Volcanic Eruptions, Not Meteor, May Have Killed The Dinosaurs

Rajahmundry Quarry. Keller's crucial link between the eruption and the mass extinction comes in the form of microscopic marine fossils that are known to have evolved immediately after the mysterious mass extinction event. The same telltale fossilized planktonic foraminifera were found at Rajahmundry near the Bay of Bengal, about 1000 kilometers from the center of the Deccan Traps near Mumbai. (Credit: Photo courtesy Gerta Keller)
Rajahmundry Quarry. Keller’s crucial link between the eruption and the mass extinction comes in the form of microscopic marine fossils that are known to have evolved immediately after the mysterious mass extinction event. The same telltale fossilized planktonic foraminifera were found at Rajahmundry near the Bay of Bengal, about 1000 kilometers from the center of the Deccan Traps near Mumbai. (Credit: Photo courtesy Gerta Keller)

A series of monumental volcanic eruptions in India may have killed the dinosaurs 65 million years ago, not a meteor impact in the Gulf of Mexico. The eruptions, which created the gigantic Deccan Traps lava beds of India, are now the prime suspect in the most famous and persistent paleontological murder mystery, say scientists who have conducted a slew of new investigations honing down eruption timing.

“It’s the first time we can directly link the main phase of the Deccan Traps to the mass extinction,” said Princeton University paleontologist Gerta Keller. The main phase of the Deccan eruptions spewed 80 percent of the lava which spread out for hundreds of miles. It is calculated to have released ten times more climate altering gases into the atmosphere than the nearly concurrent Chicxulub meteor impact, according to volcanologist Vincent Courtillot from the Physique du Globe de Paris.

Keller’s crucial link between the eruption and the mass extinction comes in the form of microscopic marine fossils that are known to have evolved immediately after the mysterious mass extinction event. The same telltale fossilized planktonic foraminifera were found at Rajahmundry near the Bay of Bengal, about 1000 kilometers from the center of the Deccan Traps near Mumbai. At Rajahmundry there are two lava “traps” containing four layers of lava each. Between the traps are about nine meters of marine sediments. Those sediments just above the lower trap, which was the mammoth main phase, contain the incriminating microfossils.

Previous work had first narrowed the Deccan eruption timing to within 800,000 years of the extinction event using paleomagnetic signatures of Earth’s changing magnetic field frozen in minerals that crystallized from the cooling lava. Then radiometric dating of argon and potassium isotopes in minerals narrowed the age to within 300,000 years of the 65-million-year-old Cretaceous-Tertiary (a.k.a. Cretaceous-Paleogene) boundary, sometimes called the K-T boundary.

The microfossils are far more specific, however, because they demonstrate directly that the biggest phase of the eruption ended right when the aftermath of the mass extinction event began. That sort of clear-cut timing has been a lot tougher to pin down with Chicxulub-related sediments, which predate the mass extinction.

“Our results are consistent and mutually supportive with a number of new studies, including Chenet, Courtillot and others (in press) and Jay and Widdowson (in press), that reveal a very short time for the main Deccan eruptions at or near the K-T boundary and the massive carbon dioxide and sulfur dioxide output of each major eruption that dwarfs the output of Chicxulub,” explained Keller. “Our K-T age control combined with these results strongly points to Deccan volcanism as the likely leading contender in the K-T mass extinction.” Keller’s study was funded by the National Science Foundation.

The Deccan Traps also provide an answer to a question on which Chicxulub was silent: Why did it take about 300,000 years for marine species to recover from the extinction event? The solution is in the upper, later Deccan Traps eruptions.

“It’s been an enigma,” Keller said. “The very last one was Early Danian, 280,000 years after the mass extinction, which coincides with the delayed recovery.”

Keller and her colleagues are planning to explore the onset of the main phase of Deccan volcanism, that is, the rocks directly beneath the main phase lavas at Rajahmundry. That will require drilling into the Rajahmundry Traps, a project now slated for December-January 2007/2008.

Keller and her collaborator Thierry Adatte from the University of Neuchatel, Switzerland, are scheduled to present the new findings on Tuesday, 30 October, at the annual meeting of the Geological Society of America in Denver. They will also display a poster on the matter at the meeting on Wednesday, 31 October.

