Journal of Asian Earth Sciences 96 (2014) 84–92
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Clay mineralogy of the riverine sediments of Hainan Island, South China Sea: Implications for weathering and provenance Bangqi Hu a,⇑, Jun Li a,⇑, Ruyong Cui a, Helong Wei a, Jingtao Zhao a, Guogang Li b, Xisheng Fang c, Xue Ding a, Liang Zou a, Fenglong Bai a a b c
Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao Institute of Marine Geology, Qingdao 266071, China Marine Engineering and Prospecting Institute of North China Sea, North China Sea Branch of the State Oceanic Administration, Qingdao 266033, China Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China
a r t i c l e
i n f o
Article history: Received 26 February 2014 Received in revised form 26 August 2014 Accepted 27 August 2014 Available online 6 September 2014 Keywords: Clay minerals Provenance Weathering Hainan Island South China Sea
a b s t r a c t Clay mineralogy of 54 ﬂuvial samples collected from 20 major rivers on Hainan Island are investigated in order to determine compositional changes of clay minerals and to assess the weathering processes. The clay mineral assemblages consist dominantly of kaolinite (31–66%), with a lesser abundance of chlorite (22–44%) and illite (4–33%), and a trace amount of smectite (0–15%). Fluvial sediments from the east and northwest of Hainan Island are characterized by a higher kaolinite content and illite chemical index and poorer illite crystallinity than those from southwest Hainan. Only minor smectite (mean of 7%) occurs in the sediments from west Hainan; smectite is total lacking in east Hainan. Compared with the adjacent basins, Hainan Island is characterized by moderate to intensive chemical weathering with strong hydrolysis. Our results suggest that rainfall is the principal factor controlling the intensity of chemical weathering on Hainan Island, with more intense chemical weathering occurring in eastern and northwestern Hainan. Another practical implication of this study is that it provides a ‘‘missing’’ end member (Hainan Island) in the provenance discrimination study focused on the northern South China Sea (SCS). Hainan ﬁne-grained sediments likely play an important role in providing clay minerals to the northern SCS carried by the South China Sea Warm Current (SCSWC) during the summer. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The South China Sea (SCS) is the largest marginal sea in the western Paciﬁc. It is bordered by the Asian continent and Taiwan Island to the north and west and by the Philippine Islands and Borneo to the east and south (Fig. 1a). Numerous rivers, including both large rivers (i.e., Pearl, Red, and Mekong Rivers) and small mountainous rivers in southwestern Taiwan, the Malay Peninsula, and Borneo, annually deliver huge amounts of ﬁne-grained sediments to the SCS (Milliman and Farnsworth, 2011), making it as a significant sediment sink. Speciﬁcally, the predominant sediment sources in the northern SCS are the Pearl River, SW Taiwan and the Luzon Islands, which together deliver more than 265 Mt/yr of sediments to the sea (Liu et al., 2008, 2010b). These river-derived
⇑ Corresponding authors. Address: Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao Institute of Marine Geology, Fuzhou South Road 62#, Qingdao, Shandong 266071, China. Tel.: +86 53285718613 (B. Hu), +86 532 85776342 (J. Li); fax: +86 532 85720553. E-mail addresses: [email protected]
(B. Hu), [email protected]
(J. Li). http://dx.doi.org/10.1016/j.jseaes.2014.08.036 1367-9120/Ó 2014 Elsevier Ltd. All rights reserved.
terrigenous sediments have formed high sedimentation-rate deposits in the geological past, especially sediment drifts on the northern slope of SCS (Bühring et al., 2004). Therefore, it is the ideal area to study high-resolution paleoenvironmental changes including the East Asian monsoon evolution (Boulay et al., 2003, 2005; Liu et al., 2003, 2004; Tamburini et al., 2003; Wan et al., 2008, 2010b), the uplift of the Himalaya (Wan et al., 2007, 2012), and the continental weathering history (Hu et al., 2012; Wan et al., 2009, 2012). Addressing the above information ﬁrst requires in-depth understanding of the sediment provenance and its possible temporal variability. Accordingly, the sediment sources and transport pathways in the northern SCS, both in the present and in geological history, have thus attracted broad attention (Liu et al., 2003, 2005, 2008, 2010b, 2010c; Liu et al., 2010a, 2011, 2012a, 2013; Li et al., 2012b; Wan et al., 2010a). Clay minerals are widely distributed in various sediment types, and their compositions have been used extensively to constrain the provenance of ﬁne-grained terrigenous sediment, to decipher the climatic changes in the source area, and to illustrate the changes of transport agents (ocean currents, winds, ice drifts) (Chamley, 1989; Fagel, 2007; Singer, 1980; Thiry, 2000; Velde,
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Fig. 1. (a) Sketch map of the South China Sea and surrounding basins. (b) Locations of the Hainan riverine samples and averaged clay mineral assemblages of three river systems. See Table 1 for detailed clay mineral contents of each river. S = smectite, I = illite, K = kaolinite, C = chlorite.
