Clues to one of Earth’s oldest craters revealed

The Sudbury Basin located in Ontario, Canada is one of the largest known impact craters on Earth, as well as one of the oldest due to its formation more than 1.8 billion years ago. Researchers who took samples from the site and subjected them to a detailed geochemical analysis say that a comet may have hit the area to create the crater.

“Our analysis revealed a chondritic platinum group element signature within the crater’s fallback deposits; however, the distribution of these elements within the impact structure and other constraints suggest that the impactor was a comet. Thus, it seems that a comet with a chondritic refractory component may have created the world-famous Sudbury basin,” said Joe Petrus, lead author of the Terra Nova paper.

Prehistoric climate change due to cosmic crash in Canada

An artist's rendition of mastodons, camels and a ground sloth before the environmental changes of the Younger Dryas led to their extinction. -  Barry Roal Carlsen, University of Wisconsin
An artist’s rendition of mastodons, camels and a ground sloth before the environmental changes of the Younger Dryas led to their extinction. – Barry Roal Carlsen, University of Wisconsin

For the first time, a dramatic global climate shift has been linked to the impact in Quebec of an asteroid or comet, Dartmouth researchers and their colleagues report in a new study. The cataclysmic event wiped out many of the planet’s large mammals and may have prompted humans to start gathering and growing some of their food rather than solely hunting big game.

The findings appear next week in the online Early Edition of the Proceedings of the National Academy of Sciences. A preprint of the article is available to journalists starting Wednesday, Aug. 28, at http://www.eurekalert.org/account.php.

The impact occurred about 12,900 years ago, at the beginning of the Younger Dryas period, and marks an abrupt global change to a colder, dryer climate with far-reaching effects on both animals and humans. In North America, the big animals all vanished, including mastodons, camels, giant ground sloths and saber-toothed cats. Their human hunters, known to archaeologists as the Clovis people, set aside their heavy-duty spears and turned to a hunter-gatherer subsistence diet of roots, berries and smaller game.

“The Younger Dryas cooling impacted human history in a profound manner,” says Dartmouth Professor Mukul Sharma, a co-author of the study. “Environmental stresses may also have caused Natufians in the Near East to settle down for the first time and pursue agriculture.”

It is not disputed that these powerful environmental changes occurred, but there has long been controversy over their cause. The classic view of the Younger Dryas cooling interlude has been that an ice dam in the North American ice sheet ruptured, releasing a massive quantity of freshwater into the Atlantic Ocean. The sudden influx is thought to have shut down the ocean currents that move tropical water northward, resulting in the cold, dry climate of the Younger Dryas.

But Sharma and his co-authors have discovered conclusive evidence linking an extraterrestrial impact with this environmental transformation. The report focuses on spherules, or droplets of solidified molten rock expelled by the impact of a comet or meteor. The spherules in question were recovered from Younger Dryas boundary layers at sites in Pennsylvania and New Jersey, the layers having been deposited at the beginning of the period. The geochemistry and mineralogy profiles of the spherules are identical to rock found in southern Quebec, where Sharma and his colleagues argue the impact took place.

“We have for the first time narrowed down the region where a Younger Dryas impact did take place,” says Sharma, “even though we have not yet found its crater.” There is a known impact crater in Quebec – the 4-kilometer wide Corossal crater — but based on the team’s mineralogical and geochemical studies, it is not the impact source for the material found in Pennsylvania and New Jersey.

People have written about many impacts in different parts of the world based on the presence of spherules. “It may well have taken multiple concurrent impacts to bring about the extensive environmental changes of the Younger Dryas,” says Sharma. “However, to date no impact craters have been found and our research will help track one of them down.”

Waiting for Death Valley’s Big Bang

Death Valley's half-mile-wide Ubehebe Crater turns out to have been created 800 years ago -- far more recently than generally thought. -  Brent Goehring/Lamont-Doherty Earth Observatory
Death Valley’s half-mile-wide Ubehebe Crater turns out to have been created 800 years ago — far more recently than generally thought. – Brent Goehring/Lamont-Doherty Earth Observatory

In California’s Death Valley, death is looking just a bit closer. Geologists have determined that the half-mile-wide Ubehebe Crater, formed by a prehistoric volcanic explosion, was created far more recently than previously thought-and that conditions for a sequel may exist today.

