About thirty years ago, the discovery of anomalously high abundance of iridium and other platinum group elements in the Cretaceous/Palaeogene (K-Pg) boundary led to the hypothesis that a 10 km asteroid collided with the Earth and caused one of the most devastating events in the history of life. The impact created the 180-kilometre wide Chicxulub crater causing widespread tsunamis along the coastal zones of the surrounding oceans and released an estimated energy equivalent of 100 teratons of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. The vapour produced by the impact could have led to global acid rain and a dramatic acidification of marine surface waters. Calcareous nanoplankton (primarily the coccolithophores) and planktonic foraminifera had the highest extinction rates among the marine plankton.
But the exact mechanism that lead to the demise of the 75% of all life on Earth including the non-avian dinosaurs still remain debated. To test the hypothesis that acid rain could have caused the extinction patterns observed, Sohsuke Ohno and colleagues at the Chiba Institute of Technology in Japan mounted natural anhydrite – the bedrock of the Chicxulub crater is largely anhydrite – in a vacuum chamber with a chemically inert, high density tantalum metal plate backed with a plastic ablator a short distance from it. The team used lasers to fire impactors into anhydrite test samples at velocities of 13 to 25 kilometres per second, very similar to the speeds expected in an asteroid impact.
The mass spectrometer analysis, showed there was more sulphur trioxide molecules than sulphur dioxide. Sulphur trioxide reacts quickly with atmospheric water vapour and form sulphuric acid aerosols. These sprays adhere to particles heavier silicates ejected into the atmosphere by the impact, returning to the surface much faster than previously thought. These results could also explain the so called fern spike right after the impact event, because ferns are one of the most tolerant plants for dealing with those conditions.
Sohsuke Ohno, Toshihiko Kadono, Kosuke Kurosawa, Taiga Hamura, Tatsuhiro Sakaiya, Keisuke Shigemori, Yoichiro Hironaka, Takayoshi Sano, Takeshi Watari, Kazuto Otani, Takafumi Matsui and Seiji Sugita, Production of sulphate-rich vapour during the Chicxulub impact and implications for ocean acidification, Nature Geoscience (2014) doi:10.1038/ngeo2095