Geochemical Evidence For The Periodic Bleaching And Recovery Of Porites Corals During The Mid-Late Holocene In The Northern South China Sea

Coral bleaching（thermal bleaching）caused by global warming has led to rapid degradation of modern coral reef ecosystems.However,the understanding of high temperature induced coral bleaching is mainly confined to modern times.Understanding the response mechanism of corals to high temperature in geological time will be helpful to scientifically evaluate modern coral bleaching events and their ecological impacts.In the present dissertation,7 modern thermal bleached corals（1 coral was collected from Sanya,Hainan in 2015,and 6 corals were collected from Weizhou Island in 2020）and 10mid-late Holocene corals（corals were dated 5.7–3.4 ka BP using U-Th dating and were collected from Wenchang,Hainan）from the northern South China Sea were used,among which the mid-late Holocene corals contain well-preserved 10 mortality surfaces and 5 growth discontinuities.Taking the widely used geochemical parameters of Sr/Ca,δ¹⁸O andδ¹³C as climate and environmental proxy indicators to explore the evidence of coral bleaching and recovery during the mid-late Holocene.In the study of coral paleoclimate,Sr/Ca symbolizing sea surface temperature（SST）,Δδ¹⁸O obtained fromδ¹⁸O after Sr/Ca-SST deducted symbolizing sea surface salinity（SSS）,andδ¹³C symbolizing the photosynthetic intensity of symbiotic algae.The main content is divided into two aspects:Firstly,we studied the skeletal Sr/Ca,δ¹⁸O andδ¹³C characteristics of modern thermal bleached corals,as a basis for observing the present to study the past.Then,we analyzed the skeleton Sr/Ca,δ¹⁸O andδ¹³C characteristics of mid-late Holocene corals to explore the climate background and causes of the formation of coral mortality surfaces and growth discontinuities.A total of 2051 samples were tested for Sr/Ca ratios,and 2002samples were tested forδ¹⁸O andδ¹³C values.The following conclusions were obtained:1)Skeleton Sr/Ca,δ¹⁸O andδ¹³C of modern bleached corals can well record environmental characteristics and the changes of photosynthesis intensity of symbiotic algae during coral bleaching.The Sr/Ca ratios at the bleaching surfaces of modern corals in Weizhou Island and Hainan Island are low and the reconstructed average SST（Sr/Ca-SST）values are 29.3°C and29.8°C,respectively,which is consistent with the instrumental measured average SST（29.5°C and 30.1°C）of the month when thermal bleaching occurs,indicating that coral Sr/Ca ratios can accurately record the SST during thermal bleaching.Sr/Ca combined withδ¹⁸O reconstructedΔδ¹⁸O values at the coral bleaching surfaces are high,indicating that the salinity at the bleaching surfaces of corals is in an annual high salinity period,which is basically consistent with the instrumental measured salinity characteristics,also indicating thatΔδ¹⁸O can relatively accurately record the salinity characteristics during coral bleaching.Skeletonδ¹³C values of 7 coral bleaching surfaces exhibit abnormal negative deviation,indicating reduced photosynthesis intensity of coral symbiont zooxanthellae,coinciding with the characteristics of a large decrease in the density of the symbiont zooxanthellae upon coral thermal bleaching,indicating thatδ¹³C can record the photosynthetic characteristics of symbiotic algae during thermal bleaching events.2)Sr/Ca indicates that the mortality surfaces and growth discontinuities of mid-late Holocene corals are basically formed in a high temperature environment.Coral Sr/Ca reconstructed SST values during 5.7–3.4 ka BP vary from 19.8°C to 33.6°C,among which the Sr/Ca-SST at the mortality surfaces of 8 corals ranges from 29.8°C to 33.1°C,all during summer high temperature period.In addition,the Sr/Ca-SST at the mortality surfaces of 2 other corals ranges from 19.8°C to 27.0°C,during winter or spring.The Sr/Ca-SST at 5growth discontinuities ranges from 30.8°C to 33.6°C,all corresponding to the summer high temperature period.Compared to the maximum monthly mean Sr/Ca-SST at that time in summer,Sr/Ca-SST is 0.1–1.2°C higher at 5 coral mortality surfaces and 0.6–1.2°C higher at 4 coral growth discontinuities.The above Sr/Ca-SST information indicates that the mortality surfaces and growth discontinuities of mid-late Holocene corals are basically formed in the summer high temperature environment.3)Δδ¹⁸O obtained from the combination of Sr/Ca andδ¹⁸O indicates that the mortality surfaces and growth discontinuities of mid-late Holocene corals are formed in a high salinity environment.By subtracting the temperature information inδ¹⁸O,using the remaining signal(Δδ¹⁸O)reconstructed SSS values during the formation of coral mortality surfaces and growth discontinuities are 31.1–38.3,mostly corresponding to the annual high salinity season.