Image copyright Science Photo Library Image caption Brachiopods like these dominated the seas prior to the Capitanian extinction

Over the past 450 million years, life on Earth has been devastated by five mass extinction events that are widely recognised by geologists.

Now, an international team of researchers proposes adding a sixth mass extinction to the list.

The team believes it has accumulated sufficient evidence to promote the Capitanian event to the rank of mass extinction.

The extinction occurred approximately 262 million years ago.

Their proposal would elevate the Capitanian, which occurred during the Middle Permian period, to sit alongside the so-called "Big Five" mass extinctions.

The researchers, led by Dr David Bond of the University of Hull, presented their case in the Geological Society of America Bulletin.

The status of the Capitanian crisis is disputed because little has been learned about it since its discovery 20 years ago. The majority of the existing literature is based on evidence unearthed in the tropics.

Braving the bears

Adding to the confusion is the End Permian extinction, the deadliest mass extinction in Earth's history. Occurring around 250 million years ago, the "Great Dying," as it is called, wiped out about 96% of all species.

Because the End Permian extinction occurred merely 12 million years after the Capitanian extinction, it has been difficult for geologists to determine if these events are separate or one-and-the-same.

In order to demonstrate that the Capitanian extinction was a separate, catastrophic and global phenomenon, Dr Bond needed evidence from elsewhere on the planet.

So, his team headed to Spitsbergen, an island roughly 890 km (553 mi) north of the Norwegian mainland.

Armed with camping gear and rifles - to keep the polar bears away - Dr Bond's team made three separate journeys to the island from 2011 to 2013, each time in the month of July when there is 24 hours of daylight and the weather is more amenable to field work.

Image copyright Getty Images Image caption Researchers braved areas populated by polar bears to study the rocks on Spitsbergen

"I love working in remote places," Dr Bond says, "it gives me a real sense of true exploration in the old-fashioned sense."

The team's destination was the Kapp Starostin Formation, a series of rock layers that are more than 400 meters thick in some locations and which span about 27 million years of the Permian period.

Their first task was to verify that the ancient rock layers agreed with the existing data extracted from the tropics.

By analysing carbon and strontium isotope ratios, as well as measuring the abundance of various trace metals and patterns of magnetic polarity recorded in the rocks, the researchers confirmed that the Norway rock layers correlated with those from the tropics.

Next, the team examined fossils of brachiopods and bivalve molluscs (the former resemble the latter but, existing in different phyla, are evolutionarily unrelated). The pre-extinction Capitanian age was dominated by brachiopods.

Precipitous fall

However, in the middle of the Capitanian strata, brachiopod diversity fell precipitously: 87% of the species vanished in a few tens of thousands of years, a mere blip on the geologic time scale. Clearly, something catastrophic had occurred.

Dr Bond believes the extinction was triggered by the eruption of the Emeishan Traps, now located in the southwestern Chinese province of Sichuan.

Volcanic eruptions release enormous amounts of carbon dioxide, which can cause ocean acidification. A depletion of seafloor oxygen may also have played a role.

Then, Dr Bond's team noticed that in younger rock layers, new brachiopod and bivalve species re-emerged. The post-extinction Capitanian age shifted from being dominated by brachiopods to being dominated by bivalves.

But, just a few million years later, these new species disappeared, apparently wiped out in yet another extinction event. Following that, the End-Permian mass extinction, represented by its own unique rock layer, devastated the planet.

The authors believe their research demonstrates that the Capitanian extinction left a distinguishable mark in both the tropics and northern latitudes.

Thus, they conclude that the Capitanian extinction was not a regional crisis, but a global phenomenon and, hence, deserving of mass extinction status.

Dissent in the ranks

Not everyone agrees.

Matthew Clapham, associate professor of palaeobiology at University of California-Santa Cruz, believes the evidence is more supportive of a separate regional extinction in the area that possibly extended to what is now Greenland.

"The bottom line is that we know the Capitanian extinction wasn't large," he says, because, "The disappearance of a few dozen species in one region doesn't make a global extinction."

Dr Clapham also suggests that the isotope data supports the notion that the extinction event identified by Dr Bond's team occurred after the Capitanian age. Still, he said, "they have definitely identified an interesting event in Spitsbergen".

Regardless of the ultimate verdict on the Capitanian, geologists will continue to shed light on the darkest ages of Earth's natural history.