Holocene Coral Reef Development At Chenhang Island,Xisha Islands,South China Sea And Its Recorded Sea Level Changes

Revealing the characteristics of sea level changes by reconstructing the development of Holocene coral reefs is of great significance for understanding the relationship between reef development and sea level,and the trend of sea level change.In this study,the Holocene strata of Well Chenke-2(coral reef drilling in Chenhang Island,Xisha Islands)are comprehensively studied by using U-Th dating,grain size analysis,biological component analysis,carbon-oxygen isotope analysis and element analysis.We established the chronological framework of the Holocene coral reefs in the Xisha Islands,obtained the initiation time,thickness,substrate and accretion rate,revealed the development process of the reef,and reconstructed the Holocene sea level change curve of Xisha Islands based on the relationship between the reef and sea level.1)The Holocene coral reefs in Chenhang Island started to develop 7900 years ago,and the age of the top coral is 4000 years ago.The Holocene reefs are 16.7 m thick,and are in unconformity with the late Pleistocene coral reefs(aged 110 ka).2)The internal U-series age and the thickness of the Holocene reef show that the reef had experienced a fast growth(6.44 m/ka)period in 7900 to 6100 a BP,a broad inflection(0,87 m/ka)period during 6100 to 4000 a BP.And the average accretion rate of the Holocene coral reefs in Chenhang Island is 3.48 m/ka.3)The size of loose clastic sediments in the whole Holocene interval of Well Chenke-2 is relatively coarse(0.66)and mainly consists of coral(59.35%)and coral algae(24.81%).And the average contents of Sr,Mg and Ca are 0.686%,0.996%and 34.926%,respectively.Although the grain size,biological composition and chemical composition of each part are different,they all show the characteristics of reef flat deposits.4)The development process of the Holocene coral reefs in Chenhang Island can be divided into four stages:initiation stage(7900 a BP),rapid accretion stage(7900-6100 a BP),stable growth stage(6100-4000 a BP)and lateral accretion stage(4000 a BP to present).The result of the top dating is 4000 years ago,which is a key time node.It reflects that the sea level of Chenhang Island has reached the limit height about 4000 years ago,and then the vertical accumulation of reefs has changed into lateral development.5)The tectonic of Xisha Islands is relatively stable since Holocene,and the subsidence rate is very small.On this premise,the Holocene sea level change curve in Xisha Islands was reconstructed by the reef development curve constructed by U-Th dating and the suitable water depth reflected by sedimentary facies.Firstly,the sea level of Xisha Islands was about 13.8 m below the low tide level of modern sea level.That is to say,the sea level of Xisha Islands was about 13.8 m below the modern sea level 7900 years ago,and the sea level has risen at least 13.8 m since 7900 years ago.And the sea level rose rapidly from 7900 to 6500 years ago,then slowed down,reached the position of modern sea level around 5500 years ago,and finally rose to the highest level about 4000 years ago,about 1.5 m above the modern sea level.