Concretions as Agents of Soft-Tissue Preservation: A Review

Abstract

Carbonate concretions may preserve exceptional soft-tissue fossils. Organismal decay in sediment can produce HCO3−faster than it diffuses away, creating a local microenvironment of high alkalinity around the decaying organism that promotes carbonate precipitation. Infilling of sediment pore-space around the decaying organism decreases permeability and inhibits decay, thus increasing preservation potential within the concretion and promoting soft-tissue preservation. Different patterns of concretion growth may promote different mechanisms of soft-tissue preservation. Other factors such as shifting salinity and clay mineral chemistry, which increase preservation potential in the depositional environment, also promote soft tissue preservation within concretions. The rate of concretion nucleation and growth affects preservation; the faster concretions nucleate and grow, the better the preservation. Concretionary preservation is biased both by concretion-promoting environments and organism size effects on concretion formation.