A spike of woody plant biomarkers in the deep-sea iridium layer at the Cretaceous/Paleogene boundary

Abstract

At the Cretaceous/Paleogene (K/Pg) boundary, 66millionyears ago, the Chixulub impact resulted in significant environmental changes and a mass extinction of dinosaurs and marine invertebrates such as ammonites. Here, we report that accumulation of woody plant biomarkers in the deep water occurred in the iridium anomaly at ~700km from the impact crater. The results reveal that the concentration of terrestrial organic molecules derived from woody plants, namely biphenyl and dibenzofuran, shows synchronized changes and increases abruptly in the red layer (fine ejecta), which has an iridium spike, above tsunami-like coarse deposits indicating a significant increase in the influx of woody plant fragments into the ocean a few years after the impact. Long-chain n-alkanes and cadalene derived from land vegetation in the tsunami-like coarse deposits prior to the transportation of trees were also transported to the deep sea. This implies that transportation of grass to the deep sea started within a few days of the bolide impact. Transportation of trees then began a few years later. A rapid increase in the concentration of dibenzothiophenes also occurs in the red layer, indicating that low-dissolved-oxygen conditions had expanded in the bathypelagic zone over the seafloor. An increase in the influx of terrestrial organic matter into the deep ocean could have resulted in the low-dissolved-oxygen conditions. Furthermore, the stratigraphic distribution of planktonic foraminifera at Beloc shows that Cretaceous planktonic foraminifera became extinct as the result of an asteroid impact.