Location of upwelling in Earth’s mantle discovered to be stable

This is a diagram showing a slice through the Earth's mantle, cutting across major mantle upwelling locations beneath Africa and the Pacific. -  C. Conrad (UH SOEST)
This is a diagram showing a slice through the Earth’s mantle, cutting across major mantle upwelling locations beneath Africa and the Pacific. – C. Conrad (UH SOEST)

A study published in Nature today shares the discovery that large-scale upwelling within Earth’s mantle mostly occurs in only two places: beneath Africa and the Central Pacific. More importantly, Clinton Conrad, Associate Professor of Geology at the University of Hawaii – Manoa’s School of Ocean and Earth Science and Technology (SOEST) and colleagues revealed that these upwelling locations have remained remarkably stable over geologic time, despite dramatic reconfigurations of tectonic plate motions and continental locations on the Earth’s surface. “For example,” said Conrad, “the Pangaea supercontinent formed and broke apart at the surface, but we think that the upwelling locations in the mantle have remained relatively constant despite this activity.”

Conrad has studied patterns of tectonic plates throughout his career, and has long noticed that the plates were, on average, moving northward. “Knowing this,” explained Conrad, “I was curious if I could determine a single location in the Northern Hemisphere toward which all plates are converging, on average.” After locating this point in eastern Asia, Conrad then wondered if other special points on Earth could characterize plate tectonics. “With some mathematical work, I described the plate tectonic ‘quadrupole’, which defines two points of ‘net convergence’ and two points of ‘net divergence’ of tectonic plate motions.”

When the researchers computed the plate tectonic quadruople locations for present-day plate motions, they found that the net divergence locations were consistent with the African and central Pacific locations where scientists think that mantle upwellings are occurring today. “This observation was interesting and important, and it made sense,” said Conrad. “Next, we applied this formula to the time history of plate motions and plotted the points – I was astonished to see that the points have not moved over geologic time!” Because plate motions are merely the surface expression of the underlying dynamics of the Earth’s mantle, Conrad and his colleagues were able to infer that upwelling flow in the mantle must also remain stable over geologic time. “It was as if I was seeing the ‘ghosts’ of ancient mantle flow patterns, recorded in the geologic record of plate motions!”

Earth’s mantle dynamics govern many aspects of geologic change on the Earth’s surface. This recent discovery that mantle upwelling has remained stable and centered on two locations (beneath Africa and the Central Pacific) provides a framework for understanding how mantle dynamics can be linked to surface geology over geologic time. For example, the researchers can now estimate how individual continents have moved relative to these two upwelling locations. This allows them to tie specific events that are observed in the geologic record to the mantle forces that ultimately caused these events.

More broadly, this research opens up a big question for solid earth scientists: What processes cause these two mantle upwelling locations to remain stable within a complex and dynamically evolving system such as the mantle? One notable observation is that the lowermost mantle beneath Africa and the Central Pacific seems to be composed of rock assemblages that are different than the rest of the mantle. Is it possible that these two anomalous regions at the bottom of the mantle are somehow organizing flow patterns for the rest of the mantle? How?

“Answering such questions is important because geologic features such as ocean basins, mountains belts, earthquakes and volcanoes ultimately result from Earth’s interior dynamics,” Conrad described. “Thus, it is important to understand the time-dependent nature of our planet’s interior dynamics in order to better understand the geological forces that affect the planetary surface that is our home.”

The mantle flow framework that can be defined as a result of this study allows geophysicists to predict surface uplift and subsidence patterns as a function of time. These vertical motions of continents and seafloor cause both local and global changes in sea level. In the future, Conrad wants to use this new understanding of mantle flow patterns to predict changes in sea level over geologic time. By comparing these predictions to observations of sea level change, he hopes to develop new constraints on the influence of mantle dynamics on sea level.

