Modern freshwater microbialite analogues for ancient dendritic reef structures

Microbialites are organosedimentary structures that can be constructed by a variety of metabolically distinct taxa(1). Consequently, microbialite structures abound in the fossil record, although the exact nature of the biogeochemical processes that produced them is often unknown(2). One such class of ancient calcareous structures(3-5), Epiphyton and Girvanella, appear in great abundance during the Early Cambrian. Together with Archeocyathids, stromatolites and thrombolites, they formed major Cambrian reef belts. To a large extent, Middle to Late Cambrian reefs are similar to Precambrian reefs(6), with the exception that the latter, including terminal Proterozoic reefs(7), do not contain Epiphyton or Girvanella. Here we report the discovery in Pavilion Lake, British Columbia, Canada, of a distinctive assemblage of freshwater calcite microbialites, some of which display microstructures similar to the fabrics displayed by Epiphyton and Girvanella. The morphologies of the modern microbialites vary with depth, and dendritic microstructures of the deep water (>30 m) mounds indicate that they may be modern analogues for the ancient calcareous structures. These microbialites thus provide an opportunity to study the biogeochemical interactions that produce fabrics similar to those of some enigmatic Early Cambrian reef structures.