Holocene Fire in Relation to Environmental Change and Human Activity Reconstructed from Sedimentary Charcoal of Chaohu Lake

Quaternary International 507 (2019) 62–73

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Holocene fire in relation to environmental change and human activity T reconstructed from sedimentary charcoal of Chaohu Lake, East China ∗ Li Wua,b,c, , Linying Lia, Hui Zhoua, Xinyuan Wangc, Guangsheng Zhangd a Key Laboratory of Earth Surface Processes and Regional Response in the Yangtze-Huaihe River Basin, School of Geography and Tourism, Anhui Normal University, Wuhu, 241002, China b State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography & Limnology, Chinese Academy of Sciences, Nanjing, 210008, China c Key Laboratory of Digital Earth Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, 100094, China d Faculty of Environment and Tourism, West Anhui University, Lu'an, 237012, China

ARTICLE INFO ABSTRACT

Keywords: Through analysis of the concentration of different-sized charcoals and magnetic susceptibility of the CH-1 core Holocene fire from Chaohu Lake, East China during the Holocene, the features of fire in relation to environmental change in Environmental change the charcoal record and the effect of human activities were revealed. Between 9870 and 6040 cal yrBP,the Charcoal record climate was still relatively dry, although it was becoming warmer and wetter. The relatively dry climate ap- Lake sediment parently created favorable conditions for natural fires to occur, which suggests that the charcoal concentration Chaohu Lake was high. The warmest and wettest period was from 6040 to 2170 cal yr BP during the Holocene, and very little East China charcoal was found in the core, because the high precipitation during this period apparently suppressed natural fires. However, human activities increased the fire frequency during the cultural phase. Archaeological investigations indicate that a large number of Neolithic and historic sites, including the Lingjiatan cultural sites (5600–5300 cal yr BP), were distributed around Chaohu Lake. As a result, value peaks in the charcoal concentration were shown. After 2170 cal yr BP, the climate became drier and cooler, and conditions were once again favorable for fires to occur naturally. The concentration of charcoal in the core greatly increasedand showed the greatest levels of fire activity, which was related to both the drier climate and the enhanced human activities. The water level of Chaohu Lake reached a minimum in this period, as the lakebed was possibly exposed and formed the substrate for ancient sites, such as the flourishing Juchao state (2090–1710 cal yr BP). During the most recent 200 years, the amount of charcoal concentration sharply decreased, and fire occurrence disappeared gradually because there was not enough biomass in the Chaohu Lake basin.

1. Introduction world, such as Indonesia, Brazil and Australia (Moritz, 2012; Dello, 2017). The fire problem has attracted great attention of the scientists As an important and unique ecological environmental factor, fire worldwide (Power et al., 2008; Bowman et al., 2009; Whitlock et al., has always been one of the main indicators of climate change (Huang 2007; Mao et al., 2011; Daniau et al., 2012). et al., 2006; Conedera et al., 2009; Daniau et al., 2010; Pechony and In order to study the regional fire history over long time scales, itis Shindell, 2010; Tan et al., 2011; Parkner and Kasai, 2014; Miao et al., both theoretically and practically important to further understand the 2017; Sorensen, 2017). The human-induced fire activities have a great spatial-temporal rule of fire and reduce the future fire risk. Studies have impact on terrestrial ecosystems (Bowman et al., 2011; Zhu et al., 2012; indicated that charcoal existing in the sediments can be considered an Moser et al., 2017; Ma et al., 2018). All the global change factors are important index reflecting the past history of fire activity (Clark, 1990; sensitive to both natural fire and human fire activity, especially the Johnson, 1992; Lü et al., 2002; Camill et al., 2003; Power et al., 2008; changes of climate and land use (Yan, 1993; Liu et al., 1995; Bowman Marlon et al., 2009; Brücher et al., 2015; Tan et al., 2015, 2018; Inoue et al., 2009; IPCC, 2013; Cheng et al., 2015; Ellis, 2015; Shen et al., et al., 2016; Wang et al., 2017; Hawthorne et al., 2018; Power and 2015; Scott et al., 2016). As the El Nino phenomenon occurs more and Vannière, 2018). Charcoal is the black inorganic carbon compound more frequently, extraordinarily serious fire events caused by the ex- produced by incomplete combustion of plant organisms (Patterson III treme weather have also occurred more frequently in many parts of the et al., 1987). It is widely stored in different periods of sediments, and

∗ Corresponding author. School of Geography and Tourism, Anhui Normal University, Wuhu, 241002, China. E-mail address: [email protected] (L. Wu). https://doi.org/10.1016/j.quaint.2018.11.035 Received 23 December 2017; Received in revised form 2 November 2018; Accepted 27 November 2018 Available online 01 December 2018 1040-6182/ © 2018 Elsevier Ltd and INQUA. All rights reserved. L. Wu et al. Quaternary International 507 (2019) 62–73