1995). Recently, Liu and his colleagues conducted comprehensive works on the clay mineralogy and geochemistry of riverine sediments surrounding the SCS, including the Pearl, Red, and Mekong Rivers in South China and the Indochina Peninsula (Liu et al., 2007a, 2007b), small mountainous rivers in southwestern Taiwan (Liu et al., 2008), major and moderate rivers in Luzon (Liu et al., 2009b), and small rivers in the Malay Peninsula, Borneo, and Sumatra (Liu et al., 2012b; Wang et al., 2011), and the controlling mechanisms of climatic, tectonic, and lithological forcing on the
weathering processes have been summarized. Based on the distinct clay mineral compositions of the surrounding basins in the SCS, the source and transport of ﬁne-grained sediments have been semiquantitatively evaluated (e.g., Liu et al., 2008; Liu et al., 2011, 2013). However, few studies have been conducted on the weathering products of Hainan Island, the second largest and highest island in the northern SCS. The island covers an area of 33,000 km2 and has likely served as an important sediment source for the
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surrounding rift basins (Lin et al., 2001; Xie et al., 2008; Yan et al., 2011). In this study, clay mineralogy has been investigated for the ﬁrst time on the ﬂuvial samples collected from 20 major rivers on Hainan Island. Our objectives are threefold: (1) to reveal the clay mineral assemblage characteristics; (2) to quantify chemical weathering states; and (3) to discuss the implications for provenance discrimination in the northern SCS. 2. Geological and climatic settings Hainan Island, the second largest island in China, is a continental-type island in the north of the SCS, separated from Mainland China by the Qiongzhou Strait (Fig. 1). Tectonically, it is located at the intersection of the Paciﬁc oceanic plate, the Indochina Block and the southern margin of the South China Block (Metcalfe, 2009). Paleozoic–Mesozoic granitic rocks outcrop extensively on Hainan Island, accounting for 40% of the island’s land area (Fig. 2) (Wang et al., 1991). Approximately 60% of these rocks are thought to be of Indosinian (Triassic) age, and the remainder of Yanshanian (Jurassic and Cretaceous) age (Li et al., 2006; Wang et al., 1991). The Indosinian granitoids are mainly unfoliated, medium- to coarse-grained monzogranite with abundant K-feldspar megacrysts that outcrop primarily on the Central Island. Strongly foliated granitoids outcrop over an area of ca. 800 km2; most of these are exposed in the central and southern parts of Hainan Island. In these rocks, abundant maﬁc magmatic and paragneissic enclaves occur in sizes from several tens of centimeters up to ten meters. Large areas of Cenozoic basalts are distributed in the north part of Hainan Island (Fig. 2). The incipient volcanism in northern Hainan likely occurred in the Late Oligocene and increased gradually in intensity toward the Miocene and Pliocene (Ho et al., 2000). Paleozoic–Mesozoic sedimentary rocks are widely developed in west Hainan and are scattered in the east Hainan. Paleozoic sedimentary rocks are shallow-marine strata and characterized by sandstone, slate, and limestone. In contrast, Mesozoic sedimentary rocks are terrestrial strata and characterized by volcanic-clastic rocks. Additionally, Quaternary sediments are widely distributed in the coastal plain around Hainan Island. The landscape of Hainan Island is characterized by a central mountainous region (attaining a maximum elevation of 1867 m) surrounded by low hills, a broad basaltic mesa (in the northern part) and coastal marine-built terraces. As a result, Hainan Island is characterized by a radial river system, mostly originating from the central mountainous area with high elevation (Fig. 1b). The
