Bacterial fossil record in whale-falls: Petrographic evidence of microbial sulfate reduction

Whale fossils from a variety of geologic ages and depositional environments were thin-sectioned and analyzed for evidence of microbial activity, comprising a combination of microborings, authigenic pyrite, botryoidal cements, and micropeloids. Borings are dense in some samples with an average diameter of 8.02 µm. Framboidal pyrite crystals with an average diameter of 61.84 µm in diameter occur within the pore spaces of the bone, in borings, and at the bone–sediment interface. The pyrite crystals are densely packed at the edges of the bone and scattered more randomly throughout the pore spaces. Aragonitic botryoidal cements line pore spaces in samples from deep bathyal settings. Potential bacterial peloids occur within pore spaces with an average size of 77.45 µm or diffuse to comprise clotted micrite. Since bacterial body fossils are extremely rare in the fossil record, recognition of past microbial ecosystems must be based on multiple lines of supporting evidence. Of the independent signatures studied, the bacterial peloids and botryoids provide the most convincing evidence of a sulfophilic stage. These fossil biosignatures suggest that a sulfophilic stage is preserved at least as far back as the Eocene.