Variation in humidity and the forcing mechanism in Asian monsoon-influenced regions indicated by hematite/goethite from Baxian Lake,southern China,since AD 800

Abstract

Understanding the potential mechanisms driving the precipitation pattern in the Asian summer monsoon (ASM) area is significant to reconstructing the environmental and hydrological conditions over the past 2000 years. However, robust and consistent conclusions have been hampered by the complex processes controlling the dynamics and diverse interconnected linkage of the Asian monsoon. Here, we present a reconstruction of variations in humidity since AD 800, based on the ratio of hematite to goethite (Hem/Goe) and other magnetic parameters in the sediments of Baxian Lake, southern China. The record indicates that a dramatic transition from dry to humid climate occurred during AD 800–950 and then returned to extreme drought during the second half of the Medieval Warm Period. The fluctuations between a relatively wet climate and weak drought were also demonstrated during the Little Ice Age (AD 1450–1800). Climate-influenced vegetation coverage may influence the flux of weathered detrital magnetic minerals in sediments, as it hinders the input of coarser detritus into Baxian Lake. The magnetic minerals in the sediments were not significantly altered during the post-depositional processes. Additionally, the Hem/Goe record indicates that an opposite precipitation pattern occurred in southern China when compared with precipitation records in northern China and India. We ascribe this result to the superimposition of land-ocean-atmosphere dynamics on the traditional model of the intertropical convergence zone (ITCZ) movement forced by the Atlantic Meridional Overturning Circulation. This is significant for enhancing our understanding of the relationship among the continent-ocean thermal gradient, ASM variation, ITCZ movement, El Niño Southern Oscillation-like events, and extreme climates in areas influenced by the ASM.

Keywords

droughts ENSO Hem/Goe precipitation pattern southern China

Introduction

The Asian Summer Monsoon (ASM) mainly controls the hydroclimatic and environmental conditions in Asia, making it one of the most important and dynamic monsoons in the global monsoon system (Wang et al., 2014). Two sub-systems transport moisture to the Asia continent, the East Asia Summer Monsoon (EASM) and India Summer Monsoon (ISM). These sub-systems are of critical significance, and variations in their intensity substantially affect human survival and prosperity in the most densely populated regions of Asia (Gadgil, 2003; Zhao et al., 2020). Although many previous studies have discussed the potential driving forces of precipitation patterns on the Asia continent (Gu et al., 2020; Sinha et al., 2015; Yamoah et al., 2016; Zhang et al., 2019, 2020), robust and consistent conclusions have been hampered by the complex control processes that drive the dynamics of the Asian monsoon and by diverse interconnected mechanisms.

Paleoclimatic records spanning the last glaciation or even longer indicate that a bi-polar see-saw mechanism influences climate on a millennial- scale (Shakun and Carlson, 2010; Stager et al., 2011; Zhang and Delworth, 2005). This hypothesis invokes the idea that the speed of the Atlantic Meridional Overturning Circulation (AMOC) was restrained by rapid iceberg melting in the North Atlantic, which subsequently led to the southward shift of the intertropical convergence zone (ITCZ) (McManus et al., 2004; Rahmstorf, 2002). This model of AMOC-forced ITCZ movements has resulted in a generally weakened ASM during the southward migration of the ITCZ period and vice versa (Dykoski et al., 2005; Wang et al., 2001). However, whether this interpretation is appropriate for climatic change on decadal to centennial timescales still needs to be explored in depth. In addition, the tropical land-ocean-atmosphere system, which is controlled by the El Niño-Southern Oscillation (ENSO) cycle (Walker, 1918) and Himalayan/Eurasian snow cover (Blanford, 1884; Webster et al., 1998), is also strongly associated with the strength of the ASM and variations in precipitation (Fasullo, 2012; Krishnamurthy and Goswami, 2000), especially during the late-Holocene (Kumar et al., 2006; Tan et al., 2018). Nevertheless, the mechanisms and conclusions related to these dynamics also remain controversial. For example, it has been suggested that the increase in sea surface temperatures (SSTs) across the tropical Pacific weakened the continent-ocean thermal contrast and the intensity of the ISM in southwest China during the Medieval Warm Period (MWP) with more frequent and intense El Niño events occurring at that time (Gu et al., 2020; Roxy et al., 2015). In contrast, other studies have argued that fewer convective activities in the western tropical Pacific have led to a stronger EASM comparably during El Niño events, thereby annulling the dry climate in Thailand and Southern China (Yamoah et al., 2016). Moreover, a recent hypothesis indicates that seven drought events that have occurred in south China over the past 60 ka BP were principally influenced by the large zonal SST gradients (ΔSST) in the tropical Pacific resulting in the westward shifts of the western Pacific subtropical high (WPSH) and the enhancement of the Hadley circulation (Zhang et al., 2020). These disagreements suggest that the complex Asian monsoon dynamics cannot be reconciled with a single mechanism, especially for the past two millennia, during which the interaction between Earth’s climate system and anthropogenic activities has become increasingly intense.

