Pollen record of precipitation changes during the Younger Dryas and Early Holocene in the North China Plain

Pollen record of precipitation changes during the Younger Dryas and Early Holocene in the North China Plain

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Quaternary International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

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Pollen record of precipitation changes during the Younger Dryas and Early Holocene in the North China Plain Wensheng Zhanga,b, Baoshuo Fana,b, Yuecong Lia,b,∗, Qinghai Xua,b, Bing Lia,b,∗∗, Guoqiang Dinga,b, Junfan Zhanga,b a b

College of Resources and Environment Science, Hebei Normal University, Shijiazhuang, 050024, PR China Key Laboratory of Environmental Evolution and Ecological Construction of Hebei Province, Hebei Normal University, Shijiazhuang, 050024, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Younger dryas Early holocene Ningjinpo lake Precipitation enhancement event Pollen assemblages Vegetation succession

The Younger Dryas event, which was the most recent millennial-scale cold event, had a major impact on the global environment. However, there were substantial regional differences in precipitation during the Younger Dryas. Ningjinpo Lake was formed at the end of Late Pleistocene and was formerly the largest lake in the North China Plain. Its sedimentary record is well suited for studying environmental changes during the Younger Dryas. We used pollen and grain-size analyses of 69 lake sediment samples, combined with 1190 modern surface samples and a radiocarbon chronology, to characterize the processes of vegetation succession and precipitation change during the Younger Dryas and Early Holocene in the Ningjinpo area of North China Plain. The results show that during the Younger Dryas the sediments were relatively coarse-grained and the pollen assemblages were dominated by herb taxa such as Poaceae, Artemisia, and Asteraceae, indicating that the vegetation was mainly meadow steppe or forest meadow, and that the climate was generally dry. The annual average precipitation was ~510 mm, which was ~100 mm lower than today. However, four events of enhanced centennialscale precipitation are evident in the record, characterized by increases in tree pollen representation: during 12,350–12,250, 12,200–12,100, 11.900–11,800 and 12,650–12,550 cal yr B.P. During these events, precipitation was ~50 mm higher than before and afterwards. In the late part of the Younger Dryas (after 12,100 cal yr B.P.), the representation of tree pollen increased, indicating an increase in forest dominated by Pinus and Betula, and precipitation increased to ~560 mm. After 11,440 cal yr B.P., at the beginning of the Holocene, the arboreal pollen percentages exceeded 60%, indicating that the regional vegetation was dominated by Pinus, Ulmus and Quercus forest, and the annual average precipitation was ~620 mm, which was close to that of today.

1. Introduction The Younger Dryas (YD) event, which was the most recent millennial-scale cold event, had a major impact on the global environment and has attracted extensive research attention (Severinghaus et al., 1998; Broecker et al., 2010). Although there is a consensus that the Younger Dryas was generally cold (Xu et al., 2017; Zhong et al., 2015), the nature of precipitation conditions remains controversial (Ding et al., 2014; Liu et al., 2019). Although in most regions the YD was dry, as in southern France, Israel, Turkey and Italy (Genty et al., 2006; Fleitmann et al., 2009; Bar-Matthews et al., 2003), it was humid in the Southwestern United States (Asmerom et al., 2010; Pigati et al., 2009).

Records from Qinghai Lake and Dalian Lake in the northwest arid region of China (Cheng et al., 2010; Ma et al., 2011), an organic carbon record from Huguangyan Maar Lake in southern China (Wang and Liu, 2001), a geochemical record from Dingnan Lake (Shang et al., 2018), and pollen records from Gonghai Lake in North China (Chen et al., 2015), all reveal a dry Younger Dryas. In contrast, however, records from Donggi Cuna Lake (Saini et al., 2017) in the southeastern Tibetan Plateau and Hani peatland (Hong et al., 2010) in Northeast china indicate that the climate was humid. In addition, many studies have documented climatic instability within the Younger Dryas; for example, the δ18O record from the Greenland GISP2 ice core reveals several centennial-scale climatic oscillations during the Younger Dryas (Stuiver

∗ Corresponding author. College of Resources and Environment Science, Hebei Normal University, No.20 Road East, 2nd Ring South, Shijiazhuang, 050024, PR China. ∗∗ Corresponding author. College of Resources and Environment Science, Hebei Normal University, No.20 Road East, 2nd Ring South, Shijiazhuang, 050024, PR China. E-mail addresses: [email protected] (Y. Li), [email protected] (B. Li).

