Cyanobacterial calcification in Cambrian Rivulariaceae oncolites: A key to understanding early carbonate systems

Ancient oncoid structures serve as important archives of past environments, providing significant insights into early ecosystems and the role of cyanobacteria in shaping Earth's early history. The Cambrian Series 2 oncoids from the Jianchang and Mantou formations in the Fuzhou-Bay section, Liaotung Peninsula, North China, are particularly noteworthy due to their distinct features. These include centimeter-scale, spherical to ellipsoidal grains, with a matrix enriched in clots and microclots of filamentous cyanobacteria, and composed of dense micrite and microspar that form heterogeneous and laminated structures. The presence of calcified cyanobacteria sheaths, especially those of the Hedstroemia group, suggests a strong connection with modern nitrogen-fixing Rivularia species, which occur in both the cortices of the oncoids and their surrounding matrix. This association provides valuable insights into the development of oncoids within photosynthetic biofilms and microbial mats. Despite the abundance of fossilized cyanobacterial filaments, interpreting the formation processes of these oncoids remains challenging due to diagenetic alterations. Significant uncertainties persist regarding the exact mechanisms of calcification in cyanobacteria-dominated biofilms and microbial mats. However, the presence of cyanobacterial filaments resembling Hedstroemia , Apophoretella , and Acuasiphonoria , which are analogous to modern nitrogen-fixing Rivulariaceae , sheds light on the early occurrence of complex cyanobacterial communities during Cambrian Series 2 (∼521–509 Ma). In conclusion, this study highlights the unique characteristics of Cambrian oncoids, their relationship with cyanobacteria, and the challenges in interpreting their formation. It provides valuable insights into the early evolution of high-order cyanobacteria and the calcification processes within microbial mats.