As one of the areas most strongly dominated by the ASM, the conditions of southern China during the MWP and the Little Ice Age (LIA) are of great interest. The spatiotemporal heterogeneity of these two intervals and the underlying mechanisms of hydroclimatic anomalies are the main focus being discussed in recent studies (Mann et al., 2009; Oppo et al., 2009; Yan et al., 2011). For example, previous reviews indicate that the ASM region has experienced a “warm-dry and cool-wet” climatic pattern during the MWP and LIA in southern and southwestern China (Gu et al., 2020; Tan et al., 2018). This is attributed to the forcing mechanism of the ENSO conditions in the tropical Pacific (Tan et al., 2018), the ITCZ shifting forced by solar periodic variation (Wang et al., 2005; Yan et al., 2015), and continent-ocean thermal contrasts (Gu et al., 2020). However, the opposite conclusion of the “warm-wet” pattern during AD 800–1300 has also been identified using the leaf wax-based hydrogen isotope record from Pa Kho Lake in northeast Thailand (Yamoah et al., 2016). The coupling mechanism of the ASM and the tropical Pacific has quite complicated effects on precipitation patterns in southern China, and further discussion is still necessary. Thus, based on the inconsistency described above, we aim to provide an enhanced understanding of the dynamic linkage between external mechanisms and spatial precipitation patterns using the record of magnetic minerals in sediments cores from Baxian Lake in South China.

Here, comparing a continuous Hem/Goe record and some magnetic parameters from Baxian Lake sediments with some proxies reflecting precipitation variabilities in North China, South China, and India, we propose a possible mechanism governing the precipitation variations in the Baxian Lake area. The hydrological and environmental changes there are influenced by the superimposition of land-ocean-atmosphere dynamics on the traditional model of AMOC forced ITCZ movement since AD 800, and the migration of western Pacific subtropical high (WPSH) which controls the route of EASM and location of precipitation.

Material and methods

Study area

Baxian Lake (23.5407°N, 109.6511°E, 213 m a.s.l.) is a small and closed lake formed after a karst collapse and located in Xuanwu County in the middle of Guangxi Province (Figure 1). This lake covers 0.01 km² with a catchment area of 2 km² and a maximum depth of 4 m; the surrounding landscape is covered by Pinus massoniana along with mixed evergreen and deciduous broad-leaved forests (Su, 1998). The bedrock of the catchment area is mainly composed of a 30–50 cm thick weathered soil layer and limestone. Because this area is hydrologically closed, the water level of this lake is mainly controlled by the amounts of precipitation and evaporation. This lake is located in the subtropical monsoon region under the influences of the ISM and the EASM. Based on climatic data from local stations, the mean annual temperature and precipitation from 1950 to 2010 were 20.5°C and 1500 mm, respectively. The warmest month was July with an average high temperature of 29.3°C while 50% of the precipitation occurs primarily between May and July (Chang et al., 2020). Thus, the Baxian Lake is an ideal place to explore the variations in precipitation and potential driving mechanisms in southern China. (China Meteorological Data Service Centre: http://data.cma.cn/en) (CMDC, 2017).

Figure 1.

Location of the study region and its climatological settings. (A) Distribution of paleoclimatic records (yellow solid circles) in the ASM region. Northern China: (a) Gonghai Lake (Chen et al., 2020); (b) Wanxiang Cave (Zhang et al., 2008); (c) Heshang Cave (Hu et al., 2008). Southern China: (d) Dongge Cave (Wang et al., 2005); (e) Baxian Lake (this study); (f) Hugangyan Maar Lake (Chu et al., 2002). Thailand: (g) Pa Kho Lake (Yamoah et al., 2016). India: (h) Mawmluh Cave (Dutt et al., 2021); (i) Dandak Cave (Berkelhammer et al., 2010); (j) Sahiya Cave (Sinha et al., 2015). (B) Location of Guangxi Province. (C) Aerial view of the Baxian Lake and core location.

Sampling and chronology

In November 2016, one sediment core 190-cm long (BXC1) and two 150-cm long (BXC2, BXC5) were collected from the center of the lake (4 m depth) using a piston corer. All cores were cut in half lengthwise in the laboratory and then photographed and described. Wet samples were sectioned at 1-cm intervals and embedded into standard 1 cm³ non-magnetic cubic boxes. The remaining samples were packaged into sample bags and refrigerated prior to further analysis. After correlating the fluctuations of magnetic susceptibility and Ti contents, we constructed a composite core approximately 212-cm long, 58 cm for BXC2 and 154 cm for BXC5 (Supplemental Figures S1–S3, available online).

The sediment sources are mainly detrital materials from the weathering of the bedrock and organic remains after vegetation succession. Therefore, the shifts of its sedimentary characteristics likely represent hydrological and environmental variations at that time. The lacustrine sediments are dominated by brown clay, and a clear organic-rich clay exists at depth of 134–144 cm. Gradual changes in lithology indicate the continuity of sedimentation and rapid changes in climate over a short period.

Five samples of plant remains were dated using accelerator mass spectrometry (AMS) at the Beta Analytic Testing Laboratory, Miami, FL, USA (Table 1). Then, a Bayesian piece-wise linear accumulation model was used to construct an age-depth model using the Bacon package in R (Blaauw and Christen, 2011). The Bacon output shows that the bottom age was extrapolated to around AD 200 (Figure 2). Because the oldest calibrated age was AD 1039–1211 (Table 1), we are cautious about the accuracy of the chronological sequence based on extrapolation earlier than AD 800 and do not discuss it in detail in this paper.

Table 1.

Radiocarbon ages of the composite core in Baxian Lake are based on the AMS technique involving plant remains.

Sample ID	Composite depth (cm)	Material	Conventional age (BP)	Calibrated age (cal. AD)
BXC2-51	17.5	Plant	110 ± 30	1694–1898
BXC2-36	36	Plant	390 ± 30	1556–1666
BXC2-18	59	Plant	430 ± 30	1427–1533
BXC5-94	99	Plant	650 ± 30	1236–1364
BXC5-77	119	Plant	890 ± 30	1039–1211

Figure 2.

The age-depth model for the core from Baxian Lake. The age model was drawn using data output from the Bacon program based on ¹⁴C dates (Table 1) and R software (Blaauw and Christen, 2011). The gray dotted lines show 95% confidence intervals, and the red dotted line denotes the best model.

