| 摘要: |
| 对采自西沙赵述岛和北岛的沉积剖面ZS2、BD1进行分析，两个剖面具有十分相似的物源组成和岩性变化。通过测定剖面中海鸟残体的14C年龄，运用传统方法以及Clam和Bacon程序建立了ZS2和BD1沉积剖面的年龄-深度模型。对比年龄-深度模型发现，Bacon程序运用MCMC作为迭代算法，排除异常数据影响，拟合得到的曲线单调相比较于Clam年龄模型显得更为合理。所建立的年代学模型为后期的气候环境演变研究提供了基础。根据Bacon程序所构建的年龄-深度模型结果，我们发现近千年来ZS2和BD1剖面处的平均沉积速率分别为0.86 mm ∙ a−1和1.26 mm ∙ a−1。采样点的沉积速率在一定程度上代表了岛屿的垂向生长速率，均低于当前海平面的平均上升速率。同时，鸟骨的14C年龄也揭示了赵述岛海鸟活动时间应不晚于AD 1172 — 1290，而海鸟开始在北岛活动的时间则不晚于AD 984 — 1140。 |
| 关键词: 14C定年 海鸟遗迹 沉积速率 赵述岛 北岛 西沙群岛 |
| DOI：10.7515/JEE182001 |
| CSTR：32259.14.JEE182001 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41402148，41376124）；中央高校基本科研业务费（JZ2017HGTB0201）；中国博士后科学基金（2014M550338） |
| 英文基金项目:National Natural Science Foundation of China (41402148, 41376124); Fundamental Research Funds for the Central Universities (JZ2017HGTB0201); China Postdoctoral Science Foundation (2014M550338) |
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| Age models of seabird remains from Zhaoshu and Beidao islands in the Xisha Archipelago and its implications |
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ZHAO Jinjun, XU Liqiang, WU Libin, LIU Xiaodong, CHEN Lin
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1. School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
2. School of Earth and Space Science, University of Science and Technology of China, Hefei 230026, China
3. Xisha Deep Sea Marine Environment Observation and Research Station, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Sansha 573199, China
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| Abstract: |
| Background, aim, and scope Some environmental and ecological records from several islands of the Xisha Archipelago, South China Sea, have been reconstructed in our earlier study. However, little is known about history of the Qilianyu in the Xisha Islands so far. Zhaoshu and Beidao are the two typical coral islands in Qilianyu of the Xisha Archipelago. To reconstruct history of the two islands, chronology analysis is a prerequisite. Well-preserved seabird subfossils were recovered from the ornithogenic sediments. It is aimed to establish chronology for two sediment profiles collected from Beidao and Zhaoshu islands based on radiocarbon dating of seabird remains. Materials and methods Ornithogenic sediments were observed during our field investigations into the Qilianyu, Xisha Islands in the year 2015. Two sediment profiles, namely ZS2 and BD1, were collected and analyzed from Zhaoshu and Beidao islands, respectively. They are 94 cm (ZS2) and 137 cm (BD1) in length. Both the two profiles consisted of three sediment units. Similarity in lithology of the cores suggests that the two islands may have a similar history and be restricted by identical environmental factors. A large number of seabird subfossil remains, including guano particles, eggshells, fish scales and bones etc., were observed and recovered in the bulk ornithogenic sediments. These biological materials imply historical occupation of these islands by seabirds. Avian subfossils were separated from the bulk sediments, and bone samples were used to investigate island history via radiocarbon (14C) analysis. Results 14C ages increased with depth for each of the profiles, suggesting well-preservation of the sediments. The 14C ages based on seabird remains are much younger than previously reported 14C ages from carbonate materials, and this could be explained by secondary transport of the carbonates. Seabird-based 14C ages reveal that the collected samples were accumulated within approximately the last 1000 years. Direct mathematic fitting, R-language based Bacon and Clam programs were employed to establish age-depth models of the two profiles. It is found that the age-depth model constructed by Bacon is more suitable for age-depth modelling than mathematic fitting models and Clam package model. Discussion The seabird subfossils in the bulk ornithogenic deposits reflect gradual accumulation of the sediments, as well as upward growth of the islands. According to the age-depth models for the two sediment profiles based on Bacon program package, it is found that the mean sedimentation rates of the two profiles within the last 1000 years are 0.86 mm ∙ a−1 and 1.26 mm ∙ a−1,respectively. This partly represents growth rates of the islands, which are significantly lower than the rate of sea level rising in the present day. This implies a potential threat to island ecosystem. In addition, the initial activity of seabirds on the Zhaoshu Island is no later than AD 1172 — 1290, and the first occupation of the Beidao Island by birds began no later than AD 984 — 1140. Conclusions The ornithogenic sediments from Zhaoshu and Beidao islands have been well-preseved and are ideal materials for studying history of the islands. Age-depth models based on Bacon program are suggested for further studies concerning the two islands. Mean upward growth rate of the islands within the last millennium is lower than that of present sea level rise. Seabird began to inhabit Qilianyu at approximately 900 — 1000 years ago. Recommendations and perspectives Chronology is essential in island history study of the Xisha Archipelago. The established chronology in the present study provides basis for further climatic, environmental and ecological records of the two islands. This study also suggests a potential threat from sea level rise on the islands, as the rising rate of sea level is greater than growth rates of the two islands, implying an underlying risk of global warming on reef islands. |
| Key words: radiocarbon dating ornithogenic sediments sedimentation rate Zhaoshu Island Beidao Island Xisha Archipelago |
