The occurrence, acoustic characteristics, and significance of submerged reefs on the continental shelf edge and upper slope, northern South China Sea

The occurrence, acoustic characteristics, and significance of submerged reefs on the continental shelf edge and upper slope, northern South China Sea

Continental Shelf Research 100 (2015) 11–24 Contents lists available at ScienceDirect Continental Shelf Research journal homepage: www.elsevier.com/...

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Continental Shelf Research 100 (2015) 11–24

Contents lists available at ScienceDirect

Continental Shelf Research journal homepage: www.elsevier.com/locate/csr

Research papers

The occurrence, acoustic characteristics, and significance of submerged reefs on the continental shelf edge and upper slope, northern South China Sea Xishuang Li a,n, Xinzhong Li b, Qiang Zhao a, Lejun Liu a, Songwang Zhou c a

The First Institute of Oceanography, State Oceanic Administration, Qingdao, China The Research Center of the China National Offshore Oil Corporation, Beijing, China c The Survey Center of China Oilfield Services Limited, Tianjin, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 22 August 2014 Received in revised form 9 March 2015 Accepted 16 March 2015 Available online 18 March 2015

Comprehensive research was performed to investigate the existence of submerged reefs on the continental shelf edge and upper slope outside the Pearl River estuary in the northern South China Sea to analyze the morphology and distribution of the coral reefs, identify their ages, and discuss their paleoenvironmental significance. The results show that the submerged reefs are distributed on the topographic high of the continental shelf and upper slope at a water depth of 140–420 m, with an outcropping area of over 700 km2. The submerged reefs are mainly topographically characterized by low mound protrusions, several to 20 m above the modern seafloor, and they exhibit three typical acoustic facies in the acoustic profile: punctate reflection, planar reflection, and mound reflection. Based on various geophysical data, we propose at least four stages (I–IV) of submerged reefs on the continental shelf edge and upper slope: The oldest stage I submerged reefs are covered by loose sedimentary strata of 4100 m thickness, a minority of stage II submerged reefs outcrop on the modern seafloor, and most stage III and IV submerged reefs outcrop on the seafloor. The submerged reefs developed on the top of the sandy or muddy substrate on the continental shelf edge and upper slope and formed during a period of low sea level. Sea-level changes controlled periodic reef formation events, with stage I, II, and IV reef formation events corresponding to three low-sea-level events, i.e., at 1450, 600, and 20 ky B.P. Based on the maximum water depth (∼220 m) where the last glacial submerged reefs appeared, we estimate that the sea level during the last glacial maximum fell by ∼132–152 m. Further geophysical surveys and sampling need to be performed to clarify the actual distribution, type, and age of the submerged reefs in the study area. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Northern South China Sea Acoustic detection Submerged reef Paleoenvironment Sea-level changes

1. Introduction Modern coral reefs are widely developed in tropical and subtropical shallow water near shore areas, with an estimated total area of about 284,300 km2 (Spalding et al., 2001; Mulhall, 2009). Previous studies have shown that there are also submerged reefs in many tropical island surroundings and continental shelves, such as the Caribbean Sea (Macintyre,1972; Lighty et al., 1978; Fairbanks, 1989; Hubbard et al., 1990; Blanchon et al., 2002; Jarrett et al., 2005), the Seychelles Islands (Stoddart, 1971), Madagascar (Pichon, 1977; Guilcher, 1988), the Bahamas Islands (Hine and Steinmetz, 1984), the edge of the western Indian continental shelf (Wagle et al., 1994; Colonna et al., 1996; Vora et al., 1996; Dullo et al., 1998; Rao n

Corresponding author. E-mail address: [email protected]fio.org.cn (X. Li).

http://dx.doi.org/10.1016/j.csr.2015.03.006 0278-4343/& 2015 Elsevier Ltd. All rights reserved.

et al., 2003) in the Indian Ocean, Papua New Guinea (Guilcher, 1988; Galewsky et al., 1996), the Fiji Islands (Guilcher, 1988), the Hawaiian Islands (Veeh, 1966; Ku et al., 1974; Szabo et al., 1994; Muhs and Szabo, 1994; Muhs, 2002; Webster et al., 2004, 2010; Hearty et al., 2007), the outer edge of the Florida continental shelf (Lighty, 1977), and the Gulf of Carpentaria (Harris et al., 2004) and the outer edge of the Great Barrier Reef (GBR) (Veeh and Veevers, 1970; Harris and Davies, 1989) in Australia in the Pacific Ocean. These submerged reefs in the continental shelves, such as the 1300 km-long submerged reefs distributed on the outer edge of the western Indian continental shelf (Vora et al., 1996), were mainly formed during the last glacial period, whereas the submerged reefs around oceanic volcanic islands, such as the Hawaiian Island of Lanai (Webster et al., 2010), have longer formation histories Compared with modern coral reefs, submerged coral reefs not only reflect the shelf terrain (Hopley et al., 2007) and marine environment (Maxwell, 1970; Veron and Hudson, 1978; Sammarco