Like it or not, uncertainty and climate change go hand in hand

Despite decades of ever more-exacting science projecting Earth’s warming climate, there remains large uncertainty about just how much warming will actually occur.

Two University of Washington scientists believe the uncertainty remains so high because the climate system itself is very sensitive to a variety of factors, such as increased greenhouse gases or a higher concentration of atmospheric particles that reflect sunlight back into space.

In essence, the scientists found that the more likely it is that conditions will cause climate to warm, the more uncertainty exists about how much warming there will be.

“Uncertainty and sensitivity have to go hand in hand. They’re inextricable,” said Gerard Roe, a UW associate professor of Earth and space sciences. “We’re used to systems in which reducing the uncertainty in the physics means reducing the uncertainty in the response by about the same proportion. But that’s not how climate change works.”

Roe and Marcia Baker, a UW professor emeritus of Earth and space sciences and of atmospheric sciences, have devised and tested a theory they believe can help climate modelers and observers understand the range of probabilities from various factors, or feedbacks, involved in climate change. The theory is contained in a paper published in the Oct. 26 edition of Science.

In political polling, as the same questions are asked of more and more people the uncertainty, expressed as margin of error, declines substantially and the poll becomes a clearer snapshot of public opinion at that time. But it turns out that with climate, additional research does not substantially reduce the uncertainty.

The equation devised by Roe and Baker helps modelers understand built-in uncertainties so that the researchers can get meaningful results after running a climate model just a few times, rather than having to run it several thousand times and adjust various climate factors each time.

“It’s a yardstick against which one can test climate models,” Roe said.

Scientists have projected that simply doubling carbon dioxide in the atmosphere from pre-Industrial Revolution levels would increase global mean temperature by about 2.2 degrees Fahrenheit. However, that projection does not take into account climate feedbacks — physical processes in the climate system that amplify or subdue the response. Those feedbacks would raise temperature even more, as much as another 5 degrees F according to the most likely projection. One example of a feedback is that a warmer atmosphere holds more water vapor, which in itself is a greenhouse gas. The increased water vapor then amplifies the effect on temperature caused by the original increase in carbon dioxide.

“Sensitivity to carbon dioxide concentration is just one measure of climate change, but it is the standard measure,” Roe said.

Before the Industrial Revolution began in the late 1700s, atmospheric carbon dioxide was at a concentration of about 280 parts per million. Today it is about 380 parts per million and estimates are that it will reach 560 to 1,000 parts per million by the end of the century.

The question is what all that added carbon dioxide will do to the planet’s temperature. The new equation can help provide an answer, since it links the probability of warming with uncertainty about the physical processes that affect how much warming will occur, Roe said.

“The kicker is that small uncertainties in the physical processes are amplified into large uncertainties in the climate response, and there is nothing we can do about that,” he said.

While the new equation will help scientists quickly see the most likely impacts, it also shows that far more extreme temperature changes — perhaps 15 degrees or more in the global mean — are possible, though not probable. That same result also was reported in previous studies that used thousands of computer simulations, and the new equation shows the extreme possibilities are fundamental to the nature of the climate system.

Much will depend on what happens to emissions of carbon dioxide and other greenhouse gases in the future. Since they can remain in the atmosphere for decades, even a slight decrease in emissions is unlikely to do more than stabilize overall concentrations, Roe said.

“If all we do is stabilize concentrations, then we will still be risking the highest temperature change shown in the models,” he said.

Scientist studies ‘fossil earthquakes,’ possible key to understanding future quakes

A Colorado State researcher is studying Earth’s ancient earthquakes, or fossil earthquakes, to get a better understanding of how and why earthquakes happen.

Geologist Jerry Magloughlin is studying the rocks that form where earthquakes actually happen. By studying these unique fossil earthquakes, scientists can learn about how and why faults slip and produce earthquakes.

These fossils are the only well-accepted evidence proving that an area was affected by ancient earthquakes. Magloughlin unearthed some of these rocks in Northern Colorado’s Poudre Canyon.

