LIU Jinqing, CHEN Xiaoying, YIN Ping, CAO Ke, GAO Fei, MENG Yuanku, QIU Jiandong, and LI Meina

Sediment Characteristics, Sources, and Transport Patterns in Kompong Som Bay, Cambodia: Indications from Grain Size and Heavy Minerals

LIU Jinqing1), 2), *, CHEN Xiaoying3), *, YIN Ping3), CAO Ke3), GAO Fei3), MENG Yuanku1), QIU Jiandong3), and LI Meina3)

1),,266590,2),,266071,3),,,266071,

In this paper, we analyzed the grain size and heavy mineral compositions of 52 surface sediment samples collected from the Kompong Som Bay of Cambodia and the adjacent rivers to depict the marine sedimentary environments and transport processes. Heavy minerals in sediments are dominated by authigenic pyrite, siderite, and tourmaline, with average percentages of 36.52%, 29.02%, and 10.94%, respectively. Two provinces can be divided according to the spatial similarity of minerals. The sediments from Province I, covered by silt grains in the northern bay, are characterized by autogenic pyrite, indicating a weakly reducing environment; whereas in Province II, covered by sand grains in the southern bay, the siderite-tourmaline-authigenic pyrite-zircon-hornblende assemblage occurs, indicating a mild reducing environment and locally oxidizing environment. Most of the sediments in the Kompong Som Bay are introduced from the Preak Piphot River and Srae Ambel River, except that some of them in the south areas come from coastal erosion. Generally, the sediments are difficult to be transported because of the low sediment loads entering the sea and weak hydrodynamic conditions. However, they are transported from the north to the south during the tide ebbing when the hydrodynamic force is much stronger. The sediment distribution and transport patterns are controlled by many factors, including submarine topography, hydrodynamic conditions, the southwest monsoon, land contours, and sediment supply.

heavy mineral; provenance; sedimentary environment; sediment transport; Kompong Som Bay; Cambodia

1 Introduction

Heavy minerals (HMs) can effectively reveal the sources, transportation routes, and the differentiation of mineral components in sediments, thus providing important information of the sedimentary environment, hydrodynamic conditions, and climate change (Chen, 2008). The composition of HMs is mainly affected by rock type, weathering, transportation, sedimentation, and diagenesis. Hydrodynamic conditions are important factors affecting the enrichment of HMs. The density, particle size, shape, and stability of HMs all restrict the hydrodynamic sorting of HMs. However, minerals with similar hydrodynamic and diagenetic properties are basically unaffected in the sedimentary cycle (Morton and Hallsworth, 1994; Liu., 2016). Therefore, analysis of HM assemblages, characte-ristic minerals, and mineral ratios in sediments can effectively trace the provenance and sedimentary environment. Coastal and nearshore sediments generally have complicated sources, such as local river inputs, coastal erosion, and distal deposition (Liu., 2018a, 2018b; Song., 2018; Liu., 2021). Consequently, identifying the se- diment source and transport pattern in coastal areas is im- portant.

Cambodia is located on the southeast coast of the Gulf of Thailand. The geological setting of the Gulf of Thailand is poorly understood, and little research on the marine environment has been done in the past few decades (Windom., 1984; Srisuksawad., 1997). In recent years, with the cooperation of the China-ASEAN Marine Joint Voyage, many research results have been acquired in terms of sedimentary characteristics, material sources, transport processes, ocean currents, and the marine environment, forming a preliminary understanding about sedi- ment flux and the fate of the Gulf of Thailand (Meksumpun., 2005; Hong., 2013; Qiao., 2015; Shi., 2015; Hu., 2016, 2017; Liu., 2016,2018c). Coarser sediments in the northern Gulf of Thailandare mainly from the local rivers including the Chao Phraya and Mae Klong Rivers. The fine sediments in the central Gulf of Thailand are contributed by the Mekong River and the South China Sea. The sediments with complex pat- terns in the coastal areas of the southwestern gulf close to Malaysia may derive from the Mae Klong River and resuspended sediments (Shi., 2015; Liu., 2016, 2018c). Terrigenous sediments mainly occurring in the coastal area of the Gulf of Thailand are characterized by hornblende, magnetite, limonite, ilmenite, and hypersthene minerals. Siderite, authigenic pyrite in foraminifera shells, and limonite from weathered siderite dominate in sediments of the south, east, and north-central Gulf of Thailand (Wang., 2014).

The influence of ocean currents on the distribution of HMs in the Gulf of Thailand is weak. The ocean current is mainly affected by wind speed, wind direction, and topography. The surface water velocity and direction are particularly affected by the monsoon. The surface water flows counterclockwise under the control of the northeast monsoon from November to March, whereas the surface water influenced by the southwest monsoon from May to October flows clockwise. Although the intensity of water flow in the bay is low, it has a great influence on the distribution and diffusion of water chemistry, microparticles, and biological species, while the effect on coarser grains is weak (Saadon., 1999; Aschariyaphotha and Wong- wise, 2012).

