New study shows 3 abrupt pulse of CO2 during last deglaciation

A new study shows that the rise of atmospheric carbon dioxide that contributed to the end of the last ice age more than 10,000 years ago did not occur gradually, but was characterized by three “pulses” in which C02 rose abruptly.

Scientists are not sure what caused these abrupt increases, during which C02 levels rose about 10-15 parts per million – or about 5 percent per episode – over a period of 1-2 centuries. It likely was a combination of factors, they say, including ocean circulation, changing wind patterns, and terrestrial processes.

The finding is important, however, because it casts new light on the mechanisms that take the Earth in and out of ice age regimes. Results of the study, which was funded by the National Science Foundation, appear this week in the journal Nature.

“We used to think that naturally occurring changes in carbon dioxide took place relatively slowly over the 10,000 years it took to move out of the last ice age,” said Shaun Marcott, lead author on the article who conducted his study as a post-doctoral researcher at Oregon State University. “This abrupt, centennial-scale variability of CO2 appears to be a fundamental part of the global carbon cycle.”

Some previous research has hinted at the possibility that spikes in atmospheric carbon dioxide may have accelerated the last deglaciation, but that hypothesis had not been resolved, the researchers say. The key to the new finding is the analysis of an ice core from the West Antarctic that provided the scientists with an unprecedented glimpse into the past.

Scientists studying past climate have been hampered by the limitations of previous ice cores. Cores from Greenland, for example, provide unique records of rapid climate events going back 120,000 years – but high concentrations of impurities don’t allow researchers to accurately determine atmospheric carbon dioxide records. Antarctic ice cores have fewer impurities, but generally have had lower “temporal resolution,” providing less detailed information about atmospheric CO2.

However, a new core from West Antarctica, drilled to a depth of 3,405 meters in 2011 and spanning the last 68,000 years, has “extraordinary detail,” said Oregon State paleoclimatologist Edward Brook, a co-author on the Nature study and an internationally recognized ice core expert. Because the area where the core was taken gets high annual snowfall, he said, the new ice core provides one of the most detailed records of atmospheric CO2.

“It is a remarkable ice core and it clearly shows distinct pulses of carbon dioxide increase that can be very reliably dated,” Brook said. “These are some of the fastest natural changes in CO2 we have observed, and were probably big enough on their own to impact the Earth’s climate.

“The abrupt events did not end the ice age by themselves,” Brook added. “That might be jumping the gun a bit. But it is fair to say that the natural carbon cycle can change a lot faster than was previously thought – and we don’t know all of the mechanisms that caused that rapid change.”

The researchers say that the increase in atmospheric CO2 from the peak of the last ice age to complete deglaciation was about 80 parts per million, taking place over 10,000 years. Thus, the finding that 30-45 ppm of the increase happened in just a few centuries was significant.

The overall rise of atmospheric carbon dioxide during the last deglaciation was thought to have been triggered by the release of CO2 from the deep ocean – especially the Southern Ocean. However, the researchers say that no obvious ocean mechanism is known that would trigger rises of 10-15 ppm over a time span as short as one to two centuries.

“The oceans are simply not thought to respond that fast,” Brook said. “Either the cause of these pulses is at least part terrestrial, or there is some mechanism in the ocean system we don’t yet know about.”

One reason the researchers are reluctant to pin the end of the last ice age solely on CO2 increases is that other processes were taking place, according to Marcott, who recently joined the faculty of the University of Wisconsin-Madison.

“At the same time CO2 was increasing, the rate of methane in the atmosphere was also increasing at the same or a slightly higher rate,” Marcott said. “We also know that during at least two of these pulses, the Atlantic Meridional Overturning Circulation changed as well. Changes in the ocean circulation would have affected CO2 – and indirectly methane, by impacting global rainfall patterns.”

“The Earth is a big coupled system,” he added, “and there are many pieces to the puzzle. The discovery of these strong, rapid pulses of CO2 is an important piece.”

Solving the puzzle of ice age climates

The paleoclimate record for the last ice age – a time 21,000 years ago called the “Last Glacial Maximum” (LGM) – tells of a cold Earth whose northern continents were covered by vast ice sheets. Chemical traces from plankton fossils in deep-sea sediments reveal rearranged ocean water masses, as well as extended sea ice coverage off Antarctica. Air bubbles in ice cores show that carbon dioxide in the atmosphere was far below levels seen before the Industrial Revolution.

