Optical properties of the Antarctic system and new radiation information

The system has an important part in the global climate due to its size, its high latitude location and the negative radiation balance of its large ice sheets. Antarctica has also been in focus for several decades due to increased ultraviolet (UV) levels caused by stratospheric ozone depletion, and the disintegration of its ice shelves.

During the summer of 1997-1998, measurements of beam absorption and beam attenuation coefficients, and downwelling and upwelling irradiance were made in the Southern Ocean along a S-N transect at 6 degrees E. The attenuation of photosynthetically active radiation (PAR) was calculated and used together with hydrographic measurements to show that the phytoplankton in the investigated areas of the Southern Ocean are not light limited.

Variabilities in the spectral and total albedo of snow were studied in the Queen Maud Land region during the summers of 1999-2000 and 2000-2001. The measurement areas were the vicinity of the South African research station SANAE 4, and a traverse near the Finnish research station Aboa. The mean spectral albedo levels at Aboa and SANAE 4 were very close to each other. The variations in the spectral albedos were due more to differences in ambient conditions than variations in snow properties.

A Monte-Carlo model was made to study the spectral albedo and to help in developing a novel nondestructive method to measure the diffuse attenuation coefficient of snow. The method was based on the decay of upwelling radiation moving horizontally away from a source of downwelling light. In the model, the attenuation coefficient obtained from the upwelling irradiance was higher than that obtained using vertical profiles of downwelling irradiance. The model results were compared to field measurements made on dry snow in Finnish Lapland and they correlated reasonably well.

Low-elevation (below 1000 m) blue-ice areas may experience substantial melt-freeze cycles due to absorbed solar radiation and the small heat conductivity in the ice. A two-dimensional (x-z) model was developed to simulate the formation and water circulation in the subsurface ponds. The model results show that for a physically reasonable parameter set the formation of liquid water within the ice can be reproduced. Vertical convection and a weak overturning circulation is generated stratifying the fluid and transporting warmer water downward, thereby causing additional melting at the base of the pond. In a 50-year integration, a global warming scenario mimicked by a decadal scale increase of 3 degrees per 100 years in air temperature, leads to a general increase in subsurface water volume.

Oceanic crust formation is dynamic after all

A research team led by Brown University studied seismic velocities -- the speed of seismic waves -- in the Gulf of California to determine that a geological phenomenon known as dynamic upwelling occurs in the Earth's mantle as oceanic crust is formed. -  Yun Wang, Brown University
A research team led by Brown University studied seismic velocities — the speed of seismic waves — in the Gulf of California to determine that a geological phenomenon known as dynamic upwelling occurs in the Earth’s mantle as oceanic crust is formed. – Yun Wang, Brown University

Imagine the Earth’s crust as the planet’s skin: Some areas are old and wrinkled while others have a fresher, more youthful sheen, as if they had been regularly lathered with lotion.

Carry the metaphor a little further and a good picture emerges of the geological processes leading to the creation of the planet’s crust. On land, continental crust, once created, can remain more or less unaltered for billions of years. But the oldest oceanic crust is only about 200 million years old, as new crust is continually forming at mid-ocean ridge spreading centers.

While geologists have known that oceanic crust continually replenishes itself, they have been unsure what occurs below the surface that leads to the resurfacing. What geodynamics are occurring in the mantle that eventually produces new crust, that new layer of skin on the ocean’s bottom?

The answer has been elusive in part because oceanic crust is difficult to reach and instruments that can measure seismic activity have not fully covered the terrain to obtain an accurate picture of forces below the surface. Now earth scientists led by Brown University have observed – in detail and at unprecedented depths – a geological phenomenon known as dynamic upwelling in the underlying mantle beneath a spreading center. Their findings, reported in this week’s Nature, may resolve a longstanding debate regarding the relative importance of passive and dynamic upwelling in the shallow mantle beneath spreading centers on the seafloor.

“We know the crust of the ocean is produced by upwelling beneath separating plates,” said Don Forsyth, professor of geological sciences at Brown. “We just didn’t know the upwelling pattern that took place, that there are concentrated upwelling centers rather than uniform upwelling.”

