February 9th, 2013 by James Ayre
The exact cause of the mass extinction 66 million years ago, that included the dinosaurs, has been a matter of considerable debate over the past three decades. There’s been a lot of evidence pointing towards asteroid or comet impacts, but also some towards the role that volcanos or changes in the climate may have had.
New research done by the University of California – Berkeley has provided strong evidence that may end this debate though, the most precise dates yet for when exactly the group of animals commonly known as dinosaurs went extinct, and also for the impact that occurred roughly around the same time period.
The new research has shown that the two dates are extremely close to each other. The researchers note that while the impact event may not be the sole cause of the dinosaur extinction, they almost definitely dealt the death blow.
“The impact was clearly the final straw that pushed Earth past the tipping point,” said Paul Renne, BGC director and UC Berkeley professor in residence of earth and planetary science. “We have shown that these events are synchronous to within a gnat’s eyebrow, and therefore the impact clearly played a major role in extinctions, but it probably wasn’t just the impact.”
Modern extinctions are much the same, rarely is it just one factor that results in an extinction. Almost always it’s a confluence of events and circumstances. In modern times, it’s been mostly a confluence of human expansion, deforestation and habitat loss and soil erosion, and introduced diseases and invasive ‘pests’.
The new dates clarify some of the still present “confusion over whether the impact actually occurred before or after the extinction, which was characterized by the almost overnight disappearance from the fossil record of land-based dinosaurs and many ocean creatures.” The impact has now been dated to 66,038,000 years ago, which is within the same error limits as the extinction event, making the events, essentially, simultaneous.
“The extinction of the dinosaurs was first linked to a comet (make sure to check out Comet ISON later this year) or asteroid impact in 1980 by the late UC Berkeley Nobel Laureate Luis Alvarez and his son, Walter, who is a UC Berkeley professor emeritus of earth and planetary science. A 110-mile-wide crater in the Caribbean off the Yucatan coast of Mexico is thought to be the result of that impact. Called Chicxulub (cheek’-she-loob), the crater is thought to have been excavated by an object six miles across that threw into the atmosphere debris still found around the globe as glassy spheres or tektites, shocked quartz and a layer of iridium-enriched dust.”
This recent research, aiming to achieve a more accurate dating of the extinction, started about three years ago when the lead researcher “noticed that the existing date conflicted with other estimates of the timing of the extinction and that the existing dates for the impact and the extinction did not line up within error margins.”
So, the researchers “went to work recalibrating and improving the existing dating method, known as the argon-argon technique. They then collected volcanic ash from the Hell Creek area in Montana and analyzed them with the recalibrated argon-argon technique to determine the date of the extinction. The formation below the extinction horizon is the source of many dinosaur fossils and one of the best sites to study the change in fossils from before and after the extinction.”
Previously dated tektites were then also gathered from Haiti and “analyzed using the same technique to determine how long ago the impact had occurred. The new extinction and impact dates are precise to within 11,000 years, the researchers said.”
“When I got started in the field, the error bars on these events were plus or minus a million years,” said paleontologist William Clemens, a UC Berkeley professor emeritus of integrative biology who has spent the last 30 years studying the Hell Creek area. “It’s an exciting time right now, a lot of which we can attribute to the work that Paul and his colleagues are doing in refining the precision of the time scale with which we work. This allows us to integrate what we see from the fossil record with data on climate change and changes in flora and fauna that we see around us today.”
As the researchers note, the seemingly synchronous occurrence of the impact and extinction event doesn’t imply that the impact was the only reason for it. “Dramatic climate variation over the previous million years, including long cold snaps amidst a general Cretaceous hothouse environment, probably brought many creatures to the brink of extinction, and the impact kicked them over the edge.”
“These precursory phenomena made the global ecosystem much more sensitive to even relatively small triggers, so that what otherwise might have been a fairly minor effect shifted the ecosystem into a new state,” he said. “The impact was the coup de grace.” This is very similar to the way that human expansion, and the accompanying loss of natural habitat and free movement for many animals, will make the effects of future climate change much more severe for many organisms.
