Chemical Weathering of the Mekong River Basin With Implication for East Asian Monsoon Evolution During the Late Quaternary: Marine Sediment Records in the Southern South China Sea

Abstract

Clay mineralogy, major-element geochemistry, and Sr-Nd isotopic compositions from Core MD05-2896 collected in the southern South China Sea have been utilized to investigate the discrimination of sediment provenance and to reconstruct a history of chemical weathering in the Mekong River basin over the last 45 ka. The results display that the clay mineral assemblage of the core is characterized by abundant smectite (27%–56%) and illite (18%–32%), with moderate kaolinite (13%–23%) and chlorite (11%–18%), and the ⁸⁷Sr/⁸⁶Sr ratio and ε_Nd value narrowly vary in the ranges of 0.7232–0.7272 and from –10.9 to –9.6, respectively. According to clay mineralogy and Sr-Nd isotopic compositions, the Mekong River is the main terrigenous sedimentary source to the southern South China Sea, with no detectable change over the time span of the study, despite having strong sea-level fluctuations. Clay mineralogy and elemental geochemistry analyses reveal that higher smectite/(illite + chlorite), smectite/kaolinite, TiO₂/K₂O, and SiO₂/K₂O ratios during Marine Isotope Stage (MIS) 3 and 1 suggest enhanced chemical weathering, whereas lower values of these ratios during MIS 2 indicate weakened chemical weathering. These proxies reveal a close relationship with the available climate records of the East Asian monsoon evolution, implying that the chemical weathering in the Mekong River basin has been strongly controlled by the East Asian monsoon rainfall.

1 Introduction

Chemical weathering of continental rocks is a primary Earth surface process, which significantly relates to atmospheric carbon dioxide consumption and climate change globally (Walker et al., 1981; Berner et al., 1983; Beaulieu et al., 2012). It is a principal influence on global geochemical cycles and provides important materials (i.e., nutrients, elements, etc) from continents to the oceans (Meybeck, 1982; Conley, 2002; Qin et al., 2006). This process on land is mainly controlled by climate conditions, tectonic activity, and lithology (Chamley, 1989; Liu et al., 2007a; Liu et al., 2012), but it is considered a significant factor responsible for the global climate over the geological timescale (Tamburini et al., 2003; Wan et al., 2007; Clift et al., 2014). Thus, investigation of the weathering process of parent rocks in the source regions and its relationship with monsoon can provide meaningful information in our understanding of the Earth’s surface and monsoon evolution (Colin et al., 1999; Wan et al., 2015; Clift et al., 2020).

Marginal sea receives terrigenous sediments from adjacent continents, which contain invaluable information on paleoclimatological and paleoceanographic evolution, making this region of particular attention to Earth scientists (Yang et al., 2003; Liu and Stattegger, 2014; Beny et al., 2018). The South China Sea is the largest marginal sea in the western Pacific, and it has been provided with approximately 700 million tons (Mt) of fluvial sediments annually from numerous surrounding rivers, including three large rivers, namely, the Mekong River (160 Mt/yr), the Red River (130 Mt/yr), and the Pearl River (84 Mt/yr) (Milliman and Syvitski, 1992; Milliman and Farnsworth, 2011) (Figure 1). Terrigenous sediment transport in this region is highly influenced by oceanic circulation, sea-level change, hydrodynamic sorting, monsoon evolution, and tectonic activities (e.g., Liu et al., 2003; Liu et al., 2007b; Liu et al., 2010; Schimanski and Stattegger, 2005; Clift et al., 2006; Colin et al., 2010; Tjallingii et al., 2010; Tjallingii et al., 2014; Yan et al., 2011; Sang and Liu, 2021). The East Asian monsoon controls seasonal changes in wind patterns, temperature, and rainfall over Southeast Asia, which strongly impact the weathering process on the land, forcing significantly terrigenous sediment variations in the South China Sea (Webster, 1994; Chu and Wang, 2003; Liu et al., 2004; Liu et al., 2005; Chen et al., 2017; Sang et al., 2019). Clay minerals, major elements, and Sr-Nd isotopes are important compositions of terrigenous sediments in this region and contain valuable information on particular characterizations of their source regions and the weathering process on the Earth’s surface (Liu and Stattegger, 2014; Liu et al., 2016). These allow them to be widely utilized to identify sediment provenance in the South China Sea and to reconstruct a history of weathering process in the source regions and its relationship to the East Asian monsoon (Clift et al., 2014; Wan et al., 2015; Zhao et al., 2018). The South China Sea, hence, is considered an ideal area in the global marginal seas for investigating East Asian monsoon evolution and paleoceanography (Wang et al., 1995; Wang et al., 1999; Boulay et al., 2007; Liu et al., 2016).

FIGURE 1

Hanebuth et al., 2011; Tjallingii et al., 2014). Paleo-river systems are displayed according to Voris (2000) and Sathiamurthy and Voris (2006).

The southern South China Sea inputs huge weathering products of parent rocks from surrounding river basins in Southeast Asia under the strong influence of the East Asian monsoon and sea-level change, and this region becomes a valuable region for studying sediment source-to-sink and land-sea interactions (Hanebuth et al., 2002; Hanebuth et al., 2009; Hanebuth et al., 2011; Steinke et al., 2008; Huang et al., 2016) (Figure 1). Offshore from the Mekong River, clay mineralogy of Cores MD01-2393, MD97-2150 (Liu et al., 2004), and SO18383-3 (Jiwarungrueangkul et al., 2019b) and Sr-Nd isotopic compositions of Core MD01-2393 (Liu et al., 2005) were employed to indicate the Mekong River is the primary source of terrigenous sediments. Liu et al. (2004) and Liu et al. (2005) showed that strengthened chemical weathering corresponds to increased sediment discharge and weakened physical erosion during interglacial periods in the Mekong River basin, whereas weakened chemical weathering is associated with reduced sediment discharge and intensified physical erosion during glacial periods since 190 ka ago. Colin et al. (2010) suggested that higher chemical weathering of detrital material during the Holocene mainly derives from the lower reach of this basin over the last 25 ka. Jiwarungrueangkul et al. (2019b) evaluated moderate chemical weathering in the Mekong River basin since the Last Glacial Maximum. Nevertheless, most of the previous studies offshore from the Mekong River only used clay mineralogy instead of robust proxies (i.e., Sr-Nd isotopic compositions) for identifying bulk terrigenous sediment provenance, and they have not exposed weathering process in the source regions and its relationship with paleoclimate evolution during MIS cycles (i.e., MIS 1, 2, and 3) due to the lack of high-resolution and/or short-term sediment records. Sediment provenance in the southern South China Sea, chemical weathering in the source regions, and its relationship with the East Asian monsoon are, therefore, still not fully understood and require more detailed investigations.