Compared with the degree of research in the Gulf of Thailand, studies in Kompong Som Bay, Cambodia are lacking. What about the sedimentary characteristics and marine environment in the bay? What about the hydrody- namic conditions and material transport patterns? Therefore, understanding modern marine sedimentation and land- sea interaction processes in Kompong Som Bay is essential. Based on a China-Cambodia maritime joint investigation voyage, we collected some seafloor sediments in Kompong Som Bay. In this study, sediments were analy- zed for HMs and the results were compared with the mineral compositions of sediments from the surrounding rivers to evaluate the sediment provenance for the whole study area. The main objectives in this study are 1) to ana- lyze the HM compositions and spatial distribution patterns in the surface sediments of Kompong Som Bay; 2) to determine the provenance of the bay sediments and sedimentary environment; and 3) to discuss the transport patterns and controlling factors.

2 Regional Setting

Cambodia is a country of forested mountains and well- watered plains. The central part of the country forms a gigantic basin for the Tonle Sap and the Mekong River, the latter of which flows down from Laos to the southern border with Vietnam. Between the Tonle Sap and the Gulf of Thailand lie the Cardamom Mountains and the Elephant Range, which rise abruptly from the sea and from the eastern plains. The Cardamom and Elephant Mountains occupy Koh Kong Province and Kampong Speu Province, running in a northwestern to southeastern direction and rising to more than 1500m. To the southwest of the southern mountain ranges extends a narrow coastal plain that contains the Kompong Som Bay area and the Sihanoukville Peninsula, facing the Gulf of Thailand.

The Bay of Kompong Som lies in southern Cambodia. It has deep water inshore and a chain of islands across the mouth that protect the bay from storms (Fig.1). The Port of Sihanoukville situated in the Bay of Kompong Som is the principal and only deep-water port of Cambodia. Extensive mangrove forests develop along the coast of the bay. The study area lies in the tropical monsoon climate zone, which has two seasons: a wet season (from May to October) and a dry season (from November to April). The southwest monsoon blows inland, bringing moisture-la- den winds from the Gulf of Thailand and Indian Ocean during the wet season, and the northeast monsoon ushers in the dry season. The heaviest precipitation appears in September and October, with the driest period occurring from January to February.

The region experienced tectonic activities and low-grade metamorphism throughout the Paleozoic, giving rise to a shift to marine conditions, under which fossil formations are preserved during the Permian and through much of the Mesozoic. Few rocks remained from the Cenozoic. Quaternary alluvium and middle Jurassic–lower Cretaceous sandstone and claystone are widely exposed in the study area. In addition, Quaternary basalts, colluvium, eluvium, and laterite and Jurassic–Cretaceous dacite and rhyolites are also distributed in the northern area (Fig.1).

Kompong Som Bay is mainly controlled by tidal wave action in the South China Sea. There is a regular tide with an average tidal range of <1m. The tides in the bay are mainly reciprocating, and the falling tide is slightly larger than the rising tide, with a maximum value of 0.2ms?1. The flow velocity near the cape is greater than that in otherareas (Anond and Pramot, 1999). The wind direction in the bay varies with the seasons. West and southwest winds prevail in the rainy season, and the north and south winds prevail in the dry season. The bay is less affected by the waves from the outer sea because of the shielding effect of the many islands at the mouth of the bay and is mainly affected by wind waves in the bay. Under normal weather conditions, the effective wave height is only 1m. Two small rivers, the Srae Ambel River (SAR) and the Preak Piphot River (PPR), flow into the bay. Their sediment loads are very limited, and the amounts of suspended particles are low. The sediments from the Mekong River are not transported into the bay because of the blockage from the southwestern peninsula of Vietnam. In addition, effective coastal sediment transport is inhibited by the shorter sandy shores separated by the capes in the bay. The nearshore area is affected by wave action, and the main tidal channels at the bay mouth are exposed to strong hydrodynamic conditions. The hydrodynamic conditions inside the bay are weak, which is conducive to sedimentation (Yao., 2012).

Fig.1 Geological sketch map of the Kompong Som Bay in Cambodia, with the location of river and seafloor samples.

3 Data and Methods

A total of 43 seafloor sediment samples were collected from Kompong Som Bay using a box sampler in 2018 (Fig.1). Each sample, collected from the surface layer of 0–5cm, was at least 2kg in weight. Another nine fluvial sediment samples were collected from the two rivers (SAR and PPR) that flow into the bay (Fig.1). The organics in the sediments were first removed with hydrogen peroxide and deionized water, then the samples were oven-dried and weighed. Each sample was divided into two subsamples for grain size analysis and mineral analysis, respectively.