Mantle upwelling and melting beneath spreading centers has been thought to be mostly a passive response to the separating oceanic plates above. The new finding shows there appears to be a dynamic component as well, driven by the buoyancy of melt retained in the rock or by the lighter chemical composition of rock from which melt has been removed.

The scientists from Brown and the University of Rhode Island based their findings on a high-resolution seismic study in the Gulf of California. In that region, there are 25 seismometers spaced along the western coast of Mexico and the Baja California peninsula, which lie on either side of the Gulf of California. Yun Wang, a Brown graduate student and the paper’s lead author, tracked the velocity of seismic waves that traveled from one station to another. She noticed a pattern: The seismic waves in three localized centers, spaced about 250 kilometers (155 miles) apart, traveled more slowly than waves in the surrounding mantle, implying the presence of more melt in the localized centers and thus a more vigorous upwelling. From that, the geologists determined the centers, located 40-90 kilometers (25 to 56 miles) below the surface, showed evidence of dynamic upwelling in the mantle.

“We found a pattern that was predicted by some of the theoretical models of upwelling in mid-oceanic ridges,” Forsyth said.

While other studies have been done of mantle geodynamics, most notably an experiment on the East Pacific Rise, the Brown-URI study imaged seismic activity, or the shear velocity of the seismic waves, some 200 kilometers (124 miles) below the surface – a far deeper seismic penetration into the mantle than previous experiments.

Brian Savage, assistant professor of geophysics at the University of Rhode Island and a contributing author on the paper, said the finding is important, because it helps to provide “a basic understanding of how a majority of the earth’s crust is formed, how it emerges from the mantle below to create the oceanic crust. It’s a basic science question that helps understand how crust is created.”

Small faults in southeast Spain reduce earthquake risk of larger ones

This image shows the reverse fault and associated fold in the Molat Albox (Almería). Its progressive development follows from the geometry and age of deformed sediments. -  Antonio Pedrera.
This image shows the reverse fault and associated fold in the Molat Albox (Almería). Its progressive development follows from the geometry and age of deformed sediments. – Antonio Pedrera.

A team of Spanish scientists, studying recent, active deformations in the Baetic mountain range, have shown that the activity of smaller tectonic structures close to larger faults in the south east of the Iberian Peninsula partially offsets the risk of earthquakes.

“There are large faults in the eastern part of the Baetic mountain range, which are active and occasionally cause moderate, low magnitude earthquakes (measuring less than 5 on the Richter scale)”, Antonio Pedrera, lead author of the study and a researcher in the Department of Geodynamics at the University of Granada (UGR), tells SINC.

The team’s research, published recently in the Journal of Quaternary Science, involved studying the La Molata sector, near Albox, in Almeria, near the southern end of the active Alhama de Murcia fault. The authors say this sector has been deformed by small faults and folds that are growing progressively.

“Although we can’t exclude the possibility that these direction faults could cause earthquakes of greater magnitude, we have shown that the formation of small tectonic structures helps to partially relax the energy associated with the convergence of plates, and reduces seismic activity in these larger faults”, says Pedrera.

The secrets of rodent fossils

By studying mammal fossils, Antonio Ruiz Bustos, co-author of the study and a researcher at the Andalusian Institute of Earth Sciences (UGR) has been able to date inverse faults and active folds near the town of Albox.

Some of the fossils found in the faults have included the molars of Mimomys Sabin (a small rodent that lived in wetland areas between 950,000 and 830,000 years ago), which have allowed him to measure the horizontal narrowing of the faults at 0.006 milimetres/year.

The scientists have combined the dating of deformed sediments with other surface geological data, such as geological mapping, cinematic analysis of the structures, geophysical prospecting and geomorphological analysis, in order to evaluate what role these faults have played in causing earthquakes during the Quaternary (from 1.8 million years ago to the present day).

Nine million years ago, the eastern part of the Baetic mountain range was deformed by numerous folds and faults, caused by the collision of the Eurasian and African plates.

Currently, some of these tectonic structures are still developing, but available data on the location of earthquakes suggest that their seismic activity is dispersed and moderate.