Up to now, geologists were vague on the age of the 600-foot deep crater, which formed when a rising plume of magma hit a pocket of underground water, creating an explosion. The most common estimate was about 6,000 years, based partly on Native American artifacts found under debris. Now, a team based at Columbia University’s Lamont-Doherty Earth Observatory has used isotopes in rocks blown out of the crater to show that it formed just 800 years ago, around the year 1200. That geologic youth means it probably still has some vigor; moreover, the scientists think there is still enough groundwater and magma around for another eventual reaction. The study appears in the current issue of the journal Geophysical Research Letters.

Ubehebe (YOU-bee-HEE-bee) is the largest of a dozen such craters, or maars, clustered over about 3 square kilometers of Death Valley National Park. The violent mixing of magma and water, resulting in a so-called phreatomagmatic explosion, blew a hole in the overlying sedimentary rock, sending out superheated steam, volcanic ash and deadly gases such as sulfur dioxide. Study coauthor Brent Goehring, (now at Purdue University) says this would have created an atom-bomb-like mushroom cloud that collapsed on itself in a donut shape, then rushed outward along the ground at some 200 miles an hour, while rocks hailed down. Any creature within two miles or more would be fatally thrown, suffocated, burned and bombarded, though not necessarily in that order. “It would be fun to witness-but I’d want to be 10 miles away,” said Goehring of the explosion.

The team began its work after Goehring and Lamont-Doherty professor Nicholas Christie-Blick led students on a field trip to Death Valley. Noting that Ubehebe remained poorly studied, they got permission from the park to gather some 3- to 6-inch fragments of sandstone and quartzite, part of the sedimentary conglomerate rock that the explosion had torn out. In the lab, Goehring and Lamont-Doherty geochemist Joerg Schaefer applied recent advances in the analysis of beryllium isotopes, which change their weight when exposed to cosmic rays. The isotopes change at a predictable rate when exposed to the rays, so they could pinpoint when the stones were unearthed. An intern at Lamont-Doherty, Columbia College undergraduate Peri Sasnett, took a leading role in the analysis, and ended up as first author on the paper.

The dates clustered from 2,100 to 800 years ago; the scientists interpreted this as signaling a series of smaller explosions, culminating in the big one that created the main crater around 1200. A few other dates went back 3,000 to 5,000 years; these are thought to have come from earlier explosions at smaller nearby maars. Christie-Blick said the dates make it likely that magma is still lurking somewhere below. He pointed out that recent geophysical studies by other researchers have spotted what look like magma bodies under other parts of Death Valley. “Additional small bodies may exist in the region, even if they are sufficiently small not to show up geophysically,” he said. He added that the dates give a rough idea of eruption frequency: about every thousand years or less, which puts the current day within the realm of possibility. “There is no basis for thinking that Ubehebe is done,” he said.

Hydrological data points the same way. Phreatomagmatic explosions are thought to take place mainly in wet places, which would seem to exclude Death Valley–the hottest, driest place on the continent. Yet, as the researchers point out, Lamont-Doherty tree-ring researchers have already shown that the region was even hotter and drier during Medieval times, when the blowup took place. If there was sufficient water then, there is certainly enough now, they say. Observations of springs and modeling of groundwater levels suggests the modern water table starts about 500 feet below the crater floor. The researchers’ calculations suggest that it would take a spherical magma chamber as small as 300 feet across and an even smaller pocket of water to produce a Ubehebe-size incident.

Park officials are taking the study in stride. “We’ve typically viewed Ubehebe as a static feature, but of course we’re aware it could come back,” said geologist Stephanie Kyriazis, a park education specialist. “This certainly adds another dimension to what we tell the public.” (About a million people visit the park each year.) The scientists note that any reactivation of the crater would almost certainly be presaged by warning signs such as shallow earthquakes and opening of steam vents; this could go on for years before anything bigger happened.

