The Devonian-Carboniferous boundary in Vietnam: Sustained ocean anoxia with a volcanic trigger for the Hangenberg Crisis?

The Devonian-Carboniferous transition (359 Ma) was a time of extreme climate and faunal change and is associated with the end-Devonian biodiversity crisis. The transition is characterized by transgressive/regressive cycles, which culminated in the onset of widespread ocean anoxia (the Hangenberg Black Shale event) and a remarkable sea-level fall close to the Devonian-Carboniferous boundary (the Hangenberg Sandstone event); together these are known as the Hangenberg Crisis. The Hangenberg Crisis has been documented around the globe, but the trigger mechanisms for its onset remain unknown. The Pho Han Formation on Cat Ba Island in northeastern Vietnam preserves the Devonian-Carboniferous transition and Hangenberg Crisis in a sediment starved basinal facies on the South China carbonate platform. Although the Hangenberg Black Shale event is generally preserved as a discrete anoxic interval in Devonian-Carboniferous boundary sections of North America and Europe, the Pho Han Formation records sustained dysoxic/anoxic conditions from the Famennian (Upper Devonian) through the Tournasian (early Carboniferous), with severe anoxia (approaching euxinia) throughout the Hangenberg Black Shale event interval (as determined by trace element proxies, increased total organic carbon, and framboidal pyrite distributions). There is also significant mercury enrichment corresponding to the Hangenberg Crisis in the Pho Han Formation. The isolated paleogeography of the region suggests that the mercury is most likely sourced from distal volcanic emissions. It is therefore possible that large-scale volcanic activity acted as a trigger mechanism for the Hangenberg Crisis and biodiversity drop at the Devonian Carboniferous transition, similar to other major mass extinction events.