To solve the research questions in the studied area, clay mineralogy, major-element geochemistry, and Sr-Nd isotopic compositions of Core MD05-2896 in the southern South China Sea are combined to 1) investigate sediment provenance discrimination, especially 2) reconstruct chemical weathering in the source regions and elucidate the relationship between chemical weathering and the East Asian monsoon over the last 45 ka.

2 Geological Background

The southern South China Sea is encompassed by the Indochina Peninsula to the west, Borneo to the east, and Sumatra and the Malay Peninsula to the south (Figure 2). In the Indochina Peninsula, the Mekong River flows from the Tibetan Plateau to the South China Sea. The upper reach of this river basin is mainly characterized by Mesozoic sedimentary rocks (meta-sandstone, shale, slate, and phyllite), with minor Precambrian metamorphic and extrusive igneous rocks. The strong physical weathering processes of the parent rocks in this area make high illite and chlorite in lithosols (Liu et al., 2003). Paleozoic–Mesozoic sedimentary rocks (metasandstone, shale, and slate) and intrusive igneous rocks (mainly granites) widely cover the middle reach forming abundantly basaltic and ferrallitic soils. The lithology of the lower reach mainly distributes Mesozoic sedimentary rocks (mostly sandstone and mudstone) with some extrusive igneous rocks (basalts) and a broad alluvial plain primarily with basaltic soils (Segalen, 1995). In Borneo, the lithology consists dominantly of Tertiary sedimentary rocks, with a less abundant distribution of Paleozoic–Mesozoic sedimentary rocks in the central part, of Paleozoic–Mesozoic granite and granodiorite and Tertiary volcanic rocks in the southwestern part, and of Quaternary sediments in the coastal plain and delta (Figure 2). The sedimentary rocks contain mostly sandy shale, partly interbedded with coal beds, sandstone, and carbonate rocks (Hutchison, 2005). Sparse Tertiary basic volcanic rocks and Mesozoic basic–ultrabasic intrusive rocks are mainly distributed in central tectonic belts. Borneo has been tectonically active throughout the Cenozoic and displayed the strongest tectonic uplift during the Oligocene–Miocene period (Hall, 2002; Hutchison, 2005). Sumatra consists mainly of Quaternary intermediate to basic volcanic rocks in the mountains and slopes of the southwest and late Tertiary sedimentary rocks in the northwest, with a minor distribution of Quaternary sediments along rivers and coastline (Liu et al., 2012) (Figure 2). Besides, this region is covered sparsely by Paleozoic–Mesozoic volcanic, intrusive, and sedimentary rocks and Tertiary basalts. This area has been forced by northeastward subduction of the Indian Ocean Plate along the Sunda Trench in the eastern Indian Ocean and the Sumatra strike-slip fault on the mountainous range of the southwestern island (Barber and Crow, 2009), causing highly tectonically active since the Tertiary. Sumatra shows an extremely active volcanic eruption history with the abundant distribution of Quaternary basic–intermediate volcanic rocks. The Malay Peninsula occurs abundantly in Paleozoic–Mesozoic granite and granodiorite and Paleozoic sedimentary rocks, with minor Mesozoic (mainly Jurassic–Cretaceous) sedimentary rocks and sparse basic volcanic rocks (Liu et al., 2012) (Figure 2). Quaternary sediments have been distributed primarily along the coastline. Mudstone, sandstone, and limestone, interbedded with andesitic–rhyolitic volcanic rocks are the main compositions of the sedimentary rocks (Hutchison, 2005; Sultan and Shazili, 2009). This Peninsula has displayed stable tectonic activities since the Mesozoic period (Hutchison, 1968).

FIGURE 2

3 Materials and Methods

3.1 Materials

Core MD05-2896 (8^o49.5′N, 111^o26.47′E; 1,657 m water depth; 11.03 m long) was collected in the southern South China Sea during the cruise of MARCO POLO in 2005 (Laj et al., 2005) (Figure 1). The upper 4.69 m of this core was used for the present study wherein the samples were collected at 4 cm intervals from surface to 3.13 m whereas from 3.13 to 4.69 m the core was sampled at 2 cm intervals. A total of 178 samples were collected for clay mineralogy and major-element geochemistry analyses. Twelve of these samples were chosen to measure Strontium (Sr) and Neodymium (Nd) isotopic compositions. The age model was established by using planktonic foraminiferal AMS ¹⁴C dating and benthic foraminiferal δ¹⁸O records (Wan and Jian, 2014; Wan et al., 2020). All these analyses were performed at the State Key Laboratory of Marine Geology, Tongji University.

3.2 Analytical Methods

Clay minerals were identified by X-ray diffraction (XRD) using a PANalytical X’Pert PRO diffractometer on oriented mounts of non-calcareous clay-sized (<2 µm) particles (Holtzapffel, 1985). The carbonate of sediment samples was removed by using 0.2 N HCl. The oriented mounts were prepared following the methods described in Liu et al. (2004). Three XRD runs were performed following pretreatment conditions of air drying, ethylene-glycol solvation for 24 h, and heating at 490°C for 2 h. To identify clay minerals, the position of the (001) series of basal reflections on the three XRD diagrams was employed. Based on the glycolated curve, the MacDiff software was used to determine semiquantitative estimates of peak areas of the basal reflections for the main clay mineral groups of smectite, including mixed-layer minerals (15–17 Å), illite (10 Å), and kaolinite/chlorite (7 Å) (Holtzapffel, 1985; Petschick, 2000). Relative proportions of kaolinite and chlorite were identified using the ratios of the 3.57/3.54 Å peak areas. Furthermore, some mineralogical characteristics of illite were identified on the glycolated curve. The illite chemistry index was calculated utilizing the ratio of the 5 Å and 10 Å illite peak areas in glycol-saturated samples (Esquevin, 1969). Ratios below 0.5 represent Fe–Mg-rich illite (biotite, mica) characterized by physical erosion; ratios above 0.5 are mainly found in Al-rich illite (muscovite) formed by strong hydrolysis (Esquevin, 1969; Gingele et al., 1998). Illite crystallinity was estimated with full width at half maximum of illite 10 Å peak. Lower values represent the higher crystallinity and indicate weak hydrolysis in continental sources and arid and cold climate conditions (Chamley, 1989; Krumm and Buggisch, 1991; Ehrmann, 1998).

