Journal of Earth Science  2019, Vol. 30 Issue (4): 728-738   PDF    
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Formation Mechanism of Beach Rocks and Its Controlling Factors in Coral Reef Area, Qilian Islets and Cays, Xisha Islands, China
Na Zhao 1,2, Dishu Shen 3, Jian-Wei Shen 1     
1. CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. Huxi Higher Education Mega Center, Chongqing City Management College, Chongqing 401331, China
ABSTRACT: Beach rock, which forms at the intertidal zone, is a natural barrier to protect beach and island from wave erosion. The formation mechanism of beach rocks is significant in the study of surface carbonate diagenesis and protection of island. Beach rocks in Qilian Islets and Cays were formed about 500-700 a BP, neither its composition nor sedimentary structure experienced intense post-reformation. Beach rocks in Qilian Islets and Cays are mainly composed of reef-building skeletal fragments and bioclasts without terrigenous sediments. This study focused on the types and morphologies of cements in beach rocks of Qilian Islets and Cays, and its cementation mechanisms and influencing factors. Biological activities, such as micritization caused by microbial activities and algae binding action, play an important role in the initial stage of bioclast transformation and promote the subsequent early marine and early meteoric cementation. Acicular aragonite induced by early marine cementation is well developed in beach rocks, especially in those samples from Medium Islet. Early meteoric cementation is not common as marine cementation, generally presenting granular and meniscus cements. The main factors affecting the formation and development of beach rocks in Qilian Islets and Cays include sediment source, hydrodynamic condition, climate, sea level change and anthropogenic impacts.
KEY WORDS: beach rock    cementation    micritization    coral reef    Qilian Islets and Cays    the South China Sea    
0 INTRODUCTION

Beach rock, a typical sedimentary rock cemented in the coastal zone, is composed of various sizes and types of sediments, such as rock sands, marine skeletal grains, wood fragments, and marine garbage (Vousdoukas et al., 2007). Chemical precipitation of carbonates and biological binding and mediation are the primary cementation processes (Vieira and De Ros, 2006; Li and Feng, 1998). Beach rocks could form in various environments, dominantly in tropical and subtropical intertidal zones (Scoffin and Stoddart, 1983). Coastal zone would rapidly response to environment changes since the influence of land- sea interaction. In such case, beach rocks could record land-sea interaction during diagenetic period and play a significant role in studying sea-level change, coastal development, protection of island, diagenetic mechanisms of surface carbonates and paleoclimatic changes (Caldas et al., 2006; Vieira and De Ros, 2006; Spurgeon et al., 2003; Xue, 2002; Cooper, 1991; Hopley, 1986; Zeng and Qiu, 1985). The studies of beach rocks were mainly carried out in the Mediterranean Sea, the Caribbean Sea, the coastal zone of tropical and subtropical Atlantic, Atolls in Pacific Ocean and Indian Ocean (Vousdoukas et al., 2007), focusing on the type of cementation, hydrodynamic condition, and factors impacting on formation and development of beach rocks (Last et al., 2012; Vousdoukas et al., 2012; Desruelles et al., 2009; Ravisankar et al., 2009; Erginal et al., 2008; Schöne et al., 2005; Calvet et al., 2003; Webb et al., 1999). Beach rocks are mainly distributed along the coast of the South China Sea, the coast of the East China Sea, and the islands in the South China Sea (Li, 1988). Chinese researchers paid much attention to the beach rocks developed on mainland shores or coastal island shore, including bedrock-coast or bio-bedrock-coast (Jiang and Yang, 1999, 1995, 1994; Wang J H, 1997, 1992; Wang S H, 1995; Zhao and Chen, 1994), but little work was done on the beach rocks along the biologic coast of coral reef islands (Shen et al., 2017).