For grain size analysis, 5mL of 10% H2O2and 5mL of 0.1molL?1HCl were added to the subsamples for 24h to remove organic and calcareous materials, respectively. Then, a sodium hexametaphosphate solution and an ultrasonic instrument were used to fully disperse the sample particles Grain size was tested by using a laser particle size analyzer (Mastersizer 2000). For mineral analysis, size fractions of 62.5–125μm were selected and separated using tribromomethane (specific gravity=2.89gcm?3). For the HMs, over 300 grains in each sample were counted using the ribbon method (Galehouse, 1971). The particle frequency of occurrence for each mineral under the microscope was counted, and the particle percentage of each mineral in the total particles identified was calculated. For detailed experimental procedures, see Liu. (2017).

A Q-mode cluster analysis was applied to group offshore samples according to their similarity in mineral abundance (Liu., 2017). In Q-mode clustering, the squared Euclidean distance was adopted to measure the similarity between samples, and the results were merged by using Ward’s minimum variance method (Wang., 2003). The cluster analysis was conducted using SPSS.

4 Results

4.1 Sediment Composition

According to Folk’s classification rules, there are eight types of sediments in the bay (Fig.2). Sandy silt, the most dominant type, is widely distributed in the whole area. The main type of shallow-water sediments in the northern bay is mud, extending in a north-to-south band. The central part of the bay has mainly silty sediments distributed in patches, while the area near the coast of Sihanoukville on the southeast side mainly has sandy sediments, including silty sand, sand, and gravel sand.

Fig.2 Types of surface sediments in the study area.

In this study, the percent abundance quoted is the mean value, unless specifically stated otherwise. The HM content refers to the weight percentages of the HM fraction. The HM data are all listed in Table 1.

The mean grain size (Mz) varies from 0.41Φ to 7.33Φ, with a mean value of 5.73Φ. The main particle size component is silt, which is widely distributed in the bay. High- value zones of Mz (>6Φ) are mainly distributed in the central part of the area, extending from the two river mouths seaward to the bay mouth, where they are covered by fine silt to coarse silt. The Mz values decrease regularly toward the eastern and western sides. Low-value zones of Mz (<4Φ) are mainly distributed in areas near the coast of Sihanoukville and Koh Rong Sanloem Island, where they are covered by very fine sand to coarse sand (Fig.3a).

The weight percentage of HMs is very low, with a mean of 0.53% and a range from 0.04% to 3.36%. High- value zones (>1.4%) are mainly distributed in the northern bay, extending from the PPR mouth seaward to the central bay (Fig.3b). Most of the rest study area has very low values of HMs (<0.4%).

HMs are dominated by authigenic pyrite (36.52%), si- derite (29.02%), and tourmaline (10.94%), which account for 78.88% of the total amount of HMs. The content of hornblende (4.41%) is approximately equal to those of zircon (4.20%) and limonite (3.62%) with relatively lower concentrations. The amounts of anatase, epidote, ilmenite, and sphene are very low, ranging from 0.87% to 2.01%, and leucoxene, garnet, apatite, and rutile occur occasionally.

4.2 HM Distribution

Hornblende contents vary from 0 to 47.67%, with a mean value of 4.41%. Hornblende is characterized by a green color, columnar and granular shapes, subangular psephicity, and strong weathering. The high concentration of hornblende (>6%) occurs at the mouth of Kompong Som Bay, but the concentration gets slightly lower in the central bay (3%–6%) and decreases to a relatively low level in the northern bay (<3%) (Fig.4a).

Table 1 The contents of heavy minerals and the percentages of each heavy mineral in sediments from the Kompong Som Bay and two rivers (ZTR index is also listed)

Fig.3 Distribution of mean grain size (a) and heavy mineral content (b) in surface sediments in the study area.

Fig.4 Distribution of major heavy minerals in surface sediments in the study area. (a), hornblende; (b), tourmaline; (c), zircon; (d), siderite; (e), limonite; (f), authigenic pyrite.

Tourmaline contents vary from 0 to 36.10%, with a mean value of 10.94%. Tourmaline is mainly characterized by a brown color, columnar and granular shapes, subangular psephicity, and obvious pleochroism. A large amount of tourmaline (>15%) occupies the bay mouth and the nearshore areas of Sihanoukville in the southeast of the study area. The low values (<9%) are located in the nor- thern and central parts of the inner bay (Fig.4b).

Zircon contents vary from 0 to 20.26%, with a mean value of 4.20%. Zircon is mainly characterized by a light pink, yellow, and transparent color, columnar and granular shapes, and subangular psephicity. A large amount of zircon grains (>8%) occupies the nearshore areas of Sihanoukville in the southeast of the study area. Low values (<2%) are found in most of the study area, especially in the bay mouth and the northern and central parts of the inner bay where the concentrations are very low (Fig.4c).

Siderite contents vary from 0 to 75.79%, with a mean value of 29.02%. Siderite is characterized mainly by a gray or gray-green color and granular and rod shapes. A large amount of siderite (>40%) occupies the southern study area. The low values (<5%) are located in the northern and central parts of the inner bay (Fig.4d), and in a large area no siderite is found. In addition, there is also a low- value area between Koh Rong Island and Sihanoukville City (Fig.4d).