Major elements were measured on fused glass by X-ray fluorescence (XRF) using a PANalytical Axios^MAX spectrometer. The samples were first reacted with 0.2 N HCl to remove carbonate and then dried under 60°C. The decarbonated samples were fused and analyzed by the XRF method to obtain major element concentrations. The glass beads were prepared using ∼0.7000 g of sediment sample combined with ∼7.000 g of Li₂B₄O₇. Chinese rock and sediment standards GRS02 and GRS04 were used to monitor the analytical precision and accuracy. Major elements were expressed as their oxides as absolute bulk contents of samples after carbonate removal. Considering the objectives of terrigenous clastic compositions of this study, total organic carbon (TOC) and total inorganic carbon (TIC) as well as the loss of ignition (LOI) were not measured separately prior to the fusion because the data were not planned for usage. However, given the fusion method of our XRF analysis, the LOI values can be roughly calculated by subtracting all the major elements from 100%. All these data are included in the Supplementary Material.

Sr and Nd isotopic compositions of bulk carbonate-free terrigenous sediments were analyzed using a Thermo Scientific Neptune Plus MC-ICP-MS. Geostandards GSR-6 and GSD-9 were used to monitor the precision and accuracy. A small subsample of the sediment samples was dried at low temperature (∼50°C) and crushed to a fine powder. The powder samples were treated with 1 N HCl to remove calcium carbonate and authigenic components. They were then heated at 600°C for 2 h to remove the organic carbon. A ∼50 mg of each sample was totally digested with a mixture of concentrated HF and HNO₃ in high-pressure Teflon bombs at 190°C for at least 48 h. Sr and Nd were separated following the procedure described in detail by Wu et al. (2021). Sr and Nd isotopic compositions were normalized to ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, respectively. The standards SRM987 (⁸⁷Sr/⁸⁶Sr = 0.710250 ± 0.000016, Lugmair et al., 1983) and JNdi-1 (¹⁴³Nd/¹⁴⁴Nd = 0.512115 ± 0.000006, Tanaka et al., 2000) were used to monitor the quality of ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd, respectively. The mean measured ⁸⁷Sr/⁸⁶Sr value of SRM987 was 0.710242 ± 12 (2σ, N = 10), and the mean measured ¹⁴³Nd/¹⁴⁴Nd of JNdi-1 was 0.512118 ± 8 (2σ, N = 10), well within the recommended values. Nd isotopic data are expressed as ε_Nd = [(¹⁴³Nd/¹⁴⁴Nd)_measured/(¹⁴³Nd/¹⁴⁴Nd)_CHUR–1] x 10⁴. The CHUR (Chondritic Uniform Reservoir) value is 0.512638 (Jacobsen and Wasserburg, 1980).

4 Results

4.1 Clay Minerals

The clay mineral assemblage of Core MD05-2896 consists of dominant smectite (27%–56%) and illite (18%–32%), with moderate kaolinite (13%–23%) and chlorite (11%–18%) (Figure 3). All these clay minerals present high variability over the past 45 ka; however, their variability can be distinguished between MIS 3, 2, and 1. Kaolinite, illite, and chlorite contents exhibit a similar pattern with decreased values during MIS 3 and 1, and increased values during MIS 2. On the contrary, smectite content displays an inverse pattern with high values during MIS 3 and 1 and low values during MIS 2. Illite crystallinity and illite chemistry index show high variations in values between 0.16 and 0.22°Δ2θ (average 0.19°Δ2θ) and from 0.39 to 0.59 (average 0.45), respectively, and they display no visible trend over the last 45 ka.

FIGURE 3

Hanebuth et al., 2000) and 1C (MWP-1C) (Tjallingii et al., 2010). MIS 1 (0–12.05 ka), MIS 2 (12.05–24.11 ka), and MIS 3 (24.11–45.00 ka) display marine isotope stages (Martinson et al., 1987). The clay mineral data were smoothed by a three-point running average (coarse).

4.2 Major Elements

In Core MD05-2896, the major elements contain dominant SiO₂, Al₂O₃, and Fe₂O₃ (in total ∼84%), with minor K₂O, MgO, TiO₂, Na₂O, CaO, MnO, and P₂O₅ (∼7% in total) (Figure 4). Generally, variations in Al₂O₃, Fe₂O₃, K₂O, and Na₂O contents present inverse correlations to those of TiO₂, SiO₂, and MgO contents over the last 45 ka. Al₂O₃, Fe₂O₃, K₂O, and Na₂O contents display increased values during MIS 3 and 2, and decreased values during MIS 1. In contrast, TiO₂, SiO₂, and MgO contents are characterized by low values during MIS 3 and 2, and high values during MIS 1. Nevertheless, P₂O₅ content exhibits little variation in values with no obvious trend during MIS 3, 2, and 1. CaO content shows high values during MIS 3 and decreased values after late MIS 3 to the present time.

FIGURE 4

Hanebuth et al., 2000) and 1C (MWP-1C) (Tjallingii et al., 2010). The major element data were smoothed by a three-point running average (coarse).

4.3 Sr-Nd Isotopes

⁸⁷Sr/⁸⁶Sr ratios of terrigenous sediments from Core MD05-2896 vary between 0.7232 and 0.7272, and ε_Nd values range from −10.9 to −9.6 (Figure 3; Table 1). Downcore variations in ⁸⁷Sr/⁸⁶Sr ratios increase in values from MIS 3 to MIS 1, and ε_Nd shows increased values from MIS 3 to middle MIS 2 and decreased values after middle MIS 2 to the present time.

