The role of coupled Redox fluctuation and fossil mineralization in the Exceptional Preservation of Jehol Biota in the Aptian Jiufotang Formation, Northeastern China

The fossils of the Jehol Biota preserved in lacustrine fine-grained sedimentary rocks exhibit well-preserved detailed features. However, the specific depositional environments that led to this exceptional fossil preservation remain unclear. This study reconstructs redox conditions and fossil mineralization in drilled Jiufotang Formation samples which contains abundant Jehol Biota fossils to assess redox variations' impact on fossil preservation. We found that the lake's redox conditions fluctuated frequently, ranging from oxic to anoxic (ferruginous). Fossils in the lower core exhibit both phosphatization and pyritization, whereas those in the middle and upper sections show only phosphatization. This difference in fossil mineralization is likely due to the lack of a phosphorus reservoir in the water column during the initial stage of redox fluctuations, thereby allowing for concomitant phosphatization and pyritization. Later, volcanic phosphorus input and fluctuating redox conditions enriched the water column with phosphorus, promoting apatite precipitation while inhibiting extensive pyrite formation. The frequent and intense environmental fluctuations in the lake and their control over mineral precipitation were key factors responsible for the preservation of fossils in different mineralization forms.