Fig. 1. Sampling site of the CH-1 core and regional topographic-drainage characteristics in Chaohu Lake. often taken to the sedimentary basin for preservation by wind and The Chaohu Lake basin is located in the core area between the flowing water from the source. In particular, the charcoal stored inlake Huaihe River and the Yangtze River of Anhui in East China. Studies sediment, due to the good sedimentary continuity, has been recognized focusing on the problems of fire and fire events over a longer temporal as a sensitive alternative index for fire-regime changes in the past (Shen scale and wider spatial scale in the basin are rare (Wu, 2008; Xie, 2009; et al., 2010; Colombaroli et al., 2018; Mustaphi and Pisaric, 2018), thus Wang and Wu, 2018), with most attention focused on floods and other becoming the main source of information for the historical record of fire disasters (Wu et al., 2012b; Cheng and Hao, 2015). Moreover, this lake environmental change (Power et al., 2008; Thevenon et al., 2010; Kaal basin has been an important area for human activities since the Neo- et al., 2011; Kehrwald et al., 2013; Tan et al., 2013, 2015). For ex- lithic Age (Wu et al., 2010). At present, from the archaeological point of ample, Chu (2001) explored the relationship between the sediment flux view, the periods of the early development of regional culture are as change of charcoal and human activities over recent 2000 years in the follows: the earliest known Neolithic culture is the Lingjiatan culture Maar Lake of Huguangyan and Wu and Liu (2013) reported the char- (5600–5300 cal yr BP), followed by the Shang and Zhou Dynasties coal recorded climate changes from Moon Lake in the late glacial (3550–2720 cal yr BP), and the Han Dynasty (2150–1730 cal yr BP), period. which lasted 4000 years (Wu et al., 2012c). Therefore, together with Magnetic susceptibility has been widely adopted as an indicator for the magnetic susceptibility, pollen concentration, archaeological find- environmental change (Balsam et al., 2011; Qiang et al., 2013; Li et al., ings and historical document records, the concentrations of different 2014; Prasad et al., 2014; Finkenbinder et al., 2015; Kang et al., 2018). sizes of charcoals in the Chaohu Lake sediments were analyzed in this Existing studies have shown that the magnetic strength of lake sedi- study. The results were correlated with a stratigraphic chronology ments can be reflected in the low-frequency magnetic susceptibility of based on AMS14C dating, which facilitated the investigation of the sediments (Zhu et al., 2003, 2004; Song et al., 2016; Hou et al., 2018). climate fluctuations and the changes in the fire environment inthe A higher magnetic susceptibility in lake sediment indicates that hy- Chaohu Lake Basin since the early Holocene. During the late Holocene, drodynamic conditions were stronger and the environment was wetter the mutual influence between the fire environment and the enhance- (Yang et al., 2004; Xie et al., 2006). Conversely, lower magnetic sus- ment of human activities will also be discussed. This study can provide ceptibility indicates that the environment was drier. The magnetic a scientific reference for effective disaster prevention and natural, social susceptibility of the lake sediment could be used to reflect the change in and economic coordinated development in the region. wet and dry climate and precipitation (Xie et al., 2006; Zhang et al., 2007). Studies have also determined that magnetic susceptibility values 2. Materials and methods reveal anthropogenic effects on the deposition rate and the land-use types in given drainage basins (Dai et al., 2009). Thus, a sharply in- 2.1. Regional setting and sampling creasing magnetic susceptibility value indicated intensified bedrock erosion caused by farming and deforestation (Sandgren and Fredskild, Chaohu Lake is located in the middle of Anhui Province and be- 1991; Beach et al., 2006; Li et al., 2014; Lin et al., 2017; Wu et al., tween the Huaihe River and the Yangtze River. The regional climate is 2017b). the transitional monsoon climate of the northern subtropical and warm

