Understanding isotope exchange at chemical equilibrium for better carbon dating

In a recent article published in the journal Geochimica and Cosmochimica Actaresearchers have discussed calcite and fluid exchange rates for carbon and oxygen isotopes during chemical equilibrium.

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Background

A window into the Earth’s past is created through the analysis of the isotopic composition of carbonate minerals, which maintain a record of historical geochemical and climatic circumstances. Calcium carbonate minerals are excellent records of past climatic conditions on the planet. Effective use of any of these proxies requires knowledge of isotopic fractionation mechanisms as well as preservation of isotopic and trace element compositions across time scales up to millions of years.

Despite the fact that numerous studies have documented the isotopic alteration of calcium carbonate and a number of other minerals while in chemical equilibrium and without visible morphological alteration at the level of electron microscopic analysis, the mechanism underlying this alteration in isotopic composition is still not fully understood.

The theory of simultaneous forward and reverse reactions has been applied in recent experimental studies to estimate near-equilibrium reaction rates of a variety of minerals, including calcite, to account for the balance of isotopic compositions between the fluid and the solid after the initial precipitation, and to explain the exchange of isotopes at or near chemical equilibrium.

It is unclear what drives dynamic equilibrium as a process of isotope exchange; however, it may be related to an isotopic imbalance. It is important to understand the method by which the isotopic composition of calcite changes. For weakly crystalline materials, Ostwald ripening is probably an important control of isotopic exchanges in the early stages of mineral precipitation and development. However, well-crystallized and old materials probably have less need for this mechanism.

About the study

In this study, the authors carried out experiments in a batch reactor in chemical equilibrium between calcite and a fluid enriched in 13^VS and 18^O relative to solid at 25°C to assess the rates and processes by which the C and O isotopes were exchanged between the calcite and the fluid. To assess the effect of mineral surface and size on the exchange rates of C and O isotopes, different grain sizes of natural and synthesized calcite were examined.

The team indicated the change in the O and C isotopic compositions of calcite at room temperature, which occurred easily on short time scales, while it was not clear how to determine the amount at which this process continued on geological time scales.

The researchers assessed the rates of change in the C and O isotopic composition of calcite at 25°C and chemical equilibrium, and to generate new information about a potential process. The ionic activity product of Ca and CO₃^2- and the solubility product of pure calcite were equivalent in this definition of chemical equilibrium. At calcite-fluid chemical equilibrium, a fluid enriched in 18^O and 13^VS versus calcite was exposed to natural and synthetic calcite of different grain sizes in a series of batch reactor studies.

Comments

The specific isotope exchange process was unclear between 72 hours and 2112 hours during the large calcite experiments and between 72 hours and the conclusion of the tiny and synthetic calcite studies at 3000 hours. The projected dissolution rate of calcite at pH 8 was four orders of magnitude lower than the exchange rate of the C and O isotopes. In tests of synthetic calcite, small and first and second large, the values ​​of the 13^VS diffusion coefficients for long-term exchange were 1.3 x 10^-241.1×10^-234.2×10^-26and 1.2×10^-25 m²/s, respectively. In studies of synthetic calcite, small and second and first large, the equivalent values ​​for 18^O diffusion coefficients were 1.3 x 10^-241.0×10^-232.0×10^-26and 1.1×10^-25 m²/s, respectively.

According to the experimental results, the exchange of C and O isotopes occurred rapidly within 72 hours for all sizes of calcite grains examined. The exchange rates of C and O isotopes slowed down after 72 hours but remained very stable over thousands of hours. For all calcite grain sizes examined, the area-normalized O and C isotope exchange rates were comparable, and C and O were exchanged at a ratio of about 3:1, corresponding to CO exchange₃^2-.

The exchange rates of O and C were about four orders of magnitude lower than the dissolution rates of calcite away from equilibrium, indicating that the exchange was either governed solely by the dissolution-precipitation of pre-existing reactive sites, or by a combination of diffusion and dissolution-precipitation in the solid/aqueous state.

The results of this study could serve as a further reminder of the importance of developing a mechanistic understanding of the mechanism of isotope exchange in chemical equilibrium and as a first step in this direction.

conclusion

In conclusion, this study indicated that the alteration of the O and C isotopic compositions of calcite at Earth surface temperatures can easily occur over short periods of time, whereas it is currently difficult to determine how far this process would continue on geologic time scales. When there was an isotopic imbalance, there was a rapid initial exchange between the fluid and the solid which was mainly due to the exchange of species at the surface.

Uncertainty remains regarding the mechanism underlying the slower rate of change observed in the C and O isotopic composition of calcite on the longer timescales in the tests performed. Longer time periods of more than 72 hours could see a combination of diffusion and solid/aqueous state dissolution-reprecipitation result in the change of carbon and oxygen isotopes.

However, the significant utility of isotopic and trace element compositions of carbonates underscores the need for a mechanistic understanding of solid-fluid exchange to enable reliable interpretation of sample selection and records.

The authors mentioned that this is the first study to examine O and C exchanges simultaneously and at bulk calcite-fluid equilibrium with high-purity calcite, opening new insights into the processes and isotopic exchange rates.

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Source

Harrison, AL, Schott, J., Oelkers, EH, et al. Hydrogel scaffolds loaded with algal biomass as a biomimetic platform with antibacterial and healing activities. Geochimica and Cosmochimica Acta (2022). https://www.sciencedirect.com/science/article/abs/pii/S0016703722003295

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