Microclots in early Paleoproterozoic stromatolites from Dashiling Formation, Wutai, North China: Implications for microbial calcification and atmospheric CO2 levels during Great Oxidation Event

Abstract

The relationship between microbial calcification and atmospheric CO₂ levels is a critical aspect of Earth’s biogeochemical evolution. This study presents the first detailed investigation of micro- and mesoscopic features of stromatolites from the 2.1 Ga Dashiling Formation of the Hutuo Group in Wutai area, North China, one of the best preserved microbialites during the Great Oxidation Event (GOE). The study identifies three types of microclots (microclots-1, -2 and -3) as key contributors to the formation of stromatolitic laminae in the Dashiling Formation. These microclots form layered, meshed, and dendritic microclotted laminae, characterized by alternation of light and dark laminae, with lighter laminae being sparsely distributed and darker laminae densely distributed. These alternations reflect the periodic growth and decay of microbial communities in response to dynamic environmental changes. The microclots and microclotted laminae in the Dashiling Formation exhibit evidence for microbial calcification, suggesting that atmospheric CO₂ levels during the early Paleoproterozoic may have been lower than previously estimated. Previous studies have proposed that microbial calcification occurs extensively only when atmospheric CO₂ declines to ∼10 times the present atmospheric level. The well-preserved microclots and calcified microbial structures in the Dashiling Formation suggest that atmospheric CO₂ levels during the GOE may have approached or fallen below this threshold. While increased atmospheric O₂ levels during the GOE may have influenced microbial calcification, our results indicate that declining CO₂ levels played a more direct role in facilitating carbonate precipitation. These findings refine the understanding of the interplay between microbial activities, atmospheric CO₂ decline, and carbonate saturation during the GOE, offering new insights into early Earth’s carbon cycle dynamics.