https://doi.org/10.1016/j.quaint.2019.10.017 Received 17 June 2019; Received in revised form 9 October 2019; Accepted 27 October 2019 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article as: Wensheng Zhang, et al., Quaternary International, https://doi.org/10.1016/j.quaint.2019.10.017

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natural vegetation is mainly deciduous broadleaved trees and coniferous and broadleaved mixed forest. However, most of the original vegetation has been destroyed and the existing vegetation mainly comprises secondary forest with Pinus, Cupressus, Populus, Salix, Ulmus, Sophora. Cultivated plants include wheat, millet and corn (Liu, 1996).

and Grootes, 2000). In addition, loess records from China indicate a dry-wet-dry pattern of monsoonal fluctuations during the Younger Dryas, against a background of overall dry conditions (Zhou et al., 2001). A stalagmite record from Kulishu Cave in northern China reveals three centennial-scale monsoon intensification events within the Younger Dryas (Ma et al., 2012), and records from Qingtian and Hulu caves in the middle and lower reaches of the Yangtze River, respectively, reveal three centennial-scale abrupt wet and dry events (Liu et al., 2013; Wang et al., 2001). As a temperate monsoon region, the North China Plain plays a key role in linking climatic events across mid-low latitudes and mid-high latitudes (Shen et al., 2018). The region is also important for studying the advance and retreat of the East Asian summer monsoon and its driving mechanism (Li et al., 2018b). Due to the limitations of sedimentary archives in the North China Plain, there have been few studies of climate change (Zhou et al., 2001; Ding et al., 2014), and hence little is known about changes in dryness/wetness within the Younger Dryas. Located in the middle of the North China Plain, Ningjinpo Lake was formed in the Late Pleistocene, which was the largest lake in the region, and dried up until the mid-late 19th century (Shi, 2007; Guo et al., 1999). The sedimentary record of the lake is potentially valuable for the high-resolution reconstruction of climate and vegetation within the North China Plain during the Younger Dryas. In this study, we obtained four AMS14C dates and conducted measurements of sediment grain-size and pollen analyses of 69 sediment samples; in addition, we analyzed 1190 modern pollen surface samples using the WA-PLS method. Our aims were to characterize at high-resolution the processes of vegetation succession and precipitation change during the Younger Dryas and Early Holocene in the Ningjinpo area of the North China Plain, and to explore short-term climatic fluctuations and their driving mechanism during the Younger Dryas.

3. Materials and methods 3.1. Sampling collection and dating The sampling site is at Dacaozhuang (DCZ) (37°30′57.5″N, 114°57′33.8″E; a.s.l. 24 m) in Ningjin County (Fig. 1) within Ningjinpo Lake. Sampling was conducted at a 2-cm resolution within the depth interval of 476–630 cm, except for shell layers which were sampled at 4-cm resolution; 69 samples were obtained. Due to the lack of plant residues, three shell samples were taken from the section and one shell sample (DCZ-A) was taken from 40 to 42 cm above the top of section (434–436 cm above the surface). Four shell samples were used for AMS 14 C dating which was conducted by the Beta Analytic Laboratory (Miami, Florida). Radiocarbon dates were calibrated to calendar years using the BetaCal3.9 program (Ramsey, 2009), with the IntCal13 database (Reimer et al., 2013). The age-depth model was established by fitting spline functions using WinBacon 2.2 (Blaauw and Christen, 2011) and R statistical software (R core team, 2017). The results are listed in Table 2 and the age-depth model for the DCZ section is shown in Fig. 3. 3.2. Grain-size analysis Grain-size analyses were conducted at the Laboratory of Environmental Evolution, College of Resources and Environmental Sciences, Hebei Normal University, using a Malvern Mastersizer 3000 (Malvern Corporation, U.K.) laser grain-size analyzer. The measurement range is 0–3500 μm. The samples were sequentially pretreated with H2O2 and HCl to remove organic matter and carbonates, respectively. A minimum of three analyses were performed for each sample and the results were averaged (Peng et al., 2005; Weiss and Frock, 1976; Liu and Luo, 2013).

