Early Aptian mineral and geochemical evidence of siderites from the Tibetan Himalaya: implications for the low sulfate concentration of Oceanic Anoxic Event

Several typical Oceanic Anoxic Events (OAEs) occurred in the greenhouse period of the Mesozoic. These OAEs were characterized by low seawater sulfate concentration ([SO4^2‒]) before and during the events, suggested by sulfur-cycle, which have been considered to play a significant role in their formation and evolution. However, there is still lack of reliable sedimentary evidence for the low [SO4^2‒] and the details how the low [SO4^2‒] impact the OAEs. Here, we present integrated sedimentologcal, mineral and geochemical study of black shale and siderites hosted in black shale and concretions during the early Aptian in the Gucuo Ⅱ section (Tibetan Himalaya). The siderites were observed throughout the section and share the similar characteristics in the black shale and concretion, which can be divided into dominant disseminated and rhombus crystals in early diagenesis and minor spherical crystals in the late diagenesis. The multiple evidence of relatively high V/Al and V/ (V+ Ni), MREE bulge pattern, minor occurrence of pyrites and the extremely low carbon-isotope values of carbonate concretion that close to organic matter indicate that siderites were formed in the Fe reduction zones by the process of Dissimilatory Iron Reduction (DIR) which required strict conditions of low [SO4^2‒], reducing environment, abundant iron and high alkalinity. Additionally, the symbiosis of siderite and pyrite may indicate that the DIR occurred close to the Microbial Sulfate Reduction (MSR) zone, and the extremely low seawater [SO4^2‒] hovered around the tipping point where pyrites could form once the seawater sulfate increase by pulse input of enhanced continental weathering and/or volcanism. Our observations supported the previous hypothesis that under the background of low [SO4^2‒], enhanced volcanic-derived sulfate input could have promoted the MSR and organic matter mineralization, which likely further enhanced nutrient recycling, and increased primary productivity and organic carbon burial, leading to more oxygen consumption and subsequently driving an expansion of the oxygen minimum zones.