“The rocks I found in northern Colorado are relatively small, humble examples of fossil earthquakes, but it’s still remarkable that these delicate rocks can be preserved for hundreds of millions of years,” Magloughlin said. “Much more interesting examples of the rock are either extremely thin or extremely thick. I’ve found microscopic ones from Ontario, possibly associated with very tiny ‘nanoquakes,’ and a 5-meter-thick rock from Scotland that must have formed about a billion years ago in a giant earthquake, or ‘megaquake.'”

When faults, or rock fractures, cut through Earth’s crust, various types of damage occur within the rock. The severity of damage depends in part on how quickly the two sides of the fault slide past each other. In some cases, this can be a very slow process resulting in so-called silent earthquakes that often go unnoticed. In other cases, the fault can slip several feet or more in a matter of seconds, producing the kinds of earthquakes capable of causing serious destruction.

In addition to studying how quickly slip occurs, the depth of the rock where the slip occurs also is studied. If the fault slips near Earth’s surface, rock can be crushed and made into a mud-like substance called fault gouge. During an earthquake – under certain conditions – the walls of the fault slip fast enough to produce frictional heat that melts the rock. The melted rock, reaching temperatures of more than 1,800 degrees Fahrenheit, solidifies into a new, distinctive rock creating a kind of fossil earthquake.

Due to the unique nature of these rocks and their ability to preserve the moment in time when earthquakes happen, scientists can better understand the mechanics of how faults operate and earthquakes occur.

Extinction Theory Falls From Favor

Doctoral student Catherine Powers traveled to fossil sites around the world, including this one in Greece, to study ancient bryozoan marine communities.
Doctoral student Catherine Powers traveled to fossil sites around the world, including this one in Greece, to study ancient bryozoan marine communities.

The greatest mass extinction in Earth’s history also may have been one of the slowest, according to a study that casts further doubt on the extinction-by-meteor theory.

Creeping environmental stress fueled by volcanic eruptions and global warming was the likely cause of the Great Dying 250 million years ago, said USC doctoral student Catherine Powers.

Writing in the November issue of the journal Geology, Powers and her adviser David Bottjer, professor of earth sciences at USC College, describe a slow decline in the diversity of some common marine organisms.

The decline began millions of years before the disappearance of 90 percent of Earth’s species at the end of the Permian era, Powers shows in her study.

More damaging to the meteor theory, the study finds that organisms in the deep ocean started dying first, followed by those on ocean shelves and reefs, and finally those living near shore.

“Something has to be coming from the deep ocean,” Powers said. “Something has to be coming up the water column and killing these organisms.”

That something probably was hydrogen sulfide, according to Powers, who cited studies from the University of Washington, Pennsylvania State University, the University of Arizona and the Bottjer laboratory at USC.

Those studies, combined with the new data from Powers and Bottjer, support a model that attributes the extinction to enormous volcanic eruptions that released carbon dioxide and methane, triggering rapid global warming.

The warmer ocean water would have lost some of its ability to retain oxygen, allowing water rich in hydrogen sulfide to well up from the deep (the gas comes from anaerobic bacteria at the bottom of the ocean).

If large amounts of hydrogen sulfide escaped into the atmosphere, the gas would have killed most forms of life and also damaged the ozone shield, increasing the level of harmful ultraviolet radiation reaching the planet’s surface.

Powers and others believe that the same deadly sequence repeated itself for another major extinction 200 million years ago, at the end of the Triassic era.

“There are very few people that hang on to the idea that it was a meteorite impact,” she said. Even if an impact did occur, she added, it could not have been the primary cause of an extinction already in progress.

In her study, Powers analyzed the distribution and diversity of bryozoans, a family of marine invertebrates.

Based on the types of rocks in which the fossils were found, Powers was able to classify the organisms according to age and approximate depth of their habitat.

She found that bryozoan diversity in the deep ocean started to decrease about 270 million years ago and fell sharply in the 10 million years before the mass extinction that marked the end of the Permian era.

But diversity at middle depths and near shore fell off later and gradually, with shoreline bryozoans being affected last, Powers said.

She observed the same pattern before the end-Triassic extinction, 50 million years after the end-Permian.

