei (Champ.) Tutch., C. sclerophylla (Lindl.) Schott., C. carlesii (Hemsl.) Hay., and Ilex chinensis Sims. Deciduous broadleaved species are Quercus fabri Hance, Q. acutissima Carruth., Q. variabilis Bl., Q. glandulifera Bl., Liquidambar formosana Hance and others (Ministry of Forestry 1993; Lu and Ye 2014). The plant communities are rich and diverse, and the main trees grow in several different types of woodland; Alnus trabeculosa Hand.-Mazz. is one of the most abundant trees in the wetland of this region (Gu et al. 2016; Wen et al. 2018).

Fig. 1
figure 1

a Study locations, 1 Qinghai lake (An et al. 2012), 2 Daihai Lake (Xiao et al. 2004), 3 core from northern Yellow Sea (Nan et al. 2017), 4 Dajiuhu peat bog (Zhu et al. 2010), 5 Lantianyan peat bog (Ma et al. 2016), 6 Shuizhuyang (SZY) core in the Pingnan peat bog (Yue et al. 2012), 7 Daiyunshan peat bog (Zhao et al. 2017), 8 Dongge Cave (Wang et al. 2005), 9 Huguangyan maar (Wang et al. 2007). b Digital elevation map of the study area, c vegetation map (Editorial Committee of Vegetation Map of China 1980), and d photo of Wangdongyang (WDY) site

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For our study, we analysed two cores, about 300 m apart, obtained from the centre of Wangdongyang which reached the bottom of the peat bog. We used a manual corer to do this sampling, obtaining two cores from several borings which were collected in PVC tubes 60 mm in diameter and 50 cm in length. Both sediment cores were sub-sampled in the laboratory at intervals of 1 cm, giving totals of 105 and 61 samples from WDY1 and WDY2, which were analysed for pollen at 2 cm intervals. Particle size analysis was done on 98 samples from WDY1, at 2 and 4 cm intervals and 184 samples at 1 cm intervals were analysed using X-ray fluorescence (XRF) for determining the elemental composition.

Dating

Nine plant macrofossil and four peat samples from WDY1 and WDY2 were submitted to Beta Analytics (USA) for AMS 14C dating (Table 1). Some dates for WDY2 were also obtained from Wen et al. (2018). All 14C dates were converted to calendar years with the IntCal20 calibration curve (Reimer et al. 2020). Bayesian chronological modelling is the most frequently used way of studying the dates and it integrates the data into a coherent framework and gives estimates of age–depth relationships objectively and optimally (Bronk Ramsey 2009). This was done using BACON v. 2.5.0 (Blaauw and Christen 2011) for the chronology (Fig. 2), also using R v.3.3.2 (R Core Team 2020).

Table 1 Details of the AMS dated samples and calibrated ages
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Fig. 2
figure 2

Age–depth model of cores WDY1 and 2. The 95% probability date ranges are within the grey area

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Proxy data analysis

The pollen and sediment particle size analyses were done at the School of Geography and Ocean Science at Nanjing University. Pollen was extracted according to the standard procedure (Fægri and Iversen 1989) and micro-charcoal particles were also studied in the pollen preparations. The sediment particle sizes were measured with a Malvern Mastersizer 2000 analyser, based on the method described by Lu and An (1998). We transformed the particle size distribution into various end-members in order to characterize the depositional environments more accurately (ESM 1; Weltje and Prins 2003). X-ray fluorescence (XRF) studies were done with an Avaatech core scanner at the XRF Laboratory at Nanjing University School of Earth Sciences and Engineering, with continuous high resolution (1 cm) micro-XRF element scanning. Principal component analysis (PCA) was done on the pollen percentages and geochemical elements with CANOCO v. 4.52 (Ter Braak and Smilauer 2003). Detailed descriptions of the procedures are in ESM 1, the data sets in ESM 2.

Results

Peat stratigraphy and age modelling

In WDY1, we identified seven distinct units of sediment, including yellowish-brown peat and brownish peat with silt (Fig. 3a); in WDY2, we identified six distinct units (Fig. 3b). No obvious sediment hiatuses or disturbances were noticed when we subsampled. The majority of the AMS dated samples in WDY1 and WDY2 were in chronological order in both cores, suggesting that the dating was good (Fig. 2; Table 1). Both records started in the Mid Holocene, with median bottom sample ages of ca. 8.3 and 8.1 ka cal bp. Sedimentation rates were moderately high in the bottom 40 cm of WDY1; the lowest sedimentation rates were at 169–140 cm, and the highest ones at 140 cm from the top of the peat section (Fig. 2). In WDY2, the lowest sedimentation rates were at 122–99 cm, moderately high at 99–69 cm and the highest rates at the top, at 69–18 cm (Fig. 2).

