Mid- to late-Holocene East Asian summer monsoon variability recorded in lacustrine sediments from Jingpo Lake,Northeastern China

Abstract

The Northeastern China involves complex interactions between the East Asian summer monsoon (EASM) circulation and the polar climate system, and plays a significant role as the bridge communicating low- and high-latitude climatic processes. High-resolution multi-proxy analysis of a robust accelerator mass spectrometry (AMS) ¹⁴C dated lacustrine sediment core recovered from Jingpo Lake in Northeastern China provides a detailed history of EASM variability and vegetation changes since ~5100 cal. yr BP. The period from ~5100 to 3600 cal. yr BP was characterized by the highest pollen percentages of Quercus, Ulmus, Juglans and Corylopsis; low Md (median grain size diameter); and high δ¹³C_org values, reflecting a relatively warm and humid period. The period between ~3600 and 2100 cal. yr BP is characterized by high Md and low δ¹³C_org values, and a rapid increase in pollen percentages of herbs, indicating cool and dry climatic conditions. From ~2100 to 150 cal. yr BP, a gradual increase in δ¹³C_org values and low Md values, and a rapid increase in Carpinus, Juglans and Corylopsis pollen percentages was observed, indicating climate change towards warmer and wetter conditions. After ~150 cal. yr BP, the highest values of total organic carbon mass accumulation rate (TOC-MAR), total nitrogen mass accumulation rate (TN-MAR) and magnetic susceptibility suggesting that the Jingpo Lake region has been severely affected by human activities. The EASM variability in Northeastern China during the mid- to late Holocene shows trends similar to EASM records in China. Furthermore, our findings indicate that the variability of the EASM during the mid- to late Holocene on the multi-decadal to centennial scale was forced by changes in both solar output and oceanic–atmospheric circulation interaction.

Keywords

climatic change climate forcing mechanisms East Asian monsoon Jingpo Lake mid- to late Holocene Northeastern China

Introduction

The Asian monsoon (AM) system consists of two important subsystems: the East Asian monsoon (EAM) and the Indian monsoon (IM). This system is an integral part of the global climatic system and plays a significant role in the global hydrological and energy cycles (Wang, 2006). Understanding the nature of AM variability during the Holocene is essential to understanding present climatic conditions, gaining insights into potential climate forcing mechanisms and predicting future climatic processes (Long et al., 2012). In recent decades, there have been numerous studies on Holocene monsoon climate changes in the EAM regions (Cosford et al., 2008; Dykoski et al., 2005; Hong et al., 2003, 2005; Ji et al., 2005; Shen et al., 2005, 2013; Wang et al., 2005, 2012a, 2012b; Yancheva et al., 2007; You and Liu, 2012). These records show a strengthened EASM during the early and mid-Holocene (~10,500–4500 cal. yr BP) and a relatively weakened EASM during the late Holocene (since ~4500 cal. yr BP), corresponding to orbital induced reduction in Northern Hemisphere (NH) summer solar insolation. However, there has still been some controversy regarding specific details of the Holocene monsoon climate changes (Long et al., 2010; Zhang et al., 2011). For instance, some recent high-resolution, well-dated monsoon reconstructions seem to suggest an abnormal increase in EASM strength during the late Holocene, against the generally weakening Holocene trend (Zhao et al., 2013). In addition, most of these investigations have been restricted to low- and mid-latitude monsoon regions, and there are no substantial high-quality and high-resolution paleoclimatic records of Holocene climate changes from the mid- and high-latitude monsoon regions (Chen et al., 2008; Herzschuh, 2006; Wang et al., 2010; Zhang et al., 2011).

Compared with the larger glacial–interglacial changes or at the beginning of the Holocene, the mid- to late Holocene (since ~5000 cal. yr BP) is of particular interest to understand the EASM because the boundary conditions of the climate driving forces in this interval did not change dramatically (Wanner et al., 2008). Global climate changes during the mid- to late Holocene show large oscillations, such as the Current Warm Period (CWP) after AD 1850, the ‘Little Ice Age’ (LIA) between AD 1350 and 1850, the ‘Medieval Warm Period’ (MWP) between AD 700 and AD 1350, the Dark Age Cold Period (DACP), the Roman Warm Period (RWP; Ge et al., 2010; Lamb, 1985; Mann and Jones, 2003; Mann et al., 2009) and the Holocene Event 3 (An et al., 2005; Bond et al., 2001; Wang et al., 2005; Wu and Liu, 2004); however, the timing and duration of such abrupt climatic events are different among various areas in the NH. Furthermore, the mid- to late Holocene climate changes and warm/cold oscillation events in the continental EAM region are still poorly understood. Thus, in order to understand the temporal and spatial variability of EASM during the mid- to late Holocene and its forcing mechanisms, more high-resolution regional paleoclimatic records are needed.

Northeastern (NE) China located in mid- to high latitudes of the Asian continent is influenced by tropical and polar climate systems. The paleoclimatic development of NE China has been of considerable interest because of the region’s sensitivity to climate fluctuations driven by the EASM and associated shifts in precipitation and temperature (Shen, 2012). Therefore, NE China is a key region for study of the Holocene EASM variability. In this study, we present the results of high-resolution multi-proxy (pollen, sedimentological, magnetic, geochemical and isotopic proxies) of a well-dated lacustrine sediment core from Jingpo Lake in Heilongjiang Province, NE China, which is utilized to reconstruct the EASM variation and vegetation history during the mid- to late Holocene. Furthermore, we compared our results to other paleoclimate records from the EAM region and other regions, aiming to better understand the history of variations in EASM intensity during the mid- to late Holocene and the possible climate forcing mechanisms.

Study region

Jingpo Lake (43°46′–44°03′N, 128°27′–129°03′E, ~350 m.a.s.l.) is located in Ning’an County, Southeastern Heilongjiang Province, NE China (Figure 1). It is China’s largest lava-dammed lake and was formed by lava flows dammed of the Mudanjiang River. The Jingpo volcanoes are located in the upper Mudanjiang River, approximately 40 km northwest of Jingpo Lake; they have been intermittently active during the Holocene (Chen et al., 2005; Fan et al., 2003). The length of Jingpo Lake from north to south is ~45 km, and the widest distance from east to west is only ~6 km. The surface area of the lake is ~92 km², and its catchment area of ~11,820 km² (Wang and Dou, 1998). Jingpo Lake is mainly fed by surface runoff and precipitation (maximum water depth: ~48.0 m; mean water depth: ~12.9 m). Approximately, 30 rivers flow into the lake; most of these rivers drain into the southern part of the lake, and the lake outflows from the northwest to the Mudanjiang River (Figure 1c). The southern region of the lake is shallow (mean water depth of ~5 m), while the northern region is the deepest (Wang and Dou, 1998). The lake is surrounded by the hills and low mountains, with bedrock mainly composed of granite, perlite and basalt (Li et al., 2011).

Figure 1.

(a) The atmospheric circulation systems in China; the black dashed line shows the modern Asian summer monsoon limit. (b) Map showing the location of the Jingpo Lake (red square), Heilongjiang Province. (c) Topographic map of Jingpo Lake catchment and the coring site; the red arrow indicates the water flowing direction of the Mudanjiang River. (d) Climate diagram from Ning’an county meteorological station near Jingpo Lake showing monthly temperature and precipitation.

