Scientists hunt for meteor crash clues in 200-million-year-old mystery

Mass extinctions are a relatively common theme in the history and evolution of life on Earth, and the most famous one is the extinction of the dinosaurs 65 million years ago. A plethora of research has been conducted to determine how the dinosaur era ended, generating theories of massive volcanic eruptions, catastrophic climate change and giant impactors from space.

However, much less is known about another remarkable extinction event that occurred roughly 135 million years earlier - an extinction that may have set the stage for the age of dinosaurs . The mass extinction that occurred just before the boundary between the Triassic and Jurassic periods wiped out much of the life on land and in the oceans, leaving the world ripe for dinosaurs to plunder. For astrobiologists, the causes of this extinction comprise one of the greatest murder mysteries of all time.

Now, a team of scientists is helping to reveal the secrets of the Triassic-Jurassic (T-J) extinction by studying geological formations around the world that bear evidence of a traumatic disruption in Earth's ecosystems some 200 million years ago. Recently, their investigation brought them to the shores of Northern Ireland's Antrim coast near the seaport of Larne. Northern Ireland is famous around the world for its stunning coastal drives and the lush forests of its glens and inlets. However, many of the locals are unaware that the quiet countryside also holds a veritable "pot of gold" beneath their feet for geologists.

The emerald coast

The team of researchers, led by Paul Olsen of Columbia University and Dennis Kent of Rutgers University, gather on a misty Irish morning in a small parking lot in Whitehead, Northern Ireland. Here, they are able to cross the train tracks that hug the coastline and scramble down to a seawall that provides a safe route along the rocky shore. As their shoes slip along the damp stone, the small cliffs come into view ahead.

There is nothing particularly dramatic about the cliffs themselves, which are nestled below the train line and an imposing seawall topped with barbed wire. But the crumbling rocks peeking out from straw grass and brambles are a rare outcrop of material from the T-J boundary. They contain physical and fossil evidence that could help determine what happened to Earth's ecosystems before, during and after the T-J extinction.

In a time of plenty

At the time of the T-J extinction, the view from Northern Ireland's Antrim Coast may have been quite similar to the one that the research team is treated to today. In the late Triassic, Earth's landmass was smashed together as the single supercontinent Pangaea, and the British Isles were positioned relative to one another in much the same way they are now.

However, the waterways of the North Atlantic that now separate Ireland and Great Britain had much less exchange with the open ocean. In fact, this body of water was more akin to a large, inland sea. As the waves of this sea rolled in and out, the sediments they gradually deposited on the floor recorded a history of the environment that can be read like a book by geologists today as they dig down through the layers.

The late Triassic was a time of plenty for Earth, and the planet was a veritable paradise for life. Even the land now known as Antarctica was temperate, moist and supported a diverse range of flora and fauna. On the shores of the ancient Irish Sea, amphibians roamed the land alongside reptiles, some of which had some distinctly mammal-like traits.

Suddenly, disaster struck. In the geological blink of the eye (i.e., 10,000 years), life on Earth began to die. Two hundred million years ago, just before Pangaea began to break apart, half of the known species on Earth disappeared. [Earth in the Balance: 7 Crucial Tipping Points]

Many of the mammal-like reptiles were wiped out along with a vast array of single-celled and multicellular creatures on sea and land. Theories have been put forth about how this could have happened, but evidence of the true cause has eluded scientists for decades.

Modern shores and ancient lake beds

Evidence of the T-J extinction has been reported by numerous researchers working in sites throughout the world. For instance, a sharp decline in organic carbon and marine organisms was reported in samples from Canada's Queen Charlotte Islands in 2001 and St. Audrie's Bay in England in 2002. With their sampling efforts in the United Kingdom, Paul Olsen and his team are hoping to add their expertise to solving the T-J question. In various locations throughout the UK, scientists have identified outcrops of rock from the T-J boundary that are uniquely exposed at the Earth's surface. These sites are like natural libraries for geologists, where they can simply walk up and pick up samples that were "written" by the Earth at specific points in its history.

The researchers chose two additional sites in the British Isles to examine. The first was in western England's Somerset County. Here, the sediments that settled on the bed of the tropical sea between Great Britain and Ireland are now visible as great sheets of rock below the cliffs of the Bristol Channel. In these cliffs are visible layers of limestone and shale that contain a myriad of fossils - lasting evidence of the T-J catastrophe.

The team's second site was Lavernock beach near Barry Island, Wales. At these sites, there is a "dead zone" where few fossils can be found at the time of extinction.

At all three sites (Northern Ireland, Somerset and Wales), the cliffs reveal a unique feature that makes the British Isles of particular interest in the story of the T-J event. Near the time of extinction, the layering has been contorted. In contrast to the surrounding sediments, the layers ripple and bend as if they were shaken and pushed out of place. Could it be a clue of some specific, violent event that befell the region?

According to Paul Olsen, "The scale of the disruption is huge, and a huge cause seems likely."

This type of deformation is not rare in the geological record. Local disturbances, such as earthquakes, often disrupt the layers of rock beneath the Earth. What is unique is that it occurs all around the United Kingdom. If this deformation was caused by an earthquake, it would have been a very large one indeed.

"Not only is this disruption seen in the UK," Olsen said, "but it also appears to be present in at least Belgium and maybe as far away as Italy, according to the work of my UK and US colleagues."

Interestingly, at the Lavernock beach site in Wales, the deformation rests just below the dead zone where few fossils are found. This raises questions about whether or not the deformation event is tied to the loss of life at the T-J boundary.

"It's extremely unusual to have such a widespread zone of deformation," Olsen said. "The fact that it occurs very close below the extinction level suggests that there might be a causal relationship between the cause of the disruption, probably a mega-earthquake, and the extinction itself."

- The Independent