Fig. 3
figure 3

a WDY1 pollen diagram, and b WDY2 pollen diagram; pollen percentages of the total sum of terrestrial pollen, including trees, shrubs and terrestrial herbs. Percentages of wetland and aquatic herbs, ferns and algae are based on the total sum of all palynomorphs and algae

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Pollen results

We identified 99 pollen and spore types consisting of 55 trees and shrubs, 31 herbs, 6 wetland and aquatic taxa, 7 ferns and 6 types of algal palynomorphs. The main results are presented in simplified pollen diagrams in Fig. 3. The WDY1 pollen diagram (Fig. 3a) is divided into five major pollen assemblage zones whose key features are described below.

Zone 1 (210–169 cm; ca. 8.3–6.7 ka cal bp) is defined by high and stable arboreal pollen (AP) percentages (based on the total sum of terrestrial pollen, including trees, shrubs and terrestrial herbs), mainly evergreen Quercus, Alnus and Taxodiaceae, and relatively low percentages of dry land herbaceous pollen, wetland and aquatic herbs and fern spores throughout the zone. Poaceae and Cyperaceae are the main non arboreal pollen (NAP) types. The total pollen concentrations are relatively high at approximately 129,000 grains g−1 but concentrations of charcoal are very low, mean 3,000 fragments g−1.

Zone 2 (169–124 cm; ca. 6.7–0.9 ka cal bp) is dominated by evergreen Quercus which is also the most abundant taxon throughout the record. Alnus, Ilex and Taxodiaceae are the dominant pollen contributors in this zone. The AP percentages are high, but slightly lower than those in the other zones. The NAP percentages are low, and the values of fern spores relatively high. The pollen concentration is at its highest, with an average of 480,000 grains g−1. There is a charcoal concentration peak of > 77,000 fragments g−1 at approximately 141 cm from the surface.

Zone 3 (124–92 cm; ca. 0.9–0.5 ka cal bp) is characterized by a dominance of tree pollen (66–91%). The amount of Taxodiaceae pollen is negligible, but Ilex pollen is significantly higher at 15–40% of the total pollen assemblage. The values of evergreen Quercus are lower, but Alnus has a peak of approximately 30%. Percentages of dry land herbs and ferns are higher than in previous zones, while those of wetland and aquatics herbs are lower. The total pollen and charcoal concentrations are moderate to low, but are relatively higher in the upper part of this zone (ca. 0.5 ka cal bp).

Zone 4 (92–50 cm; ca. 0.5–0.25 ka cal bp) is distinctly different from the others because of its markedly lower AP percentages, in which evergreen Quercus and Alnus remain dominant. Although the percentages of wetland and aquatic herbs remain low, those of dry land herbs and ferns are higher in the upper part above 75 cm (ca. 0.4 ka cal bp). The total pollen concentration is relatively low (approximately 70,000 grains g−1), and there is a distinct charcoal concentration peak of > 68,000 fragments g−1 at approximately 83 cm.

Zone 5 (50–0 cm; ca. 0.25–0 ka cal bp) is also characterized by a dominance of evergreen Quercus (4% mean), but other trees including Castanea, Alnus and Ilex are relatively scarce. The highest proportions of Poaceae and Pinus occur in this zone. Fern spores are relatively abundant, and the percentages of wetland and aquatics and dry land herbs are notably higher. The total pollen concentration is very low (approximately 57,000 grains g−1), and there is a moderate charcoal concentration peak of 26,000 fragments g−1 at about 33 cm.

The WDY2 pollen diagram is divided into three major zones which are described below (Fig. 3b).

Zone 1 (122–56 cm; ca. 8.1–1.0 ka cal bp) is characterized by stable and high AP percentages (79.5% mean), mainly evergreen Quercus, Alnus and Taxodiaceae and relatively small amounts of dry land herbs, wetland and aquatic herbs and fern spores. Ranunculaceae is a major NAP taxon found in this zone. The total pollen concentrations are high (approximately 129,000 grains g−1) and the charcoal concentrations are very low.

