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Planetary News: Earth (2004)The Great Dying: Does a Submerged Crater off Northwest Australia Hold the Key to an Ancient Riddle?By Amir Alexander14 May 2004 There never was a darker time in the history of life. Within a period of less than 160 thousand years, the blink of a geological eye, 90% of marine life and 70% of land life perished from the face of the Earth. It happened 252 million years ago, at the boundary of the Permian and Triassic geological ages. This gave the catastrophe its scientific name – the P/T Extinction. More commonly, and perhaps more accurately, it is known as “the Great Dying.” The cause of the Great Dying has been one of the great riddles of paleontology, and many answers have been suggested over the years. Some scientists have pointed the finger at tectonic shifts, which created a giant supercontinent at the expense of shallow seas, and wiped out rich habitats. Others have suggested that a period of rapid global warming brought about the mass extinctions, and others still have pointed to massive volcanic eruptions on an unprecedented scale that were taking place in Siberia at the end of the Permian period. None of these theories, however, have been generally accepted by the scientific community as sufficient to explain a disaster of such global proportions. In recent years a new explanation has been gaining ground: the Great Dying, some scientists believe, was caused by the massive impact of a comet or asteroid. After all, it is now widely accepted that just such a scenario brought about another great extinction at the Cretaceous – Tertiary (K/T) boundary 65 million years ago. At that time, a massive space rock slammed into the Mexican coast, creating the now famous Chicxulub crater and bringing about the demise of the dinosaurs. If an impact was the cause of this more recent extinction, couldn’t a similar event have been the culprit in the Great Dying? But while the notion itself appears plausible, proving it is no easy matter. 252 million years are a long time for evidence of a massive impact to erode and dissolve into the Earth. In fact, the oceanic tectonic plates, which make up 70% of the Earth’s surface, are themselves younger than 252 million years. Any evidence that they may have once held, is long gone. Despite the difficulties, however, some researchers are undeterred. At a NASA press conference on Thursday, May 13, and in a paper posted on the Science Magazine website, a group of scientists including Dr. Luann Becker of U.C. Santa Barbara, Robert Poreda and Asish Basu of the University of Rochester, Kevin Pope of Geo Eco Arc Research, and Mark Harrison of the Australian National University in Canberra, announced they have found the impact crater of a giant space rock that hit the Earth right at the time of the Great Dying. The site of the supposed crater is known as Bedout High, and lies off the coast of northwest Australia at the very edge of the continental plate. This location, explained Becker, is highly fortunate, since had the impact occurred on the oceanic plate itself no trace of it would have survived to our day. Becker and her colleagues’ interest in the site was first sparked by the work of geologist John Gorter, who in 1996 suggested that the unique geological formation of Bedout High is in fact an impact crater 200 kilometers wide. Gorter analysis however, was not based on a direct geological analysis of the bedrock at the site, which Becker’s team considered crucial for establishing an impact. Fortunately, they found that in the late 70s and early 80s commercial companies had drilled cores deep into the bedrock of Bedout High, in search of pockets of oil and natural gas. These cores were still preserved in Australia and available for study. At the very deepest part of the core, around 3,000 meters below the surface, the oil company geologists detected a layer they identified as “volcanic breccia.” But when Becker and her team inspected the cores, they saw something rather different: “the moment we saw the cores we thought they looked like an impact breccia” she said. In particular Becker and Poreda noted the presence of glassy fragments in the core, which, they argue, could only be the result of a massive impact. A volcanic eruption, they explained, could also melt the minerals, but the slow cooling process would give the molecules time to reform into ordered crystals. Only the pressure from an enormous and sudden impact would leave in its wake the disordered molecular structure that characterizes glass. Once the researchers had satisfied themselves that Bedout High was indeed the site of a massive impact, the next stage was to date the event as precisely as possible. This, they concede in the paper, was no easy task, given the many geological transformations the area had undergone over hundreds of millions of years. Nevertheless, team member Mark Harrison managed to date a sample from one of the cores at around 250.1 million years, with an accuracy of plus or minus 4.5 million years. If this dating indeed proves representative of the Bedout High site as a whole, it would place the impact right at the time of the Great Dying. The researchers then turned to geographical evidence to establish their case. Over the past decade Becker’s group and others claimed to have discovered debris from a large impact at various Permian-Trassic boundary sites around the globe. Just as iridium deposits at K/T sites all over the world helped convince scientists that a massive space rock had indeed slammed into the Earth 65 million years ago, so these other impact deposits found around the world could serve as evidence that of a more ancient P/T impact. These markers include shocked quartz grains, meteoritic shards, and minute mineral deposits of extraterrestrial origin, and they were found in locations from China to Antarctica. At the end of the Permian age, Becker explains, the landmass of the Earth was concentrated in a single massive supercontinent called Pangaea. If one looks at a map of the world as it was then, and plots the sites where impact markers had been discovered in the P/T boundary, a clear pattern emerges: all of the locations are clustered around Bedout High, and in its relative vicinity. This is precisely the pattern one would expect for the distribution of debris from a massive impact. Finally, team member Kevin Pope pointed out a pattern in the shocked quartz granules created by the K/T impact 65 million ears ago. As one moved further away from the site of the impact at Chicxulub, the grains of shocked quartz from the impact were found to be smaller and smaller. When plotted on a graph, the diminishing size of the granules neatly correlates with the increasing distance from the impact site. This pattern is hardly surprising, since the smaller granules would naturally be carried further away by the winds than the larger and heavier granules, and it served to confirm that the Chicxulub was indeed the source of the shocked quartz. What is true of Chicxulub, according to Pope, is also true of Bedout High. Becker’s team had previously found traces of shocked quartz from the P/T boundary at Graphite Peak in Antarctica, and Pope had recently found larger grains at Fraser Park in Australia. Although these are only two sites, the pattern they establish closely parallels the pattern for the Chicxulub crater: the larger granules come from Fraser Park, the site that was closer to Bedout at the end of the Permian age; the smaller granules were found in Antarctica, which was much further away. Even more importantly, the size of the granules in the P/T layer diminished with the distance from Bedout at almost exactly the same rate as the granules in the K/T layer diminished with the increasing distance from Chicxulub. This, Pope Argues, is strong evidence that Bedout is the source of the shocked quartz at Permian-Triassic boundary sites. Not all scientists are convinced by the evidence presented by Becker’s team. When the group announced its discovery of meteoritic shards in Antarctica last November, many geologists argued that the shards could not possibly have survived for hundreds of millions of years. Other scientists, such as Andrew Glikson of the Australian National University, who had studies the Bedout cores, expressed skepticism about the group’s finding of impact breccia at Bedout. Did a giant space rock slam into what is now northwest Australia 251 million years ago, triggering the greatest mass extinction in the history of life? We are unlikely to know any time soon, as scientific consensus seems currently out of reach. As long as the mystery of the Great Dying endures, the debate will continue. |
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