1. Emma A. A. Versteegha | Stuart Blackb | Matthew G. Cantic | Mark E. Hodsond
Department of Geography and Environmental Science
An oxygen-isotope palaeothermometer
Many earthworm species secrete calcite granules of unknown physiological function
through specialised glands. These granules are frequently found in archaeological finds
and buried soils. We investigated their utility as a palaeothermometer by means of
δ18O analyses of calcite produced at different temperatures under controlled
laboratory conditions. Results yield the palaeotemperature relationship:
1000 ln α = 19.55 (103 T-1) – 36.28
δ18O values of calcite granules produced by Lumbricus terrestris reflect soil solution
δ18O values and temperature, but are enriched in 18O by 1.5 ‰ in comparison to
equilibrium.1
An SEM picture of an earthworm-produced calcite granule; The palaeotemperature relationship.
Silbury Hill - England – Neolithic
Silbury Hill is the largest prehistoric man-made mound in Europe. It is part of the
Stonehenge, Avebury and Associated Sites UNESCO World Heritage Site. Existing 14C
dates place the base of the mound at 4365-4140 cal BP. During this time the climate
was likely wetter and 0.5-1.5 °C warmer than the present day.3
Silbury Hill, Wiltshire, England; Third and final phase of building (c 2490-2340 BC) showing spiral
method of construction, possibly used for access to summit. Copyright English Heritage, Picture Credit
Judith Dobie.
U-Th isochron for the Silbury Hill granules.
Reconstructed temperatures range from 2.3 to 12.3 °C and overlap with modern
values (4.1-16.4 °C), but are on average lower. This is in contradiction with existing
reconstructions, which suggest similar or higher temperatures at Silbury Hill during its
construction.
Maastricht Belvédère - Netherlands - Middle Palaeolithic
The Belvédère quarry is the type location for the Belvédère interglacial, which has
previously been dated to 0.324-0.338 Ma BP (MIS 9), and contains abundant Middle
Palaeolithic artefacts. Climate reconstructions based on vertebrate and molluscan
fauna indicate high annual rainfall and mean summer temperatures probably
exceeding those of today.2
Reconstructed temperatures for Maastricht Belvédère; error bars for U-Th dates are 2σ; grey area
indicates the modern range for mean monthly air temperatures.
The Belvédère quarry near Maastricht, the Netherlands.
Discussion
Both the Silbury Hill and the Belvédère samples produce lower than expected
reconstructed temperatures, reflecting relatively high δ18O values of the granules.
Alternative explanations for this observation could be:
1. A different composition (source) of precipitation. This is unlikely for the Holocene,
but more plausible for older time intervals;
2. A large influence of evapotranspiration. Unlikely in a high-rainfall climate;
3. Preferential production of calcite granules by the earthworms during cooler
episodes and/or while deep in the soil.
The latter explanation seems most plausible. On-going work investigates this by
analysing recent granules and down soil profiles.
Contact information
a) Department of Geography and Environmental Science, University of Reading, Whiteknights, Reading RG6 6DW
e.a.versteegh@reading.ac.uk | https://sites.google.com/site/emmaversteegh/
b) Department of Archaeology, University of Reading, Whiteknights, Reading RG6 6AB, UK
c) English Heritage, Centre for Archaeology, Fort Cumberland, Eastney, Portsmouth PO4 9LD, UK
d) Environment Department, University of York, Heslington, York YO10 5DD, UK
References
1. Versteegh, E.A.A., Black, S., Canti, M.G., Hodson, M.E., 2013. Earthworm-produced calcite granules: a new terrestrial
palaeothermometer? Geochimica et Cosmochimica Acta.
2. Van Kolfschoten, T., Roebroeks, W., Vandenberghe, J., 1993. The Middle and Late Pleistocene sedimentary and
climate sequence at Maastricht-Belvédère: The type locality of the Belvédère interglacial. Mededelingen Rijks
Geologische Dienst 47, 81-91.
3. Whitehead, P.G., Edmunds, W.M., 2012. Modelling and reconstruction of the River Kennet palaeohydrology and
hydrogeology: Silbury Hill and Avebury in 4,400 BP. International Water Association, London, 9 pp.
Acknowledgements
We thank Yan Gao for helping with the isotopic analysis, and Tom Meijer (Naturalis Biodiversity Center) for providing
the Maastricht Belvédère granules.
Pleistocene and Holocene temperature
reconstructions using earthworm-produced calcite
13 granules were analysed for δ18O values
and were individually U-Th dated. U-Th
dates are consistent with the Belvédère
interglacial. Average δ18O values of modern
precipitation from the nearby GNIP station
Beek were used to calculate reconstructed
soil temperatures. Reconstructed
temperatures overlap with, but are on
average lower than modern temperatures,
in contrast with existing reconstructions.
10 granules were used to construct
an U-Th isochron, which yielded an
age of 4670 ± 440 years, agreeing
well with the existing 14C data.
8 granules were analysed for δ18O
values. Average δ18O values of modern
precipitation from the nearby GNIP
station Wallingford were used to
calculate tentative temperature
reconstructions.