2. Regional setting Ningjinpo Lake (37°–37°30′N,114°40′–115°15′E) (Fig. 1) is in the warm, temperate semi-humid region of the monsoon zone of China. Several small rivers sourced from the western hills enter the lake, including the rivers Fuyang, Li, Wu and Huai, while the Zhang River enters the lake from the south and the Hutuo River enters from the north. These rivers have deposited diluvial fans within the lake basin. Due to differences in sediment accumulation, a relatively low-lying area was formed within the region (Guo et al., 1999; Shao et al., 1989). Precipitation and temperature in the Ningjinpo area are strongly seasonal; the annual average temperature is 12–13 °C and the annual average precipitation is 600–700 mm (Yang et al., 2018). The modern

3.3. Pollen analysis Samples for pollen analysis were sequentially pre-treated using a modified HCL-NaOH-HF procedure (Fægri et al., 1989). For each sample, 30 g of sediment was weighed before chemical treatment and one tablet of Lycopodium spores (27,560 grains) was added to enable

Fig. 1. Location of the study area in China and the sampling site (DCZ) within Ningjinpo Lake (Zhang, 1999). 2

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Table 1 Results of the calibration model using WA-PLS model errors are expressed as root mean square error (RMSE), and prediction errors as root mean square error of prediction (RMSEP), calculated by bootstrapping. R2 is the coefficient of determination. Variable

Code

RMSE

R2

RMSEP

%Change

Rand. t-test

Pann

Component 1 Component 2 Component 3 Component 4 Component 5

86.1 77.2 72.6 69.7 68

0.858 0.886 0.899 0.907 0.911

88.4 83.1 82.8 84.2 85.9

– 6.025 0.327 −1.715 −2.02

– 0.001 0.001 0.332 0.841

Table 2 Results of AMS14C dating of the DCZ profile from Ningjinpo Lake. Sample

Depth (cm)

Material

Radiocarbon age (yr B.P.)

2δ calibrated age (cal.yr B.P.)

Median age (cal.yr B.P.)

DCZ-A DCZ-19 DCZ-26 DCZ-69

434–436 510–514 542–544 628–630

Shell Shell Shell Shell

8790 ± 50 9960 ± 40 10350 ± 30 11310 ± 60

9603–9953 11835–11875 12033–12250 13069–13279

9778 11855 12141 13175

Ningjinpo lake. the calculation of the pollen concentration. After chemical treatment, pollen and spores were extracted using heavy liquid flotation. The procedures were carried out at the College of Resources and Environmental Sciences of Hebei Normal University. Pollen identifications were made at ×400 under an Imager A2 optical microscope with the aid of standard pollen reference publications for China and reference material preserved in the Key Laboratory of Environmental Evolution and Ecological Construction of Hebei Normal University. A minimum of 300 grains were counted for each sample, except for samples with very low abundance. A pollen diagram was plotted using Tilia 1.7.16 and pollen assemblage zones were defined using stratigraphically-constrained cluster analysis (CONISS) (Grimm, 2011).

3.5.2. Approaches of reconstruction In this study the WA-PLS method was implemented and the pollen data were square-root transformed using C2 software (Steve et al., 2007). The advantage of the method is that, given the non-linear relationship between pollen spectra and climate (Braak and Juggins, 1993), pollen assemblages (rather than a small number of pollen types) are used to establish a quantitative relationship between pollen and climate, which improves the reliability of the reconstruction. The results showed that the best model was the 2nd component for annual average precipitation (Pann), annual average temperature (Tann), average temperature of the coldest month (MTco) and average temperature of the warmest month (MTwa) (Table 1 and Fig. 2).

3.4. Principal components analysis

4. Results

Principal components analysis (PCA) is widely applied to the analysis of multivariate datasets in the environmental sciences. Its main aim is to reduce the dimensionality of a complex multivariate dataset (Weng et al., 1993; Jiang et al., 2013; Davies and Fall 2001). Canoco 5 software was used for the analysis and only taxa with an average representation of at least 1% were selected for analysis and the pollen data were square-root transformed (Braak and Smilauer, 2002, 2012).

4.1. Lithology and chronology The sediment lithology was mainly yellow-gray and black-gray clayey silt, with horizontal bedding; several shell layers are present (Fig. 3). Based on the age-depth model derived from the AMS 14C dates, the age of each sample was estimated by linear interpolation and extrapolation. The results are listed in Table 1 and the age-depth model for the DCZ section is shown in Fig. 3. The calibrated age of the top of the profile (476 cm) is ~10,900 cal yr B.P., and the age of the base (630 cm) is ~13,200 cal yr B.P. Previous research has indicated that the hard water effect is negligible in the region (Shen et al., 2018).

