Magnetic Minerals Of Coral Reefs In The South China Sea And Their Response To Climatic Events

Because of its early origin and the large time span of coral reef drilling cores,coral reefs have become valuable materials for rebuilding and restoring the long-scale history of climate and environmental change.The measurement of magnetic parameters has the advantages of simple,rapid and non-destructive,which are widely used in the research of loess,lakes and other fields.However,there are few studies on coral reef magnetism.Therefore,it is very important to study the magnetic minerals and magnetic properties of coral reefs and the response of coral reef magnetic parameters to climate and environment.In order to solve the above problems,magnetic methods such as rock magnetism and environmental magnetism and non-magnetic methods such as major and trace element anaylsis,X-ray diffraction analysis and optical microscope analysis are used to explore the magnetic minerals and their magnetic properties of surface sediments of different coral reefs in the South China Sea,and the magnetic minerals and magnetic properties of LYJ3 core in the South China Sea.These results are compared with ancient and modern climatic events to explore the types,possible sources,magnetic characteristics and response to climate and environmental changes of magnetic minerals in coral reefs.The conclusions of this study are as follows:（1）The magnetic minerals in the surface sediments of coral reefs in the South China Sea belong to ferromagnetic minerals,which are similar to those of magnetite.In most of the surface sediments of coral reefs in the South China Sea,the magnetic domains of magnetic minerals are pseudo-single domains,a few of them are multi-domains,and the particle sizes of magnetic minerals are small.The surface sediments of coral reefs in the South China Sea contain magnetic minerals such as magnetite,pyrrhotite and ilmenite.After comparing the magnetic parameters and magnetic mineral composition of the surface sediment samples of coral reefs in the South China Sea with those of the surrounding sediments,it is preliminarily concluded that the source of magnetic minerals of these samples is terrigenous clastic brought by sea water,but it is not excluded the probability that they are from volcanic eruption detritus at the bottom of the South China Sea.（2）The northern part of the South China Sea is dominated by low coercivity magnetic minerals such as magnetite,and the dominant position of low coercivity magnetic minerals gradually decreases from northwest to southeast.Chinese mainland and Taiwan Island may be the main sources of magnetic minerals for coral reefs in the South China Sea.（3）The magnetic parameters of LYJ3 core in Lingyang Reef show that the environment around Lingyang Reef has changed during the seven periods of-74～1500 cal yr BP,1500～2375 cal yr BP,2375～2500 cal yr BP,2500～2650 cal yr BP,2650～2850 cal yr BP,2850～3290 cal yr BP,3290～3531 cal yr BP,resulting in changes in particle content,magnetic domain and mineral types of magnetic minerals in LYJ3 core of Lingyang Reef.The properties of magnetic minerals in LYJ3 samples of Lingyang Reef are similar to those of magnetite and hematite.The magnetic domains of magnetic minerals in most samples are pseudo-single domain,a few are single domain and multi-domain,and contain magnetic minerals such as magnetite（titanomagnetite）,hematite and goethite.The mineral composition is mainly detrital magnetite,indicating that the magnetic minerals of coral reefs are greatly affected by detrital sediments.Based on the time series analysis of the remanence intensity of the obtained magnetic minerals,it is inferred that very significant climate change events occurred in the periods of～2000 cal yr BP,～2500 cal yr BP and～2800 cal yr BP.These periods coincide with the Roman Warm Period（about 2200～1550 a BP）,the dry climate period in the Spring and Autumn Period and the Warring States Period（2.45～2.35 ka BP）and 2.8 ka event.（4）By comparing the data of LYJ3 core log ARM of Lingyangreef with the variation of TOC flux andδ¹³C_org,the magnetic susceptibility of loess and the Ti content of Huguangyan Maar lake in the northern shelf of the South China Sea,it is found that the log ARM of LYJ3 core is affected by monsoon precipitation,and these datas have different responses at 2800 a BP.The change of log ARM sequence of Lingyang Reef core is consistent with its chemical weathering index（K/Ti,K/Al）,and both have an obvious peak at～2800 a BP,which means that there may be a transition/abrupt event of East Asian summer monsoon at～2800a BP.This corresponds to the 2.8 ka event of the eight IRD discovered by Bond,indicating that the magnetic characteristics of tropical marine coral reefs may be used to deduce the evolution of the East Asian summer monsoon,and the evolution of the East Asian summer monsoon may be related to the temperature change at high latitudes in the northern hemisphere.Futher more,the log ARM records of tropical marine coral reefs change almost synchronously with the high latitude temperature of the northern hemisphere within the error range,which further indicates that there is a correlation between the evolution of the East Asian summer monsoon and the high latitude temperature change of the northern hemisphere.Comparing the LYJ3 core log ARM record with the stalagmiteδ¹⁸O record,it is found that the coral reef ARM value may also record the strength of the summer monsoon.When the ARM increases,the summer monsoon strengthens,on the contrary,the summer monsoon weakens.The power spectrum analysis of the log ARM sequence of LYJ3 cores shows that there are 1193a,813a,526a,136a,130a,79a,72a,51a,40a,37a,35a,34a,30a and 27a cycles in log ARM records beyond the 95%confidence degree.These periods are consistent with the solar activity cycle,the earth climate change cycle and the tide-generating force variation cycle of astronomical object,indicating that the magnetic properties of coral reef boreholes may indicate weak monsoon events in Asia and solar radiation changes,record global climate fluctuations and astronomical tidal force changes.