The hydrothermal explosion craters of yellowstone and how they came to be

GSA's new Special Paper 459, Hydrothermal Processes above the Yellowstone Magma Chamber: Large Hydrothermal Systems and Large Hydrothermal Explosions,
by Lisa A. Morgan, W.C. Pat Shanks III and Kenneth L. Pierce, uses new mapping, sampling, and analysis techniques to document a broad spectrum of ages and geologic settings for these events. -  The Geological Society of America
GSA’s new Special Paper 459, Hydrothermal Processes above the Yellowstone Magma Chamber: Large Hydrothermal Systems and Large Hydrothermal Explosions,
by Lisa A. Morgan, W.C. Pat Shanks III and Kenneth L. Pierce, uses new mapping, sampling, and analysis techniques to document a broad spectrum of ages and geologic settings for these events. – The Geological Society of America

Yellowstone National Park is widely known for its more than 10,000 thermal features. Among these features are at least 20 large (100 to greater than 2,500 meters in diameter) hydrothermal explosion craters, produced during the past 16,000 years. Although large hydrothermal explosions are rare on a human time scale, the potential for future explosions in Yellowstone is not insignificant, and events large enough to create even a 100-m-wide crater may be expected every 200 years.

Using new mapping, sampling, and analysis techniques, this new volume from The Geological Society of America documents a broad spectrum of ages and geologic settings for these events and considers additional processes and alternative triggering mechanisms not previously explored.

The book’s authors, all from the U.S. Geological Survey, present the information in a clear and compelling manner and include 50 figures (most in color) and several tables to help illustrate the data. Details on several lakes, basins, and explosion craters are synthesized in order to help determine the timing, distribution, and possible causes of hydrothermal explosions in Yellowstone and thereby aid in mitigation planning.

Using new technique, scientists find 11 times more aftershocks for 2004 quake

Using a technique normally used for detecting weak tremor, scientists at the Georgia Institute of Technology discovered that the 2004 magnitude 6 earthquake along the Parkfield section of the San Andreas fault exhibited almost 11 times more aftershocks than previously thought. The research appears online in Nature Geoscience and will appear in print in a forthcoming edition.

“We found almost 11 times more events in the first three days after the main event. That’s surprising because this is a well-instrumented place and almost 90 percent of the activity was not being determined or reported,” said Zhigang Peng, assistant professor at Georgia Tech’s School of Earth and Atmospheric Sciences.

In examining how these aftershocks occurred, Peng and graduate research assistant Peng Zhao discovered that the earliest aftershocks occurred in the region near the main event. Then with time, the aftershocks started migrating. Seeing how the aftershocks move from the center of the quake outward lends credence to the idea that it’s the result of the fault creeping, said Peng.

“Basically, the big event happens due to sudden fault movement, but the fault doesn’t stop after the main event. It continues to move because the stress has been perturbed and the fault is trying to adjust itself. We believe this so-called fault creep is causing most of the aftershocks,” he said.

Peng and Zhao used a method known as the matched filter technique, rather than the standard technique to examine the aftershocks. The traditional way of determining a location of an earthquake is that a human analyst has to go through each seismic recording, determine the order of events and their location. This takes time and if there are many events, or if some of them occur at the same time, it’s hard for the analyst to figure out which came first.

“Because of these difficulties, only the largest aftershocks are located, with many small ones missing. So, we used the matched filter technique because it allows us to use a computer to automatically scan the seismic records to detect events when their patterns are similar. There is no need to manually pick out the aftershocks after the mainshock,” said Peng.

The team chose the 2004 Parkfield quake to test the matched filter technique because the quake is on the San Andreas fault. The San Andreas is one of the most heavily instrumented places in the world, owing to the famous Parkfield, California, earthquake prediction experiment in the 1980s.

Peng is currently using the matched filter technique to work with several other research groups to detect early aftershocks of recent large earthquakes in Japan and China.