For perspective, Yellowstone National Park, further east, is loaded with explosion craters made by related processes, plus the world’s largest concentration of volcanically driven hot springs, geysers and fumaroles. The U.S. Geological Survey expects an explosion big enough to create a 300-foot-wide crater in Yellowstone about every 200 years; there have already been at least 20 smaller blowouts in the past 130 years. Visitors sometimes are boiled alive in springs, but no one has yet been blown up. Death Valley’s own fatal dangers are mainly non-geological: single-vehicle car accidents, heat exhaustion and flash floods. Rock falls, rattlesnakes and scorpions provide extra hazards, said Kyriazis. The crater is not currently on the list. “Right now, we’re not planning to issue an orange alert or anything like that,” she said.

Cold case: Siberian hot springs reveal ancient ecology

Albert Colman, assistant professor in geophysical sciences, researches a class of bacteria that consumes the carbon monoxide both produced by other microbes and derived from the volcanic gases bubbling up in the hot springs in eastern Siberia's Kamchatka Peninsula. -  Courtesy of Albert Colman
Albert Colman, assistant professor in geophysical sciences, researches a class of bacteria that consumes the carbon monoxide both produced by other microbes and derived from the volcanic gases bubbling up in the hot springs in eastern Siberia’s Kamchatka Peninsula. – Courtesy of Albert Colman

Exotic bacteria that do not rely on oxygen may have played an important role in determining the composition of Earth’s early atmosphere, according to a theory that UChicago researcher Albert Colman is testing in the scalding hot springs of a volcanic crater in Siberia.

He has found that bacteria at the site produce as well as consume carbon monoxide, a surprising twist that scientists must take into account as they attempt to reconstruct the evolution of Earth’s early atmosphere.

Colman, an assistant professor in geophysical sciences, joined an American-Russian team in 2005 working in the Uzon Caldera of eastern Siberia’s Kamchatka Peninsula to study the microbiology and geochemistry of the region’s hot springs.

Colman and his colleagues focused on anaerobic carboxydotrophs – microbes with a physiology as exotic as their name. They use carbon monoxide mostly for energy, but also as a source of carbon for the production of new cellular material.

This carbon monoxide-based physiology results in the microbial production of hydrogen, a component of certain alternative fuels. The research team thus also sought to probe biotechnological applications for cleaning carbon monoxide from certain industrial waste gases and for biohydrogen production.

“We targeted geothermal fields,” Colman says, “believing that such environments would prove to be prime habitat for carboxydotrophs due to the venting of chemically reduced, or in other words, oxygen-free and methane-, hydrogen-, and carbon dioxide-rich volcanic gases in the springs.”

The team did discover a wide range of carboxydotrophs. Paradoxically, Colman found that much of the carbon monoxide at the Kamchatka site was not bubbling up with the volcanic gases; instead “it was being produced by the microbial community in these springs,” he says. His team began considering the implications of a strong microbial source of carbon monoxide, both in the local springs but also for the early Earth.

The Great Oxidation Event


Earth’s early atmosphere contained hardly any oxygen but relatively large amounts of carbon dioxide and possibly methane, experts believe. Then during the so-called Great Oxidation Event about 2.3 to 2.5 billion years ago, oxygen levels in the atmosphere rose from vanishingly small amounts to modestly low concentrations.

“This important transition enabled a widespread diversification and proliferation of metabolic strategies and paved the way for a much later climb in oxygen to levels that were high enough to support animal life,” Colman says.

The processing of carbon monoxide by the microbial community could have influenced atmospheric chemistry and climate during the Archean, an interval of Earth’s history that preceded the Great Oxidation Event.

Previous computer simulations rely on a primitive biosphere as the sole means of removing near-surface carbon monoxide produced when the sun’s ultraviolet rays split carbon dioxide molecules. This theoretical sink in the biosphere would have prevented substantial accumulation of atmospheric carbon monoxide.

“But our work is showing that you can’t consider microbial communities as a one-way sink for carbon monoxide,” Colman says. The communities both produce and consume carbon monoxide. “It’s a dynamic cycle.”

Colman’s calculations suggest that carbon monoxide may have nearly reached percentage concentrations of 1 percent in the atmosphere, tens of thousands of times higher than current concentrations. This in turn would have exerted influence on concentration of atmospheric methane, a powerful greenhouse gas, with consequences for global temperatures.