Zone 2 (56–46 cm; ca. 1.0–0.9 ka cal bp) is dominated by Alnus pollen, as well as evergreen Quercus, Ilex and Castanopsis. The AP percentages are very high, but slightly lower than those in zone 1. The NAP percentages are low and the fern spores relatively high. Pollen concentrations are the highest in this zone (average of 1,910,000 grains g−1). The > 50 µm and < 50 µm charcoal concentration peaks at an approximate depth of 48 cm are 133,000 fragments g−1 and 937,000 fragments g−1, respectively.

Zone 3 (46–0 cm; ca. 0.90 ka cal bp) has significantly lower percentages of AP, mainly Ilex, Alnus and Castanopsis. Taxodiaceae pollen is present in small amounts, but Poaceae is higher than in the other zones at 15–60% of the total pollen assemblage. Evergreen Quercus is lower, while dry land herbs, wetland and aquatic herbs and fern spores are higher; Pinus pollen peaks at approximately 32% in this zone. The total pollen and charcoal concentrations are moderate to low, but are relatively higher in the upper part (ca. 0.4 ka cal bp).

A detailed description of the principle components analysis (PCA) of the pollen results is given in ESM 1, Fig. S1.

Particle size and geochemistry results

In our study area, the East Asian monsoon brings most of the precipitation, which has over the years accumulated water in the basin to form a peat bog. The particle size of the mineral input is influenced by its source and the relative changes in the water level. The changes in particle size mainly reflect the influence of either dry or wet climates. When the EASM is stronger and brings more precipitation, there is more erosion washing in fine silt material.

The particle size distribution from WDY1 is relatively uniform, with most samples having particle sizes of 4–64 μm and only a few were < 4, or larger at 70–120 μm. The median particle size fluctuates greatly in the 6–40 µm range, and a few samples showed peaks between 40 and 130 µm. End member modelling analysis was done to distinguish the different groups of particle sizes (ESM 1). R2 indicates the linear correlations between end members and the original data set. The higher value of R2, the better the fit and the representativeness between end members and the original data set. The mean total R2 between the modelled and original data is generally > 0.9, based on this analysis. The end members EM1 to EM4 explain 68, 22.9, 4.5 and 1.6% of the data variance and show particle sizes that are in the ranges of 0.3–1000, 0.8–600, 10–2000 and 15–2000 μm, with peaks at ~ 1.4, ~ 11, ~ 40 and ~ 560 μm (Fig. 4). EM1 and EM2 consist of fine silt, EM3 coarse silt and EM4 coarse sand. Since there were more of them, we concentrated on the EM1 and EM2 components. These are most sensitive to climate change and they are relatively stable between ca. 8.3 and 1.5 ka cal bp, but show high variability after ca. 1.5 ka cal bp (Fig. 5h, i).

Fig. 4
figure 4

Sediment particle size distributions and geochemistry of WDY1; a results of optimal end-member modelling, b particle size distributions, and c geochemical element diagram

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Fig. 5
figure 5

Summary plot of evergreen tree pollen percentages in (a) WDY1, (b) WDY2, (c) principal component analysis (PCA) axis 1 of geochemical elements in WDY1, elements in WDY1, (d) Ti, (e), Al, (f) Si, (g) Fe, (h, i) particle classes EM1 and EM2 in WDY1

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The geochemical analysis studied the main minerals in the sediment. Macroelements are the main chemical components of rocks and sediments, and their content changes are mainly those of the main minerals, which can show the source of the sediments. Al, Si, Ti, and Fe represent rock fragments brought in from elsewhere (Schreiber et al. 2014; Rothwell and Croudace 2015). These minerals are sensitive to climate change and have often been used to show the climatic environment of the source region (Cuven et al. 2010; Kylander et al. 2011). In our samples they were present in large amounts, indicating more material brought in from the soil by high precipitation.

The geochemical elements are relatively uniform in WDY1 with high Al, Si, Ti and Fe concentrations from ca. 8.3 to 1.0 ka cal bp, representing minerals transported from elsewhere (Figs. 4c, 5d–g). This indicates increased amounts of material derived from various rock fragments in slope surface runoff and the formation of marshlands under a moist climate. The Al, Si, Ti, and Fe concentrations decrease noticeably from ca. 1.0 to 0.25 ka cal bp and then increase after ca. 0.25 ka cal bp, indicating a transition from low to high input of this material, which is most likely to have been in response to a climate change from dry to wet conditions. A detailed description of the principle components analysis (PCA) results for geochemical elements and their palaeoclimatic significance is provided in ESM 1, Fig. S2.

