In two separate studies, researchers using different methods of high-precision dating attempted to illuminate the series of events that led to the demise of vast swaths of life on Earth nearly 66 million years ago. Both studies agree that Deccan Traps volcanism - not an asteroid alone - played a role in the mass extinction event, but they differ in their emphasis on the volcanism's triggering power. The work to understand this major climate-driven extinction event informs understanding of present-day human-driven climate change. The history of life on Earth has been punctuated by five mass extinction events. Perhaps the most well-known of these extinctions occurred at the end of the Cretaceous period, which spelled the end of the long reign of non-avian dinosaurs. What triggered Cretaceous-Paleogene (K-Pg) mass extinction, however, remains debated - was it an asteroid, or a series of massive volcanic eruptions over thousands of years within the Deccan Traps volcanic province of India, or both? A better understanding of the timing of the Deccan Trap volcanic eruptions, particularly as compared to the Chicxulub asteroid impact, could help resolve this debate.
Blair Schoene and colleagues used uranium-lead (U-Pb) dating to construct a precise timeline of eruption within the Deccan Traps. U-Pb dating can achieve far more precise ages than previously used techniques by measuring the ratio of uranium and lead isotopes within individual crystals of zircon, a mineral commonly formed in cooling magma. Like a newly wound watch ticking precisely to the second, radioactive isotopes of uranium contained within zircon begin to decay as soon as the mineral crystalizes, slowly yet steadily transforming into lead isotopes at known rates. Thus, the current ratio of the two isotopes within an individual zircon can be used to determine its age with an uncertainty as low as ±40,000 years. Schoene et al. sampled nine of the largest and most complete Deccan formations, collecting zircon from the volcanic ash beds and soils embedded between each of the ancient lava flows. The method allowed the authors to determine dates for each eruptive event. The results indicate that the Deccan Traps erupted in four high-volume events - each lasting roughly 100,000 years in length and unleashing vast amounts of magma and climate-transforming greenhouse gases, into the environment. The results suggest that the onset of these eruptive events began tens-of-thousands of years before the Chicxulub bolide impact. According to the authors, the amounts of methane, carbon dioxide and sulfur dioxide belched into the atmosphere during the Deccan eruptions could have dramatically altered the planet's climate and environments, triggering mass extinctions long before the asteroid's arrival.
Courtney Sprain and team used a different geochronological technique: argon-argon (40Ar/39Ar) dating. This approach to dating relies on the measurement of radiogenic argon gas released from a sample following nuclear radiation. While less precise, the method addresses some of the limitations of U-Pb dating. Sprain et al. conclude that much of the Deccan lava volume (over 75%) erupted nearly 600 thousand years after the K-Pg extinction event, which limits the responsibility of magmatism as a primary trigger of the extinction, they say. In a related Perspective, Seth Burgess discusses each study's findings and highlights how different methodologies, while each more precise than their predecessors and uniquely valuable in their own way, can lead to significantly different interpretations of the same geological event. "Ever better datasets permit ever more detailed interrogation of the rock record, which answers some questions, but also generates new ones," writes Burgess.