Seeking a Pot of Geological Gold
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 port 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’s Lamont-Doherty Earth Observatory and Dennis Kent, also of Lamont-Doherty with a secondary appointment at 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.
Behind the team lies the modern incarnation of this body of water - the Irish Sea that separates Northern Ireland from the coast of Scotland. Today, the frigid waters are still and dark. Patches of light break across the calm surface as the clouds overhead give way to a brilliant moment of sunshine. As the misty rain stills, a rainbow falls over the steep shoreline in the distance. It's a quintessential Irish scene, and hopefully a good sign that the team will uncover some geologic gold in the samples they collect today.
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 'Pangea' and the British Isles were positioned relative to one another in much the same way they are now.
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, four-legged reptiles roamed the land alongside amphibians and even 'mammal-like' reptiles.
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 Pangea began to break apart, half of the known species on Earth disappeared. 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 Somerset England in 2002.
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?
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," comments Olsen, "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.
“Its extremely unusual to have such a widespread zone of deformation," according to Olsen. "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.”
Many of the typical explanations for mass extinction have been put forth as potential culprits in the T-J event. These include dramatic climate change or the evolution of new and more competitive life forms. However, geological evidence from the British Isles provides little support for these theories.
The organic-rich shale that is widespread in the UK is evidence of a period of anoxia (no oxygen) – but these rocks were laid down seemingly later than the actual extinction event. Evidence for disruption of the carbon cycle due to global warming is also present, but again it postdates the main extinction event. In the UK, the extinction appears to time more closely to geological evidence of sea-level fall and loss of shallow marine habitats.
So what could cause a large-scale disruption in the geological record - rippling and twisting layers of rock over an area the size of the British Isles - and cause global repercussions that could change the course of life's evolution at the planetary scale? Could the dinosaur age have been ushered in by the same type of event that brought it to a close - namely an impact from space?
The only known impact that is thought to have occurred around the time of the T-J extinction, and in a location near the Britsh Isles, struck the Earth near the village of Rochechouart, France. Today, the Rochechouart crater is so heavily eroded that no surface features are visible at the Earth's surface today. Disruptions in the rocks surrounding the impact were discovered in the early 19th century, but it wasn't until 1969 that the French geologist François Kraut proved an actual crater existed underground.
The Rochechouart crater is relatively small with an estimated diameter of only 21 km, although before erosion it may have been about twice that diameter. Unfortunately for Paul Olsen and his team, it is unlikely that the Rochechouart impact was large enough to cause a mass extinction all on its own. By expanding their sampling efforts, the researchers hope to gain a larger view of the events surrounding the T-J extinction. Rochechouart may not be the primary culprit, but maybe its just one piece in a series of catastrophes - from impacts to earthquakes and the eruption of a giant pulse of lava flows - that befell the Earth at this point in history.
The samples collected from Northern Ireland will now make their way back to Columbia University where they can be carefully studied in the laboratory.
"The samples will be analyzed for platinum group elements (PGEs), the specific ratios of which can help us distinguish between causes related to impacts or volcanic eruptions," said Olsen.
On Earth, the elements referred to as PGEs (such as iridium, platinum, palladium and Osmium) were concentrated in the core shortly after our planet was formed, leaving the crust depleted in these metals. This means that there is a lower proportion of iridium at the Earth's surface relative to debris objects (like asteroids and comets) left over from formation of the Solar System.
Asteroids and comets retain higher levels of PGEs and have distinct PGE signatures, generally with more iridium relative to platinum and palladium. Most lavas have very low levels of PGEs with crustal ratios of the elements (although there are some exceptions), and volcanic and magmatic processes occurring on the way to the surface can also result in distinct PGE signatures.
"We have already found platinum group element evidence of an impact in eastern North America and Morocco," Olsen continued, "and if we find it here in the UK we will make a concerted effort to find more definitive evidence of an impact such as shocked quartz. But searching for that is very laborious and we need to know which specific layer to focus on."
The Antrim coast will help the team build on previous studies from far-flung locations. Each of these locations offers a new perspective on the disaster that befell life at the boundary of the Triassic and Jurassic periods. With every new sampling effort, Paul Olsen and his team are getting closer and closer to understanding this pivotal period in life's evolution on Earth.