5 Discussion

5.1 Sediment Provenance Discrimination

Generally, ways of transportation, weathering processes in source regions, and sea-level change can induce variability of clay mineral assemblage and Sr-Nd isotopic compositions of Core MD05-2896. Clay mineral assemblages and Sr-Nd isotopic compositions of marine sediments have been then extensively utilized to elucidate potential sediment provenance (Clift et al., 2002; Liu et al., 2008; Lupker et al., 2013; Cai et al., 2020), resulting in tracking sediment transport in the sea (Liu et al., 2007b; Liu et al., 2010; Zhao et al., 2018). Terrigenous sediments in the southern South China Sea derive mainly from the surrounding rivers, such as the Mekong River and small rivers from Borneo (42 Mt/yr), Sumatra (498 Mt/yr, model data), the Malay Peninsula (35 Mt/yr), and Thailand (19 Mt/yr) (Figure 1) due to the trivial contribution of eolian fluxes in this region (Wehausen et al., 2003). The small mountainous rivers in central Vietnam mainly provide huge terrigenous sediments to the central Vietnam shelf (Schimanski and Stattegger 2005; Sang et al., 2019), whereas they contribute a minor volume of terrigenous fine detrital materials to the western South China Sea (Liu et al., 2007b) and offshore from the Mekong River (Liu et al., 2004; Liu et al., 2005). Hence, terrigenous sediment contributions from these rivers in central Vietnam will not be discussed more thereafter.

5.1.1 Evidence From Clay Minerals

To determine the provenance of clay minerals in the region during the late Quaternary, all clay mineral samples of Core MD05-2896 are grouped into two intervals: Holocene and Pleistocene samples. The proportion of kaolinite, illite + chlorite, and smectite at this core is compared to the potential sources mentioned above and referred to as Cores SO18383-3, MD01-2393, and MD97-2150 offshore from the Mekong River, respectively (Figure 5). Clay mineral assemblage of Core MD05-2896 in comparison with the potential sediment sources shows that clay minerals in this region may originate from multiple sources due to no distinct overlap (e.g., Liu et al., 2007b; Steinke et al., 2008; Wang et al., 2015; Chen et al., 2017; Zhao et al., 2018). However, some robust evidence below indicates that the Mekong River can provide mainly clay minerals to this studied area. Clay mineral assemblages of Holocene and Pleistocene samples at Core MD05-2896 are overlayed, indicating no major changes in clay mineral sources during the late Quaternary. Additionally, the clay mineral assemblage of Core MD05-2896 and the referred cores are situated nearly each other, implying that these cores can have the same source of clay minerals. According to Liu et al. (2004) and Jiwarungrueangkul et al. (2019b), clay minerals of Cores MD01-2393, MD97-2150, and SO18383-3 primarily come from the Mekong River, suggesting that this river can be the most crucial source of clay minerals at Core MD05-2896 as well.

FIGURE 5

Jiwarungrueangkul et al., 2019b) are plotted for comparison. Orange dashed line and arrow imply the major contribution from the Mekong River with increasing differential setting of smectite.

Illite and chlorite are mainly formed by the physical erosion processes in the source regions under cold and arid climatic conditions (Chamley, 1989). Furthermore, illite and chlorite at Core MD05-2896 display a similar pattern over the last 45 ka (Figure 3). Based on their formation in the source regions and patterns during MIS 3, 2, and 1, illite and chlorite are generally similar to sediment sources (e.g., Liu et al., 2007b; Liu et al., 2008). Surrounding the southern South China Sea, the Mekong River and rivers in North Borneo are characterized by high illite (average 35% and 56%, respectively) and chlorite (average 26% and 25%, respectively) in sediments (Liu et al., 2007a; Liu et al., 2012). In North Borneo, strong physical erosion of parent rocks has been caused by active tectonic setting since the Mesozoic and especially the rapid uplift during the Oligocene-Miocene (Rangin et al., 1990; Hutchison, 2005) (Figure 2) and the abundant seasonal precipitation (Liu et al., 2012), yielding high illite and chlorite in river sediments. In the Mekong River basin, metamorphic and granitic parent rocks extensively occur in the eastern Tibetan Plateau under cold and dry climate conditions where illite and chlorite can be produced highly in weathering products (Liu et al., 2004) (Figure 2). Nevertheless, this studied area can receive primarily illite and chlorite from the Mekong River, with negligible contributions from rivers in North Borneo due to some probable evidence. First, rivers in North Borneo are characterized by small drainage areas, short length, and no significant sediment discharge, implying that they cannot contribute an essential amount of terrigenous sediments to this region (Figure 1). In contrast, the Mekong River drains a large catchment of 790,000 km², supplying the highest amounts of terrigenous sediments to the southern South China Sea. Additionally, this river mouth is much closer to the location of Core MD05-2896 than those of rivers in North Borneo. These suggest a higher contribution from the Mekong River compared to rivers in North Borneo. Second, the illite chemistry index and illite crystallinity of marine sediments have been extensively employed to trace the source of illite in the South China Sea (Liu et al., 2007b; Liu et al., 2008; Liu et al., 2016; Jiwarungrueangkul et al., 2019a; Sang et al., 2019). Illite chemical index and illite crystallinity are determined by the type and intensity of weathering in river basins (Chamley, 1989), but they show no obvious trends between MIS 1, 2, and 3 in this study (Figure 3), implying significant relation to the source of illite instead of the chemical weathering process in the source regions. To elucidate a vital source of illite in this region, the illite chemistry index and illite crystallinity of Core MD05-2896 are plotted compared with those of referred Cores MD01-2393, MD97-2150, and SO18383-3 and potential sources (Figure 6). Most of the samples at Core MD05-2896 are close to the Mekong River samples, while they display long distances with other potential sources, especially rivers in North and West Borneo. Additionally, illite at Cores MD01-2393, MD97-2150, and SO18383-3 is provided chiefly by the Mekong River (Liu et al., 2004; Jiwarungrueangkul et al., 2019b), but their samples are less close to the Mekong River samples than those of Core MD05-2896. The Mekong River is, therefore, the most significant source of illite and chlorite in the studied region.

