Distribution and morphological features of the patch reefs at Xiaonanxun Reef, Zhenghe Atoll of the Nansha Islands, South China Sea
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摘要: 点礁是在空间上呈点状分布的礁体，是一种重要的珊瑚暗礁地貌类型，南海南沙群岛北部海区的一系列环礁发育有众多的珊瑚点礁。目前，对点礁地貌的形态特征、发育过程和机制的研究相对匮乏，南海地区的珊瑚点礁地貌定量研究尚为空白。以南沙群岛北部郑和环礁西南端的小南薰礁发育的点礁为研究对象，利用多波束水深实测数据结合Google Earth WorldView-2遥感影像，对所识别的123个点礁的顶底水深、高度、顶底直径等形态参数进行量测与统计分析,结果表明，其底部最大水深为35.05 m，顶部最小水深为7.89 m，平均高度2.62 m。其中，88个点礁（占71.5%）分布于水深25～35 m（深水点礁），27个（占17.9%）分布于水深10～18 m（浅水点礁）。点礁顶部和底部直径的平均值分别为8.49 和19.13 m，形状因子和顶底关系因子的平均值分别为0.15和0.42，其剖面形态呈峰丘型（21.1%）和礁坪型（占78.9%），且多呈孤立型分布，亦见少数以群落型分布。浅水点礁以礁坪型为主，其顶部和底部直径较大，而形状因子较小，反映了波浪动力的控制作用；峰丘型仅见于深水点礁，其形状因子较大（＞0.23），顶底关系因子较小（＜0.25），高度和直径呈正相关，反映了珊瑚生长作用对点礁地貌的影响。点礁底部直径集中呈NEE-N向延伸，显示了海域盛行风向与海水运动方向的控制作用。Abstract: Coral reef is an efficient indicator for paleoenvironment reconstruction, as it may provide abundant information on paleoclimate changes, sea-level fluctuations and marine environmental variations. Patch reef is a kind of submerged reefs, widely distributed in isolation on reef flat in patch shape in the atolls of the northern Nansha Islands area of the South China Sea. However, the morphological features and formation mechanisms of these patch reefs remain unclear up to present. In this paper, Google Earth WorldView-2 images and multi-beam bathymetric data are processed and carefully studied for the Xiaonanxun Reef, which is located in the southwestern Zhenghe Atoll. Patch reefs are widely distributed there in secondary lagoons, tidal channels, lagoonal slopes and basin floors. Based on the data mentioned above, 123 patch reefs are recognized and measured. The maximum depth of the rim and the minimum depth of the top of the patch reefs are 35.05 m and 7.89 m, and the average height, bottom diameter and top diameter of the patch reefs are 2.62 m, 19.13 m and 8.49 m on average respectively. Most of the patch reefs is distributed in isolation in shapes of “pancake” and “knoll”, and only 3 groups are found combined as colony developed in the area in the water depth of 29 ~ 35 m. The patch reefs developed in the water depth less than 18 m are predominated by the “pancake” type, with the average height and the bottom and top diameters in 1.85 m, 21.98 m and 11.79 m, respectively. The shape index is rather small (less than 0.23), but the relational factor between top and bottom is rather large (87% between 0.30 and 0.75). Wave dynamics are believed the dominant factor to the morphological features. In contrast, “knoll” type of patch reefs is mainly developed in the area under water depth deeper than 25 m, and the average height and bottom and top diameters are 2.74 m, 17.44 m and 6.89 m respectively. The bottom diameter and height of patch reefs are positively correlated and the correlation coefficient R is up to 0.827, which indicates the biological processes of coral growth is the dominant factor to the morphological features. Furthermore, the long axis direction is mainly in NEE to N direction for 82% of the bottoms of the patch reefs, being consistent with the prevailing wind wave direction and perpendicular to the reef crest, which indicate the impacts of the wave dynamic processes as well.
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Keywords:
- submerged reef /
- patch reef /
- morphological fatures /
- wave dynamics /
- Nansha
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图 1 郑和环礁与小南薰礁的位置与地形
a为南海诸岛，据自然资源部标准地图<审图号：GS（2020）4617号>，图中黑色方框为b范围；b为郑和环礁，据Dong Y等[22]的水深、干出礁和暗礁的范围改绘。
Figure 1. Location and topography of Xiaonanxun Reef and Zhenghe Atoll in the Nansha Islands, South China Sea
a shows the islands in South China Sea, after the Standard Maps of Ministry of Natural Resources of the People’s Republic of China: GS(2020)4617.b shows the Zhenghe Atoll, modified from reference [22].The black box in a shows the location of b.
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图 3 小南薰礁地貌类型和代表性点礁位置
图中黑色方框2为图4范围；蓝色方框1为图6a位置；绿色方框3为图6b位置；绿色和蓝色框中的白色直线为图6中两个剖面的位置。
Figure 3. Geomorphological classification and locations of the representative patch reefs of Xiaonanxun Reef
The black box 2 shows the location of Fig.4, the blue and green boxes show the locations of Fig.6a and 6b, the white lines inside of the green and blue boxes show the location of profiles in Fig.6.
