Oldest bit of crust firms up idea of a cool early Earth

A 4.4 billion-year-old zircon crystal is providing new insight into how the early Earth cooled from a ball of magma and formed continents just 160 million years after the formation of our solar system, much earlier than previously believed. The zircon, pictured here, is from the Jack Hills region of Australia and is now confirmed to be the oldest bit of the Earth's crust. -  John Valley
A 4.4 billion-year-old zircon crystal is providing new insight into how the early Earth cooled from a ball of magma and formed continents just 160 million years after the formation of our solar system, much earlier than previously believed. The zircon, pictured here, is from the Jack Hills region of Australia and is now confirmed to be the oldest bit of the Earth’s crust. – John Valley

With the help of a tiny fragment of zircon extracted from a remote rock outcrop in Australia, the picture of how our planet became habitable to life about 4.4 billion years ago is coming into sharper focus.

Writing today (Feb. 23, 2014) in the journal Nature Geoscience, an international team of researchers led by University of Wisconsin-Madison geoscience Professor John Valley reveals data that confirm the Earth’s crust first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system. The work shows, Valley says, that the time when our planet was a fiery ball covered in a magma ocean came earlier.

“This confirms our view of how the Earth cooled and became habitable,” says Valley, a geochemist whose studies of zircons, the oldest known terrestrial materials, have helped portray how the Earth’s crust formed during the first geologic eon of the planet. “This may also help us understand how other habitable planets would form.”

The new study confirms that zircon crystals from Western Australia’s Jack Hills region crystallized 4.4 billion years ago, building on earlier studies that used lead isotopes to date the Australian zircons and identify them as the oldest bits of the Earth’s crust. The microscopic zircon crystal used by Valley and his group in the current study is now confirmed to be the oldest known material of any kind formed on Earth.

The study, according to Valley, strengthens the theory of a “cool early Earth,” where temperatures were low enough for liquid water, oceans and a hydrosphere not long after the planet’s crust congealed from a sea of molten rock. “The study reinforces our conclusion that Earth had a hydrosphere before 4.3 billion years ago,” and possibly life not long after, says Valley.

The study was conducted using a new technique called atom-probe tomography that, in conjunction with secondary ion mass spectrometry, permitted the scientists to accurately establish the age and thermal history of the zircon by determining the mass of individual atoms of lead in the sample. Instead of being randomly distributed in the sample, as predicted, lead atoms in the zircon were clumped together, like “raisins in a pudding,” notes Valley.

The clusters of lead atoms formed 1 billion years after crystallization of the zircon, by which time the radioactive decay of uranium had formed the lead atoms that then diffused into clusters during reheating. “The zircon formed 4.4 billion years ago, and at 3.4 billion years, all the lead that existed at that time was concentrated in these hotspots,” Valley says. “This allows us to read a new page of the thermal history recorded by these tiny zircon time capsules.”

The formation, isotope ratio and size of the clumps – less than 50 atoms in diameter – become, in effect, a clock, says Valley, and verify that existing geochronology methods provide reliable and accurate estimates of the sample’s age. In addition, Valley and his group measured oxygen isotope ratios, which give evidence of early homogenization and later cooling of the Earth.

“The Earth was assembled from a lot of heterogeneous material from the solar system,” Valley explains, noting that the early Earth experienced intense bombardment by meteors, including a collision with a Mars-sized object about 4.5 billion years ago “that formed our moon, and melted and homogenized the Earth. Our samples formed after the magma oceans cooled and prove that these events were very early.”

Giant mass extinction may have been quicker than previously thought

The largest mass extinction in the history of animal life occurred some 252 million years ago, wiping out more than 96 percent of marine species and 70 percent of life on land – including the largest insects known to have inhabited the Earth. Multiple theories have aimed to explain the cause of what’s now known as the end-Permian extinction, including an asteroid impact, massive volcanic eruptions, or a cataclysmic cascade of environmental events. But pinpointing the cause of the extinction requires better measurements of how long the extinction period lasted.