Studies on the beach rocks in Xisha Islands were mostly done around the late 1970s, including geographical distribution, sedimentary structure, grain features, cementation and chemical composition of beach rocks (He et al., 1985; Huang et al., 1978). Away from the China mainland, so there is almost no terrigenous composition in beach rocks, and nearly all of the sediments are sourced from adjacent reef flat. This paper focused on the cementation of beach rocks and its influence factors in coral reef area of Qilian Islets and Cays, Xisha Islands. The types and morphologies of cements in beach rocks of Qilian Islets and Cays were identified, its cementation mechanisms were determined, and the factors affecting the formation of beach rocks in reef islands were discussed, which provided a theoretic basis to the future research of the beachrock formation and protection of islands.

1 STUDY AREA

Xisha Islands, situated in the western continental slope of the South China Sea, is the largest archipelago in this area (8 km2), comprising more than 30 islands, reefs and cays, including Xuande Atolls, Yongle Atolls, Huaguang Atolls, Dongdao Atolls, and some smaller islands (Wang, 2001). Qilian Islets and Cays is located in Xuande Atolls (16°53′N-17°0′N, 112°11′E-112°22′E), including Zhaoshu Islet, North Islet, Medium Islet, South Islet, and several other cays (Fig. 1). It was possibly developed on the gneiss basement (Wei et al., 2008; Wang, 2001), forming a carbonate platform, mainly composed of coral reefs. Qilian Islets and Cays are small in Xisha Islands and far away from the continent. There is no river or surface runoff on the islets. Human activities are not common (only a few fishermen lived occasionally on Zhaoshu Islet), and beach rocks are very pure carbonates without terrigenous sediments.

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Figure 1. Sketch maps showing the study area and the sampling locations (modified from Shen et al., 2017, 2013; ). (a) Modified after http://bzdt.nasg.gov.cn/, No. GS(2016)2891.
2 METHODS

Research work was conducted on the beach rocks around Zhaoshu Islet, North Islet and Medium Islet of the Qilian Islets and Cays (Fig. 1). The distribution of the outcrops and submerged extention of beach rocks were measured by GPS positioning and mapped according to the subaerial and underwater measurement data. In order to study on the development of beach rocks under different situations (wind direction, hydrodynamics, coastal topography, etc.), beach rocks in different locations were observed and measured. The AMS14C dating data was used to determine the age of beach rock formation. Skeletal content and porosity were analyzed under polarizing microscope and calculated statistically by using software Photoshop CS 14. In addition to field observation and microscopic study of thin sections, the scanning electron microscope and Raman spectroscopy were used to determine the morphology and mineralogy composition of cements.

3 LITHOLOGIC FEATURES

In the study area, beach rocks are commonly deposited along the coast of four islets, including Zhaoshu Islet, North Islet, Medium Islet, and South Islet. Sorting and roundness of the skeletal grains in beach rocks are variable from place to place. Statistics of grains based on the microscope studies of thin sections indicate that average sizes of skeletal grains vary from 1 to 6 mm, but tridacna fragments can be up to 10 cm in outcrops.

3.1 Occurrence of Beach Rocks and Characteristics of Outcrops

The surface of beach rocks is dark gray or dark brown, and the inner beds are light gray. Some beach rocks are covered by boring algae mats. Beach rock beds with loose textures are composed of poorly-sorted skeletal grains and fragments. Most of the beach rocks beds are outcropped on the coast, and two or three beds are submerged in the water. The average thickness of beach rock beds is about 18.7 cm. The dip angle of the strata is approximately ten-degree downwards to the sea. Striking direction of the beds is almost parallel to coastline. Beach rocks in the three islets are different in the amount of beds, thickness of each bed and widths of outcrops (Table 1).

Table 1 Characteristics of the outcrops
3.2 Composition and Characteristics of Skeleton Grains

Microscopic studies show that beach rocks in the study area consist primarily of marine skeleton grains, with no terrigenous and non-marine basal deposition. The skeleton grains are mainly composed of coral fragments and grains, followed by bioclasts of algae, bivalves, gastropod, foraminifera, echinoderm, as well as a small amount of bryozoan and sponges (Table 2). The different contents of skeletal grains in beach rocks were counted by using the method of Adobe Photoshop quantitative analysis (Zhang et al., 2009), then the percentage of grain contents were calculated (the area of grains in the thin section).