63 L. Wu et al. Quaternary International 507 (2019) 62–73 temperate zones. Standing at the line dividing the north and south was recorded. This method was able to greatly improve work efficiency. climates, and with only one river connecting to the Yangtze River, For micro-charcoal samples, an appropriate amount of sample was Chaohu Lake forms a quasi-closed lake, where the continuous and taken using a glass rod and glycerinum was added to prepare a slide. stable lacustrine sediments are rich in information relating to paleo- The slide was carefully observed with an optical microscope (set at climate and environmental change. From April 2006 to July 2015, our 40×), and the areal concentration (cm2/ml) of charcoals was recorded research team carried out more than 10 separate field investigations (Clark, 1983). A total of 500 non-overlapping viewing ranges were and a series of drilling samplings for lake sediment core in the Chaohu selected on the slide, reading the total coordinate point number covered Lake area, including a 4.9 m continuous sedimentary core from 3 m by the charcoal on the slide with 11 coordinate points (0–10) marked in water depth of 31°33′44.60″N, 117°23′39.40″E in the west lake area ocular micrometers. All the Lycopodium spores added within the 500 (No. CH-1, sampling in May 2006, with the coring rate of 98%, Fig. 1), viewing ranges was counted. The areal concentration (cm2/ml) of the where the lithology is the cyan-grey mud, pertaining to the lacustrine charcoal was calculated as below: sediment. Y = C × L /(M × l × V), In the laboratory, 98 charcoal samples were obtained by cutting in 5 cm intervals and 245 samples were obtained by sampling in 1 cm where Y is the area concentration (cm2/ml) of charcoal, L is the con- interval for magnetic susceptibility analysis. After being dried naturally centration of the Lycopodium spore added, l is the statistical number of in the chamber, the samples were tested and analyzed for charcoal Lycopodium spore, V is the volume of sample, C is the statistical number concentration and magnetic susceptibility. of coordinate points of charcoal and M is the micrometer coordinate point in recorded viewing range (generally 5500). 2.2. Charcoal concentration analysis There are many approaches to quantifying charcoal accumulation (Whitlock and Larsen, 2001; Higuera et al., 2010; Kelly et al., 2011). Two statistical methods were applied to grade the charcoal in the Here, a 500 yr period was divided into a total of 20 stages to calculate samples, screening and pollen flow, combining the advantages of the the average charcoal deposition rate since the Holocene and plot the two analysis methods (Li et al., 2006). Screening was applied to con- moving average curve (Fig. 3), with the method Fc = Ct/500 × 100%, 2 duct extraction statistics for the macro-charcoal (> 125 μm particle where Ct is the total charcoal concentration (grains/ml or cm /ml) of diameter), whereas pollen flow was applied to obtain the micro-char- the sample in every 500 yr after age interpolation, and Fc is the average coal (< 125 μm particle diameter). The pre-preparation process was as value of charcoal deposition rate (grains/ml·yr or cm2/ml·yr), which follows: (1) A ∼5 ml sample was taken and shaken in a measuring can reflect the changes of charcoal deposition content per unit volume cylinder to ensure the sample was compacted, and the accurate volume per year over a certain period of time. was recorded. (2) Next, 10% HCl was added to remove CaCO3. The mixture was transferred to a centrifuge tube after the initial reaction, and centrifuged for 5 min at 2000 r/min to remove supernatant liquid. 2.3. Magnetic susceptibility testing (3) Two to three times the sample volume of 10% KOH was added, and then heated for 15 min in a water bath to remove organic matter. (4) The samples were naturally dried, numbered and packaged into a The sample was filtered in the centrifuge tube using a nylon sieve witha standard plastic box, measured with a Bartington MS2 portable mag- pore diameter of 125 μm, and the remainder of the sample was removed netic susceptibility instrument, to test the samples of low-frequency from the sieve and placed into a culture dish. (5) A Lycopodium spore magnetic susceptibility (χlf) and high-frequency magnetic susceptibility tablet was added into the sample. The pore diameter was less than (χhf) values, and to calculate the frequency magnetic susceptibility 125 μm to ensure full dissolution. The sample was centrifuged for an (χfd) for the samples with the formula χfd = (χlf − χhf)/χlf × 100%, additional 5 min at 2000 r/min, cleaned two times using the centrifuge where 693 valid data points were obtained. method, and the supernatant liquid was removed. (6) Heavy liquid (ratio = 2.2) was added, and then centrifuged for 30 min at 2500 r/ min. The upper heavy liquid was poured into a beaker, and diluted with 2.4. AMS14C dating 6 times the sample volume of distilled water. (7) Glacial acetic acid was added into the solution to dissolve any flocculate content in the solu- Because the whole CH-1 lacustrine core is composed of relatively tion. The solution was left to stand for approximately 12 h. (8) Two pure cyan-grey medium-sized silt and fine silt, mingled with a small thirds of the supernatant fluid was poured out. The base solution was amount of very fine silt, not including the inclusions and plant residues, transferred to a centrifuge tube, centrifuged for 5 min (2000 r/min) seven total organic samples from lake sediments were selected to test each time, and finally cleaned twice with distilled water. AMS14C chronology, completed by the State Key Laboratory of Nuclear When the sample preparation finished, the Leitz optical projection Physics and Technology, Beijing University (Table 1). Except for the microscope was used for charcoal graded statistics under the micro- first measurement data point (at depth of 0.87 m), the linear relation- scope to calculate the concentration of macro- and micro-charcoal, re- ship of the other six data points were very good. Considering the whole spectively, and Fig. 2 and 3 were plotted for the obtained charcoal core did not include the sedimentary discontinuity traces, it was statistical results. A stereoscopic microscope was used to record and speculated that the lower six 14C data points were affected by the count macro-charcoal. A 1 × 1 cm quadrille paper was placed below “carbon reservoir effect” (Shen, 2009; Hou et al., 2012; Timothy Jull the culture dish, taking the entire square number covered by the culture et al., 2013). Therefore, the tree-ring calibration and linear regression dish. The squares were numbered in turn, placing samples in the culture calibration were carried out in turn for the six chronology data points dish, and counting using the stereoscopic microscope. If there was under 1.27 m core depth (published in other paper, without giving minimal charcoal content in the sample, all charcoals in the culture dish unnecessary details herein) (Stuiver et al., 1998, 2005; Zhang, 2007; could be directly counted. When the charcoal content was large, 100 ml Wang et al., 2008a, 2008b). After the corrections, the seven calendar of distilled water was added to the sample and fully mixed. After mixing chronological sequences had a good linear relationship with the depth with distilled water, 10 ml of the mixture was taken and poured into a (R2 = 0.9775), where the extrapolation and interpolation methods culture dish. Squares were numbered randomly until squares from each were used to calculate the chronology of other core depths. line had been selected, and then the number of total squares was recorded. The total content (grains/ml) of charcoal in the selected squares

64 L. Wu et al. Quaternary International 507 (2019) 62–73

Fig. 2. Changes of environmental proxies of charcoal, pollen and magnetic susceptibility of the CH-1 core in Chaohu Lake. F/C is the ratio of the concentration of micro-charcoal (< 125 μm) to macro-charcoal (> 125 μm).