Nandu (length of 311 km), Changhua (230 km), and Wanquan (163 km) Rivers are the three largest rivers on Hainan Island: their catchment areas together account for 47% of the island area. In total, rivers on Hainan Island deliver approximately 31 km3 of water and 4 Mt of sediment into the SCS (Zhang et al., 2013), most of which occurs during Typhoon season (July–October) (Yang et al., 2013). The climate of Hainan Island is dominated by tropical monsoons, with a dry season from November to April (winter) and a wet season from May to October (summer). The vegetation consists of typical tropical broadleaf forests and grass-forb communities in the high elevation areas (>500–1000 m), and scrubdominated and cultural crop landscapes in the lowlands. The annual average temperature of Hainan Island is within the range of 22.8–25.8 °C and the annual rainfall ranges between 960 and 2140 mm/a (Fig. 2), with 80% of that occurring between May and October. There is a large rainfall difference between the west and east sides of Hainan, resulting from the orographic effect of the south central highlands. Annual precipitation tends to decrease westward from more than 2000 mm in central and east Hainan to approximately 1200 mm in southwest Hainan (Fig. 2). In general, the eastern part of Hainan is dominated by a warm-wet-windy climate, whereas there is a relatively dry-windy climate in northwest Hainan and a dry-hot-windy climate in southwest Hainan (He and Zhang, 1985; Xu et al., 2013). Hainan Island is frequently and seriously affected by tropical cyclones (TC) (Liu and Chan, 2003). From 1959 to 2000, the island was impacted by an annual mean of 7.9 TCs and was directly hit by 2.6 TCs (Huang, 2003). TCs mainly impact Hainan Island between June and November. The associated precipitation is Hainan Island’s main water source, accounting for more than one-third of the total precipitation (Wu et al., 2007). 3. Sampling and analytical methods A total of 54 riverine samples were collected downstream of estuary sites from 20 major rivers on Hainan Island in November 2013 (Fig. 1b). These samples were obtained from surface muddy channels or bed deposits to avoid contamination from bank sediments. All samples were analyzed for clay minerals at the Key Laboratory of Marine Sedimentology and Environmental Geology, State Oceanic Administration of China, using a D/max-2500 diffractometer with Cu Ka radiation (40 kV and 100 mA). Three XRD runs were performed on oriented mounts of non-calcareous clay sized
Paleozoic-Mesozoic granites Paleozoic sedimentary rocks Mesozoic sedimentary rocks Cenozoic sedimentary rocks Cenozoic volcanics Rainfall (mm/a)
Fig. 2. Geological map (modiﬁed by Shi et al., 2011) of Hainan Island with spatial distribution of annual averaged rainfall (modiﬁed by Long et al., 2011).
B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92 Table 1 Clay mineral assemblages of surface sediments from the Hainan Island. Regions
NDJ-1 NDJ-3 NDJ-4 NDJ-5 NDJ-6 NDJ-7 WCH-1 WJH-1 WJH-2 WQH-6 WQH-7 WQH-8 JQJ-1 JQJ-2 LGH-1 LGH-2 LTH-2 LSH-1 LSH-2 STH-1 STH-2 TQH-3
Wenchang R. Wenjiao R. Wanquan R. Jiuqu R. Longgun R. Longtou R. Lingshui R. Shentian R. Tengqiao R. SW Hainan
Baisha R. Nangang R. Gan’en R.
Paipu R. Beimen R.
Chun R. Wenlan R.
Illite chemical index
Illite crystallinity (°D2h)
0 0 2 1 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 6 1
25 17 24 24 15 13 10 4 6 12 16 19 33 25 31 26 6 8 8 4 6 10
43 59 46 52 56 50 61 61 54 58 54 51 36 40 35 44 59 57 58 66 57 52
32 23 28 22 27 38 29 36 40 30 29 29 31 35 35 30 35 35 34 28 32 37
0.63 0.78 0.81 0.67 0.71 0.82 1.02 1.06 0.92 0.93 0.65 0.80 0.84 1.17 1.07 0.43 1.03 0.56 0.57 0.70 0.74 0.67
0.37 0.36 0.30 0.37 0.29 0.31 0.39 0.29 0.31 0.31 0.36 0.33 0.81 0.59 0.40 0.36 0.33 0.43 0.44 0.37 0.38 0.35
NYH-1 NYH-2 NYH-3 BSH-1 BSH-2 NGH-1 NGH-2 GEH-1 GEH-2 CHJ-1 CHJ-2 CHJ-3 CHJ-4 CHJ-5 CHJ-6
5 3 4 11 15 10 12 4 8 7 6 5 8 8 9
18 24 21 13 10 16 17 31 28 20 23 20 22 20 22
48 50 50 50 49 49 38 31 35 51 43 41 46 43 41
29 23 26 26 26 24 33 34 29 22 28 34 25 29 28
0.51 0.49 0.49 0.38 0.47 0.55 0.57 0.52 0.56 0.41 0.36 0.42 0.44 0.50 0.45
0.39 0.37 0.34 0.32 0.31 0.36 0.33 0.27 0.33 0.34 0.41 0.37 0.39 0.37 0.38
ZBJ-1 ZBJ-2 ZBJ-3 ZBJ-4 PPQ-1 PPQ-2 BMJ-1 BMJ-2 BMJ-3 CJ-1 CJ-2 WLJ-1 WLJ-2 WLJ-3 WLJ-4 WLJ-5
2 4 4 4 8 8 1 1 2 7 15 6 8 4 15 4
20 21 18 18 16 18 12 14 15 16 17 19 15 5 10 7
