Volcanism-driven lacustrine redox fluctuations were responsible for the formation of the Jehol Lagerstätte: Evidence from a high-resolution Aptian sedimentary core, Northeast China

While a “volcanic Pompeii” model is well accepted as the cause of mass mortality and exceptional preservation of the Jehol Biota, no consensus has been reached thus far regarding the cause of mass mortality of fossils found in interbedded finely laminated sediments. We propose that volcanically driven, lacustrine redox fluctuations were responsible for the formation of the Jehol Lagerstätte, based on data obtained from a new drill core of Lower Cretaceous sedimentary rocks in the Chaoyang Basin, Northeast China. We found manganese-rich carbonates are interbedded with molybdenum-rich black mudstones and tuffs, and phosphatized fossils in the drill core. The elemental enrichment factors for molybdenum are much higher than for uranium in the black mudstones, suggesting the role of the manganese particulate-shuttle in deposition of this element. The manganese-rich carbonates are formed by the reaction of manganese oxides with organic matter, indicating an oxidized water column. This process accelerates the enrichment of molybdenum under anoxic non-sulfidic conditions. The interbedded manganese carbonates and molybdenum-rich mudstones thus record redox fluctuations in the lacustrine depositional environment. Given the numerous tephra in the black mudstones and soft-sediment deformation structures ostensibly associated with seismicity, we propose that the redox fluctuations were caused by lake eutrophication and turnover driven by volcanism that was ultimately related to the destruction of the North China Craton. These volcanically induced redox fluctuations, along with the toxicity of the volcanic gases, may have caused the simultaneous mass mortality of aquatic, terrestrial, and avian fauna. Micro-X-ray fluorescence, energy dispersive X-ray spectroscopy, and Raman mapping show that the preservation of fossils was mainly due to phosphatization. Fluctuating redox conditions facilitated phosphate enrichment in sediments. Both the oxic and anoxic non-sulfidic conditions were conducive to the preservation of organisms by phosphatization, and oxidation was favorable for phosphate mineralization, both contributing to the exceptional preservation of the Jehol Biota fossils.