An international team of scientists has synchronized key climate records from the Atlantic and Pacific Oceans to unravel the sequence of events during the last million years before the extinction of the dinosaurs at the Cretaceous/Paleogene boundary. For the first time, these new high-resolution geochemical records reveal when and how two major eruption phases of gigantic flood basalt volcanism had an impact on climate and biota in the late Maastrichtian era 66 to 67 million years ago.

On ten thousand to million-year time scales, climate dynamics on the Earth’s surface are driven by both external and internal processes. Earth’s interior provides heat from radioactive decay and chemical compounds by volcanic degassing, such as sulfur dioxide (SO₂) and carbon dioxide (CO₂).

Quasiperiodic changes in Earth’s orbit around the sun regulate the amount of incoming solar radiation on the planet’s surface as well as its distribution across latitudes, affecting the length and intensity of the seasons. The interplay of both processes through complex geochemical interactions on the surface of our planet shape and regulate the climate we live in.

“Just like a metronome, we used the rhythmic changes in solar insolation imprinted in geological data to synchronize geological climate archives from the South Atlantic and the Northwest Pacific. These key records span the last million years of the Cretaceous and are synchronized down to 5,000 years or less, geologically a blink of an eye, 66 million years ago,” says lead author of the article published in Science Advances, Thomas Westerhold from MARUM—Center for Marine Environmental Sciences at the University of Bremen.

To unravel causality arguments in Earth climate history across regions, this kind of synchronization is essential.

“So, we had the geological records perfectly lined up in time, and observed that two major changes in climate and biota occurred at the same time in both oceans. But we had to find a way to test if these changes are caused by large-scale volcanic eruptions related to the Deccan Traps in India,” says Westerhold.

The up to two-kilometers-thick basaltic rocks of the Deccan Traps cover a large part of western India. This large-scale volcanism flooding entire landscapes is referred to as Large Igneous Province by geoscientists.

Several times in Earth’s history these caused mass extinction events of life on the surface of the planet. In particular, the release of volcanic gases like carbon and sulfur dioxide during the formation of the flood basalts may have played a key role.

“The formation of the flood basalts and its subsequent weathering will leave a geochemical fingerprint in the ocean. Therefore, we measured the Osmium isotope composition of the South Atlantic and the Northwest Pacific deposits. They should show the same fingerprint at the same time,” says co-author Junichiro Kuroda (University of Tokyo, Japan), who conducted the geochemical analyses.

“To our surprise, we found two steps in the Osmium isotope composition in both oceans contemporaneous with major eruption phases of the Deccan Traps in the latest Cretaceous. And even more surprising, those steps had different impacts on the environment as recorded by fossil remains in the drill cores,” says Westerhold.

The new data were difficult to understand, but geochemical modeling helped to unravel their secrets. “The volume of the erupted flood basalt must have been much larger than previously thought during this early phase of Deccan Trap volcanism. And the related distinct emissions of carbon and sulfur dioxide had diverse effects on the global climate system,” says Don Penman (Utah State University, U.S.), who did the geochemical modeling.

According to the new finding, it seems plausible that at the onset of major Deccan Trap volcanism, independently dated 66.288 million years by radioisotopic methods, an initial pulse with sulfur-rich eruptions occurred, stressing the ecosystem locally and possibly also globally.