Powers’ work was funded by the Geological Society of America, the Paleontological Society, the American Museum of Natural History and the Yale Peabody Museum, and supplemented by a grant from USC’s Women in Science and Engineering program.

Geology is published by the Geological Society of America.

Methane Bubbling From Arctic Lakes, Now And At End Of Last Ice Age

UAF researcher Katey Walter lights a pocket of methane on a thermokarst lake in Siberia in March of 2007. Igniting the gas is a way to demonstrate, in the field, that it contains methane. (Credit: Photo by Sergey Zimov)
UAF researcher Katey Walter lights a pocket of methane on a thermokarst lake in Siberia in March of 2007. Igniting the gas is a way to demonstrate, in the field, that it contains methane. (Credit: Photo by Sergey Zimov)

A team of scientists led by a researcher at the University of Alaska Fairbanks has identified a new likely source of a spike in atmospheric methane coming out of the North during the end of the last ice age.

Methane bubbling from arctic lakes could have been responsible for up to 87 percent of that methane spike, said UAF researcher Katey Walter, lead author of a report printed in the Oct. 26 issue of Science. The findings could help scientists understand how current warming might affect atmospheric levels of methane, a gas that is thought to contribute to climate change.

“It tells us that this isn’t just something that is ongoing now. It would have been a positive feedback to climate warming then, as it is today,” said Walter. “We estimate that as much as 10 times the amount of methane that is currently in the atmosphere will come out of these lakes as permafrost thaws in the future. The timing of this emission is uncertain, but likely we are talking about a time frame of hundreds to thousands of years, if climate warming continues as projected.”

Ice cores from Greenland and Antarctica have shown that during the early Holocene Period–about 14,000 to 11,500 years ago–the levels of methane in the atmosphere rose significantly, Walter said. “They found that an unidentified northern source (of methane) appeared during that time.”

Previous hypotheses suggested that the increase came from gas hydrates or wetlands. This study’s findings indicate that methane bubbling from thermokarst lakes, which are formed when permafrost thaws rapidly, is likely a third and major source.

Walter’s research focused on areas of Siberia and Alaska that, during the last ice age, were dry grasslands atop ice-rich permafrost. As the climate warmed, Walter said, that permafrost thawed, forming thermokarst lakes.

“Lakes really flared up on this icy permafrost landscape, emitting huge amounts of methane,” she said.

As the permafrost around and under the lakes thaws, the organic material in it–dead plants and animals–can enter the lake bottom and become food for the bacteria that produce methane.

“All that carbon that had been locked up in the ground for thousands of years is converted to potent greenhouse gases: methane and carbon dioxide,” Walter said. Walter’s paper hypothesizes that methane from the lakes contributed 33 to 87 percent of the early Holocene methane increase.

To arrive at the hypothesis, Walter and her colleagues traveled to Siberia and northern Alaska to examine lakes that currently release methane. In addition, they gathered samples of permafrost and thawed them in the laboratory to study how much methane permafrost soil can produce immediately after thawing.

“We found that it produced a lot very quickly,” she said.

Finally, she and other researchers studied when existing lakes and lakes in the past formed and found that their formation coincided with the early Holocene Period northern methane spike.

“We came up with a new hypothesis,” she said. “Thermokarst lake formation is a source of atmospheric methane today, but it was even more important during early Holocene warming. This suggests that large releases from lakes may occur again in the future with global warming.”

Co-authors on the paper include Mary Edwards of the University of Southampton and the UAF College of Natural Science and Mathematics; Guido Grosse, an International Polar Year postdoctoral fellow with the UAF Geophysical Institute; Sergey Zimov of the Russian Academy of Sciences; and Terry Chapin of the UAF Institute of Arctic Biology. Funding was provided by the National Science Foundation, the Environmental Protection Agency and the National Aeronautics and Space Administration.

Seismologists See Earth’s Dynamic Interior as Interplay of Temperature, Pressure, Chemistry

Surface topography and bathymetry around South America (top) overlays variable topography in Earth's upper mantle at 410 kilometers and 660 kilometers depth. - Credit: Arizona State University, Nicholas Schmerr/Edward Garnero
Surface topography and bathymetry around South America (top) overlays variable topography in Earth’s upper mantle at 410 kilometers and 660 kilometers depth. – Credit: Arizona State University, Nicholas Schmerr/Edward Garnero

Seismologists have recast their understanding of the inner workings of Earth from a relatively homogeneous environment to one that is highly dynamic and chemically diverse.

