| 计画期限 | 2012-12-31 ~ 2013-12-30 |
|---|---|
| 研究名称 | 东沙环礁全新世地质演化研究 |
| 受讬单位 | 台湾珊瑚礁学会 |
| 计画主持人 | 宫守业 |
| 协同主持人 | 王士伟 |
| 参与计画人员 | 李韦廷 |
| 关键词 | 东沙环礁、东沙岛、国家公园、全新世、南中国海 |
| 摘要 | 一、研究缘起 东沙环礁位于广东外海，高出周围海床约400公尺，直径约25公里，面积约500平方公里。行政院于民国95年12月19日正式核定成立东沙环礁国家公园，为我国第一座海洋型国家公园。虽然东沙环礁极具生态保育及学术研究的价值，但目前对东沙的珊瑚礁地质了解仍然十分有限。 二、研究方法 为了进一步了解东沙环礁及东沙岛的环境与地质作用的变化，本计划以地质钻探的方式，分别取得东沙岛之全新世地层岩芯标本3孔以及东沙环礁礁台岩芯1孔、分析岩心组成，并进行碳十四或铀钍同位素定年，以建立东沙环礁及东沙岛自全新世之地层纪录。 三、 研究成果 1. 本计划执行东沙岛上地质钻探岩芯3孔。DS-2位于东沙岛西端，岩芯长度分别为22公尺，于14.9公尺钻入更新世。DS-3位于接近东沙岛东半部的中央，岩芯长度为20公尺，于17.25公尺钻入更新世。DS-4位于东沙岛东端，岩芯长度为23.3公尺，于20.6公尺钻入更新世。本计划执行东沙环礁北侧礁台钻探岩芯1孔，岩芯长度为21.2公尺，于12.3公尺钻入更新世。 2. 本计划执行东沙岩芯描述、XRD矿物分析19件，其中3件已再结晶，16件为保存原始矿物组成之珊瑚化石。 3. 本计划执行东沙岛上岩芯中全新世珊瑚化石之AMS碳十四定年12件。东沙岛上最近之年代为 785± 95 cal BP，最早之年代为 8150± 130 cal BP。环礁上最近之年代为6430± 90 cal BP，最早之年代为 7915± 85 cal BP。 4. 东沙岛形成方式完全为沈积物堆积而成的沙洲岛，并非由珊瑚礁构成的基底。东沙岛的形成在礁台内侧（潟湖侧），并非因为该处基底之水深较浅，而是该处沈积物堆积较快。礁台堆积方式大致为上积(aggradation)，并无由外向内或由内向外的前积(progradation)现象。 5. 东沙岛更新世古地形面由西向东下降，由东沙岛西侧礁台边缘的-13.6mMSL逐渐下降到东沙岛东端的-17.1 mMSL。这个现象符合Purdy(1974)提出的假设，全新世环礁的基底在更新世时期受喀斯特作用影响，形成外缘高于内侧的环状古地形。 6.DS-5位于东沙环礁礁台北侧边缘之礁顶(reef crest)，但并未钻遇珊瑚礁架构，该地之礁台仍为碎屑堆积所形成。珊瑚礁架构可能在更为外缘处。由DS-5岩芯中最老的珊瑚年代推估，礁台北侧珊瑚礁大约在八千年前已经开始形成，略早于礁台西侧边缘最早的年代7339±40 yr（宫守业等，2012）。 7.不论在资料比较充分的礁台西侧，或资料比较不充分的礁台北侧，礁后堆积在过去大约八千年内，形成了宽2000-3000公尺的礁台，此沈积速率可能为全球环礁之冠。 四、建议 1. 海洋国家公园以往推动的调查或研究计画，对东沙岛附近交通相对便利的范围已经有很好的成果，但是对于礁台的认识相对较少，建议今后的调查、研究的方向应在以往的基础上扩大范围，往环礁礁台发展。 2. 海洋国家公园以往推动的调查或研究计画，对东沙环礁的生物学、生态学和海洋学，已经有很好的成果，但是对于环礁地质的认识相对较少，建议今后除继续生物学、生态学和海洋学的研究、调查之外，应加强推动珊瑚礁地质的相关研究。 3. 为了集中资源、统合调查、研究成果，建议海洋国家公园应至少每两年举行一次东沙研究成果发表会，促进赴东沙调查、研究的学者之间的交流合作。 1. Purpose The Dongsha Atoll is located on the Dongsha Rise that is outside of continental shelf offshore Canton. The atoll stands about 400 meters above the surrounding sea floor with a diameter of about 25 km and area of 500 km2. The Executive Yuan of ROC officially approved on December 19, 2006 the establishment of the Dongsha Atoll Marine National Park, the first marine national park in our country. Although the park has tremendous values in ecological conservation and academic research, our understanding about reef geology of Dongsha is very limited due to its remote ocality. 2. Methods In order to understand the geological processes and environmental changes, this project plans to drill 3 cores on the Dongsha Island and 1 cores on the reef flat of atoll through he Holocene sequence. The Holocene stratigraphic records of the atoll and the island will be established by analyzing the cores and dating fossil corals or mollusks by C14 or U-Th isotopes. 