Novel palaeoecological proxies and methods are always interesting, offering new or better insights into the past environment. This week I found out from the BBC that earthworm poo can be used as a climate proxy. The BBC article even links to the open access article. I think the quirkiness of the study is responsible for the coverage this proxy received. I want to consider the utility of the proxy.
Versteegh, E.A.A, Black, S., Matthew G. Canti, M.G., Hodson, M.E. (2013) Earthworm-produced calcite granules: a new terrestrial palaeothermometer? Geochimica et Cosmochimica Acta
Biominerals are one of the most important carriers of palaeoecological proxies. For example, the isotopic composition of calcium carbonate in foraminiferal tests, corals and mollusc shells can be related to the temperature and salinity of the water in which the organisms grew. So it is not surprising that someone has explored the potential of the small calcite granules produced by earthworms as palaeoenvironmental proxies.
Although the function of these calcite granules, first described by Darwin (1881), is unclear, Versteegh et al show that there is a strong relationship between temperature and their oxygen isotope composition. The relationship is similar, but noisier, to that found in calcite from other biomineralisers. In principle, this relationship can be used to estimate past temperatures from the isotopic composition of earthworm granules found in fossil soils. I don’t think this is going to be trivial.
The first problem is the noise. Versteegh et al did their experiments under ideal controlled conditions, but still the noise in the relationship between temperature and isotopic composition is about twice that for mussels, giving an average error of 0.7°C.
This error is much smaller that what it will be possible to achieve in practice. To estimate temperature from the oxygen isotopes in calcite, you need to know the isotopic composition of the soil water at the time the granule was formed. It is possible to make some assumptions about the soil water’s isotopic composition from modern studies and speleothems in caves, especially in soggy climates where evaporation usually has a relatively minor influence on the isotopic composition of soil water. This weak constraint on soil water isotopic composition will increase the uncertainty on the temperature reconstructions in wet climates, and make the proxy almost unusable in arid climates.
Most proxies don’t record exactly what we are interested in. For example, the isotopic composition of foraminifera tests reflect that at the time and depth of calcification, not the summer (or annual) sea surface temperature that we want to know. Earthworm calcite granules will record soil temperature at the time of granule formation. Soil temperature will vary with depth, and on the amount of shade at the site, being warmer on a sunny summer’s day beneath a grassland than a forest. Soil temperature will also vary through the year. Granule production will presumably vary seasonally, as earthworm activity will vary with temperature (and soil moisture). These factors will result in a large amount of isotopic variation, as Versteegh and co-workers have found, and potential biases, that will limit the proxy’s utility.
The final factor that will limit the utility of earthworm granules is the availability of suitable fossil soils. Continuous profiles of lake and ocean sediment spanning thousands to millions of years are available from many sites, in contrast to the typically fragmentary record of fossil soils (except for loess deposits which usually form in arid environments). Certainly high-resolution reconstructions of earthworm calcite-temperatures will be impossible. Earthworms will have an advantage over terrestrial molluscs as smaller sample sizes will be needed to collect enough material for analysis, but due to their small size, might be more prone to dissolution, especially in more acid soils.
I think that despite the media attention, earthworm calcite-derived temperature estimates are going be a niche temperature proxy. They will probably be more useful in conjunction with other isotopic proxies. For example, earthworm calcite-isotopes could be paired with isotopes from lakes to understand the contribution of the catchment to the isotopic signal in the lake. Or the earthworms could be paired with the isotopic composition of tree-rings. In both cases, finding and dating suitable archives for the worms will be the limiting factor.