Using High-Resolution X-Ray Computed Tomography to Test the Preparation of Stalagmites for Paleo-Climate Reconstruction

Natasha Sekhon, Jay L. Banner, Dan O. Breecker, and Darrel M. Tremaine

ABSTRACT:

Stalagmites being prepared for paleoclimate analysis should typically be slabbed along the central growth axis. This is an important first step because it allows for the highest resolution of sampling with minimal over- or under-sampling of the growth layers. Further, stable isotope ratios and trace element concentrations along the central growth axis most closely record climate variability. Choosing how to slab to best expose the central growth axis for geochemical sampling is challenging based on external morphology alone. High-resolution X-ray computed tomography (XRCT) can provide the ability to discern the internal growth morphology of stalagmites non-destructively, inexpensively, and rapidly. These data can inform selection of optimal slabbing plane(s) and can help identify locations for preliminary U-series dating. We develop a conceptual screening model to assess rapidly the internal morphologies of uncut stalagmites. The spe cifics of screening the internal morphologies through XRCT scans include investigating the internal porosity of the sample, the number and size of voids and hiatuses, and the presence and absence of growth layers and growth axes. We demonstrate that XRCT scans capture the migration of center of growth in uncut stalagmites of both simple and complex internal morphologies. XRCT scanning facilitates the investigation of stalagmites with complex internal growth banding, opening up avenues to work on such samples when stalagmites with simpler internal morphologies are not available. Further, screening stalagmites for paleoclimate reconstructions using XRCT improves the sustainability of speleothem science by helping researchers select which stalagmites should be returned to caves without destructive slabbing, thereby minimizing impact on caves.

SIMPLE LANGUAGE SUMMARY:

When scientists study stalagmites (those pointy things that grow up from the floor of a cave) to learn about past climates, they usually cut a slice down the middle. This helps them see the layers inside, kind of like how tree rings show a tree's age and growth history. These layers can tell us a lot about climate changes over time. The problem is, deciding exactly where to cut based on just looking at the outside can be tricky. That's where a cool technique called high-resolution X-ray computed tomography (XRCT, think of it like a super detailed CT scan) comes in. XRCT allows scientists to see inside the stalagmites without damaging them, and it's fast and not too expensive. It helps to find the best place to cut for further study, and it can also help identify spots for a type of dating method. In this study, scientists created a model for quickly checking out the inside structures of stalagmites using XRCT. They looked for things like porous areas, voids, breaks in growth, and the existence and location of growth layers and axes. The scientists found that XRCT can effectively capture the shift of growth centers in stalagmites, even in ones that have complex inner structures. This method helps scientists work with stalagmites that have complicated internal banding, which is useful when simple stalagmites are not available. What's also great is that using XRCT makes studying stalagmites more sustainable. It helps researchers decide which stalagmites to put back in the cave without needing to cut them open, which minimizes the impact on the cave environment.