磷酸盐氧同位素组成的测定方法及分馏机理研究进展

磷酸盐氧同位素组成在古气候和磷的生物地球化学循环研究中都具有十分重要的意义.测定方法和同位素的分馏机理是该类研究的基础.国际上已开展了一系列磷酸盐氧同位素的测定方法和分馏机理研究.在测定方法上,由初期的间接法,经高温还原/裂解法到氟化法,再演化到改进后的高温还原法(包括TC/EA-IRMS法),甚至激光原位技术,样品由实验室纯化学试剂扩展到各种复杂地质样品,在测量精确度、测量速度、样品用量、安全性和技术要求方面都有巨大改进.在分馏机理上,①尽管Longinelli等建立的关系式已获得了天然样品的验证,并认为是平衡分馏,但实验室模拟结果与其还存在较大差异(即没有达到平衡分馏).②在地表温度和pH条件下,无机过程均不会造成水体中溶解态磷酸盐和水之间的氧同位素交换.在高温(>70℃)及不同pH条件下,即使没有生物作用也会造成溶解磷酸盐和水分子之间进行氧的同位素交换,但不同实验室之间结果不一致.③在生物作用存在下,溶解无机磷酸盐和水之间在地表环境会发生强烈氧同位素交换,但除了PPase外,其余均没有达到平衡值.④磷灰石的氧同位素组成要比形成它的溶解态磷酸盐的值高1‰～1.4‰,因此在把Longinelli等关系式用于溶解态磷酸盐和水体系时,需要考虑该因素.同位素平衡分馏和条件有关,认为无机条件下的高温(>70℃)实验结果不一致,以及有生物参与的培养实验结果偏离平衡值,都是实验条件不同所致,包括pH、磷酸盐浓度、生物种类、生物量等.

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

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.