While ice ages are set into motion by Earth’s slow wobbles in its transit around the sun, researchers agree that the solar-energy decrease alone wasn’t enough to cause this glacial state. Paleoclimatologists have been trying to explain the actual mechanism behind these changes for 200 years.

“We have all these scattered pieces of information about changes in the ocean, atmosphere, and ice cover,” says Raffaele Ferrari, the Breene M. Kerr Professor of Physical Oceanography in MIT’s Department of Earth, Atmospheric and Planetary Sciences, “and what we really want to see is how they all fit together.”

Researchers have always suspected that the answer must lie somewhere in the oceans. Powerful regulators of Earth’s climate, the oceans store vast amounts of organic carbon for thousands of years, keeping it from escaping into the atmosphere as CO2. Seawater also takes up CO2 from the atmosphere via photosynthesizing microbes at the surface, and via circulation patterns.

In a new application of ocean physics, Ferrari, along with Malte Jansen PhD ’12 of Princeton University and others at the California Institute of Technology, have found a new approach to the puzzle, which they detail in this week’s Proceedings of the National Academy of Sciences.

Lung of the ocean

The researchers focused on the Southern Ocean, which encircles Antarctica – a critical part of the carbon cycle because it provides a connection between the atmosphere and the deep ocean abyss. Ruffled by the winds whipping around Antarctica, the Southern Ocean is one of the only places where the deepest carbon-rich waters ever rise to the surface, to “breathe” CO2 in and out.

The modern-day Southern Ocean has a lot of room to breathe: Deeper, carbon-rich waters are constantly mixing into the waters above, a process enhanced by turbulence as water runs over jagged, deep-ocean ridges.

But during the LGM, permanent sea ice covered much more of the Southern Ocean’s surface. Ferrari and colleagues decided to explore how that extended sea ice would have affected the Southern Ocean’s ability to exchange CO2 with the atmosphere.

Shock to the system

This question demanded the use of the field’s accumulated knowledge of ocean physics. Using a mathematical equation that describes the wind-driven ocean circulation patterns around Antarctica, the researchers calculated the amount of water that was trapped under the sea ice by currents in the LGM. They found that the shock to the entire Earth from this added ice cover was massive: The ice covered the only spot where the deep ocean ever got to breathe. Since the sea ice capped these deep waters, the Southern Ocean’s CO2 was never exhaled to the atmosphere.

The researchers then saw a link between the sea ice change and the massive rearrangement of ocean waters that is evident in the paleoclimate record. Under the expanded sea ice, a greater amount of upwelled deep water sank back downward. Southern Ocean abyssal water eventually filled a greater volume of the entire midlevel and lower ocean – lifting the interface between upper and lower waters to a shallower depth, such that the deep, carbon-rich waters lost contact with the upper ocean. Breathing less, the ocean could store a lot more carbon.

A Southern Ocean suffocated by sea ice, the researchers say, helps explain the big drop in atmospheric CO2 during the LGM.

Dependent relationship

The study suggests a dynamic link between sea-ice expansion and the increase of ocean water insulated from the atmosphere, which the field has long treated as independent events. This insight takes on extra relevance in light of the fact that paleoclimatologists need to explain not just the very low levels of atmospheric CO2 during the last ice age, but also the fact that this happened during each of the last four glacial periods, as the paleoclimate record reveals.

Ferrari says that it never made sense to argue that independent changes drew down CO2 by the exact same amount in every ice age. “To me, that means that all the events that co-occurred must be incredibly tightly linked, without much freedom to drift beyond a narrow margin,” he says. “If there is a causality effect among the events at the start of an ice age, then they could happen in the same ratio.”

Soil production breaks geologic speed record

This is a photo of the researcher hiking down the ridge at Rapid Creek to collect soil samples.  The dense bush and heavy 10 kilogram soil samples slowed uphill progress to less than 200 meters per hour. -  Andre Eger
This is a photo of the researcher hiking down the ridge at Rapid Creek to collect soil samples. The dense bush and heavy 10 kilogram soil samples slowed uphill progress to less than 200 meters per hour. – Andre Eger

Geologic time is shorthand for slow-paced. But new measurements from steep mountaintops in New Zealand show that rock can transform into soil more than twice as fast as previously believed possible.