Now researchers at MIT have determined that the end-Permian extinction occurred over 60,000 years, give or take 48,000 years – practically instantaneous, from a geologic perspective. The new timescale is based on more precise dating techniques, and indicates that the most severe extinction in history may have happened more than 10 times faster than scientists had previously thought.

“We’ve got the extinction nailed in absolute time and duration,” says Sam Bowring, the Robert R. Shrock Professor of Earth and Planetary Sciences at MIT. “How do you kill 96 percent of everything that lived in the oceans in tens of thousands of years? It could be that an exceptional extinction requires an exceptional explanation.”

In addition to establishing the extinction’s duration, Bowring, graduate student Seth Burgess, and a colleague from the Nanjing Institute of Geology and Paleontology also found that, 10,000 years before the die-off, the oceans experienced a pulse of light carbon, which likely reflects a massive addition of carbon dioxide to the atmosphere. This dramatic change may have led to widespread ocean acidification and increased sea temperatures by 10 degrees Celsius or more, killing the majority of sea life.

But what originally triggered the spike in carbon dioxide? The leading theory among geologists and paleontologists has to do with widespread, long-lasting volcanic eruptions from the Siberian Traps, a region of Russia whose steplike hills are a result of repeated eruptions of magma. To determine whether eruptions from the Siberian Traps triggered a massive increase in oceanic carbon dioxide, Burgess and Bowring are using similar dating techniques to establish a timescale for the Permian period’s volcanic eruptions that are estimated to have covered over five million cubic kilometers.

“It is clear that whatever triggered extinction must have acted very quickly,” says Burgess, the lead author of a paper that reports the results in this week’s Proceedings of the National Academy of Sciences, “fast enough to destabilize the biosphere before the majority of plant and animal life had time to adapt in an effort to survive.”

Pinning dates on an extinction


In 2006, Bowring and his students made a trip to Meishan, China, a region whose rock formations bear evidence of the end-Permian extinction; geochronologists and paleontologists have flocked to the area to look for clues in its layers of sedimentary rock. In particular, scientists have focused on a section of rock that is thought to delineate the end of the Permian, and the beginning of the Triassic, based on evidence such as the number of fossils found in surrounding rock layers.

Bowring sampled rocks from this area, as well as from nearby alternating layers of volcanic ash beds and fossil-bearing rocks. After analyzing the rocks in the lab, his team reported in 2011 that the end-Permian likely lasted less than 200,000 years. However, this timeframe still wasn’t precise enough to draw any conclusions about what caused the extinction.

Now, the team has revised its estimates using more accurate dating techniques based on a better understanding of uncertainties in timescale measurements.

With this knowledge, Bowring and his colleagues reanalyzed rock samples collected from five volcanic ash beds at the Permian-Triassic boundary. The researchers pulverized rocks and separated out tiny zircon crystals containing a mix of uranium and lead. They then isolated uranium from lead, and measured the ratios of both isotopes to determine the age of each rock sample.

From their measurements, the researchers determined a much more precise “age model” for the end-Permian extinction, which now appears to have lasted about 60,000 years – with an uncertainty of 48,000 years – and was immediately preceded by a sharp increase in carbon dioxide in the oceans.

‘Spiraling toward the truth’


The new timeline adds weight to the theory that the extinction was triggered by massive volcanic eruptions from the Siberian Traps that released volatile chemicals, including carbon dioxide, into the atmosphere and oceans. With such a short extinction timeline, Bowring says it is possible that a single, catastrophic pulse of magmatic activity triggered an almost instantaneous collapse of all global ecosystems.

To confirm whether the Siberian Traps are indeed the extinction’s smoking gun, Burgess and Bowring plan to determine an equally precise timeline for the Siberian Traps eruptions, and will compare it to the new extinction timeline to see where the two events overlap. The researchers will investigate additional areas in China to see if the duration of the extinction can be even more precisely determined.

