Decoding the end-Devonian Hangenberg oceanic anoxia event: Insights from ironstones in South China

The Devonian-Carboniferous Hangenberg event (HBE) was a global oceanic anoxia event. The HBE had profound impacts on the surface environments and caused the sixth biggest bio-extinction in Phanerozoic, with ending the long-lasting greenhouse climate in the Devonian. However, possible mechanisms that triggered the HBE have long been a matter of debate. Enhanced continental weathering or oceanic upwelling has been proposed to be responsible for the HBE. Here, valuable information about the HBE is offered from chamosite and siderite ironstones with structures indicative of paleo-storms in South China. Based on sedimentology, stratigraphy and calcite U-Pb dating, we confirm that the chamosite ironstones and siderite ironstones were formed before and during the main anoxia interval of the HBE, respectively. The concentrations of rare earth elements, U and Mo, V/Cr ratio and Fe and C isotopes of the ironstones are collectively indicative of a redox-stratified ocean under a greenhouse climate at the beginning of the HBE. Subsequently, storm-induced upwelling likely led to surface water eutrophication and the expansion of the oxygen minimum zone, resulting in severe shelf anoxia. The genetic model proposed for the HBE may have implications for the trigger mechanisms of other OAEs in the context of greenhouse climates.