3. Results

3.1. Charcoal concentration

In total, 196 statistical data points were obtained for the charcoal experiments. The concentration of the macro-charcoal was 5.757–355.343 grains/ml, and the concentration of the micro-charcoal was 0.180–22.302 cm2/ml. The characteristics of changes in charcoal concentration and the CONISS cluster analysis results for pollen percentage of the CH-1 core (Fig. A1 and A2) indicated that the section, from bottom to top, is divided into five zones (Fig. 2), for which the analysis results are described as follows: Zone I: At 490–337 cm (9870–6040 cal yr BP), the concentration of macro-charcoal was 21.457–195.727 grains/ml with an average of 62.451 grains/ml. There were four obvious peak values at 465, 455, 380 and 370 cm. The concentration of micro-charcoal was 0.645–7.380 cm2/ml with an average of 3.307 cm2/ml. In general, the concentration of macro-charcoal was higher than that of micro-charcoal. Zone II: At 337–247 cm (6040–4860 cal yr BP), the concentration of Fig. 3. Changes of deposition rates of macro-charcoal (> 125 μm) concentra- the macro-charcoal was 13.797–138.446 grains/ml with an average of tion (a) and micro-charcoal (< 125 μm) concentration (b) in the CH-1 core in 60.594 grains/ml and a larger peak at 255 cm. The concentration of the Chaohu Lake basin. The bars represent the calculated charcoal deposition 2 2 rates and the lines represent the moving average curves of charcoal deposition micro-charcoal was 1.029–6.022 cm /ml with an average of 3.233 cm / rate. ml. The charcoal concentration in this zone was slightly reduced relative to Zone I. Zone III: At 247–117 cm (4860–2170 cal yr BP), the concentration of

Table 1 The AMS14C ages and their calibrated results of CH-1 core in the Chaohu Lake basin.

Lab. No. Depth (m) AMS14C age (yr BP) 2σ calibrated age Linear regression calibrated age (cal. yr BP)

BA061038 0.87 1065 ± 35 890 AD (95.4%) 1030 AD 1040 ± 70 BA061039 1.27 4855 ± 35 3710 BC (79.2%) 3630 BC 2550 ± 40 BA061040 1.89 5955 ± 50 4970 BC (95.4%) 4710 BC 3720 ± 130 BA061041 2.27 6795 ± 40 5740 BC (95.4%) 5630 BC 4565 ± 55 BA061042 2.87 7785 ± 40 6690 BC (95.4%) 6500 BC 5475 ± 95 BA061043 3.87 8685 ± 50 7840 BC (93.9%) 7580 BC 6590 ± 130 BA061044 4.87 10825 ± 40 10935 BC (95.4%) 10850 BC 9770 ± 40