The Jingpo Lake catchment is climatically controlled by the EAM system, causing distinct seasonal variations in regional heat-moisture (Figure 1d). In the summer, warm and humid air masses are transported by southeasterly winds from the western tropical Pacific; whereas in the winter, the climate is affected by the very dry and cold northwesterly winter monsoon caused by the Mongolia–Siberian high-pressure system (Gao, 1962; Wang, 2006). The 62-year (AD 1951–2012) meteorological records available from the Ning’an County meteorological station (~40 km north of Jingpo Lake) indicate a mean annual temperature (MAT) of 4.3°C with a maximum monthly mean of 22.1°C in July and a minimum of −17.7°C in January (Figure 1d). The study area has a mean annual precipitation (MAP; including snowfall) of ~545 mm, about 81% falling between May and September in association with the EASM, and the peak precipitation of ~123 mm in August (Figure 1d). Jingpo Lake is covered with ~70 cm of ice in winter, from December to March.

The modern native vegetation of the study area belongs to the temperate mixed-forest flora of Changbai Mountain and is dominated by C3 plants (Qian et al., 2003; Tan et al., 2009). The native vegetation is characterized by a Pinus koraieusis mixed conifer and broadleaved forest zone. The coniferous trees are dominated by P. koraieusis and also include Picea jezoensis, Picea koraiensis and Abies nephrolepis. The deciduous trees mainly consist of Betula costata, Betula platyphylla, Fraxinus mandschurica, Quercus mongolica, Phellodendron amurense, Juglans mandshurica, Ulmus propinqua, Carpinus cordata, Tilia amurensis, Tilia mandshurica, Populus ussuriensis, Acer mono and Acer mandshurica (Li et al., 2011; Qian et al., 2003). The shrub layer includes species such as Corylopsis, Corylus, Philadelphus, Spiraea, Lonicera and Rhododendron. Herbaceous species include Artemisia, Chenopodiaceae, Gramineae and Cyperaceae (Li et al., 2011; Zhou, 1997).

Materials and methods

Materials and dating method

In August 2012, three parallel sediment cores (JP-A, JP-B and JP-C, 43°59′N, 129°02′E) were collected from a water depth of 30.9 m in the Northeastern part of Jingpo Lake (Figure 1c), using Austria UWITEC™ piston coring system with PVC tubes in 2.0 m length and 60 mm diameter. The sediment cores were placed in PVC tubes and transported to the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. All sediment cores were stored at 4°C until analysis. In this study, the longest sediment core JP-C (1272 cm) was selected for analyses. Core JP-C was split in half, photographed and visual stratigraphy recorded. Core JP-C was mainly composed of black-brown silty clay, dark brown clayey silt and grey-brown sand, except for the core base part (1232–1272 cm), which mainly consisted of coarse sands and gravels. Sediments were sampled at 2-cm intervals, and all subsamples were dried with a vacuum freeze-dryer. This study will focus on the upper sediment (0–1154 cm) of core JP-C to study paleoenvironmental and paleoclimatic changes.

Reliable and robust chronology for lacustrine sediment cores is crucial for paleoclimate and paleoenvironment reconstructions (He et al., 2013). In this study, a total of 13 samples from the JP-C core were collected for accelerator mass spectrometry (AMS) ¹⁴C dating at Beta Analytic Inc., Miami, US, 9 from terrestrial plant fragments (leaves) and 4 from bulk organic matter (OM). All AMS ¹⁴C dates were calibrated to calendar years before present (0 cal. yr BP = AD 1950) using CALIB 6.0 (Stuiver and Reimer, 1993) with IntCal 09 calibration dataset (Reimer et al., 2009).

Laboratory analyses

Pollen analysis of lake sediments is commonly used to deduce paleovegetation and paleoclimatic changes. A total of 80 pollen samples were taken at 4–20-cm intervals. Each sample was treated with 10% HCl, 10% KOH, HF and a hot acetolysis mixture, in accordance with the standard methods (Faegri et al., 1989). At least 400 terrestrial pollen grains were counted per sample to ensure statistical consistency. Terrestrial pollen percentages were based on a sum of pollen excluding aquatic herbs, while percentages of aquatic herbs, ferns and algae were calculated using the total pollen and spore sum. Lycopodium spores with known numbers were added to each sample for the calculation of pollen concentrations before the chemical treatment. The program TILIA 1.18 (Grimm, 1993) and TGVIEW were used to calculate and plot the pollen data.

The grain size composition of sediments is generally used to describe changes of the sedimentary environment related to hydrologic energy variations (Lambiase, 1980). For grain size analyses, samples were pretreated with 10–20 mL of 30% H₂O₂ to remove OM, followed by several rinses with deionized water. The residue was treated with 10% HCl to remove carbonate, and followed by further rinses with deionized water. Biogenic silica was removed with a 20-mL 1M NaOH digestion (8 h at 60°C) and then treated with 20 mL of 0.05 M (NaPO3)₆ on an ultrasonic vibrator for 10 min to facilitate dispersion (Bird et al., 2014). Grain size distributions between 0.02 and 2000 µm were determined using a Malvern Mastersizer 2000 analyzer. Magnetic susceptibility was analysed using a Bartington Ltd MS2 Magnetic Susceptibility Meter linked to an MS2B Dual Frequency Sensor (470 and 4700 Hz).

Lacustrine sediment contains a number of reasonably reliable geochemical and isotopic proxies that help to characterize changes in past climate and ecosystems (Leng and Marshall, 2004). Total organic carbon (TOC) content and total nitrogen (TN) content were measured using a CE Model 440 Elemental Analyzer after removing carbonates with 10% HCl. Replicate analyses of well-mixed samples showed that precision was ca. ±<0.1% (1 standard deviation (SD)). Atomic C/N values were derived from these data. The pretreated samples for TOC measurements were also used for organic carbon isotope (δ¹³C_org) determinations. The δ¹³C_org values were determined using a Thermo Finnigan Delta^plus Isotope Ratio Mass Spectrometer. All isotopic values are presented in standard δ-notation in per million (‰) relative to the Vienna Pee Dee Belemnite (V-PDB). Analyses of replicates revealed a precision of ca. ±<0.1‰ (1 SD). All these methods were analysed at 4-cm intervals. All these analyses were performed in the State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences.

Numerical analyses

The pollen data presented here are complex and multivariate. Ordination provides an effective way to identify and visualize the main directions of variance. Detrended correspondence analysis (DCA) with detrending by segments and down-weighting of rare taxa helped to estimate the amount of pollen compositional change within the data. To summarize structural changes in the pollen data, DCA was performed using a normalized dataset with species/taxon abundances >1% of the total count. The DCA results revealed a maximum gradient length of 0.544, which is less than 2.5 SDs, suggesting that most of the underlying responses are linear or at least monotonic to the underlying latent variables (ter Braak and Smilauer, 2002). Therefore, principal components analysis (PCA) was used to analyse the pollen assemblages in relationship to environmental factors by using inter-species correlations and no transformation of pollen percentages. We used the percentages of 13 terrestrial pollen types with values greater than 1% in at least two samples for numerical analyses using the CANOCO program version 4.5 (ter Braak and Smilauer, 2002). The PCA results of 13 terrestrial pollen types with values greater than 1% from 80 samples reveal that the first and second principal components (PCA axis 1 and PCA axis 2) have eigenvalues of 0.472 and 0.158, explaining 47.2% and 15.8% of the total variance of pollen data, respectively.

Stratigraphic zones were mainly determined on the pollen data, grain size, χ_lf, total organic carbon mass accumulation rate (TOC-MAR), total nitrogen mass accumulation rate (TN-MAR), C/N and δ¹³C_org analyses results of the sediment core JP-C with CONISS in the TILIA program (Grimm, 1987). In the EAM area, evidence of cyclic monsoonal oscillations has been mainly derived from the δ¹⁸O records of well-dated stalagmites (Dykoski et al., 2005; Wang et al., 2005) and a few lacustrine sediments (Liu et al., 2009). Therefore, spectral analyses of detrended δ¹³C_org values (δ¹³C_{org-detrended}) were carried out using the REDFIT program (Schulz and Mudelsee, 2002). TOC-MAR and TN-MAR are better measures of delivery and preservation of OM than %TOC content (Lücke et al., 2003; Meyers and Lallier-Vergés, 1999; Wang et al., 2013b) and expressed as mass of TOC and TN per unit of lake bottoms area per unit of time, typically g cm⁻² yr⁻¹. TOC and TN were calibrated to TOC-MAR and TN-MAR using the sedimentation rate and dry bulk density.