Table 2 Contents and microscopic features of various skeletal grains

The composition and grain size of beach rocks from three islets are different (Table 2). The broken gastropod shells are difficult to be distinguished from bivalves, so these two kinds of skeletal grains were counted in total. Besides, there were two forms of algae in the samples, bioclasts and binders. Algal binders generally encrust other grains or overgrow them. Such kind of algae was not included in algae grains.

4 CEMENTATION CHARACTERISTICS OF BEACH ROCKS

Beach rocks are formed in intertidal zone, submerged during high-tide period and exposed during low-tide period. Some beach rocks are eroded when exposed to the surface of the water, while some beach rocks are submerged in marine water are cemented by seawater.

Beach rocks in the study area are loosely cemented. Average porosity is 15%, with maximum averaged porosity 24% in North Islet and minimum averaged porosity 5% in Medium Islet. Various morphologies and types of cements were observed under microscope, such as dark-colored micrite cements, sometimes with acicular aragonite perpendicular to grain surface. By comparing characteristics of cementation from the three islets, it can be conclude that: (1) the most well-developed micritization is in beach rocks of Zhaoshu Islet. Grain surface is commonly surrounded by thick micrite, while the poorly- developed micritization is in beach rocks of North Islet; (2) algae binding action is the main cementation in beach rocks of North Islet; (3) the most well-developed marine water cementation is in Medium Islet, and the cements in beach rocks of Medium Islet are characterized by obvious multi-generations.

4.1 Binding Action of Algae

Since 1960s, scientists have discussed algae binding action in sedimentary rocks (Frantz et al., 2015; Davies, 1970; Neumann et al., 1970; Scoffin, 1970; Monty, 1967). Davies and Kinsey (1973) presented algae binding action in beach rocks of Heron Island in Great Barrier. Near-shore beach rocks are bound by blue-green algae while off-shore beach rocks are mostly bonded by red algae. Algae are commonly aggregated on the surface of beach rocks and form dark-grey algae mats, which have also been observed on beach rocks in North Islet and Zhaoshu Islet.

The surface of beach rocks in the study area is overgrown by boring algae mats. Algal binders, as an encrusting and binding forms of algae around skeletal grains, is very common in the beach rock of Qilian Islets and Cays. The algal binder can stabilize and consolidate beach sediments, which is a prerequisite for the formation of beach rocks in Qilian Islets and Cays (Fig. 2).

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Figure 2. Algae binding action. (a) Algal bioclasts and algae bonded other skeletal grains (from Section 1 of North Islet); (b) algae occur on the surface of acicular aragonite cement, which would cause micritization (from Medium Islet); (c) algal bioclasts and binders (from Section 3 of Zhaoshu Islet); (d) algae encrusted other bioclast (from Section 3 of Zhaoshu Islet). ① Algal binders (brown); ② algal bioclasts; ③ acicular aragonite cement; ④ coral; ⑤ foraminifera.

Algae occur between other grains in some samples of North Islet (Fig. 2a). Micritization, early marine cementation and early meteoric cementation are poorly developed in these samples. It is hardly found micrite envelopes and granular calcite cements due to early meteoric cementation. All these phenomena supported that the biological genesis play an important role in beach rock formation, especially the binding action of algae. In Zhaoshu Islet, algae also aggregated to cover and bind the small grains besides the binding action of algae (Figs. 2c-2d). In Medium Islet, early marine cementation is dominated, and algae binding action is not common. Algae generally overgrow the isopachous aragonitic cements (Fig. 2b), forming one or two interlayers of algal encrusts and aragonitic cements (Fig. 4c).

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Figure 4. Early marine cement. (a) Acicular aragonite cement of the beach ock in North Islet (from Section 2 of North Islet); (b) acicular aragonite cement of the beach rock in Zhaoshu Islet (from Section 7 of Zhaoshu Islet); (c) acicular aragonite cement of the beach rock in Medium Islet; (d) scanning electron microscope photographs of beach rock in Medium Islet (modified from Shen et al., 2017); (e) scanning electron microscope photographs of acicular aragonite cement; (f) Raman spectroscopy analysis of acicular aragonite cement. ① Aragonite cements; ② grains; ③ pore.
4.2 Micritization

Micritization of skeletal grains in beach rocks are common in beach rocks of Qilian Islets and Cays. Micritization includes four stages: (1) algae drilling; (2) forming of micrite nuclei, micrite net and radiation pattern; (3) micrite envelope; and (4) micrite cast (Wei, 1995). All these stages can be observed from the microscopic photos of thin sections (Fig. 3).