65 L. Wu et al. Quaternary International 507 (2019) 62–73 the macro-charcoal was 34.218–295.130 grains/ml with an average that time is likely to be the main cause of the rapid increase of charcoal was 66.170 grains/ml, with a larger peak at 185 cm and smaller peak deposition rate for nearly 2000 yr. values at 170 and 150 cm. The concentration of micro-charcoal was 0.180–3.154 cm2/ml with an average of 0.633 cm2/ml. The charcoal 3.3. Magnetic susceptibility concentration of macro-charcoal in this zone was slightly increased relative to Zone II; however, the concentration of micro-charcoal was According to the test results (Fig. 2 and Table 2), the low-frequency the lowest in the whole section. magnetic susceptibility curve under 117 cm (2170 cal yr BP) was rela- Zone IV: At 117–87 cm (2170–1040 cal yr BP), the concentration of tively stable, which shows a slightly decreasing trend at 160–117 cm, macro-charcoal was 38.465–250.379 grains/ml, with an average value whereas the low-frequency magnetic susceptibility above 117 cm of 146.469 grains/ml and two larger peaks at 110 and 100 cm. The started to increase sharply. In contrast, the frequency magnetic sus- concentration of micro-charcoal was 0.279–16.415 cm2/ml with an ceptibility (χfd) showed a different amplitude of oscillation, with six average of 3.429 cm2/ml and a larger peak at 100 cm. Both the macro- clear peaks at 385, 345, 295, 247, 107 and 7 cm, where the lowest value charcoal and micro-charcoal concentrations increased rapidly, thus of the whole core occurred at 237 cm (4700 cal yr BP). In general, the representing the high-charcoal stage of the section. frequency magnetic susceptibility was higher below 237 cm than above Zone V: At 87–0 cm (1040 cal yr BP to present), the concentration of 237 cm. These results indicate that the climate in the early Holocene macro-charcoal was 5.757–355.343 grains/ml with an average of 73.37 transited towards a warm and wet climate, but it was still relatively dry. grains/ml and two peaks at 70 and 45 cm. The peak value at 70 cm had The climate in middle Holocene was warm and wet, and the climate in the maximum charcoal concentration in the section. The concentration the late Holocene might be more strongly affected by the higher level of of micro-charcoal was 0.274–22.302 cm2/ml with an average of human activities. 7.15 cm2/ml and three peak values at 80, 45 and 20 cm. The change in concentration of macro-charcoal was large, having a more significant 4. Discussions decrease in the upper part of this zone. The concentration of micro- charcoal was further increased in the upper part of this zone. In order to make better use of charcoal in discussing the fire in re- The trend of concentration change of the sedimentary charcoal was lation to environment change and human activity of the Chaohu Lake also analyzed (Fig. 2). Firstly, the concentration fluctuation of macro- area since the Holocene, the aspects of climate, source and transporting charcoal was large with multiple peaks, whereas the concentration of mechanism, combined with historical documents and archaeological micro-charcoal showed changes from relative stability to considerable data, were made available for detailed comparative research. The fluctuation. Secondly, by contrast, the charcoal concentration was high comprehensive changes in environmental proxies of the core are shown in the early-middle Holocene, and lowest from 4860 to 2170 cal yr BP. in Fig. 2. The charcoal concentration substantially increased during the most recent 2000 years; since the Holocene, the concentration of charcoal 4.1. Proxy indicator of fire has presented an overall increasing trend. Thirdly, the change in concentration of macro-charcoal was greater than that of micro-charcoal. The charcoal in the sediment was used as the alternative indicator for fire. The frequency, intensity and change of the fire in the geological 3.2. Charcoal deposition rate history could be reconstructed by the quantitative statistics and mor- phological analysis of charcoal (Tan et al., 2015). A higher concentra- Fig. 3 shows that there were clear changes in the charcoal deposition of charcoal indicated an intense fire activity, whereas a lower tion rate and the moving average curve presented peak–valley alter- concentration reflected a weak fire activity (Li et al., 2005). In contrast, nation. The two most obvious peaks appeared at 7500–6000 cal yr BP in the climatic conditions were important factors in the occurrence of fire the early-middle Holocene and the last 2000 years of the late Holocene. events and the higher intensity fires were often associated with arid The climatic factor was the main factor controlling the peak of charcoal climatic conditions (Cao et al., 2007). The more arid the climate, the deposition rate in the early-middle Holocene. The pollen record showed more likely fires would occur at a higher frequency (Zhang et al., 1997; that, in the area of Chaohu Lake around 7700 cal yr BP, the proportions Tan et al., 2008). Moreover, since the change in charcoal deposition of Pinus, Quercus, Castanopsis/Lithocarpus, Ulmus, Cyclobalanopsis and flux reflected the intensity of human activities, the occurrence offire wetland herbs suddenly decreased (Fig. A1). Pollen concentrations also can be considered to be closely related to human activities (Chu, 2001). substantially reduced to the minimum (Fig. A2). This change of pollen Therefore, the change of charcoal concentration could indicate not only assemblage indicates a significant drought event once occurred, so that the change of dry and wet climate but also the change of population the climate of this area developed from the previous warm and wet quantity and activity intensity in the study area (Xu et al., 2002). climate status towards warm and dry. At that time, in addition to the After the fire activities, a small amount of charcoal (about mild and dry climate, the mixed forest vegetation with a higher content 1.5%–2%) would have been lofted with the smoke, and then spread of xerophytic herbs providing ample and combustible biomass for with the wind. The majority of charcoal was deposited in the original natural fires. Therefore, the natural fire occurred frequently andmore place, which was carried to the lake basin by runoff (Kershaw, 1985). intensely. Furthermore, the charcoal deposition rate started rising ra- According to the laws of the distribution and deposition of charcoal, the pidly from 7500 cal yr BP and hit the peak (Fig. 3). The rapid rise of propagation distance of macro-charcoals was relatively short, and the charcoal deposition rate in the last two millennia might be the result of source area was closer to the depositional site. The source area of superposition of climate and human activity (Vannière et al., 2008; Wu micro-charcoals was relatively further away and the propagation dis- et al., 2010; Roos et al., 2016; Snitker, 2018). The archaeology and tance was relatively longer, so the size of the charcoal particles could pollen data showed that the climate in this area during this period of indicate the relative distance between the fire source area and the se- time developed towards the cool and dry climate (Fig. A1 and A2), dimentary area (Sun et al., 2000). While this is often true, recent studies whereas the human activity was greatly strengthened since the Han have looked at the specific factors that influence this relationship, such Dynasty (2150–1730 cal yr BP) (Wang et al., 2008a, 2008b; Wu et al., as the type of fires (crown fire or surface fire), fire intensity, particle 2010, 2012c). Moreover, the forest vegetation shrank with rapid suc- morphology, atmospheric conditions, vegetation type and completeness cession of grassland. These findings indicate that large-scale defor- of combustion (Tinner et al., 2006; Shen et al., 2010; Roberts, 2014; estation or burning by humans for the development of agriculture at Hawthorne and Mitchell, 2016; Leys et al., 2017; Li et al., 2017; Inoue

66 L. Wu et al. Quaternary International 507 (2019) 62–73

Table 2 The main parameters of magnetic susceptibility of CH-1 core in the Chaohu Lake basin.

Parameter of magnetic susceptibility χlf (×10−8 m3/kg) χhf (×10−8 m3/kg) χfd (%) Ratio of frequency magnetic susceptibility to low frequency magnetic susceptibility (%)

