Research Method of Carbon and Oxygen Isotopic Composition of Carbonste in Loess Sediment

In 1947 Urey presented a poper concerning the thermodynamics of isotopic systems and suggested that variations in the temperature of precipitation of calcium carbonate from water should lead to measurable varistions in the O/O ratio of the calcium carbonate.He Postulated that the determination of temperature of the ancient oceans should be possible, in principle, by measuring the O-content of fossil calcite shells.In 1950 McCrea concluded that isotopic fidelity was optimally obtained by utilizing a procedure in which 100% phosphoric acid at 25℃ dissolved the carbonate.Shortly afterwards, with the recognition of consistent isotopic signals in the sedimentary records of many different areas, Emiliani (1955, 1966 ) defined isotopic stages.Since then geochemistry of stable isotope of carbonate has widely been used in the Studies of paleoclimate.In recent years, with the deepgoing of global change research, many scientists try to obtain more derail information on poleoclimatic variations through the study of isotope of carbonates in loess sediments by the t6chnique of phosphoric acid decomposition.However, the carbonates in loess sediments are a mixture of primary carbonates and secondary carbonates.In the mixture, only secondary carbonates can provide the poleoclimatic information during deposition.Unfortunately, no effective technique can, so far, separate the secondary carbonates from samples and the carbon and oxygen isotopic compeitions measured by the technique of phosphoric acid decompeition are those of total Carbonates in loess sediments.In order to study the relationship between isotope of carbonates in loess sediments and Paleoclimate,stepwise heating technique was used.The resultS show that the carbonates ftom different sources and different genesises have different carbon and oxygen isotopic compeitions and also different tracer significance.The carbon isotopic composition of 700-800℃is more sensitive to paleoclimatic change than that of phosphoric acid decomposition technique.Therefore, phosphoric acid decompeition technique is not fit to analyse the isotopic compositions of carbonates in loess sediments.