Rapid communicationA new form of needle-fiber calcite produced by physical weathering of shells

- Needle-fiber calcite can be produced by physical weathering of shell valves.
- Freeze-thaw cycles promote the breakdown of the outer calcite layer of shells.
- Calcite needles in soils with shells may indicate cold climates and frost action.

Needle-fiber calcite is a common crystal form in soils and sediments from diverse environmental settings, and it has been used as evidence of a specific soil development either past or present. However, it can have either a physicochemical or a biological origin and its ubiquity prevents straightforward use as an environmental proxy. In this paper, we present a new form of needle-fiber calcite, derived neither from biologically mediated mineralization in the soil nor from physicochemical precipitation. This needle-fiber calcite is monocrystalline and prismatic, and is associated with the physical weathering of Mytilus edulis (Linnaeus) bivalve shells found in soils from anthropic shell middens located on the northern coast of the Beagle Channel (Argentina). The effects of freeze-thaw cycles can be observed in the local soils and would be responsible for the release of the calcite crystals that make up the outer layer of the shell. In this respect, the new form of needle-fiber calcite would be specific for this process in anthropogenic soils in cold climates, and could provide information on past climatic conditions.