Maximum value 64.49 61.81 11.17 0.86 Minimum value 6.59 4.07 0 0 Average value 18.44 17.84 −0.74 0.21 et al., 2016; Vachula and Richter, 2018). Because of a lack of data, we charcoal presented four peak values, and the charcoal deposition considered the normal conditions and employed multiple methods (not rate showed an increasing trend and rapid increase to the peak, rely upon charcoal method only), to increase the robustness of the re- indicating that the fire activities were frequent. Moreover, the low- search conclusions. The concentration of macro-charcoal represented frequency magnetic susceptibility value was low, with an average the local fire activity events, while the concentration of micro-charcoal value of 14.86 × 10−8 m3/kg, and the frequency magnetic sus- reflected the regional fire activity events (Clark, 1988; Clark and ceptibility was also at a relatively low stage. The total concentration Royall, 1995; Clark et al., 1998; Li et al., 2006). From the perspective of of pollen in this zone was 111613 grains/ml (Fig. A2), which was in the change trend of the concentration of macro-charcoal and micro- a relatively higher stage. All of the above indicated that this period charcoal in the core, the macro-charcoal concentration changed more was the temperature fluctuation recovery stage after the late glacial, substantially, with a stronger indicative function for the characteristics but the climate condition was still relatively dry. There may have of local fire activities. been a climate of frequent alternation of dry and wet, especially in The ratio of the concentration of micro-charcoal to that of the the mid-late part of this stage (Wang et al., 2008b; Song et al., macro-charcoal (F/C) was used to indicate the distance between the 2017). This is also in accordance with the peripheral records of the sedimentary site and charcoal source area (Huang and Sun, 2004; Luo Dajiuhu Basin of the Shennongjia Mountains in Hubei, the Xianghu et al., 2006). The greater the F/C value, the greater the distance be- section in northern Zhejiang and the Nanyi Lake in southeastern tween the sedimentary site and charcoal source area (Luo et al., 2006). Anhui (Gu et al., 2006; Zhu et al., 2006; Ma et al., 2008; Li et al., 13 Of course, macro-charcoal particles might break into fine particles 2016; Liu et al., 2018). In the records of Dajiuhu peat, δ Corg, LOI, during the process of secondary transportation, or sediment and other TOC and TN appeared high frequency changes after 9400 cal yr BP taphonomic effects (Luo et al., 2001). Supposing that the physical and showed several climate events before 6.1 cal yr BP, with pollen broken coefficient of the charcoal particle remains constant, as well concentrations fluctuating frequently at 11000-6000 cal yr BP(Zhu considering that the study area is a quasi-closed lake basin (Wang et al., et al., 2006, 2010; Ma et al., 2008). Variations in the distribution 2008b; Wu et al., 2010), the size of charcoal particle could still reflect and compound-specific stable carbon isotopic compositions of n-al- the relative distance for charcoal to travel from the source to the se- kanes recorded in the Nanyi Lake sediments also revealed significant diment area (Sun et al., 2000; Luo et al., 2001). Fig. 2 shows that the F/ fluctuations in temperature and humidity since 8000 cal yrBP(Liu C value of Chaohu Lake CH-1 core was small in general, with an et al., 2018). As the temperature increased, the combustible material average of only 0.096, indicating that the charcoal in the Chaohu Lake became dried and easily burnt, and the increase of biomass accu- sediment was mainly derived from the surrounding areas very close to mulation also provided a high possibility of fire occurrence. The the lake, which could very well represent the fire history in the study relatively drier climate led to frequent fire activities, with increased region. However, there was also a clear change at 60–0 cm. It is rea- charcoal storage. sonable to suggest that the charcoal at 60–0 cm may result from re- 2) From 6040 to 2170 cal yr BP (including Zone II and III), the time gional fires, whereas the charcoal at 490–60 cm may result fromlocal period for Zone II was the warmest and wettest period in the Chaohu fires. Lake basin (Wang et al., 2008b; Chen et al., 2009; Wu et al., 2010). The vegetation changed to the mixed forest of deciduous broadleaf 4.2. Time series features of Holocene fire in relation to environmental trees and evergreen broadleaf trees with Quercus, Castanea, Cyclo- change and human activity balanopsis and Castanopsis/Lithocarpus as the main components. Al- though the temperature and the vegetation coverage rate in that The comprehensive analysis of charcoals, magnetic susceptibility period increased greatly, the rich precipitation not only caused air and related environmental proxies for the CH-1 core of Chaohu Lake humidity increase but also caused the combustible material (Fig. 2, A1, and A2) indicated the following: flammability to decrease. This weakened the fire activities greatly because the combustible materials had absorbed plenty of water 1) Between 9870 and 6040 cal yr BP (Zone I), the climate transited to from the precipitation. The analysis results for the macro- and an overall warm and wet condition from the cold and drought cli- micro-charcoal concentrations showed that the charcoal content mate of the last glacial period (Wang et al., 2008b; Jiang et al., was lower than that in the Zone I, and the charcoal deposition rate 2015; Guan et al., 2017; Song et al., 2017). The mixed forest of was in a relatively smooth valley, so the fire activity was weak. The deciduous broadleaf trees and evergreen broadleaf trees (mainly wet climate caused low frequency of fire activity, with less charcoal Fagaceae) occupied most areas of the Chaohu Lake basin. The pa- stored in the sediment. At this stage, the frequency magnetic sus- lynological analyses of the Nanling core also indicates that an ceptibility (χfd) values alternated with peaks and valleys and vio- evergreen and deciduous mixed broad-leaved forest dominated by lently vibrated, indicating that the climate had several major fluc- Cyclobalanopsis and Quercus existed from ca. 10500 cal yr BP and tuations. However, there was a peak at 255 cm (4990 cal yr BP) for became fully developed between 8250 and 7550 cal yr BP (Chen the macro-charcoal concentration, which could be associated with et al., 2009). The concentration of both macro-charcoal and micro- human activities. The archaeological excavation data showed that charcoal was slightly high in general. The concentration of macro- this period was an important stage for the formation and

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Table 3 The number and density of sites from the mid-late Neolithic Age to the Han Dynasty in the Chaohu Lake basin.

Period Date range (cal. yr BP) Number of sites Density of sites (sites/100 km2)

