What caused Earth's largest mass extinction event? New evidence from the Permian-Triassic boundary in northeastern Utah

摘要

The discovery of a Permian-Triassic boundary section in northeastern Utah provided an opportunity to study the chemistry and geology associated with this event that led to one of the greatest mass extinctions on the planet. From 83% to 97% of the species living on the planet went extinct during this relatively short interval of geological time that defines the major geological boundary between the Paleozoic and Mesozoic Eras. The cause and resulting sequence of events that led to this extinction have puzzled geologists for years. The new stratigraphic section in Utah provided a framework for reconstruction of the chemistry that was in the ocean during the mass extinction. Geochemical analysis of the section demonstrates, moving across the boundary from the Paleozoic to the Mesozoic, a significant drop in carbonate and total organic carbon, and a delayed occurrence of pyrite. Carbon isotope ratios (delta C-13) show a negative excursion in carbonate with ratios dropping from 2.59 parts per thousand to - 3.63 parts per thousand. Elevated mercury is present at the boundary with a 4-fold increase from background levels. The boundary layer also has elevated zinc, lead, strontium, and nickel, but not at high enough levels to indicate a volcanic ash source. There is no evidence in concentrations of siderophile and chalcophile elements for an extraterrestrial impact. The stratigraphic section in Utah supports the theory of a massive release of carbon dioxide, resulting in the acidification of the oceans. High levels of mercury, as well as elevated levels of zinc and lead, implicate a coal source triggered by the contemporary Siberian Traps sill complex. The delay of enriched sulfur and barium content in the stratigraphic record indicates an anoxic ocean and upwelling of methane hydrates from depth. Further study of this stratigraphic section will aid our understanding of the global impact these catastrophic events had on life.