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Fig. 1. (a) Location of the study area. (b) Regional topographic contour map, showing the track of the geophysical voyage survey lines. The map is drawn based on USGS 30arcsec data. (c) 3D topographic map of the study area drawn based on multibeam bathymetric data and bathymetric data derived from seismic data. The black pentacle marks the station where previous researchers recorded in situ coral reefs (Zhu, 1995); the fine pink, white, and black lines represent the multichannel seismic track, the subbottom profile, and the single-channel seismic track, respectively. (d) Interpretation map of the seismic reflection profile running through the continental shelf–continental slope (adapted from Pang et al., 2005). This profile crosses three tectonic units, i.e., the Zhu 1 depression, the Panyu low uplift, and the Baiyun Sag. DWF represents the deepwater fan, ①–⑤ represent the sedimentary stratum sequence interfaces of 10.5, 13.1, 16.5, 23.8, and 32 Ma, respectively, and ⑥ is the Cenozoic sedimentary basement.

et al., 1991; Hopley, 2006) during their growth but also record very valuable information on the relative sea-level change (Steinen et al., 1973; Carter et al., 1986; O’Leary et al., 2008; Fürstenau et al., 2010) and tectonic movement. However, research on submerged coral reefs remains quite scarce, compared with that on modern

coral reefs; in particular, as yet no published reports on submerged reefs on the outer edge of a wide continental shelf are available. Compared with narrow shelves, wide continental shelves are unsuitable for the development of coral reefs, because they are usually supplied with large amounts of sediment from rivers and

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have smaller shelf gradients, making their shorelines move landward faster during sea-level rising. Modern coral reefs in the northern South China Sea (SCS) are dominated by fringing reefs and are mainly distributed around Hainan Island and Weizhou Island and in offshore areas to the south of Leizhou Peninsula and Taiwan Island (Yu and Zhao, 2009). In the early 1990s, native coral reefs had been sampled (from the surface at E114°50.31′, N20°18.20′, at a water depth of 148 m, and ∼220 km from the Pearl River estuary) (see Fig. 1c) at station ZD36-13 on the outer shelf outside the Pearl River estuary (Liu et al., 1993), but no further investigation had been conducted, so there is no related report on submerged coral reefs on the outer edge of the continental shelf in the northern SCS. In recent years, a large number of acoustic sounding data (including multibeam bathymetry, subbottom profile, side-scan sonar, and seismic data) have been collected from the outer continental shelf near ZD36-13. These data show some similar acoustic characteristics to those of submerged or buried coral reefs and knolls on other continental shelves and continental slopes, such as the GBR (Orme and Salama, 1988; Abbey et al., 2011; Webster et al., 2012), the submerged coral reefs on the outer edge of the western Indian continental shelf (Vora et al., 1996), and the cold-water corals on the continental slope of the Ionian Sea (Savini and Corselli, 2010), etc. Using previous research discoveries of native coral reefs (e.g., Liu et al., 1993) and based on various collected geophysical data, in this paper we aim to 1. describe in detail the morphology and distribution of submerged reefs on the shelf edge and upper slope in the study area, 2. identify their formation age, environment, and control factors, and 3. discuss their paleoenvironmental significance. The study will aid in obtaining an in-depth understanding of the Quaternary paleoenvironmental evolution in the northern SCS and will enrich research on submerged coral reefs on the continental shelf edge.

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Fig. 2. Relative PRMB sea level (a) and global sea level (b) changes since the Neogene (based on Qin (1996) and Haq et al. (1987), respectively). The depth in A is the modern water depth at the continental shelf slope of the northern SCS, but that in B indicates the changes in sea level compared with the modern sea level (0 m). The trend in sea-level change in the PRMB is basically similar to that of the global change since 25 Ma.