3.5. Quantitative climatic reconstruction 3.5.1. Surface pollen samples and meteorological data The surface pollen dataset comprised 2486 surface samples, including 2411 samples from the surface pollen database of China (Zheng et al., 2008) together with 75 samples collected by our research group. These data cover a wide environmental gradient, including tropical and subtropical regions. The fossil pollen assemblages studied in the profile are dominated by temperate plants, and therefore we screened the original data to remove samples from the southern part of the northern subtropics, together with samples with large errors. Specifically, taking the modern precipitation and temperature conditions in the Ningjinpo area into account, we only selected samples with annual average temperature within the range of 2–16 °C, and with annual average precipitation of < 1000 mm to ensure reliable estimates of the precipitation and temperature optima and tolerances of the major pollen taxa, and hence a robust precipitation and temperature reconstruction. Application of this screening procedure produced a final total of 1190 samples. Most of the pollen surface sampling sites were distant from weather stations, which made it difficult to acquire meteorological data that corresponded exactly to the sites. Therefore, based on the systematic spatial variation of different climatic parameters, we processed the meteorological data of 375 weather stations from the China Meteorological Data Sharing Service System (http://cdc.cma.gov.cn) using Polation software, which conducts linear spatial interpolation of meteorological elements. This enabled us to obtain estimates of annual average precipitation (Pann), annual average temperature (Tann), average temperature of the coldest month (MTco) and the average temperature of the hottest month (MTwa) at all the sampling sites, which enabled the quantitative reconstruction of the paleoclimate of

4.2. Grain-size distributions The grain-size results show that the sediments consist mainly of silt (4–63 μm) (average of 65%), followed by sand (> 63 μm) (average of 23.1%), and clay (< 4 μm) (average of 11.9%). The grain-size characteristics can be used to divide the sequence into four zones which are illustrated in Fig. 4 and described below. Zone 1 (630-606 cm, 13,200~12,900 cal yr B.P.) - The median grainsize ranges from 24.4 to 37.7 μm (average of 31.8 μm), which is the lowest within the profile, and the clay content ranges from 12.6 to 15.5% (average of 13.7%), which is the highest within the profile. The average silt content is 60.3% (> 55% in most intervals) and the average sand content is ~26% (> 23% in most intervals). Zone 2 (606-542 cm, 12,900~12,130 cal yr B.P.) - The average median grain-size is 34.2 μm and the median grain-size increases to the maximum within the profile. The contents of clay and sand are substantially lower than in zone 1, with average values of 11.2% and 21.9%, respectively. The average silt content is higher than in zone 1, ranging from 48 to 78.6% (average of 66.9%). Zone 3 (542-496 cm, 12,130~11,440 cal yr B.P.) - The average median grain-size is 34 μm and the values are relatively uniform. The silt content is substantially lower than in Zone 2, reaching 62%, while the clay and sand content are substantially increased, averaging 12.3% and 25.7%, respectively. Zone 4 (496-476 cm, 11,440~10,900 cal yr B.P.) - The median grainsize is slightly lower than in zone 3, with an average of 33.6 μm. The 3

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Fig. 2. Distribution of residuals and the relationship between the observed and estimated values of Pann using the 2nd component of WA-PLS.