Mid-late Neolithic Age 6000–4000 52 0.797 Shang and Zhou Dynasties 3550–2720 114 1.312 Han Dynasty 2150–1730 60 0.891

development of Neolithic culture in Chaohu Lake basin, where many the Qin and Han Dynasties (221 BC–220 AD), the level of agri- Neolithic sites appeared (Chen et al., 2009; Wu et al., 2010, 2012a, cultural productivity increased greatly, and in the feudal society, 2017a), represented by Lingjiatan (5600–5300 cal yr BP) (Wang mankind was constantly destroying the forest to reclaim wasteland. et al., 2009; Shuo, 2015; Wu et al., 2015), WeiGang From the Three Kingdoms Period (220–280 AD) to the Ming and (5600–5300 cal yr BP) (Shuo et al., 2015) and Gugeng (ca. Qing Dynasties (1368–1912 AD), the artificial dike paddy fields 6000 cal yr BP, 4600–4000 cal yr BP) (Yang and Yang, 1985; Wu increased from 200 to 1500 km2 in the Chaohu Lake basin (Lu, et al., 2012a, 2017a), so the intensification of ancient human ac- 2007; Chen, 2009). This kind of man-made fire, caused by large- tivities caused the frequency of fire activities in the basin to in- scale production activity, had been quite prominent during this crease. After entering Zone III, the concentration of macro-charcoal period; however, the drying of the climate was also apparent. It is showed a larger fluctuation, with three peaks, whereas the con- particularly noteworthy that ca. 2170 cal yr BP was within a period centration of micro-charcoal was the lowest in the whole section. of considerable drought in the Chaohu Lake area in the Holocene, The magnetic susceptibility values decreased and the climate be- and the lake surface area had reduced once again (Jia et al., 2006; came drier. The research on the phytolith fossils of the Nanling core Wang et al., 2008b). Based on comprehensive analysis of 14C in Chaohu Lake also indicates that the climate showed a relatively chronology results 2090 ± 130 cal yr BP for Tangzui Site's cultural warm and dry trend with a decrease in temperature and humidity layer with a higher level of carbon (Wu et al., 2012c), it has been during this period (Fan et al., 2006). High-resolution stalagmite speculated that the smallest lake surface at Chaohu Lake in its his- δ18O records from Shanbao Cave at Shennongjia also suggest that an tory occurred at around 2170 cal yr BP. At this time, the local areas arid period prevailed at this time (Shao et al., 2006; Wang et al., of the lake basin might have been exposed and the lake beach area 2008c). In particular, a maximum value occurred at 185 cm was expanded, with a thriving economic and cultural condition in (3645 cal yr BP) since the Holocene, up to 295.130 grains/ml. These the Tangzui site (i.e. the ancient Juchao State, existed in might be closely related to burning forest, hunting, heating and 2090–1710 cal yr BP) (Wu et al., 2012c). Many other large city sites slash-and-burn cultivation caused by the increase in the frequency and tombs also appeared in this area during phases of intensive of human activities during the Shang and Zhou Dynasties human inhabitation in the basin low-lying land formed due to lake (3550–2720 cal yr BP) (Wu et al., 2012c). Available archaeological shrinkage (Wu et al., 2010, 2012c). Furthermore, according to data have shown that the local ancient culture during the Shang and Chorography of Chao County and Chaohu Lake (Qing Dynasty) com- Zhou Dynasties was relatively developed, as more than 100 settle- piled in the Kangxi Period (Lu et al., 2007), during more than 100 ment sites were discovered, with a very heavy distribution density years from the second reign year of Emperor Jiajing of Ming Dynasty (Wu et al., 2010). The largest density site was found in the Shang (1523AD) to the tenth reign year of Emperor Kangxi of Qing Dy- and Zhou Dynasties, whereas it was comparatively small in the nasty (1671AD), there were nine major drought events, with up to mid–late Neolithic Age and the Han Dynasty, corresponding to the 48 recorded fire and drought events. Therefore, the intensification number of settlement sites (Table 3). These archaeological metric of fire activity during this period did not only correspond tothe data also indicated that human activity was an important fire-gen- drought phase of the climate but was also closely related to the erating factor. intense interference of human activities. Under the dual effects of 3) After 2170 cal yr BP (including Zones IV and V), the climate had a climate change and human activity, the frequency of fire increased, tendency towards arid, but there was a clear temperature fluctua- so that the amount of charcoal stored in the sediment was highest tion. The concentration of both macro- and micro-charcoal was during the whole Holocene, especially in the Little Ice Age greatly increased, with the occurrence of several large peaks, and (1500–1900 AD) of the Ming and Qing Dynasties (Mann et al., 2009; the charcoal deposition rate rapidly increased to the maximum Ren et al., 2017; Zhang et al., 2017). Statistical analysis of the ex- value in the section, indicating that the fire activity was quite fre- tensive reference to the ancient and modern local annals in Chaohu quent during this period. Into this period, the pollen combination Lake area such as Chorography of Chao County and Chaohu Lake (Lu showed the mixed forest of deciduous broadleaf and evergreen et al., 2007), Chorography of Hefei County (Zuo, 2006), Chorography broadleaf was quickly destroyed, with succession into mainly of Wuwei State (Gu and Wu, 2011), Chorography of Hanshan County Gramineous grassland (Fig. A1 and A2). The total concentration of (Zhao, 2008), Brief Chorography of Chaohu Region (Chorography pollen showed considerable increase, and the magnetic suscept- Compilation Committee of Chaohu Region, 1995), and Chorography ibility value had an abnormal increase. In this period, there was a of Hefei City (Chorography Compilation Committee of Hefei City, population density increase from an estimated 5–10 persons/km2 in 1999) has revealed that the fire and drought events were closely the Taikang Period (280–289 AD) of the Jin Dynasty to 300–500 related to the charcoal concentration in the Ming and Qing Dynas- persons/km2 at the Jiaqing Period (1796–1821 AD) of the Qing ties and were far greater in number than in other historical periods Dynasty (Lu and Teng, 2000; Goldewijk et al., 2010). The vegetation (Table 4), as a major fire and drought event occurred in every 5–6yr of the Chaohu Lake basin was strongly influenced by human activ- on average. In particular, in the Kangxi Period (1662–1723 AD) and ities, which was mainly manifested in the cutting or burning forest Qianlong Period (1736–1796 AD) of the Qing Dynasty, frequent fire and enhancement of agricultural planting activity (Zuo, 2006; Lu and drought events can also be tied to the charcoal and magnetic et al., 2007; Zhao, 2008; Chen et al., 2009; Gu and Wu, 2011; Wu susceptibility data from the CH-1 core. Toward the end of this stage, and Ji, 2013; Zhang and Wang, 2017; Wang and Wu, 2018). Since the concentration of macro-charcoal showed a greater decrease,