The research, conducted by scientists Nicholas Schmerr and Edward Garnero of Arizona State University in Tempe, is published in the October 26 issue of the journal Science.

This view of Earth’s inner workings depicts the inner planet as a living organism where events that happen deep within can affect what happens at its surface, like the rub and slip of tectonic plates and the rumble of volcanoes.

The new research into these inner workings shows that in Earth’s upper mantle (an area that extends down to 660 kilometers), more than temperature and pressure play a role.

“It has long been a goal of seismologists to distinguish between thermal and chemical effects on seismic velocities in Earth’s deep interior,” said Robin Reichlin, program director in the National Science Foundation (NSF)’s division of earth sciences, which funded the research. “This work is a tantalizing step toward solving this important problem, necessary for understanding the internal structure and dynamics of our planet.”

The simplest model of the mantle–the layer of the Earth’s interior just beneath the crust–is that of a convective heat engine. Like a pot of boiling water, the mantle has parts that are hot and welling up, as in the mid-Atlantic rift, and parts that are cooler and sinking, as in subduction zones. There, crust sinks into the Earth, mixing and transforming into different material “phases,” like graphite turning into diamond.

“A great deal of past research on mantle structure has interpreted anomalous seismic observations as due to thermal variations within the mantle,” Schmerr said. “We’re trying to get people to think about how the interior of the Earth can be not just thermally different but also chemically different.”

Schmerr’s and Garnero’s work shows that Earth’s interior possesses an exotic brew of material that goes beyond simply hot and cold convection currents.

To make their observations, Schmerr and Garnero used data from the USArray, which is part of the NSF-funded EarthScope project.

“The USArray is 500 seismometers deployed in a movable grid across the United States,” Schmerr said. “It’s an unheard of density of seismometers.”

Schmerr and Garnero used seismic waves from earthquakes to measure where phase transitions occur in the interior of Earth by looking for where waves reflect off these boundaries.

They studied seismic waves that reflect off the underside of phase transitions halfway between an earthquake and a seismometer. The density and other characteristics of the material they travel through affect how the waves move, and give geologists an idea of the structure of the inner Earth.

Beneath South America, Schmerr’s research found the 410-kilometer phase boundary bending the wrong way. The mantle beneath South America is predicted to be relatively cold due to cold and dense former oceanic crust and the underlying tectonic plate sinking into the planet from the subduction zone along the west coast. In such a region, the 410-kilometer boundary would normally be upwarped, but using energy from far away earthquakes that reflect off the deep boundaries in this study area, Schmerr and Garnero found that the boundary significantly deepened.

They postulate that either hydrogen or iron concentrations are responsible for the observed deflection of the 410 discontinuity.

“This study lets us constrain the temperature and composition to a certain degree, imaging this structure inside the Earth and saying: ‘These are not just thermal effects — there’s also some sort of chemical aspect to it as well,'” Schmerr said.

New Way To Measure Ancient Ocean Temperatures Refined

Spanish researcher Carme Huguet further refined the recently developed TEX86 paleothermometer during her doctoral research at the Royal Netherlands Institute for Sea Research (NIOZ). The thermometer measures seawater temperature dependent changes in the cell wall composition of archeabacteria.

Real thermometers have been available since the 17th century. For all periods before this, researchers depend on signs from nature. For such determinations, geochemists resort to molecules from microorganisms whose structure is well preserved in seabeds.

The TEX86 index has recently been developed at Royal Netherlands Institute for Sea Research (NIOZ). It is based on temperature-dependent changes in the lipid composition of the cell walls of certain types of archeabacteria. Their cell membranes are composed from special lipids of which the number of carbon rings in the molecule changes with the temperature of the surrounding seawater. These organisms therefore adjust the degree of fluidity of their membranes to the prevailing conditions. Carme Huguet studied several aspects of this in greater detail and made significant improvements to the determination.

