Clumped Isotope Fractionation In Biogenic Carbonates

Based on the relationship between the relative abundance of ¹³C-¹⁸O bonds in carbonate crystal,the clumped isotope(quantified byΔ₄₇)can be used to indicate temperature without knowing the chemical and isotopic composition of the water where the carbonates precipitated,making it a promising temperature proxy for palaeoclimatic and palaeoenvironmental studies.However,the clumped isotopic signatures and their relationship with oxygen isotopes(δ¹⁸O)are not well-understood.The clumped isotopic compositions in biogenic carbonates（especially land snail shells and coral skeletons）and their disequilibrium fractionation signatures due to“vital effect”remain unsolved so far.To better use the clumped isotope as a powerful thermometer,we need to better understand theΔ₄₇ signature and its fractionation mechanism in biogenic carbonates.For this reason,this dissertation has devoted to firstly review the basic rationale,analytical methodology,and relevant applications for carbonate clumped isotope system.Secondly,the method for carbonateΔ₄₇measurement has been established via setting up a self-developed CO₂ purification system and adjusting the analytical method on a Thermo Fisher 253 Plus isotope ratio mass spectrometer.TheΔ₄₇ value scale has been anchored through CO₂ gas standards with knownΔ₄₇ values and the long term reproducibility ofΔ₄₇ values for several carbonate standards has reached the international advanced level.We have investigated the clumped isotope kinetic fractionation during diagenetic alterations of coral aragonites under different experimental conditions at 25 and 90°C.The results showed that the rate of the ¹³C-¹⁸O bond reordering is significantly lower than that of oxygen isotope equilibrium during the transition of coral aragonite to calcite in solution state,in which reprecipitated calcite preserved the originalΔ₄₇signals from the aragonite but significantly differentδ¹⁸O values.These results suggest that the clumped isotope thermometry may be more reliable than traditional oxygen isotope thermometry when they are applied on diagenetic carbonates.We have analyzed shellΔ₄₇ values of modern land-snails collected from several regions influenced by the East Asia monsoon in China.The results showed that the shellΔ₄₇ generally indicates the main snail body temperature during its activity periods and show no robust correlation to the annual mean air temperatures.Being subjected to the influence of physiological and behavioral adaptation,the shellΔ₄₇cannot record the environmental temperature at extreme conditions.Coupled with oxygen isotope thermometry and model calculations,we found that the snail body fluidδ¹⁸O derived fromΔ₄₇ temperature can better indicate rainδ¹⁸O at high relative humidity（⁹0%）,suggesting that theΔ₄₇ signature of snail shells are able to be applied in palaeohydrological studies for continental environments.We have measured theΔ₄₇ signature in Porites sp.coral carbonates from the South China Sea and found that the coralΔ₄₇ generally presented higher-than-expected values,which are likely associated with coral vital effects.Even so,the coralΔ₄₇ still reliably preserved its temperature dependence,making it possible to indicate surface seawater temperature.At annual time scale,inter or intra-colony differences have no significant effect on the coralΔ₄₇ values.At seasonal time scale,however,intra-colony difference has a significant effect on coralΔ₄₇ values accompanying with remarkable variations inδ¹³C,suggesting that the excess metabolic CO₂ during coral calcification may drive theΔ₄₇ away from the equilibrium fractionation condition.