“We’ve refined our approach, and now we have higher accuracy and precision,” Bowring says. “You can think of it as slowly spiraling in toward the truth.”

Newly discovered flux in the Earth may solve missing-mantle mystery

It’s widely thought that the Earth arose from violent origins: Some 4.5 billion years ago, a maelstrom of gas and dust circled in a massive disc around the sun, gathering in rocky clumps to form asteroids. These asteroids, gaining momentum, whirled around a fledgling solar system, repeatedly smashing into each other to create larger bodies of rubble – the largest of which eventually cooled to form the planets.

Countless theories, simulations and geologic observations support such a scenario. But there remains one lingering mystery: If the Earth arose from the collision of asteroids, its composition should resemble that of meteoroids, the small particles that break off from asteroids.

But to date, scientists have found that, quite literally, something doesn’t add up: Namely, the Earth’s mantle – the layer between the planet’s crust and core – is missing an amount of lead found in meteorites whose composition has been analyzed following impact with the Earth.

Much of the Earth is composed of rocks with a high ratio of uranium to lead (uranium naturally decays to lead over time). However, according to standard theories of planetary evolution, the Earth should harbor a reservoir of mantle somewhere in its interior that has a low ratio of uranium to lead, to match the composition of meteorites. But such a reservoir has yet to be discovered – a detail that leaves Earth’s origins hazy.

Now researchers in MIT’s Department of Earth, Atmospheric and Planetary Sciences have identified a “hidden flux” of material in the Earth’s mantle that would make the planet’s overall composition much more similar to that of meteorites. This reservoir likely takes the form of extremely dense, lead-laden rocks that crystallize beneath island arcs, strings of volcanoes that rise up at the boundary of tectonic plates.

As two massive plates push against each other, one plate subducts, or slides, under the other, pushing material from the crust down into the mantle. At the same time, molten material from the mantle rises up to the crust, and is ejected via volcanoes onto the Earth’s surface.

According to the MIT researchers’ observations and calculations, however, up to 70 percent of this rising magma crystallizes into dense rock – dropping, leadlike, back into the mantle, where it remains relatively undisturbed. The lead-heavy flux, they say, puts the composition of the Earth’s mantle on a par with that of meteorites.

“Now that we know the composition of this flux, we can calculate that there’s tons of this stuff dropping down from the base of the crust into the mantle, so it is likely an important reservoir,” says Oliver Jagoutz, an assistant professor of geology at MIT. “This has a lot of implications for understanding how the Earth evolved through history.”

Jagoutz and his colleague Max Schmidt, of the Swiss Federal Institute of Technology in Zurich, have detailed their results in a paper published in Earth and Planetary Science Letters.

A mantle exposed


Measuring the composition of material that has dropped into the mantle is a nearly impossible task. Jagoutz estimates that such dense rocks would form at a depth of 40 to 50 kilometers below the surface, beyond the reach of conventional sampling techniques.

There is, however, one place on earth where such a depth of the crust and mantle is exposed: a region of northern Pakistan called the Kohistan arc. Forty million years ago, this island arc was crushed between India and Asia as the two plates collided.

“When India came in, it slammed into the arc, and the arc extended and rotated itself,” Jagoutz says. “Because of that, we now have a cross-section of the mantle-to-crust transition. This is the only place on Earth where this exists.”

On various trips from 2000 to 2007, Jagoutz trekked through the Kohistan arc region, collecting rocks from various parts of the arc’s crust and mantle. Bringing them back to the lab, he analyzed the rocks’ density and composition, discovering that some were “density-unstable” – much denser than the mantle. These denser rocks could potentially sink into the mantle, creating a hidden reservoir.

Adding up to an asteroid origin


The researchers measured the rocks’ composition, and found that the denser rocks contained much more lead than uranium – exactly the ratio predicted for the missing reservoir of material. Jagoutz then performed a mass balance (a simple conservation-of-mass calculation) to determine how much dense rock drops into the mantle, based on the composition of the region’s crust, rocks and mantle: Essentially, the mass of the Kohistan arc, minus whatever material drops into the mantle, should equal the material that comes out of the mantle.