A billion-year-old piece of North America traced back to Antarctica

The Franklin Mountains in West Texas were once part of Coats Land in Antarctica, according to Staci Loewy, a geochemist at California State University, Bakersfield, et al. This photo shows Coats Land with its only rock outcrops, Littlewood (L) and Bertrab (B) nunataks. -  Ian Dalziel
The Franklin Mountains in West Texas were once part of Coats Land in Antarctica, according to Staci Loewy, a geochemist at California State University, Bakersfield, et al. This photo shows Coats Land with its only rock outcrops, Littlewood (L) and Bertrab (B) nunataks. – Ian Dalziel

An international team of researchers has found the strongest evidence yet that parts of North America and Antarctica were connected 1.1 billion years ago, long before the supercontinent Pangaea formed.

“I can go to the Franklin Mountains in West Texas and stand next to what was once part of Coats Land in Antarctica,” said Staci Loewy, a geochemist at California State University, Bakersfield, who led the study. “That’s so amazing.”

Loewy and her colleagues discovered that rocks collected from both locations have the exact same composition of lead isotopes. Earlier analyses showed the rocks to be the exact same age and have the same chemical and geologic properties. The work, published online 5 August (ahead of print) in the September issue of the journal Geology, strengthens support for the so-called SWEAT hypothesis, which posits that ancestral North America and East Antarctica were joined in an earlier supercontinent called Rodinia.

The approximately 1.1 billion year old North American Mid-continent Rift System extends across the continent from the Great Lakes to Texas. Volcanic rocks associated with the rift, which appears to represent an aborted tectonic attempt to split the ancestral North American continent of Laurentia, are well exposed in the Keweenaw Peninsula of the Upper Peninsula of Michigan from which they take their name, the Keweenawan large igneous province. The rift extends in the subsurface beneath Minnesota, Iowa, Nebraska, Kansas and Oklahoma to the Franklin Mountains near El Paso, Texas where related rocks are exposed. In this latest report, Loewy, Ian Dalziel, research professor at The University of Texas at Austin, Richard Hanson of Texas Christian University and colleagues from several overseas institutions, find that rocks barely peeking through the ice in Coats Land, a remote part of the Antarctic continent south of the Atlantic Ocean basin, reflect a former continuation of the North American rift system. Loewy began her collaboration with Dalziel several years ago as a graduate student at the University of Texas at Austin.

Loewy et al. use new lead (Pb) isotopic data from the 1.1-billion-year-old rocks from Coats Land, to constrain the positions of Laurentia (ancestral North America) and Kalahari (ancestral southern Africa) in the 1-billion-year-old supercontinent, Rodinia. The Coats Land rocks are identical in age to both the Keweenawan large igneous province of the North American mid-continent rift and the contemporaneous Umkondo large igneous province of southern Africa. Comparison of the isotopic compositions, however, unequivocally links the Coats Land rocks with the Keweenawan province. Together with paleomagnetic data this suggests that the Coats Land block was a piece of Laurentia near west Texas 1.1 billion years ago. Furthermore, the Coats Land block collided with the Kalahari Precambrian craton of Africa during a 1-billion-year-old collision. Based on this reconstruction, Laurentia collided with Kalahari along Antarctica’s Maud mountain belt, which would represent a continuation of the 1-billion-year-old Grenville mountain belt of eastern and southern North America.

Thus the tiny Coats Land block of Antarctica is a ‘tectonic tracer’ providing critical clues to the geographic relationships between three of the major continents of the planet in the time interval 1.1 – 1.0 billion years ago, just prior to the opening of the Pacific Ocean basin, the hypothesized ‘Snowball Earth’ glaciations, and the rise of multi-cellular life.

New ancient fungus finding suggests world’s forests were wiped out in global catastrophe


Tiny organisms that covered the planet more than 250 million years ago appear to be a species of ancient fungus that thrived in dead wood, according to new research published today (Thursday 1 October 2009) in the journal Geology.

The researchers behind the study, from Imperial College London and other universities in the UK, USA and The Netherlands, believe that the organisms were able to thrive during this period because the world’s forests had been wiped out. This would explain how the organisms, which are known as Reduviasporonites, were able to proliferate across the planet.

