Reconstruction Of The Environmental Changes In The Kiribati Coral Reef System By Giant Clam,tridacna Gigas

In recent years,the giant clam skeleton has become an ideal material for studying paleoenvironmental changes in low-and mid-latitude coral reef ecosystems due to their long life,high resolution,complete record of environmental information and clear growth bands.The giant clams of this study were taken from the Marakei Atoll(2°00'N,173°16'E)in western Kiribati,which is located in the central equatorial Pacific Ocean and on the eastern edge of western Pacific warm pool.The area is a typical maritime climate,strongly influenced by ENSO(El Nino-Southern Oscillation)and the human activities is limited,making it an ideal site for studying low-and mid-latitude climates.Via measuring the carbon and oxygen isotope(?13C,?180),Sr/Ca ratio,and skeleton-bound organic nitrogen isotope,the study systematically discusses the application of the above indicators in retrieving the environmental information of the coral reef system.The main findings are as follows:1)The growth rate of giant clam is positively correlated with the proxy of solar insolation(Sr/Ca ratio),which indicates that the growth rate is inhibited by light.2)The positive deviation of the ?180 of the clam shell is related to the low precipitation event in this research area.During low precipitation period,the seawater evaporates more than the rainfall,and more seawater 16O is evaporated,resulting in an increase in the ratio of 180/160,which increases the shell ?180 of the giant clam living therein.The large positive deviations of the ?18O(?180 changes more than 0.5‰)are related to the occurrence of La Nina phenomenon.Among the period of La Nina,the rainfall in Marakei atoll significantly decreases and the drought event frequently occurs.The small 8180 positive deviation event(?18O changes less than 0.5‰)may be related to the high evaporation in the dry season.3)The skeleton-bound organic ?15N is consistent with the trend of the Southern Oscillation Index(SOI,ENSO indicator),indicating that ?15N may records the oscillation events of the East and West Pacific water caused by the changes of ENSO in the Pacific Ocean.The ?15N of the nitrate in the East Pacific water is heavier,while the ?15N in the West Pacific is smaller because of the accumulation of nitrogen fixation signal.When El Nino occurs,the east Pacific water with high ?15N value of nitrate retreats eastward,and the West Pacific water intrudes,causing the skeleton-bound organic ?15N to be negatively biased.In addition,we also compared the differences between the east and west samples in Marakei Atoll,and explore the differences in local regional environments under the context of ENSO,The conclusions are as below:4)The Sr/Ca ratio of the giant clam in the east of Marakei Atoll is less than that in the west,which indicates that the light intensity in the water of east side is stronger.The strong insolation will inhibit the growth of giant clam,which shows that the growth rate of giant clam in the east of the atoll is lower than that in the west.The ?80 in the west side of the atoll is 0.2‰ lighter than the east side,which is presumably caused by the difference in the signals recorded by ?15O due to the different hydrological environments on both sides.The seasonal stratification of the water body in the west of the atoll and the slow exchange rate of the water body make the ?180 in the west side(W01 and W04)reflect the rainfall signal.By contrast,the east side is well mixed,and the water from the bottom dilutes the rainfall.Therefore,the 8 80 of E01(the sample from east side)is more likely to reflect the signal of seawater masses,which have greater 6180 value than rainfall.In addition,skeleton-bound organic ?15N of E01 varies from 3.9-11.0‰,with an average of 8.6‰,and the skeleton-bound organic 8 5N of W04 is changed between 4.6-11.7‰,with an average of 8.9‰.There is no significant difference in these 8 5N of E01 and W04,indicating that the water masses in the east and west side of Marakei Atoll are the same.However,there are significant differences in the fluctuations of skeleton-bound organic 8 5N between E01 and W04:the fluctuation of ?15N of E01 is relatively flat,mainly with low-frequency signals.The?15N of W04 fluctuates more frequently,and mix high-frequency signals in the low-frequency signals.The low frequency signals of ?15N of these two samples are caused by oscillation of the East and West Pacific water masses.For the high-frequency variation of W04,we speculate that the reason is:in the west of the Marakei Atoll,due to the blocking effect of the atoll,the seasonal variation of the stormy wave will influence the water ventilation,which leads to seasonal changes in ?15N of nitrate in the surface water and then results to seasonal fluctuations of skeleton-bound organic ?15N.The east side of the atoll is long-term exposed to the high hydrodynamic conditions,making the water mixed well and thus no seasonal frequency fluctuation in the skeleton-bound organic ?15N in E01.It is worth mentioning that this study is the first to successfully establish a method for measuring the skeleton-bound organic nitrogen isotope of the giant clam,which is of great significance for studying the nitrogen cycle of coral reefs.Meanwhile,the XRF core scanner was used to measure to Sr/Ca ratio of the giant clam shell for the first time,extending the application of non-destructive analysis of trace elements of giant clam in paleoenvironment reconstruction.