Stalagmites with ‘drip cups’ – are they suitable for palaeotemperature reconstructions?

Abstract

Stalagmites (carbonate deposits in caves) are excellent archives of palaeoenvironmental changes, as they are not only amenable to high-precision radiometric dating but host numerous proxy variables that reflect past changes in climate conditions related to hydrology, temperature, and atmospheric dynamics. In most stalagmites, the convex shape of the apex leads to rapid runoff of dripwater, which results in a thin film of water on top of the stalagmite, fast CO2 degassing, and depletion of the dissolved inorganic carbon (DIC) pool. These kinetic dynamics lead to carbon and oxygen isotope fractionation, hampering quantitative estimation of the carbonate formation temperature and reconstruction of original dripwater composition using, e.g., clumped isotope systematics (Affek et al. 2008; Affek & Zaazur 2014). Equilibrium conditions can be achieved however in subaqueously-formed speleothems (Daëron et al. 2019). Whether stalagmites that possess a ‘drip cup’, i.e., a concave apex, show near-equilibrium conditions and constitute archives of past temperature should be testable using isochronous clumped isotope analyses. We hypothesized that the deeper water body inside the ‘drip cup’ limits CO2 degassing and DIC depletion, enabling near-equilibrium conditions compared to the outer slope of the rim wall surrounding the ‘drip cup’ that is likely affected by kinetic fractionation. We tested this hypothesis using multiple isochronous samples from stalagmite MAYA-22-7, taken at Cenote Ch’en Mul, Mayapán, Yucatán (México). The actively growing stalagmite was collected in 2022. Air temperature in the cave chamber is very stable, at 25.7 ± 0.6°C year-round. The stalagmite shows a prominent and several-mm-deep ‘drip cup’ in its lower growth section. U-series dating showed that this interval is ca. 500 years old. Very thin growth layers inside the ‘drip cup’ and thick growth layers at the rim wall indicate different growth conditions across the stalagmite at the time of formation, likely with suppressed DIC depletion and reduced kinetic isotope fractionation in the ‘drip cup.’ Δ47 values were predicted to be highest inside the drip cup (if kinetic isotope fractionation increases with distance from the apex) or to remain constant (if kinetic effects are negligible). We milled seven subsamples from growth layers of the same age for Δ47 analysis. Each sample integrates several years (< 10) because of the required sample mass (~ 4.5 mg). Clumped isotopes were measured at Northumbria University on a NU Instruments Perspective IRMS. We discuss the clumped isotope dynamics along a single growth interval and implications for palaeotemperature reconstructions. Finding near-equilibrium conditions inside the ‘drip cup’ would offer the opportunity to reconstruct past temperatures, if not continuously, then at least for some time intervals.