Mid-Holocene Relative Sea-Level Reconstruction In Northern South China Sea Based On Porites Corals

An accurate understanding of the Holocene sea-level change in northern South China Sea can better understand the evolution process and influence of climate and environment in northern South China Sea in the past,and has important scientific and practical significance for predicting the future trend of sea-level change.Although there have been a large number of studies on sealevel change in the region,there are still large differences on the historical reconstruction results of Holocene sea-level change in northern South China Sea,and there is a lack of reliability assessment of the reconstruction results,which cannot provide historical reference for sea-level change trends in the context of modern climate change.The large fossil porites and microatolls in the reef area of Leiqiong in north of the South China Sea provide ideal research data for the study of past sea-level changes.Based on this,this paper first carried out a field investigation on the coral reef area along the Wenchang-Qionghai coast in the east of Hainan Island.Meanwhile,the upper top surface elevation of 27 modern live porites was measured to establish the relationship between the upper top surface elevation and sea-level,and clarify the sea-level indicator significance of modern live porites.Secondly,the elevation measurement and sampling of 3 large fossil porites in the coral reef area along the Wenchang-Qionghai coast in the eastern part of Hainan Island were carried out.The extracted coral was polished and then subjected to indoor digital X-ray imaging.According to the results of indoor digital X-ray imaging,2 samples from each coral were selected for high precision U-Th dating,so as to obtain new past sea-level elevation data.Finally,we recalculate the original elevation data of 59 native propocoral/microatolls published in recent years according to the coral sea-level indicator significance,quantitative sea-level reconstruction method and tectonic uplift correction,and then obtain the sea-level change sequence of the northern South China Sea in the Middle Holocene.Finally,this paper compares the above sea-level change sequence with the East Asian monsoon intensity,sea surface temperature(SST)in the northern South China Sea,ENSO and other climate records to analyze the causes of sea=level change in the northern South China Sea.The results show that:(1)The average height of sea-level change indicated by 27 modern porites in the eastern coastal area of Hainan Island is-0.431 m,and the corresponding maximum range of change is 0.17 m and the range of change is ± 0.083 m.These height data indicate that the growth range of the upper top surface of the local modern waterfront coral is narrow,the height is almost close,and the vertical relationship with the reference tide can be quantified,which is a relatively ideal sea-level indicator.The relative sea-level in northern South China Sea calculated in this paper is relative to the local average tide level,and the newly added coral records show that the sea-level elevation in Tongguling,Hainan fluctuates between 1.62 ～ 1.72 m and 1.37 ～ 1.47 m,respectively,from 5314 ～ 5253 cal a BP and 3613 ～ 3485 cal a BP.(2)The highest relative sea-level in northern South China Sea appears at 6,500～ 5,500 cal a BP,which is 2.37 m.During 5846 ～ 5518 cal a BP,the sea-level in northern South China Sea decreased rapidly in the range of 1.47 ～ 2.16 m,with a fluctuation range of 0.69 m.From 5250 to 4386 cal a BP,the rate of sea-level decline in northern South China Sea was further weakened,and the fluctuation range of sea-level was small,and the fluctuation range was 0.63 m in the range of 1.45 to 2.08 m.During 3914 ～ 3084 cal a BP,the sea-level in northern South China Sea showed a rapid decline from 2.01 m to 1.33 m at a rate of 7.7 mm/a.(3)The difference of sea-level reconstruction results in the Middle Holocene is mainly caused by the difference of sea-level indicator meanings of porites.Under the influence of glacial isostatic adjustment,the sea-level of the eastern coast of the Malay Peninsula,the South China Sea and the East China Sea all changed synchronously during the Holocene.We believe that the past sea-level indicated by the porites in this paper is accurate.In addition,the Middle Holocene facies of the northern South China Sea is positively correlated with the intensity of the East Asian monsoon and the sea surface temperature in northern South China Sea,and negatively correlated with ENSO activities.The relationship between them can be divided into two stages: the climate suitable period(8000 ～ 6000 cal a BP),the sea-level rise rate in northern South China Sea gradually weakened,and the sea-level was higher than the modern sea-level,which may represent the high weathering intensity and warm and humid climate at that time;At the end of the Holocene suitable period,about 6000 cal a BP,the fluctuation amplitude of sea-level in northern South China Sea significantly weakened,and the highest point appeared at 5000 ～ 4000 cal a BP.