Siderite from the Tibetan Himalaya: Evidence for a low sulphate ocean during Oceanic Anoxic Event 1a (Early Aptian)

Mesozoic oceanic anoxic events were characterized by relatively low seawater sulphate concentrations ([SO42-$$ {{\mathrm{SO}}_4}<^>{2-} $$]), which likely regulated the development and evolution of these major palaeoceanographic phenomena. However, there is little reliable sedimentary evidence for low [SO42-$$ {{\mathrm{SO}}_4}<^>{2-} $$] in ancient marine waters and understanding of how such a seawater chemistry potentially impacted oceanic anoxic events is limited. This study presents an integrated sedimentological, mineralogical and geochemical investigation of the mineral siderite hosted in dark grey shale and sideritic concretions of Early Aptian (coeval with Oceanic Anoxic Event 1a) from the Tibetan Himalaya. Siderite is present throughout the section and possesses similar morphological characteristics whether in dark grey shale or concretions. Siderite can be present as disseminated and rhombic crystals formed during early diagenesis, or minor spherical crystals formed during late diagenesis. The evidence from redox elements, middle rare-earth element bulge patterns and extremely low carbon-isotope values of the sideritic concretions indicates that the iron carbonate was formed in the Fe-reduction zones by the process of dissimilatory iron reduction. This process would have required conditions of low [SO42-$$ {{\mathrm{SO}}_4}<^>{2-} $$], reducing environment, abundant iron and high alkalinity. Additionally, the coexistence of siderite and pyrite may indicate that dissimilatory iron reduction occurred close to the microbial sulphate reduction zone, with seawater [SO42-$$ {{\mathrm{SO}}_4}<^>{2-} $$] hovering around the tipping point at which pyrite could form once seawater sulphate increased. Such an increase during Oceanic Anoxic Event 1a could have resulted from basalt-seawater interaction and associated enhanced continental weathering, and/or hydrothermal activity. This study's observations support the previous hypothesis that low [SO42-$$ {{\mathrm{SO}}_4}<^>{2-} $$] for Oceanic Anoxic Event 1a was probably caused by massive gypsum burial in the proto-South Atlantic. Subsequently, enhanced sulphate input could have promoted microbial sulphate reduction and accompanying oxidation of organic matter, which likely further enhanced nutrient recycling, increased primary productivity and organic-carbon burial, leading to more oxygen consumption and expansion of oxygen minimum zones, as reconstructed for many oceanic anoxic events.