Jagoutz and Schmidt solved the equation for 10 common elements. From their calculations, they found that 70 percent of the magma that rises from the mantle must ultimately drop back down, relatively heavy with lead. Applying this statistic to other island arcs in the world – such as the Andean volcanic belt and the Cascade Range – they found that the amount of material dropped into the mantle globally equals the composition and quantity of the so-called missing reservoir – a finding that suggests that Earth did indeed form from the collision of meteorites.

“If we are right, one of the questions we have is: Why is the Earth capable of hiding something from us? Why is there never a volcano that brings up these rocks?” Jagoutz adds. “You’d think it’d come back up, but it doesn’t. It’s actually interesting.

Borneo stalagmites provide new view of abrupt climate events over 100,000 years

Georgia Tech researchers Stacy Carolin (Ph.D. candidate), Jessica Moerman (Ph.D. candidate), Eleanor Middlemas (undergraduate), Danja Mewes (undergraduate) and two caving guides (Syria Lejau, Jenny Malang) climb out from Cobweb Cave in Gunung Mulu National Park after a day of rock and water sample collection during the Fall 2012 field trip. -  Credit: Kim Cobb
Georgia Tech researchers Stacy Carolin (Ph.D. candidate), Jessica Moerman (Ph.D. candidate), Eleanor Middlemas (undergraduate), Danja Mewes (undergraduate) and two caving guides (Syria Lejau, Jenny Malang) climb out from Cobweb Cave in Gunung Mulu National Park after a day of rock and water sample collection during the Fall 2012 field trip. – Credit: Kim Cobb

A new set of long-term climate records based on cave stalagmites collected from tropical Borneo shows that the western tropical Pacific responded very differently than other regions of the globe to abrupt climate change events. The 100,000-year climate record adds to data on past climate events, and may help scientists assess models designed to predict how the Earth’s climate will respond in the future.

The new record resulted from oxygen isotope analysis of more than 1,700 calcium carbonate samples taken from four stalagmites found in three different northern Borneo caves. The results suggest that climate feedbacks within the tropical regions may amplify and prolong abrupt climate change events that were first discovered in the North Atlantic.

The results were scheduled to be published June 6 in Science Express, the electronic advance online publication of the journal Science, and will appear later in an issue of printed publication. The research was supported by the National Science Foundation.

Today, relatively subtle changes in the tropical Pacific’s ocean and atmosphere have profound effects on global climate. However, there are few records of past climate changes in this key region that have the length, resolution and age controls needed to reveal the area’s response to abrupt climate change events.

“This is a new record from a very important area of the world,” said Kim Cobb, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. “This record will provide a new piece of the puzzle from the tropical Pacific showing us how that climate system has responded to forcing events over the past 100,000 years.”

Among the findings were some surprises that show just how complicated the Earth’s climate system can be. While the stalagmite record reflected responses to abrupt changes known as Heinrich events, another major type of event – known as Dansgaard-Oeschger excursions – left no evidence in the Borneo stalagmites. Both types of abrupt climate change events are prominently featured in a previously-published stalagmite climate record from China – which is only slightly north of Borneo.

“To my knowledge, this is the first record that so clearly shows sensitivity to one set of major abrupt climate change events and not another,” said Cobb. “These two types of abrupt change events appear to have different degrees of tropical Pacific involvement, and because the tropical Pacific speaks with such a loud voice when it does speak, we think this is extremely important for understanding the mechanisms underlying these events.”

The researchers were also surprised to discover a very large and abrupt signal in their stalagmite climate records precisely when super-volcano Toba erupted nearby, roughly 74,000 years ago.