51 50 56 56 46 45 57 50 49 42 36 50 49 61 50 59
26 25 22 22 30 29 30 36 35 34 32 26 27 30 25 30
0.74 0.72 0.63 0.63 0.63 0.64 0.75 0.56 0.46 0.68 0.61 0.56 0.61 1.18 0.72 1.06
0.42 0.39 0.40 0.40 0.40 0.39 0.42 0.38 0.56 0.52 0.42 0.39 0.36 0.33 0.32 0.36
Table 2 Average clay mineral assemblages of various sub-provinces in the Hainan and surrounding drainage basins of the Northern South China Sea. Locations
Illite chemical index
Illite crystallinity (°D2h)
E Hainan SW Hainan NW Hainan Taiwan Pearl Luzon Mekong Red
1 8 6 5 5 87 11 7
16 20 15 53 31 1 35 44
52 44 50 5 46 5 28 26
32 28 29 37 18 7 26 24
0.80 0.48 0.70 0.33 0.62 – 0.47 0.40
0.38 0.35 0.40 0.16 0.30 – 0.21 0.20
(<2 lm) particles, following air-drying, ethylene–glycol solvation for 24 h, and heating at 490 °C for 2 h. Identiﬁcation of clay minerals was made mainly according to the position of the (0 0 1) series
Liu et al. (2010b)
of basal reﬂections on the three XRD spectra. Semi-quantitative estimation of clay mineral abundances is based on the peak areas of smectite (17 Å), illite (10 Å), and kaolinite/chlorite (7 Å) on the
B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92
le ica Ph
e urc so
N Borneo NW Borneo NE Borneo Red Mekong
g rin the ea lw
e urc so
SW Hainan NW Hainan
Pearl SE Borneo
Smectite Chemical weathering intensity
Pearl+S Borneo +Hainan
Red+Mekong+ NE&NW Borneo Taiwan +N Borneo
(b) Chemical weathering intensity
Illite chemical index
Kaolinite/(Illite+Chlorite) Fig. 3. (a) Ternary diagram of the major clay mineral groups illite + chlorite, kaolinite, and smectite and (b) Correlation of kaolinite/(illite + chlorite) with illite chemical index, showing the different forcing processes on the clay mineral assemblages in the surrounding areas of the SCS. Data of the Pearl, Red and Mekong Rivers from Liu et al. (2007b), the SW Taiwan rivers from Liu et al. (2008), the Luzon rivers from Liu et al. (2009b), and the Malay Peninsula, Sumatra, and Borneo rivers from Liu et al. (2012b).
According to the climatic and geological conditions of Hainan Island, we grouped the 20 studied rivers into three different provinces: east Hainan (10 rivers), southwest Hainan (5 rivers), and northwest Hainan (5 rivers) (Table 1). The clay mineral assemblages of 22 samples from ten rivers in east Hainan are dominated by kaolinite (35–66%, average 52%), with moderate chlorite (22–40%, average 32%), less abundant illite (4–33%, average 16%), and very scarce smectite (0–6%, average <1%). The illite chemical index in east Hainan varies between 0.43 and 1.17, with most higher than 0.6, with a mean of 0.80. Illite crystallinity ranges from 0.29 to 0.81°D2h, with an average value of 0.38°D2h. The southwest Hainan riverine sediments (15 samples from ﬁve rivers) consist of relatively less kaolinite (31–51%, average 44%), comparable illite (10–31%, average 20%), and higher chlorite (22– 34%, average 28%) and smectite (3–15%, average 8%) compared with those of east Hainan. The illite chemical index in southwest Hainan is usually low, approximately 0.36–0.57, with a mean of 0.48. Illite crystallinity in this province varies slightly from 0.27 to 0.47°D2h, with an average value of 0.35°D2h. In northwest Hainan, the clay mineral components of 16 samples from ﬁve rivers are generally similar to those of southwest Hainan, with slightly more kaolinite (36–61%, average 50%) and less illite (5–21%, average 15%). However, the values of the illite chemical index (0.46–1.18, mean of 0.70) and illite crystallinity (0.32–0.56°D2h, mean of 0.40°D2h) are increased relative to those of southwest Hainan. In summary, the clay mineral assemblages of the Hainan riverine sediments display three distinct characteristics (Table 2): (1) high kaolinite (35–66%), illite chemical index (0.43–1.18), and illite crystallinity (0.29–0.81°D2h) in east and northwest Hainan; (2) low kaolinite (31–51%), illite chemical index (0.36–0.57), and illite crystallinity (0.27–0.41°D2h) in southwest Hainan, and (3) minor smectite (mean of 7%) in west Hainan (including southwest and northwest Hainan), with smectite being nearly absent in east Hainan.