With a new detection method the analytical reproducibility of the TEX86 paleothermometer was brought to ±0.3 °C and the deviation in the results measured was reduced to 5% of the average. The TEX86 values for organic material out of the water column and from the uppermost layer of the floor sediment best match the temperature of the uppermost 100 m of seawater.

However, the small cells of Crenarchaeota cannot sink to the floor by themselves; they are far too light for that. This is, however, achieved more rapidly if the cells of Crenarchaeota are eaten, for example, by crustaceous zooplankton. Fortunately, the time spent in the gastrointestinal tract of the crustaceans does not harm the molecules. Once they have landed on the sea floor, the preservation of the original fat molecules takes place best in anaerobic sediments.

In modern, anaerobic sediments from a side branch of the Oslo fjord, the measured TEX86 values accurately reflected the average spring-autumn air temperature in Oslo. Temperature estimations of the transition from the last ice age to the present interglacial period were made using two cores drilled from the Arabian Sea. The TEX86 temperatures were compared with values from a British index; the Uk37.

The index differences can be explained by differences in the growing season of the archeabacteria and algae that the Uk37 index is dependent on. The upwelling dynamic of the seawater in the Arabian Sea also exerts an influence. This dynamic is strongly dependent on the monsoon season in this area.

Carme Huguet’s research makes it clear that climate reconstructions should always be based on comparisons of several types of parallel measurements to prevent unexpected scientific blunders. Determining the surface seawater temperatures in oceans and coastal waters is essential for the reconstruction of historic climate changes and changes in ocean currents. This information is, in turn, vital for perfecting current climate models.

This research was funded by NWO.

Researchers probe undersea earthquake zone

D/V CHIKYU drilling system
D/V CHIKYU drilling system

Over the next five years, an international team of scientists will drill deep into the Earth’s crust off the shore of Japan to understand how undersea earthquakes are generated and to establish a series of permanent undersea observatories on the plate boundary.

The scientists, part of the International Ocean Drilling Program’s Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) aboard the specially built Japanese vessel, Chikyu, began their expedition in September. The project will eventually include between 150 to 200 scientists from Japan, Europe, China, South Korea and the United States.

The Nankai Trough, located off the southwest coast of Japan, has been the site of large earthquakes and tsunamis for millions of years, including events in 1944 and 1946 that measured 8.1 and 8.3, respectively.

“We want to understand what happens at the transition from one tectonic plate to another,” said Demian Michael Saffer, associate professor of geosciences, Penn State. “We would like to be able to apply what we learn there to other parts of the world.”

In the Nankai Trough, the Philippine Plate is moving under the Eurasian Plate. Subduction occurs when two plates meet and one slides beneath the other. This causes volcanic activity inland from the plate edge. The Japanese islands sit on the edge of the Eurasian plate. Subduction causes Japanese volcanoes including Mt. Fuji.

“The Pacific Northwestern coast in the United States is similar to the southwestern portion of Japan,” said Saffer, who will join the expedition at the end of October.

He is an expert on water and rock mechanical properties and the specialty coordinator in these disciplines for all of the planned expeditions on NanTroSEIZE, which is the largest single project in the history of marine science. He will arrive at the end of the first drilling expedition and begin the second, serving as a bridge for the other expedition participants. Saffer is also the lead scientist on the first shallow seabed observatory that will be installed later in the project.

The ultimate aim of NanTroSEIZE is to drill 3.75 miles into the fault zone to establish permanent monitoring of the area. While deep drilling is the ultimate goal, the initial phase of the project, currently under way, is surveying drilling sites and shallowly probing locations in the study area. They will drill seven holes up to the leading edge of the plate taking measurements as they go, but not collecting any materials. These holes will be a half-mile deep. The second group will also drill shallow holes but will collect the rock for laboratory studies in the coming years.

“Another goal of the project is to try to understand what happens when an earthquake generates a tsunami,” said Saffer. “We think that direct observation of the earthquake zone will help us understand exactly where and what is happening.”