2. Regional Geological background The continental shelf in the northern SCS exhibits an NE-SW strike in general, being higher in the north and lower in the south, and dipping southward gently. The shelf is 150–300 km wide and is broad and flat, with a slope gradient of about 1%, and there are several clear underwater terraces from the inner shelf to the outer one (Feng et al., 1982; Liu et al., 1993). From the water depth of ∼200 m, the slope gradient of the seafloor terrain increases significantly, and the seafloor becomes a continental slope in geomorphology. The study area is located on the modern outer continental shelf edge outside the Pearl River estuary, with water depth of ∼140–800 m, where the bathymetric contours protrude southward in general, exhibiting an underwater highland protruding and dipping southward. (See Fig. 1). Some scholars believe that this underwater highland might be the ancient Pearl River Delta formed during the lowest-sea-level period (Feng et al. 1998; Chen et al., 1990; Bao, 1995). On the southwest side of the study area, there is an “S”-shaped wide and gentle groove extending from the shelf to the deep sea with a width of ∼20–30 km (referred to as the “Pearl River Submarine Canyon” by Jin (1989)). On the continental slope of the southern edge of the study area, there are a dozen short, closely arranged submarine canyons (Zhu et al., 2010) (see Fig. 1c). The Pearl River Mouth Basin (PRMB) is the biggest of a series of NE–ENE strike rift basins developed on the continental shelf edge in the northern SCS. The basin entered the depression stage after the Middle Miocene, with tectonic activity weakening, and it was basically in a relatively stable tectonic subsidence period (Wang and Zhao, 1987; Gong and Li, 1997). The study area is located on the edge of the Panyu low uplift, a secondary tectonic unit in the PRMB, and has had a low regional tectonic subsidence rate of about 40–60 m/Ma since the Middle Miocene (Liao et al., 2011).

Since the Middle Miocene, the sea level in the northern SCS has been consistent with global sea level, exhibiting a fluctuating downward trend (Haq et al., 1987; Qin, 1996). From the Pleistocene to present (∼2.5 Ma to present), there were three major sea-levelfluctuation events in the northern SCS (see Fig. 2), and the sealevel drop was close to or 4100 m during the lowest-sea-level period. The lowest sea level lasted a short time during the last glacial minimum (LGM), but the drop might be 4120 m (Qian, 1994; Chen, 1998). Since the Middle Miocene, the sedimentary stratum sequences of the PRMB have been mainly controlled by the change of sea level (see Fig. 1d), and the main part of the sedimentary stratum sequences consists of progradational deltas formed during the regressive period, with the maximum thickness of sedimentary stratum sequences occurring at the modern shelf break (Pang et al., 2005; Wu et al., 2008).

3. Geophysical data We collected bathymetric, sub-bottom profile, single-channel seismic, and high-resolution multichannel seismic data, as well as a small amount of side-scan sonar images (see Fig. 1c). The bathymetric data include shipborne multibeam bathymetric data and bathymetric data derived from initial seafloor waves in seismic data from the China National Offshore Oil Company (CNOOC). Multibeam bathymetric data were collected from two voyage surveys in 2010 and 2013 by the China Oilfield Services Limited (COSL). A digital elevation model (DEM) of 10 m resolution was used. The spatial density of the bathymetric data derived from the seismic data was 12.5 m, based on a DEM of 20 m resolution. The sub-bottom profile

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was obtained by using an IXSEA ECHOES 3500 sub-bottom profiler synchronously with the multibeam bathymetric survey, with a basic acoustic pulse frequency of 3.5 kHz. A few side-scan sonar images were obtained from the equipped autonomous underwater vehicle platform by COSL. The seismic data were obtained from the seismic voyage in 2013 by the First Institute of Oceanography (FIO). Sparker sources were used and the signals were received, respectively, by a single-channel streamer and a digital streamer with 64 channels and 6.25 m channel space. The time migration section was obtained after processing the multichannel seismic data using the software package (MBP2.0).

4. Results 4.1. Bathymetric data The bathymetric data show (see Fig. 3) that there is a large area of rough terrain distributed within a water depth range of 160– 420 m on the continental shelf edge in the study area. On the outer continental slope zone of the rough terrain area, the water depth increases rapidly, the slope gradient is large, and the head end of the canyon can be seen, but the seafloor surface is relatively smooth in general. On the shelf with water deptho220 m, some low underwater mound-like protrusions exhibit a patchy

Fig. 3. 3D topographic map and terrain section showing the morphology of the reefs outcropping on the seafloor on the outer continental shelf edge. (a) 3D topographic map, for the location shown in Fig. 1c. The white dotted line encircles the reef distribution area, and the black dotted lines represent the arc belt with an intensive distribution of reefs. (b) and (c) Enlarged figures of local terrain, with locations being shown in (a). The white lines in the figure show the stretching direction of the reefs. (d) and (e) Terrain sections, with locations being shown in (a). The short arrows in the section are reefs outcropping on the seafloor.

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Fig. 4. Shadow map (a), stereogram (b), and sections (c) and (d) showing the terrain characteristics of reefs on the upper continental slope. (a) and (b) are drawn by using multibeam bathymetric data (with positions being shown in Fig. 3a). The positions of terrain sections (c) and (d) are shown in (b).