5. Discussion 5.1. Clay mineralogical changes in the Hainan Island Rivers
glycolated curve using the MDI Jade 6.5 software. Relative proportions of kaolinite and chlorite were determined based on the ratio of the 3.57/3.54 Å peak areas. In order to compare with literature data from other rivers surrounding the SCS and the SCS shelf/slope (Liu et al., 2007b, 2008, 2009a,b, 2010b, 2012b), the Biscaye weighting factors are not used when calculating relative weight percentages of each clay mineral. The error on the reproducibility of semi-quantitative evaluation is estimated to be ±5% for each clay mineral. In addition, the illite chemistry index (the ratio of illite 5 Å and 10 Å peak areas) and illite crystallinity [the full width at half maximum height (FWHM) of the illite 10 Å peak] were determined on the glycolated curve. Illite chemistry indexes below 0.15 represent Fe–Mg-rich illites (biotite, mica) characterized by physical erosion, whereas indexes above 0.4 are primarily found in Al-rich illites (muscovite) formed by strong hydrolysis (Petschick et al., 1996). Lower (higher) values of illite crystallinity represent good (poor) crystallinity and indicate weak (strong) hydrolysis and arid and cold (humid and warm) climate conditions in continental sources (Chamley, 1989; Ehrmann, 1998). These parameters have been widely used to track source regions and to estimate the intensity of chemical weathering (Petschick et al., 1996).
Generally, the differences in clay mineral compositions are related to the weathering intensity, which is mainly determined by climatic conditions (rainfall, temperature) and geological settings (lithology and morphology) (Chamley, 1989; Garzanti et al., 2014; Li et al., 2012a; Liu et al., 2007b, 2012b; Wang et al., 2011; Yang, 1988). Clay minerals in soils originate mainly from the weathering products of parent rocks (Wilson, 1999). Both the physical and chemical weathering processes play important roles in the formation of clay minerals: physical weathering leads to rock fragmentation, whereas chemical weathering involves the subtraction of ions, which in turn produces new minerals. During the chemical weathering processes, the more mobile ions (e.g., Na, K, Ca, Mg, and Sr) are expelled ﬁrst from minerals in the parent rocks (bisialitization), the transitional elements (Mn, Ni, Cu, Co, and Fe) tend to be evacuated later (monosialitization), and Si is removed last (alitization) (Chamley, 1989). Kaolinite is usually found in monosialitic soils, representing the intensive hydrolysis under warm and humid climate conditions (Galán and Ferrell, 2013). Kaolinite contents of the Hainan riverine sediments vary from 33% to 66%, with a mean of 49%. Under the combined effects of a warm and humid monsoon climate, stable tectonics, and good vegetation conditions, parent rocks enriched
B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92
in alkali and alkaline elements (e.g., granite, granodiorite) on Hainan are easily and intensively weathered to form kaolinite. Similar situations also occur in the Pearl River (Liu et al., 2007a, 2007b) and in tropical Southeast Asian (Malay Peninsula, Sumatra, and South Borneo rivers) (Liu et al., 2012b; Wang et al., 2011). Illite and chlorite are primary minerals and are interpreted to form by weak hydrolysis and/or strong physical erosion of bedrock under relatively dry climatic conditions (Galán and Ferrell, 2013). Illite and chlorite together contribute 46% of the Hainan riverine sediments, which is comparable with the Pearl River (49%), but much lower than the Mekong (61%), Red (68%), and Taiwanese (90%) Rivers (Table 2). Illite in the Hainan riverine sediments could be derived from the direct physical erosion of metamorphic and granitic parent rocks that result from the intense seasonal precipitation. Moreover, the relative high contribution of chlorite is likely related to the bedrock types of Hainan, where metamorphic, metasedimentary and magmatic rocks outcrop widely and can produce abundant chlorite by precipitation-caused physical erosion in the highlands. Although they are the abundant primary minerals on Hainan Island, illite and chlorite actually suffer moderate to
ste rn NE SC S
Western NE SCS Luzon offshore
Illite crystallinity (°Δ2θ)
(b) Hainan K=49% IC=0.38
Taiwan K=5% IC=0.17
Pearl K=46% IC=0.22 Western NE SCS
Eastern NE SCS
strong chemical weathering as conﬁrmed by the high illite chemistry index and illite crystallinity (Table 2). Smectite is a secondary mineral in soil and is formed in the early weathering stage of unstable Fe–Mg–Ca-rich minerals in igneous or metamorphic rocks (Galán and Ferrell, 2013). Formation of smectite in soil requires warm and wet climatic conditions with poor drainage, which leads to soil solutions that have high pH and are concentrated in silica and basic cations (Righi and Meunier, 1995; Velde, 1995). Although the sedimentary lithology settings between east and west Hainan are almost the same, moderate to minor smectite (1–15%) mostly occurs in west Hainan, where the annual evaporation signiﬁcantly exceeds precipitation (Zhang et al., 2006). Strong heavy rainfall caused by TCs may cause more efﬁcient formation of kaolinite in the east Hainan (Table 2). The Hainan-sourced clay mineral assemblage is similar to that of the Pearl River in South China: both of them consist dominantly of kaolinite (50%) and illite + chlorite (46%) and are scarce in smectite (4%) (Table 2 and Fig. 3a). In contrast, the Hainan riverine clay mineral compositions are obviously different from other basins (Fig. 3a). Different forcing processes (climatic, tectonic, and lithological) play competitive roles in the formation and preservation of clay minerals in these basins, which will be further discussed in the next section. 5.2. Weathering intensity on Hainan Island compared with other surrounding areas in the northern SCS
Kaolinite (%) Fig. 4. (a) Ternary diagram of the clay mineral groups illite + chlorite, kaolinite, and smectite of the northern SCS surface sediments and potential source areas end members. (b) Correlation of illite crystallinity and kaolinite of surface sediments in the northern SCS and surrounding drainage basins. Dash lines indicate linear correlations between Taiwan and Pearl/Hainan end members. In addition to the Hainan, other clay mineral data are from Liu et al. (2010b). IC = illite crystallinity (°D2h), K = kaolinite (%).