Δδ¹⁸O indicates that the SSS values（32.8–38.3）prior to 7 coral mortality surfaces and 2 coral growth discontinuities are 0.1–3.4 higher than the summer monthly mean SSS values during their growth period,indicating that coral death and interrupted growth occurred in high salinity environments,which are presumed to be related to strong evaporation and less precipitation.The SSS values（31.1–34.8）prior to 3 coral mortality surfaces and 3 coral growth discontinuities are slightly lower than the summer monthly mean SSS values during their growth period.4)δ¹³C indicates that the photosynthesis intensity of symbiotic algae is basically reduced during the formation of mid-late Holocene coral mortality surfaces and growth discontinuities.Coral skeletonδ¹³C values at 5 mortality surfaces and 5 growth discontinuities are in the low value period,whereδ¹³C values at 2 mortality surfaces and 3 growth discontinuities show abnormal negative deviation（with an amplitude of up to 1.8‰）,indicating that the photosynthesis intensity of coral symbiotic algae at the mortality surfaces and growth discontinuities is significantly weakened.δ¹³C values at mortality surfaces of the other 5 corals are in the annual high value period.5)The seasonal variation characteristics ofδ¹³C values show that the summer insolation during the formation of mortality surfaces and growth discontinuties of mid-late Holocene corals is stronger than that in modern times.Compared with modern times,coralδ¹³C seasonality in the mid-Holocene is 18.8%higher;coralδ¹³C seasonality since the mid-Holocene shows a long-term decreasing trend,which is consistent with the change trend as orbital-induced（precession）insolation seasonal amplitude.By excluding potential influencing factors of kinetic fractionation and river runoff on coral skeletonδ¹³C,we suggest that the higher coralδ¹³C seasonality over the mid-Holocene than modern period is primarily driven by the metabolic effect caused by the enhanced insolation seasonality.This conclusion on the one hand highlighted that coralδ¹³C has the potential to record past insolation information,on the other hand indicates that summer insolation was stronger in mid-late Holocene than in modern times.Coral geochemical indicator records spanning approximately 2214 years（5648–3434 BP）in northern South China Sea indicate that 2 coral mortality surfaces（4568 a BP and 3434 a BP）and 3 coral growth discontinuities（4370 a BP,4350 a BP and 3654 a BP）,exhibit higher SST（30.3–33.6°C）,higher SSS（34.3–38.3）and abnormally negatively biased coral skeletonδ¹³C,indicating that they were formed in high temperature,high salinity（or strong evaporation）environment and the photosynthesis intensity of coral symbiotic algal was significantly reduced.The evidence implies that the mortality surfaces and growth discontinuities are the consequence of severe coral bleaching under abnormally high temperature.In addition,3 mortality surfaces（5309 a BP,4499 a BP,and 4432 a BP）and 2 growth discontinuities（5314 a BP and 4360 a BP）are formed in the summer high temperature environment and correspond to decreased coral skeletonsδ¹³C,similar to the environmental and physiological characteristics during modern coral bleaching,presumably should also be the product of high temperature induced coral bleaching;the SSS may be related to factors such as rainfall at that time.The environmental backgrounds for the other 5 coral mortality surfaces are as follows:abnormally high temperature and salinity environment（5648 a BP）;summer high temperature period（4225 a BP and 3650 a BP）;winter or spring（4335 a BP and 3582 a BP）,presumably related to other factors such as environmental stress or natural death.In general,coral bleaching events occurred repeatedly in the mid-late Holocene when the summer insolation was stronger,the rainfall was less in summer and the climate was warmer than that in modern times,and the resilience of corals has meant that corals still exist today.