The findings were published Jan. 16 in the early online edition of Science.

“Some previous work had argued that there were limits to soil production,” said first author Isaac Larsen, who did the work as part of his doctoral research in Earth sciences at the University of Washington. “But no one had made the measurements.”

The finding is more than just a new speed record. Rapidly eroding mountain ranges account for at least half of the total amount of the planet’s weathering and sediment production, although they occupy just a few percent of the Earth’s surface, researchers said.

So the record-breaking production at the mountaintops has implications for the entire carbon cycle by which the Earth’s crust pushes up to form mountains, crumbles, washes with rivers and rainwater to the sea, and eventually settles to the bottom to form new rock.

“This work takes the trend between soil production rates and chemical weathering rates and extends it to much higher values than had ever been previously observed,” said Larsen, now a postdoctoral researcher at the California Institute of Technology in Pasadena.

The study site in New Zealand’s Southern Alps is “an extremely rugged mountain range,” Larsen said, with rainfall of 10 meters (33 feet) per year and slopes of about 35 degrees.

To collect samples Larsen and co-author André Eger, then a graduate student at Lincoln University in New Zealand, were dropped from a helicopter onto remote mountaintops above the tree line. They would hike down to an appropriate test site and collect 20 pounds of dirt apiece, and then trek the samples back up to their base camp. The pair stayed at each of the mountaintop sites for about three days.

“I’ve worked in a lot of places,” Larsen said. “This was the most challenging fieldwork I’ve done.”

Researchers then brought soil samples back to the UW and measured the amount of Beryllium-10, an isotope that forms only at the Earth’s surface by exposure to cosmic rays. Those measurements showed soil production rates on the ridge tops ranging from 0.1 to 2.5 millimeters (1/10 of an inch) per year, and decrease exponentially with increasing soil thickness.

The peak rate is more than twice the proposed speed limit for soil production, in which geologists wondered if in places where soil is lost very quickly, the soil production just can’t keep up. In earlier work Larsen had noticed vegetation on very steep slopes and so he proposed this project to measure soil production rates at some of the steepest, wettest locations on the planet.

The new results show that soil production and weathering rates continue to increase as the landscape gets steeper and erodes faster, and suggest that other very steep locations such as the Himalayas and the mountains in Taiwan may also have very fast soil formation.

“A couple millimeters a year sounds pretty slow to anybody but a geologist,” said co-author David Montgomery, a UW professor of Earth and space sciences. “Isaac measured two millimeters of soil production a year, so it would take just a dozen years to make an inch of soil. That’s shockingly fast for a geologist, because the conventional wisdom is it takes centuries.”

The researchers believe plant roots may be responsible here. The mountain landscape was covered with low, dense vegetation. The roots of those plants reach into cracks in the rocks, helping break them apart and expose them to rainwater and chemical weathering.

“This opens up new questions about how soil production might happen in other locations, climates and environments,” Larsen said.

The biggest mass extinction and Pangea integration

Relationships between geosphere disturbances and mass extinction during the Late Permian and Early Triassic are shown. -  ©Science China Press
Relationships between geosphere disturbances and mass extinction during the Late Permian and Early Triassic are shown. – ©Science China Press

The mysterious relationship between Pangea integration and the biggest mass extinction happened 250 million years ago was tackled by Professor YIN Hongfu and Dr. SONG Haijun from State Key Laboratory of Geobiology and Environmental Geology, China University of Geosciences (Wuhan). Their study shows that Pangea integration resulted in environmental deterioration which further caused that extinction. Their work, entitled “Mass extinction and Pangea integration during the Paleozoic-Mesozoic transition”, was published in SCIENCE CHINA Earth Sciences.2013, Vol 56(7).

