Fossil evidence provides new insights into the origin of the Mesoproterozoic ministromatolites

The Proterozoic millimeter-sized ministromatolites have been recognized for their potential biological origin, yet concrete biotic proof has been elusive, which has sparked debates about their possible chemical origins. Recently, unique ministromatolites were identified in a microbial dolostone layer within the Mesoproterozoic Wumishan Formation at Lingyuan in Northern China. Examination of thin sections of these ministromatolites has uncovered a wealth of coccoidal microfossils, measuring 10 to 30 micrometers in size. Petrofabric analysis indicates that these ministromatolites underwent three episodes of diagenetic silicification, with the microfossils being exclusively preserved in early diagenetic chert. This finding supports the hypothesis that silicification occurring concurrently with mat development is crucial for the exceptional preservation of microfossils, which suggests that the scarcity of microfossils in most Precambrian carbonate stromatolites may be attributed to the absence of hydrochemical conditions conducive to early silicification. While chemically-formed fibrous carbonate minerals predominate in these ministromatolites, the abundance of microfossils suggests that the contribution of microorganisms to the formation of ministromatolites was previously underestimated. Consequently, this study proposes that Mesoproterozoic ministromatolites are not merely chemical byproducts but rather intricate mixtures of both biological and chemical components.