Late Cretaceous Climate Before the Chicxulub Impact

The end of the Cretaceous is often pictured as a single instant: an asteroid strikes, darkness spreads and the dinosaurs disappear. The boundary event was abrupt in geological terms, but the ecosystems approaching it had a longer history. In parts of the Late Maastrichtian, climate and marine communities were already changing. Fossil shells from Seymour Island, Antarctica, preserve one unusually detailed local record of warming before the Chicxulub impact. It offers evidence for environmental stress, while leaving open how widely that signal applied.

A climate record inside fossil shells

Researchers analysed well-preserved shells of Late Cretaceous bivalves from the López de Bertodano Formation on Seymour Island. Their oxygen and clumped-isotope measurements provide estimates of past seawater temperature. In that record, temperatures rose sharply around 66.3 million years ago, before the Cretaceous–Paleogene boundary. The study estimated a warming of roughly 8°C at the site, with substantial uncertainty. This is a proxy-based reconstruction from a particular region and set of shells, not a direct thermometer reading for the whole planet.

The same section records two pulses of species loss: one before the boundary and another at it. Their timing overlaps, respectively, with the earlier warming and the Chicxulub event. The pattern supports the possibility that volcanic climate effects contributed to ecological stress before the impact, then the impact delivered an additional severe shock. It does not show that every species responded in the same way, or that warming alone would have caused the global extinction.

What the Deccan Traps may have changed

The Deccan Traps are a vast volcanic province in what is now India. Eruptions and underground magma intrusions released gases that could affect climate and ocean chemistry. Carbon dioxide tends to warm climate over longer intervals; sulphur dioxide can cause shorter-lived cooling and acid rain. Which effect dominated depended on the amount, timing and rate of emissions. Dating volcanic rocks and comparing them with boundary sediments lets researchers test the sequence, but the details of eruption tempo and gas release remain actively studied.

At Seymour Island, the pre-boundary warming aligns in time with a major phase of Deccan volcanism. That coincidence and the fossil turnover make a causal contribution plausible. Researchers have also proposed other stresses, including ocean acidification and changes in productivity. The Antarctic section is valuable because it preserves shells across the boundary, but regional conditions such as sea ice and local ocean circulation may amplify or alter a signal. Other basins and land records are needed to assess its geographic reach.

Why the impact still matters

The Chicxulub impact left a global boundary layer rich in impact-related material and coincides with a much broader turnover in the fossil record. The impact’s rapid physical effects – including heat, ejecta, darkness and disruption of food webs – differ from the slower and mixed climate effects associated with volcanism. The two processes were close in time, and their relative importance is not identical for every place or group. The broader comparison of Chicxulub and Deccan timing explains how those lines of evidence are evaluated together.

Pre-impact warming therefore changes the story from “nothing happened until the asteroid” to a more nuanced sequence of environmental pressure followed by an abrupt catastrophe. It does not replace the impact explanation or prove a simple two-cause formula for every extinction. The strongest conclusion is local and testable: a well-studied Antarctic section records warming and biological change before the boundary, at a time consistent with Deccan activity. How much this weakened ecosystems elsewhere remains uncertain.