End-Cretaceous devastation of terrestrial flora in the boreal Far East

Abstract

A continuous marine sedimentary sequence spanning the Cretaceous/Tertiary (K/T) boundary has been identified in eastern Hokkaido, Japan, on the basis of planktonic foraminifera. The K/T boundary is marked by a 6–10-cm-thick layer of pyrite-rich, fossil-poor, greyish-black claystone¹, which is lithologically similar to the well-known ‘boundary clay’ of the classical European K/T boundary sections^2–4. We show here that terrestrial palyno-morphs from the Hokkaido section record sudden changes in the floristic composition precisely at the base of the boundary claystone layer. In particular, both angiosperm and gymnosperm pollen abundances decline markedly and these changes are accompanied by an abrupt rise in the proportion of fern spores. Return to the original, moderate level of fern dominance occurs within a few centimetres above the boundary layer. A sharp increase of fern spores has also been reported at the palynologically defined K/T boundary in the western interior of North America⁵, which coincides with the top of an iridium-rich clay layer^6–7. The possibly synchronous occurrence of analogous floral changes at these widely separated regions implies a devastation of the land flora which, although geologically brief, was intercontinental in scope, and which was probably caused by a major catastrophic event such as the impact of a massive meteorite⁷.