Results

Lithology and chronology

Lithological change in JP-C generally coincides with median grain size (Md) and colour variation throughout the core (Figure 2). The lacustrine sediment of the upper 1154 cm can be divided into three lithological units: (1) From 1154 to 850 cm, the lithology consists primarily of black-brown silty clay and with the Md fluctuates between 2.9 and 8.8 µm with a mean of 5.1 µm; (2) dark brown clayey silt from 850 to 490 cm, the Md values range 3.4–11.0 µm (mean 7.8 µm), and there is a clear trend of increasing values upwards; and (3) black-brown silty clay above 490 cm, the Md fluctuates between 3.0 and 15.9 µm (mean 5.0 µm), with relatively small fluctuations, it first decreases, then increases gradually.

Figure 2.

Lithology, median grain sizes and age–depth model of the sediment core JP-C. Circles and squares represent the ages derived from terrestrial plant fragments and bulk organic matter, respectively. The open square marks the depth of the AD 1963 137Cs peak, and the solid diamond marks the modern sediment–water interface (AD 2012). The squares are dates that were not used in the age–depth model.

AMS ¹⁴C dating results for the core are presented in Table 1 and Figure 2. According to the results of four bulk OM, samples indicated that the hard-water effect could be 740–1380 years (Table 1; Figure 2). Terrestrial plant fragments (such as leaves, twigs and fruits) from lake sediments for AMS ¹⁴C dating results will not be influenced by the hard-water effect (Walker, 2005) and thus can yield reliable radiocarbon dates. The upper 40-cm sediments were analysed for ²¹⁰Pb/¹³⁷Cs dating (Liao et al., unpublished data). The dates were calculated using the constant rate of supply (CRS) model. The initial rise in ¹³⁷Cs activity occurs at 29 cm and can probably be ascribed to the onset of significant fallout in AD 1954 in the NH. A single maximum ¹³⁷Cs occurs at 23 cm, suggesting this dates to AD 1963 in accordance with the cessation of atmospheric nuclear weapons testing (Figure 2). Therefore, the chronological sequence for the core JP-C was established based on the two adjacent ²¹⁰Pb/¹³⁷Cs and/or terrestrial plant fragments calendar year ages using linear interpolation. The basal age of the upper 1154 cm lacustrine sediments is ~5100 cal. yr BP, and the age–depth model shows a linear relationship (r² = 0.996; Figure 2). The sedimentation rates of the upper 1154 cm varied from ~0.169 to 0.495 cm yr⁻¹, with a nearly constant sedimentation rate of ~0.228 cm yr⁻¹. In this study, the 4-cm interval corresponds to ~18-year resolution.

Table 1.

AMS ¹⁴C dates for JP-C core together with sampling location and dating materials. All ages were calibrated using CALIB 6.0 (Stuiver and Reimer, 1993) with IntCal 09 dataset (Reimer et al., 2009).

Lab number	Sample name	Core depth (cm)	Dating material	Age (¹⁴C yr BP ± 1σ)	¹³C/¹²C ‰	Calibrated age (cal. yr BP); 2σ ranges	Median age (cal. yr BP)
Beta-330281	JPP-1	28.5	Leaf fragment	Modern carbon	NA	Modern carbon	Modern
Beta-330291	JPP-11	100	Leaf fragment	110 ± 30	−29.8	10–270	140
Beta-330292^a	JPPS-1	100	Bulk organic matter	1000 ± 30	−27.0	800–960	880
Beta-330282	JPP-2	282	Leaf fragment	1150 ± 30	−26.8	980–1170	1075
Beta-330283	JPP-3	519	Leaf fragment	2210 ± 30	−27.0	2140–2330	2235
Beta-330293^a	JPPS-2	551	Bulk organic matter	2980 ± 30	−27.2	3070–3260	3165
Beta-330285	JPP-5	700	Leaf fragment	2790 ± 30	−25.4	2800–2960	2880
Beta-330286	JPP-6	750	Leaf fragment	2830 ± 30	−27.2	2860–3040	2950
Beta-330294^a	JPPS-3	750	Bulk organic matter	3840 ± 30	−26.5	4150–4410	4280
Beta-330287	JPP-7	815	Leaf fragment	3160 ± 30	−27.6	3350–3440	3395
Beta-330289	JPP-9	965	Leaf fragment	3850 ± 30	−27.1	4150–4410	4280
Beta-330290	JPP-10	1154	Leaf fragment	4420 ± 30	−28.0	4870–5260	5065
Beta-330295^a	JPPS-4	1154	Bulk organic matter	5670 ± 40	−26.3	6350–6540	6445

AMS: accelerator mass spectrometry.

The dates were not used for building up the age framework of JP-C core.

Multi-proxy records

Results of pollen, grain size, χ_lf, TOC-MAR, TN-MAR, C/N and δ¹³C_org analyses for the core JP-C are compiled in Figures 3 and 4. The JP-C record was divided into four obvious zones based on pollen data, geochemical and physical values using CONISS cluster analyses. The characteristics of these zones are described from bottom to top as follows.

Zone A (1154–852 cm; ~5100–3600 cal. yr BP). In this zone, the pollen percentages of conifer trees, broadleaf trees and shrubs were highest (means of 14.3% and 74.1%, respectively), such as the pollen percentages of Pinus, Quercus, Ulmus, Carpinus, Juglans and Corylopsis are highest for the entire core (Figure 3). On the contrary, the pollen percentages of herbs were lowest for the entire core (mean of 11.7%; Figure 3). The fractions of clay, silt and sand fluctuated between 21.0% and 62.1%, 37.1% and 77.3%, and 0.4% and 18.8%, averaging 43.8%, 53.4%, and 2.8%, respectively. The Md values range from 3.0 to 8.9 µm with a mean value of 5.1 µm. The χ_lf values were relatively low and range from 17.7 to 35.2 × 10⁻⁸ m³ kg⁻¹, with an average value of 23.7 × 10⁻⁸ m³ kg⁻¹. The TOC-MAR and TN-MAR values were relatively high: TOC-MAR ranged from 2.08 to 4.24 × 10⁻³ g C cm⁻² yr⁻¹ with an average value of 2.83 × 10⁻³ g C cm⁻² yr⁻¹ and TN-MAR ranged from 0.20 to 0.37 × 10⁻³ g C cm⁻² yr ⁻¹ with an average value of 0.27 × 10⁻³ g C cm⁻² yr⁻¹ (Figure 4). C/N ratios and δ¹³C_org values were highest for the entire core (means of 12.2 and −26.4‰, range from 11.3 to 14.3 and −27.4‰ to −25.9‰, respectively; Figure 4).

Zone B (852–492 cm; ~3600–2100 cal. yr BP). This zone was notable for the decrease in the pollen percentages of conifer trees, broadleaf trees and shrubs (means of 12.6% and 68.9%, respectively), and increase in the pollen percentages of herbs (mean of 18.5%; Figure 3).This zone was characterized by the lowest clay fraction content (mean of 32.2%, range from 15.2% to 57.4%) and χ_lf values (mean of 21.8 × 10⁻⁸ m³ kg⁻¹, range from 17.5 to 31.2 × 10⁻⁸ m³ kg⁻¹; Figure 4). The Md values ranged from 3.3 to 11 µm with a mean value of 6.8 µm. TOC-MAR and TN-MAR values were lowest for the entire core (means of 2.7 and 0.26 × 10⁻³ g C cm⁻² yr⁻¹, range from 1.8 to 4.1 × 10⁻³ g C cm⁻² yr⁻¹ and from 0.18 to 0.42 × 10⁻³ g C cm⁻² yr⁻¹, respectively; Figure 4). C/N ratios (mean of 11.8, range from 10.9 to 15.3) and δ¹³C_org values (mean of −27.1‰, range from −27.8‰ to −26.4‰) decreased slightly in this period (Figure 4).