One cause for the climate variability that preceded the asteroid impact “could have been a sustained series of volcanic eruptions in India that produced the extensive Deccan Traps. Renne plans to re-date those volcanic rocks to get a more precise measure of their duration and onset relative to the dinosaur extinction.”
“This study shows the power of high precision geochronology,” said coauthor Darren F. Mark of the Scottish Universities Environmental Research Center, who conducted independent argon-argon analyses on samples provided by Renne. “Many people think precision is just about adding another decimal place to a number. But it’s far more exciting than that. It’s more like getting a sharper lens on a camera. It allows us to dissect the geological record at greater resolution and piece together the sequence of Earth history.”
The new research was just published February 8th in the journal Science.
The Chicxulub impact crater in the Yucatan peninsula is one of the largest confirmed impact craters on the Earth. It’s no surprise that an impacter as large as the one that caused the crater could have such large effects, the impact crater is more than 110 miles in diameter. The impacter must have been at least 6 miles wide.
“The crater was discovered by Glen Penfield, a geophysicist who had been working in the Yucatán while looking for petroleum during the late 1970s. Penfield was initially unable to obtain evidence that the unique geological feature was in fact a crater, and gave up his search. Through contact with Alan Hildebrand, Penfield was able to obtain samples that suggested it was an impact feature. Evidence for the impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas.”
“The impactor had an estimated diameter of 10 km (6.2 mi) and delivered an estimated energy equivalent of 100 teratons of TNT (4.2×1023 J). By contrast, the most powerful man-made explosive device ever detonated, the Tsar Bomba, had a yield of only 50 megatons of TNT (2.1×1017 J), making the Chicxulub impact 2 million times more powerful. Even the most energetic known volcanic eruption, which released approximately 240 gigatons of TNT (1×1021 J) and created the La Garita Caldera, was substantially less powerful than the Chicxulub impact.”
“The impact would have caused some of the largest megatsunamis in Earth’s history. A cloud of super-heated dust, ash and steam would have spread from the crater, as the impactor burrowed underground in less than a second. Excavated material along with pieces of the impactor, ejected out of the atmosphere by the blast, would have been heated to incandescence upon re-entry, broiling the Earth’s surface and possibly igniting wildfires; meanwhile, colossal shock waves would have triggered global earthquakes and volcanic eruptions. The emission of dust and particles could have covered the entire surface of the Earth for several years, possibly a decade, creating a harsh environment for living things. The shock production of carbon dioxide caused by the destruction of carbonate rocks would have led to a sudden greenhouse effect. Over a longer period, sunlight would have been blocked from reaching the surface of the earth by the dust particles in the atmosphere, cooling the surface dramatically. Photosynthesis by plants would also have been interrupted, affecting the entire food chain. A model of the event developed by Lomax et al. suggests that net primary productivity rates may have increased to higher than pre-impact levels over the long term because of the high carbon dioxide concentrations. A long-term effect of the impact was the creation of the sedimentary basin which ‘ultimately produced favorable conditions for human settlement in a region where surface water is scarce.'”
“In February 2008, a team of researchers led by Sean Gulick at the University of Texas at Austin’s Jackson School of Geosciences used seismic images of the crater to determine that the impactor landed in deeper water than was previously assumed. They argued that this would have resulted in increased sulfate aerosols in the atmosphere. According to the press release, that ‘could have made the impact deadlier in two ways: by altering climate (sulfate aerosols in the upper atmosphere can have a cooling effect) and by generating acid rain (water vapor can help to flush the lower atmosphere of sulfate aerosols, causing acid rain).'”
If such an asteroid or comet were to strike the Earth today, the effects seem difficult to clearly guess. It would completely reshape the world though, on a scale probably similar to those seen in the disaster stories common to many mythologies around the world.
Image Credits: Don Davis; NASA; Chicxulub via Wikimedia Commons
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