The Pangea was integrated at about the beginning of Permian, and reached its acme during Late Permian to Early Triassic. Formation of the Pangea means that the scattered continents of the world gathered into one integrated continent with an area of nearly 200 million km2. Average thickness of such a giant continental lithosphere should be remarkably greater than that of each scattered continent. Equilibrium principle implies that the thicker the lithosphere, the higher its portion over the equilibrium level, hence the average altitude of the Pangea should be much higher than the separated modern continents. Correspondingly, all oceans gathered to form the Panthalassa, which should be much deeper than modern oceans. The acme of Pangea and Panthalassa was thus a period of high continent and deep ocean, which should inevitably induce great regression and influence the earth’s surface system, especially climate.

The Tunguss Trap of Siberia, the Emeishan Basalt erupted during the Pangea integration. Such global-scale volcanism should be evoked by mantle plume and related with integration of the Pangea. Volcanic activities would result in a series of extinction effects, including emission of large volume of CO2, CH4, NO2 and cyanides which would have caused green house effects, pollution by poisonous gases, damage of the ozone layer in the stratosphere, and enhancement the ultra-violet radiation.

Increase of CO2 concentration and other green house gases would have led to global warming, oxygen depletion and carbon cycle anomaly; physical and chemical anomalies in ocean (acidification, euxinia, low sulfate concentration, isotopic anomaly of organic nitrogen) and great regression would have caused marine extinction due to unadaptable environments, selective death and hypercapnia; continental aridity, disappearance of monsoon system and wild fire would have devastated the land vegetation, esp. the tropical rain forest.

The great global changes and mass extinction were the results of interaction among earth’s spheres. Deteriorated relations among lithosphere, atmosphere, hydrosphere, and biosphere (including internal factors of organism evolution itself) accumulated until they exceeded the threshold, and exploded at the Permian-Triassic transition time. Interaction among bio- and geospheres is an important theme. However, the processes from inner geospheres to earth’s surface system and further to organism evolution necessitate retardation in time and yields many uncertainties in causation. Most of the processes are now at a hypothetic stage and need more scientific examinations.

Ancient trapped water explains Earth’s first ice age

The North Pole area, Pilbara, Western Australia, where the samples came from. -  University of Manchester
The North Pole area, Pilbara, Western Australia, where the samples came from. – University of Manchester

Tiny bubbles of water found in quartz grains in Australia may hold the key to understanding what caused the Earth’s first ice age, say scientists.

The Anglo-French study, published in the journal Nature, analysed the amount of ancient atmospheric argon gas (Ar) isotopes dissolved in the bubbles and found levels were very different to those in the air we breathe today.

The researchers say their findings help explain why Earth didn’t suffer its first ice age until 2.5 billion years ago, despite the Sun’s rays being weaker during the early years of our planet’s formation.

“The water samples come from the Pilbara region in north-west Australia and were originally heated during an eruption of pillow basalt lavas, probably in a lake or lagoon environment,” said author Dr Ray Burgess, from the University of Manchester’s School of Earth, Atmospheric and Environmental Sciences.

“Evidence from the geological record indicates that the first major glaciations on Earth occurred about 2.5 billion years ago, and yet the energy of the Sun was 20 per cent weaker prior to, and during, this period, so all water on Earth should already have been frozen.

“This is something that has baffled scientists for years but our findings provide a possible explanation.”

The study, done in collaboration with the CRPG-CNRS, University of Lorraine and the Institut de Physique du Globe de Paris, revealed that the ratio of two argon isotopes – 40Ar, formed by the decay of potassium (40K) with a half-life of 1.25 billion years, and 36Ar – was much lower than present-day levels. This finding can only be explained by the gradual release of 40Ar from rocks and magma into the atmosphere throughout Earth’s history.

The team used the argon isotope ratio to estimate how the continents have grown over geological time and found that the volume of continental crust 3.5 billion years ago was already well-established being roughly half what it is today.

Dr Burgess said: “High levels of the greenhouse gas carbon dioxide in the early atmosphere – in the order of several percent – which would have helped retain the Sun’s heat, has been suggested as the reason why the Earth did not freeze over sooner, but just how this level was reduced has been unexplained, until now.

“The continents are a key player in the Earth’s carbon cycle because carbon dioxide in the atmosphere dissolves in water to form acid rain. The carbon dioxide removed from the atmosphere by this process is stabilised in carbonate rocks such as limestone and if a substantial volume of continental crust was established, as revealed by our study, then the acid weathering of this early crust would efficiently reduce the carbon dioxide levels in the atmosphere to lower global temperatures and lead to the first major ice age.