Discussion

Vegetation and other responses to changes in the East Asian summer monsoon

The Wangdongyang peat bog is in an enclosed basin without an outflow. The pollen sources are both local and regional so that the local pollen reflects the nearby surrounding vegetation and climate, which is important for understanding changes to the local vegetation (Prentice 1985; Xu and Zhang 2013). The regional pollen source is an area of 100–2,000 m from the centre of the basin and the pollen spectra also represent regional changes in the vegetation and thus the climate (Nielsen and Sugita 2005; Xu and Zhang 2013).

In this study, vegetation history of the Wangdongyang region during the Holocene has been reconstructed based on the pollen records. The high percentage of trees and the dominance of evergreen taxa during the Mid Holocene interval suggest a subtropical evergreen broadleaved forest vegetation (Fig. 5a, b). The high percentages of Castanopsis and evergreen Quercus and extremely low percentages of Pinus indicate a wet and warm climate, which favours evergreen broadleaved forests. The pollen records suggest a slight reduction in forest cover since ca. 2.0 ka cal bp, which is indicative of a relatively cool and dry climate. The concentration of charcoal increases and fluctuates then, which may indicate a phased drying of the climate. The high values in Axis 1 in the WDY1 and WDY2 PCAs also indicate a cool and dry climate (ESM 1, Fig. S1).

The high contents of Al, Si, Ti and Fe combined with the stable particle sizes for this part of the record (ca. 8.3/8.1–2.0 ka cal bp) suggest a warm and moist climate (Fig. 5d–g). The pollen, particle size and geochemical data indicate a stronger EASM during this period, which is supported by other pollen records from the lakes Gonghai (Chen et al. 2015), Daihai (Xiao et al. 2004) and Qinghai (Shen et al. 2005). These findings suggest that this region had a wetter climate and stronger summer monsoon during this part of the Holocene than today, which are also consistent with the pollen records from Fujian to the south of WDY (Zhao et al. 2016, 2017) and Taiwan to the southeast (Liew et al. 2006; Lee et al. 2010).

In the period ca. 7.5–7.2 ka cal bp there were significant reductions of sediment particle sizes and concentrations of Si, Al, Ti and Fe, indicating a drier and cooler climate, consistent with the pollen records from the East Asian monsoon region of China (Zhu et al. 2010; Zhao et al. 2017). This period is also characterized by low values of evergreen taxa in response to the climate change.

In the period ca. 6.7–2.0 ka cal bp the large amounts of Al, Si, Ti, Fe and the fine EM1 and EM2 sediment components indicate a high and stable input of mineral materials (Fig. 5), suggesting a warm and moist climate. The obvious peaks in the pollen percentages of evergreen Quercus, Ilex and Alnus reflect the shorter term climate fluctuations over centuries superimposed on the more gradual longer term changes over millennia (Fig. 3). In WDY1, Quercus pollen percentages rapidly increase to a maximum of 29% during this period, which indicates a localized spread of oaks under wetter and warmer climate conditions.

In the Late Holocene (ca. 2.0 ka cal bp), the amounts of Al, Si, Ti and Fe and the finer particle size fractions EM1 and EM2 decrease and fluctuate strongly, indicating a low and unstable input of mineral materials under cool and dry climate conditions. The highest percentages of xerophytic taxa with Aster, Artemisia, Taraxacum and Chenopodiaceae occur in this part of WDY1 and WDY2 (Fig. 3) and they also indicate drier conditions, which is consistent with other precipitation reconstructions of the East Asian monsoon region (Chen et al. 2015; Lu et al. 2019; Sun et al. 2019). The WDY1 and WDY2 pollen records indicate a slight reduction in forest cover, which is supported by a slight increase in the percentages of herbs and fern spores. Additionally, the other Late Holocene records cited in this study suggest that these areas received less precipitation during this period than in the Mid Holocene and were covered by deciduous trees.