Zone C (492–104 cm; ~2100–150 cal. yr BP). The pollen percentages of Pinus, Ulmus, Carpinus, Juglans and Corylopsis at the beginning of Zone C were relatively high, then gradually decreased, while the pollen percentages of herbs (mean of 23.0%) increased obviously (Figure 3). The Md values (mean of 5.0 µm, range from 3.1 to 16.0 µm) and silt fractions content (mean of 50.1%, range from 37.6% to 62.0%) decrease obviously (Figure 4). Notable features include the significant increase in the clay fraction (mean of 45.0%, range from 17.1% to 58.6%), sand fraction (mean of 5.0%, range from 0.7% to 36.1%) and χ_lf values (mean of 30.9 × 10⁻⁸ m³ kg⁻¹, range from 22.0 to 44.9 × 10⁻⁸ m³ kg⁻¹; Figure 4). The TOC-MAR, TN-MAR, C/N ratios and δ¹³C_org values increased slightly. TOC-MAR ranged from 1.94 to 3.92 × 10⁻³ g C cm⁻² yr⁻¹, with an average value of 2.96 × 10⁻³ g C cm⁻² yr⁻¹; TN ranged from 0.20 to 0.38 × 10⁻³ g C cm⁻² yr⁻¹, with an average value of 0.29 × 10⁻³ g C cm⁻² yr⁻¹ (Figure 4). C/N ratios range from 11.0 to 13.4 with an average value of 11.8 and δ¹³C_org values ranging from −28.3‰ to −25.8‰ with a mean of −27.0‰ (Figure 4). Notably, a period of fluctuation in almost all proxies was recorded at 268–328-cm interval (~1250–1050 cal. yr BP or ~AD 700–900; Figures 3 and 4).

Zone D (104–0 cm; after ~150 cal. yr BP). The pollen percentages of Pinus and Quercus decreased slightly, and the pollen percentages of herbs increased slightly (mean of 25.8%; Figure 3). The most striking features of this zone were the sharp increases in TOC-MAR and TN-MAR values (means of 6.6 and 0.67 × 10⁻³ g C cm⁻² yr⁻¹, range from 4.7 to 8.2 × 10⁻³ g C cm⁻² yr⁻¹ and 0.50 to 0.80 × 10⁻³ g C cm⁻² yr⁻¹, respectively) and χ_lf values (mean of 46.5 × 10⁻⁸ m³ kg⁻¹, range from 34.0 to 65.1 × 10⁻⁸ m³ kg⁻¹; Figure 4). The Md values drops to an average of 6.8 µm (range from 3.0 to 7.7 µm; Figure 4). C/N ratios (mean of 11.4, range from 10.8 to 12.3) and δ¹³C_org values (mean of −27.3‰, range from −28.1‰ to −26.8‰) decreased slightly (Figure 4).

Figure 3.

Simplified pollen percentage diagram and PCA sample axis 1 scores of core JP-C from Jingpo Lake. Exaggeration (×2) is indicated by grey shading. Black bars indicate terrestrial plant fragments AMS ¹⁴C dates of core JP-C with 2σ error.

Figure 4.

Variations in TOC-MAR, TN-MAR, C/N, δ¹³C of total organic matter, grain size proxies including the sand, clay, silt fractions and median grain size values, and magnetic susceptibility values of core JP-C in Jingpo Lake. The thick lines indicate 10-point smoothing results. Black bars indicate terrestrial plant fragments AMS ¹⁴C dates of core JP-C with 2σ error.

Discussion

Proxy interpretations

Pollen data

Modern Quercus, Carpinus, Corylus, Juglans and Ulmus pollen frequencies are highly positively correlated with the MAT and precipitation in NE China (Li et al., 2011). Broadleaved taxa (such as Quercus, Ulmus, Juglans, Corylus and Carpinus) and Corylopsis have negative loadings on PCA axis 1, whereas Betula and almost all herb taxa, such as Artemisia, Chenopodiaceae, Gramineae, Cyperaceae and Humulus, have positive side of PCA axis 1 (Figure 5a). The climate of the Jingpo Lake region is typically controlled by the temperate-humid oceanic monsoon climate. Meteorological data from northern China indicate that precipitation and temperature are key factors responsible for the spatial differentiation of modern vegetation (Gansert, 2004; Xu et al., 2014). Furthermore, broadleaved trees and Corylopsis favour a warmer and humid environment, whereas the herbs are able to tolerate cooler and drier conditions than broadleaved forest in the mountainous region of NE China (Zheng et al., 2007, 2008). In addition, several recent pollen record studies from Long Gang Volcanic Field maar and crater lakes (Lake Erlongwan, Lake Sihailongwan and Lake Xiaolongwan) in NE China indicate that broadleaved trees (Quercus, Ulmus, Juglans and Carpinus) and Corylopsis expanded corresponding to increase the intensity of the EASM (climate became warmer and wetter), however, increased the herbs (Artemisia, Chenopodiaceae, Gramineae and Cyperaceae) and Betula suggests that the climate became cooler and drier (Li et al., 2013; Stebich et al., 2009; Xu et al., 2014). Therefore, these results suggest that the PCA axis 1 generally represents the regional climate variations which were influenced by the EASM intensity, with the sample scores on PCA axis 1 showing positive values related to cold and dry climate conditions and the negative values related to warm and wet climate conditions. In summary, the high abundance of Quercus, Ulmus, Juglans, Carpinus, Corylopsis and the low PCA axis 1 scores suggest warm and wet climatic conditions, whereas high proportion of Artemisia, Chenopodiaceae, Gramineae and Cyperaceae and the high PCA axis 1 indicate cool and dry conditions (Figure 5a).

Figure 5.

(a) PCA ordination with the pollen percentage data. (b) Grain size distributions of different zones of sediment core JP-C. A, B, C and D represent the zone A, B, C and D, respectively. Scatter plots of core JP-C TOC-MAR, TN-MAR, C/N and δ¹³C_org data. (c) TOC-MAR versus TN-MAR. (d) C/N versus δ¹³C_org.

Grain size and magnetic susceptibility

Grain size composition variability in lacustrine sediment can indicate changes in the sedimentary environment (Lambiase, 1980) related to variations in hydrologic energy, such as transport capacity (Hakanson and Jansson, 1983; Liu et al., 2008) and lake levels (An et al., 2012). The core is mainly composed of clay and silt fraction, whereas the sand fraction generally less than 10% (Figure 4). Most frequency curves of grain size distribution for the sediments in core JP-C are unimodal (Figure 5b), because of the influence of a single sedimentary or transportation process. Jingpo Lake catchment is located in the central part of NE China surrounded by the Zhangguangcai mountains and Laoye mountains (in the Changbai Mountains System). The area is covered by flourishing vegetation and rarely aeolian activities. Thus, the sediments in core JP-C are most likely transported by river discharge, not by aeolian activities. Thus, grain size can be used to describe changes in the sedimentary environment related to hydrologic energy variations. According to the principle of lake sediment sorting (Lerman, 1978), the grain size of lacustrine sediments becomes finer from the shore to the centre, and sediment belts of different grain size fractions can be distinguished, namely, in lacustrine environments grain size variations through time can indicate water level fluctuations. Hence, the median of the grain size distribution is believed to be an indicator of paleoshoreline proximity. We assume that as the lake level rises, the median particle diameter of the sediment in the centre of the lake (i.e. the coring location) becomes smaller because of the greater distance from the shoreline (inflow) to the coring location. On the contrary, when the lake level is low, the shorter distance between the river mouth and the coring site may be the main reason for the coarser grain size fraction at the core site (An et al., 2012). Therefore, in this study, we interpret that the grain size distribution of Jingpo Lake sediments most likely reflects changes in the EASM precipitation, that is, the finer grain size fraction and lower median grain size might be explained by higher lake levels, which in turn reflects increased monsoonal precipitation over the lake region, and vice versa.