Measurements of mineral magnetism

We measured the low-(χ_lf) and high-frequency magnetic susceptibility (χ_hf) at frequencies of 976 and 15,616 Hz, using an MFK1-FA Kappabridge meter (Agico, Brno, Czech Republic). Then, the frequency-dependent susceptibility (χ_fd) was defined using the formula: [(χ_lf – χ_hf) /χ_lf × 100] (Dearing et al., 1997), which can be used to quantify the relative contribution of fine-grained superparamagnetic (SP) particles. The anhysteretic remanent magnetization (ARM) was measured using a JR-6A dual-speed spinner magnetometer (Agico). A decaying alternating field (peak field of 80 mT) and a superimposed direct bias field (DC field) of 50 μT were imparted to measure the ARM. Then, the value of anhysteretic susceptibility (χ_ARM) was calculated by ARM dividing the value of the DC field (Evans and Heller, 2003). The IRM was measured using a 2 G-760 impulse magnetizer with a field of 1T (SIRM) and −300 mT (IRM_−300mT). The hard fraction of IRM (HIRM) was defined as [(SIRM + IRM_−300mT)/2] to quantify the absolute concentration of high-coercivity components using the equation (Thompson and Oldfield, 1986). In addition, the ratios of SIRM/χ_lf, ARM/SIRM, and χ_ARM/χ_lf were also calculated to distinguish magnetitic grain sizes (Snowball, 1991; Thompson and Oldfield, 1986).

Representative samples were selected from lake sediments to study their magnetic mineralogy and magnetic grain size. Thermomagnetic analysis (χ–T) was carried out in an argon atmosphere, heating the dried samples from room temperature to 700℃ and then cooling back to room temperature. The experimental instrument was an MFK1-FA Kappabridege and CS-4 temperature control system at frequencies of 976 Hz and an alternating field of 200 A/m. Magnetic hysteresis loops were measured using a MicroMag 3900 Vibrating Sample Magnetometer (VSM, Princeton Measurements Corp., Princeton, NJ, USA). Then, the hysteretic parameters, including Coercivity (B_c), remanent coercivity (B_cr), remnant magnetization (M_rs), and saturation magnetization (M_s) were obtained after diamagnetic and paramagnetic corrections, which were used to make a Day plot to distinguish the magnetic grain sizes. To further estimate the magnetic domains of magnetic minerals, the first-order reversal curves (FORCs) of representative samples were measured using the VSM 3900. For each sample, 120 FORCs were measured with a field of 1T; the FORC diagrams were processed in IGOR PRO software using the FORCinel program. All experiments above were completed in the magnetic laboratory of Sun Yat-Sen University and the Institute of Geology and Geophysics, China Academy of Sciences.

A total of 170 samples were prepared for the diffuse reflectance spectrophotometry (DRS) experiment to measure the content of hematite (α-Fe₂O₃) and goethite (α-FeOOH). The pretreatment procedures employed the methods described by Deaton and Balsam (1991). First, all samples were freeze-dried thoroughly and then ground under a 200 mesh in an agate mortar. Then, ground samples were placed on rectangular glass slides (7 cm × 4 cm × 2 mm) and mixed into a homogeneous slurry with ultrapure water (Deaton and Balsam, 1991). After being smoothed and dried at room temperature, each prepared sample was measured on a Lambda 950 UV-VIS-NIR spectrophotometer (PerkinElmer) with a wavelength of 350–700 nm and a scanning interval of 0.5 nm at the Laboratory in Sun Yat-Sen University, China. Finally, the initial reflective curve was converted into a Kubella-Munk (K-M) function curve, and then the second derivative curve (Torrent and Barrón, 2008). The amplitudes between the characteristic troughs and their adjacent peaks were determined to semi-quantify the content of goethite and hematite in each sample (Supplemental Figure S4, available online, Torrent et al., 2007). A dry and warm environment is conducive to the formation of hematite by thermal dehydration from goethite or other dehydration processes (Jiang et al., 2012). In contrast, goethite can be generated in a relatively wet and cold environment (Ji et al., 2004; Zhang et al., 2007). Experimental results show that the transformation into hematite/goethite mixtures in the systems with gibbsite, illite, kaolinite, and smectite can be completed efficiently in 16 years or less at a temperature of 25℃ (Schwertmann et al., 2000). Studies also show that variations of magnetic minerals respond quickly to precipitation, even on an interannual scale (Liu et al., 2013; Nie et al., 2017). Thus, the content of two iron minerals and their ratio can be governed by climatic factors (Schwertmann, 1988), and be used to restore the record of changes in precipitation rates (Hyland et al., 2015; Long et al., 2011). To avoid this, the low content hematite information will be masked when summing the goethite and hematite content with different orders of magnitude, the value of [Rel _{(Hem + Goe)}] was calculated, which represents the relative content of hematite and goethite after normalization based on the following formula developed by Zhang et al. (2018):

n \times (\frac{I_{H e m_{i}}}{\sum_{i = 1}^{n} I_{H e m_{i}}} + \frac{I_{G o e_{i}}}{\sum_{i = 1}^{n} I_{G o e_{i}}}),

where I_Hem and I_Goe indicate the intensity of hematite and goethite, and n represents the number of samples.

Results

Magnetic mineralogy of representative samples

The IRM acquisition curves show that all representation samples rise to more than 70% of the saturated remanence at an applied field of 100 mT, and to approximately 90% at an applied field of 300 mT, except for sample 7 and sample 8, which present at around 55% and 75% of the saturated remanence at same respective fields (Figure 3b). The remanent coercivities of samples 7 and 8 are near 70–80 mT, approximately twice that of the other samples (35–45 mT, Figure 3a). These data indicate the dominance of low-coercivity ferrimagnetic minerals, mixed with some high-coercivity minerals in the sediments of Baxian Lake (Rawat et al., 2015).