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Figure 3. Micritization. (a) Boring of endolithic algae and microbes (from Section 5 of Zhaoshu Islet), endolithic algae drilled holes within coral skeletal fragments; (b) algae further eroded coral skeletal fragments, forming radial patterns on the edge of grains (from North Islet); (c) micritic rims around bivalve skeletal grains, forming typical micritic envelope (from Section 4 of Zhaoshu Islet); (d) algae eroded and micritized the entire grain, forming micrite casting (from Section 6 of Zhaoshu Islet). ① Micro drilling; ② micrite net; ③ micrite envelope; ④ micrite casting.

Microbes induced micrite cements are significantly different from carbonate autogenous micrite cements precipitated from pore water. Micritization induced by microbes is a process of transformation, and carbonate materials of micrite derives from the grains themselves. Irregular eroded rims of the skeletal grains, especially bivalve bioclasts can be observed, showing distinct boundary between it and micrite. Nearly all the micrite cements in the study area are microbially induced.

Micritization is well developed in the beach rocks of Zhaoshu Islet, but is poorly formed in the beach rocks of North Islet. The isopachous micrite rim is formed around grains with thickness about 0.2-0.3 mm. Some grains are totally micritized, but remained the original morphologies of grains, forming micrite casts. Isopachous aragonitic cements were formed on the surface of micritic rims due to early marine cementation.

The micrite envelope in North Islet is thinner, showing uneven thickness. Most of the grains are in the form of micrite net, even micritization cannot be observed on some grains. Micritization is also common in the beach rocks of Medium Islet. Most of the grains are rimmed by isopachous micrite, forming micritic envelope. Sometimes, the edges of micrite cements are surrounded by sparry aragonite, forming multiple generations of cementation.

Micritization formed by endolithic algae and microbes are significant agents in the formation of beach rocks in Qilian Islets and Cays. The micrite induced from micritization of skeletal grains is not only an important role in consolidation of beach sediments, but also a component for the early marine cementation and early meteoric cementation.

4.3 Early Marine Cementation

In some beach rock samples in the study area, especially in Medium Islet, early marine cementation is well developed. Previous studies considered that shallow marine carbonates were mostly cemented by aragonite and high-magnesium calcite (Morse and Arvidson, 2002). According to the test and analysis of scanning electron microscopy and Raman spectroscopy, aragonite is almost the unique mineral in marine cements in beach rocks of Qilian Islets and Cays.

Acicular aragonite cements grew perpendicularly to the surface of micrite envelope that coated skeletal the grains. Some of the aragonites show isopachous characteristics, and other aragonite presents sporadically growth on the surface of grains.

In beach rocks of Zhaoshu Islet, the thickness of isopachous aragonite is 0.1 to 0.25 mm. In some thin sections, marine cementation is poorly developed, but there is distinct embryonic acicular aragonite within the pore of grains, while inter grain pore is still filled with micrite (Fig. 4b). Although the acicular aragonite within pore of grains did not play a role in the cementation of beach rocks, it indicates that the pore water is supersaturated with calcium carbonate.

In the beach rocks near to the sea surface, such as those in North Islet, acicular aragonite cements are well developed and occur almost in every sample. Isopachous aragonite can be observed in some thin sections of beach rocks, the thickness of aragonite rim is uneven, no more than 0.2 mm. Some neogenetic aragonite on the wall of grain-voids can grow to 0.2-0.3 mm thick (Fig. 4a). For those beach rocks above the sea water, micritization is poorly developed. Most of the grains are in contact and cemented. In the pore, dispersed acicular aragonite, as an early form of isopachous aragonitic cements, can be observed.