The team recovered the stalagmites from caves in Gunung Mulu and Gunung Buda National Parks, in northern Borneo, which is located a few degrees north of the Equator in the western Pacific. Back at their Georgia Tech lab, they analyzed the stalagmites for the ratio of oxygen isotopes contained in samples of calcium carbonate, the material from which the stalagmites were formed. That ratio is set by the oxygen isotopes in rainfall at the site, as the water that seeped into the ground dissolved limestone rock and dripped into the caves to form the stalagmites. The stalagmites accumulate at a rate of roughly one centimeter every thousand years.

“Stalagmites are time capsules of climate signals from thousands of years in the past,” said Stacy Carolin, a Georgia Tech Ph.D. candidate who gathered and analyzed the stalagmites. “We have instrumental records of climate only for the past 100 years or so, and if we want to look deeper into the past, we have to find records like these that locked in climate signals we can extract today.”

In the laboratory, Carolin sawed each stalagmite in half, opening it like a hot dog bun. She then used a tiny drill bit to take samples of the calcium carbonate down the center at one-millimeter steps. Because the stalagmites grew at varying rates, each sample represented as little as 60 years of time, or as much as 200 years. The precise ages of the samples were determined by measuring uranium and thorium isotope ratios, an analysis done with the help of Jess F. Adkins, a professor at the California Institute of Technology and a co-author of the study.

Rainfall oxygen isotopic ratios are good indicators of the amount of rainfall occurring throughout the region, as determined by a modern-day calibration study recently published by another graduate student in Cobb’s lab.

Merging data from the four different stalagmites provided a record of precipitation trends in the western Pacific over the past 100,000 years. That information can be compared to stalagmite and ice core climate records obtained elsewhere in the world.

“This record, which spans the entire last glacial period, adds significantly to the understanding of how various climate forcings are felt by the western tropical Pacific,” Carolin added.

Climate scientists are interested in learning more about abrupt climate changes because they indicate that the climate system may have “tipping points.” So far, the climate system has responded to rising carbon dioxide levels at a fairly steady rate, but many scientists worry about possible nonlinear effects.

“As a society, we haven’t really thought enough about the fact that we are moving Earth’s climate system toward a new state very quickly,” said Cobb. “It’s important to remember that the climate system has important nonlinearities that are most evident in these abrupt climate events. Ultimately, we’d like to be able to reproduce the global signatures of these abrupt climate events with numerical models of the climate system, and investigate the physics that drive such events.”

For Carolin, studying the half-meter-long stalagmites brought an awareness that the Earth has not always been as we know it today.

“You have to be impressed with the scope of what you are studying, and recognize that the state our climate is in today is incredibly different from Earth’s climate during the last Ice Age,” she said. “As we consider how humans may be affecting climate, dissecting what was going on tens of thousands of years ago in all regions of the globe can help scientists better predict how the Earth will respond to modern climate forcings.”

Arctic current flowed under deep freeze of last ice age, study says

Arctic sea ice formation feeds global ocean circulation. New evidence suggests that this dynamic process persisted through the last ice age. -  National Snow & Ice Data Center
Arctic sea ice formation feeds global ocean circulation. New evidence suggests that this dynamic process persisted through the last ice age. – National Snow & Ice Data Center

During the last ice age, when thick ice covered the Arctic, many scientists assumed that the deep currents below that feed the North Atlantic Ocean and help drive global ocean currents slowed or even stopped. But in a new study in Nature, researchers show that the deep Arctic Ocean has been churning briskly for the last 35,000 years, through the chill of the last ice age and warmth of modern times, suggesting that at least one arm of the system of global ocean currents that move heat around the planet has behaved similarly under vastly different climates.

“The Arctic Ocean must have been flushed at approximately the same rate it is today regardless of how different things were at the surface,” said study co-author Jerry McManus, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory.

Researchers reconstructed Arctic circulation through deep time by measuring radioactive trace elements buried in sediments on the Arctic seafloor. Uranium eroded from the continents and delivered to the ocean by rivers, decays into sister elements thorium and protactinium. Thorium and protactinium eventually attach to particles falling through the water and wind up in mud at the bottom. By comparing expected ratios of thorium and protactinium in those ocean sediments to observed amounts, the authors showed that protactinium was being swept out of the Arctic before it could settle to the ocean bottom.From the amount of missing protactinium, scientists can infer how quickly the overlying water must have been flushed at the time the sediments were accumulating.

