NEW EVIDENCE FOR BACTERIAL MEDIATION OF THE SILICIFICATION OF CARBONATE FOSSILS

Microbial degradation of organic matter in the shell has long been suspected of playing a key role in silicification of carbonate fossils. Direct evidence was lacking until we reported on silicified biofilm and framboidal pyrite molds found on trilobite sclerites from the Cambrian Weeks Formation, Utah. In this material, silicification commonly extended past the edge of the fossils into the surrounding matrix, suggesting involvement of organic matter from the sediment, not just from within the fossil. We hypothesize that the Weeks Formation trilobite sclerites are not unique and predict that silicified biofilm and extension of silicification into the matrix is common for silicified fossils. To test this hypothesis, we examined a variety of Paleozoic silicified fossils (mostly trilobites) at high magnification (>1500X).

Although silicification does generally stop at the edge of the fossil, all samples are also partly coated by silica-cemented matrix, consistent with the Weeks' observations. Most, but not all, fossils have rare crystalline-looking braided threads and ribbons that are composed of Si-O, with minor C and Al. More common are smooth ribbons (sometimes braided) and mats mainly composed of C, O and trace N. One specimen had framboid molds in the silica. A Permian silicified brachiopod is encrusted by numerous, roughly oval chambers covered by thin silicified crusts. The chambers vary in diameter from <50 µm to nearly 1 mm and extend tens of microns above the shell. The crust is always very thin (1-2 µm) and the chamber beneath is partly filled with euhedral crystal molds (possibly dolomite, not silica morphology).

We interpret the crystalline-looking features as silicified biofilm replaced at the same time as the underlying fossil. The smooth features are likely modern biofilm and perhaps fungi. The rare framboid molds are interpreted as formerly framboidal pyrite, supporting bacterial sulfate reduction (BSR) associated with silicification. These data support a widespread presence of microbial degradation of organic matter associated with silicification of carbonate fossils and the surrounding matrix. In addition, we speculate that the thin crust on areas of the Permian brachiopod shell is silicified biofilm that coated the living chambers of soft-bodied encrusting organisms.