Challenges of initial Thorium and Approaches to Robust Speleothem Age Models: A case study from the Yucatán peninsula, Mexico

Speleothems, such as stalagmites and flowstones, are invaluable archives of past environmental and climatic conditions due to their layered growth and suitability for precise 230 Th/U dating. In tropical karst settings, however, elevated and variable detrital thorium contamination can severely compromise age accuracy, especially for young speleothems. Here, a suite of stalagmites from Áaktun Kóopo' Cave from the Yucatán Peninsula is used as a case study to develop and evaluate a multi-method framework for constraining initial thorium and constructing robust age–depth models. We combine extensive 230 Th/U dating with local isochron analysis, stratigraphic approaches, and annual Sr/Ca layer counting to constrain elevated and initial (230Th/232Th) activity ratios and resolve pronounced chronological inversions. High uranium concentrations in the speleothems (average ca. 1 ppm) allow precise measurements, yet we infer a high and largely unsystematic variability of high initial (230Th/232Th) activity ratios in space and time, with values spanning between 4 and 68, thus far exceeding standard bulk earth values. This variability demonstrates that elevated and temporally variable initial Th may be common in tropical karst settings and that multiple, independent constraints on initial (230Th/232Th) activity ratios are essential for reliable chronologies in such environments. Applying this framework to Áaktun Kóopo' Cave stalagmites yields internally consistent stalagmite age–depth models spanning the past 2.7 kyr, as well as evidence for earlier glacial and interglacial growth phases. These chronologies provide the basis for high-resolution, multi-proxy reconstructions of climatic and environmental changes in the northeastern Yucatán Peninsula during the entire era of Maya cultural evolution, and they illustrate how robust speleothem chronologies can be obtained even in cave systems affected by elevated and highly variable initial thorium.