Figure 3.

The Backfield demagnetization curves (a) and the isothermal remanent magnetization acquisition curves (b) for representation samples at different depths. The gradient of the acquisition plots (GAP) of IRM ((c–l) red lines); the orange, blue and green lines represent the three components of coercivity. The processing program was pyIRM (https://github.com/botaoxiongyong/pyIRM). Temperature dependence of magnetic susceptibility (m–q); the red and blue curves represent the heating and cooling processes, respectively.

To further discriminate the minerals components based on their different mineral coercivity, the cumulative log-Gaussian (CLG) method was carried out to analyze the IRM data (Kruiver et al., 2001). As shown in the gradient of the acquisition plots (GAP, Figure 3f–l) and Table 2, three coercivity components were derived from IRM data of representative samples. Components 1 and 2 may be low coercivity minerals of different particle sizes, and component 3 is interpreted as the contribution of high coercivity minerals, such as hematite (Kruiver et al., 2001; Peters and Dekkers, 2003). Significantly, the mean coercivity (B_1/2) of the three components derived from 117 and 128 cm samples are all higher than that from other samples (Figure 3j and k), indicating more magnetic minerals with high coercivity in sediments from those samples, whereas other samples are characterized by the absolute dominance of component 2 and a relatively lower B_1/2.

Table 2.

Numbers and parameters of IRM coercivity components of samples at different depths.

Sample ID (cm)	Component 1			Component 2			Component 3
	C_IRM (%)	B_1/2 (mT)	DP	C_IRM (%)	B_1/2 (mT)	DP	C_IRM (%)	B_1/2 (mT)	DP
2.5	9	11.22	0.30	80	37.20	0.27	9	387.58	0.29
13	19	17.78	0.39	71	39.75	0.24	9	423.43	0.29
30	19	17.78	0.31	65	39.81	0.25	15	405.87	0.36
35	6	11.08	0.33	73	35.11	0.28	20	353.11	0.31
55	6	7.08	0.34	73	36.18	0.27	19	365.79	0.33
87	4	8.91	0.25	70	34.16	0.30	24	380.49	0.37
112	8	8.91	0.24	74	36.72	0.29	17	322.29	0.40
117	26	23.24	0.30	46	72.44	0.36	26	469.92	0.29
128	23	17.78	0.30	46	70.79	0.34	30	427.40	0.28
143	13	14.13	0.38	67	48.68	0.28	18	303.78	0.35

C_IRM represents the contribution of each coercivity component to the IRM curve; B_1/2 is the field at which half of the SIRM is reached, reflecting the mean coercivity of each component; DP is the width of the distribution, given by one standard deviation of the logarithmic distribution (Kruiver et al., 2001).

Representative χ-T curves of the samples are shown in Figure 3m to q. The heating curves show a small peak at around 250℃, and then gradually decrease until 300℃ (Figure 3o–q). This process may reflect the transformation of lepidocrocite and goethite to maghemite and then to hematite with increasing temperature (Deng et al., 2001); it may also indicate the presence of small amounts of iron sulfide minerals (Roberts, 1995). The rapid increase in susceptibility between 400℃ and 500℃ for all samples is likely attributed to the neoformation of magnetite transformed from iron-containing silicates, clays, carbonates (Pan et al., 2000). All curves exhibit a distinctive Curie-point of 580°C, indicating the main magnetic carrier of magnetite in sediments. The cooling curve is much higher than the heating curve after the temperature drops below 580℃ which also indicates the neoformation of magnetite (Ortega et al., 2006).

FORC diagrams and grain-size dependent plots of representative samples

All FORC diagrams are characterized by mostly-closed outer contours that diverge along the Bu axis (Figure 4a–e). Divergence of the contours along the Bu axis is less than ~20 mT, indicating quite a low interaction field among the magnetic grains. The center of the closed contours along the B_c axis occurs between 20 and 30 mT, suggesting the dominance of low-coercivity minerals in the samples, compatible with the dominance of magnetite. These characteristics indicate the dominant contribution of single-domain (SD) particle components with a mixture of a small amount of superparamagnetic (SP) and pseudo-single-domain (PSD) components (Roberts et al., 2014).

Figure 4.

FORCs diagram (a–e) of representative samples at different depths in Baxian Lake; (f) semi-quantitative magnetic granulometry plot (Dearing et al., 1997); (g) King plot for detecting grain-size variation of the magnetite in Baxian Lake sediments (King et al., 1982); (h) hysteresis ratios for samples on a Day plot (Day et al., 1977).

Day and Dearing plots are efficient tools for identifying the domain state of magnetic minerals (Day et al., 1977; Dearing et al., 1997). As shown in Figure 4h, the domain states of all the samples since AD 800 are clustered in the PSD region. Considering the limitation of the Day plot, we used the Dearing plot to identify the domain state more accurately (Dearing et al., 1997). As illustrated in Figure 4f, the domain state of magnetite is coarse SD dominated during AD 800–1150 (red dots), and during AD 1150–1450 (purple dots), it becomes finer, dominated by fine SD. After AD 1450, almost all samples are located in the SD-SP section in the Dearing plot (blue dots). The King plot can determine the specific particle size range of magnetite as shown in Figure 4g. As a whole, the domain state of samples from AD 800 to the present tended to gradually be finer.

Variations of magnetic parameters with age

Three remarkable stages (black arrows) are demonstrated in Figure 5. The clear and consistent fluctuations of these magnetic parameters indicate that environmental and climatic conditions in Baxian Lake underwent rapid changes. Based on the interpretations of these magnetic parameters, we classified them into three different categories: the magnetic grain size, magnetic concentration, and magnetic mineralogy.