Clay minerals in riverine sediments can provide valuable information about weathering types and intensity at the basin scale (Chamley, 1989; Fagel, 2007; Singer, 1980; Thiry, 2000). Recent studies focused on other basins surrounding the SCS have shown that multiple factors (climatic, tectonic, and lithological) play signiﬁcant roles in the weathering processes (Fig. 3) (Liu et al., 2007b, 2009b, 2012b; Wang et al., 2011). In principle, kaolinite is generally formed by chemical weathering, while illite and chlorite are mostly inherited from parent rocks by physical erosion. Thus, the kaolinite/(illite + chlorite) ratio indicates the type of weathering (chemical weathering vs. physical erosion) that affected the erodible sediments, with increased values suggesting strong chemical weathering and weak physical erosion, and vice versa (e.g., Alizai et al., 2012; Boulay et al., 2007; Sionneau et al., 2010; Colin et al., 2010; Huang et al., 2011; Thamban et al., 2002; Wan et al., 2010a). In addition, two crystallographic indices of illite (illite chemical index and illite crystallinity) are also widely used to assess the chemical weathering intensity and to constrain sediment provenances (Chamley, 1989; Ehrmann et al., 2007; Liu et al., 2007b, 2008, 2012b; Petschick et al., 1996; Wan et al., 2010a, 2010b, 2012; Wang and Yang, 2013). A series of recent studies have conﬁrmed that these indices are correlated to the chemical weathering state of the riverine sediments surrounding the SCS (Liu et al., 2007b, 2008, 2009b, 2012b). Values of the illite chemistry index range between 0.36 and 1.18 for all river samples on Hainan Island (Table 1), with higher values occurring in east Hainan (mean of 0.80), moderate values in northwest Hainan (mean of 0.70), and lower values in southwest Hainan (mean of 0.48) (Table 2). Except for one sample from the Changhua River (CHJ-2, 0.36), all values above 0.40 indicate that Al-rich illites found on Hainan are predominately formed by strong hydrolysis. Thus, Hainan Island, from a clay mineralogy standpoint, is characterized by moderate to intensive weathering processes (Fig. 3). This is also conﬁrmed by the poor illite crystallinity (mean of 0.38°D2h) and high kaolinite contents (mean of 49%) of the Hainan riverine sediments (Table 2). Compared with other basins surrounding the SCS (Fig. 3b), the weathering status of Hainan is comparable with those of the Pearl River in South China (Liu et al., 2007b) and the Malay Peninsula, Sumatra and
B. Hu et al. / Journal of Asian Earth Sciences 96 (2014) 84–92
Ta iw an
30% 10 %
South China Sea 14°N 108°E
Fig. 5. Spatial distribution of the illite + chlorite contents (%) (white line) and the associated ocean currents in the northern SCS (modiﬁed by Liu et al., 2010b). Detailed data points of surface sediment can be found in Liu et al. (2010b). GDCC = Guangdong Coastal Current, SCSWC = South China Sea Warm Current, SCSBK = South China Sea Branch of Kuroshio, KC = Kuroshio Current, DWC = Deep Water Current.
