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 delta O-18 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 degrees C) for an acclimatisation period of three weeks followed by transfer to identical treatments and cultivation for a further four weeks. Earthworm-secreted calcite granules were collected from the second set of soils. delta O-18 values were determined on individual calcite granules (delta O-18(c)) and the soil solution (delta O-18(w)). The delta O-18(c) values reflect soil solution delta O-18(w) values and temperature, but are consistently enriched by 1.51 (perpendicular to 0.12)parts per thousand in comparison to equilibrium in synthetic carbonates. The data fit the equation 1000ln alpha = [20.21 +/- 0.92] (10(3) T-1) - [38.58 +/- 3.18] (R-2 = 0.95; n = 96; p < 0.0005). 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. (C) 2013 The Authors. Published by Elsevier Ltd. All rights reserved.