The magnetic susceptibility of lake sediments may indicate the relative proportion of magnetic minerals in the sediments. This proxy has been successfully used to reconstruct the climatic and environmental evolution of lacustrine sediments (Chen et al., 2013; Hodell et al., 1999). The bedrock surrounding the Jingpo Lake region consists mainly of granite, perlite and basalt, and the magnetic minerals in lake sediments derived from these volcanic rocks in the catchment are found in finer sediments. Therefore, variations in the χ_lf values were associated with catchment erosion, that is, increased χ_lf values indicated increase in erosive input concurrent with stronger EASM climate conditions. Specifically, the rise in catchment soil erosion was associated with comparatively high χ_lf values, which were correlated with stronger EASM periods (apart from the period ~1050–1250 cal. yr BP and ~150 cal. yr BP to present, which may have been seriously affected by human activities; Figure 4).

TOC-MAR, TN-MAR, C/N and δ¹³C_org of OM

The type and amount of lacustrine sedimentary OM reflects past fluctuations in lake productivity and terrestrial inputs linked to climate-induced environmental changes, catchment-related processes and lake internal changes (Leng et al., 2006; Meyers, 1997). Variations in TOC-MAR and TN-MAR values represent changes in primary organic productivity or changes in OM (Olsen et al., 2013). Lacustrine OM has two major types of source of OM in lake sediments: autochthonous and allochthonous (Meyers, 1994; Meyers and Teranes, 2001). C/N ratios and δ¹³C_org of lacustrine OM may be used to distinguish between different sources of OM and interpret related biogeochemical changes (Leng et al., 2006; Meyers and Teranes, 2001). The C/N ratios of algae and phytoplankton are only about 5–12 and are generally less than 10. Submerged and floating aquatic macrophytes or OM from mixed sources have C/N ratios between 10 and 20 (Hedges et al., 2002; Meyers, 1994). In contrast, C/N ratios in terrestrial vascular plants are greater than 20 (Meyers, 1994; Talbot and Lærdal, 2000). The C/N ratios in core JP-C were mostly less than 13 (mean of 11.9, range from 10.8 to 15.3) are slightly greater than aquatic source (<10). The significant positive relationship between TOC-MAR and TN-MAR (r² = 0.98, p < 0.001; Figure 5c) observed for Jingpo Lake sediments suggests that the inorganic nitrogen content in Jingpo Lake sediment was negligible (~0.05%). Therefore, in Jingpo Lake sediments, C/N ratios are mostly less than 13 (Figure 4), which indicates that OM of the core was mainly derived from a mixture of sources, especially algae and aquatic vegetation (both submerged and floating) and a low contribution from terrestrial vegetation (Leng et al., 2006; Meyers, 1997; Meyers and Lallier-vergés, 1999). Therefore, the TOC-MAR values reflect the primary productivity of aquatic plants and terrigenous material (Hodell and Schelske, 1998; Hollander et al., 1993; Meyers and Teranes, 2001).The primary isotopic signature of OM incorporated during its synthesis is not significantly altered during this process, and, most importantly, the relative isotope variations are well preserved (Hodell et al., 1999; Meyers and Lallier-vergés, 1999). Therefore, the δ¹³C_org values of lacustrine sedimentary OM have frequently been used as environmental proxies because these changes are directly or indirectly linked to changes in the environment and climate of the lake catchments (Leng and Marshall, 2004; Lücke et al., 2003); however, the δ¹³C_org values of lake sediments are complex and influenced by many factors, such as the OM derived from allochthonous or autochthonous sources (Meyers, 1994), the differential fractionation associated with the inorganic carbon source (CO₂, ${HCO}_{3}^{-}$ ) used for photosynthesis (Leng et al., 2006) and variable productivity (Leng and Marshall, 2004). In Jingpo Lake, all the samples have δ¹³C_org values below −25‰ (Figure 4), indicating the dominance of C3 plants as the principal carbon source. C3 plants utilize RuBisCO (ribulose-1, 5-bisphosphate carboxylase) to fix atmospheric CO₂, which results in δ¹³C_org values between −22‰ and −33‰ (O’Leary, 1988). This photosynthetic pathway is the most common among plants and is used by most trees, shrubs, aquatic vegetation and algae. The C/N ratios of Jingpo Lake indicate that the sedimentary OM is of mixed origin, and thus changes in the carbon isotope ratio would instead reflect changes in the contribution of different OM sources to bulk sedimentary OM. Variations in δ¹³C_org values are frequent because of the changes in primary organic productivity or changes in the fraction of allochthonous to autochthonous OM (Leng and Marshall, 2004). The δ¹³C_org values were not correlated with C/N ratios (Figure 5d), suggesting that δ¹³C_org is not controlled by changes in OM sources (i.e. from allochthonous to autochthonous sources). Therefore, primary productivity in lakes can influence OM δ¹³C_org values. Algae and aquatic plants preferentially utilize ¹²C to produce lacustrine OM (Hodell and Schelske, 1998); however, increased primary productivity in lakes and exhaustion of available light δ¹³C within the lake carbon pool result in the limitation of dissolved atmospheric CO₂ (δ¹³C = −7‰) and force algae and aquatic plants to utilize ¹³C-enriched dissolved ${HCO}_{3}^{-}$ (δ¹³C = 1‰) in the lake as their carbon source (Meyers and Teranes, 2001). In Jingpo Lake, most of the high/low δ¹³C_org values coupled with relatively high/low TOC-MAR from the core (Figure 4) may be attributable to primary productivity of lakes. Accordingly, we suggest that primary productivity of lakes (TOC-MAR) may play a significant role in driving the variability of δ¹³C_org values (apart from the interval ~1050–1250 cal. yr BP and ~150 cal. yr BP to present, which may have been significantly affected by human activities). Therefore, variations in δ¹³Corg values of autochthonous OM in Jingpo Lake can serve as a proxy of lacustrine paleoproductivity, that is, higher TOC-MAR and higher δ¹³C_org values indicate higher primary productivity and stronger EASM intensity and vice versa.

In addition, the strong correlation between χ_lf and the TOC-MAR and with δ¹³C_org values indicates a relationship between terrigenous input and carbon isotope composition. The influence of allochthonous OM on the δ¹³C_org values can be tested by applying the δ¹³C_org values of the plant material used for ¹⁴C dating as an estimate of the allochthonous OM δ¹³C_org endpoint. The average δ¹³C_org value of the macrofossil plant material is −27.4±1.2% (Table. 1). Using these δ¹³C_org values as an isotopic endpoint for allochthonous OM suggests that the sediment may contain allochthonous OM. Carbon isotope values of terrestrial plants reflect the balance between photosynthesis and stomatal conductance and vary with environmental conditions. Temperature is an important influential factor for plant δ¹³C (Wang et al., 2013a). Studies of C3 plants showed mostly positive correlations between δ¹³C and temperature (Pearman et al., 1976; Stuiver and Braziunas, 1987; Treydte et al., 2007; Wilson and Grinsted, 1977). Recently, a work derives a quantitative pattern between plant δ¹³C and temperature by measuring δ¹³C of a large number of plants growing along the 400 mm isoline of MAP in north and northeast China. After correcting for the effects of precipitation, a strong positive relationship was found between site-averaged δ¹³C of C3 terrestrial plants and MAT (Wang et al., 2013a). Therefore, the variation in the δ¹³C_org values of the allochthonous OM generally closely follows the mean annual air temperature. Jingpo Lake sediment δ¹³C_org values can be assumed to reflect MAT. In general, increase in δ¹³C_org values of sedimentary OM in Jingpo Lake may reflect compound climate signals resulting from the two kinds of forcing factors: the increasing lacustrine paleoproductivity related to stronger EASM intensity and the MAT rising attributed to the intensification of EASM.

