Girvanella Clumps in Columnar Stromatolites from the Cambrian (Early Miaolingian) of North China: Evidence for Microbial Calcification and a Marine Resurgence

Ancient stromatolites have experienced substantial alterations in their structure over time due to diagenesis, creating challenges in interpreting these formations and understanding their role in the evolution of life on Earth. To shed some lights on this issue, we examined exceptionally preserved stromatolites from the early Miaolingian-aged (510⁓506 Ma) at Jinzhou Bay section of the Liaoning province, North China Platform. The uppermost part of the early Miaolingian Maozhuang Formation comprises small column-like stromatolites of open tidal-flat sedimentary facies with highstand limestone, distinguishing it from the Maozhuang Formation in the rest of the North China sections, where it predominantly comprises restricted tidal-flat facies i.e., highstand dolostone. The stromatolite matrix primarily comprises dark micrite laminae, along with occasional micrite clumps that indicate the presence of calcified sheaths of filamentous cyanobacteria (Girvanella). The abundance of filamentous cyanobacteria along with pyrite grains indicate the direct microbial evidence in the growth of columnar stromatolites. Furthermore, the matrix of stromatolites represents potential resurgence of stromatolites in a normal marine environment during Miaolingian, which was previously thought as the time interval with relatively low abundance of stromatolites. Further, Girvanella within matrix of columnar stromatolites provide new insights concerning the complex and diverse biological traits of cyanobacteria, including large cell diameters, motility, filamentous growth, sheath evolution, nitrogen fixation, and exact calcification known as a hard life, particularly during the Cambrian period. As a result, the studied stromatolites not only highlight the resurgence and cyanobacterial calcification event associated with the formation of stromatolite, but also distinctive from the lithified discrete stromatolite buildups in Shark Bay's Hamelin Pool, which is dominated by coccoid cyanobacteria and evolved in a low-energy environment.