Limonite contents vary from 0 to 28.70%, with a mean value of 3.62%. Limonite is characterized mainly by a yellow-brown color and granular shapes. The low values (<4%) are widely distributed in the northern and central parts inside the bay, and only a small part near the coast of Sihanoukville exhibits high amounts of limonite (> 10%), locally reaching 28% (Fig.4e).

Authigenic pyrite contents vary from 0 to 98.36%, with a mean value of 38.92%. It is characterized mainly by a yellow copper or gray-brown color and granular, framboidal, and biological membrane shapes. The high concentrations of authigenic pyrite (>50%) are found in the central and northern parts of the bay, especially in the northern area where high concentrations of >90% extend over a large area. The low values (<10%) are distributed in the south of the bay (Fig.4f).

4.3 Zircon-Tourmaline-Rutile and Fe-Ti Oxide Minerals

For the convenience of research, here we discussed the zircon-tourmaline-rutile (ZTR) maturity index and treated ilmenite, limonite, siderite, rutile, leucoxene, and sphene as Fe-Ti oxide minerals. The ZTR index is regarded as a valid indicator to assess the degree of sediment recycling. These minerals tend to be concentrated during recycling owing to their high chemical and mechanical stability. How-ever, source-rock weathering, intense intrastratal solutions,and fine particle grains also increase the ZTR values. Con- sequently, the ZTR index can only effectively indicate mineral maturity when these other factors can be eliminated (Hubert, 1962; Morton, 1985). Fe-Ti oxide minerals generally represent the composition characteristics of the parent rocks in the watershed, and they are usually secondary minerals or altered minerals of intermediate-acid volcanic rocks and low-grade metamorphic rocks (Song., 2018; Liu., 2019).

The ZTR values of the bay sediments vary from 0 to 56.36%, with a mean value of 15.20%. The high concentrations of ZTR (>25%) are distributed in the nearshore areas of Sihanoukville in the southeast of the study area. The eastern area of Koh Rong Island also has high values of ZTR. The northern and central areas of the bay have extremely low values of ZTR (Fig.5a).

The contents of Fe-Ti oxide minerals in the bay sediments vary from 0.30% to 80.79%, with a mean value of 37.43%. The high concentrations (>50%) are distributed in the southern study area, where siderite overwhelmingly dominates. In addition, the mouth of the SAR also has high values (>35%). The northern and central areas of the bay have extremely low values (<30%) (Fig.5b).

Fig.5 Distribution of the values of ZTR (a) and Fe-Ti oxide minerals (b) in sediments in the Kompong Som Bay.

4.4 Mineral Province

To distinguish the source differences of sediments in the study area, we subjected the contents of HMs, including hornblende, epidote, sphene, zircon, tourmaline, rutile, il- menite, anatase, apatite, and garnet to a Q-mode cluster analysis. As a result, two mineral assemblage provinces can be divided (Table 1 and Fig.6).

Province I consists of 15 samples from the northern bay near the river mouths of the PPR and SAR with relatively high amounts of HMs (1.16%) and comparatively low percentages of hornblende (2.93%), tourmaline (1.92%), and zircon (0.83%). The contents of secondary minerals such as siderite (4.3%) and limonite (0.04%) are also very low. This province is characterized by extremely high amounts of authigenic pyrite (86.10%) (Fig.4) and low values of ZTR (2.75%) and Fe-Ti oxide minerals (6.46%) (Fig.6).

Province II consists of 28 samples distributed in most areas, extending from the mouth of the SAR toward the southwest along the eastern nearshore area until the bay mouth. This province has relatively high contents of tour- maline (15.78%), zircon (6.01%), hornblende (5.20%), and anatase (3.06%) and relatively low content of HMs (0.2%) (Fig.6). Siderite is dominant (42.26%) in this area, and the content of authigenic pyrite is also high (13.65%). Therefore, Province II is characterized by high values of Fe-Ti oxide minerals (54.03%) and ZTR (21.88%).

Fig.6 Different geographical locations of two Provinces. Ant, anatase; Hbl, hornblende; Tur, tourmaline; Zrn, zircon; Rt, rutile; Sd, siderite; Ilm, ilmenite; Lm, limonite; Leu, leucoxene; Ep, epidote; Spn, sphen.

5 Discussion

5.1 Provenance Discrimination

Because of the small watershed areas and lush vegetation, the sediment supply from the coastal rivers (SAR and PPR) into the bay are relatively limited. The HMs in PPR sediments are mainly characterized by zircon (25.43%),tourmaline (24.60%), leucoxene (18.55%), anatase (13.42%), rutile (6.10%), and limonite (4.87%) (Table 1; Fig.6). The HMs in SAR sediments are mainly characterized by limonite (33.83%), leucoxene (23.36%), tourmaline (13.84%),zircon (13.71%), anatase (6.79%), and siderite (4.33%) (Ta- ble 1; Fig.6). A comparison of the HM compositions of the two rivers shows that the average ZTR value of the PPR is much higher (56.13%) than that of the SAR (28.24%). However, the average contents of leucoxene and limonite from the PPR are significantly lower than those of the SAR.