FIGURE 6

Jiwarungrueangkul et al., 2019b) are plotted for comparison.

The formation of kaolinite usually relates to monosialitization of parent rocks, characterizing intensive chemical weathering (Chamley, 1989). Kaolinite is easily formed by parent rocks enriched in alkali and alkaline elements (e.g., granite, granodiorite, and intermediate-acid volcanic rocks) under warm and humid climates. In the potential source regions, kaolinite abundantly occurs in river sediments from the Malay Peninsula (average 80%), Sumatra (average 57%), West Borneo (average 47%), Thailand (average 32%), and the Mekong River (average 28%) (Liu et al., 2007a; Liu et al., 2012). However, the Mekong River can be the most important potential source of kaolinite at Core MD05-2896 (average 16%) due to its location and sediment discharge, as well as the differential setting tendency of kaolinite in saline water. Paleozoic-Mesozoic sedimentary rocks (i.e., mudstone, sandstone, and limestone, interbed with andesitic-rhyolitic volcanic rocks) and Paleozoic-Mesozoic granite and granodiorite rocks in the Malay Peninsula easily form kaolinite under warm and humid East Asian-Australian monsoon climate and stable tectonics since the Mesozoic (Hutchison, 1968; Sultan and Shazili, 2009; Liu et al., 2012) (Figure 2). In Sumatra, the majority of Quaternary intermediate-basic volcanic rocks highly yield kaolinite in weathering products under the strong influence of the monsoon climate with the warm temperature and dominant precipitation because of the well-developed monosialitization (Liu et al., 2012) (Figure 2). The monosialitiation of intrusive and volcanic rocks in West Borneo produces high kaolinite in river sediments under the warm and humid monsoon climate. In Thailand, parent rocks mainly contain granite, granodiorite, and intermediate-acid and basic volcanic rocks, which are undergone warm and humid climate conditions, producing high kaolinite in river sediments (Liu et al., 2016) (Figure 2). However, kaolinite tends to deposit close to its sources because it is a relatively large size (0.5–8.0 µm in diameter) and strongly flocculated in alkaline seawater (Gibbs, 1977; Johnson and Kelley, 1984; Patchineelam and Figueiredo, 2000). As a consequence, kaolinite is distributed highly in sediments from surrounding fluvial systems in the South China Sea (i.e., the Malay Peninsula, South China, central Vietnam, and West Borneo) (Liu et al., 2016), implying that kaolinite in this area can come mainly from proximal sources (i.e., the Mekong River) instead of remote sources (i.e., rivers in Borneo, Sumatra, the Malay Peninsula, and Thailand) (Figure 1). The Mekong River mouth is much closer to Core MD05-2896 and displays much higher terrigenous sediment contribution than those of other small rivers surrounding the southern South China Sea. Furthermore, more kaolinite in the Malay Peninsula, Sumatra, Thailand, and West Borneo should be supplied to the region during the Holocene because of increased southwesterly summer monsoon currents. Nevertheless, kaolinite displays lower content during the Holocene than during glacial MIS 2. Therefore, rivers in Sumatra, the Malay Peninsula, West Borneo, and Thailand can contribute kaolinite insignificantly to the studied site. It is then reasonable to conclude that the Mekong River is the main source of kaolinite transported to the Core MD05-2896. Variations of kaolinite proportions display a good positive correlation with those of illite and chlorite (Figure 3). This suggests that all these clay minerals derive from physical erosion processes of different parts of the Mekong River basin. According to Liu et al. (2004), kaolinite could originate from active erosion of inherited clays of ferrallitic soils in a lower part of the middle reach in the Mekong River basin. Additionally, kaolinite can be transported by reworking from the exposed continental shelf during the sea-level low-stand.

Smectite may be formed by the chemical weathering of volcanic rocks in the source regions, which contains basic materials such as Fe-Mg species and rhyolitic materials under warm and humid climate conditions (Chamley, 1989). Smectite is dominant in rivers from Thailand (average 42%), moderate in Sumatra rivers (average 19%), and low in West Borneo rivers (average 14%) and the Mekong River (average 11%) (Liu et al., 2007a; Liu et al., 2012). Nevertheless, the Mekong River emerges to play the most critical role in providing smectite to this studied area. Smectite displays the highest content in river sediments from Thailand due to the abundant basaltic lithology in these drainage systems under hot-warm climate conditions (Figure 2). However, rivers in Thailand do not yield much terrigenous sediments to the sea annually, and most of the terrigenous sediments are trapped within the Gulf of Thailand, and little exchange of clay minerals happens between gulf of Thailand and the southern South China Sea because of the strong clockwise surface circulation in the entrance region during the Holocene (Liu et al., 2016) (Figure 1). These imply that smectite in the region could not come mainly from rivers in Thailand. In Sumatra, the majority of Quaternary intermediate-basic volcanic rocks are considered potential parent rocks for producing moderate smectite in river sediments (Liu et al., 2012) (Figure 2). Furthermore, the occurrence of basic Tertiary volcanic rocks in West Borneo can produce smectite in river sediments, too, but these rivers contribute only minor terrigenous sediments to the sea. Kaolinite shows dominant contents in river sediments in Sumatra and West Borneo, but the increase of smectite is not associated with the increase of kaolinite (Figure 3). Additionally, the temperature is mostly stable in the tropical region (Warrier and Shankar 2009), and the winter monsoon primarily supplies the precipitation in tropical lands surrounding the southern South China Sea, implying that the enhanced winter monsoon may bring much more precipitation to those regions (Sun et al., 2002; Hu et al., 2003; Wang et al., 2009). These climatic conditions allow parent rocks in these tropical regions to produce more smectite in weathering products during the last glacial period. Nevertheless, smectite at Core MD05-2896 displays lower values during the last glacial period (MIS 2) than during the Holocene (MIS 1) (Figure 3). These suggest that rivers in Sumatra and West Borneo cannot provide a valuable amount of smectite to this region. Therefore, rivers in Thailand, Sumatra, and West Borneo can transport only negligible amounts of smectite to the studied area, and smectite at Core MD05-2896 mainly originates from the Mekong River. Sediments in the Mekong River are characterized by low smectite, but this river can be sufficiently a source for smectite of Core MD05-2896 due to the close relationship between the differential setting effect of smectite in more saline and distal waters and the South China Sea basin-wide clay mineral distribution (Liu et al., 2016). Smectite in this region can be weathering products of parent aluminosilicate and ferromagnesian silicates in the middle to lower reaches of the Mekong River under warm and humid climate conditions (Liu et al., 2004) (Figure 2).