He added: “The signs of the Earth’s evolution in the distant past are extremely tenuous, only fragments of highly weathered and altered rocks exists from this time, and for the most part, the evidence is indirect. To find an actual sample of ancient atmospheric argon is remarkable and represents a breakthrough in understanding environmental conditions on Earth before life existed.”

Cracking the ice code

UWM geosciences professor John Isbell (left) and postdoctoral researcher Erik Gulbranson, University of Wisconsin, Milwaukee, look over some of the many samples they have brought back from Antarctica. The two are part of an international team of scientists investigating the last extreme climate shift on Earth, which occurred in the late Paleozoic Era. -  Troye Fox
UWM geosciences professor John Isbell (left) and postdoctoral researcher Erik Gulbranson, University of Wisconsin, Milwaukee, look over some of the many samples they have brought back from Antarctica. The two are part of an international team of scientists investigating the last extreme climate shift on Earth, which occurred in the late Paleozoic Era. – Troye Fox

What happened the last time a vegetated Earth shifted from an extremely cold climate to desert-like conditions? And what does it tell us about climate change today?

John Isbell is on a quest to coax that information from the geology of the southernmost portions of the Earth. It won’t be easy, because the last transition from “icehouse to greenhouse” occurred between 335 and 290 million years ago.

An expert in glaciation from the late Paleozoic Era, Isbell is challenging many assumptions about the way drastic climate change naturally unfolds. The research helps form the all-important baseline needed to predict what the added effects of human activity will bring.

Starting from ‘deep freeze’

In the late Paleozoic, the modern continents were fused together into two huge land masses, with what is now the Southern Hemisphere, including Antarctica, called Gondwana.

During the span of more than 60 million years, Gondwana shifted from a state of deep freeze into one so hot and dry it supported the appearance of reptiles. The change, however, didn’t happen uniformly, Isbell says.

In fact, his research has shaken the common belief that Gondwana was covered by one massive sheet of ice which gradually and steadily melted away as conditions warmed.

Isbell has found that at least 22 individual ice sheets were located in various places over the region. And the state of glaciation during the long warming period was marked by dramatic swings in temperature and atmospheric carbon dioxide (CO2) levels.

“There appears to be a direct association between low CO2 levels and glaciation,” he says. “A lot of the changes in greenhouse gases and in a shrinking ice volume then are similar to what we’re seeing today.”

When the ice finally started disappearing, he says, it did so in the polar regions first and lingered in other parts of Gondwana with higher elevations. He attributes that to different conditions across Gondwana, such as mountain-building events, which would have preserved glaciers longer.

All about the carbon

To get an accurate picture of the range of conditions in the late Paleozoic, Isbell has traveled to Antarctica 16 times and has joined colleagues from around the world as part of an interdisciplinary team funded by the National Science Foundation. They have regularly gone to places where no one has ever walked on the rocks before.

One of his colleagues is paleoecologist Erik Gulbranson, who studies plant communities from the tail end of the Paleozoic and how they evolved in concert with the climatic changes. The information contained in fossil soil and plants, he says, can reveal a lot about carbon cycling, which is so central for applying the work to climate change today.

Documenting the particulars of how the carbon cycle behaved so long ago will allow them to answer questions like, ‘What was the main force behind glaciation during the late Paleozoic? Was it mountain-building or climate change?’

Another characteristic of the late Paleozoic shift is that once the climate warmed significantly and atmospheric CO2 levels soared, the Earth’s climate remained hot and dry for another 200 million years.

“These natural cycles are very long, and that’s an important difference with what we’re seeing with the contemporary global climate change,” says Gulbranson. “Today, we’re seeing change in greenhouse gas concentrations of CO2 on the order of centuries and decades.”

Ancient trees and soil

In order to explain today’s accelerated warming, Gulbranson’s research illustrates that glaciers alone don’t tell the whole story.

Many environmental factors leave an imprint on the carbon contained in tree trunks from this period. One of the things Gulbranson hypothesizes from his research in Antarctica is that an increase in deciduous trees occurred in higher latitudes during the late Paleozoic, driven by higher temperatures.

What he doesn’t yet know is what the net effect was on the carbon cycle.