distribution or form a low, narrow, and long underwater dam mainly trending northeastward. Station ZD36-13 where in situ coral reefs were recorded is located near an underwater protrusion with a NE trend (see Fig. 3a). These low protrusions are 1–5 m higher than the surrounding seafloor, and the surface of these protrusions is rough. On the outer edge of the patchy or narrow long-belt protrusion area (the side toward the deep water), there is an arc belt protruding southeastward, with an average width of 5.5 km and length of 4 100 km. On the inner side of the arc belt (the side toward the shallow water), the seafloor surface presents intensive broken morphology. Pointy protrusions are distributed intensively on the terrain profile, giving the seafloor surface a serrated appearance. The height of these protrusions is usually r5 m and their distribution density is about 5/km. In the middle of the arc belt, there are some dome-shaped underwater protrusions, exhibiting a NW-SE trend or isolated (see Fig. 4); these protrusions are 5–20 m higher than the surrounding seafloor, and some isolated ones have diameters of 150–300 m. On the outside of the arc belt (the side toward the deep water), there are 4–5 terrain steps trending NE, but their continuity is poor, with interruption seen frequently, and with distances between two adjacent steps of ∼200–800 m. The patchy or zonal low protrusion terrains in the study area, especially the terrain within the arc belt, have similar characteristics to those of the underwater coral reefs of Australia’s GBR (Abbey et al., 2011; Webster et al., 2012), the Lophelia pertusa deep water reefs on the west side of Scotland (Roberts et al., 2005), and the Santa Maria di Leuca cold-water corals in the Ionian Sea (Savini and Corselli, 2010). In middle of the arc belt, a small underwater bay developed in the north-south direction; it is ∼7 km long and stretches ∼3 km to the shallow water.

see three common reflection facies: punctate strong reflection, planar strong reflection, and mound reflection. Punctate strong reflection exists in the seafloor or loose sediments with evident diffraction (see Fig. 5b), and the lower reflection structure is unknown because of shielding; the punctate strong reflection is mainly distributed in the rough terrain with water depth of o200 m. Planar strong reflection exists in the seafloor or loose sediments, roughly parallel to the seafloor, with nonevident diffraction on the edge, and the lower reflection structure is unknown because of shielding; the planar strong reflection is also dominantly distributed in the area with water depth of o200 m. At the top of mound reflection, there is an undulating or domeshaped strong reflection surface, and the lower reflection structure is unknown because of shielding (see Fig. 5d). The diffraction phenomena on both sides of the reflection body with a dome mainly occur in the area with water depth of 220–420 m, the reflection body is several to 420 m higher than the surrounding seafloor, and the apparent width of the profile ranges from about dozens of meters to 300 m. The reflection inside of the mound reflection body in the seafloor sediments is extremely weak, exhibiting a strong contrast with parallel or progradational reflection characteristics in the loose sediments. The characteristics and distribution of various acoustic reflection facies are listed in Table 1. The above common acoustic reflection facies are comparable with the acoustic reflection facies of submerged or buried coral reefs found on other continental shelf or slope areas, such as Australia’s GBR (Orme and Salama, 1988; Webster et al., 2012), the buried coral reef mounds on the Tyrrhenian continental slope (Remia and Taviani, 2005), and the Santa Maria di Leuca coldwater corals in the Ionian Sea (Savini and Corselli, 2010). 4.3. Seismic data

4.2. Sub-bottom profile On the sub-bottom profile of the study area, the loose sediments have typical weak amplitude with parallel reflection or progradational reflection inner structures. In addition, we can also

On the seismic profile, there are usually punctate or mound protrusion reflections near the seafloor surface. The upper part has a low-frequency strong-reflection top interface and evident diffraction; the inside has a weak and disordered reflection structure

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Fig. 5. Acoustic reflection characteristics of the outcropping or buried coral reefs shown by 3.5 kHz sub-bottom profile. (a) and (c) Shadow maps based on multibeam bathymetric data (with positions shown in Fig. 1c). (b) and (d) Sub-bottom profiles and their interpretations (with profile positions shown in (a) and (c), respectively). The orange solid lines on the interpretation profile represent the outcropping part of the coral reefs on the seafloor, the orange dashed lines represent the coral reefs buried by sediments, the green solid lines represent sandy or muddy seafloor, and the blue fine lines represent the acoustic reflection interfaces inside the sediments. R. and B.R. represent the identified outcropping and buried coral reefs, respectively, and II–IV represent stage II–IV coral reefs.

or does not have a reflection structure. These reflection facies are also similar to those of outcropping and buried coral reefs (Orme and Salama, 1988; Vora et al., 1996; Van Rooij et al., 2011). In the 250-ms strata beneath the seafloor of the study area, there are two main reflection interfaces (Rf2 and Rf1, respectively, from the top down; see Fig. 6). Rf2 is located within the range of 50–100 ms under the seafloor. It is a low-frequency strong-reflection interface inclined toward the deep water and becomes shallow toward the

continental shelf, with the fluctuation turning gentle. At ∼400 ms, however, its continuity degrades, and it drops rapidly toward the deep water, with a slope gradient of over 4°. Rf1 has similar reflection and fluctuation characteristics to those of Rf2 (see Fig. 6b). It is located within the range of 150–200 ms beneath the seafloor, and it drops rapidly toward the deep water from ∼450 ms. These two interfaces have similar morphology to that of the modern seafloor, and their rapid drop-off position may be located at the

