Evaluating the limits and potential of fluid-calcite δ¹⁸O and Δ₄₇ thermometry in modern speleothems from Borneo

Speleothems are among the most important terrestrial climate archives, combining precise chronologies with sensitivity to temperature and hydrology, yet reconstructing absolute temperatures remains challenging. While fluid inclusion microthermometry provides the most direct temperature constraints, it requires large, well-preserved inclusions and is not applicable to all samples. Emerging approaches such as TEX₈₆ may require site specific calibrations and remain in early stages of development. Consequently, stable isotope-based approaches including fluid-calcite δ¹⁸O thermometry and clumped isotope Δ₄₇ thermometry represent promising options for speleothem paleothermometry, despite both being affected by isotopic disequilibrium inherent to speleothem growth. Here, we evaluate whether empirical calibrations that incorporate mean disequilibrium effects can yield meaningful temperature estimates in a tropical setting. Using actively growing speleothems from caves in Borneo, we assess whether δ¹⁸O and Δ₄₇ based thermometers can be applied despite expected disequilibrium, whether disequilibrium effects are consistent among samples, and whether particular speleothem morphologies are better suited for clumped isotope thermometry.

We measured calcite δ¹⁸O and Δ₄₇, together with fluid inclusion δ¹⁸O, in stalagmites, stalactites, flowstones, soda straws, and pool carbonates. We find that oxygen isotope derived temperatures calculated using a speleothem specific calibration such as Tremaine et al. (2011) agree well with modern cave temperatures, with a 1σ spread of approximately 1.5 °C across 22 growth layers from nine different stalagmites. In contrast, Δ₄₇ based temperatures show large sample dependent disequilibrium effects, with an approximately 4 °C 1 σ catter across 14 specimens. Only pool carbonates record Δ₄₇ values consistent with isotopic equilibrium. These results indicate that for Holocene samples fluid-calcite δ¹⁸O thermometry can provide meaningful absolute temperatures with an inherent uncertainty of ± 1.5°C, whereas Δ₄₇ disequilibrium effects are highly variable and indicate that an empirical calibration incorporating mean disequilibrium would not yield robust temperature estimates. Pool carbonates emerge as the only speleothem type consistently suitable for clumped isotope thermometry