(excluding algal spores) were counted, with an average of 308 per sample and an average concentration of 147 grains/g. Common tree pollen types are Pinus, Picea, Quercus, Ulmus, Betula, and Carpinus; common shrub pollen types are Spiraea and Rosaceae; and common herb pollen types are Artemisia, Chenopodiaceae, Poaceae, Asteraceae, Cyperaceae, Typha, Myriophyllum, Urtica, and Humulus. Fern spores are mainly Selaginellaceae and Triletes. The Ningjinpo profile can be divided into four pollen zones based on CONISS analysis, which are shown in Fig. 5 and described below. Pollen zone 1 (630-606 cm, 13,200~12,900 cal yr B.P.) - This zone consists of 12 samples. Pollen concentrations average 335 grain/g, which is the maximum within the profile. The tree pollen representation ranges from 7.8 to 50% (average of 25%) and is dominated by Pinus (21.1%). The average representation of broadleaved trees is 3.5%, including Quercus, Ulmus, Betula and Carpinus. The herb pollen representation ranges from 46.5 to 90.2% (average of 71.4%), dominated by Poaceae (39.7%). The representations of Chenopodiaceae (9%), Typha (9.2%), Cyperaceae (2.40%) and Myriophyllum (2.9%) are the highest within the profile. Shrubs are poorly represented (0.4%). Fern spores average 3.2% and are dominated by Triletes (1.6%). Pollen zone 2 (606-542 cm, 12,900~12,130 cal yr B.P.) - This zone consists of 32 samples. The average pollen concentration is 118 grain/g, substantially lower than in zone 1. The tree pollen content is less than 30% throughout, with an average of 27.4%. Broadleaved tree pollen decreases to ~2.1%, which is the minimum within the profile. The average herb pollen content is 67.5%, dominated by Poaceae (23.39%). Artemisia and Asteraceae, averaging 14.9% and 8.3%, respectively, and they reach maxima within the profile. The representation of the hygrophilous/aquatic taxa Typha, Cyperaceae and Myriophyllum decreases to 3%, 0.4% and 0.1%, respectively. Pollen zone 3 (542-496 cm, 12,130~11,440 cal yr B.P.) - This zone consists of 15 samples. The average pollen concentration is 132 grain/g, substantially higher than in zone 2. Tree pollen representation (average of 55.8%) increases and is dominated by Pinus (generally > 40%.). Broadleaved trees increase substantially (maximum of 6.6%), and Betula (1.2%) and Carpinus (1.3%) reach their highest values within the zone. Herb pollen decreases substantially (generally < 50%) and is dominated by Poaceae (23%), Artemisia (7.4%) and Asteraceae (1.9%). There is an increased representation of hygrophilous/aquatic taxa such as Cyperaceae (0.5%) and Myriophyllum (0.4%). Pollen zone 4 (496-476 cm, 11,440–10,900 cal B.P.) - This zone consists of 10 samples. The pollen concentration decreases to 31 grain/ g. The tree pollen representation increases to 67%, reaching the highest

Fig. 3. Age-depth model of the Ningjinpo DCZ profile.

contents of clay and sand decrease substantially, with averages of 11.2% and 20%, respectively, and are the lowest within the profile. The silt content increases to 69%, which is the maximum within the profile. 4.3. Pollen assemblages A total of 66 fossil pollen and spore types were identified in 69 samples, including 18 tree pollen types, 7 shrub pollen types, 36 herb pollen types and 5 fern spore types. A total of 21,283 pollen and spores 4

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Fig. 4. Profiles of selected grain-size parameters for the DCZ profile.

(Fig. 6). Therefore, we conclude that the first principal component mainly reflects changes in moisture or precipitation, with positive values representing wetter conditions, and negative values reflecting drier conditions. The variable loadings on PCA axis 2 do not exhibit a clear pattern and it is difficult to determine its environmental significance.

of the profile, and is dominated by Pinus (generally > 50%). The representation of broadleaved trees increases substantially (up to 7.5%), dominated by Ulmus (4%) and Quercus (1.9%); however, Betula decreases to 0.7%. The shrub pollen representation increases to 4.4%, reaching the highest within the profile, and is dominated by Spiraea (3.7%). The representation of herb taxa decreases sharply (average of 24%) and reaches the minimum within the profile; among them, Poaceae (9.7%) and Chenopodiaceae (1.1%) decrease to profile minima, while the representation of Cyperaceae (1.7%) and Myriophyllum (0.5%) increase.

4.5. Quantitative reconstruction of annual average precipitation (Pann) during the Younger Dryas and the Early Holocene in the north China Plain A statistically significant relationship could be established between pollen assemblages and Pann (R2Boot = 0.87; RMSEP = 83 mm), Tann (R2Boot = 0.63; RMSEP = 2.0 °C), MTco (R2Boot = 0.67; RMSEP = 2.9 °C), and MTwa (R2Boot = 0.50; RMSEP = 2.5 °C). The R program was used to test and verify the reliability of the screened surface soil pollen data for quantitative paleoclimatic reconstruction. The results show that only those for Pann are significant (Fig. 7) and therefore the other climatic factors were excluded from further analysis. Moreover, the relationship between observed and predicted Pann is statistically significant which indicates that the model is reliable and suitable for application to the fossil pollen data. The results of quantitative reconstruction of Pann during the Younger Dryas and Early Holocene in the North China Plain are presented in Fig. 8. The sequence can be divided into four periods, which are described below.