Beach rocks in Medium Islet are also well cemented. Acicular aragonite cements are well developed, forming ~0.4 mm thick isopachous rims that coated the grains (Fig. 4c). Multiple generations of cements can be observed in some thin sections. The pores among grains are also filled by acicular aragonite, forming 0.4 to 0.5 mm thick cements.

Single aragonite crystal of cements in beach rocks of Medium Islet is 40 μm in length and 5 μm in width. Crystals are attached by micro-crystals, which indicate aragonite cements experienced erosions (Figs. 4d-4e).

Raman spectroscopy, a method to analyze molecular vibrations, can identify minerals non-destructively. Minerals with same elemental composition but different lattice structure can be differentiated. The primary difference in Raman spectra of aragonite and calcite is the frequencies of the internal [CO3] bending lattice modes. The Raman shift of internal [CO3] bending of aragonite is 707 cm-1 with sharp peak, while for calcite is 715 cm-1 with some degree of broadening.

The thin sections of beach rocks in Medium Islet were horizontally placed on X-Y platform of Horiba Xploara micro- Raman spectroscopy. Using 10× and 100× objective lens, Raman spectra were obtained and bright field observations were conducted both under reflective light source and transmission light source. Semiconductor laser with wavelength of 532 nm was used to excite Raman scattering. The spectra were obtained using 1 800 gr/mm gratings, with resolution of 0.5 cm-1 in the range of 0 to 4 000 cm-1. Raman spectroscopy was corrected with single crystal silicon. The spectra were analyzed using GRAMS software. The Raman spectra presented that most of the carbonate cements in beachrocks of Qilian Islets and Cays are aragonite (Fig. 4f). No evidence of other minerals is found according to the analysis of acicular cements.

4.4 Early Meteoric Cementation

Beach rocks are formed in the sea-land interactive zones. In addition to acicular aragonite cements formed by early marine cementation, low-magnesium granular calcite related to meteoric cementation may also present. In the study area, granular calcites occur in some samples from Zhaoshu Islet and North Islet.

The morphologies of granular calcites in thin sections are irregular, showing subhedral and anhedral crystals. The size of single crystal is only a few micron meters, showing bright white granular shapes under orthogonal polarized light microscope. Under scanning electron microscopy, granular calcite is irregularly aligned around acicular aragonite cements or grows toward pore center of grains (Figs. 5a-5b).

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Figure 5. Granular calcite cement of beach rocks (from section 6 of North Islet). (a) Single polarization microscope photographs of granular calcite cement; (b) cross-polarized optical microscopy photographs of granular calcite cement; (c)-(f) scanning electron microscope photographs of granular calcite cement; (g) Raman spectroscopy analysis of granular calcite cement. ① Micrite envelope; ② acicular aragonite; ③ granular calcite; ④ pore.

There are two types of granular calcite cements in the beach rocks of Qilian Islets and Cays. One is those growing on the acicular aragonite, showing a cementation succession of micritic rims, isopachous acicular aragonite and granular calcite from grain surface toward pore center (Figs. 5c-5d). This cementation succession indicates that beach rocks are initially experienced early marine cementation in marine setting, then they exposed from the sea and granular calcite representing early meteoric cementation were formed in atmospheric setting. (2) The other is those granular calcite formed in the inter-grain pores without obvious acicular aragonite but with micrite envelope only (Figs. 5e-5f), indicating no early marine cementation occur in these beach rocks. In addition, meniscus calcite cements are visible in the concave of grains or connections between grains in beach rocks of North Islet. The meniscus or bridge-shaped cement is typically formed in the freshwater vadose setting, is caused by uneven distribution of pore fluid under gravity. Beach rocks in study area were mostly developed in coastal zone and were submerged in sea water from time to time, especially during extra high tide or storm. Probably not commonly exposed on the land, early meteoric cementation is not common.

5 PRIMARY INFLUENCING FACTORS ON FORMATION AND DEVELOPMENT OF BEACH ROCKS

Beach rocks are formed in intertidal zone, are generally affected by various factors, including sedimentary source, climate and environment, sea level change, hydrodynamic regime, intertidal biological productivity and human activities.