In the period ca. 2.0–0.9 ka cal bp the Al, Si, Ti and Fe concentrations, evergreen Quercus pollen percentages and the fine EM1 and EM2 particle sizes are relatively high, indicating a continuing relatively warm and wet climate. At ca. 0.9–0.2 ka the percentages of Alnus pollen in WDY1 and WDY2 increase significantly, accompanied by a marked reduction of evergreen Quercus pollen. This suggests a fairly warm and wet climate during the Medieval Warm Period (MWP), despite the relatively lower temperatures and precipitation than in the previous period. There is a pronounced reduction in Al, Si, Ti and Fe concentrations and in the EM1 and EM2 particle size components during the period ca. 0.5–0.25 ka cal bp, indicating drier and cooler conditions during the early Little Ice Age (LIA), compared to the MWP. During the LIA, the percentages of evergreen Quercus, Ilex and Alnus pollen decrease markedly, representing a reduction in the wooded areas and indicating drier and cooler climate conditions. Pollen evidence from the Daiyunshan mountain peat bog (Zhao et al. 2016, 2017) also suggests a reduction in the forest cover as the climate became cooler and drier from the MWP to the LIA. This climatic pattern has also been observed in regional and global records (Mann et al. 2008; Ge et al. 2013; PAGES 2k Consortium 2019). Interestingly, the Al, Si, Ti and Fe concentrations and EM1 and EM2 particle size classes increased after ca. 0.25 ka cal bp, coinciding with a reduction in the tree pollen percentages. This suggests that the climate was relatively dry and cold during the late LIA, resulting in strong climatic fluctuations and inconsistent inputs of mineral materials.

A strong reduction in AP percentages after 2 ka cal bp may reflect climate more than human impact. However, there is not enough direct evidence to suggest a large human population and significant regional scale impact on the vegetation and landscape of the study area before 2 ka cal bp. The research focus of this paper is on climate change and vegetation change rather than on human activity. In a future paper we will combine the results from several peatland research sites in southeastern China to discuss human activities after 2 ka cal bp in detail.

Changes in solar radiation

This study also investigated the factors, which may have caused the climate and vegetation changes since the Mid Holocene. During this period, stronger solar radiation in summer caused greater thermal gradients between land and sea, which increased the intensity of the monsoon circulation, thereby increasing the flow of water vapour to the middle and lower Yangtze River area, which increased summer precipitation there (Laskar et al. 2004). Moreover, solar radiation can influence the intensity of atmospheric circulation (Gray et al. 2010). The associated energy decreases the amount of radiation reaching the ocean surface, which reduces moisture evaporation and transport and affects the EASM directly (Meehl et al. 2009). Similarly, the weakening of the EASM from lower solar radiation during the Late Holocene has been well recorded (Fig. 6g; Wang et al. 2005). A TraCE transient simulation of the lower Yangtze River area suggested a decrease in the average regional summer precipitation from the Early to Late Holocene, in response to the changes in the summer insolation (Lu et al. 2019). The dominant cause of climate change during the Mid to Late Holocene, when compared with that of the modern climate, is probably linked to the orbital variability of incoming solar radiation (Fig. 6f; Berger 1978). The evidence of significant evergreen and deciduous forest decrease after 2.0 ka cal bp is a response to a cool and dry climate during this period resulting from the reduction in solar radiation. In summary, the results of this study suggest that vegetation and palaeoclimate variations in southeast China since the Mid Holocene have been driven by changes in insolation.

Fig. 6
figure 6

Comparison of WDY1 and 2 pollen and other records; (a) pollen percentages of evergreen trees in WDY1, (b) in WDY2, (c) pollen percentages of xerophytic taxa in WDY1, (d) in WDY2; other records, (e) mean annual precipitation reconstruction from Dajiuhu (Sun et al. 2019), (f) June to August insolation at 65° N (Berger 1978); (g) δ18O record of Dongge Cave (Wang et al. 2005), (h) ENSO (El Niño Southern Oscillation) events per 100 years (Moy et al. 2002), (i) Ti concentration record from Cariaco Basin, which indicates movement of the ITCZ (inter tropical convergence zone) (Haug et al. 2001)

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The effects of ice in the northern hemisphere and CO2

