Ocean Acidification Following The Dino-Killing Asteroid Caused Much Of The Last Great Extinction

Ocean acidification upset pH levels, killing the majority of marine life 66 million years ago, lending weight to the dino-killing asteroid theory as the reason for one of Earth's biggest mass extinctions. Rich Carey/Shutterstock

The oceans took millions of years to recover their pH balance after the asteroid strike that ended the Cretaceous Era a new study has found. The surge of acidity, and alkaline rebound, was probably responsible for most of the marine extinctions of the era, and its ripple effects likely wiped out many of the land-based species that had survived the initial impact.

Although an asteroid has been widely blamed for the disappearance of so much of the planet's richness 66 million years ago, many paleontologists have questioned how its impact could have been so widespread. The alternative theory, that Deccan trap volcanism was the extinction's true cause, has its own problems when it comes to explaining the disappearance of so many marine lifeforms from the fossil record.

Dr Michael Henehan of the GFZ German Research Center for Geosciences has conducted a detailed study of foraminifera, tiny calcifying algae fossils, to draw a picture of what happened in the oceans at the time.

Henehan's first conclusion is that the algae confirm the asteroid theory, which has been facing more vocal challenges in recent years. Volcanic eruptions, even those that happen quickly by geological standards, would do their damage over a much longer span of time than the asteroid, something advocates of this theory report in particular ecosystems' fossil record.

"Our data speak against a gradual deterioration in environmental conditions 66 million years ago," Henehan said in a statement

Heterohelix globulosa foraminifera - calcifying algae fossils - from the K-Pg boundary clay at Geulhemmerberg in the Netherlands (8x magnification). Michaekl J Henehan

Instead, Henehan reports in the Proceedings of the National Academy of Sciences, a sudden change in foramanifera shell calcification indicative of the oceans dropping at least 0.25 pH units. For 40,000 years the oceans were acidic enough to interfere with calcium carbonate shell formation. After this there was a rebound, where the oceans became more alkaline than normal.

The loss of so many photosynthesizing organisms halved the ocean's biological absorption of carbon dioxide from the atmosphere, which meant food webs that had these organisms as their base collapsed.

It took 80,000 years after the event for ocean pH to be similar to levels before the asteroid impact, but it took several million years for marine biodiversity to stage a similar recovery.

Foraminifera are heavily used by paleontologists to track ancient environmental changes, but most sites accumulate slowly, leaving deposits too thin to reveal sudden shifts precisely. Henehan hit paydirt with the discovery of rocks laid down around the time of the Cretaceuous-Palaeogene boundary in a cave in the Netherlands. "In this cave, an especially thick layer of clay from the immediate aftermath of the impact accumulated, which is really quite rare,” he said. Deposits that capture a longer period of time from two North American sites and deep-sea drills confirmed the Dutch cave reflects global changes, not some local effect.

The dark layer of clay marks the Cretaceous-Paleogene era boundary, revealing changes in ocean algae known as foraminifera on either side. The rate of accumulation is higher than from other deposits of the same era, providing unprecedented observations of short term changes. Michael Henehan

As the pH of the oceans falls at the fastest pace since the end of the Cretaceous, Henehan's findings spell another grim warning of the dangers we face.

Closer view of the boundary from the cave. Michael J Henehan

 

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