Toxic concentrations


Furthermore, such high carbon monoxide concentrations would have been toxic for many microorganisms, placing evolutionary pressure on the early biosphere.

“A much larger fraction of the microbial community would’ve been exposed to higher carbon monoxide concentrations and would’ve had to develop strategies for coping with the high concentrations because of their toxicity,” Colman says.
Colman and UChicago graduate student Bo He have conducted fieldwork in both Uzon and California’s Lassen Volcanic National Park. Colman has most recently journeyed to Kamchatka for additional fieldwork in 2007 and 2010.

“This fantastic field site has a wide variety of hot springs,” he says. “Different colors, temperatures, chemistries, different types of micro-organisms living in them. It’s a lot like Yellowstone in certain respects.”

Lassen’s springs have a narrower range of acidic chemistries, yet microbial production of carbon monoxide appears to be widespread in both settings.

Collaborator Frank Robb of the University of Maryland, Baltimore, lauds Colman for his “boundless enthusiasm” and for his “meticulous preparation,” much-needed qualities to ensure the safe transport of delicate instruments into the field.

Some of the microbial life within the caldera’s complex hydrothermal system may survive in even more extreme settings than scientists have observed at the surface, Colman says.

“One thing we really don’t know very well is the extent to which microbial communities beneath the surface influence what we see at the surface, but that’s possible as well,” Colman says.

“We know from culturing deep-sea vent microbes that they can live at temperatures that exceed the temperatures we’re observing right at the surface, and some of the turn out to metabolize carbon monoxide.”

Seismic Images Show Dinosaur-killing Meteor Made Bigger Splash





A new study reveals that the asteroid that killed the dinosaurs landed in deeper water than once thought, perhaps explaining why its effects were so severe. Inset: 1996 (black) and 2005 (red) seismic surveys are shown over the Bouguer gravity anomaly map showing the buried Chicxulub impact crater. (Credit: Map from Nature Geoscience / Illustration courtesy of NASA)
A new study reveals that the asteroid that killed the dinosaurs landed in deeper water than once thought, perhaps explaining why its effects were so severe. Inset: 1996 (black) and 2005 (red) seismic surveys are shown over the Bouguer gravity anomaly map showing the buried Chicxulub impact crater. (Credit: Map from Nature Geoscience / Illustration courtesy of NASA)

The most detailed three-dimensional seismic images yet of the Chicxulub crater, a mostly submerged and buried impact crater on the Mexico coast, may modify a theory explaining the extinction of 70 percent of life on Earth 65 million years ago.



The Chicxulub crater was formed when an asteroid struck on the coast of the Yucatan Peninsula. Most scientists agree the impact played a major role in the “KT Extinction Event” that caused the extinction of most life on Earth, including the dinosaurs.



According to Sean Gulick, a research scientist at the Institute for Geophysics at The University of Texas at Austin’s Jackson School of Geosciences and principal investigator for the project, the new images reveal the asteroid landed in deeper water than previously assumed and therefore released about 6.5 times more water vapor into the atmosphere.



The impact site also contained sulfur-rich sediments called evaporites, which would have reacted with water vapor to produce sulfate aerosols. According to Gulick, an increase in the atmospheric concentration of the compounds 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). Earlier studies had suggested both effects might result from the impact, but to a lesser degree.



“The greater amount of water vapor and consequent potential increase in sulfate aerosols needs to be taken into account for models of extinction mechanisms,” says Gulick.



An increase in acid rain might help explain why reef and surface dwelling ocean creatures were affected along with large vertebrates on land and in the sea. As it fell on the water, acid rain could have turned the oceans more acidic. There is some evidence that marine organisms more resistant to a range of pH survived while those more sensitive did not.



Gulick says the mass extinction event was probably not caused by just one mechanism, but rather a combination of environmental changes acting on different time scales, in different locations. For example, many large land animals might have been baked to death within hours or days of the impact as ejected material fell from the sky, heating the atmosphere and setting off firestorms. More gradual changes in climate and acidity might have had a larger impact in the oceans.