Mid- to late Holocene EAM variations and regional comparisons

High-resolution multi-proxy analyses of pollen data, grain size, magnetic susceptibility, geochemistry and stable carbon isotope in sediments from core JP-C in Jingpo Lake provide a detailed history of changes in the paleoenvironment and paleoclimate since ~5100 cal. yr BP.

During the period of ~5100–3600 cal. yr BP, the highest pollen percentages of Ulmus, Juglans and Corylopsis and the lowest pollen percentages of Artemisia and Chenopodiaceae indicate relatively strong EASM during this period (Figure 3). The increase in TOC-MAR and δ¹³C_org values are likely a result of increased primary organic productivity, but may also result from increased allochthonous OM content because the relatively high χ_lf values would be expected with increased runoff. This is also indicated by the relatively high C/N values (Figure 4). Therefore, high δ¹³C_org values coupled with relatively high TOC-MAR may be attributed to a relatively warm and humid period (Figure 4). The relatively high clay content and low Md value indicates that the lake level was high and the coring site was situated away from the shore. This relatively strong EASM period has also been inferred from δ¹⁸O record of Dongge Cave in eastern China (Dykoski et al., 2005; Figure 6c), the δ¹³C record of Hani Peat in Northeastern China (~4700–3700 cal. yr BP; Hong et al., 2005; Figure 6d), the redness record of Qinghai Lake close to the present EASM limit in northwest China (~5000–4000 cal. yr BP; Ji et al., 2005; Figure 6e) and the reconstructed Holocene sea surface temperatures (SSTs) in the western tropical Pacific Ocean (~5100–3600 cal. yr BP; Stott et al., 2004; Figure 6f). Around ~3600 cal. yr BP, the rapid increase in Artemisia and Chenopodiaceae pollen percentages and significantly reduced Ulmus, Juglans and Corylopsis pollen percentages suggest a weakened EASM circulation from ~3600 to 2100 cal. yr BP (Figure 3). The decrease in C/N ratios during this interval is consistent with χ_lf values, indicating a decline in delivery of OM from the terrestrial environment (Figure 4). In addition, the rapid decreases in the TOC-MAR, δ¹³C_org values, clay content and χ_lf values suggest dry and cold climate conditions that may have been caused by a weakening of the EASM intensity until ~2100 cal. yr BP (Figure 4). Similar results have also been obtained from the stalagmite δ¹⁸O record of Dongge Cave in eastern China (Dykoski et al., 2005), where climate became cool and dry during ~3500–2000 yr BP (Figure 6c). At Hani Peat in Northeastern China, the gradual increase in δ¹³C values of OM suggests a weakened EASM and wetter climate between ~3700 and 1700 cal. yr BP (Hong et al., 2005; Figure 6d). At Qinghai Lake, decreased sediment redness indicates a decreased EASM rainfall (~4000–2200 cal. yr BP; ~4000–2200 cal. yr BP; Ji et al., 2005; Figure 6e). In Taiwan, the desiccation of the subalpine Retreat Lake around ~4500–2100 cal. yr BP strongly indicates decreased EASM rainfall (Selvaraj et al., 2011). Decreased abundance of Pulleniatina obliquiloculata in the northwestern Pacific Ocean also suggests a weakening of EASM rainfall during ~4100–1900 cal. yr BP (Jian et al., 2000; Figure 6g). These cool and dry periods have also been inferred from the reconstructed Holocene SSTs in the western tropical Pacific Ocean (~3700–1800 cal. yr BP; Stott et al., 2004; Figure 6f).

Figure 6.

(a) Correlation of δ¹³C_org values from Jingpo Lake (dark blue, this study) with the (b) NH summer solar insolation at 45^oN (red dash line, Laskar et al. 2004), (c) a stalagmite δ¹⁸O record from Dongge Cave (green, Dykoski et al., 2005), (d) cellulose δ¹³C record from Hani peat (purple, Hong et al. 2005), (e) redness data from Qinghai Lake (brown, Ji et al. 2005), (f) sea surface temperature reconstructed on the Mg/Ca ratio of Globigerinoides ruber from MD81 core in the western tropical Pacific (red, Stott et al., 2004) and (g) Pulleniatina obliquiloculata abundance represent the path and intensity of the warm Kuroshio Current in the northern Okinawa Trough (light blue, Jian et al., 2000). Green bars indicate terrestrial plant fragments AMS ¹⁴C dates of core JP-C with 2σ error.

During the period of ~2100–100 cal. yr BP, the rapid increase in Carpinus, Juglans and Corylopsis pollen percentages indicate warm and humid conditions caused by a strengthening of the EASM (Figure 3). A gradual trend towards higher C/N ratios during this period is consistent with wetter conditions because an increase in terrestrial sources content would be expected with increased runoff (Figure 4). In addition, δ¹³C_org values gradually increase (Figure 4) becoming more enriched and combined with higher OM content, suggesting increasing trend of EASM intensity. However, this period is characterized by increase in χ_lf values, sand content; and Artemisia, Gramineae and Chenopodiaceae pollen percentages; and a gradual decrease in Pinus pollen percentages values (Figures 3 and 4). Human activities most likely caused these dramatic changes. In forest region, increases in the pollen percentage of Artemisia and Gramineae generally indicate more open landscapes and intensified human activities (Li et al., 2011; Sun et al., 1997), and the Betula is regarded as a pioneer species in the Changbai Mountains, because it colonizes the land rapidly after slash-and-burn activity (Jiang et al., 2008). This hypothesis is supported by abrupt increase in χ_lf values and sharp increase in sand fraction in lake sediments. Therefore, these changes in pollen percentages may indicate the impact of human activity on the regional vegetation.

Numerous studies indicate that the Holocene climate change was characterized by generally cooling and drying trend in Asian summer monsoonal regions (Fleitmann et al., 2003, 2007; Wang et al., 2005), following the gradual decrease in NH summer insolation over the last ~11,500 years (Kutzbach and Street-Perrott, 1985). Nevertheless, a recent review of high-resolution, well-dated monsoon proxy records seems to suggest an abnormal increase in Asian summer monsoon strength during the late Holocene, in contradiction with the generally weakening Holocene Asian summer monsoon trend (Zhao et al., 2013). In this study, we used multi-proxy records of the core in Jingpo Lake, which suggest warm and humid conditions that may have been caused by a strengthening of the EASM intensity during ~2100–100 cal. yr BP. This strengthening of the EASM during late Holocene has also been inferred from the stalagmite sediments δ¹⁸O record of Dongge Cave in eastern China (~2000–150 yr BP; Dykoski et al., 2005; Figure 6c), the δ¹³C record of Hani Peat in Northeastern China (since ~1700 cal. yr BP; Hong et al., 2005; Figure 6d), the TOC record from the subalpine Retreat Lake in Taiwan (after ~1900 cal. yr BP; Selvaraj et al., 2011) and the increased sediment redness values in Qinghai Lake (since ~2200 cal. yr BP; Ji et al., 2005; Figure 6e). Similar results have also been obtained from the reconstructed Holocene SSTs in the western tropical Pacific Ocean (after ~1800 cal. yr BP; Stott et al., 2004; Figure 6f), and the abundance of P. obliquiloculata in the northwestern Pacific Ocean (since ~1900 cal. yr BP; Jian et al., 2000; Figure 6g). Moreover, in this period, our δ¹³C_org record provides a significantly more detailed several centennial-scale warm/cool phases in the Jingpo Lake that may correspond to the late Holocene climate anomalies, including warm periods of ~AD 700–1300 (MWP) and ~AD 1800–present (CWP), and cool periods of ~AD 400–700 (DACP) and ~ AD 1300–1800 (LIA; Figures 6 and 7).