Ice volumes in high northern latitudes have a dominant effect on the variability of the EASM in glacial and interglacial timescales (Ding et al. 1995, 2002). The ice volume in the northern hemisphere was relatively high during the Early Holocene and subsequently decreased since the Mid to Late Holocene (Dyke 2004). This decrease in the ice volume increased the ocean to land moisture transport distance, causing decreased summer precipitation (Ding et al. 2005). During the Holocene, summer precipitation in southern China was mainly controlled by the EASM and movement of inter tropical convergence zone (ITCZ). When the ITCZ rapidly moved southwards, it caused the EASM to weaken since the Late Holocene (Fig. 6i; Haug et al. 2001) and the summer precipitation decreased in southern China. Alternatively, higher atmospheric CO2 concentrations may have raised the ocean surface temperatures and influenced the summer monsoon rainfall (Hu et al. 2000). The increasing effect of the El Niño-Southern Oscillation (ENSO, Fig. 6h; Moy et al. 2002) during the Holocene was accompanied by a gradual increase of the CO2 concentration, indicating a possible relationship between CO2 levels and precipitation. In contrast to the gradual decrease in insolation during the Late Holocene, the EASM declined abruptly at ca. 3.0 ka cal bp, which is attributed to tropical ocean conditions. During El Niño events, the tropical eastern Pacific Ocean warming increases the temperatures in the troposphere and causes an eastward propagation of the Kelvin wave, which increases the tropospheric temperature of the western Pacific Ocean (Chiang and Sobel 2002; Xie et al. 2009) and reduces the land to sea temperature difference and the effects of the EASM (Chen et al. 2015). These simulation results suggest a higher frequency of the El Niño event during the Late Holocene, resulting in the weakening of the EASM. Therefore, solar radiation has probably been the dominant factor controlling climate changes during the Early to Mid Holocene. We can deduce that the higher frequency of the El Niño event and increased CO2 concentration, together with a rapid southward movement of the ITCZ during the Late Holocene, have contributed to the weakening of the EASM. These changes to the EASM and other climatic factors led to a reduction in the evergreen broadleaved forest cover since the Mid Holocene.

Abrupt climatic event at ca. 7.5–7.2 ka cal bp

The most notable changes which have been noticed for the period ca. 7.5–7.2 ka cal bp are the decreased Al, Si, Ti and Fe concentrations, the reductions in the EM1 and EM2 fine mineral particle classes and the reduced evergreen broadleaved tree cover, which together imply a drier and cooler climate (Fig. 5). This is confirmed by peat bog pollen records from Dajiuhu (Fig. 7e; Zhu et al. 2010), Lantianyan (Fig. 7f; Ma et al. 2016), Pingnan (Fig. 7g; Yue et al. 2012) and Daiyunshan (Fig. 7i; Zhao et al. 2017) and the Hugangyan Crater Lake (Fig. 7j; Wang et al. 2007) in southeast China, which also show a significant decrease in evergreen and other trees in this period, coinciding with a weakened EASM, as interpreted from the oxygen isotope ratio (δ18O) from a stalagmite obtained from the Dongge cave speleothem (Fig. 7d; Wang et al. 2005). The precipitation reconstructions from the Dajihu peat bog (Sun et al. 2019) and Xinjie regions (Lu et al. 2019) in the lower reaches of the Yangtze River in the East Asian monsoon region, indicate low precipitation from ca. 7.5–7.2 ka cal bp and both these studies show lower precipitation and temperatures in southeast China during this period, in response to a weaker EASM. Moreover, the low tree pollen percentages (Fig. 7a; Xiao et al. 2004) and decreased ocean surface temperature (Fig. 7b; Nan et al. 2017) in northern China at ca. 7.2–7.0 ka cal bp also occurred in response to the weakening of the EASM, as inferred from the Asian summer monsoon index obtained for Qinghai lake (Fig. 7c; An et al. 2012) and the δ18O speleothem records obtained from the Nuanhe (Wu et al. 2011) and Wangjiawei Caves (Dong and Zhang 2012), suggesting that this abrupt climate event occurred later in northern than in southeastern China.

Fig. 7
figure 7

Selected palaeoclimate results demonstrating the cooling events since the Mid Holocene; (a) pollen percentages from Daihai Lake (Xiao et al. 2004), (b) sea surface temperature (SST) change in Huanghai Sea (Nan et al. 2017), (c) Asian summer monsoon index at Qinghai Lake (An et al. 2012), (d) δ18O speleothem record from Dongge Cave (Wang et al. 2005), (e) pollen percentages from Dajiuhu (Zhu et al. 2010), (f–i) pollen percentages of evergreen broadleaved trees, (f) from Lantianyan (Ma et al. 2016), (g) Shuizhuyang (SZY) core in Pingnan peat (Yue et al. 2012), (h) WDY1 (current study), (i) Daiyunshan (Zhao et al. 2017), (j) tree pollen percentages from Hugangyan Crater Lake (Wang et al. 2007)

