Abstract
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 δ18O 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°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. δ18O 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 1000lnα=[20.21±0.92] (103T−1)−[38.58±3.18] (R2=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.
Highlights
Many organisms form reliable archives of palaeotemperature or water composition, by precipitating calcium carbonate (CaCO3) in oxygen isotopic equilibrium with their environment (Wanamaker et al, 2007; Ullmann et al, 2010; Versteegh et al, 2010)
The d18O composition of CaCO3 produced by some biomineralising taxonomic groups shows a systematic offset from equilibrium, and as such is still useful as a proxy (Ziveri et al, 2003; Rosenheim et al, 2009; Ford et al, 2010)
Higher soil solution d18Ow values yield higher d18Oc values, and higher temperatures result in lower d18Oc values (Fig. 2)
Summary
Many organisms form reliable archives of palaeotemperature or water composition, by precipitating calcium carbonate (CaCO3) in oxygen isotopic equilibrium with their environment (Wanamaker et al, 2007; Ullmann et al, 2010; Versteegh et al, 2010). The d18O composition of CaCO3 produced by some biomineralising taxonomic groups shows a systematic offset from equilibrium, and as such is still useful as a proxy (Ziveri et al, 2003; Rosenheim et al, 2009; Ford et al, 2010). The CaCO3 minerals produced by other taxonomic groups are not in equilibrium with the environment, but exhibit deviations, called vital effects (Owen et al, 2002; Juillet-Leclerc et al, 2009; Correa et al, 2010).
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