Figure 7.

Comparison of the detrended δ¹³C_org record (blue line) with the residual Δ¹⁴C record (purple line; Reimer et al., 2004) and total solar irradiance reconstructions based on the smoothed ¹⁰Be record from the South Pole for the last 1200 years (red line; Bard et al., 2000).

It is worth noting the sharp increase in χ_lf, C/N and δ¹³C_org values; the notable decrease in Pinus, Quercus, Ulmus and Juglans pollen percentages; and the rapid increase in Betula, Artemisia and Chenopodiaceae pollen percentages during the period of ~1250–1050 cal. yr BP (or ~AD 700–900; Figures 3 and 4); these changes may be indicative of open landscapes and intensified human activities. A number of historical files and archaeological documents show human activities in Jingpo Lake area since mid-Holocene. The Yinggeling Phase I culture and Yinggeling Phase II culture (Neolithic) occupied the south of Changbai Mountains along the Mudanjiang River and Tumenjiang River areas around ~4000 cal. yr BP and 3500 cal. yr BP, respectively (Jia, 2005; Zhao, 2011). These Neolithic archaeological sites in Jingpo Lake area excavated a number of stone tools (include hunting tools, gathering tools, woodcutting tools and simple agricultural tools) revealing that the native people relied on collecting, hunting and primitive agriculture (Jia, 2005). Therefore, in the Neolithic Age, human activity that altered the original natural environment pattern was very limited. However, during the early Tang Dynasty, the Sumo Mohe (or Sokmal Malgal) established an independent kingdom in AD 698 called Bohai or Balhae Kingdom in the northeast of ancient China. The Kingdom of Bohai existed for over 200 years (AD 698–926), roughly coincided with the Tang Dynasty. During most of that time, it dominated the river basins of the Mudanjiang River, Songhuajiang River and portions of the Heilongjiang (Amur) River (Garcia, 2012). The Kingdom of Bohai had five capital cities; the supreme capital ‘Shangjing’ was located on the northern margin of Jingpo Lake at Dongjingcheng Town, Mudanjiang City, Heilongjiang Province (Figure 1c; Sun, 2009). The Tang dynasty has been described as the peak of classical Chinese Civilization, a golden age of agriculture, literature and art. Bohai Kingdom actively imported aspects of the political and agriculture system of Tang Dynasty, including many agricultural tools, irrigation facilities and crops such as foxtail millet, broomcorn millet, rice and soybean (Lu, 2009). Thus, human activities, especially the agricultural activities intensified in this period, with cropland expansion through the destruction of the native vegetation, causing soil erosion. Thus, from ~1250 to 1050 cal. yr BP (~AD 700–900), the rapid increase in χ_lf values indicates enhanced soil erosion throughout the lake region. The anomalously high C/N ratios and the most positive shifts in δ¹³C_org indicate a greater contribution of terrestrial OM, which could be a result of intensified human activities, including the destruction of native vegetation. Furthermore, our study also shows that a rapid decrease in Pinus and Quercus and a significant increase in Betula, Artemisia and Chenopodiaceae occurred at this period (Figure 3). This selective vegetation change based on pollen data indicates that intensified human activities could be a result of forested areas that were cleared for agriculture and for the development of cities (Li et al., 2011). Decrease in Pinus and Quercus in the Changbai Mountains was possibly because they are preferred as major construction timber. Betula is regarded as the pioneer species in the Changbai Mountains, because it colonizes the land rapidly after slash-and-burn activity (Jiang et al., 2008).

Since ~150 cal. yr BP, a rapid increase in clay content indicates that the lake level rose (Figure 4). The highest values of TOC-MAR/TN-MAR and χ_lf, and the low C/N ratios imply a significant enhancement of aquatic productivity (Figure 4). We hypothesize that this could be affected by intensification of human activities such as deforestation, agriculture, industry and soil erosion, which can lead to abrupt increase in magnetic susceptibility values and sharp increase in nutrient levels in lake sediments.

Possible forcing mechanism for mid- to late Holocene EAM variability

Jingpo Lake is located in the region of the EAM, the moisture/rainfall associated with monsoon circulation resulting from seasonal changes in the land–sea atmospheric pressure distribution (Gao, 1962; Wang, 2006), which are triggered by variations in solar insolation and solar irradiance over decadal to millennial time scales. In this study, multi-proxy results indicated a general weakening of EASM intensity in the study area, generally following the gradual decrease in NH summer solar insolation at 45^oN over the last ~5,100 years (Figure 6b). This synchronicity suggests that solar insolation may have influenced the climate of the study region on the orbital scale. However, our results demonstrate an abnormal increase in EASM strength since ~2100 cal. yr BP. This phenomenon cannot be explained by long-term change in NH summer insolation, because Holocene NH summer insolation shows a continuous and gradual decrease trend without any reversal (Laskar et al., 2004). Moreover, the decadal to centennial variability in the EASM cannot be explained by changes in solar insolation. Previous paleoclimatic studies suggested a possible link between change in solar activity (solar irradiance) and EASM intensity (Liu et al., 2009; Selvaraj et al., 2007; Wang et al., 2005). Numerical simulations suggest that monsoon intensity could be sensitive to relatively small solar activity changes (Shindell et al., 2001). To test this link, we compared the δ¹³C_{org-detrended} record from Jingpo Lake with the residual atmospheric ¹⁴C (Δ¹⁴C) and ¹⁰Be records (Bard et al., 2000; Reimer et al., 2004), which can be used to represent solar activity (Figure 7). We found that the variations in EASM intensity indicated by δ¹³C_{org-detrended} generally correspond to residual Δ¹⁴C and total solar irradiance (TSI) reconstructions based on the smoothed ¹⁰Be record fluctuations on multi-decadal and centennial scales. Generally, enhanced EASM (higher δ¹³C_{org-detrended}) corresponds to higher solar irradiance (smaller Δ¹⁴C), whereas weakened EASM (lower δ¹³C_{org-detrended}) corresponds to lower solar irradiance (larger Δ¹⁴C). Moreover, within chronological uncertainty, weakened EASM intensity could be identified for the Oort, Wolf, Spörer, Maunder and Dalton solar minima (Figure 7). These correlations suggest a possible link between solar activity and EASM variability in NE China during the mid- to late Holocene.

To further confirm this correlation, we used the program REDFIT (Schulz and Mudelsee, 2002), and the spectral analysis of the δ¹³C_{org-detrended} record in Jingpo Lake revealed six statistically significant cycles centred on 201, 81, 65, 56, 43 and 36 years (above the 90% confidence level; Figure 8). The 201-year cycle and 65-year cycle are very similar to the well-known de Vries (Suess) solar activity cycle of ~200 years (Usoskin and Mursula, 2003) and Gleissberg solar activity cycle of ~70 years (Beer et al., 2000), respectively. The 200-year de Vries oscillation has been reported in AM controlled region, for example, δ¹⁸O values from Dongge Cave (Dykoski et al., 2005; Wang et al., 2005) and Qunf Cave (Fleitmann et al., 2003), detrended TOC content in Kusai Lake (Liu et al., 2009) and the iron oxide content signal in Qinghai Lake (Ji et al., 2005). The records from monsoon controlled region such as δ¹⁸O values from Dongge Cave (Dykoski et al., 2005) and Qunf Cave (Fleitmann et al., 2003) also show a significant ~70-year periodicity. Similar periodicities of 81, 56, 43 and 36 years were found in the residual Δ¹⁴C record (Reimer et al., 2004) and other AM region records (Dykoski et al., 2005; Fleitmann et al., 2003; Wang et al., 2005). These similar cycles demonstrate that solar irradiance may have played a role in the variation of EASM intensity.

