Emma A. A. Versteegha | Mark E. Hodsonb | Stuart Blackc | Matthew G. Cantid

Department of Geography and Environmental Science

Earthworm secreted CaCO3 granules

Many earthworm species are true biomineralisers, secreting granules of intricately zoned calcium carbonate. These granules are frequently found in archaeological finds and buried soils.

SEM pictures of a Lumbricus terrestris calcite granule: (a) entire granule, (b) surface calcite crystal structure1, (c) individual calcite crystal.

(a) The amount of granules produced by one L. terrestris in one month; (b) EBSD image of a granule cross section showing crystallographic orientation of calcite crystals. Image courtesy of Prof. Martin Lee, University of Glasgow.

Methods

Individual L. terrestris earthworms were kept in bags of 300 g of moist soil. Two different types of soil were used, and three isotopically different types of mineral water. The experiment was performed at three different temperatures. After 28 days the stable isotopic composition of the soil pore water and CaCO3 granules was measured.

δ13C and δ18O values

Higher soil solution δ18O values yield higher CaCO3 δ18O values, and higher temperatures result in lower CaCO3 δ18O values. δ13C values show a wide range, possibly related to different food sources (peat / manure ).

A new terrestrial palaeothermometer?

Predicted equilibrium δ18O values2 are plotted with the average measured δ18O value per replicate. A linear regression shows that earthworm granules are systematically enriched in 18O by 1.51 ‰ in comparison to equilibrium.

Regression analysis between the fractionation factor α and 103 T-1 (K) yields the palaeotemperature relationship:

1000 ln α = 20.21 (103 T-1) - 38.58

with R2 = 0.95; n = 96; p < 0.0005. Results are compared to equations for synthetic carbonates2 and Mytilus edulis3. δ18O values of calcite granules produced by L. terrestris reflect soil solution δ18O values and temperature, but are enriched in 18O in comparison to equilibrium.

Ongoing and future work

In combination with U-Th series dating, the δ18O composition of earthworm secreted calcite granules enables the reconstruction of past temperatures. δ13C values are possibly a proxy for vegetation and soil organic matter composition. Preliminary temperature reconstructions on granules from several Pleistocene time intervals yield realistic values.

References 1. Lambkin D.C., Gwilliam K.H., Layton C., Canti M.G., Piearce T.G., Hodson M.E. (2011) Production and dissolution

rates of earthworm-secreted calcium carbonate. Pedobiologia 54, S119-S129.

2. Kim S.-T., O'Neil J.R. (1997) Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates. Geochimica et Cosmochimica Acta 61, 3461-3475.

3. Wanamaker A.D., Jr., Kreutz K.J., Borns Jr. H.W., Introne D.S., Feindel S., Funder S., Rawson P.D., Barber B.J. (2007) Experimental determination of salinity, temperature, growth, and metabolic effects on shell isotope chemistry of Mytilus edulis collected from Maine and Greenland. Paleoceanography 22, 12.

Acknowledgements We thank Yan Gao for helping with the isotopic analysis, and Martin Lee for taking the EBSD image.

Earthworm-secreted calcite: A new palaeoenvironmental proxy

a b

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) Environment Department, University of York, Heslington, York YO10 5DD, UK

c) Department of Archaeology, University of Reading, Whiteknights, Reading RG6 6AB, UK

d) English Heritage, Centre for Archaeology, Fort Cumberland, Eastney, Portsmouth PO4 9LD, UK