The Geochemistry of Mass Extinction

The boundaries between geologic periods often mark major turnovers in the types (species, genera, and families) of organisms that inhabit the planet, that is, mass extinctions. Geochemistry is used to investigate these events, to determine both the trigger for the event (asteroid/comet impact and volcanism) and its cause (abrupt warming, anoxia, trace-metal poisoning, loss of habitat by sea-level fall, etc.). Because they are fractionated by biogeochemical processes, the stable isotopes are important geochemical clues. Although the long-term Phanerozoic trends in stable isotope ratios reveal no consistent indication of progressively worsening conditions, detailed analysis of the geochemical and isotopic record of sedimentary successions spanning mass extinctions generally indicate some combination of a loss of biologic productivity, expansion of low-oxygen zones, and increased continental weathering. Anoxia (the loss of oxygen) and euxinia (the buildup of hydrogen sulfide) clearly played a role in the Late Devonian and end-Permian mass extinctions, as revealed by the various geochemical proxies of anoxia and the presence of biomarkers for anoxygenic sulfur phototrophs. Only the Cretaceous-Paleogene extinction has conclusive evidence of asteroid impact; the other events seem to be events driven by endogenic processes in Earth system.