The second phase of drilling, set to begin next fiscal year, focuses on using riser drilling for the first time in scientific deep-sea drilling. The hole will be about two miles deep and will penetrate a major fault that the team believes ruptured in the 1944 earthquake. Riser drilling, unlike methods used in the past that used water in the drilling process, creates a pipe leading from the hull of the ship into the borehole, allowing the use of drilling muds and pressure control during drilling.

Next, the researchers will drill 3.75 miles into the subducting plate boundary and install temporary monitoring equipment. Finally, the scientists will install a long-term observatory system into two ultra-deep boreholes. The subsea bed observatory would record seismology, strain, tilt, water pressure and temperature within the hole. The researchers also plan to establish a number of shallow observatory bore holes to add to the data collected.

“The Japanese have funded a cabled network,” said Saffer. “We hope our observatory will be connected to this network and that by 2010-2011, scientists may be able to see real-time data on their computer anywhere in the world.”

The IODP is a marine research initiative jointly funded by Japan’s Ministry of Education, the U.S. National Science Foundation, a consortium of European Countries, the People’s Republic of China and South Korea.

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.

Ancient Fossil Evidence Supports Carbon Dioxide As Driver Of Global Warming

A team of American and Canadian scientists has devised a new way to study Earth’s past climate by analyzing the chemical composition of ancient marine fossils. The first published tests with the method further support the view that atmospheric CO2 has contributed to dramatic climate variations in the past, and strengthen projections that human CO2 emissions could cause global warming.

In the current issue of the journal Nature, geologists and environmental scientists from the California Institute of Technology, the University of Ottawa, the Memorial University of Newfoundland, Brock University, and the Waquoit Bay National Estuarine Research Reserve report the results of a new method for determining the growth temperatures of carbonate fossils such as shells and corals. This method looks at the percentage of rare isotopes of oxygen and carbon that bond with each other rather than being randomly distributed through their mineral lattices.

Because these bonds between oxygen-18 and carbon-13 form in greater abundance at low temperatures and lesser abundance at higher temperatures, a precise measurement of their concentration in a carbonate fossil can quantify the temperature of seawater in which the organisms lived. By comparing this record of temperature change with previous estimates of past atmospheric CO2 concentrations, the study demonstrates a strong coupling of atmospheric temperatures and carbon dioxide concentrations across one of Earth’s major environmental shifts.

According to Rosemarie Came, a postdoctoral scholar in geochemistry at Caltech and lead author of the article, only about 60 parts per million of the carbonate molecular groups that make up the mineral structures of carbonate fossils are a combination of both oxygen-18 and carbon-13, but the amount varies predictably with temperature. Therefore, knowing the age of the sample and how much of these exotic carbonate groups are present allows one to create a record of the planet’s temperature through time.

“This clumped-isotope method has an advantage over previous approaches because we’re looking at the distribution of rare isotopes inside a single shell or coral,” Came says. “All the information needed to study the surface temperature at the time the animal lived is stored in the fossil itself.”

In this way, the method contrasts with previous approaches that require knowledge of the chemistry of seawater in the distant past–something that is poorly known.

The study contrasts the growth temperatures of fossils from two times in the distant geological past. The Silurian period, approximately 400 million years ago, is thought to have been a time of highly elevated atmospheric CO2 (more than 10 times the modern concentration), and was found by the researchers to be a time of exceptionally warm shallow-ocean temperatures–nearly 35 degrees C. In contrast, the Carboniferous period, roughly 300 million years ago, appears to have been characterized by far lower levels of atmospheric carbon dioxide (similar to modern values) and had shallow marine temperatures similar to or slightly cooler than today-about 25 degrees C. Thus, the draw-down of atmospheric CO2 coincided with strong global cooling.

“This is a huge change in temperature,” says John Eiler, a professor of geochemistry at Caltech and a coauthor of the study. “It shows that carbon dioxide really has been a powerful driver of climate change in Earth’s past.”

The title of the Nature paper is “Coupling of surface temperatures and atmospheric CO2 concentrations during the Paleozoic era.” The other authors are Jan Veizer of the University of Ottawa, Karem Azmy of Memorial University of Newfoundland, Uwe Brand of Brock University, and Christopher R. Weidman of the Waquoit National Estuarine Research Reserve, Massachusetts.