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Table 1 Acoustic reflection facies types and characteristics of submerged coral reefs. Acoustic image

Acoustic reflection facies types

Facies characteristics

Distribution area

Area with water depth of o200 m

Main types

Subtypes

I

I1

Punctate strong reflection, evident diffraction, unknown lower reflection structure, outcropping on seafloor

I1

Punctate strong reflection, obvious diffraction, unknown lower reflection structure, located Area with water depth in sediments of o200 m

II1

Planar strong reflection, no diffraction on the edge, unknown lower reflection structure, outcropping on seafloor

Area with water depth of o200 m

II2

Planar strong reflection, no diffraction on the edge, unknown lower reflection structure, roughly parallel to seafloor, located in sediments

Area with water depth of o200 m

III1

Mound reflection outline, strong fluctuation reflection interface, nonevident diffraction, unknown lower reflection structure, outcropping on seafloor

Area with water depth between 200 and 220 m

III2

Mound reflection outline, dome-shaped strong reflection interface, evident diffraction, unknown lower reflection structure, outcropping on seafloor

Area with water depth between 220 and 420 m

III3

Mound reflection outline, strong fluctuation reflection interface, nonevident diffraction, unknown lower reflection structure, located in sediments

Area with water depth of o200 m

II

III

paleo shelf break. When the shelf break positions of the same interface on different seismic profiles are connected, two lines running approximately parallel to the modern slope break line and protruding southeastward can be obtained. We suspect that they were paleo shelf breaks in the formation periods of Rf2 and Rf1, respectively, with the slope break line in the Rf1 formation period being closer to the continent than that in the Rf2 formation period. The sediment between the seafloor and the above two reflection interfaces exhibits high-angle progradational reflection, showing a cutting reflection termination mode with the top interface and an overlap termination mode with the bottom interface. Multichannel seismic data show clearly that, along the Rf2 and Rf1 interfaces, there are also mound reflection bodies (see Fig. 7), which have characteristics such as a strong-reflection top interface, and unknown reflection structure below the top interface because of

shielding, making them similar to those of the mound reflection bodies near the modern seafloor.

4.4. Side-scan sonar images Although there are only a few side-scan sonar images, they can still help us understand the sonar image characteristics of the reefs outcropping on the seafloor. On the side-scan sonar images, the outcropping reefs are very easy to distinguish from other geomorphic units. They exhibit a combination of irregular bright spots and shadows and show significant contrast to the surrounding flat seafloor (for which there is almost no reflection) (see Fig. 8). The bright spots and shadows are not the same size, indicating that these outcropping reefs are of different scales.

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Fig. 6. Single-channel seismic data (sparker) revealing the reflection facies on the continental shelf edge and upper slope. The location of the seismic profile position is shown in Fig. 1c. Rf1 and Rf2 are two main reflection interfaces (200 ms below the seafloor) on the continental shelf edge and upper slope. Their morphology is similar to that of the modern seafloor surface, and the sedimentary strata between the seafloor and the two reflection interfaces mainly show a high-angle progradational reflection inner structure ( 42°) and exhibit cutting reflection termination. R. marks coral reef outcropping on the seafloor, B.R. is the speculated buried coral reef, I–IV represent stage I–IV coral reefs, and the black triangles mark the positions of the modern and paleo shelf breaks.

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Fig. 7. Multichannel high-resolution seismic data revealing the mound reflection characteristics of buried reefs beneath the shelf break. The locations of the seismic profiles are shown in Fig. 1c. The meanings of R, B.R., and I–III are the same as those in Fig. 6. A and B are enlarged images of mound seismic facies near the Rf1 and Rf2 interfaces, respectively. The black fine lines are the reflection interfaces inside the sediments.