South Borneo basins in tropical Southeast Asia (Liu et al., 2012b). All of these basins are beneﬁted by long-term warm and humid climatic conditions and stable tectonic settings, which produces high kaolinite contents (generally >50%) and illite chemical indexes (ca. 0.5–0.7), indicating dominant chemical weathering with strong hydrolysis. In contrast, Taiwan and North Borneo, and to a lesser extent, the Red and Mekong Rivers are characterized by physical erosion due to the combined effects of tectonic uplift and abundant monsoon precipitation (Li et al., 2012a; Liu et al., 2007b, 2008, 2012b). Illite and chlorite are the dominant clay minerals in these basins (generally >60%), and they are extremely high in Taiwan and N Borneo (up to 90%), with an illite chemical index lower than 0.4 (Fig. 3). Furthermore, obvious differences exist in the three sub-provinces of Hainan Island, with higher values of kaolinite, illite chemical index, and illite crystallinity in east and northwest Hainan than those of southwest Hainan (Table 2), which indicates that more intense chemical weathering occurs in east and northwest Hainan. This regional discrepancy is most likely due to the contrast in annual precipitation (Fig. 1b) that results from frequent typhoonderived heavy rainfall in east and northwest Hainan (Guo et al., 1993), as well as the orographic effect of the south-central highlands that blocks water vapor from the SCS on the east side. Therefore, our clay mineralogy results suggest that rainfall could be the primary factor controlling the chemical weathering intensity on Hainan Island. Similar meteorology situations can also be found on Taiwan Island, however, they produce opposite results. According to Li et al. (2012a), the values of the illite chemical index in the west Taiwan Rivers are higher than those in east Taiwan, which implies that chemical weathering intensity is greater in the west Taiwan. The east Taiwan Rivers are more mountainous, with steeper erosional gradients, shorter transport distances, and shorter residence times. Therefore, typhoon-derived heavy rainfall triggers more intensive physical erosion, while chemical weathering is diminished (Li et al., 2012a). This scenario suggests the tectonic uplift and associated steeper relief have substituted climatic forcing (i.e., rainfall) as being the controlling factor of the weathering processes on Taiwan Island.
5.3. Implications for sediment provenance discrimination in the Northern SCS In recent years, many works have focused on the clay mineral assemblages of surface sediments (e.g., Li et al., 2012b; Liu et al., 2010a, 2011, 2012a, 2013; Liu et al., 2008, 2010b) and core sediments (e.g., Boulay et al., 2005, 2007; Huang et al., 2011; Hu et al., 2012; Liu et al., 2003, 2004, 2005, 2010c; Steinke et al., 2008; Wan et al., 2008, 2010a, 2010b) in the SCS, aiming to constraint the clay mineral sources and transport pathways, as well as to elucidate information about the related paleoenvironmental and paleoclimatic conditions. As for the northern SCS, where Hainan Island is situated, three discriminative clay mineral end members, i.e., Taiwan, Pearl River, and Luzon, have been recognized, and the detrital ﬁnegrained sediment contributions from each of them have been semi-quantitatively evaluated (Liu et al., 2008, 2010b). The basic interpretation of Liu et al. (2008, 2010b) is that most smectite in the Northern SCS derives from the Luzon arc system and most kaolinite sources from the Pearl River, whereas illite and chlorite originate from both Taiwan (with low illite crystallinity) and the Pearl River (with high illite crystallinity) (Fig. 4 and Table 2). Accordingly, Liu et al. (2008) estimate that the contributions of clay minerals are 52% from the Pearl River, 29% from Taiwan, and 19% from Luzon to the South China shelf, and 31% from the Pearl River, 23% from Taiwan, and 46% from Luzon to the South China slope. However, subsequent studies (Liu et al., 2010a, 2011, 2013) suggest that the Pearl River sediments are mostly limited to the area between the Pearl River mouth and the southeast of Hainan Island due to the blocking effect of ocean currents. Recent works (Ge et al., 2014; Liu et al., 2009a, 2014) indicated that the Pearl River has formed a 400 km elongated, shore-parallel Holocene mud deposit, extending from the Pearl River delta southwestward off the Guangdong coast to the Leizhou Peninsula (Fig. 5). The formation mechanism is similar to that of the Yellow and Yangtze River-derived sediments in the East China Sea (i.e., summer deposition and winter transport) (Yang et al., 1992). During the summer, the Pearl River sediments entering the estuary are mostly trapped within the shelf area near the estuary. In the