5.1.2 Evidence From Sr-Nd Isotopes

Bulk terrigenous sediments of Core MD05-2896 display high ⁸⁷Sr/⁸⁶Sr ratios and low ε_Nd values (Table 1), indicating that terrigenous sediments in this region can come mainly from old continental materials. Sr-Nd isotopic compositions in weathering products depend on crustal domains and tectonic backgrounds (Mclennan et al., 1990; McLennan and Hemming, 1992). Nd is more immobile than Sr during chemical weathering, suggesting that Nd can be enriched in weathering products, whereas Sr will be removed easily from parent rocks during chemical weathering. In addition, ε_Nd values and ⁸⁷Sr/⁸⁶Sr ratios can be influenced by size fractions of the sediments, as decreased grain size of sediments can cause increased ⁸⁷Sr/⁸⁶Sr ratios, and ε_Nd will be more radiogenic (Revel et al., 1996; Innocent and Fagel, 2000). Naturally, variations in Sr-Nd isotopic compositions of terrigenous sediments at Core MD05-2896 may depend on intensive chemical weathering and grain size fractions, but they are still helpful for tracing sediment provenance (e.g., Walter et al., 2000; Colin et al., 2006; Wei et al., 2012; Lupker et al., 2013; Wan et al., 2015; Wan et al., 2017).

To elucidate the terrigenous sediment provenance, Holocene and Pleistocene Sr-Nd isotopic compositions of Core MD05-2896 are plotted to compare with those of the potential sources and referred to Core MD01-2393 (Figure 7). Surrounding this region, ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd values of the Mekong River and rivers in Thailand are much close to those of Core MD05-2896 and surface sediments of these rivers are mainly weathering products of old continental materials. Taking into consideration the short distance of the Mekong mouth to the studied site and the huge sediment discharge of the Mekong River, we can hypothesize that the Mekong River is the main sedimentary source. Although ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd values of river sediments from Thailand also are quite close to those of the cores, these rivers can contribute insignificantly terrigenous sediments to the sea and their remote locations (Figure 1). Additionally, if rivers in Thailand would supply crucial amounts of terrigenous sediments to the region, rivers in the Malay Peninsula and Sumatra can also transport much fluvial sediments to this studied area. Nevertheless, rivers from the Malay Peninsula and Toba-volcanic rocks in southwestern Sumatra clearly show much different ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd values compared with those of the core. This suggests that rivers from these areas cannot be important sources of terrigenous sediments to the Core MD05-2896. As a result, rivers in Thailand cannot transport a significant amount of terrigenous sediments to the studied area. In addition, river sediments from North Borneo and these cores also differ in ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd values, suggesting terrigenous sediments offshore from the Mekong River cannot come significantly from rivers in North Borneo as well. As illustrated in Figure 7, the ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd values of Cores MD05-2896 and MD01-2393 are adjacent to each other, determining that these cores originate from the same source. Provenance analysis based on Sr-Nd isotopic compositions of Core MD01-2393 indicated that the Mekong River is considered the primary source of this core (Liu et al., 2005), suggesting this river also provides mainly terrigenous sediments of Core MD05-2896. The ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd values of the Holocene and Pleistocene samples of both cores are imbricated clearly, suggesting no change in terrigenous sediment sources since the late Pleistocene. Consequently, the Mekong River emerges as the most important source contributing bulk terrigenous sediments to the southern South China Sea without significant change in sediment sources during the late Quaternary. Furthermore, ε_Nd shows little higher values during MIS 2 than MIS 3 and 1, implying that stronger physical weathering of parent rocks with more positive Nd isotopes in the eastern Tibetan Plateau during MIS 2 can provide more terrigenous sediments from the highland part to the Core MD05-2896. This hypothesis is consistent with provenance analysis based on clay mineralogy above, implying bulk and clay-fraction sediments are the same source and mainly originate from the Mekong River.

FIGURE 7

Chesner, 2012; Lai et al., 2021) for potential sediment sources and sediment samples at referred Core MD01-2393 (Liu et al., 2005) are plotted for comparison.