Supervolcano eruption – in Sumatra – deforested India 73,000 years ago

University of Illinois anthropology professor Stanley Ambrose and his colleagues found that central India was deforested after the Toba eruption, some 73,000 years ago. -  Photo by L. Brian Stauffer, University of Illinois News Bureau.
University of Illinois anthropology professor Stanley Ambrose and his colleagues found that central India was deforested after the Toba eruption, some 73,000 years ago. – Photo by L. Brian Stauffer, University of Illinois News Bureau.

A new study provides “incontrovertible evidence” that the volcanic super-eruption of Toba on the island of Sumatra about 73,000 years ago deforested much of central India, some 3,000 miles from the epicenter, researchers report.

The volcano ejected an estimated 800 cubic kilometers of ash into the atmosphere, leaving a crater (now the world’s largest volcanic lake) that is 100 kilometers long and 35 kilometers wide. Ash from the event has been found in India, the Indian Ocean, the Bay of Bengal and the South China Sea.

The bright ash reflected sunlight off the landscape, and volcanic sulfur aerosols impeded solar radiation for six years, initiating an “Instant Ice Age” that – according to evidence in ice cores taken in Greenland – lasted about 1,800 years.

During this instant ice age, temperatures dropped by as much as 16 degrees centigrade (28 degrees Fahrenheit), said University of Illinois anthropology professor Stanley Ambrose, a principal investigator on the new study with professor Martin A.J. Williams, of the University of Adelaide. Williams, who discovered a layer of Toba ash in central India in 1980, led the research.

The climactic effects of Toba have been a source of controversy for years, as is its impact on human populations.

In 1998, Ambrose proposed in the Journal of Human Evolution that the effects of the Toba eruption and the Ice Age that followed could explain the apparent bottleneck in human populations that geneticists believe occurred between 50,000 and 100,000 years ago. The lack of genetic diversity among humans alive today suggests that during this time period humans came very close to becoming extinct.

To address the limited evidence of the terrestrial effects of Toba, Ambrose and his colleagues pursued two lines of research: They analyzed pollen from a marine core in the Bay of Bengal that included a layer of ash from the Toba eruption, and they looked at carbon isotope ratios in fossil soil carbonates taken from directly above and below the Toba ash in three locations in central India.

Carbon isotopes reflect the type of vegetation that existed at a given locale and time. Heavily forested regions leave carbon isotope fingerprints that are distinct from those of grasses or grassy woodlands.

Both lines of evidence revealed a distinct change in the type of vegetation in India immediately after the Toba eruption, the researchers report. The pollen analysis indicated a shift to a “more open vegetation cover and reduced representation of ferns, particularly in the first 5 to 7 centimeters above the Toba ash,” they wrote in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. The change in vegetation and the loss of ferns, which grow best in humid conditions, they wrote, “would suggest significantly drier conditions in this region for at least one thousand years after the Toba eruption.”

The dryness probably also indicates a drop in temperature, Ambrose said, “because when you turn down the temperature you also turn down the rainfall.”

The carbon isotope analysis showed that forests covered central India when the eruption occurred, but wooded to open grassland predominated for at least 1,000 years after the eruption.

“This is unambiguous evidence that Toba caused deforestation in the tropics for a long time,” Ambrose said. This disaster may have forced the ancestors of modern humans to adopt new cooperative strategies for survival that
eventually permitted them to replace neandertals and other archaic human species, he said.

Rich ore deposits linked to ancient atmosphere

Much of our planet’s mineral wealth was deposited billions of years ago when Earth’s chemical cycles were different from today’s. Using geochemical clues from rocks nearly 3 billion years old, a group of scientists including Andrey Bekker and Doug Rumble from the Carnegie Institution have made the surprising discovery that the creation of economically important nickel ore deposits was linked to sulfur in the ancient oxygen-poor atmosphere.

These ancient ores — specifically iron-nickel sulfide deposits — yield 10% of the world’s annual nickel production. They formed for the most part between two and three billion years ago when hot magmas erupted on the ocean floor. Yet scientists have puzzled over the origin of the rich deposits. The ore minerals require sulfur to form, but neither seawater nor the magmas hosting the ores were thought to be rich enough in sulfur for this to happen.

