Rapid thermal resetting of clumped isotope in coral aragonite

Abstract

Biogenic aragonite serves as a critical archive of geological signals, and its clumped isotope analysis can provide paleoenvironmental information. However, the thermal resetting of clumped isotope (Δ₄₇) presents a major challenge, especially in deep-time carbonate samples subjected to diagenetic alteration or burial heating. Laboratory heating experiments and kinetic modeling offer important insights into constraining the initial clumped isotope composition and thermal history of carbonates. Thermal resetting has been extensively studied in calcite, dolomite, and apatite. However, mineralogical phase transitions during heating, and the resulting multiphase mixtures complicate the fitting of heating experiment data to thermally reset kinetic models for aragonite. Here, we present heating experiments designed to evaluate the thermal resetting kinetic parameters of aragonite prior to phase transitions. We treated coral aragonite materials in three forms—fragmented, powdered, and oxidized and heat them at 120–240 °C—to evaluate the role of different types of internal water on thermal resetting. We observed a rapid decrease in the Δ₄₇ values of aragonite during the initial stage of the experimental heating, which slowed significantly around 40 min. Meanwhile, we observed a decrease in oxygen isotope values during heating suggesting the isotopic exchanges between the internal water and the solid carbonate. We observed no significant difference in the thermal resetting rates between fragmented coral and powdered coral; however, oxidized coral thermal resetting rate exhibited limited acceleration compared to the other two sample types. We infer that the limited accelerated thermal resetting rate in oxidized aragonite may be due to the partial removal of organic matter during oxidative pretreatment, which increases the surface-to-volume ratio, facilitating the migration of internal water—potentially sourced from nanoscale organic-associated water or structural water—through newly formed microfractures and microchannels. The thermal resetting kinetic parameters of coral aragonite were derived by fitting the heating data to the disordered kinetic model. The mean activation energy (μ_E) is 132.0 ± 12.4 kJ mol⁻¹, the activation energy distribution width (σ_E) is 10.9 ± 0.7 kJ mol⁻¹, and the pre-exponential factor (ν₀) is 30.5 ± 3.6 min⁻¹. To better understand the long-term behavior, we simulated the thermal history of coral aragonite across a temperature range of 20–200 °C. The simulations indicate that, at temperatures above 100 °C, the Δ₄₇ values of coral aragonite reach complete equilibrium within one year. Meanwhile, below 50 °C, the clumped isotope signal of coral aragonite is susceptible to alteration. These results suggest that the use of coral aragonite clumped isotopes in deep-time studies should be approached with caution. However, coral aragonite’s high sensitivity to thermal resetting makes its clumped isotope signals useful as a diagenetic indicator. When analyzed alongside coexisting carbonate phases, the T(Δ₄₇) of aragonite can provide additional constraints on the interpretation of T(Δ₄₇) values in those coexisting minerals.