5.1 Sedimentary Source

Sedimentary source mainly affects the composition of beach rocks. The study area is far away from mainland without rivers and runoffs, so no siliciclastic sediments and other terrigenous sediments occur in beach rocks. According to the field observations and microscopic studies, the sediments in beach rocks of Qilian Islets and Cays are mainly sourced from associated reef flat and reef ridges, including (1) coral skeletal fragments and bioclasts; (2) various kinds of algae; (3) marine organism shells such as bivalve, gastropods and foraminifera; (4) tropical plants and marine garbage transported by tidal and currents. The composition of beach rocks is commonly related to coastal types. For example, beach rocks in Hainan Island and East China Sea coastline were formed in a bedrock coast or muddy and sandy coast, quartz, feldspar and other siliciclastic sediments are dominated in beach sediments (Shen and Wang, 1999; Zhao et al., 1978), such as beach rocks in Taiping Harbor, Shandong Peninsula, where white quartz and feldspar are major sediments in beach rocks (Wu and Wang, 1995).

Sedimentary source also affects the thickness of beach rock beds. The decline of corals on the reef flat will enhance the hydrodynamic regime as well as the wave erosion on the sediments over reef flat, and consequently increasing sediments transportation. These changes will increase the thickness of beach rock beds. For example, in the Section 7 of North Islet, the up four beds are thicker than the lower four beds. It indicates that the sediment supply increased in the late stage of beach rock formation, which may be related to a continually decline of coral reef development and intensified wave erosion on the sediments of reef flat in recent decades.

5.2 Hydrodynamic Regime

Strikes of beach rocks in the study area are generally parallel to the coastal line, and beach rock beds have a gentle slope down to the sea. Surface of beach rocks are more or less flat due to continuous wave abrasion. The directions and strengths of tide and wave controlled and modified the occurrence and distribution of beach rocks.

Hydrodynamic regime can affect the sorting, roundness and grain size of the beach sediments, as well as the species of organisms in the sediments (Youssef and El-Sorogy, 2015). Grain sizes in beach rocks vary largely due to different hydrodynamic reworking. Poorly sorted beach rocks contain skeletal grains and fragments with different sizes from millimeters to several centimeters. The foraminifera are conserved with primitive morphology without fractures. Roundness of other small grains is often very poor. It indicates the grains of beach rocks rapidly deposited without sorting and rounding due to current and they are local or carried over from short distance.

The roundness and sorting of sedimentary grains also affect beach rock cementation. The retaining of pore water is more easily in the sediments with uniform size and good roundness, which promotes marine and meteoric cementation and lithification. Study area is affected by tropical monsoon climate. The northeast coast and the southwest coast of Zhaoshu Islet as well as the sampling regions of North Islet are on windward positions, while the northwest coast and southeast coast of Zhaoshu Islet and the sampling region of Medium Islet are on leeward positions (Fig. 1). The studies on thin sections of beach rocks shows that the developments of early marine cementation in beach rocks on North Islet and Medium Islet are largely different, although these two islets are closed and human activities are rare on both islets. Early marine cementation is well developed in beach rocks of Medium Islet and forms thick and uniform aragonite rims around grains. However, early marine cementation is not well developed in beach rocks of North Islet, the fibrous aragonites only occur in beach rocks submerged in the sea and forms thin and uneven rims. The sampling location of North Islet is on windward position with strong hydrodynamic regime, where sediments with poor sorting and roundness hardly retain pore water. Although wave and tide bring much marine water into sediment, the condition is not suitable for cementation. However, the hydrodynamic regime in the sampling location of Medium Islet is relatively weak, pore water is uniformly distributed in beach sediments with good sorting and roundness, providing a suitable condition for cementation.