The occurrence of a large amount of zircon mineral indicates that the source area is developed with medium- acid volcanic rocks, and the high contents of leucoxene and tourmaline indicate the presence of granite and low- grade metamorphic rocks in the source area. Leucoxene is not an independent mineral but is a secondary mineral aggregate formed after decomposition of titanium minerals (containing rutile, ilmenite, brookite, anatase,.). The appearance of leucoxene indicates that the source rocks have undergone a certain degree of weathering, and it often occurs in loose sediments where ultrabasic rocks, amphibolites, and greenschist developed. The heavy minerals in these two river sediments are mainly ZTR and Fe-Ti oxide minerals, which are formed as secondary minerals of sandstone, claystone, rhyolite, dacite, and loose alluvium exposed widely in the river basins.

It can be seen that the detrital minerals in river sediments of this area have high contents of Fe-Ti oxide mi- nerals and ZTR. The amounts of hornblende and epidote are extremely low. This is obviously different from the HMs in the sediments of rivers flowing into the sea in eastern China. For the Yellow River, HM minerals are characterized by high contents of flaky minerals (55.8%), with biotite content reaching 47.44% and hornblende (13.5%) and epidote (6.5%) contents both being low (Lin., 2003). In contrast, Yangtze River transporting minerals are characterized by high contents of flaky minerals (25.5%), dolomite (26.0%), hornblende (24.2%), and epi- dote (8.0%) (Chen, 2008). In addition, there are some characteristic minerals such as Fe-Ti oxide minerals, garnet, biotite, and zircon (Yang., 2009). The differences in the composition of HMs in these rivers reflect the differences in lithology of the parent rocks in their river basins. Therefore, the continental shelf sediments in eastern China sea usually have characteristics of terrestrial detrital mineral assemblages, such as flaky minerals, hornblende, and epidote. The minerals with large specific gravity such as hornblende and epidote are generally deposited in coarse-grained sediments nearby and will not be transported too far as a result of gravity differentiation (Liu., 2017).

Therefore, to determine the sources of sediments in the bay, we selected ZTR, Fe-Ti oxide minerals, hornblende+ epidote groups to form a three-terminal diagram (Fig.7). Among them, the higher values of ZTR indicate that the sediments have higher maturity, stronger weathering, and greater transportation distance. The higher contents of Fe- Ti oxide minerals indicate that there are more terrestrial detrital materials imported from rivers. The higher contents of hornblende and epidote indicate that mineral weathering is weaker and that the transportation distance is shorter, which is the result of the near-source input.

It can be found that most of samples in Province I and Province II fall in the field of the coastal rivers, being relatively closer to the SAR, indicating that the sediments of Kompong Som Bay mainly come from two rivers in the northern coast. The material contribution of the SAR is more than that of the PPR. In addition, there are a few sample points distributed discretely outside the field of the rivers, demonstrating that they are not imported by the rivers. The relatively high contents of hornblende and epidote appearing near the southern bay entrance region are likely to come from nearby littoral erosion (Fig.4), because hornblende and epidote with higher density cannot easily be transported too far.

Fig.7 Three-terminal discrimination diagram with ZTR, Fe-Ti oxide minerals, and hornblende-epidote group. PPR, Preak Piphot River; SAR, Srae Ambel River.

Therefore, regardless of the authigenic minerals, only from the perspective of mineral inheritance, the sediments along the northern coast of Kompong Som Bay mainly come from the inputs of the two rivers. Some sediments near the southern bay mouth come from littoral erosion. The difference in mineral composition is mainly due to thefractionation of authigenic and secondary minerals under the different sedimentary environments. Province I is lo- cated in a region of tidal flats and subaqueous delta with shallower water and fine sediments. Because of less runoff and lower sediment load of the rivers flowing into the sea, coupled with the weak marine dynamics, a large amount of authigenic pyrite has developed here.

5.2 Sedimentary Environment

Authigenic pyrite is widely distributed in the continental shelf sediments. Framboidal pyrite formed in forami- niferal shells is common in offshore sediments. Its formation is related to sulfate-reducing bacteria, dissolved oxy- gen concentration, and organic matter content, and it is commonly found in organic-rich muddy sediments and represents a low-energy, weak alkaline reducing environ- ment (Chu., 1995). According to the particle size, structure, and composition of the authigenic pyrite, the formation of authigenic pyrite in the study area may be related to the reduction of sulfate and the degradation of organic matter, which is similar to the genesis of authigenic pyrite in the South Yellow Sea and the East China Sea (Liu., 2017, 2018b; Song., 2018; Zhang., 2019).