Figure 5.

Magnetic parameter changes for the core in Baxian Lake versus age. The blue and yellow shaded areas indicate two stages of the Medieval Warm Period. The gray bar indicates the beginning of the Little Ice Age. Note that there is a gap between AD 900 and AD 1000 where the sediment sequence is rich in organic remains and some magnetic parameters cannot be obtained. (a) SIRM/χ_lf, (b) χ_ARM/χ_lf, (c) ARM/SIRM, (d) χ_fd, (e) χ_lf, (f) HIRM, (g) Rel _{(Hem + Goe)}, and (h) Hem/Goe.

At the first stage (AD 800–950), dramatic fluctuations of all magnetic parameters occurred as shown in Figure 5. The concentration-dependent parameters χ_lf display a rapid trend of decrease (Figure 5e), possibly suggesting dramatic changes in the flux of magnetic minerals imported from the catchment into lake sediments. The value of Rel _{(Hem + Goe)} exhibits a clear similar trend with the HIRM, suggesting a rapid decrease of high-coercivity components (e.g. hematite, goethite). The grain size-dependent magnetic parameters (SIRM/χ_lf, χ_ARM/χ_lf, ARM/SIRM, and χ_fd) display a similar pattern, indicating a rapid shift in grain size from coarse to fine. The ratio of Hem/Goe exhibited a distinct increasing trend and peaked during this period, suggesting a rapid shift in hydrological conditions from dry to wet.

During the second stage (AD 950–1200), all parameters showed opposite trends when compared with the previous stage. The SIRM/χ_lf, χ_ARM/χ_lf, ARM/SIRM, and χ_fd experience decreasing trends from the relatively high values. Conversely, the value of Rel _{(Hem + Goe)} exhibited an overall increasing trend, which coincided with the value of HIRM, suggesting that more magnetic minerals with high-coercivity components were brought to lake sediments and deposited. Similarly, the level of χ_lf was maintained at a relatively high level and almost increased to the maximum in around AD 1200. This characteristic indicates that the variation tendencies of low-coercivity and high-coercivity magnetic minerals in lake sediments are similar.

Since around AD 1200, the overall variation tendency was similar to that during AD 800–950, though all of these parameters exhibited dramatic fluctuations from AD 1450 to around AD 1600. The decreasing χ_lf and HIRM indicate a gradual decrease in the concentrations of ferrimagnetic minerals and high-coercivity magnetic minerals, respectively. However, the magnetic grain-size parameters (SIRM/χ_lf, χ_ARM/χ_lf, ARM/SIRM) exhibited a gradually increasing trend despite AD 1450–1600, suggesting that greater amounts of fine magnetic minerals had been carried into the sediments and deposited.

Discussion

Controls of magnetic minerals in Baxian Lake

The magnetic minerals in lake sediments are generally considered to be terrestrial detrital minerals brought into the lake by surface run-off or eolian processes (Chen et al., 2013; Wei et al., 2020), and usually respond well to the climatic environment. However, post-depositional processes, for instance, the formation of biomagnetosome and authigenic iron sulfide, may change the sedimentary characteristics and thus interfere with the interpretation of the original environmental information (Roberts et al., 2018; Snowball and Thompson, 1988). Therefore, whether these processes influenced the magnetic records needs to be evaluated.

Previous studies have shown that more active iron-rich non-magnetic minerals (hydrous ferric oxides) and iron-oxyhydroxides (lepidocrocite and goethite) will dissolve prior to magnetite in anoxic environments with high organic materials (Liu et al., 2012). H₂S occurs when more active hydrous ferric oxides and lepidocrocite are consumed in large quantities. Then, less active minerals such as goethite, magnetite and hematite will react with H₂S to form iron sulfides (Liu et al., 2012). However, our χ-T curves show a small peak at around 250℃ (Figure 3o–q), indicating the scarcity of iron sulfide minerals (Roberts, 1995). The value of SIRM/χ_lf in Baxian Lake increased upward from 1.68 to 30.88 k Am⁻¹ throughout the whole core (Figure 5a), which was also much lower than the threshold value that is usually higher than 70 k Am⁻¹ for the presence of ferrimagnetic iron sulfide (Snowball, 1991). In addition, the existence of authigenic greigite is characterized by its peak coercivity at ~60–70 mT with strong magnetostatic interactions in FORC diagrams (Roberts et al., 2018). However, these characteristics did not appear in our FORC results (Figure 4). Therefore, we conclude that the dissolution of magnetic minerals and the formation of ferrimagnetic iron sulfide may have occurred during post-deposition, but the lake was still in the primary iron (hydrogen) oxide-reducing environment.

Bacterial magnetosomes are another important source of authigenic magnetic minerals (Liu et al., 2012). The magnetosome chains produced by magnetotactic bacteria are characterized by a narrow and specific SD grain distribution with weak or negligible magnetic interaction. The collapsed chains may result in a high proportion of interacting SD grains, which may lower the ARM ratio and distort its interpretation (Lascu and Plank, 2013). Our FORC diagrams indicate the dominance of SD particles with a mixture of a small amount of SP and PSD components (Figure 4a–e). However, whether this is presented by magnetosomes or SD detrital ferromagnetic minerals still needs to be determined. Our previous study shows that no long chain-shaped or bullet-shaped magnetite was found in the soil samples and surface lake sediments samples (Supplemental Figure S5, available online; Wei et al., 2021), which are typical shapes of magnetic minerals biomineralized by magnetotactic bacteria (Kim et al., 2005). In addition, the ratio of ARM/SIRM for SD magnetosome is generally between 0.15 and 0.25 (Moskowitz et al., 1993), far higher than the extremely low value of ARM/SIRM (<0.042) throughout the core in Baxian Lake (Figure 4c). Hence, we conclude that even if there are magnetite grains produced by biogenic processes, a very small amount of biomagnetosome does not affect the environmental information recorded by magnetic parameters.