Fig. 8. Side-scan sonar image (a) and 3.5 kHz sub-bottom profile and interpretation (b) showing the acoustic characteristics of the submerged reefs below the continental shelf slope break. The location of the sonar image is shown in Fig. 1c, and the location of the sub-bottom profile is shown in (a). The meanings of the orange solid lines, green solid lines, and blue fine lines on the interpretation profile are the same as those in Fig. 5. The sub-bottom profile shows that the sedimentary strata on the upper slope had been eroded, which exposed the early-buried reefs. The side-scan sonar image shows that a part of these reefs are covered by thin sandy sediments. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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5. Discussion 5.1. Development stages and distribution of the submerged reefs Based on the seafloor terrain, the reflection facies on the acoustic profile, and the comparison with the terrain and acoustic characteristics of submerged coral reefs on other continental shelves and continental slopes, and with in situ coral reefs in the study area (although only at one station currently) being taken into account, we suspect that there are large areas of submerged reefs outcropping on the seafloor of the study area and that there are also buried reefs. Our preliminary estimation by using the bathymetric data shows that the total area of the submerged reefs outcropping on the modern seafloor is 4700 km2. This value might be far less than the actual one because of the lacking geophysical data in the northern part of the study area. The seismic data reveal that there are two seismic reflection interfaces (Rf1 and Rf2) in the sedimentary strata with thickness of 200 m (Vp E1,600 m/s) beneath the seafloor in the study area and that there are small mound reflection bodies along the above interfaces, whose reflection characteristics are comparable with the seismic reflection patterns of the coral reefs on other continental shelves and continental slopes (Huvenne et al., 2003; Van Rooij et al., 2011). The sub-bottom profile and bathymetric data indicate that coral reefs also exist in the shallow sedimentary strata near the seafloor throughout most of the study area. Some of them directly outcrop from the seafloor, whereas others are covered by thin sediments with thickness of 0–2 m. According to the precedence of strata where reefs exist, the submerged reefs can be divided into four stages as a whole: stages I–IV, respectively, from old to new. Stage I coral reefs are at the depth of 120–160 m beneath the seafloor and are distributed along the Rf1 reflection interface; because this is revealed only by a small number of seismic lines, we suspect that the coral reefs are

mainly distributed on the shoreward side of the shelf break line that is coetaneous with Rf1 (see Fig. 9). Stage II coral reefs are distributed along the Rf2 reflection interface, and the seismic data show that they are mainly distributed on the shoreward side of the shelf break line that is coetaneous with the Rf2; most of them are covered by sediments ∼50 m thick, but some reefs in the southern part of the study area outcrop from the modern seafloor (see Fig. 9). In the area with water depth of 200–220 m, there are dense outcropping submerged reefs on the seafloor. These are in an arc belt distribution in general, forming a dense coral distribution area with a width of ∼1 km. On the shallow continental shelf side of this area, most submerged reefs are in punctate or belt distribution, and the reefs not outcropping on the seafloor are covered only by sediments with a thickness of 0–2 m. The previous sampling shows that the thickness of the Holocene sediments here is usually only 1–2 m (Zhu, 1995; Xu, 1995; Feng et al., 1996; Chen et al., 2005). Therefore, we conclude that the above submerged reefs were formed during the LGM, belonging to the latest stage IV. In the upper slope with water depth of 300–420 m, there are some submerged reefs that are isolated or in discrete distributions. These reefs are in strata of different ages and outcrop on the modern seafloor as the strata erode. The strata where the above submerged reefs of different periods are located are later than Rf2 in time but earlier than the last glaciation, so the above submerged reefs are classified as stage III as a whole. The distribution of the submerged reefs in stage III cannot be determined because of the lack of sufficient geophysical data. The only fact about the reefs in stage III is that their ages are younger than that of Rf2 and older than that of the submerged reefs during the LGM. 5.2. Formation environment of submerged reefs The sub-bottom profile shows that the submerged reefs formed in the latest age are covered by sediments with a thickness of 0–

Fig. 9. Distribution of submerged or buried reefs in the study area obtained from summarization of geophysical data. The black dotted lines represent the distribution outer edge boundaries of stage I, II, and IV coral reefs. The submerged reefs outcropping on the seafloor belong to three stages, i.e., stage II, stage III, and stage IV. The outer edge of the submerged reef (III) is not given because of the lack of sufficient geophysical data.