Limonite is a very common secondary mineral under oxidizing conditions and is an oxidation product of iron- bearing minerals (such as ilmenite, magnetite, and siderite). It is widely distributed in the coastal area, and it can be found in the eastern seas and estuaries of China (Wang., 2003; Chen, 2008; Liu., 2017, 2018b; Song., 2018). Siderite is a carbonate mineral that can be produced mostly in sedimentary and hydrothermal en- vironments (Gautier, 1982; Ellwood., 1988; Mozley, 1989; Bernard and Symonds, 1992; Mozley and Wersin, 1992; Sapota., 2006; Mortimer., 2011) and also found in some igneous pegmatites. Although the siderites produced in different environments have different isotopic compositions, they are all related to reduction and anaerobic environments. The occurrence of siderite is con- sidered as an index of intermediate oxidation conditions, interposed between sulfide-facies (reducing conditions) and hematite-magnetite facies iron formations (oxidizing conditions). The siderite has the highest content of HMs in the sediments of the Gulf of Thailand, with an average of 41.66% and a maximum of 99.7%. High-content areas are distributed in the northeastern and central parts of the Gulf of Thailand, approximately strip-shaped spreading in the north-to-south direction. The southeast Gulf of Thailand has an oxidative deposition environment, which cau- ses most of the siderite to be weathered into red opaque limonite pellets (Wang., 2014).

Therefore, the high content of siderite near the bay mouth in the southern study area is consistent with that of the Gulf of Thailand. The area near the bay mouth can be considered as a mild restoration environment, but the high content of limonite near the Sihanoukville coast indicates an oxidizing environment.

In summary, the sediment types in Province I are mainly tidal flat and subaquatic deltaic sediments and are characterized by authigenic deposition, indicating a weak and alkaline reduction depositional environment. The sediment types in Province II are mainly subaqueous bank slope and tide channel sediments, mostly derived from river inputs, indicating a slightly strong hydrodynamic environment. It is worth noting that the relatively high contents of siderite located in the southern bay entrance area, that is, on the northern and southern sides of the line of Koh Rong Island and Sihanoukville City, indicate a mild marine reduction environment with relatively low contents of organic matters and a low deposition rate (Wang., 2014). The areas with high limonite contents, located near the entrance, are mainly deep-water tide channels with slightly stronger hydrodynamic forces, which are considered to be an oxidizing environment.

5.3 Sediment Transport Pattern

The ZTR index represents the degree of mineral wea- thering. The change of this value from low to high can indicate the direction of sediment transport. Generally, as the distance increases, the mineral maturity becomes higher and the ZTR value gradually increases, especially near the southeast coast, where the ZTR value reaches its ma- ximum. We have found that most of the bay sediments come from the river input. The ZTR value gradually increases from north to south, indicating that sediments are transported from north to south. As the distance increases, the mineral maturity becomes higher. The southern bay is a relatively strong hydrodynamic environment with coar- ser sediments, which undergo a strong degree of reworking, resulting in a high maturity. In contrast, the northern bay is mainly subaqueous delta and tidal flat (with a water depth of <10m) with finer sediments and weak hydrodynamic conditions, leading to a low degree of mineral erosion and relatively low mineral maturity.

The flow velocity of the ebb tide is slightly higher than that of the flood tide in the bay. The sediments are mainly from the supply of northern rivers and coastal erosion. They are transported southward under the influence of the ebb tide. When they encounter the southern shoreline turning and the islands, the ocean hydrodynamic forces weaken and the sediments are deposited. Meanwhile, some sediments outside the bay and near the bay mouth can be reworked, suspended, and transported into the central and northern bay by the flood tide and the southwest monsoon (Fig.8). As a result, most of the terrigenous sediments are transported to the southeast coast by currents and monsoons.

6 Conclusions

The dominant HMs in sediments from Kompong Som Bay are authigenic pyrite, siderite and tourmaline, with the average percentages of 36.52%, 29.02%, and 10.94%, respectively.

Two provinces can be identified according to the spatial similarity of minerals. Province I is characterized by extremely high amounts of authigenic pyrite and low con- tents of ZTR and Fe-Ti oxide minerals. Province II is cha- racterized by high contents of Fe-Ti oxide minerals and ZTR. The bay sediments mainly come from the inputs of the two rivers along the northern coast. Some sediments near the southern bay mouth come from littoral erosion.

Sediments from different sedimentary environments exhibit significant differences in mineral compositions. The sediments from Province I are characterized by auto- genic pyrite, indicating a weakly reducing environment; whereas Province II is mainly characterized by authgenic deposition of siderite, which represents a mild reduction environment, with the local oxidizing environment for the high content of limonite.

