西沙群岛琛航岛全新世珊瑚礁的起始发育时间及其海平面指示意义

The Initiation Time of the Holocene Coral Reef at the Chenhang Island (Xisha Islands) and Its Significance as a Sea Level Indicator

以西沙群岛琛航岛珊瑚礁钻孔为材料,通过高精度铀系测年技术测定了全新世底界的年代;同时利用MAT-253同位素质谱仪和电感耦合等离子体质谱仪测定了0~20m层段δ^13C、δ^18O和锶元素的质量分数,以U-Th年龄为基础,结合δ^13C、δ^18O和锶元素的质量分数均在16~17m之间明显降低这一特点,得到琛航岛全新世珊瑚礁的起始发育时间为距今7900a前,不整合于晚更新世的珊瑚礁体(年代老于110ka)之上,全新世礁体的厚度为16.7m。考虑研究区域新构造活动相对稳定,以及南海现代珊瑚礁的礁坪面与大潮低潮面基本一致,琛科2井的钻孔井位高于现代礁坪约2.9m,推测其起始发育时的位置在现代海平面大潮低潮面之下约13.8m,即西沙群岛海域7900a前的海平面在现代海平面以下约13.8m,近7900a以来海平面上升了至少13.8m。这一结果为理解全新世南海珊瑚礁的发育历史及海平面变化提供了新的信息。

This study relies on analyses of the top layer of the material extracted from Well Chenke-2,which is partof a coral reef drilling program in the south-east of Chenhang Island,Xisha Islands.The age of the material at thebottom of the borehole was determined by high-precision uranium dating technology.Five coral samples wereselected for the U-Th dating.The five groups of data can be clearly dated to the Holocene or to the Pleistocene.TheHolocene part includes samples CK2-15,CK2-16,and CK2-17.Their age was determined as 7914±67,7552±73,and 7584±55 years,respectively.The Pleistocene part includes samples CK2-18 and CK2-19.Their age was determinedas 112700±700 and 128100±1000 years,respectively.Furthermore,a MAT-253 isotope mass content ofδ13C,δ18O,and strontium in 21 whole rock samples extracted in the depth interval 0-21 m.Based on the contents of these threeelements,the interval can be divided into two parts.The content ofδ13C,δ18O,and strontium in the depth interval0-16 m is higher,with average values of 0.647‰,-3.392‰,and 0.843%,respectively,than that in the depth interval17-21 m,with average values of-3.185‰,-7.994‰,and 0.174%,respectively.On the basis of the U-Th age,andconsidering that the contents ofδ13C,δ18O,and strontium decrease evidently between 16 and 17 m,it can beconcluded that the boundary between the Holocene and the Pleistocene corals is located at a depth between 16 and17 m in Well Chenke-2,and that the age of the corals determined by uranium dating in the upper part of the boundaryrepresents the initiation time of the Holocene reefs.Therefore,it is concluded that the Holocene coral reefs inChenhang Island started to develop 7900 years ago,basically simultaneously with most of the Holocene reefs inthe Indo-West Pacific,Central Pacific,and Caribbean.The Holocene reefs are 16.7 m thick,and are in unconformitywith the late Pleistocene coral reefs(aged 110 ka).Considering the relative stability of the Neotectonic activities inthe study area,as well as the fact that the reef flat of the modern coral reefs in the South China Sea is basicallylocated at the low tide height,and that the borehole of Well Chenke-2 is located approximately 2.9 m above themodern reef flat,it is speculated that the top of Well Chenke-2 was originally located about 13.8 m below the lowtide level of the modern sea.In other words,the sea level at Xisha Islands 7900 years ago was about 13.8 m belowthe modern sea level.This result provides newinformation for understanding the development history of the Holocene coral reefs in the South China Sea and thesea level changes in the area.