New research has lent credence to a long-held theory about the last mass extinction, and how it affected the world's oceans.

The study also offers theories for how the diversity of marine flora and fauna recovered from the planet-wide extinction event. The widespread dying-off is inevitable if human activity continues to turn the oceans progressively more acidic, it adds.

Mass extinctions are no longer an enduring mystery. Researchers think these events are an intrinsic feature of the carbon cycle. When levels of dissolved carbon dioxide in the oceans reach a threshold level, life under the water (and on land) undergoes dramatic, catastrophic changes.

Ice in the Arctic ocean set adrift in sea. Image credit: Wikimedia Commons/AWeith

The first evidence gathered of the Cretaceous-Paleogene extinction event, which took place 66 million years ago, overlaps with periods of sharp pH drops of the oceans — pH levels are an indicator of acidity or alkalinity. This strongly indicates a rise in ocean acidity at the time. Based on statistical reasoning and evidence in the marine sediments, a US mathematician argues that the seas are once again becoming too acidic for marine organisms to form their carbonate shells. This, in their view, is the beginning of a cascade towards extinction.

Fossil fuel use combined with the widespread destruction of forests could be building up to extinction on a massive scale — enough to be visible in fossil records of these years, hundreds or millions of years from now.

Like many times in the past before humans came along, researchers think it is quite possible the carbon cycle will take over, and decide the fate of life's direction. It could happen again, as per the new study in Proceedings of the National Academy of Sciences.

"Once we are over the threshold, how we got there may not matter," Daniel Rothman from the Massachusetts Institute of Technology, said in a statement. "Once you get over it, you’re dealing with how the Earth works, and it goes on its own ride."

The more closely researchers look at evidence of catastrophic extinctions in the past, the more evidence once finds of ocean acidification tipping the balance in favour of extinction, the statement adds. The most unequivocal of these die-offs is the "Great Dying" at the close of the Permian era.

Still hotly debated, atmospheric conditions during the Great Dying have been repeatedly invoked, with researchers repeatedly examining the past to help craft today's to-do list for dealing with climate change.