Chemo-biostratigraphic study on the Devonian Frasnian-Famennian event

Study on the Nandong Section of South China reveals that the Devonian Frasnian-Famennian (F-F) bio-event was a stepwise mass-extinction. The devastating diversity loss occurred in the latest linguiformis Zone, which was 0.1 Ma in duration. Across the F-F boundary there are three nickel-iridium moderate anomaly beds, which are also enriched in chalcophile elements, and also accompanied with the carbon isotope negative anomalies and microtektites. The last Ni-Ir anomaly was followed immediately by tsunamis. In fact, the F-F event is an integrative event - biotic, geochemical, sedimentologic. Such integrative events occurred five times in the Middle and Upper Devonian with a frequency of 4.6 ?? 10-6 a-1 (year-1). This frequency approximates to the frequency of the impact of 1 km-sized asteroids on Earth, estimated by Shoemaker (1990). The possible cause of the F-F mass-extinction is herein discussed. Asteroids (probably 1 km-sized) struck the Earth with a frequency of 4.6 ?? 10-6 a-1. These impacts intensified the global (or vast regional) hydrothermal activities on fracture zones, with anomalous Ni-Ir and chalcophile deposits ensuing. The main Ni-Ir excess might have a hydrothermal origin. Hydrothermal metal toxicity might be one of the kill mechanisms in mass-extinction. Sea level change and regressive anoxia excursion were ongoing phenomena which might create appropriate stagnant condition for the preservation of element excess, but might not be the main causative factor of mass-extinction.EI02-3363-3693