Investigation of the Global Meteoric Water Line in the past using speleothem fluid inclusions

The paleoclimate role of speleothem fluid inclusions – and namely its direct link to past precipitation – has been recognized since decades (Affolter et al., 2025). Water isotopes contained in speleothem fluid inclusions offer a unique opportunity to reconstruct and study the past water cycle. The analyses of water extracted from the speleothem calcite matrix allow a precise determination of its hydrogen (δ²H) and oxygen (δ¹⁸O) isotopes. The relationship between δ²H and δ¹⁸O isotope ratios in meteoric waters is called the Global Meteoric Water Line (GMWL). Here, we explore this relationship in the past based on speleothem fluid inclusion water.

Based on a compilation of existing data covering essentially mid- to low-latitude area, as well as new data, we developed a Global Meteoric Water Line for the past (paleo-GMWL). Results show a remarkable similarity between the paleo-GMWL inferred from the speleothem fluid inclusion water in the past, and the present-day GMWL. This demonstrates the long-term GMWL stability, at least during temperate periods when speleothems at mid- and high-latitudes grew, i.e. when mean annual air temperatures and cave air temperatures were above the freezing point.

Similarly, we analyzed the spatial distribution of δ²H by comparing speleothem fluid inclusion and recent rainfall δ²H isotope values. Results show the suitability of fluid inclusion water isotopes for the reconstruction of a global network of isotopes in precipitation in the past. However, challenges such as temporal and spatial gaps in the speleothem record remain in many regions of the world.  Nevertheless, our study highlights the high potential of speleothem fluid inclusions to investigate the spatial and temporal variability of water isotopes in the past. The fact that speleothem fluid inclusion water isotopes fall on the GMWL delineates the quality of this proxy and its ability to reconstruct the past water cycle.

 

Affolter, S., Kipfer, T., Hofmeister, E., Leuenberger, M. and Fleitmann, D., 2025. Paleoclimatic significance of water isotopes in speleothem fluid inclusions. Earth-Science Reviews, 261: 105026.