Reconstructing climate dynamics on terrestrial environment using the stable isotope composition of earthworm calcite granule: An experimental approach

Reconstructing past climate dynamics on terrestrial environment remains a major challenge in paleoclimate research. Improving our understanding of how continental ecosystems responded to abrupt climate oscillations is essential for assessing future climate impacts on terrestrial environments and human societies. While ice-core and marine archives document large-scale and rapid climate variability, the links between climate and continental surface processes remain poorly constrained. Identifying robust climate proxies in continental sedimentary records is therefore crucial.

Fossil earthworm calcite granules preserved in loess–paleosol sequences have recently emerged as promising archives of past climate conditions, providing insights into temperature and precipitation during the last glacial period in Western Europe. However, the climatic interpretation of these proxies requires a robust calibration based on modern earthworm calcite granules to better constrain the environmental and biological parameters controlling granule formation, such as temperature, soil moisture, and litter composition.

Here, we present an experimental calibration approach using modern earthworms (Lumbricus terrestris) reared under controlled environmental conditions. Soil temperature and food sources were systematically varied to assess their influence on granule production and isotopic signatures. Calcite granules were analysed for δ¹⁸O and δ¹³C, while δ¹³C was also measured in soil organic matter and litter. For the first time, clumped isotope (Δ₄₇) measurements were performed on earthworm calcite granules, allowing direct temperature estimates independent of past soil-water δ¹⁸O.

This experimental approach provides new constraints on vital effects and isotopic fractionation in earthworm calcite granules and improves their use as quantitative paleoclimate proxies. Our results complement previously established empirical relationships between (i) the oxygen isotopic composition of meteoric water, granules, and temperature, and (ii) the δ¹³C of litter and the δ¹³C of granules, strengthening the potential of earthworm calcite granules for reconstructing past terrestrial climate dynamics.