Pyritization of Soft Tissues in the Fossil Record: An Overview

Abstract

Authigenic pyrite preserves non-biomineralized tissues in the fossil record under exceptional circumstances. Diagenetic models and taphonomic experiments demonstrate that active, localized sulfate reduction in iron-rich pore waters results in a strong concentration gradient, which confines pyrite precipitation to decaying organic matter. The locus and timing of pyrite precipitation is also influenced by the original composition of the organic matter. In recent decades, new sites with three-dimensional pyritized soft tissues have been discovered, although the Hunsrück Slate (Devonian) and Beecher's Trilobite Bed (Ordovician), known since the late 1800s, remain the primary examples in terms of diversity, abundance, and quality of preservation. Sedimentological and geochemical analyses at these sites have shown that rapid burial in fine-grained, reworked sediments sets up the high iron, low organic carbon conditions necessary for soft-tissue pyritization. Soft-tissue pyritization may also occur in association with other taphonomic modes, in particular with Burgess Shale-type preservation and carbonaceous preservation in lakes, although many of these specimens are now weathered. Continued comparison among sites and between specimens with variable degrees of preservation could help clarify the limits to soft-tissue pyritization and its distribution in ancient sediments.