5.2 Effect of Sea-Level Change

Generally, global sea-level change can impact coastline configuration (i.e., location of estuary system) and offshore sediment transport concerning river power and marine forcing, causing sediment source changes and/or variations in terrigenous sediment discharge. Terrigenous sediment input to the southern South China Sea off the Sunda Shelf can be influenced strongly by sea-level changes (Steinke et al., 2008; Huang et al., 2016; Jiwarungrueangkul et al., 2019a), whereas the terrigenous sediment variations offshore from the Mekong River should be less sensitive to sea-level fluctuations. In this region, the significant sea-level rise was as much as 16 m from 14.6 to 14.3 ka at the meltwater pulse 1A (MWP-1A) (Hanebuth et al., 2000) (Figure 8), forming a rapid retreat of the Paleo-Chao Phraya River mouth toward the land, whereas there is no important change in the location of the Paleo-Mekong River estuary (Voris, 2000; Sathiamurthy and Voris, 2006) (Figure 1). This is due to the continental shelf at the Paleo-Mekong River mouth being characterized by a high gradient and not by large morphology (Tjallingii et al., 2010), making this paleo-river retreat insignificantly on the continental shelf during the MWP-1A. Furthermore, between 9.5 and 8.5 ka, Tjallingii et al. (2010) found a rapid migration of the Paleo-Mekong River mouth which was influenced by the enhanced rate of sea-level rise (25 mm/yr) from 34 to 9 m below the modern sea level corresponding to the sea-level jump of the meltwater pulse 1C (MWP-1C). Nevertheless, sea-level fluctuations may not greatly impact terrigenous sediment variations of sediment cores that are close to their sources (e.g., Liu et al., 2010; Wan et al., 2015; Sang et al., 2019). In comparison with sea-level fluctuations in this region, no important variations of clay minerals, major elements, and Sr-Nd isotopic compositions at Core MD05-2896 are observed evidently during the MWP-1A and -1C (Figures 3, 4), suggesting that the sea-level change may not have strongly affected the terrigenous sediment variations of the studied core over the last 45 ka. In addition, clay mineral assemblage and Sr-Nd isotopic compositions of Holocene and Pleistocene samples at the core are located adjacently (Figures 5, 7), indicating no obvious change in sediment sources under the effect of sea-level change since the late Pleistocene. These suggest that sea-level fluctuations cannot alter the overall trend of terrigenous sediment input in this region, and it only causes a trivial influence on terrigenous sediment variations. These hypotheses are also suitable for investigations on clay mineralogy and elemental geochemistry of Cores MD01-2393, MD97-2150, and SO18383-3 offshore from the Mekong River, which showed no apparent variations of terrigenous sediments under the impact of sea-level change during the MWP-1A and -1C (Liu et al., 2004; Liu et al., 2005; Colin et al., 2010; Jiwarungrueangkul et al., 2019b). Undoubtedly, the Mekong River still contributes large terrigenous sediments to this region without any significant changes in sediment sources and terrigenous sediment variations under the influence of sea-level fluctuations over the last 45 ka.

FIGURE 8

Dykoski et al., 2005). Sea-level data are from Geyh et al. (1979), Waelbroeck et al. (2002), and Hanebuth et al. (2000); Hanebuth et al. (2009); Hanebuth et al. (2011). Dashed magenta lines indicate the meltwater pulses 1A (MWP-1A) (Hanebuth et al., 2000) and 1C (MWP-1C) (Tjallingii et al., 2010).

5.3 Chemical Weathering and East Asian Monsoon Evolution

Based on the provenance analysis above, the Mekong River is the dominant source of terrigenous sediments to the Core MD05-2896. In addition, sea-level fluctuations could not have significantly impacted terrigenous sediment variations of this core over the last 45 ka. This implies that terrigenous sediment variations of this core can relate intimately to the weathering processes in the source regions instead of sea-level changes in the southern South China Sea. Thus, clay minerals and major elements from this core can be used to investigate past changes in the chemical weathering intensity in the Mekong River basin. In Core MD05-2896, illite, chlorite, and kaolinite contents relatively present inverse correlations to smectite content (Figure 3), suggesting that they could be produced by different weathering processes in the Mekong River. In this basin, illite, chlorite, and kaolinite are derived from physical weathering and erosion, while smectite originates from chemical weathering. For these reasons, smectite/(illite + chlorite) and smectite/kaolinite ratios can be used as reasonable proxies for characterizing chemical weathering in the Mekong River basin (Figure 8). Thus, higher smectite/(illite + chlorite) and smectite/kaolinite ratios indicate enhanced intensive chemical weathering conditions, whereas lower ratios represent the weakened intensity of chemical weathering conditions.

In the source regions, the chemical weathering of the parent rocks generally motivates the enrichment of immobile elements (e.g., Al, Si, Ti, and Fe) and movement of mobile elements (e.g., Ca, Na, K, and Mg) in weathering products (Nesbitt et al., 1980) and their behavior can be conserved in terrigenous sediments in the sea (Clift et al., 2014; Liu et al., 2016). Based on the behavior of major elements during the chemical weathering on the Earth’s surface, the chemical index of alteration (CIA) (Nesbitt and Young, 1982), the weathering index of parker (WIP) (Parker, 1970), the plagioclase index of alteration (PIA) (Fedo et al., 1995), the index of compositional variability (CIW) (Harnois, 1988), A-CN-K and A-CNK-FM ternary diagrams (Nesbitt and Young, 1984; Nesbitt and Young, 1989), and their oxide ratios (e.g., Al₂O₃/K₂O, SiO₂/K₂O, TiO₂/K₂O) have been widely utilized to identify the altered pathway and to evaluate the intensive chemical weathering in the source regions (e.g., Liu et al., 2004; Clift et al., 2014; Wan et al., 2015; Jiwarungrueangkul et al., 2019b; Sang et al., 2019). Nevertheless, the geochemical compositions of fluvial sediments are controlled by complex factors such as provenance geology, weathering and erosion processes, and hydrodynamic sorting (McLennan, 1993; Nesbitt et al., 1996; Garzanti and Resentini, 2016; Guo et al., 2018). Furthermore, the fluvial sediments in the large catchments may not be derived directly from the weathering of igneous rocks (i.e., first cycle) instead of recycled sedimentary rocks (Dellinger et al., 2014; Garzanti and Resentini, 2016). These imply that these weathering indices may reflect the integrated weathering history rather than the instantaneous weathering condition at the time of source-to-sink transport processes of fluvial sediments and these indices should be applied carefully to investigate the weathering history in a specific catchment. Examining Figure 4, variations in Al₂O₃, Fe₂O₃, TiO₂, and SiO₂ contents don’t display similar patterns over the last 45 ka, and variations in Na₂O, K₂O, CaO, and MgO contents also show different trends during MIS 3, 2, and 1. This may indicate that not all of these elemental oxide variations at this studied core relate significantly to the intensity of chemical weathering in this basin. Additionally, stronger physical erosion of inherited clays of ferrallitic soils in a lower part of the middle reach in the Mekong River basin could provide higher Al₂O₃ in fluvial sediments during MIS 2 than during MIS 3 and 1 (Figure 4). This event may impact CIA, WIP, PIA, and CIW values and Al₂O₃/mobile element oxide ratios, indicating that these proxies could not display temporal changes in the intensive chemical weathering of the Mekong River basin over the last 45 ka.