River inputs in the north of bay are mainly transported to the south by the ebb tide, and the possible influence of the southwest monsoon. Many factors–such as the bay mouth opening toward the southwest, wind waves caused by the southwest monsoon, the islands barrier in the bay mouth, and the land outline of Sihanoukville extending southwest–control the sediment distribution patterns and transport processes.

Fig.8 Transport patterns of surface sediments based on the heavy mineral assemblage and current system in the study area.

Acknowledgements

We appreciate two anonymous reviewers for their constructive comments on our original manuscript. This study was jointly funded by China-ASEAN Maritime Cooperation Fund: China-ASEAN Marine Geoscience Research and Disaster Reduction and Prevention Initiatives, and the National Natural Science Foundation of China (Nos. 4170 6074 and 41506107).

Anond, S., and Pramot, S., 1999.. Samutprakan Press, Samutprakan, 54-72.

Aschariyaphotha, N., and Wongwises, S., 2012. Simulations of seasonal current circulations and its variabilities forced by runoff from freshwater in the Gulf of Thailand., 37 (5): 1389-1404.

Bernard, A., and Symonds, R. B., 1989. The significance of si- derite in the sediments from Lake Nyos, Cameroon., 39 (2-3): 187-194.

Chen, L. R., 2008.. Ocean Press, Beijing, 476pp.

Chu, F. Y., Chen, L. R., Shen, S. X., Li, A. C., and Shi, X. F., 1995. Origin and environmental significance of authigenic py-rite from the South Yellow (Huanghai) Sea sediments., 26 (3): 227-233 (in Chinese with English abstract).

Ellwood, B. B., Chrzanowski, T. H., Hrouda, F., Long, G. J., and Buhl, M. L., 1988. Siderite formation in anoxic deep-sea se- diments: A synergetic bacteria controlled process with important implications in paleomagnetism., 16 (11): 980- 982.

Gautier, D. L., 1982. Siderite concretions; indicators of early dia- genesis in the Gammon Shale (Cretaceous)., 52 (3): 859-871.

Hong, G. H., Kim, C. J., Yeemin, T., Siringan, F. P., Zhang, J., Lee, H. M., Choi, K. Y., Yang, D. B., Ahn, Y. W., and Ryu, J. H., 2013. Potential release of PCBs from plastic scientific gear to fringing coral reef sediments in the Gulf of Thailand., 96: 41-49.

Hu, L. M., Shi, X. F., Bai, Y. Z., Fang, Y., Chen, Y. J., Qiao, S. Q., Liu, S. F., Yang, G., Kornkanitnan, N., and Khokiattiwong, S., 2016. Distribution, input pathway and mass inventory of black carbon in sediments of the Gulf of Thailand, SE Asia., 170: 10-19.

Hu, L. M., Shi, X. F., Qiao, S. Q., Lin, T., Li, Y. Y., Bai, Y. Z., Wu, B., Liu, S. F., Kornkanitnan, N., and Khokiattiwong, S., 2017. Sources and mass inventory of sedimentary polycyclic aromatic hydrocarbons in the Gulf of Thailand: Implications for pathways and energy structure in SE Asia., 575: 982-995.

Hubert, J. F., 1962. A zircon-tourmaline-rutile maturity index and the interdependence of the composition of heavy mineral assemblages with the gross composition and texture of sandstones., 32 (3): 440-450.

Lin, X. T., Li, W. R., and Shi, Z. B., 2003. Characteristics of mineralogy in the clastic sediments from the Yellow River provenance, China., 23 (3): 17-21 (in Chinese with English abstract).

Liu, J. Q., Cao, K., Yin, P., Gao, F., Chen, X. Y., Zhang, Y., and Yu, Y. Y., 2018a. The sources and transport patterns of mo- dern sediments in Hangzhou Bay: Evidence from clay minerals., 17 (6): 1352-1360.

Liu, J. Q., Song, H. Y., Yin, P., Zhang, Y., and Cao, Z. M., 2018b. Characteristics of heavy mineral assemblage and its indication of provenance in the mud area off the southern coast of Weihai since the late Pleistocene., 40 (3): 129-140 (in Chinese with English abstract).

Liu, J. Q., Yin, P., Chen, X. Y., and Cao, K., 2019. Distribution, enrichment and transport of trace metals in sediments from the Dagu River Estuary in the Jiaozhou Bay, Qingdao, China., 9: 545.

Liu, J. Q., Yin, P., Zhang, Y., Song, H. Y., Bi, S. P., Cao, Z. M., and Liu, S. S., 2017. Distribution and provenance of detrital minerals in southern coast of Shandong Peninsula., 16 (5): 747-756.

Liu, S. F., Shi, X. F., Yang, G., Khokiattiwong, S., and Korn- kanitnan, N., 2016. Distribution of major and trace elements in surface sediments of the western Gulf of Thailand: Implications to modern sedimentation., 117: 81-91.