“The water couldn’t have been stagnant, because we see the export of protactinium,” said the study’s lead author, Sharon Hoffmann, a geochemist at Lamont-Doherty.

The upper part of the modern Arctic Ocean is flushed by North Atlantic currents while the Arctic’s deep basins are flushed by salty currents formed during sea ice formation at the surface. “The study shows that both mechanisms must have been active from the height of glaciation until now,” said Robert Newton, an oceanographer at Lamont-Doherty who was not involved in the research. “There must have been significant melt-back of sea ice each summer even at the height of the last ice age to have sea ice formation on the shelves each year. This will be a surprise to many Arctic researchers who believe deep water formation shuts down during glaciations.”

The researchers analyzed sediment cores collected during the U.S.-Canada Arctic Ocean Section cruise in 1994, a major Arctic research expedition that involved several Lamont-Doherty scientists. In each location, the cores showed that protactinium has been lower than expected for at least the past 35,000 years. By sampling cores from a range of depths, including the bottom of the Arctic deep basins, the researchers show that even the deepest waters were being flushed out at about the same rate as in the modern Arctic.

The only deep exit from the Arctic is through Fram Strait, which divides Greenland and Norway’s Svalbard islands. The deep waters of the modern Arctic flow into the North Atlantic via the Nordic seas, contributing up to 40 percent of the water that becomes North Atlantic Deep Water-known as the “ocean’s lungs” for delivering oxygen and salt to the rest of world’s oceans.

One direction for future research is to find out where the missing Arctic protactinium of the past ended up. “It’s somewhere,” said McManus. “All the protactinium in the ocean is buried in ocean sediments. If it’s not buried in one place, it’s buried in another. Our evidence suggests it’s leaving the Arctic but we think it’s unlikely to get very far before being removed.”

Scientists refine Earth’s clock

New research has revealed that some events in Earth’s history happened more recently than previously thought. Scientists from the British Geological Survey and the Massachusetts Institute of Technology, publishing this week in the journal Science, have refined the data used to determine how much time has passed since a mineral or rock was formed. They report uranium isotopic composition of minerals, used to date major geological events, which are more accurate than previously published. The major effect of this is to reduce previous age determinations by up to 700,000 years.

Minerals naturally capture uranium when they form, which in turn undergoes a chain of radioactive decays to other elements, ending with lead. This new research has shown that, by more accurately measuring the relative amount of the uranium isotopes 238U and 235U, we now have a better understanding of how much time has passed since a mineral or rock has formed.

A major effect of this work will be to decrease all previous uranium-lead (U-Pb) age determinations, by up to 700,000 years for samples that are about 4.5 billion years old – the age of the Earth. In particular, the new 238U/235U ratio will allow geologists to place more accurate limits on the exact timing of a broad range of geological processes, from the initial formation of our planet, continents and economic mineral deposits, to past evolutionary events and climate change.

Blair Schoene, a geologist from Princeton University said “This new determination will not only improve the accuracy of each U-Pb age but ultimately our understanding of events in Earth history.”

For over 35 years, a 238U/235U ratio of 137.88 has been used to calculate U-Pb dates, from the oldest rocks that formed four billion years ago, to much younger rocks that are hundreds of thousands of years old. When scientists recently evaluated the measurements used to arrive at the 137.88 value, they came to a dead end: the value could not be traced back to standard units such as the kilogram. This new study shows that many naturally occurring uranium-rich minerals, such as zircon, actually have a lower 238U/235U value with an average of 137.818 ± 0.045 (the uncertainty assigned to this value relates to the variation observed between different samples). Agreement between these results, other rocks, and meteorites indicate the new average 238U/235U value and uncertainty may also be representative of the Earth’s ‘bulk’ uranium isotopic composition.</P