Volcanism-Driven Lacustrine Redox Fluctuation Was Responsible for Jehol Lagerstätte

Abstract

While a 'volcanic Pompeii' model is well accepted as the cause of mass mortality and exceptional preservation of the Jehol Biota in volcanic debris, no consensus has been reached thus far regarding those in finely laminated sediments. Here we propose volcanism-driven lacustrine redox fluctuation as another vital cause of the Jehol Lagerstätte based on a new drilling archive of the Early Cretaceous Chaoyang Basin, North China. Mn-carbonates interbedded with Mo-enriched black shales, volcanic ashes, and phosphate mineralization fossils were found in our drilling. MoEF is much larger than UEF in black shale, and we found that the 'particulate shuttle' pattern controls this deposition process. Mn-carbonate was derived from the reduction reaction of Mn-oxides with organic matter, representing an oxidized water column. This process, in turn, results in Mo enrichment under anoxic non-sulfidic conditions and forming Mo-enriched black shales. The repetition of interbedded Mn-carbonate/Mo-shale hence monitors redox fluctuation in the lacustrine deposition environment. Given multilayered volcanic ashes in black shale and numerous soft sedimentary deformation structures associated with seismicity in Mn-carbonate layers, the redox fluctuation is attributable to lake eutrophication and water column turnover by frequent and intensive volcanism probably related to the destruction of the North China Craton. This volcanism-induced redox fluctuation in lacustrine water, together with the toxicity of the atmosphere due to volcanically released gases, may have simultaneously caused mass mortality of aquatic, terrestrial, and aviate organisms. The micro-X-ray fluorescence mapping, energy dispersive X-ray, and Raman mapping show that the preservation of fossils in the cores is mainly phosphatization. Fluctuating redox conditions facilitate phosphate enrichment by 'Fe-P particulate shuttle'. Both the oxic and anoxic non-sulfidic conditions are conducive to the phosphatization preservation of organisms, and oxidation are most favorable for phosphate mineralization, leading to the exceptional preservation of the Jehol Biota.

Keywords: redox fluctuation, tectonics and volcanism, Fe-Mn particulate shuttle, phosphate mineralization, Jehol Biota