Connections between magnetic properties and paleoclimatic changes since AD 800

The ratio of hematite and goethite is an effective indicator to record local precipitation rates (Hyland et al., 2015; Long et al., 2011). We compared the Hem/Goe with other rainfall indicators from southern China, northern China, and India, and then analyzed the rainfall patterns at different stages over the past 1200 years.

From AD 800 to AD 950, a rapid transition to a humid climate was recorded by Hem/Goe (Figure 6e), similar with other indexes (Figure 6d, f and g), reflecting a gradually strong ASM in southern China and Thailand (Chu et al., 2002; Wang et al., 2005; Yamoah et al., 2016). However, the obvious opposite trend was observed based on precipitation proxies in northern China (Figure 6a–c), indicating a continuous decrease in precipitation (Chen et al., 2020; Hu et al., 2008; Zhang et al., 2008), while similar drought environments were also shown by ¹⁸O analysis from Dandak Cave (Figure 6i) and Sahiya Cave (Figure 6j) (Berkelhammer et al., 2010; Sinha et al., 2015). During AD 950–1200, as recorded by Hem/Goe, the intensity of precipitation in Baxian Lake decreased continuously and caused the most extreme drought around AD 1200 (Figure 6e). This was the opposite to what occurred in northern China and India, where climates shifted from a pattern of “north drought-south wet” to “north wet-south drought.” This opposite precipitation pattern remained in northern China and India until AD 1900 (Figure 6, black arrows). As demonstrated by Berkelhammer et al. (2010), the records of Wanxiang (north China, Figure 6b) and Dandak caves (India, Figure 6i) displayed a strong correlation during AD 800–1420, reflecting a common forcing mechanism that may be different from that of southern China (Berkelhammer et al., 2010).

Figure 6.

Comparisons of Hem/Goe records in this study with selected representative humidity records from northern China (blue lines), India (green lines), and southern China and Thailand (red lines). The gray shaded areas represent the phase of “north drought-south wet” and the yellow shaded areas represent the phase of “north wet-south drough.” (a) Pollen-based quantitative precipitation records from Gonghai Lake (Chen et al., 2020); (b) δ¹⁸O record from Wanxiang Cave (Zhang et al., 2008); (c) δ¹⁸O record from Heshang Cave (Hu et al., 2008); (d) δ¹⁸O record from Dongge Cave (Wang et al., 2005); (e) Hem/Goe record from Baxian Lake (this study); (f) content of CaCO₃ from Hugangyan Maar Lake (Chu et al., 2002); (g) leaf wax-based hydrogen isotope record from Pa Kho Lake (Yamoah et al., 2016); (h) δ¹⁸O record from Mawmluh Cave (Dutt et al., 2021); (i) δ¹⁸O record from Dandak Cave (Berkelhammer et al., 2010); (j) δ¹⁸O record from Sahiya Cave (Sinha et al., 2015).

The hydrological conditions in the Baxian Lake area may influence the process of vegetation evolution, erosion, and weathering; these, in turn, can influence the variations of magnetic minerals. We infer that the wet climate during AD 800–950 might have promoted high vegetation coverage in the Baxian Lake basin, and consequently hindered the flux of coarse detritus into sediments. Both the content of low-coercivity and high-coercivity magnetic minerals were correspondingly decreased (Figure 5e–g). Then, conversely, lush vegetation gradually evolved toward a sparse condition during an increasingly dry climate, which lead to the concentration and grain sizes of magnetic minerals increasing from AD 950 to AD 1200 (Figure 5). This can be confirmed from the result that the grain sizes of magnetic minerals are significantly different between 850 and 900 AD (<0.1 μm, wet) and 1000–1150AD (0.2–1 μm, dry, Figure 3l). Our previous studies have shown that the magnetic minerals in the surface soil samples are larger than those in the lake sediments and the shapes are more irregular, indicating the formation of detrital magnetite by weathering in the process of pedogenesis and the filtration of vegetation (Supplemental Figure S5, available online, Wei et al., 2021).

After 1200 AD, the slow and consistent change in the general trend of magnetic parameters indicated that the climatic conditions had returned to relatively normal levels after extreme drought events (Figure 5).

Potential mechanisms of different precipitation patterns

In previous studies, the linkage among solar activities, AMOC, and ITCZ movements has been shown to dominate the precipitation pattern in the ASM area (Wang et al., 2005; Yan et al., 2015; Zhang et al., 2019). This “see-saw” mechanism on seasonal precipitation moving between the northern and southern hemispheres works regularly on a millenium scale, but cannot harmonize the environmental changes smoothly on centennial and decadal scales. Thus, based on the relationship between different precipitation proxies, the land-ocean thermal contrast, ΔSST variations, and shifts of the ITCZ during the past 1200 years, potential mechanisms in three main stages are discussed below.

ITCZ shifts influenced the inverse wet pattern between north and south China during the MWP

During the initial stage of the MWP (AD 800–950; Figure 7), the similar tendencies of the ITCZ, χ_lf and Hem/Goe records indicate that the dramatic transformation from a dry to a wet environment in Baxian Lake appears to be controlled by the strengthened ITCZ–influenced ASM (Figure 7e–g). Consequently, the vegetative conditions and hydrological environment were substantially improved in southern China. The emergence of this optimal period was rapid and undoubtedly influenced by both high temperature and humidity. Due to the subsequent southward shift of the ITCZ and the advent of La Niña-like conditions (AD 950–1200), the newly restored environment in the Baxian Lake area returned to sparse vegetation coverage again. This long and destructive dry period has been proven to have existed in many studies and historical data (Chu et al., 2002; Yancheva et al., 2007). For example, the demise of Dali kingdom at AD 1253 was directly related to a change to an extremely arid climate (Tan et al., 2018).