Hydrodynamic strength also influences the dissolve oxygen, light intensity and distribution of nutrients in sea water. These factors generally affect the growth rate of algae and other microbes that may change the composition of beach rocks and algae binding action (Liang et al., 2013; Cheng et al., 2012; Wu and Li, 2010; Devercelli, 2006). Possibly due to rich oxygen and sufficient light in sea water, algae binding action is commonly observed in the beach rocks of in North Islet and on the windward of Zhaoshu Islet. However, the algae binding action is not common in the beach rocks of Medium Islet and on leeward of Zhaoshu Islet, and the micritization is well developed in beach rocks on two localities. Microbial carbonates are commonly formed at low depositional rate (Teng and Shen, 2008). Weak hydrodynamic is more suitable for the growth of anaerobes microbes that either fix sediments or produce micrite cements.

5.3 Sea-Level Change

The sea-level change generally affects the distribution, textures and cementation of beach rock (Pirazzoli, 1996; Kindler and Bain, 1993; Hopley, 1986). The sea-level controls the top position of marine cementation.

When sea-level rose, beach rocks will be destroyed by wave and tide. If the conditions (hydrodynamics and sedimentary supply) are suitable for beach rock formation, the beach sediments above beach rocks were consolidated and been lithification through microbial activities and marine chemical cementation. In beach rocks of Zhaoshu Islet and Medium Islet, common multiple generations of cements are probably related to sea-level fluctuations. If the condition is not good for beach rock formation, wave erosion will continue. When the sea-level falls, marine cementation will stop, beach rocks will be exposed on the land and erosion from rainfall, and human damages will destroy beach beds. The microbes in void start to reform grains by drilling through their secreted organic acid, and at same produce micrite. The low sea-level period also promotes meteoric cementation, forming granular calcite cements. If beach rocks are subaerial for a long time, various dissolution pores and voids would occur. Aragonite and high- Mg calcite cements formed during earlier marine cementation will also be dissolved or recrystallized. Some beach rocks samples of Zhaoshu Islet and North Islet present dirty surface of grains under microscope, which is resulted from dissolution during the lower sea-level.

5.4 Climatic Condition

Climatic and environmental changes influence carbonate deposition (Li et al., 2015). Most of previous studies consider that beach rocks are only formed in tropical coastal, hot humid and arid to semi-arid areas. Recently studies revealed that beach rocks also occur in middle latitudes, where temperature of sea surface was about 5 ℃, although beach rocks prefer to form in tropical and subtropical areas (Rey et al., 2004; Kneale and Viles, 2000). The cement of beach rocks in middle latitudes is mainly acicular and bladed calcite, and the main cementation mechanism is inorganic diagenesis, such as degassing of groundwater CO2 and supersaturation due to the direct evaporation of seawater in the vadose zone, lacking of micritization and binding action of algae (Rey et al., 2004). Sea surface temperature is higher in tropical and subtropical areas than that in middle and high latitudes, resulting in increase of evaporation and benefit for sedimentation of carbonate cements. Also, warm marine water is valuable for microbes breeding and subsequently for providing rich microbiological grains. Algae and other microbes also play roles in promotion of micritization.

5.5 Anthropogenic Impacts

The influence of human activities on beach rock formation in Qilian Islets and Cays should not be neglected, especially in Zhaoshu Islet where fishmen live seasonally. Zhaoshu Islet is the important typhoon shelter for fishermen since the Song Dynasty. During the fishing season, some fishermen settle temporarily in Zhaoshu Islet. In order to build fishing ports, fishermen destroy some beach rocks facing the reef flat. Fishing snails and other mollusks near the coastline and on reef flat affect the growth of reef-building corals, leading to decline of reefs (Shen et al., 2017; Yang et al., 2014; Shi et al., 2011). The decline of reef growth will affect the supply of beach sediments.

Human activities also have a significant impact on beach rock cementation. The micritization in the beach rocks of Zhaoshu Islet is much more developed than in the beach rocks of North Islet. It is due to human beings in Zhaoshu Islet who discharge garbage and waste into surrounding marine water. The increased nutrition affects the organism abundance and diversity in coast and reef flat, resulting in eutrophic sea water for microbial bloom. Beach sediments in a setting with abundant microbes are easily micritized. However, the human activity is less in North Islet, the algal bindings in beach rocks are more common than the micritization there.