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2 m, and the thickness of the Holocene sediments is usually not more than 2 m (Zhu, 1995; Feng et al., 1996; Chen et al., 2005). With the water depth at the location of the reefs ( 4160 m) being taken into account, we believe that the latest submerged reefs (stage IV) were formed in the LGM. A paleoceanographic study shows that the sea surface temperature (SST) in the northern SCS has been between 24 °C and 30 °C since 28 ka B.P. (Yang, 2008) and was between 24 °C and 29.3 °C in the last glaciation (Huang et al., 1997; Pelejero and Grimalt., 1999; Oppo and Sun, 2005; Wei et al. ,2007). This indicates that the seawater temperature during the glacial period was suitable for the development of coral reefs. The reflection termination relationships between the Rf1 and Rf2 interfaces and their top and bottom sedimentary strata indicate that these two reflections represent denudation surfaces formed during the low-sea-level period. Because the coral reefs formed in different periods have similar morphology and distribution characteristics, we surmise that the submerged reefs in the study area have roughly similar formation environments, and all of them were formed during the low-sea-level period. Usually, the early existing topographic high is favorable to the development of coral reefs (Shinn et al., 1977; Vora et al., 1996). As can be seen from the topographic map, the submerged reef distribution areas are underwater plateaus evidently protruding toward the sea. Because the reefs themselves are very hard, being bonded together with the surrounding loose sediments, the stability of the continental shelf edge is enhanced greatly (De Mol et al., 2009). Therefore, the existence of early coral reefs plays an important role in keeping the topographic high here, which is more favorable to the development of the later coral reefs. The sub-bottom profile and seismic data indicate that the ancient coral reefs of different periods were developed on sandy or muddy loose sediment substrates. Similar situations can also be found for some coral reefs near the shore of the Red Sea (Hayward, 1982), southern Thailand (Tudhope and Terence, 1994), and Australia’s Queensland area (Hopley et al., 1983) and GBR (Johnson and Risk, 1987), and for many cold-water coral reefs (Savini and Corselli, 2010). Based on the position relationship with the shoreline, coral reefs can be divided into three basic types: fringing reefs, barrier reefs, and atoll reefs (Laad, 1977). The submerged reefs in the study area might belong to fringing reefs or barrier reefs, but further work, such as detailed geophysical surveys and sampling, are needed to confirm this. 5.3. Factors controlling submerged reef development Suitable seawater temperature, light, transparency, salinity, and hydrodynamic environment are important conditions affecting the growth of coral reefs (Montaggioni, 2005), and such factors as sealevel change, tectonic subsidence or uplift, and river flows may generally lead to a change of the above conditions. Tectonically, the study area is located on the northern edge of Baiyun Sag, a substructure of the PRMB; according to the calculations from Zhao et al. (2011) and Liao et al. (2011), the tectonic subsidence rate in the study area has been very low (avg.  0.05 mm/y) since the Neogene, far less than the accumulation rate of coral reefs (2– 20 mm/y) (Macintyre et al., 1977; Glynn et al., 1979; Davies and Hopley, 1983; Schlager, 1999; Montaggioni, 2005). The tectonic subsidence may have some influence on the accumulation of the coral reefs, but based on the periodic occurrence of stage IV coral reef layers, the influence of the tectonic subsidence can basically be ignored. As mentioned above, the coral reefs formed during different periods have similar formation environments; that is, all of them were formed during the low-sea-level period. The seismic data show that the sedimentary strata split by the coral reefs formed

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during different periods have high-angle oblique crossing progradational reflection structure or “S”-shaped progradational structure. This kind of reflection structure was mainly formed in the regressive period and suggests a relatively high sediment supply rate (Vail et al., 1977). Therefore, in the regressive period, the environment was not favorable to the development of coral reefs owing to the large amount of sediments in seawater. Whereas in the low-sea-level period and at the early stage of sealevel rising, the sea level was relatively stable, because of the high terrain around the study area, we speculate that the ancient Pearl River would not enter into the sea from here and there would be no fresh water and less sediment around the study area. Therefore, the marine environment was favorable to the development of coral reefs. With the rise in sea level, coral reefs would stretch toward the land until the marine environment was altered to stop their development, and their development pattern might be similar to that of drowned reefs on the other outer shelf edges in the Indo-Pacific province (Montaggioni, 2005). The coral reefs became submerged reefs and were then covered by later sediments. When the next low-sea-level period was reached, roughly the same story was repeated. Obviously, the sea-level change in the northern SCS controlled the periodic development of the coral reefs in the study area (see Fig. 10). Because the growth time of coral reef layers (mainly in the lowest-sea-level period, the early stage of sea-level rising) is relatively short, and because the coral reefs were developed on unconsolidated sandy or muddy substrates, the thickness of the preserved coral reefs that we can observe now is small (Tudhope and Terence,1994). 5.4. Significance of the submerged reefs on the sedimentary environment 5.4.1. Lowest SEA level during the LGM The lowest-sea-level position in the northern SCS during the LGM has been debated for a long time (Huang et al., 1995). Three main viewpoints have been presented: the paleo shoreline at that time was (1) 120–130 m lower than the current sea level (Chen et al., 1990; Qian, 1994; Chen,1998), (2) 150 m lower than the current sea level (Zhang and Zhao, 1990; Bao, 1995), and (3) at least 180 m lower than the current sea level (Zhu, 1995). Coral reefs are good indicators for the ancient sea level and its change (Steinen et al., 1973; Chappell, 1974; Lighty et al., 1982; Hopley, 1986; Toscano and Lundberg, 1999; O’Leary et al., 2008; Fürstenau et al., 2010). The study area has a stable tectonic setting, with a small tectonic subsidence rate, so the maximum water depth position of the latest submerged reefs offers position

Fig. 10. Speculated schematic diagram of the periodic development pattern of the submerged reefs under the control of sea level on the continental shelf edge and upper slope in the study area. The submerged reefs (I, II, and IV) were mainly developed during the low-sea-level period, and the outermost edge is likely in the vicinity of the continent shelf slope break. The sea-level curve is referenced to Qin (1996) and there is no low-sea-level period between 0.6 and 1.5 Ma when the submerged reefs (III) developed.

