Using MicroCT to Improve 230Th/U Dating of Speleothems

Isotopic and elemental proxy records from cave deposits, or speleothems, provide crucial information for understanding changes in past climates. However, in order to extract the most useful information from speleothems, these proxy records must be accompanied by precise and accurate chronologies. Speleothems can be accurately and precisely dated using the ²³⁰Th/U technique, but the dates will be degraded if the analyzed carbonate is contaminated by U- or Th-bearing foreign materials. Such contaminants can be difficult or impossible to detect using conventional microscopic approaches when they are below the speleothem surface.

To address this problem, we have developed a microCT-based method to identify clean, dense carbonate samples for ²³⁰Th/U dating that is non-destructive and rapid. We applied our technique to a Holocene stalagmite from White Moon Cave (WMC) on the central California coast. Previous studies of stalagmites from this cave demonstrate that they record useful information about past rainfall variability in California. However, the stalagmites contain sub-mm to mm-sized pockets of silicate detritus that, if analyzed, greatly reduce the precision of the resulting ²³⁰Th/U ages. The microCT reveals different minerals based on density, allowing pockets of silicate detritus below the surface to be detected. Using the microCT, we scanned pieces of stalagmite WMC2 that had been previously cut for trace element analysis by laser ablation ICP-MS. We then used the microCT images to select volumes to sample for ²³⁰Th/U analyses. These images also allowed us to identify the contaminating phases as mica and quartz, which are present in trace amounts in the cave's marble host rock. Using this strategy, we were able to improve the precision of detritus-corrected ages by up to 70-100% relative to analyses where this screening method was not employed. This method shows promise for optimizing the selection of material for ²³⁰Th/U or other proxy analysis and can thus help provide improved speleothem paleoclimate records.