Pallab Roy , Amzad H. Laskar , Sangbaran Ghoshmaulik , S.K. Bhattacharya , Ravi Rangarajan , Mao-Chang Liang
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引用次数: 0
Abstract
The triple oxygen isotope composition of carbonate deposits provides a proxy for understanding changes in the past hydrological cycle. The reconstruction relies on knowing the temperature dependence of 17O and 18O fractionation factors, 17α and 18α, and θ = ln17α/ln18α, associated with the carbonate precipitation which can take place at various temperatures in nature. To determine these parameters, we synthesized carbonates at temperatures ranging from 10 °C to ∼50 °C from various substrate waters. The oxygen isotope composition of the waters was obtained using the CO2-H2O exchange method at 25 °C and the composition of the carbonate was derived from that of CO2 generated by reacting the carbonate with phosphoric acid at 25 °C. In both cases, the δ17O and δ18O values of the CO2 were measured. To convert the δ17O and δ18O values of the water equilibrated CO2 to the corresponding values of the water phase, a prior knowledge of the 17O and 18O fractionations during the CO2-H2O exchange is necessary. Liang et al. (2023) previously determined these fractionations by equilibrating CO2 with VSMOW2 (δ17O and δ18O = 0 ‰) at 25 °C and measuring the resulting isotope values. Building on this work, we present rescaled and refined fractionation values: 17α = 1.02135 ± 0.00001, 18α = 1.04122 ± 0.00002, and θ = 0.5230 ± 0.0001. With this knowledge, we calculated the values of θ relating the CO2 derived from acid–reacted carbonates (precipitated at different temperatures) and the parent water (θCO2(carb)-w). The θCO2(carb)-w (mean ± SE) at 10 °C, 25 °C, and 35 °C are 0.5232 ± 0.0001, 0.5233 ± 0.0003, and 0.5229 ± 0.0002, respectively. We also did these experiments at 50 °C. However, the θCO2(carb)-w (0.5129 ± 0.0004) at 50 °C was likely affected by kinetic isotope fractionation. We also determined the acid digestion fractionations at various temperatures ranging from 0 °C to 110 °C. We did not observe significant temperature dependence in Δ′17O values of the acid digested CO2 across the above temperature range, the triple oxygen isotope slope (relating δ’17O and δ’18O) is 0.5220 ± 0.0002.
期刊介绍:
Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry.
The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry.
Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry.
The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.