Liu, S. F., Zhang, H., Zhu, A. M., Wang, K. S., Chen, M. T., Khokiattiwong, S., Kornkanitnan, N., and Shi, X. F., 2018c. Distribution of rare earth elements in surface sediments of the western Gulf of Thailand: Constraints from sedimentology and mineralogy., 527: 52-63.

Liu, Y. L., Liu, J. Q., Xia, X. F., Bi, H. B., Huang, H. J., Ding, R. W., and Zhao, L. H., 2021. Land subsidence of the Yellow River Delta in China driven by river sediment compaction., 750: 142165.

Meksumpun, S., Meksumpun, C., Hoshika, A., Mishima, Y., and Tanimoto, T., 2005. Stable carbon and nitrogen isotope ratios of sediment in the Gulf of Thailand: Evidence for understand- ing of marine environment., 25 (15): 1905-1915.

Mortimer, R. J., Galsworthy, A. M., Bottrell, S. H., Wilmot, L. E., and Newton, R. J., 2011. Experimental evidence for rapid biotic and abiotic reduction of Fe (III) at low temperatures in salt marsh sediments: A possible mechanism for formation of modern sedimentary siderite concretions., 58 (6): 1514-1529.

Morton, A. C., 1985. Heavy minerals in provenance studies. In:. Zuffa, G. G., eds., Springer, Dordrecht, 249-277.

Morton, A. C., and Hallsworth, C. R., 1994. Identifying provenance-speci?c features of detrital heavy mineral assemblages in sandstones., 90: 241-256.

Mozley, P. S., and Wersin, P., 1992. Isotopic composition of si- derite as an indicator of depositional environment., 20 (9): 817-820.

Mozley, P. S., 1989. Relation between depositional environment and the elemental composition of early diagenetic siderite., 17 (8): 704.

Qiao, S. Q., Shi, X. F., Fang, X. S., Liu, S. F., Kornkanitnan, N., Gao, J. J., Zhu, A. M., Hu, L. M., and Yu, Y. G., 2015. Heavy metal and clay mineral analyses in the sediments of upper Gulf of Thailand and their implications on sedimentary pro- venance and dispersion pattern., 114: 488-496.

Saadon, M. N., Rojana-anawat, P., and Snidvongs, A., 1999. Phy- sical characteristics of water mass in the South China Sea, Area I: Gulf of Thailand and east coast of Peninsula Malaysia., Area I: 1-5.

Sapota, T., Aldahan, A., and Alaasm, I. S., 2006. Sedimentary facies and climate control on formation of vivianite and siderite microconcretions in sediments of Lake Baikal, Siberia., 36 (3): 245-257.

Shi, X. F., Liu, S. F., Fang, X. S., Qiao, S. Q., Khokiattiwong, S., and Kornkanitnan, N., 2015. Distribution of clay minerals in surface sediments of the western Gulf of Thailand: Sources and transport patterns., 105: 390-398.

Song, H. Y., Liu, J. Q., Yin, P., Zhang, Y., and Chen, X. Y., 2018. Characteristics of heavy minerals and quantitative provenance identification of sediments from the muddy area outside the Oujiang Estuary since 5.8kyr., 17 (6): 1325-1335.

Srisuksawad, K., Porntepkasemsan, B., Nouchpramool, S., Yam- kate, P., Carpenter, R., Peterson, M. L., and Hamilton, T., 1997. Radionuclide activities, geochemistry, and accumulation rates of sediments in the Gulf of Thailand., 17 (8): 925-965.

Wang, K. S., Shi, X. F., and Lin, Z. H., 2003. Assemblages, provinces and provenances of heavy minerals on the shelf of the southern Yellow Sea and northern East China Sea., 21 (1): 31-40 (in Chinese with Eng- lish abstract).

Wang, K. S., Shi, X. F., Liu, S. F., Qiao, S. Q., Yang, G., Hu, L. M., Narumol, K., and Somkiat, K., 2014. Spatial distribution of heavy minerals in the surface sediments from the Western Gulf of Thailand: Implications for sediment provenance and sedimentary environment., 34 (3): 623- 634 (in Chinese with English abstract).

Windom, H. L., Silpipat, S., Chanpongsang, A., Smith, R. G., and Hungspreugs, M., 1984. Trace metal composition of and accumulation rates of sediments in the upper Gulf of Thailand., 19 (2): 133-142.

Yang, S. Y., Wang, Z. B., Guo, Y., Li, C. X., and Cai, J. G., 2009. Heavy mineral compositions of the Changjiang (Yangtze Ri- ver) sediments and their provenance-tracing implication., 35 (1): 56-65.

Zhang, K. D., Li, A. C., Huang, P., Lu, J., Liu, X. T., and Zhang, J., 2019. Sedimentary responses to the cross-shelf transport of terrigenous material on the East China Sea continental shelf., 384: 50-59.

April 26, 2020;

June 15, 2020;

July 17, 2020

? Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2021

. E-mail: jinqingliu@hotmail.com

E-mail: hawk0412@163.com

(Edited by Chen Wenwen)