3. Results (1) This project drilled 3 cores on the Dongsha Island. DS-2 is located at the west end, 22 m in length, and reaches the Pleistocene at 14.9 m. DS-3 is located near the center of the eastern part of the island, 20 m in length, and drills into the Pleistocene at 17.25 m. DS-4 is located at the eastern end, 23.3 m in length, and meets the Pleistocene at 20.6 m. It also drilled 1 core near the margin of the northern reef flat, 21.2 m in length, and reaches the Pleistocene at 12.3 m. (2) The cores were described; 19 fossil corals were analyzed by XRD for mineral identification, 3 were crystallized and 16 were pristine. (3) 12 corals were dated by AMS C14 method by Beta Lab. The youngest age at the island is 785± 95 cal yrBP and the oldest 8150± 130 cal yrBP; at the northern reef flat, youngest age is 6430± 90 cal yrBP and the oldest 7915± 85 cal yrBP. (4) The Dongsha Island is a sand cay that formed by deposition entirely. There is no Holocene coral reef underneath. The island formed on the inner (lagoon) side of the reef flat not because that’s where the water depth is shallower, but the deposition rate is faster. The build-up of reef flat is mainly aggradational. There was no sign of progradation, either from lagoon outward nor from ocean inward. (5) The top surface of the underlying Pleistocene limestone descends from -13.6 mMSL on the outer (ocean) to -17.1 mMSL on the inner (lagoon) side. The topography supports the model of Purdy (1974), in which he proposed Holocene atolls developed on antecedent platform of Pleistocene karsts. (6)DS-5 is located at reef crest on the northern reef flat of Dongsha Atoll, but did not recored reef framework in situ. The core consists of detrital sediments entirely. Reef framework may occur further seaward of the site. The coral reef of the northern side is estimated to start about 8 kyr ago based on the the oldest coral in DS-5, which is slightly earlier than the oldest age 7339±40 yr of DS-1 (Gong et al., 2012). (7)Either on the western atoll where there are more data, or on the northern atoll where there is less data, backreef sedimentation has built a reef flat of 2000-3000 me in width. Such sedimentation rate may be the highest among atolls worldwide. 4.Suggestions (1) Most previous projects funded by the National Marine Park have had good results for the areas near the Dongsha Island, but few about the reef rim. On the basis of previous works, more efforts should be placed on the reef front and reef flat to expand out knowledge about Dongsha atoll. (2) Most previous projects funded by the National Marine Park are bout biology, ecology or oceanography. Relatively speaking, we know little about the geology of the atoll. While continue the studies of biology, ecology or oceanography, more efforts should be placed on the reef geology. (3) To maximize the resources and coordinate results of research and survey, National Marine Park may host a symposium of Dongsha research at least once every two years. Such symposium will promote the communication and collaboration between scientists who are interested in Dongsha. |
102年,全新世,东沙岛,东沙环礁,南中国海,国家公园