4.4. Ecological significance of the pollen types In order to interpret the ecological significance of the pollen results, we performed principal components analysis on the 69 pollen types from the DCZ profile. Only taxa with a representation of > 1% were used: Pinus, Quercus, Ulmus, Betula, Carpinus, Spiraea, Rosaceae, Artemisia, Chenopodiaceae, Poaceae, Asteraceae and Cyperaceae. The first and second principal components account for 54% and 14.2% of the total variance respectively. Tree pollen types (e.g. Pinus and Ulmus) and hygrophilous taxa (e.g. Cyperaceae) have positive loadings on the first principal component axis (PCA axis 1), and drought-tolerant taxa (e.g. Poaceae and Asteraceae) have negative loadings on PCA axis 1 5

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Fig. 5. Pollen percentage and concentration diagram for the Ningjinpo DCZ profile.

very similar to those of Period 1, but there are fluctuations and at least two distinct stages of precipitation enhancement are evident. Pann ranges from 427 to 583 mm, with an average of 496 mm. Period 3 (12,130~11,440 cal yr B.P.) - Pann ranges from 467 to 617 mm, with an average of 556 mm, The values are generally above 530 mm and are substantially higher than during Period 2. Period 4 (11,440~10,900 cal yr B.P.) - Pann ranges from 570 to 668 mm with an average of 623 mm, which represents an increase of ~70 mm compared with Period 3. The values are 580 mm and are the highest within the studied interval. 5. Discussion 5.1. Vegetation succession during the Younger Dryas and Early Holocene in the Ningjinpo area 5.1.1. Younger Dryas During the Younger Dryas interval, herb pollen is dominant, especially that of drought-resistant herb taxa such as Artemisia and Poaceae, indicating that the vegetation was meadow steppe or forest meadow. The quantitative reconstruction results showed that the annual average precipitation was low, averaging ~510 mm. Nevertheless, there are substantial changes in the pollen spectra, which are described below. Pre-Younger Dryas (13,200~12,900 cal yr B.P.)- The pollen concentration is the highest within the entire sequence. The herb pollen percentages are generally higher than 65% and are dominated by drought-resistant taxa such as Artemisia, Chenopodiaceae, Poaceae, Asteraceae; however, hygrophytic taxa such as Typha and Cyperaceae are also represented. The vegetation was dominated by forest meadow, indicating that the climate had become dry during the late BollingAllerod warm interval, but with pronounced dry and wet fluctuations. The stalagmite δ18O record from Linyi Cave in Shandong Province

Fig. 6. Variable loadings on the first and second axes of a PCA of the major pollen types of the DCZ profile.

Period 1 (13,200~12,900 cal yr B.P.) - The annual average precipitation (Pann) ranges from 423 to 547 mm, with an average of 487 mm, and is the lowest recorded within the studied interval. Period 2 (12,900~12,130 cal yr B.P.) - The precipitation values are 6

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previous period. The increased representation of broadleaved trees to a maximum of 6.8%, dominated by Quercus, Betula, Carpinus and Ulmus, indicates that the proportion of forest vegetation increased, and that the vegetation was open forest or forest meadow and that the climate had become warm and humid. Pann increased by at least 50 mm, with an average of 558 mm (460–620 mm). Moreover, a 34-cm-thick shell layer was deposited within this period, mainly composed of Lamprotula and Unio. The clam shells were relatively intact and in a closed state. The sharp upper boundary of the shell layer indicates the abrupt disappearance of mollusks, which may be related to increased of precipitation, the increase of lake water level, and the decrease of water transparency and dissolved oxygen (Xu, 2013). Such conditions are also reported for other regions: for example, pollen studies of Lake Gonghai in Shanxi showed that the coverage of pioneer tree taxa (e.g. Betula and Ulmus) increased during 12,000–11,100 cal yr B.P., while herb taxa began to decrease (Xu et al., 2017; Chen et al., 2015). 5.1.2. Early Holocene (11,440–10,900 cal yr B.P.) With the opening of the Holocene, the tree pollen content increased substantially (to > 60%), especially broadleaved trees such as Ulmus and Quercus, and reached the maximum within the profile. However, Betula decreased substantially, together with Artemisia and Poaceae, indicating that the climate became more warm and humid. Pann increased to an average of 623 mm, with a range of 600–650 mm. Vegetation development continued towards broadleaved forest dominated by Quercus and Ulmus, with Pinus representation, and the forest coverage was higher. However, the pollen concentration was low during this period, possibly because of the increased precipitation, and the resulting strengthened hydrodynamic forces (indicated by the increased silt content) may have adversely affected the accumulation or preservation of pollen (Li et al., 2018c). The vegetation characteristics at this time are also evident elsewhere in North China, such as at Daihai Lake (Xu et al., 2004), and in Northeast China at Sihailongwan Lake (Martina et al., 2009), Moon Lake (Wu et al., 2016) and Hulun Lake (Zhang et al., 2018).