A new study reveals that the asteroid that killed the dinosaurs landed in deeper water than once thought, perhaps explaining why its effects were so severe. Inset: 1996 (black) and 2005 (red) seismic surveys are shown over the Bouguer gravity anomaly map showing the buried Chicxulub impact crater. (Credit: Map from Nature Geoscience / Illustration courtesy of NASA)
A new study reveals that the asteroid that killed the dinosaurs landed in deeper water than once thought, perhaps explaining why its effects were so severe. Inset: 1996 (black) and 2005 (red) seismic surveys are shown over the Bouguer gravity anomaly map showing the buried Chicxulub impact crater. (Credit: Map from Nature Geoscience / Illustration courtesy of NASA)

Gulick and collaborators originally set out to learn more about the trajectory of the asteroid. They had hoped the crater’s structure in the subsurface would hold a tell-tale signature. Instead, the structure seemed to be most strongly shaped by the pre-impact conditions of the target site.



“We discovered that the shallow structure of the crater was determined much more by what the impact site was like before impact than by the trajectory of the impactor,” says Gulick.



If scientists can determine the trajectory, it will tell them where to look for the biggest environmental consequences of impact, because most of the hazardous, shock-heated and fast-moving material would have been thrown out of the crater downrange from the impact.



Researchers at Imperial College in London are already using computer models to search for possible signatures in impact craters that could indicate trajectory regardless of the initial surface conditions at the impact site.



“As someone who simulates impact events using computers, this work provides valuable new constraints on both the pre-impact target structure and the final geometry of the cratered crust at Chicxulub,” says Gareth Collins, a research fellow at Imperial College.



The study “Importance of pre-impact crustal structure for the asymmetry of the Chicxulub impact crater” appears in the February 2008 print edition of the journal Nature Geoscience.



Collaborators on the project included Gail Christeson of the Institute for Geophysics, Penny Barton at the University of Cambridge, Joanna Morgan and Mike Warner at Imperial College, and several graduate students.

Arctic Impact Crater Lake Reveals Interglacial Cycles in Sediments





The coring equipment and other instrumentation was set up using a tripod over the hole in the ice. The scientists were able to extract a core of the topmost 8.5 meters of sediment.
The coring equipment and other instrumentation was set up using a tripod over the hole in the ice. The scientists were able to extract a core of the topmost 8.5 meters of sediment.

A University of Arkansas researcher and a team of international scientists have taken cores from the sediments of a Canadian Arctic lake and found an interglacial record indicating two ice-free periods that could pre-date the Holocene Epoch.



Sonja Hausmann, assistant professor of geosciences in the J. William Fulbright College of Arts and Sciences at the University of Arkansas, and her colleagues will report their preliminary findings at the American Geophysical Union meeting this week.



The researchers traveled by increasingly smaller planes, Ski-doos and finally sleds dragged on foot to arrive at the Pingualuit Crater, located in the Parc National des Pingualuit in northern Quebec. The crater formed about 1.4 million years ago as the result of a meteorite impact, and today it hosts a lake about 267 meters deep. Its unique setting – the lake has no surface connection to other surrounding water bodies – makes it a prime candidate for the study of lake sediments.



Scientists study lake sediments to determine environmental information beyond historical records. Hausmann studies diatoms, unicellular algae with shells of silica, which remain in the sediments. Diatoms make excellent bioindicators, Hausmann said, because the diatom community composition changes with environmental changes in acidity, climate, nutrient availability and lake circulation.



By examining relationships between modern diatom communities and their environment, Hausmann and her colleagues can reconstruct various historic environmental changes quantitatively.



However, most sediments of lakes in previously glaciated areas have limitations – they only date back to the last ice age.


“Glaciers are powerful. They polish everything,” Hausmann said. Glaciers typically carve out any sediments in a lake bed, meaning any record before the ice age is swept away.



However, the unique composition of the Pingualuit Crater Lake led Michel A. Bouchard to speculate in 1989 that the sediments beneath its icy exterior might have escaped glacial sculpting. So in May of this year, Hausmann and her colleagues donned parkas, hauled equipment on ski-doos and slogged through sub-zero temperatures for three weeks so they could core sediments and collect data from the lake.



