Analysis Of The Oxygen Isotopic Compositions Of Coral By Secondary Ion Mass Spectrometry: Insights Into The Potential Origins And Their Palaeoclimatic Implications

The secondary ion mass spectrometer（SIMS）is one of in situ analytical techniques for measuring trace element abundance and isotopic ratios of geological samples atμm-size.Along with the development of analytical menthods,this micro-analytical capability is important for biogeochemistry applications.In situ measurement of stable isotopic compositon is a key to understand the biological processes of corals.Although in situδ¹⁸O measurements have been extensively applied in silicate minerals and glasses.Up to now,there are still several analytical challenges exist inδ¹⁸O measurements of the corals.Unlike other minerals（e.g.zircon,olivine,and quazts）,the absence of a suitable and available certified reference material is a crucial obstacle in the field of biocarbonates.So this fact is a serious burden for SIMS as no control is possible for instrumental mass fractionation.Meanwhile,the analytical artifacts from sample preparation are still poorly kown for in situ oxygen analyses.In particular,the major limitation is the coral matrix effect(IMF_coral).In order to overcome these limitations and obstacls,improved SIMS methods are required for further developing and strengthening applications of in situδ¹⁸O in biogeochemistry.Thus,this dissertation focus on finding of reference materials for the carbonate oxygen isotope,development of making the coral target and in situδ¹⁸O measurement of coral samples by SIMS.In addition,the latest achievements of micrao element-isotope studies of the corals are also summarized.Then,we apply these developed SIMS in situ methods to provide spatially and temporally highlyδ¹⁸O_daily records from three modern Porites lutea corals,which respectively collected from Hainan Island,Xisha Islands and the Great Barrier Reef.Besides the variations and protential origins of coralδ¹⁸O_daily will be discussed in this work.The main conclusions are as follow.1.The homogeneity of the protential standard materials is checked by ion microprobe measurements among at least 100 spots analyses.As a result,the oxygen isotopic compositions of the NBS-18 and Carrara marble could be considered homogeneous with the uncertainly.This homogeneity is also found in standard materials GIG-Dolo01 and GIG-Dolo02.IMF values for calcite is-0.80‰,and for dolomite is 9.46‰.The IMF values for aragonite is 0.6‰greater than that for calcite.2.Combining with the coral microstructures,δ¹⁸O_daily is determined at high spatial resolution（beam diameter of²0μm）by SIMS across 3 modern Porites lutea coral skeletions from Saya Bay,Qilianyu and Arlington reef,respectively.The internal reproducibility was typically around 0.1‰and External reproducibility was typically around 0.25‰.The oxygen isotope results for Carrara marble data demonstrate no obvious instrument drift over 12 h.Studies of long term in situ oxygen isotope records are benefit to infer oxygen isotopic fractionation mechanism and its respond to vital effect or environmental factors,which can provide scientifical insights for forecasting the trend of environment evolution in future.3.Analytical artifacts from sample preparation,such as organic matter contamination,sampling depth and thickness,gas bubbles,X-Y effect and sample topography,are examined in detail.The results indicate that sample preparation has a significant influence on the calibration of coral matrix effect.In an attempt to minimize these influences,these techniques for coral sample prepraration are optimizd.The results obtained using the new technological process show an obvious reduction of these artifacts when comparted with the old process.Thus we can obtain precise matrix effect,which is one of the key parts of SIMS and determines the veracity of the analytical data.This work guarantees the accuracy of in situδ¹⁸O measurements of coral and facilitates the SIMS application in coral geochemistry study.4.The coral matrix effect is the major limitation in the in situδ¹⁸O measurements of coral.By coupling with DIC content of an extracellular