Fig. 7. The statistical significance testing results of different climatic factors. Pann is the annual average precipitation, Tann is the annual average temperature, Mtwa is the average temperature of the hottest month, and Mtco is the average temperature of the coldest month. The solid black lines indicate the proportion of the variance explained by different environmental variables. The red dotted line indicates the proportion of variance below which 95% of the random data-trained calibration functions could be explained. The black dotted line indicates the proportion of variance explained by the first axis of a principal components analysis of the fossil pollen data. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

indicates a positive bias and reduced precipitation during 13,200–12,900 cal yr B.P (Li et al., 2018a). Early Younger Dryas (12,900~12,130 cal yr B.P.)- The pollen concentration is substantially lower than in the previous period. The representation of herb pollen is similar to that of the previous period, but the percentages of drought-resistant Artemisia are substantially increased. These changes indicate that the aridification trend had further intensified; this period was the driest within the entire profile, and the vegetation was dominated by meadow steppe. Similar vegetation and climatic characteristics are evident elsewhere in North China, such as at Gonghai Lake (Xu et al., 2017), Moon Lake (Wu et al., 2016) and Hulun Lake (Zhang et al., 2018) in Northeast China. Later Younger Dryas (12,130~11,440 cal yr B.P.)- Tree pollen percentages (mainly > 50%) are substantially increased compared to the

5.2. Evidence for precipitation events in the Younger Dryas in the north China Plain During the Younger Dryas, the sediment grain size of the DCZ section was generally coarse; sample scores on PCA axis 1 were mainly within the range of −1 and 0; and the Pann was 510 mm. These characteristics indicate that the climate was generally dry. However, the Younger Dryas was not consistently dry and there was evidence of fluctuations in drier and wetter conditions, against the background of an overall dry climate. Specifically, four events of increased precipitation on the centennial scale are evident, characterized by increases in tree pollen content to more than 30%, a finer sediment grain-size with a high clay content, and a precipitation increase of ~50 mm compared to

Fig. 8. Quantitative reconstruction of annual average precipitation (Pann) during the Younger Dryas and Early Holocene for the Ningjinpo DCZ profile. 7

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Fig. 9. Climatic records of the Younger Dryas and Early Holocene. (a) δ18O record from stalagmite H82 from Hulu Cave (red, Wang et al., 2001); (b) δ18O record from stalagmite QT16 from Qingtian Cave (green, Liu et al., 2013); (c) Precipitation reconstruction in Gonghai Lake (dark blue, Chen et al., 2015); (d) Pann reconstruction from Ningjinpo Lake (this study); (e) sample scores on PCA axis 1 of the pollen record from Ningjinpo Lake (this study); (f) 10Be flux from the Greenland GRIP and GISP2 ice cores (pink, Muscheler et al., 2004); (g) SST record from the northern margin of the western Pacific subtropical high (WPSH) (purple, Sun et al., 2005); (h) Mg/Ca-based SST record from the core of the western tropical Pacific Warm Pool: core MD98-2181 (orange, Stott et al., 2007) and core MD97-2141 from the Sulu Sea (blue, Rosenthal et al., 2003). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

