A NEW international study led by Yale University in collaboration with the Universities of St Andrews and Bristol confirms a long-held theory about the last great mass extinction event in history and how it affected Earth’s oceans.

The new study, published in the journal Proceedings of the National Academy of Sciences, reveals the first direct evidence that the Cretaceous-Paleogene extinction event, which occurred 66 million years ago, coincided with a sharp drop in the pH levels of the oceans, indicating a rise in ocean acidity.

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The Cretaceous-Paleogene die-off, also known as the K-Pg mass extinction event, occurred when a meteorite slammed into Earth at the end of the Cretaceous period. The impact and its aftereffects killed almost 75% of the animal species on the planet, including whole groups such as the non-avian dinosaurs and ammonites.

To test this idea, the team made measurements of the composition of tiny fossil shells from before, during, and after the K-Pg event.

Previous K-Pg research had shown that some marine calcifiers, animal species that develop shells and skeletons from calcium carbonate, were disproportionately wiped out in the mass extinction.

The new study suggests that higher ocean acidity (lower pH) may have prevented these calcifiers from creating their shells.

This was important because these calcifiers made up an important part of the first rung on the ocean food chain.