the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Apr 2026
Challenges of initial Thorium and Approaches to Robust Speleothem Age Models: A case study from the Yucatán peninsula, Mexico
Abstract. Speleothems, such as stalagmites and flowstones, are invaluable archives of past environmental and climatic conditions due to their layered growth and suitability for precise 230Th/U dating. These natural formations record hydroclimatic variability over time, offering insights into how environmental changes have influenced ecosystems and human societies. In the context of Mesoamerica, speleothem records provide an opportunity to explore potential links between climate variability and sociopolitical transformations during Maya cultural evolution. Paleoclimate archives from this region document severe dry conditions during the Terminal Classic Period (∼800–1000 AD), a time marked by societal decline and urban abandonment among the Maya. Yet, existing records often suffer from limited chronological precision, and high-resolution multi-proxy datasets from the area remain scarce. This study presents extensive 230Th/U dating of several speleothems from Áaktun Kóopo Cave, Yucatán, revealing continuous carbonate deposition over the past 2.7 kyr, encompassing the entire era of Maya cultural evolution, as well as evidence of speleothem growth during earlier glacial and interglacial periods. High uranium concentrations in the speleothems (averaging 1 ppm) enabled precise dating, despite challenges from elevated and variable detrital thorium contamination. These challenges were addressed by combining multiple techniques, including isochron analysis, stratigraphic approaches, and annual layer counting, to constrain elevated and initial (230Th/232Th) activity ratios. Notably, 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. Still, our approach yields stalagmite chronologies from Áaktun Kóopo Cave that provide a robust foundation for detailed multi-proxy reconstructions of hydroclimate and vegetation changes over the past 2.7 kyr. These chronologies address a critical gap in high-resolution data for this region and enable future studies to better resolve environmental conditions throughout Maya history.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Geochronology.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 04 Jun 2026)
- CC1: 'Comment on egusphere-2026-2284', Giacomo Medici, 04 May 2026 reply
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RC1: 'Comment on egusphere-2026-2284', Anonymous Referee #1, 13 May 2026
reply
This paper presents a large number (n > 150) of MC-ICP-MS U-series dating of stalagmites from a cave in the northeastern Yucatán Peninsula, Mexico. These stalagmites are particularly important because they cover the entire period of Maya cultural evolution and can be used to reconstruct a continuous, high-resolution climate record over the past 2700 years. These samples are characterized by high U and low Th concentrations, which is generally favorable for precise U-series dating. However, dating results reveal significant age inversions. These inversions are attributed to elevated initial 230Th/232Th ratio, ranging from 4 to 68, as estimated through multiple approaches, including U-series dating of modern carbonate and drip water, isochron analyses, stratigraphic constraints, and lamina counting. This paper demonstrates the potential of large variations of initial 230Th/232Th ratio in speleothem, which may pose a substantial challenge for accurate U-series dating. I think this is well-organized study supported by convincing evidence.
My major comments are given below:
- The study focuses on stalagmites from Áaktun Kóopo Cave, located in the northeastern Yucatán Peninsula, and cites several other stalagmites records from elsewhere on the Peninsula. Is the elevated initial 230Th/232Th ratio specific for Áaktun Kóopo Cave, or does it reflect a regional characteristic?
- Samples with (230Th/232Th) activity ratio <200 were excluded from age modeling, and their corrected ages are not presented in Table S3. However, their initial (230Th/232Th) activity ratios are still presented. I suggest either presenting both the corrected ages and the initial values, or removing both sets of values. In the main text, the initial (230Th/232Th) activity ratios are reported between 4 and 68, but the corrected ages corresponding the higher initial values (>50) were discarded. Does this imply that the derived high initial values (>50) are unreliable?
- The sample EO-C-81.5 has the lowest Th concentration (~0.00013 ppb) and the highest (230Th/232Th) activity ratios (213749), yet its corrected age is still not consistent with the stratigraphic order.
- In Table S3, there are two rows of data for sample E6-31.5 and three rows of data for E6-220. Are these replicated analyses? Given the challenging of the Áaktun Kóopo Cave samples, it would be beneficial to present replicate analyses to show the reproducibility.
Citation: https://doi.org/10.5194/egusphere-2026-2284-RC1
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General comments
Very good research on paleo-research with angle on the climate of the past. Please, follow my straightforward comments to improve the manuscript.
Specific comments
Line 14. “Natural formations”. I would use another term like “natural features”. “Formation” is a term used in lithostratigraphy with another meaning.
Lines 33-35. “Reconstructing past environmental and climatic variability relies on archives that combine continuity, sensitivity to climate, and robust dating methods. Speleothems fulfil these criteria, as their layered carbonate growth can be precisely dated using U-series disequilibrium dating”. Insert two recent papers on speleothems in carbonate deposits with a paleoclimate link:
- Koltai, G., Scholger, R., Knipping, M., Wang, J., Cheng, H., Frisia, S., ... Spötl, C. 2026. Ancient speleothem giant preserved in a high-Alpine cave (Dolomites, N Italy): Rare insights into the Neogene. Palaeogeography, Palaeoclimatology, Palaeoecology, 113619.
- Medici, G., Marianelli, D., Cornacchia, I., Gori, F., Brandano, M. (2026). Multi-disciplinary approach to paleokarst occurrence in the Eocene–Oligocene succession of the Apulia Carbonate Platform (Salento, Italy). Facies, 72(2), 17.
Line 103. Clarify and describe the specific objectives of your research by using numbers (e.g., i, ii, and iii).
Line 113. Insert more detail on the depositional paleoenvironment for all the sedimentary deposits described in this research.
Line 205. “Subsamples”. This terminology is unclear. I suggest a change.
Figures and tables
You might want to add images of the study site, and the specific cave.
Figure 1. Issues of graphic resolution. The coordinates are un-readable.
Figure 1. The current version of this figure is not very informative. The scope of Figure 2b is unclear.
Figure 2a. I would add a correlation/regression equation and relative line/curve.
Figure 4. I would increase the size of numbers and letters. They are difficult to read.
Figure 4. You might consider inserting a spatial scale for the speleothems.