They carefully carved squares of ice out to make a small hole for equipment, then began a series of investigations that included pulling up a core of the topmost 8.5 meters of sediment. An echosounder indicated that the lake bottom may have more than 100 meters of relatively fine-grained sediments altogether. During the time since the expedition, researchers have examined the physical, magnetic and sedimentological properties of the sediment core.



The sediment core contains mostly faintly laminated silts or sandy mud with frequent pebble-size rock fragments, which is typical of deposits found in water bodies covered by an ice sheet. Sandwiched in the middle of the faintly laminated silts and sandy mud, the researchers found two distinct and separate layers containing organically rich material that most likely date back well before the Holocene, representing earlier ice-free periods. The samples they found contain the remains of diatoms and other organic material, suggesting that they represent ice-free conditions and possibly interglacial periods.



“There are no paleolimnological studies of lakes that cover several warm periods in this area,” Hausmann said. The terrestrial record will be complementary to marine records or to long ice-core records from Greenland.



The international team of researchers in the field included Guillaume St-Onge; Reinhard Pienitz, principal investigator; Veli-Pekka Salonen of the University of Helsinki, Finland; and Richard Niederreiter, coring expert. Please visit http://www.cen.ulaval.ca/pingualuit/index.html for more information.

New meteorite impact site discovered in the north west province of South Africa





Aeromagnetic image of Setlagole-Madibogo meteorite impact site showing the circular ring structure and cross-cutting dykes
Aeromagnetic image of Setlagole-Madibogo meteorite impact site showing the circular ring structure and cross-cutting dykes

A spectacular megabreccia (a coarse rock assemblage composed of large angular-to-rounded fragments, some over 6m in length, held together by a mineral cement – in this particular case by melted rock in the form of fine crystalline glassy material) in the Kraaipan granite-greenstone terrane, located roughly midway between Mafikeng and Vryburg, has provided the first clues to the recognition of a new meteorite impact locality. The discovery adds a sixth impact site to the list previously recorded in southern Africa and is exceeded in size only by the Vredefort and Morokweng impact structures.



Geological mapping in the Archaean granite-greenstone terrane of the North West province has revealed the megabreccia which crops out sporadically in an otherwise poorly exposed part of the west-central region of the Kaapvaal Craton. Younger Kalahari sand and calcrete blankets much of the Archaean basement, which consists of remnants of the Kraaipan greenstone terrane and a variety of granitoid rocks.



The Kraaipan granite-greenstone basement in the Setlagole-Madibogo area of the North West province acted as a target environment for a meteorite impact event. The megabreccia, formed by an impacting bolide (an exploding or exploded meteor or meteorite), contains countless rock fragments and microscopic particles of the Archaean basement. The 300km diameter Vredefort Structure, 240km to the east, is known to be about 2020 million years old, whereas the 70-80km diameter of the Morokweng impact structure, about 135km to the west, has been dated at approximately 145 million years.



The exact dimensions of the Setlagole impact structure have not yet been accurately determined, but preliminary estimates put the diameter at about 25km. When exactly the event occurred also still remains to be determined. What is known at this stage is that the ring structure, defined aeromagnetically (a magnetic survey of the earths surface carried out with an airborne magnetometer) by Dr Edgar Stettler, is cut by one or possibly two dyke events. One of the dykes, which is not exposed is possibly of Karoo age. This may suggest that the impact structure is at least older than Karoo magmatism dated at about 180 million years. It is also older than the Morokweng impact structure.



There are other linear features transecting the Setlagole ring structure, several in the northwest and north and another in the southeast. These may represent either faults or dykes that are strongly remanently magnetized. It remains to be determined if some dykes may represent feeders to Ventersdorp volcanism present in the region. If they are linked with the Ventersdorp event, the impact structure could have a Neoarchaean age (about 2700 million years) making it possibly the oldest known impact structure on Earth.