In Core MD05-2896, variations in TiO₂ and SiO₂ contents inversely correlate to K₂O content (Figure 4), indicating that these elements relate to chemical weathering intensity in the Mekong River basin. SiO₂ and TiO₂ are relatively resistant to weathering (Nesbitt and Young, 1982; Wei et al., 2003), and they are mainly the terrigenous detrital origin of secondary minerals, including clay minerals in marine sediments (Zhang et al., 2002; Bastian et al., 2017). In contrast, K₂O is highly mobile during weathering processes, implying easy depletion in weathering products (Nesbitt et al., 1980; Condie et al., 1995). Based on the behavior of immobile elements (i.e., Si⁴⁺ and Ti⁴⁺) and mobile elements (i.e., K⁺) during chemical weathering, SiO₂/K₂O and TiO₂/K₂O ratios can be used to reconstruct the intensity of chemical weathering in the source region. Variations in SiO₂/K₂O and TiO₂/K₂O ratios of Core MD05-2896 show similar changes over the last 45 ka (Figure 8), and an increase in these ratios indicates strengthened chemical weathering, while a decrease in the ratios suggests weakened chemical weathering.

During the last glacial MIS 3, the Mekong River basin is characterized by slightly strengthened chemical weathering, indicated by higher smectite/(illite + chlorite) (0.8–1.8) and smectite/kaolinite (2.0–4.2), but negligibly increased SiO₂/K₂O (16.9–17.9) and TiO₂/K₂O (0.2–0.24) ratios. Variations in these weathering proxies display a correspondence with benthic foraminiferal C. wuellerstorfi δ¹⁸O records at Core MD05-2896 (Wan et al., 2020) and the stalagmite δ¹⁸O records of Hulu Cave (Wang et al., 2001). After this period, the mineralogical proxies displayed evidently lower values, whereas the elemental ratios displayed unclear decreased values, implying that the degree of chemical weathering between the last glacial MIS 3 and MIS 2 are the unobvious difference and the clay mineralogical proxies may be more sensitive with variations in intensive chemical weathering than the elemental proxies.

During the last glacial MIS 2, the lowest values of 0.5–1.4 smectite/(illite + chlorite) and 1.2–3.2 smectite/kaolinite ratios, while no distinct decreased 17.0–17.8 SiO₂/K₂O and 0.23–0.24 TiO₂/K₂O ratios indicated the weakest chemical weathering in the Mekong River basin. SiO₂/K₂O and TiO₂/K₂O ratios during this period displayed trivial lower values than the last glacial MIS 3, implying no significant weakened chemical weathering during this period. Owing to the geochemistry of K being complicated, it is enriched in weathering products under moderate chemical weathering of the parent rocks in the source regions, while depleted under more intensive chemical weathering (Balxland, 1974; Condie et al., 1995; Nesbitt et al., 1980; Nesbitt et al., 1997). The decrease in chemical weathering correlates well to a weak monsoon intensity during this period reaching maximum values in the benthic C. wuellerstorfi δ¹⁸O records at Core MD05-2896 and the stalagmite δ¹⁸O records of the Hulu Cave and the Dongge Cave (Dykoski et al., 2005). At the end of this period, the mineralogical and elemental proxies displayed increased trends, suggesting enhanced chemical weathering in the source region with higher smectite and immobile elements (i.e., Si, Ti) in weathering products, while less abundance of illite, chlorite, and kaolinite, as well as stronger leaching of mobile elements (i.e., K).

During the Holocene (MIS 1), the East Asian monsoon displays strengthened summer monsoon and weakened winter monsoon, causing higher precipitation and hot-warm temperature in the source regions (Wang et al., 2001; Dykoski et al., 2005). In the Mekong River basin, the climatic setting is significantly controlled by the East Asian monsoon (Webster, 1994; Chu and Wang, 2003), suggesting that this river basin could be characterized by hot and warm climate conditions during the Holocene. The Mekong River basin has experienced strengthened chemical weathering as indicated by the increasing 0.67–1.8 smectite/(illite + chlorite), 1.7–4.3 smectite/kaolinite, 17.2–19.2 SiO₂/K₂O, and 0.2–0.3 TiO₂/K₂O ratios.

The mineralogical proxies (smectite/(illite + chlorite) and smectite/kaolinite) and the geochemical proxies (SiO₂/K₂O and TiO₂/K₂O) for chemical weathering characterization of Core MD05-2896 in the southern South China Sea are effective indicators for reconstructing the history of chemical weathering in the Mekong River basin and the relationship between weathering process and East Asian monsoon evolution (Figure 8). In general, the smectite/(illite + chlorite), smectite/kaolinite, SiO₂/K₂O, and TiO₂/K₂O ratios showed an evident cyclicity with higher values during the last glacial MIS 3 and the Holocene (MIS 1), while the lower values during the last glacial MIS 2. Variations in these mineralogical and geochemical proxies present a close correlation to the benthic C. wuellerstorfi δ¹⁸O records at Core MD05-2896 (Wan et al., 2020) and the stalagmite δ¹⁸O records of the Hulu Cave (Wang et al., 2001) and the Dongge Cave (Dykoski et al., 2005). These suggest that the chemical weathering in the Mekong River basin has been strongly forced by the East Asian monsoon evolution over the last 45 ka. The last glacial MIS 3 and the Holocene MIS 1 are characterized by warm and humid climatic conditions, which facilitate strengthened chemical weathering in the Mekong River basin, yielding more secondary clay minerals (i.e., smectite) and enrichment of immobile elements (i.e., Si, Ti) in weathering products. On the contrary, the last glacial MIS 2 corresponds to cold and dry climatic conditions, causing weakened chemical weathering with higher occurrences of primary clay minerals (i.e., illite, chlorite) and mobile elements (i.e., K) in weathering profiles. As the 7th largest river in the world and the largest river in Southeast Asia, the Mekong River transports huge amounts of terrigenous sediments to the South China Sea under the strong influence of the East Asian monsoon. In this basin, source-to-sink transport processes of fluvial sediments highly relate to the weathering process and monsoon evolution. This study revealed the chemical weathering in the Mekong River basin and its relationship with the East Asian monsoon during the late Quaternary that can boost our understanding of the Earth’s surface and monsoon evolution as well as the complex interaction between continental chemical weathering and climatic conditions globally, especially in South China and Southeast Asia regions.

6 Conclusion

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