Marine tourism may affect the beach rock on the coast of islet and island in tropical coral reef area, as beach rocks are destroyed and removed for tourism demand such as building ports, hotels, restaurants and leisure facilities. Some local governments in tropical and subtropical coastal areas are devoted to develop tourism. For instance, since the foundation of Sansha City, the beautiful tropical coral reefs in Qilian Islets and Cays are an attractive scenic spot to tourists and many tourism constructions on Qilian Islet and Cays have been put on the agenda. However, the beaches of Qilian Islets and Cays are only in the width from 3 to 4 m, these beach rocks may be removed completely due to tourism activities.

These factors generally interact with each another. Sea-level variation changes the depth of sediments, and affects the hydrodynamic strength of waves. The hydrodynamic strength is reflected on the broken degree of reef flat and determines the distance of the sediment transportation, and consequently affects the sedimentary source and the characteristic of grains; the climate will change human behaviors, and so on. All these factors impact the water condition (e.g., hydrodynamic, oxygen content, biodiversity and productivity), and the formation and development of beach rocks. Zhaoshu Islet, Medium Islet and North Islet are all on the same reef flat, under similar climatic and sea-level variation conditions, as well receiving the same depositional source. The differences of hydrodynamics between the windward (North Islet) and leeward (Medium Islet) of the islets lead to the difference of the lithologic features of beach rocks. Moreover, anthropogenic impacts caused the cementation characteristics of beach rocks on Zhaoshu Islet different from those on the other two islets.

6 CONCLUSIONS

(a) Beach rocks are well-bedded and widely developed in Qilian Islets and Cays of reef area with well-exposed outcrops. Primary compositions of beach rocks are biological skeletons. The most common skeletal grains are coral fragments and bioclasts (averagely 40%), and other biological skeletal grains are algae, bivalves, gastropods, foraminifera, echinoderms, bryozoans and calciferous sponge. No terrigenous sediments occur in the beach rocks of Qilian Islets and Cays.

(b) The cementation mechanisms of beach rocks in the study area include: (1) microbial and algal binding and drilling on skeletal fragments and bioclasts; (2) micritization formed by boring algae and microbes; (3) early marine cementation dominated by acicular and fibrous aragonite; and (4) early meteoric cementation characterized by granular calcite and meniscus cements. Cementation is commonly promoted in a marine setting, including organic processes such as algae binding action and micritization, and inorganic processes such as early marine cementation. Organic processes are important in cementation of beach rocks, forming the foundation of early marine cementation.

(c) Micritization is an important mechanism for beach rock formation in Qilian Islets and Cays, as it is commonly observed in every sample with different development. The micritization is well developed in the beach rocks of Zhaoshu Islet, and it poorly developed in the beach rocks of North Islet. The anthropogenic activities in Zhaoshu Islet caused algal and microbial bloom and subsequently resulted in a common micritization of skeletal grains in beach rocks.

(d) The formation mechanisms of beach rocks are different in the three islets: (1) micritization is a dominated formation mechanism in beach rocks of Zhaoshu Islet; (2) algae binding action is a major formation mechanism in beach rocks of North Islet; (3) early marine cementation is a main formation mechanism in beach rocks of Medium Islet.

(e) The main factors affect the formation and development of beach rocks are sediment source, hydrodynamic regime, climatic condition, sea level change, anthropogenic impact. The most important factor is hydrodynamic regime that resulted in the difference of beach rock cementation between Medium Islet (windward) and North Islet (leeward). Due to the different hydrodynamic regime between windward and leeward of the islets, the sedimentary composition and cementation of beach rocks are also different. The hydrodynamic strength may affect the micritization around skeletal grains, and generally stable hydrodynamic regime is more suitable for micritization of skeletal grains.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Nos. 41672107, 41372119, 41440040). We thank the facilities and supports provided by the Marine Environmental Observation and Research Station at the Xisha Islands, the South China Sea Institute of Oceanology, CAS during the fieldwork. We also would like to thank Dr. Menghan Wang and Physics College of Shenzhen University for providing Raman spectroscopic analysis. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0981-3.


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