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information of the paleo shorelines during the last glaciation. The bathymetric data and sub-bottom profile reveal that the outermost edge of the submerged reefs formed during the last glaciation is located near the current water depth of 220 m; therefore, the lowest-sea-level shoreline during the last glaciation may be located at a position on the shelf with a modern water depth of 200– 220 m (according to the average 0–20 m from coral reefs to sea surface). Other indicators for the paleo shoreline, such as beach rocks, were also found in this range (Liu et al., 1993). However, for such a broad continental shelf in the northern SCS, the hydrostatic equilibrium resulting from seawater and sediment loads after transgression should be taken into account (Huang et al., 1995). We use the formula from Newman et al. (1980) to calculate the shelf subsidence (ΔH) resulting from hydrostatic equilibrium:

(2)

ΔH = (Tw ρW + Ts ρS )/ρM , where Tw is the current water depth, ρW is the seawater density, taken as 1.03 g/cm3, Ts is the thickness of sediment, taken as 2 m, ρS is the bulk density, taken as 1.9 g/cm3, and ρM is the mantle density, taken as 3.3 g/cm3. According to the above formula, the shelf subsidence at a water depth of 220 m is about 68 m. Thus, the paleo shoreline should be in the range of 152 to 132 m, which means that the sea level during the LGM had relatively dropped by ∼132–152 m. 5.4.2. Low-sea-level events occurring in the northern SCS since the quaternary The reflection termination relationships show that Rf2 and Rf1 are denudation surfaces formed during the low-sea-level period (Vail et al., 1977). Based on the study on the sea-level change during the PRMB in the northern SCS in the Cenozoic (Qin, 1996), we propose ages of the Rf2 and Rf1 of ∼600 and ∼1450 ky B.P. Analysis of the growth environment for the coral reefs in the study area indicates that reef formation events may correspond to lowsea-level events and that the three reef formation events at stages I, II, and IV should correspond to the low-sea-level events occurring at ∼1450, ∼600, and ∼20 ky B.P. This result is consistent with the study result of the ancient sea-level change of the Yongshu Reef in the southern SCS (Zhao et al., 1996). However, the development of stage III coral reefs indicates that, between 600 and 20 ky B.P., there might be multiple high-frequency sea-levelchange events, and the relative drop in amplitude of sea level was equivalent to or larger than that during the last glaciation. Nonetheless, the outermost edge of the coral reefs moved gradually toward the sea from old to new as a whole, which also coincides with the overall downward trend of the sea level since the Quaternary.

6. Conclusions Through comprehensive analysis of the geophysical data including bathymetric, sub-bottom profile, and single and multichannel seismic data and side-scan sonar images in the study area, and comparison of these data with the terrain and acoustic characteristics of the submerged reefs on other continental shelves and continental slopes, we confirmed the existence of submerged reefs on the continental shelf edge and upper slope in the northern SCS (E114°26′–115 °15′, N19°52′–20 °29′). The following conclusions have been obtained: (1) The outcropping submerged reefs in the study area have a total area of possibly 4700 km2 and are mainly in mound shape. The reefs with outcropping height ofo 5 m from the seafloor are mainly in punctate or belt shape, and most of

(3)

(4)

(5)

them are located in the area with a water depth ofo 220 m. The reefs with outcropping height of 45 m are mainly distributed in a SW–NE trending arc belt at a water depth of 220– 420 m. The submerged reefs exhibit three kinds of typical seismic facies on their acoustic profile: punctate strong reflection, planar strong reflection, and mound strong reflection. Through summarization of multiple kinds of geophysical data, the submerged reefs identified in the study area can be divided into four stages (I–IV). The earliest stage I buried reefs were developed along the Rf1 interface and had been completely covered by sediments. Stage II coral reefs were developed along the Rf2 interface, with only a small number of them outcropped on the southern seafloor in the study area, and most of them were covered by sediments. Stage III submerged reefs outcropped on the seafloor as a result of erosion to the overlying strata. Stage IV submerged reefs mainly outcropped on the modern seafloor. The submerged reefs were developed periodically and controlled significantly by the sea-level change, with reef formation events occurring during low-sea-level periods. The three reef formation events at stages I, II, and IV correspond to the low-sea-level events at 1450, 600, and 20 ky B.P., respectively. Based on the maximum water depth of the submerged reef distribution in the LGM of 220 m, we estimated that the relative sea-level drop was ∼132–152 m. Further studies, such as geophysical surveys and sampling, need to be conducted to clarify the accurate distribution range, type, and precise formation age of the submerged reefs in the study area.

Acknowledgments We thank all the scientists, technicians, and crew who participated in the three geophysical voyage surveys. This paper is funded by the National Science and Technology Major Project no. 2011ZX05056-001-02 and Natural Science Foundation of China (NSFC) Project no. 41106064.

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