Rationale Application of clumped isotope palaeothermometry to speleothems has been restricted largely to phreatic samples because of kinetic fractionation processes that occur during vadose speleothem formation, which leads to erroneously high inferred temperatures. Speleothems are spatially near-ubiquitous terrestrial archives that can be dated accurately over million-year timescales. Thus, wider application of the clumped isotope technique in speleothems could dramatically increase understanding of terrestrial thermal history. In this study we assess the potential of speleothem drip cups (concave depressions at a stalagmite apex in which dripwater accumulates to create a phreatic environment) for reliable palaeotemperature inference. Methods We sampled along two isochronous layers that extend across both sides of a pronounced drip cup in stalagmite MAYA 22-7 from Ch'en Mul, Mexico, which was dated to 1650 CE ± 23 years. We measured bulk stable (δ 18O and δ 13C) and clumped (Δ 47) isotope values at increasing distances from the drip cup centre to test for kinetic fractionation effects. Results Higher δ 18O, δ 13C and Δ 47 values were obtained from the drip cup's central phreatic zone compared with the vadose flanks, demonstrating reduced (Δ 47) isotope fractionation in the phreatic zone. Average clumped isotope temperatures (T Δ47) inferred from phreatic samples fall within the range of modern cave temperatures but are on average 3.6°C warmer than estimated formation paleotemperatures, suggesting a small degree of kinetic effects persists. Conclusions We propose that drip cup carbonates have the potential to yield reliable palaeotemperatures and describe a widely applicable test for clumped isotope equilibrium precipitation in speleothem drip cups by sampling across isochronous layers.
Umbo et al. (Wed,) studied this question.