Origin of calcium carbonate granules found in buried soils and Quaternary deposits

Abstract

The ovoid calcium carbonate granules of less than 2 mm in size commonly found by molluscan specialists and soil micromorphologists are usually interpreted as coming from either earthworms or arionid slugs, or both. In this paper the granules are examined in detail, with a view to better understanding their origins and dynamics. Through collection and experiments, it is shown conclusively that both animals produce calcium carbonate granules of the right size range. The two types have distinctive morphologies, however, and most of the granules observed in soils and sediments are clearly identifiable as coming from earthworms. This arises from a far greater production rate: worms are capable of shedding granules at the rate of 1-3 per week, whereas Arion ater only yields around 40 granules >250 μm on death. The granules survive digestive processes, so heavy predation of earthworms by various animals and birds means that there is a constant recycling of the granules in the biosphere.