Earthworm-produced calcite granules : a new terrestrial palaeothermometer?

In this paper we show for the first time that calcite granules, produced by the earthworm Lumbricus terrestris, and commonly recorded at sites of archaeological interest, accurately reflect temperature and soil water δ1818 O values. Earthworms were cultivated in an orthogonal combination of two different (granule-free) soils moistened by three types of mineral water and kept at three temperatures (10, 16 and 20 ºC) for an acclimatisation period of three weeks followed by transfer to identical treatments and cultivation for a further 4 weeks. Earthworm secreted calcite granules were collected from the second set of soils. 18O values were determined on individual calcite granules (δ18Oc) and the soil solution (δ18Ow). The δ18Oc values reflect soil solution δ18Ow values and temperature, but are consistently enriched by 1.51 (±0.12) ‰ in comparison to equilibrium in synthetic carbonates. The data fit the equation 1000 ln α = [20.21 ± 0.92] (10^3 T^-1) - [38.58 ± 3.18] (R^2 = 0.95; n = 96; p < 0.0005). Preliminary temperature reconstructions obtained by applying our equation to granules from the Holsteinian interglacial and assuming soil solution δ18 Ow values follow modern precipitation values, yield realistic temperatures. As the granules are abundant in modern soils, buried soils and archaeological contexts, and can be dated using U-Th disequilibria, the developed palaeotemperature relationship has enormous potential for application to Holocene and Pleistocene time intervals.