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Table 4 from the areas surrounding the lake, which can represent the change of The statistics of historical document records of fire and drought disasters from the fire history in the basin. The charcoal content change is aresultof the Tang Dynasty to the Republic of China (618–1949 AD) in the Chaohu Lake superposition of regional natural changes with human activity factors, area. which can not only indicate the dry–wet climate change but can also Dynasty Year No. Age (AD) Duration (yr) Number of fire reflect the changes of population and the intensity of human activities and drought in the study region. disasters The information of Holocene fire in relation to environment change

Tang Qianfeng 666–668 3 1 and human activity in the Chaohu Lake area was extracted in this study. Zhenyuan 785–805 11 2 Between 9870 and 6040 cal yr BP, though the climate transited from Yuanhe 806–820 21 1 original cold and dry status to warm and wet, it was still relatively dry Song Dazhongxiangfu 1008–1016 9 1 with comparatively high charcoal content and fire activities that oc- Shaoxing 1131–1162 32 3 curred frequently with the occasional occurrence of local and regional Chunxi 1174–1189 16 2 Shaoxi 1190–1194 5 3 fires. The warmest and wettest period was from 6040 to 2170 calyrBP Qingyuan 1195–1200 6 1 during the Holocene, especially 6000–5000 cal yr BP, which reflected Chunyou 1241–1252 12 1 the most warm-moist climate condition. Namely, the mixed forest of Xianchun 1265–1274 10 1 deciduous broadleaf trees and evergreen broadleaf trees occupied most Yuan Dade 1297–1307 11 1 Taiding 1324–1328 5 2 areas of this basin. The charcoal content was low and fire activity was Tianli 1328–1330 3 1 weak. However, during the period of cultural flourishing, the charcoal Ming Xuande 1426–1435 10 1 content peaked as a result of human activities. After 2170 cal yr BP, in Chenghua 1465–1487 23 3 the context of a drying and cooling climate, the increase in human Hongzhi 1488–1505 18 3 activity intensified the frequency and intensity of fire activities, leading Zhengde 1506–1521 16 3 Jiajing 1522–1566 45 14 to a greatly increased charcoal content and rapidly decreased pollen Longqing 1567–1572 6 2 concentrations of Cyclobalanopsis and Quercus. The vegetation also Wanli 1573–1620 48 6 changed from the mixed forest of deciduous broadleaf trees and ever- Tianqi 1621–1627 7 1 green broadleaf trees to the grassland with Gramineae as the main Chongzhen 1628–1644 17 5 Qing Shunzhi 1644–1662 18 4 components. The decrease in the charcoal concentration in the recent Kangxi 1662–1723 61 16 200 years might be related to the lack of biomass to burn in the vicinity Yongzheng 1723–1736 13 2 of the lake area. Qianlong 1736–1796 60 12 Jiaqing 1796–1821 25 6 Daoguang 1821–1851 30 2 Xianfeng 1851–1862 11 3 Acknowledgments Guangxu 1875–1909 13 2 Xuantong 1909–1912 24 3 This study was supported by grants from the National Natural Republic of – 1912–1949 27 12 Science Foundation of China (No. 41771221), the China Postdoctoral China Science Foundation (No. 2018M632403), the Public Geological Survey Project of Anhui Province (No. 2016-g-3-32), the National Innovation whereas the concentration of micro-charcoal was further increased. Training Program for College Students (Nos. 201810370207, Thus, the F/C value increased, with the maximum value in the 201810370202), and the Open Research Fund of Key Laboratory of section, indicating that there was not sufficient biomass near the Digital Earth Science, Institute of Remote Sensing and Digital Earth, lake to cause fire, but regional fires often occurred further away,as Chinese Academy of Sciences (No. 2015LDE012). Special thanks go to the amount of charcoal from the surrounding areas increased. Prof. Enlou Zhang, Dr. Xiayun Xiao, Dr. Houchun Guan, Dr. Yulian Jia, Dr. Qingfeng Jiang, Jun Xun (Senior Engineer), Rui Ke, Wei Xie and 5. Conclusions Guiping Lin for their support and help with field sampling and ex- perimental analysis. The charcoal in the sediment of Chaohu Lake mainly originated

Appendix A

The pollen spectrum of the CH-1 core has been only partially published (Wang et al., 2008a, 2008b; Wu et al., 2010), thus we list the complete percentage and concentration diagrams of sporopollens and charcoals as follows (Fig. A1 and A2). Original sporopollen and charcoal data in EXCEL format can be freely downloaded in the East Asian Paleoenvironmental Science Database, Chinese Academy of Sciences (http://paleo-data.ieecas.cn/ en/shujushow.asp?pid=572).

69 L. Wu et al. Quaternary International 507 (2019) 62–73

Fig. A1Comprehensive diagram of sporopollen percentage, total pollen grains and charcoal concentrations from the sediment of CH-1 core in the Chaohu Lake basin.

Fig. A2Diagram of sporopollen concentration from the sediment of CH-1 core in the Chaohu Lake basin.

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