Figure 7.

Hem/Goe and χ_lf records in this study compared with multi-proxies, and humidity variations during the MWP and the LIA period. (a) Hematite-strained grains from North Atlantic sediments (Bond et al., 2001); (b) temperature anomalies from China based on numerous proxy-based studies (Ge et al., 2013); (c) composite temperature anomalies for the Northern Hemisphere (Moberg et al., 2005); (d) reconstructed temperature in Baxian Lake based on a branched glycerol dialkyl glycerol tetraethers (Chang et al., 2020); (e) Hem/Goe record in Baxian Lake (this study); (f) χ_lf record in Baxian Lake (this study); (g) shifts of ITCZ based on Ti content in Cariaco Basin sediments (Haug et al., 2001); (h) zonal SST gradient based on eastern equatorial Pacific SST and western equatorial Pacific SST (Conroy et al., 2010); (i) Indo Pacific Warm Pool (IPWP) SST reconstruction (Oppo et al., 2009).

The teleconnection pattern induced by North Atlantic ice-rafting and the variations in the AMOC are considered as the potential mechanism inducing the shifts of the ITCZ (Figure 7a; Wang et al., 2005) which controls the longitudinal shift of WPSH. When the higher latitudes of northern China were covered by the WPSH, the process of transporting water vapor to the north will be inhibited by high pressure there, thus forming a pattern of “north drought-south wet” (Figure 8b). In contrast, as the WPSH moved to southern China driven by the southward movement of the ITCZ, the long-distance ASM will transmit water vapor from the western edge of the WPSH to northern China, forming a precipitation pattern of “north wet-south drought” (Figure 8c). Thus, in the two stages of the MWP, the routes of the EASM in conveying water vapor were controlled by the longitudinal shifts of the WPSH, which dominated the opposing precipitation trends in northern and southern China (Figure 6, blue and red lines, respectively). This opposite rainfall pattern has also been recorded in historical documents. During the North Song Dynasty (AD 960–1127), a warm and wet climate may have promoteed the northward movement of the agro-pastoral zone in northern China (Zhang et al., 2021), while southern China was threatened by drought (Tan et al., 2018).

Figure 8.

Schematic diagram of the mechanisms influencing the rainfall patterns in the ASM region. The white dotted lines represent the location of the intertropical convergence zone (ITCZ). The orange shaded ovals represent the areas influenced by the western Pacific subtropical high (WPSH). (a) Extreme drought controlled by coupling of ITCZ and land-ocean-air system. Warm SST in the western Pacific Ocean promotes convection, resulting in the occurrence of La Niña-like conditions, a negative IOD phase, and the strong WPSH influenced by the strengthened Hadley circulation. This process led to high pressure in the lower troposphere and hampered the northward transport of water vapor from the Pacific, resulting in an extremely dry climate. Indicators (b and c) show that the longitudinal movement of the WPSH was forced by the shifts of ITCZ, and the routes of transmitting vapor water were influenced by WPSH during MWP. (d) shows that the coverage of WPSH and its intensity were gradually weakening during LIA, prompting water vapor to be transmitted to southern China through relatively strong continent-ocean thermal contrast.

In addition, similar precipitation trends have been observed between India and northern China by comparing the variations of ¹⁸O values from Dandak and Sahiya caves during the MWP (Figure 6i and j). These trends might be associated with the weak (strong) continent-ocean thermal contrast caused by the strengthened (weakened) temperature in the Indian Ocean when the ITCZ moves to the north (south), subsequently decreasing (increasing) the strength of the ISM (Gu et al., 2020; Krishnamurthy and Goswami, 2000; Roxy et al., 2015), and transporting less (more) water vapor to north China during AD 800–950 (AD 950–1200).

La Niña-like conditions weakening the precipitation rates of south China

Previous studies have suggested that frequent ENSO activities dominate the drought events in southwest China and south China (Gu et al., 2020; Tan et al., 2018; Yan et al., 2011), and make a difference in precipitation rates between central and northeastern India over the last 2000 years (Myers et al., 2015). The continuous dry period (AD 950–1200) in southern China coincided with a large ΔSST and an increase in SST in the tropical Pacific (Figure 7e, h and i). In the western tropical Pacific, change in SST between AD 1000 and AD 1300 (Figure 7h), as Conroy et al. (2010) reconstructed, may have inhibited El Niño events (Mann et al., 2005). Although it is difficult to analyze variations in the ENSO due to limited proxies and the complexity of tropical coupled ocean-air systems (Conroy et al., 2010), the larger ΔSST in the tropical Pacific would have been expected to promote the development of the Walker circulation and the occurrence of La Niña-like conditions in the MWP (Mann et al., 2009; Zhao et al., 2020). A recent study shows that the enhanced Walker and Hadley circulations would induce westward shifts of the WPSH and suppress the EASM in southern China (Zhang et al., 2020). The westward extension of the WPSH may also cover southwest China and even northeast India, which is probably the dominant cause of abnormal droughts in those regions (Gu et al., 2020; Tan et al., 2018). Therefore, these La Niña-like conditions might have hampered the movement of a supply of water vapor from the warm Pacific to southern China, resulting in a prolonged drought in our study area during the second stage of MWP. In addition, increased convection in the western Pacific would have caused a warm and wet ASM to arrive in northern China from the west side of the WPSH, leading to increased precipitation from AD 950 to 1200 in northern China (Figure 6). The large ΔSST in the tropical Pacific would be expected to promote the occurrence of a negative IOD phase (Li and Mu, 2001; Li et al., 2002), which can also enhance the WPSH (Li and Mu, 2001), and then further strengthen the difficulty of transporting water vapor to the WPSH control area.

