Morphodynamic characteristics and genetic mechanism of the submarine sandbanks in Xuande Atoll, Xisha Islands
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摘要:
海底地形地貌研究可以揭示海底表面形态特征，探索海底物质结构及其形成、演化和分布规律。通过单波束测深、单道地震、海底表层沉积物取样和潮汐水文水动力实测等方法，开展了宣德环礁水文要素和水动力条件定量化研究，对宣德环礁水下沙洲动力地貌特征及动力作用下形成机制进行了探讨。研究结果表明：宣德环礁潟湖内水深50～55 m及60 m发育的礁滩体和埋藏珊瑚礁，与全新世50～60 m的低海平面时期发育的系列特征地貌应为同时期形成，水深50～55 m处的地貌边界很可能保留了早全新世时期古潟湖礁盘地貌轮廓形态；宣德环礁潟湖发育5类动力地貌特征形态的水下沙洲，环礁底层涨落潮流是塑造水下沙洲的主要现代动力因素。在常态天气下，涨落潮流输运的砂质沉积物足以在原有地形的基础上沉积和发育形成现今水下沙洲地貌形态。影响水下沙洲发育的主要因素由强至弱依次为气候变化引起的海平面升降，地形基础（珊瑚礁格架、礁盘岸线、口门）和涨落潮流。
Abstract:Submarine geomorphology can reveal the topographic characteristics of seafloor, which is helpful to understand the composition, formation, evolution, and distribution of the sediment. The hydrologic elements and hydrodynamic conditions in Xuande Atoll, Xisha, Islands, South China Sea were quantitatively investigated by the combination of single beam sounding, single-channel seismology, sampling of surface sediments, and hydrodynamic measurements. Furthermore, the morphodynamic characteristics and genetic mechanism of submarine sandbanks were discussed in detail. Results show that the coral reef frame and buried corals that developed in water depth of 50～55 m and 60 m in the atoll lagoon were formed in the same period of typical landforms that formed in the 50～60 m depth during the lower sea level period of the Holocene in South China Sea. It is likely that the boundary of the submarine geomorphology developed in water depth of 50～55 m retained the paleo-lagoon reef morphology of the Early Holocene. Five types of submarine sandbanks with own morphodynamic characteristics were identified in the atoll lagoon where tidal currents at the bottom are the main modern dynamic factors that shape the submarine sandbanks. Under normal weather condition, tidal currents in the study area are strong to transport sandy sediments and shape the geomorphology of present submarine sandbanks on the previous topography formed during sea level fluctuations. The sea level fluctuations are induced by climate change, followed by topographic foundation (coral frame, reef shoreline, reef gate) and tidal currents, and are the main factors that control the development of submarine sandbanks in the study area.
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Key words:
- submarine sandbanks /
- morphodynamic /
- hydrodynamic conditions /
- genetic mechanism /
- Xuande Atoll
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表 1 研究区6个调查站位实测涨落潮流最大流速、流向及平均流速、流向
Table 1. The measurements of the maximum, average flow velocity, and flow direction of flood and ebb tides at six survey sites in the study area
站号 水深/m 流速/(m/s) 流向/(°) 表层 中层 底层 表层 中层 底层 CL1 24 涨潮最大 0.13 0.16 0.14 33.2 308.8 80.3 落潮最大 0.12 0.08 0.14 330.8 241.8 50.5 涨潮平均 0.07 0.07 0.05 130.6 259.3 164 落潮平均 0.05 0.04 0.06 132 174.8 161.4 CL2 50 涨潮最大 0.35 0.38 0.42 55.5 28.8 348.7 落潮最大 0.32 0.36 0.39 98.8 92.4 327.7 涨潮平均 0.19 0.21 0.23 154.7 144.4 227.5 落潮平均 0.12 0.25 0.25 188.7 236.8 220.2 CL3 16 涨潮最大 0.46 0.31 0.4 222.7 108.1 195.4 落潮最大 0.39 0.31 0.29 239.7 262.6 155.8 涨潮平均 0.29 0.25 0.19 164 163.6 150.6 落潮平均 0.26 0.16 0.11 216.8 212.2 209.6 CL4 30 涨潮最大 0.44 0.46 0.47 229.7 218.9 216.3 落潮最大 0.48 0.48 0.49 209.3 206.6 91.8 涨潮平均 0.24 0.25 0.21 230.5 233.2 0.2 落潮平均 0.15 0.19 0.2 101.5 158.7 154.3 CL5 22 涨潮最大 0.49 0.6 0.47 70.5 69.3 60.4 落潮最大 0.56 0.51 0.44 57.9 78.3 53.9 涨潮平均 0.25 0.28 0.23 145.9 122.8 133.1 落潮平均 0.29 0.32 0.26 70.9 89.7 97.7 CL6 56 涨潮最大 0.3 0.27 0.3 127.1 94.5 91.7 落潮最大 0.26 0.29 0.31 295.4 83.1 300.5 涨潮平均 0.19 0.17 0.17 188.9 199.2 202.2 落潮平均 0.14 0.11 0.12 191.7 162.7 206.5 
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表 2 研究区6个调查站位实测底层流参数
Table 2. Parameters of bottom flows measured at six survey sites in the study area
站号 最大流速/(m/s) 流向 平均流速/(m/s) 流向 CL1 0.14 E 0.06 SE CL2 0.42 NW 0.24 SW CL3 0.4 S 0.15 SE/S CL4 0.49 SW/E 0.21 N/SE CL5 0.47 NE 0.25 SE CL6 0.3 E/W 0.15 SW
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表 3 研究区水下沙洲参数
Table 3. Parameters of submarine sandbanks in the study area
水下沙洲 长/km 宽/km 水深/m 走向 A1 17 1 45~50 NW-SE A2 12.1 0.8 45~50 NW-SE B1 5.4 2 45~50 NE-SW B2 1.9 0.9 45~50 W-E C 10.3 4.8 43~53 NE-SW D1 4.7 3.1 45~55 W-E D2 1.6 1.1 45~55 W-E E1 11.5 1.3 50~55 NW-SE E2 5.4 0.9 50~55 NW-SE
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表 4 表层沉积物粒度参数
Table 4. Parameters of submarine surface sediments in the study area
站号 水深/m 平均粒径/ϕ 分选系数 偏态 峰态 a1 52 2.96 2.38 0.12 1.5 a2 50 3.14 1.37 0.33 1.39 b1 49 3.13 2.25 0.42 1.43 b2 50 0.42 2.28 0.64 0.92 c1 44 0.15 1.54 0.55 0.64 c2 48 1.13 1.61 -0.37 1.2 c3 46 0.96 2.16 0 0.88 e1 55 1.1 1.54 -0.31 0.69 e2 51 1.43 3.5 0.91 0.85
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