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The ca. 7.5–7.0 ka cal bp event has featured in palaeoclimatic records worldwide. For example, the argon and nitrogen isotope records from the Greenland ice core GISP2 suggest a significant temperature decrease from ca. 7.5 to 7.0 ka cal bp (Kobashi et al. 2017) and North Atlantic deep sea sediments suggest a peak of ice-rafted debris at ca. 7.5–7.2 ka cal bp, indicating a significant cooling period (Bond et al. 2001). Temperature decreases in North America (Hughes et al. 2006; Ersek et al. 2012) and Europe (Korhola et al. 2000) have also been identified during this period, which agree with the reconstruction of the temperature of Greenland (Cuffey and Cow 1997; Jansson and Kleman 2004). Various records have also identified reduced precipitation in the southeast Arabian peninsula (Neff et al. 2001; Fleitmann et al. 2003), the Mediterranean coast (Frisia et al. 2006) and Africa (Thompson et al. 2002; Zielhofer et al. 2017) since ca. 7.5–7.2 ka cal bp. On the whole, these results suggest that the ca. 7.5–7.0 ka cal bp event was a global phenomenon involving decreasing temperature and precipitation.

Previous research has identified a close relationship between the variations in the EASM intensity and solar radiation (Gupta et al. 2005; Wang et al. 2005). Moreover, some studies have identified the influence of insolation on the extent and strength of the Walker circulation, which was found to further amplify climate change. Solar forcing peaks (the difference between the energy absorbed by the Earth and that radiated back into space) increase the energy input into the surface of the ocean, which increases evaporation and moisture transport to convergence zones by the trade winds (Meehl et al. 2009; Gray et al. 2010). This leads to greater precipitation, strengthening the Hadley and Walker circulations in the troposphere (Meehl et al. 2009). Solar forcing peaks also increase ultraviolet radiation, which stimulates additional stratospheric ozone production and ultraviolet absorption. This results in the differential warming of the stratosphere according to latitude, so that the tropical tropospheric circulation is modified and precipitation increased towards the poles (Meehl et al. 2009). Measurements of the cosmogenic radionuclide 10Be in the GISP2 ice cores revealed a significant reduction in total solar radiation (Steinhilber et al. 2009) and Laskar et al. (2004) noted a reduction in solar activity at ca. 7.5–7.0 ka cal bp. These findings suggest that changes in solar radiation were the dominant cause of the abrupt climatic events during this period. This cooling event coincided with North Atlantic ice-rafting events (Bond et al. 2001), suggesting that insolation influences hemispheric and global centennial changes through its impact on meridional circulation (in a north–south direction). Additionally, some studies have identified a significant rise in sea level in the period ca. 7.5–7.2 ka cal bp (Yu et al. 2007; Zong et al. 2007; Zhao et al. 2021), which is consistent with the observed fresh water inflow to the sea due to melting ice. This large influx of fresh water would have altered the sea surface temperature and salinity, which probably slowed the North Atlantic Meridional Overturning Circulation (AMOC; Törnqvist and Hijma 2012; Bakker et al. 2017), resulting in reduced atmospheric temperatures in the high northern latitudes. Additionally, heat would have been transported northwards by the Hadley cell of global tropical circulation, which probably caused a southward movement of the ITCZ that weakened the EASM (Haug et al. 2001; Schneider et al. 2014). These results suggest that solar radiation and North Atlantic fresh water inflow were the main causes for the cooling event that occurred around ca. 7.5–7.2 ka cal bp.

Conclusions

The high resolution multi-proxy records from the two WDY peat cores analysed in our study suggest a wet and warm climate and a stronger EASM than at present from ca. 8.3 to 2.0 ka cal bp, followed by a cooler and drier period and a weakened EASM from ca. 2.0 to 0 ka cal bp. Thus, we can conclude that precipitation changes in southeast China since the Mid Holocene have occurred in response to variations in the EASM. Insolation has also probably been a dominant factor controlling climate change during the Mid Holocene. Other factors such as more frequent El Niño events, a greater CO2 concentration and the rapid southward movement of the ITCZ during the Late Holocene may also have resulted in the weakening of the EASM. This study identified an abrupt cooling event at ca. 7.5–7.2 ka cal bp, which occurred earlier in southeastern than in northern China. A reduction in the forest cover occurred in response to this event, and its primary causes may have been influx of fresh meltwater into the North Atlantic and changes in solar activity.