EARTHWORM SECRETED CALCIUM CARBONATE – A NEW PALEOTHERMOMETER?

The chemical composition of calcium carbonate (CaCO₃) skeletons produced by a range of aquatic and terrestrial organisms has been proven to be a useful tool in paleoenvironmental reconstructions. Oxygen isotope ratios (δ¹⁸O values) are generally found to be in equilibrium with the environment, solely depending on oxygen isotopic composition of ambient water and temperature. Stable carbon isotope ratios (δ¹³C values) of biogenic CaCO₃ are generally the result of mixing of different C sources ingested by the animal, and can shed light on ecological variables like vegetation composition and food sources.

Although they do not form skeletons, many earthworm species are true biomineralizing animals, secreting calcium carbonate granules, predominantly of calcite but also containing aragonite and amorphous calcium carbonate. These granules are frequently found in archaeological finds and buried soils. We are currently engaged in research investigating the utility of stable isotope compositions of earthworm secreted calcite granules for reconstructing past environments.

Experiments were designed in which individual earthworms (Lumbricus terrestris) were kept in bags of soil for up to four months at a constant temperature. Three different types of soil (with both C₃ and C₄ vegetation) were used which were air-dried and then moistened with three (isotopically) different types of mineral water. The experiment was performed at three different temperatures. δ¹³C and δ¹⁸O values were measured for the soil organic matter, soil pore water, food (manure), soil air, earthworm tissues and CaCO₃ granules. Preliminary results show that the δ¹⁸O values of the granules accurately reflect those of the pore water in the soils, in a similar way as for inorganically precipitated calcite. δ¹³C values appear to reflect those of food (manure) offered to the earthworms.

Our initial results suggest that, in combination with U/Th dating of the granules, the stable isotope composition of earthworm secreted calcite granules can help in the reconstruction of past temperatures, vegetation and soil organic matter composition. As such it provides a much needed new terrestrial proxy for the reconstruction of past environments in archaeological and geological contexts.