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winter, these initially deposited sediments can be re-suspended and reworked by winter storms, and then be transported to the southwest by the strengthened Guangdong Coastal Current (GDCC) (Fig. 5) (Ge et al., 2014; Liu et al., 2014). Thus, the modern Pearl River ﬁne-grained sediments were mostly trapped in the delta/ estuary area and/or delivered southwestward to form the mud deposit (Ge et al., 2014; Liu et al., 2009a, 2014). This implies that when sea level reached a certain depth during the glacialinterglacial cycles, the contribution of the Pearl River ﬁne-grained sediments to the northern SCS shelf and slope should be largely reduced, if not terminated. This is further evidenced by the clay mineral changes of core KNG05, located on the northern SCS slope (Fig. 5), which indicate that the Pearl River was the major clay mineral source during the late glacial period (17.5–14.5 ka), when sea level dropped by approximately 100 m, whereas Taiwan has been the major contributor since 12.5 ka (Huang et al., 2011). Assuming the Pearl River ﬁne-grained sediments have been trapped within the coastal area (< 50 m water depth), as suggested by Ge et al. (2014), then other substituted clay mineral end members with high kaolinite contents are necessary to reconcile the spatial distribution of kaolinite in the surface sediments of the northern SCS (Liu et al., 2010b), because both Taiwan and Luzon provide only scarce kaolinite (Table 2) (Liu et al., 2008, 2009b). Considering the clay mineral results in this study and the regional oceanic current pattern in the northern SCS, Hainan Island, which has been ignored in the previous studies, may be the most probable ‘missing’ end member. As shown in Fig. 5, the spatial distribution of illite + chlorite contents in the northern SCS displays a double tongue-shaped pattern extending from Taiwan offshore to the southwest along the 100 m and 2000– 2500 m isobaths, which corresponds well to the ﬂow routes of winter Guangdong Coastal Current (GDCC) and to the subsurface South China Sea Branch of Kuroshio (SCSBK) and Deep Water Current (DWC) through the Luzon Strait, respectively (Fig. 5) (Liu et al., 2010b). However, there is also a small but distinct tongue of illite + chlorite extending from southeast Hainan northeastwards along the continental slope (100–200 m isobath). This most likely indicates the dispersal pathway of the Hainan ﬁne-grained sediments, transported by the northeastward South China Sea Warm Current (SCSWC, Fig. 5). High kaolinite content has been documented on the southeast of Hainan Island and is preliminarily thought to be delivered from the Pearl River (Liu et al., 2010a, 2011). However, a local anti-cyclonic eddy exists throughout the year on the east Leizhou Peninsula (Guan and Yuan, 2006; Yang et al., 2002), which effectively traps the southward Pearl Riverderived ﬁne-grained sediments and forms a distal depocenter (Ge et al., 2014). The high kaolinite contents near the southeast part of Hainan Island are most likely sourced from the small rivers on Hainan Island. Moreover, the spatial variability of clay minerals in the northern SCS, especially in its western territory, can also be well explained by changing the respective contributions from Hainan, Taiwan, and Luzon (Fig. 4). Our results thus suggest that Hainan Island may be an important clay mineral source area, especially during the high sea level stage (e.g., Holocene). Further investigations of the geochemical characteristics (e.g., REE, Sr-Nd-Pb isotopes, trace elements) and other provenance proxies of Hainan Island riverine sediments are needed to conﬁrm this proposal.
6. Conclusion Our results indicate that the clay mineral assemblages of Hainan consist dominantly of kaolinite (31–66%), with a lesser abundance of chlorite (22%–44%) and illite (4%–33%), and scarce smectite (0–15%). The east and northwest Hainan riverine sediments are characterized by higher kaolinite contents, illite
chemical index, and illite crystallinity than those of southwest Hainan. Minor smectite (mean of 7%) occurs in west Hainan and smectite is nearly absent in east Hainan. As a whole, Hainan Island is characterized by moderate to intensive chemical weathering with strong hydrolysis. Rainfall is the principal forcing factor controlling the chemical weathering intensity on Hainan Island, with more intense chemical weathering occurring in east and northwest Hainan. By combining the clay mineral compositions of surrounding basins and surface sediments in the northern SCS, we suggest that the Hainan ﬁne-grained sediment carried by the South China Sea Warm Current in the summer likely plays an important role in providing clay minerals to the northern SCS. Acknowledgments We thank Zuofen Chen, Xiaohui Han and Yuankai Xiong from the Marine Geology Survey of Hainan for assistance in collecting the samples. We especially thank Editor J.G. Liou and anonymous reviewers for their constructive reviews on the early version of this paper. This study was jointly supported by the National Key Basic Research Program of China (2013CB429701), Ministry of Land and Resources (1212011088112), the National Natural Science Foundation of China (41206049, 41476052, 41306066, and 41376079). References Alizai, A., Hillier, S., Clift, P.D., Giosan, L., Hurst, A., VanLaningham, S., Macklin, M., 2012. Clay mineral variations in Holocene terrestrial sediments from the Indus Basin. Quatern. Res. 77, 368–381. Boulay, S., Colin, C., Trentesaux, A., Pluquet, F., Bertaux, J., Blamart, D., Buehring, C., Wang, P., 2003. Mineralogy and sedimentology of Pleistocene sediment in the South China Sea (ODP Site 1144), In: Prell, W.L., Wang, P., Blum, P., Rea, D.K., Clemens, S.C. (Eds.), Proceedings ODP, Scientiﬁc Results, pp. 1–21.
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