The dolomite nodules enclosing fossil marine vertebrates in the East Pisco Basin, Peru: Field and petrographic insights into the Lagerstätte formation

The Mio-Pliocene Pisco Formation (Peru) is a world-famous marine vertebrate Lagerstätte. Several fossil specimens are wrapped up in dolomitic nodules. Some others lie in the sediment displaying dolomite only in bone cavities (e.g., mesorostral canal and endocranium). With the aim to understand whether the precipitation of the dolomitic nodules influenced the formation of the Lagerstätte, we collected field data on a high number of fossil vertebrates and conducted petrographic and mineralogical analyses on samples representative of the variable development of concretions. Our results revealed positive relationships between size, completeness and articulation of skeletons and the presence of an external nodule. Clear evidence of chemoautotrophic communities that thrived on the carcasses is scarce. Microborings are often found in the cortical bone tissues together with iron oxides; the former are left by microorganisms feeding on the carcass, the latter are traces of former Fe sulphides, a product of organic matter degradation. We suggest that an early burial of the skeletons was a determinant factor in the development of dolomite concretions, since it allowed methanogenesis and anaerobic sulphate reduction exploiting the lipids in the bones and the organic matter dispersed in the sediments. Dolomite precipitation was driven by the same bacteria operating during the suphophilic stage of whale-fall communities. Textural observations imply that dolomite precipitated shortly after the burial of carcasses. The increase of alkalinity generated by sulphate reduction and methanogenesis caused a rapid precipitation of the dolomite within skeletal cavities and prevented the degradation of the bones and diagenetic compression of skeletons; the nodules themselves prevented erosion of fossils after exhumation. Therefore, nodule formation had a crucial role in the development of the Pisco Lagerstätte.