Researchers had previously been unsure as to whether Reduviasporonites were a type of fungus or algae. By analyzing the carbon and nitrogen content of the fossilized remains of the microscopic organisms, the scientists identified them as a type of wood-rotting fungus that would have lived inside dead trees.

Fossil records of Reduviasporonites reveal chains of microscopic cells and reflect an organism that lived during the Permian-Triassic period, before the dinosaurs, when the Earth had one giant continent called Pangaea.

Geological records show that the Earth experienced a global catastrophe during this period. Basalt lava flows were unleashed on the continent from a location centered on what is present day Siberia. Up to 96 per cent of all marine species and 70 per cent of land species became extinct. Traditionally, scientists had thought that land plants weathered the catastrophe without much loss.

Today’s findings suggest that much of the vegetation on Pangaea did not survive and that the world’s forests were wiped out, according to the researchers. Geological records show that there was a massive spike in the population of Reduviasporonites across Pangea as the Permian period came to an end. The scientists suggest that this means that there was in increase in the supply of wood for them to decay.

Professor Mark Sephton, one of the authors of the study from the Department of Earth Science and Engineering at Imperial College London, comments:

“Our study shows that neither plant nor animal life escaped the impact of this global catastrophe. Ironically, the worst imaginable conditions for plant and animal species provided the best possible conditions for the fungi to flourish.”

The team suggest that the basalt lava, which flowed during Permian-Triassic catastrophe, unleashed toxic gases into the air. The gases had a dual effect, producing acid rain and depleting the ozone layer. The outcome was the destruction of forests, providing enough rotting vegetation to nourish Reduviasporonites so that they could proliferate across Pangaea.

The team reached their conclusions by analysing the carbon and nitrogen content of Reduviasporonites using a High Sensitivity Mass Spectrometer and comparing the results with those from modern fungi. They discovered that Reduviasporonites and modern fungi show similar chemical characteristics.

In the future, the team plan to carry out further comparisons between Reduviasporonites and potential counterparts among modern fungi, which they hope will provide further clues about how Reduviasporonites lived.

Cold and Ice, and Heat, Episodically Gripped Tropical Regions 300 Million Years Ago

Unaweep Canyon in the Rocky Mountains is the site of a deep gorge that reveals ancient landscapes and sediments. The inset image is of a 'dropstone' from an eons-old glacier. - Credit: Gerilyn Soreghan
Unaweep Canyon in the Rocky Mountains is the site of a deep gorge that reveals ancient landscapes and sediments. The inset image is of a ‘dropstone’ from an eons-old glacier. – Credit: Gerilyn Soreghan

Look into “deep time” sheds light on period considered analogous to today’s climate

Geoscientists have long presumed that, like today, the tropics remained warm throughout Earth’s last major glaciation 300 million years ago.

New evidence, however, indicates that cold temperatures in fact episodically gripped these equatorial latitudes at that time.

Geologist Gerilyn Soreghan of Oklahoma University found evidence for this conclusion in the preservation of an ancient glacial landscape in the Rocky Mountains of western Colorado. Three hundred million years ago, the region was part of the tropics. The continents then were assembled into the supercontinent Pangaea.

Soreghan and colleagues published their results in the August 2008, issue of the journal Geology.

Climate model simulations are unable to replicate such cold tropical conditions for this time period, said Soreghan. “We are left with the prospect that what has been termed our ‘best-known’ analogue to Earth’s modern glaciation is in fact poorly known.”

“This study is an example of the wealth of untapped climate information stored in Earth’s ‘deep time’ geologic record millions of years ago,” said H. Richard Lane, program director in NSF’s Division of Earth Sciences, which funded the research. “These kinds of discoveries may greatly improve our understanding and prediction of modern climate change.”

As a result of the close proximity of the ancient tropical glaciers to the sea, the toes of the glaciers were likely less than 500 meters above sea level–much lower than the tropical glaciers of Earth’s recent glacial times.