In brief, we propose that during the MWP, the longitudinal movement of the WPSH forced by the shift of the ITCZ and the variation in the strength of the WPSH forced by the Hadley circulation and the negative IOD phase jointly dominated the opposite precipitation patterns in southern China versus those in northern China and India (Figure 8a).

“Cold-wet” pattern in the LIA caused by the continent-ocean thermal contrast

During the LIA cooling period (AD 1450–1800), the La Niña-like conditions were no longer dominating the precipitation patterns with the decreasing SSTs in the Pacific (Figure 7h and i). The decrease of SST and ΔSST indicate that the Walker and Hadley circulations were gradually weakening after AD 1300, and reached the weakest zonal SST gradient during AD 1600–1800 (Figure 7h and i; Mann et al., 2009). This is the opposite of a typical La Niña status and is trending toward El Niño-like status. These conditions, together with the strong continent-ocean thermal contrast caused by the decrease in temperatures in the tropical Pacific, ensured a stable and suitable climate in our study area, even though the southward shift of the ITCZ represents the general weakening of the ASM during AD 1300–1600.

This “cold-suitable” pattern is consistent with the “cold-wet” pattern previously documented during the LIA in southern and southwestern China (Gu et al., 2020; Roxy et al., 2015). The precipitation pattern in southern China is inversely related to temperature, which means the climate is wetter during the LIA comparing with the dry MWP (Zhao et al., 2020). After AD 1600, the climate in Baxian Lake turned slightly to dry resulting from a dramatically southward shift of ITCZ (Figure 7g); however, the overall decreasing trend of χ_lf shows that relatively suitable and stable climatic conditions were not affected intensely (Figures 5e and 7f). In total, the intensity and coverage of the WPSH, as well as its suppression to the EASM, were gradually weakened during the LIA, which prompt more water vapor to be transmitted to southern China (Figure 8d).

Coupling effect of the ITCZ and land-ocean-atmosphere system in the North Hemisphere

Based on the discussion above, we believe two main driving factors affected the paleoclimate and paleohydrological environment in the ASM region. First, the shifts of the ITCZ induced by solar insolation and AMOC (McManus et al., 2004; Rahmstorf, 2002), predominated the overall precipitation trend in ASM-influenced regions. Then, another driving factor, the land-ocean-atmosphere system, influenced precipitation in the ASM region through ENSO activities and the continent-ocean thermal contrast. These two can promote and restrict each other. The shifts of ITCZ have changed the SSTs in the tropical Pacific and Indian Ocean (Mitchell and Wallace, 1992), and therefore influence latitudinal and longitudinal atmospheric circulation and the continent-ocean thermal contrast. In turn, the land-ocean-atmosphere system is also tied to the location of the ITCZ; for example, ENSO activities lead to shifts of the ITCZ over the tropical Pacific (Deser and Wallace, 1990). Thus, the superposition of two driving factors led to extreme drought in the second stages of MWP and a more desirable climate for humans in the LIA in southern China.

Conclusions

Based on Hem/Goe data and other environmental magnetic records from sediments in Baxian Lake, we reconstructed the precipitation history in southern China over the past ~1200 years. The magnetic records represent obvious fluctuations during the MWP and the LIA periods. The climatic-influenced vegetation evolution impacted the variations of magnetic minerals by controlling the flux of weathered detrital magnetic minerals and obstructing the input of coarser detritus into Baxian Lake. The post-depositional processes were insignificant.

Our results show a broadly opposite precipitation pattern when compared with those in northern China and India. We believe two main driving factors affected the paleoclimate and paleohydrological environment in the ASM region. First, the shifts of the ITCZ induced by solar insolation and AMOC predominated the overall precipitation trend in southern China. Then, another driving factor, the land-ocean-atmosphere system, influenced precipitation in the ASM region through ENSO activities and the continent-ocean thermal contrast. The hydrological and environmental changes in our study areas are influenced by the superimposition of these two factors since 800 AD, and the migration of western Pacific subtropical high (WPSH) which controls the route of EASM and location of precipitation.

Our low-resolution samples and a lack of sufficient age-dating data mean that a more in-depth discussion is limited. Further research regarding the complex mechanism of the nature of the tropical ocean in the ASM region will be needed in the future.

Supplemental Material

sj-docx-1-hol-10.1177_09596836221101250 – Supplemental material for Variation in humidity and the forcing mechanism in Asian monsoon-influenced regions indicated by hematite/goethite from Baxian Lake, southern China, since AD 800

Supplemental material, sj-docx-1-hol-10.1177_09596836221101250 for Variation in humidity and the forcing mechanism in Asian monsoon-influenced regions indicated by hematite/goethite from Baxian Lake, southern China, since AD 800 by Shengtan Shang, Xiaoqiang Yang, Enlou Zhang, Ruomei Wei, Tingwei Zhang and Qixian Zhou in The Holocene

Footnotes

Acknowledgements

We would like to thank two reviewers and the editor for their constructive suggestions on this manuscript.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Second Expedition to the Tibetan Plateau (Grant Nos. 2019QZKK0707) and the National Natural Science Foundation of China (Grant No. 41030366).

ORCID iDs

Shengtan Shang

Enlou Zhang

Tingwei Zhang

Supplemental material

Supplemental material for this article is available online.

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