spanning the Younger Dryas and Early Holocene, reveals vegetation changes from meadow steppe or forest meadow to forest landscape, with gradually increasing precipitation. Herb pollen was abundant before 11,440 cal yr B.P., dominated by drought-resistant taxa such as Poaceae, Artemisia and Asteraceae, indicating that the vegetation was meadow steppe or forest meadow. A quantitative paleoclimatic reconstruction indicates that the annual average precipitation was ~510 mm, which was ~100 mm lower than today. In the later part of the Younger Dryas (after 12,130 cal yr B.P.), the tree pollen representation was more than 50%, dominated by Betula, indicating an increase in the area of forest. Precipitation increased to 560 mm, but the climate was unstable and the precipitation was highly variable. The vegetation was forest or forest meadow, dominated by Pinus, Quercus and Betula. After 11,440 cal yr B.P., in the early Holocene, the tree pollen representation exceeded 60%, and the vegetation was broadleaved forest dominated by Pinus, Ulmus and Quercus. The precipitation reached 620 mm, close to that of today. (2) The climate of the Younger Dryas in the North China Plain was generally dry, but there were several centennial-scale events of increased precipitation. Three events of enhanced precipitation occurred during 12,350–12,250 cal yr B.P., 12,200–12,100 cal yr B.P. and 11,900–11,800 cal yr B.P., and may have been related to enhanced solar activity. A fourth event of increased precipitation, during 12,650–12,550 cal yr B.P., may have been related to movement of the subtropical high in the Western Pacific.

the preceding period. These events occurred during 12,350–12,250 cal yr B.P., 12,200–12,100 cal yr B.P., 11900–11,800 cal yr B.P. and 12,650–12,550 cal yr B.P. Except for the event at 12,650–12,550 cal yr B.P., the other precipitation events correspond well with monsoon enhancement events recorded in the eastern monsoon region of China (Ma et al., 2012; Wang et al., 2001; Liu et al., 2013) (Fig. 9). The Greenland NGRIP ice core record (Rasmussen et al., 2006), a δ18O record from Ammersee in southern Germany, and the stalagmite δ18O record from Chauvet Cave in southern France (Grafenstein von et al., 1999; Genty et al., 2006) also exhibit centennial-scale humidity enhancement events within the Younger Dryas. In addition, a marine sediment record from Santa Barbara Basin in the USA and a grayscale record from Cariaco Basin in South America both exhibit substantial wet and dry fluctuations within the Younger Dryas (Hendy et al., 2002; Hughen et al., 1996). Previous studies have shown that solar activity was strong during these three periods (Ma et al., 2012; Liu et al., 2013) (Fig. 9). However, the precipitation enhancement event during 12,650–12,550 cal yr B.P. has received relatively little mention. Most of the existing studies documenting precipitation enhancement during this period are from East Asia. For example, the pollen record of Hulun Lake in Northeast China reveals a substantial increase in deciduous forest dominated by Betula, Quercus and Alnus (Zhang et al., 2018); a peat cellulose δ13C record from the Northeastern Hani peatland indicates increased precipitation (Hong et al., 2010); a quantitative pollen-based climatic reconstruction from Ningwu Gonghai Lake in North China shows a precipitation increase of ~20 mm (Chen et al., 2015) (Fig. 9); and the stalagmite δ18O record from Linyi Cave indicates a negative bias and a precipitation increase during 12,600–12,500 cal yr B.P. (Li et al., 2018a). In addition, lake sediment records of grain size, chemical index of alteration (CIA) and δ13CVPDB from Jeju Island, South Korea, indicate a relatively humid climate and increased precipitation during 12,700–12,600 cal yr B.P (Park et al., 2014). Notably, however, there was no increase in solar activity at this time and we suggest that the driving mechanism for the increased precipitation during this event may have been related to movement of the subtropical high in the Western Pacific. Modern meteorological data and climate-model simulations suggest that high SSTs in the western tropical Pacific would enhance the upper-level convection, shift the western Pacific subtropical high (WPSH) over East Asia northward, and subsequently cause a northward shift of the monsoon front and rain band, which would result in increased summer precipitation in northern China and decreased summer precipitation in southern China (Liu et al., 2003; Huang et al., 2004). The intensification of monsoon convection caused by increasing SST in the western tropical Pacific (Stott et al., 2007; Rosenthal et al., 2003) during 12,650–12,550 cal B.P. would have forced the WPSH over East Asia to move northwards (Sun et al., 2005) and increase the meridional heat and water vapor transport from tropical regions to high latitudes (Liu et al., 2003), hence resulting in increased precipitation in northern China (Fig. 9). Notably, a stalagmite δ18O record from Dongge Cave (Dykoski et al., 2005), a grain size record from Dingnan Dahu Lake (Shang et al., 2018), and a pollen record from Dajiuhu Lake (Shi et al., 2008), showed that the climate in southern China was relatively dry during this period, which supports our inference.

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