calcifying fluid(DIC_ecf),its possible driving mechanisms are investigated.The result reveals a weak negative relationship between the IMF_coral and DIC_ecf,indicating the influence of the metabolism processes on coral matrix effect.More active metabolism processes can make the lattice parameter of an organic aragonite cell closer to that of an inorganic aragonite cell,which may reduce the value of IMF_{polycrystalline},and consequently result in decrease of IMF_coral.However,these IMF_coral data is mainly from coral samples collected from Sanya bay,which may not be necessarily conclusive and representative for global coral reefs.Nevertheless,the currer conclusion is helpful to understand the coral matrix effect in the in situδ¹⁸O measurements using SIMS.5.Due to the non-stationary random matrix effect between reference materials and coral samples,we corrected the SIMSδ¹⁸O_PDB data of coral by the multi-support calibration,which could efficiently and accurately evaluate matrix effect.To validate the accuracy of oxygen isotopic measurements on corals,we compared in situδ¹⁸O_PDBDB data with the bulk powder analyzedδ¹⁸O_PDB data.Our preliminary comparison indicated a positive correlation between coralδ¹⁸O_PDB data estimated by SIMS and by conventional mass spectrometry.It suggests that this calibration way could simply and efficiently increase the reliability of oxygen isotopic measurements on corals.Therefore,our newly developed method has important implications for understanding of oxygen isotopic equibibrium and vital effect,as well as paleoceanography restructions.6.There are several abnormal depletedδ¹⁸O_minin data among threeδ¹⁸O records observed in different coral skeletions.Further,the distance between aδ¹⁸O_min spot and aδ¹⁸O^*_avg spot is very narrow.Assuming that the skeletal extension rate is constant throughout the three years,the distance is nominally equivalent to periods of about 2¹7days.This short-term heterogeneity is too large to be generated by variations in SST at the study site.We estimates thatδ¹⁸O_min is not driven directly by SST.The deviation ofδ¹⁸O^*_avg andδ¹⁸O_minin ranges from 0.9‰to 3.8‰,which is basically consistent with the observation in Juillet-Leclerc et al.（2009）that fiberδ¹⁸O values were always 1‰³.9‰higher compared to the COC isotopic signature.We therefore conclude thatδ¹⁸O_min,at this spatial resolution,is contributed by both the oxygen isotopic composition of COC and fibres.7.Theseδ¹⁸O_daily distributions in 10AR2-1 are similar to that observed in 15XS15-1 and 10SY-1.In short tem,amplitude values ofδ¹⁸O_daily fluctuated ranged between 0.8‰and 1.5‰in 10SY-1,between 0.3‰and 1.5‰in 15XS15-1,and bwteen 0.5‰and1.5‰in 10AR2-1.Similar to the society resolution,SST is a crucial origin ofδ¹⁸O_dailyaily variation at the high spatial resolution.In addition temperature changes of ten days and12h may also be superimposed onδ¹⁸O_daily variation,leading to a high dδ¹⁸O/dT slope（-0.302‰/℃）which goes beyond the upper limits reported in Gangon et al.（2012）.Apart from temperature,δ¹⁸O_seawater and biological processes may also be important contributions toδ¹⁸O_daily variations at microscale.8.Weak positive relationship is observed between the SST and oxygen isotopic composition of an extracellular calcifying fluid(δ¹⁸O_cf),implying the influences from metabolic CO₂ concentration and the proportion of hydration and hydroxylation.So variation of oxygen isotopic composition in the extracellular calcifying fluid,which induced by biological processes,may be another important origin ofδ¹⁸O_daily variation at the high spatial resolution.With temperature increasing,pH of the extracellular calcifying fluid is gradually decreased as a result of the progressed production of the metabolic CO₂ concentration.Consequently,the ratio of hydration/hydroxylation is elevated,since it is pH dependent.The weak positive relationship found between SST andδ¹⁸O_cf can thus be explained,as oxygen isotope of HCO₃^-formed by hydration is enriched than that by hydroxylation.But this mechanism still remains unclear.Thus we may need in situ B/Ca-δ¹¹B-δ¹³C data to verify this mechanism in the future study.