While trees soak in CO2 and give off oxygen, there are other environmental processes to consider, says Gulbranson. For example, CO2 emissions also come from soil as microbes speed up their consumption of organic matter with rising temperatures.

“The high latitudes today contain the largest amount of carbon locked up as organic material and permafrost soils on Earth today,” he says. “It actually exceeds the amount of carbon you can measure in the rain forests. So what happens to that stockpile of carbon when you warm it and grow a forest over it is completely unknown.”

Another unknown is whether the Northern Hemisphere during this time was also glaciated and warming. The pair are about to find out. With UWM backing, they will do field work in northeastern Russia this summer to study glacial deposits from the late Paleozoic.

The two scientists’ work is complementary. Dating the rock is essential to pinpointing the rate of change in the carbon cycle, which would be the warning signal we could use today to indicate that nature is becoming dangerously unbalanced.

“If we figure out what happened with the glaciers,” says Isbell, “and add it to what we know about other conditions – we will be able to unlock the answers to climate change.”

Carbon cycle reaches Earth’s lower mantle, Science study reports

<IMG SRC="/Images/347637653.jpg" WIDTH="350" HEIGHT="175" BORDER="0" ALT="Like an insect in amber, mineral inclusions trapped in diamonds can reveal much about the Earth's deep interior. The study by Walter et al. in Science reveals mineral inclusions that originated in oceanic crust subducted into the lower mantle. – Image © Science/AAAS”>
Like an insect in amber, mineral inclusions trapped in diamonds can reveal much about the Earth’s deep interior. The study by Walter et al. in Science reveals mineral inclusions that originated in oceanic crust subducted into the lower mantle. – Image © Science/AAAS

The carbon cycle, upon which most living things depend, reaches much deeper into the Earth than generally supposed-all the way to the lower mantle, researchers report.

The findings, which are based on the chemistry of an unusual set of Brazilian diamonds, will be published online by the journal Science, at the Science Express Web site, on 15 September. Science is published by AAAS, the non-profit, international science society.

“This study shows the extent of Earth’s carbon cycle on the scale of the entire planet, connecting the chemical and biological processes that occur on the surface and in the oceans to the far depths of Earth’s interior,” said Nick Wigginton, associate editor at Science.

“Results of this kind offer a broader perspective of planet Earth as an integrated, dynamic system,” he said.

The carbon cycle generally refers to the movement of carbon through the atmosphere, oceans, and the crust. Previous observations suggested that the carbon cycle may even extend to the upper mantle, which extends roughly 400 kilometers into the Earth. In this region, plates of ocean crust-bearing a carbon-rich sediment layer-sink beneath other tectonic plates and mix with the molten rock of the mantle.

Seismological and geochemical studies have suggested that oceanic crust can sink all the way to the lower mantle, more than 660 kilometers down. But actual rock samples with this history have been hard to come by.

Michael Walter of the University of Bristol and colleagues in Brazil and the United States analyzed a set of “superdeep” diamonds from the Juina kimberlite field in Brazil. Most diamonds excavated at Earth’s surface originated at depths of less than 200 kilometers. Some parts of the world, however, have produced rare, superdeep diamonds, containing tiny inclusions of other material whose chemistry indicates that the diamonds formed at far greater depths.

The Juina-5 diamonds studied by Walter and colleagues contain inclusions whose bulk compositions span the range of minerals expected to form when basalt melts and crystallizes under the extreme high pressures and temperatures of the lower mantle.

Thus, these inclusions probably originated when diamond-forming fluids incorporated basaltic components from oceanic lithosphere that had descended into the lower mantle, the researchers have concluded.

If this hypothesis is correct, then the carbon from which the diamonds formed may have been deposited originally within ocean crust at the seafloor. A relative abundance of light carbon isotopes in the Juina-5 diamonds supports this idea, since this lighter form of carbon is found at the surface but not generally in the mantle, the authors say.

The diamond inclusions also include separate phases that appear to have “unmixed” from the homogenous pool of material. This unmixing likely happened as the diamonds traveled upward hundreds of kilometers into the upper mantle, the researchers say.

After the diamonds formed in the lower mantle, they may have been launched back near the surface by a rising mantle plume, Walter and colleagues propose.