Figure 8.

Results of spectral analysis for the detrended δ¹³C_org record from Jingpo Lake over the past ~5100 years. Peaks are labelled with their period in years above 90% (solid line) confidence level. Dashed line shows 95% confidence levels.

Recent studies have demonstrated that the AM climate was sensitive to small changes in solar irradiance during the Holocene (He et al., 2013; Liu et al., 2009; Selvaraj et al., 2007); thus, small variations in solar irradiance may have led to significant changes in the monsoon climate. Variations in solar irradiance were considered responsible for decadal to centennial changes in the monsoon climate by affecting the thermal contrast between the tropical Pacific Ocean and the Asian continent. Therefore, we suggest that ocean–atmosphere interacting processes, particularly changes in the Western Pacific Warm Pool (WPWP) SST and the Kuroshio Current originating from the North Equatorial Current in the western Pacific, may play a significant role as driving forces in driving the variability in monsoon circulation (Wen et al., 2010; Xiao et al., 2008).

Prior to ~3600 cal. yr BP, the EASM intensity in the study region was strong. During the period between ~3600 and 2100 cal. yr BP, drier and colder climate conditions show that the EASM weakened. Since ~2100 cal. yr BP, multi-proxy results indicated warm and humid conditions caused by strengthening of the EASM intensity. It is noticeable that the researchers combine oxygen isotope and Mg/Ca ratios of planktonic Globigerinoides ruber retrieved from sediment cores in the western tropical Pacific to reconstruct the Holocene SST of WPWP (Stott et al., 2004; Figure 6e). The SST of WPWP in the western tropical Pacific were mostly above 29°C before ~3700 cal. yr BP, a notable decrease between ~3700 and 2000 cal. yr BP shows less than 29°C and a remarkable increase was since ~2000 cal. yr BP (Figure 6e). This coincidence suggests that the SST of WPWP in the western tropical Pacific may have influenced the EASM of the study region. The change in the SST of WPWP in the western tropical Pacific is fundamentally linked to variability in the El Niño Southern Oscillation (ENSO). Strikingly, between ~3600 and 2100 cal. yr BP, the decreased EASM as revealed by the Jingpo Lake record and other paleoclimatic records from the EAM region (Figure 6) corresponds to a sharp decrease in reconstructed SST in the western tropical Pacific, which is an indication of an intense El Niño phase. Previous studies demonstrated that the modern El Niño phase is associated with weakened EASM intensity (Selvaraj et al., 2007, 2011). Decreased SST in the western tropical Pacific could reduce the formation of water vapour over the source area of the EASM, thereby decreasing the moisture available for transport via monsoon circulation from the western tropical Pacific onto the Asian inland and leading to a weakened summer monsoon. These results agree with the finding of decreased transport of moisture and heat from the western tropical Pacific to the northern North Pacific, as inferred from a substantial decrease in the abundance of the warm-water planktonic foraminifera P. obliquiloculata, an indicator species of the path and intensity of the warm Kuroshio Current, in the sediment cores of northern Okinawa Trough between ~4100 and 2100 cal. yr BP (Jian et al., 2000; Figure 6g). These data suggest that changes in EASM precipitation on millennial to centennial scales could be related to ocean–atmosphere interactions in the tropical Pacific within age uncertainties. A weakened, eastward shifted Kuroshio Current and decreased SST of WPWP in the western tropical Pacific could shift warm tropical waters eastward, because the convection in the North Pacific Ocean further east and reduce the formation of water vapour over the source area of the EASM, thus decreasing the moisture available for transport via monsoon circulation from the low-latitude western tropical Pacific onto the Asian inland and resulting in a weakened summer monsoon.

In summary, this study indicates that changes in EASM intensity on decadal to centennial scales at mid- to high-latitudes in NE China during the mid- to late Holocene might have been related to orbital induced insolation changes, changes in solar output and changes in oceanic–atmospheric circulation. However, more well-dated, multi-proxy and high-resolution paleoclimatic records, as well as paleoclimatic modelling, are needed to confirm this possible solar-ocean–atmosphere interaction.

Conclusion

Paleoclimatic reconstruction based on multi-proxy data (pollen, sedimentological, magnetic, geochemical and carbon isotopic data) from a well-dated lacustrine sediment core of Jingpo Lake in the NE China has provided a high-resolution record of EASM variability over the past ~5100 cal. yr BP. The highest pollen percentages of Quercus, Ulmus, Juglans and Corylopsis; low Md values; and high δ¹³C_org values reflect relatively warm and humid climate conditions during the period from ~5100 to 3600 cal. yr BP. Between ~3600 and 2100 cal. yr BP, high Md and low δ¹³C_org values, and the rapid increase in herbs pollen percentages indicate cool and dry climatic conditions. From ~2100 to 150 cal. yr BP, a gradual increase in δ¹³C_org values, low Md values and the rapid increase in Carpinus, Juglans and Corylopsis pollen percentages suggest that the climate changed towards warmer and wetter conditions. After ~150 cal. yr BP, the highest values of TOC-MAR, TN-MAR and χ_lf suggest that the Jingpo Lake region has been severely affected by human activities. The variability of EASM intensity in Jingpo Lake strongly correlated with other EASM paleoclimatic records in China. This study demonstrates that solar irradiance may have played a role in the variation in EASM intensity on multi-decadal to centennial scale, that is, increase in EASM intensity is generally correlated with increases in solar irradiance, whereas decreased EASM intensity corresponds to reductions in solar irradiance. Additionally, a spectral analysis of the δ¹³C_{org-detrended} record reveals significant periodicities centred on 201, 81, 65, 56, 43 and 36 years. These periodicities were found in solar activity records and other EASM region paleoclimatic records. Furthermore, the variation in EASM intensity coincides with the reconstructed SST in the western tropical Pacific and the path and intensity of the Kuroshio Current. Therefore, this study indicates that both changes in solar output and changes in oceanic–atmospheric circulation played a significant role in the variability of the EASM at mid- to high latitudes in Northeastern China during the mid- to late Holocene.

Footnotes

Acknowledgements

We are grateful to two anonymous reviewers for the very helpful comments. We wish to thank Qingfeng Jiang and Hao Long for their help during field coring, and we also thank Hao Long and Luo Wang for their helpful discussions and suggestions during the preparation of the manuscript.

Funding

This study is supported by the Chinese Academy of Sciences (KZZD-EW-TZ-08), the National Basic Research Program of China (2012CB956100) and the National Natural Sciences Foundation of China (NSFC 40873056, 41173080 and 41172151).

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Mid- to late-Holocene East Asian summer monsoon variability recorded in lacustrine sediments from Jingpo Lake,Northeastern China

Abstract

Keywords

Introduction

Study region

Materials and methods

Materials and dating method

Laboratory analyses

Numerical analyses

Results

Lithology and chronology

Multi-proxy records

Discussion

Proxy interpretations

Pollen data

Grain size and magnetic susceptibility

TOC-MAR, TN-MAR, C/N and δ13Corg of OM

Mid- to late Holocene EAM variations and regional comparisons

Possible forcing mechanism for mid- to late Holocene EAM variability

Conclusion

Footnotes

Acknowledgements

Funding

References

TOC-MAR, TN-MAR, C/N and δ¹³C_org of OM