Advances in the Measurement Methods and Fractionation Mechanism of the Oxygen Isotope Composition of Phosphate

Abstract

Oxygen isotope composition of phosphate is an important tracer for paleoclimatology and phosphorus cycle studies.The measurement methods and the fractionation mechanism are the basis of this kind of study.There have been a lot of these kinds of research.The measurement methods are enveloped from the indirectly methods,to high temperature reduction or pyrolyze,then to fluorination,to improved high temperature reduction,at last,to TC/EA-IRMS,and even to the laser in situ technology.The samples are extended from pure chemical regents to the geology samples.The measurement precision,sample amount,data output,safety and technical requirement are all improved greatly.For the isotope fractionation mechanism,① Although Longinelli′s formula was confirmed by nature samples,and was regarded as equilibrium fractionation,but the results of culture experiments did not agree with the formula(not equilibrium).②For the nature temperature and pH of the earth surface,non-biological processes can not make the oxygen exchange between the phosphate and water.But at high temperature(70℃),phosphate and water can exchange oxygen intensively at different pH,even without biological activity.And the results of different laboratory disagree with each other.③ The phosphate can exchange oxygen water intensively under biological process.But all are away from equilibrium fractionation except the PPase.④ The oxygen isotope of apatite was higher than the dissolved phosphate by 1‰～1.4‰.Therefore,when the Longinelli′s formula was used to the dissolved phosphate attention should be paid to this.The equilibrium fractionation is condition depended,so the disagreement of non-biological experiment results at high temperature(70℃),and the non-equilibrium fractionation of the biological culture experiments,are all the results of condition change,including pH,concentration of phosphate,kinds of organism,biomass and so on.