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图 4 水深图(a)和坡度图(b)中的点礁
位置见图3黑色方框1；黑色曲线表示点礁的外边界，蓝色曲线表示点礁的顶部，紫色线为其底部直径，青色线为其顶部直径。
Figure 4. Patch reefs in bathymetric map (a) and gradient map (b)
See black box 1, Fig.3 for location. Black lines are the outer boundaries of patch reefs, blue lines define the tops of the patch reef, purple lines define the basal diameters of patch reefs, while the cyan line shows the top diameters of patch reefs.
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图 5 小南薰礁的点礁形态参数分布特点
Figure 5. The distribution pattern of morphological parameters of Xiaonanxun Reef
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图 6 典型礁坪型点礁(a)和峰丘型点礁(b)及纵剖面（c)
位置见图 3。
Figure 6. Typical “pancake” (a) , “knoll” patch reef (b) and the profiles(c)
See Fig.3 for locations.
表 1 小南薰礁地貌形态特征及其典型影像
Table 1 Morphological characteristics of Xiaonanxun Reef and representative images
地貌名称 形态特征 面积/km2 所占比例/% 典型影像 干出礁 水深较浅、破浪带激浪作用活跃，造礁石珊瑚等生长较好，部分低潮时出露[18,28] 0.89 19.4 
次成潟湖 礁坪上的低洼地，低潮不出露，以生物碎屑堆积为主[17,28] 0.45 9.8 
潮汐通道 相对低洼、穿过礁坪的槽道，是潟湖与外海水体交换的通道，一般较少碎屑沉积，与“口门”相比，规模更小，水深更浅[17,29,33] 0.38 8.3 
脊槽 线性的脊和槽相间组成，影像中绿色为脊部，一般为珊瑚聚集生长区；槽部为白色，一般为珊瑚贝壳碎屑堆积区[14,30,33] 0.89 19.4 
珊瑚礁垄* 带状凸起区域，与脊槽相比，规模更大，主要由活珊瑚覆盖 1.98 43.1 
注：*根据其形态特征，参照其他地貌名称进行定义。 
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表 2 小南薰礁的点礁地貌形态参数统计
Table 2 Statistics of morphological parameters of patch reefs of Xiaonanxun Reef
形态参数 平均值 最小值 最大值 点礁
（123个）顶部深度H1/m 24.10 7.89 33.34 底部深度H2/m 26.71 10.42 35.05 高度H/m 2.62 0.30 10.35 底部直径D/m 19.13 3.54 71.11 形状因子H/D 0.15 0.03 0.47 顶部直径d/m 8.49 1.31 50.06 顶底关系因子d/D 0.42 0.13 0.89 深水点礁（88个） 顶部深度H1 /m 27.82 19.70 33.34 底部深度H2 /m 30.56 25.17 35.05 高度H/m 2.74 0.41 10.35 底部直径D/m 17.44 3.54 71.11 形状因子H/D 0.16 0.04 0.47 顶部直径d/m 6.89 1.31 29.83 顶底关系因子d/D 0.39 0.13 0.77 浅水点礁（27个） 顶部深度H1 /m 13.88 7.89 16.89 底部深度H2 /m 15.72 10.42 17.71 高度H/m 1.85 0.30 4.06 底部直径D/m 21.98 4.95 60.12 形状因子H/D 0.11 0.03 0.21 顶部直径d/m 11.79 2.26 50.06 顶底关系因子d/D 0.49 0.24 0.89
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表 3 小南薰礁的点礁形态参数相关分析结果
Table 3 Correlation of morphological parameters of patch reefs of Xiaonanxun Reef
相关系数 顶部深度 底部深度 高度 底部直径D 形状因子 顶部直径d d/D 所有点礁 顶部深度 底部深度 0.960** 高度 −0.154 0.127 底部直径D −0.361** −0.165 0.697** 形状因子 0.163 0.264** 0.358** −0.254** 顶部直径d −0.444** −0.299** 0.519** 0.878** −0.239** d/D −0.330** −0.311** 0.070 0.186* 0.041 0.558** 深水点礁 顶部深度 底部深度 0.776** 高度 −0.603** 0.036 底部直径D −0.524** −0.002 0.827** 形状因子 0.190 0.072 0.392** −0.094 顶部直径d −0.535** −0.124 0.691** 0.844** −0.068 d/D −0.144 −0.167 0.017 −0.008 0.094 0.481** 浅水点礁 顶部深度 底部深度 0.824** 高度 −0.346 0.039 底部直径D −0.125 0.137 0.666** 形状因子 0.123 0.066 0.159 −0.669** 顶部直径d −0.153 0.090 0.620** 0.936** −0.549** d/D −0.270 −0.150 0.339 0.413* 0.041 0.644** 注：**在0.01水平（双侧）上显著相关，*在0.05水平（双侧）上显著相关。
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