Oxygen isotope fractionation in phosphates: the role of dissolved complex anions in isotope exchange (original) (raw)
Professor Dr Jochen Hoefs on the occasion of his 75th birthday. Oxygen isotope fractionation factors for phosphates were calculated by means of the increment method. The results suggest that Ag 3 PO 4 and BiPO 4 are enriched in 18 O relative to AgPO 4 , and the three phosphates are consistently depleted in 18 O relative to Ba 3 [PO 4 ] 2 ; fluorapatite and chlorapatite exhibit a similar behaviour of oxygen isotope fractionation with consistent enrichment of 18 O relative to hydroxya-patite. The valence, radii and coordination of metal cations play a quantitative role in dictating the 18 O/ 16 O partitioning in these phosphates of different compositions. The calculated fractionation factors for the Ag 3 PO 4 –H 2 O system are in agreement with experimental determinations derived from enzyme-catalysed isotope exchange between dissolved inorganic phosphate and water at the longest reaction durations at low temperatures. This demonstrates that the precipitated Ag 3 PO 4 has completely captured the oxygen isotope fractionation in the dissolved inorganic phosphate. The calculated fractionation factors for the F/Cl-apatite–water systems are in agreement with the enzyme-catalysed experimental fractionations for the dissolved phosphate–water system at the longest reaction durations but larger than fractionations derived from bacteria-facilitated exchange and inorganic precipitation experiments as well as natural observations. For the experimental calibrations of oxygen isotope fractionation involving the precipitation of dissolved phosphate species from aqueous solutions, the fractionation between precipitate and water is primarily dictated by the isotope equilibration between the dissolved complex anions and water prior to the precipitation. Therefore, the present results provide a quantitative means to interpret the temperature dependence of oxygen isotope fractionation in inorganic and biogenic phosphates.
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RATIONALEThe integrity of the biological phosphate oxygen isotope (δ18Op) signal is thought to be contingent upon the complete removal of competing sources of oxygen such as associated organic matter. A range of pretreatment methods to purify phosphate material from competing sources of oxygen has been reported, with contradictory evidence on the usefulness and efficiency of one or another. Yet, a systematic comparison of these techniques for bioapatite phosphate has not been conducted.The integrity of the biological phosphate oxygen isotope (δ18Op) signal is thought to be contingent upon the complete removal of competing sources of oxygen such as associated organic matter. A range of pretreatment methods to purify phosphate material from competing sources of oxygen has been reported, with contradictory evidence on the usefulness and efficiency of one or another. Yet, a systematic comparison of these techniques for bioapatite phosphate has not been conducted.METHODSChemical and thermal pretreatment techniques were tested for their effectiveness at removing organic matter and the likelihood that they modify original δ18O values. The test was performed in inorganic (synthetic apatite and a phosphorite rock) and organic (bone and tooth tissues) phosphate materials for which we had an expectation of the actual original δ18Op value. Analysis of nitrogen content (wt.%), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy were employed.Chemical and thermal pretreatment techniques were tested for their effectiveness at removing organic matter and the likelihood that they modify original δ18O values. The test was performed in inorganic (synthetic apatite and a phosphorite rock) and organic (bone and tooth tissues) phosphate materials for which we had an expectation of the actual original δ18Op value. Analysis of nitrogen content (wt.%), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy were employed.RESULTSWe detected variable efficiency at removing organic matter between pretreatment methods with no correlation to any specific structural change. The δ18Op results showed considerable variation between samples pretreated with the different methods and the untreated samples, with a compositional range of up to 4.5 ‰ in the bone samples. Variations of the δ18Op values within error were found for tooth enamel, phosphorite rock and inorganic apatite.We detected variable efficiency at removing organic matter between pretreatment methods with no correlation to any specific structural change. The δ18Op results showed considerable variation between samples pretreated with the different methods and the untreated samples, with a compositional range of up to 4.5 ‰ in the bone samples. Variations of the δ18Op values within error were found for tooth enamel, phosphorite rock and inorganic apatite.CONCLUSIONSWe recommend a cautious approach when interpreting and comparing δ18Op data from bone samples treated with different pretreatment protocols. In general, the untreated samples seem to show δ18Op values closer to the expected ones. According to our results, pretreatment is completely unnecessary in highly mineralized tissues. Copyright © 2012 John Wiley & Sons, Ltd.We recommend a cautious approach when interpreting and comparing δ18Op data from bone samples treated with different pretreatment protocols. In general, the untreated samples seem to show δ18Op values closer to the expected ones. According to our results, pretreatment is completely unnecessary in highly mineralized tissues. Copyright © 2012 John Wiley & Sons, Ltd.
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