余克服2,3,4,,,
王瑞2,3,4,
颜廷礼2,3,4
1. 广西大学资源环境与材料学院, 广西 南宁 530004
2. 广西大学广西南海珊瑚礁研究重点实验室, 广西 南宁 530004
3. 广西大学珊瑚礁研究中心, 广西 南宁 530004
4. 广西大学海洋学院, 广西 南宁 530004
基金项目: 国家重大科学研究计划项目(批准号：2017YFA0603300)和广西科技计划项目(批准号：AA17204074和AD17129063)共同资助
详细信息
作者简介: 刘文会, 女, 30岁, 硕士研究生, 从事珊瑚礁生态环境研究, E-mail:whliu@cug.edu.cn
通讯作者: 余克服, E-mail:kefuyu@scsio.ac.cn
中图分类号: P722.7;P731.23收稿日期:2019-12-17
修回日期:2020-03-08
刊出日期:2020-05-25
Uranium-series ages of Beigang beachrock at Weizhou Island and their significance in recording sea level variations
Liu Wenhui1,2,3,4,,Yu Kefu2,3,4,,,
Wang Rui2,3,4,
Yan Tingli2,3,4
1. College of Resources, Environment and Materials, Guangxi University, Nanning 530004, Guangxi
2. Guangxi Laboratory on the Study of Coral Reefs in the South China Sea, Guangxi University, Nanning 530004, Guangxi
3. Coral Reef Research Center of China, Guangxi University, Nanning 530004, Guangxi
4. School of Marine Sciences, Guangxi University, Nanning 530004, Guangxi
More Information
Corresponding author: Yu Kefu,E-mail:kefuyu@scsio.ac.cn
MSC: P722.7;P731.23--> Received Date: 17 December 2019
Revised Date: 08 March 2020
Publish Date: 25 May 2020
摘要
摘要:海滩岩是古海平面高程的重要标志。本文利用高精度铀系测年技术对北部湾涠洲岛北港海滩岩中保存完好的珊瑚枝进行了测年，得出珊瑚枝的年龄分布范围介于距今(2017 A.D.)1780~836年，表明涠洲岛北港海滩岩是晚全新世时期的产物。电子探针元素含量分析得出该海滩岩的胶结物是低镁方解石，其成岩环境为大气淡水环境，但相对高含量的锶则指示其成岩过程中经历了海水的冲刷作用。综合铀系年代、海滩岩的高程及邻近区域的古海平面数据，涠洲岛北港晚全新世时期的海滩岩指示出距今1780~836年海平面高于现今海平面约为0.91~1.16 m，为晚全新世时期高海平面提供了新的信息。
关键词: 海滩岩/
铀系年代/
海平面/
新构造/
晚全新世/
涠洲岛
Abstract:Beachrock is an important indicator of the sea level in the past period. This paper studied the age and sedimentary feature of the beachrock at Beigang Village, Weizhou Island. Weizhou Island, which is located in the middle of Beibu Gulf, northern part of South China Sea, is at an elevation of 20~40 m. The coastline around the island is about 20 kilometers long, and coral reefs distribute in the north, east and southwest of the island at a depth of 2 m to 14 m. This paper examined the beachrock's sedimentary structure, clastic composition, organic components and relative elevation, and measured its contents of main elements(Mg, Na, Fe, Sr and Mn) using EPMA(Electon Probe MicroAnalysis) technology. The results showed that the cement of the beachrock is LMC(low magnesium calcite), indicating that it was primarily formed in atmospheric fresh water. However, the relatively high content of strontium indicated that seawater also contributed to the diagenetic process.
This paper also collected four well-preserved coral branches from the beachrock, and dated them with U-Tu technology. Although the U-Tu age of coral branches represents the age of coral growth, the well-preserved surface structure of the coral branches indicated a series of process growth, breakage, transportation to beach, sedimentation to intertidal zone, and cementing in the beachrock would not last long. Therefore, the ages of coral branches in the beachrock of Weizhou Island can approximately represent the maximum age of the beachrock formation. To avoid any possible interference factors, the coral branches were finely polished before being dated with U-Tu technology. The results showed that the ages of these coral branches- and thus the approximate maximum age of the beachrock-ranged from 1780 a B.P. to 836 a B.P.(years before 2017 A.D.), in the Late Holocene.
The elevation of the beachrock was surveyed relative to the modern sedimentary zone, where beachrock is most likely to form. The results showed that the bottom and top layers of the beachrock are 0.91 m and 1.16 m above the modern sedimentary zone, respectively. This paper, therefore, concludes that the sea level at 1780~836 a B.P. was about 0.91~1.16 m higher than it is at present. Considering that the width of the mentioned present sedimentary zone ranges from 1.5 m to 2.5 m, and that the corresponding elevation is 0.4 m to 0.7 m, the paper suggests the uncertainty of the estimated sea level elevation to be ±0.35 m.
This reconstructed sea level is consistent with a previous publication from Leizhou Peninsula, which is about 110 km away from Weizhou Island. The geological structure of Leizhou Peninsula has been stable since the mid-Holocene, and the sea level at Leizhou Peninsula was 80cm higher than present(+80 cm) at 1700 a B.P., +104 cm at 1500 a B.P., and +128 cm at 1235 a B.P.
In summary, this paper concluded that the relative sea level at 1780~836 a B.P. was about 0.91~1.16 m higher than the present sea level, and that U-series dating of coral branches is a good method for obtaining the forming age of beachrock.
Key words:beachrock/
U-series dating/
sea-level/
neotectonics/
Late Holocene/
Weizhou Island
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