The way forward






Dyke of fine-grained presumably impact 'melt rock', intruded into the Setlagole Megabreccia
Dyke of fine-grained presumably impact ‘melt rock’, intruded into the Setlagole Megabreccia

It has taken a wide range of specialist Earth scientists many decades to unravel the history and evolution of impact structures present around the world, including the Vredefort structure. Even here there is not always consensus of views and ongoing debates still persist on various issues. The Setlagole impact structure will doubtless also engender divergent views and opinions as time passes. Work still in progress and being planned is aimed at diminishing the speculative aspect of the issues relating to the impact structure and a number of collaborative studies have been initiated.



Dr Edgar Stettler, one of the joint discoverers of the structure employing aeromagnetic techniques, has made available the geophysical data set to Prof Gordon Cooper (Geophysics, Wits University) for the enhancement and analysis of magnetic signatures using filtering techniques involving fractional derivative and circular-shaded relief algorithms. Sue Webb (Geophysics, Wits University) is committed to undertaking a gravity survey that may eventually define more accurately the true width of the structure and other internal features.



The relative age of the structure, the intruding dykes and the metamorphic overprinting is also receiving attention together with details of geochemistry and petrology of the megabreccia and associated basement granitoids. Finally, the potential for mineralisation associated with the impact warrant consideration particularly in the light of Ni-Cu (nickel-copper) and PGMs (platinum group metals) mineral deposits being directly linked to impact structures such as the famous Sudbury Structure in Canada.



Carl Anhaeusser is Professor Emeritus in the Economic Geology Research Institute in the Wits School of Geosciences. The text is an edited version of an article authored by Prof. Anhaeusser, which was first published in June 2007 in ‘Geobulletin’ Volume 50, No 2, a quarterly publication of the Geological Society of South Africa.



Prof. Anhaeusser will be delivering a Geotalk in the Geology Department on Thursday, 18 October at 16:30 in Room 101, 1st Floor Geosciences Building, East Campus on this topic. All welcome.

Comet May Have Exploded Over North America 13,000 Years Ago





A 'black mat' of algal growth in Arizona marks a line of extinction at 12,900 years ago; Clovis points and mammoth skeletons were found at the line but not above it. - Photo Credit: Allen West, UCSB
A ‘black mat’ of algal growth in Arizona marks a line of extinction at 12,900 years ago; Clovis points and mammoth skeletons were found at the line but not above it. – Photo Credit: Allen West, UCSB

New scientific findings suggest that a large comet may have exploded over North America 12,900 years ago, explaining riddles that scientists have wrestled with for decades, including an abrupt cooling of much of the planet and the extinction of large mammals.



The discovery was made by scientists from the University of California at Santa Barbara and their colleagues. James Kennett, a paleoceanographer at the university, said that the discovery may explain some of the highly debated geologic controversies of recent decades.



The period in question is called the Younger Dryas, an interval of abrupt cooling that lasted for about 1,000 years and occurred at the beginning of an inter-glacial warm period. Evidence for the temperature change is recorded in marine sediments and ice cores.



According to the scientists, the comet before fragmentation must have been about four kilometers across, and either exploded in the atmosphere or had fragments hit the Laurentide ice sheet in the northeastern North America.



Wildfires across the continent would have resulted from the fiery impact, killing off vegetation that was the food supply of many of larger mammals like the woolly mammoths, causing them to go extinct.



Since the Clovis people of North America hunted the mammoths as a major source of their food, they too would have been affected by the impact. Their culture eventually died out.


The scientific team visited more than a dozen archaeological sites in North America, where they found high concentrations of iridium, an element that is rare on Earth, and is almost exclusively associated with extraterrestrial objects such as comets and meteorites.



They also found metallic microspherules in the comet fragments; these microspherules contained nano-diamonds. The comet also carried carbon molecules called fullerenes (buckyballs), with gases trapped inside that indicated an extraterrestrial origin.



The team concluded that the impact of the comet likely destabilized a large portion of the Laurentide ice sheet, causing a high volume of freshwater to flow into the north Atlantic and Arctic Oceans.



“This, in turn, would have caused a major disruption of the ocean’s circulation, leading to a cooler atmosphere and the glaciation of the Younger Dryas period,” said Kennett. “We found evidence of the impact as far west as the Santa Barbara Channel Islands.”



NSF’s Paleoclimate Program funded the research.