[M-IS12] Paleoclimatology and paleoceanography

[MIS12-11] An advanced method of temperature reconstruction from stalagmites with dual clumped isotopes thermometry

*Hirokazu Kato¹, Akira Murata², Taiki Mori⁴, Ryoko Senda³, Akihiro Kano² (1.Faculty of Education & Human Sciences, Teikyo University of Science, 2.Department of Earth and Planetary Science, The University of Tokyo, 3.Graduate School of Social and Cultural Studies, Kyushu University , 4.Chuo Kaihatsu Corporation)

Keywords:dual clumped isotopes, carbonate clumped isotope, stalagmite, paleo temperature, paleo climate

Carbonate clumped isotope (Δ₄₇) thermometry is a promising approach to reconstruct the temperature of carbonate formation. The principle of the method is based on temperature dependency of abundance anomaly of ¹³C-¹⁸O bond relative to the stochastic abundance. It is however problematic to estimate temperatures from Δ₄₇ values of stalagmites which depositions were induced by CO₂ degassing from the parent water. CO₂ degassing leads to isotopic disequilibrium and results in smaller Δ₄₇ values, which lead to overestimation of deposition temperatures.
Recent theoretical studies suggested that dual clumped isotopes (combination of Δ₄₇ and Δ₄₈) analyses can identify such kinetic biases (Guo and Zhou, 2019; Guo, 2020). In early stage of CO₂ degassing, Δ₄₇ of carbonate shows negative departure from the expected equilibrium value, whereas Δ₄₈ shows positive departure. The relation between the Δ₄₇ and Δ₄₈ departures can be theoretically predicted (approximately −1 vs. 1). Using the theoretical relation between Δ₄₇ and Δ₄₈ departures, the stalagmite Δ₄₇ values can be corrected to the equilibrium values which associated to quantitative absolute temperatures. In this way, the dual clumped isotope thermometry eliminates the obstacle of disequilibrium effect on stalagmite Δ₄₇ (and Δ₄₈) values.
We calculated dual clumped isotopes (Δ₄₇ and Δ₄₈) of two stalagmites from Hiroshima Prefecture (Hiro-1 stalagmite; 4.5–18.1 ka) and Gifu Prefecture (OT02 stalagmite; 2.6–8.8 and 34.8–63.5 ka). The time-series changes of terrestrial temperature based on Δ₄₇ values of these stalagmites are already reported (Kato et al., 2021; 2023). In addition, we newly analyzed dual clumped isotopes from 60 layers of a stalagmite KA03 (5.2–13.2 ka and 22.6–83.4 ka; Mori et al., 2018) from Mie Prefecture at Kyushu University. We found negative Δ₄₇ departure (approx. −0.010–−0.015‰) and positive Δ₄₈ departure in these Japanese stalagmites, which probably attributed to disequilibrium induced by CO₂ degassing. The stalagmite Δ₄₇ values were corrected to the equilibrium values and converted to absolute temperatures. The temperature history revealed from the dual clumped isotopes of KA03 exhibit cyclic cooling corresponding to Heinrich stadials in the latest Pleistocene, cooling to the last glacial maximum and warming in the Holocene.
The dual clumped isotopes (Δ₄₇ and Δ₄₈) thermometry is a progressive method which overcomes the weakness of Δ₄₇ thermometry applied to stalagmites and makes it possible to reconstruct paleo terrestrial environments in higher reliability.

References
Guo, W. & Zhou, C. 2019, GCA 267, 196–226.
Guo, W. 2020. GCA 268, 230–257.
Kato et al. 2021. QSR 253, 106746.
Kato et al. 2023. ChemGe 622, 121390.
Mori et al., 2018. QSR 192, 57–58.