Clumped Isotope Temperature Reconstruction Using Stalagmite Drip Cups

Rationale
Application of clumped isotope palaeothermometry to speleothems (carbonate cave deposits, e.g., stalagmites and flowstones) has been restricted largely to subaqueous samples because of kinetic fractionation processes that occur during subaerial speleothem formation, which lead 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 our understanding of terrestrial thermal history. In this study, we assessed the potential of speleothem drip cups (concave depressions at a stalagmite apex in which dripwater accumulates to create a subaqueous environment) to yield reliable palaeotemperature inferences.

Methods
We sampled along two isochronous layers that extend across both sides of a pronounced drip cup in stalagmite MAYA 22-7 from Cenote Ch'en Mul, Yucatán, Mexico, which was dated to 1650 ce ± 23 years. We measured bulk stable (δ18O and δ13C) and clumped (Δ47) isotope values at increasing distances from the drip cup centre to test for kinetic fractionation effects.

Results
Lower δ18O, δ13C, and higher Δ47 values were obtained from the drip cup's central subaqueous zone compared with the subaerial flanks, demonstrating reduced isotope fractionation in the subaqueous zone. Average clumped isotope temperatures (TΔ47) inferred from subaqueous drip cup samples are 1°C–2°C higher than modern cave temperatures and 3°C–7°C warmer than estimated formation paleotemperatures derived from nearby regional reconstructions and TEX86 analysis of our sample. This suggests a persistent degree of clumped isotope kinetic effects.

Conclusions
Despite persistent kinetic effects, lower inferred temperatures from subaqueous drip cup samples suggest closer to equilibrium precipitation compared with subaerial samples. We propose that drip cup carbonates have the potential to yield reliable palaeotemperatures and describe a widely applicable test for clumped isotope kinetic effects in speleothem drip cups by sampling across isochronous layers.