Development Process Of Holocene Coral Reefs In The Weizhou Island And Its Potential As A Coral Refuge In The South China Sea

Under the background of large-scale bleaching of corals and rapid degradation of global coral ecosystem caused by climate warming,relatively high latitude sea area is considered as a potential refuge for corals,because the high temperature in summer generally does not exceed the upper temperature limit of coral growth.In this context,reconstructing the development process of coral reefs in relatively high latitude sea areas is of great scientific significance for understanding the response of corals to the past climate and evaluating the development trend of coral reefs in this sea area,but so far,there is little research on the development process of coral reefs in relatively high latitude.Weizhou Island in Beibu Gulf is located in the northern edge of the tropics.Coral reef here belongs to relatively high latitude reefs.This paper focuses on the scientific problem of the response of the development process of relatively high latitude coral reefs to past climate change.In this paper,the ages of 32 corals were measured by high-precision uranium dating technology with 6 full cores(GS-1,5.90 m;GS-2,10.72 m;GS-3,8.20 m;GS-4,9.70 m;GS-5,7.65 m;GS-6,15.44 m)in the coral reef area of Weizhou Island.The chronological framework of coral reef development in Weizhou Island is established.The material composition was revealed by particle size analysis(161 samples),biological component analysis(81 samples)and mineral composition analysis(82 samples).According to the development basement,initial development time,development thickness and accretion rate of coral reefs in each borehole,the development process of coral reefs is reconstructed.Combined with the climate background of reef development period,this paper discusses the corresponding relationship between reef development and climate,evaluates the potential of Weizhou Island coral reef as a coral refuge in the South China Sea under the background of global warming,and draws the following main conclusions:(1)The initial development age of the coral reef and the thickness of the coral reef layer in Weizhou Island vary in different parts.It is measured that the oldest age is 8252 a BP.The maximum thickness of the coral reef is 7.7 m.The basement of the coral reef is quartz sandy deposit.The thickness of the coral reef layer in each core,the age of the maximum uranium series and the water depth of the initial development point of the coral reef are as follows:GS-1,3.8 m,571 a BP,3.18 m,GS-2,6 m,1740 a BP,7.02 m,GS-3,7.6 m,8061 a BP,11.91 m,GS-4,7.7 m,8252 a BP,12.15 m,GS-5,5.1 m,7378 a BP,12.32 m,GS-6,5.4 m,2772 a BP,1.07 m.(2)The particle size of the coral reef sediments in Weizhou Island is medium fine-fine sand.The biological components are mainly composed of corals,molluscs and coral algae.The mineral components are mainly aragonite,quartz and calcite.Generally,the coral reef carbonate component is the main component(69.68%).Based on the results of particle size analysis of 161 samples,biological component analysis of 81 samples and mineral composition analysis of 82 samples,it is concluded that the overall particle size of coral reef sediments in Weizhou Island is medium fine sand fine sand;The contents of corals,molluscs and coralline algae in the biological components of sediments were 52.50%,32.16% and 14.13% respectively;The average contents of aragonite,quartz and calcite are 52.86%,30.32%and 16.82% respectively.(3)The average accretion rate of coral reefs in Weizhou Island is 1.11m/ka,but there are obvious temporal and spatial differences.Coral reefs developed slowly at 8252～4193 a BP and 1645～571 a BP,with an average accretion rate of 0.19 m/ka(varying between 0.02～0.50 m/ka)and 0.73 m/ka(varying between 0.32～0.95 m/ka)respectively.At 4193～1645 a BP and 571 a BP to present,it develops rapidly,and the accretion rates are 1.73 m/ka(varying between 0.92～6.75 m/ka)and 3.37 m/ka(varying between1.86～10.15 m/ka)respectively.Spatially,the average accretion rate of coral reefs in 6 core from shore to sea are 2.85 m/ka,10.15 m/ka,1.75 m/ka,0.91m/ka,0.92 m/ka and 0.49 m/ka.(4)According to the spatial distribution of the initial development time,reef thickness and accretion rate of coral reefs,the development process of coral reefs in Weizhou Island can be divided into four stages,including the reef germination and slow development stage(about 8252～4193 years ago),the rapid development towards the shore stage(about 4193～1645 years ago),the slow development stage(about 1645～571 years ago)and the lateral high-speed development stage(about 1645～571 years ago).The coral reefs in Weizhou Island developed slowly in reef germination and slow development stage.It began to germinate at 2000 m from the current coastline.The average accretion rate was 0.19 m/ka.In the rapid development towards the shore stage,the average accretion rate of reef is 1.73 m/ka.Reef developed rapidly and extends developed 1500 m towards the shore.In the slow development stage,the average accretion rate of reef is 0.73 m/ka.It developed slowly and mainly developed bioclastic deposits.In the lateral high-speed development stage,the average accretion rate of reef is 3.37 m/ka.Reef developed is dominated by coral growth.Reef developed rapidly laterally to the modern coastline.(5)Comparing the accretion rate of coral reefs in Weizhou Island with the climate background in the same period,it is concluded that the accretion rate of coral reefs in Weizhou Island is generally regulated by cold and warm climate.The coral reefs developed fast in warm period and slow in cold period.The rapid development stage of coral reefs in Weizhou Island roughly corresponds to the late Megathermal Period(8500～3000 years ago),Roman Warm Period(2250～1650 years ago)and Current Warm Period(30 years ago).The slow development stage of coral reefs roughly corresponds to the cold period of the Dark Ages Cold Period(1650～1150 years ago)and the Medieval Climate Anomaly(1050～550 years ago).The accretion rate of each stage of coral reef has a good corresponding relationship with the climate background of warm and cold climate,Sea surface temperature and temperature change in the same period.(6)Weizhou Island coral reef has the potential to serve as a refuge for coral reefs in the South China Sea under the background of climate warming.On the one hand,Weizhou Island is located in a relatively high latitude.Under the background of global warming,the seawater warming in winter is conducive to alleviate the threat of low temperature of seawter in winter to the growth of coral in Weizhou Island.On the other hand,the fact that the development history of coral reefs in Weizhou Island shows that the rapid development stage in the past 8252 years basically corresponds to the warm period and the slow development stage corresponds to the cold period indicating that the warm climate is conducive to the development of coral reefs in Weizhou Island.