“The Late Paleozoic tropical climate was not buffered against cold from the high latitudes, as everyone had thought,” said Soreghan. “The evidence we found indicates that glaciers were common at this time, even in tropical latitudes. This calls into question traditional assumptions of long-lasting equatorial warmth in the Late Paleozoic, and raises the possibility of large-scale and unexpected climate change in the tropics during that time.”

Scientists reveal secrets of ancient ocean in new book

The Rheic Ocean separated two major land masses 430 million years ago. - art by: Christina Ullman, Ullman Design
The Rheic Ocean separated two major land masses 430 million years ago. – art by: Christina Ullman, Ullman Design

Call it the ocean that time forgot. About 400 million years ago, the Rheic Ocean played a big role in Earth’s history. When this massive body of water closed, the Appalachians were lifted to Himalayan heights and the planet’s continents slammed together to form the supercontinent of Pangaea. Dinosaurs and early mammals evolved to traverse the large swath of land, spreading life to every corner of the globe.

But the Rheic Ocean doesn’t get much attention in the field of geology today. In fact, American texts give usually credit to an older ancient sea, the Iapetus, for creating the Appalachians.

Ohio University geologist Damian Nance and colleagues now hope to set the record straight with a new book published this fall by the Geological Society of America. It pulls together recent data from a team of UNESCO-funded scientists in the United States, Germany, Britain, Portugal, Turkey and several former Eastern Block countries who have spent years combing for better geological evidence of this ancient ocean and its legacy.

The Rheic Ocean opened 480 million years ago and, by 430 million years ago, separated two major land masses. To the north was Laurentia, which comprised North America, Europe, Greenland and part of Asia. To the south lay Gondwana, which comprised Africa, South America, Antarctica, Australia and India.

The sea closed some 340 million years ago, which pushed the continents together and created two mountain ranges: the Appalachian mountains of North America and the Variscan Belt of Europe, which runs across southern Europe and North Africa from Ireland to the Czech Republic and from Morocco to the Black Sea. Both mountain belts have eroded greatly over time, “shadows of their former selves,” Nance noted.

When the Atlantic Ocean opened and pushed the pieces of Pangaea apart again, geological evidence of the Rheic Ocean’s line of closure became buried – or was carried half a world away to Europe. That’s why scientists in areas ranging from Texas to Turkey have had to puzzle together remaining scraps of information to reconstruct this sea’s important history.

Nance, for example, has found evidence of the Rheic Ocean in rock formations in Mexico. That also suggests that the range is a bit longer than previously thought, said the scientist, who recently served as an expert on the topic for a National Geographic television program that will air in early 2008.

The creation and break up of supercontinents is one of the hot topics in geological sciences, Nance said, as these major plate movements impact climate change and lead to extinctions. “All hell breaks loose when they get together. When they break up, they are responsible for massive sea level changes,” he said.

The Earth’s land masses have merged into supercontinents at least twice – 300 million years ago to create Pangaea and 1 billion years ago to form Rodinia – and may have done so repeatedly through Earth history. Scientists project that in 250 million years, the closure of the Atlantic Ocean will merge North and South America with Africa and Eurasia, forging what some call “Pangaea Ultima.”

Sound like science fiction? Researchers already see evidence of this future world, Nance said. The Atlantic is about as old and as wide as an ocean can get. As it ages, the floor of an ocean becomes heavier and colder, eventually sinking into the Earth’s interior like a plank of waterlogged wood. Geologists see signs of seabed collapse off Gibraltar and anticipate that the Atlantic floor off the American East Coast and African west coast will be the first to sink.

Learning more about the ancient oceans and continental shifts not only helps scientists predict future geological changes, Nance added, but can suggest where on Earth certain natural resources may lie.

Ancient maps of the world also explain certain synergies in what are now disparate locations – the similar feel of Newfoundland and Scotland, for example, despite their separation by the Atlantic Ocean – and also some surprises. For much of Earth’s history, our Sunshine State was part of Africa. “If it wasn’t for the closure of the Rheic Ocean,” Nance says, “we wouldn’t have Florida. It actually belongs to Mauritania.”