“These nickel deposits have sulfur in them arising from an atmospheric cycle in ancient times. The isotopic signal is of an anoxic atmosphere,” says Rumble of Carnegie’s Geophysical Laboratory, a co-author of the paper appearing in the November 20 issue of Science.

Rumble, with lead author Andrey Bekker (formerly Carnegie Fellow and now at the University of Manitoba), and four other colleagues used advanced geochemical techniques to analyze rock samples from major ore deposits in Australia and Canada. They found that to help produce the ancient deposits, sulfur atoms made a complicated journey from volcanic eruptions, to the atmosphere, to seawater, to hot springs on the ocean floor, and finally to molten, ore-producing magmas.

The key evidence came from a form of sulfur known as sulfur-33, an isotope in which atoms contain one more neutron than “normal” sulfur (sulfur-32). Both isotopes act the same in most chemical reactions, but reactions in the atmosphere in which sulfur dioxide gas molecules are split by ultraviolet light (UV) rays cause the isotopes to be sorted or “fractionated” into different reaction products, creating isotopic anomalies.

“If there is too much oxygen in the atmosphere then not enough UV gets through and these reactions can’t happen,” says Rumble. “So if you find these sulfur isotope anomalies in rocks of a certain age, you have information about the oxygen level in the atmosphere.”

By linking the rich nickel ores with the ancient atmosphere, the anomalies in the rock samples also answer the long-standing question regarding the source of the sulfur in the ore minerals. Knowing this will help geologists track down new ore deposits, says Rumble, because the presence of sulfur and other chemical factors determine whether or not a deposit will form.

“Ore deposits are a tiny fraction of a percent of the Earth’s surface, yet economically they are incredibly important. Modern society cannot exist without specialized metals and alloys,” he says. “But it’s all a matter of local geological circumstance whether you have a bonanza — or a bust.”

Mysteriously warm times in Antarctica

A new study of Antarctica’s past climate reveals that temperatures during the warm periods between ice ages (interglacials) may have been higher than previously thought. The latest analysis of ice core records suggests that Antarctic temperatures may have been up to 6°C warmer than the present day.

The findings, reported this week by scientists from the British Antarctic Survey (BAS), the Open University and University of Bristol in the journal Nature could help us understand more about rapid Antarctic climate changes.

Previous analysis of ice cores has shown that the climate consists of ice ages and warmer interglacial periods roughly every 100,000 years. This new investigation shows temperature ‘spikes’ within some of the interglacial periods over the last 340,000 years. This suggests Antarctic temperature shows a high level of sensitivity to greenhouse gases at levels similar to those found today.

Lead author Louise Sime of British Antarctic Survey said,

“We didn’t expect to see such warm temperatures, and we don’t yet know in detail what caused them. But they indicate that Antarctica’s climate may have undergone rapid shifts during past periods of high CO2.”

During the last warm period, about 125,000 years ago, sea level was around 5 metres higher than today.

Ice core scientist Eric Wolff of British Antarctic Survey is a world-leading expert on past climate. He said,

“If we can pin down how much warmer temperatures were in Antarctica and Greenland at this time, then we can test predictions of how melting of the large ice sheets may contribute to sea level rise.”

Antarctica glacier retreat creates new carbon dioxide store

Large blooms of tiny marine plants called phytoplankton are flourishing in areas of open water left exposed by the recent and rapid melting of ice shelves and glaciers around the Antarctic Peninsula. This remarkable colonization is having a beneficial impact on climate change. As the blooms die back phytoplankton sinks to the sea-bed where it can store carbon for thousands or millions of years.

Reporting this week in the journal Global Change Biology, scientists from British Antarctic Survey (BAS) estimate that this new natural ‘sink’ is taking an estimated 3.5 million tonnes* of carbon from the ocean and atmosphere each year.

Lead author, Professor Lloyd Peck from BAS says,

“Although this is a small amount of carbon compared to global emissions of greenhouse gases in the atmosphere it is nevertheless an important discovery. It shows nature’s ability to thrive in the face of adversity. We need to factor this natural carbon-absorption into our calculations and models to predict future climate change. So far we don’t know if we will see more events like this around the rest of Antarctica’s coast but it’s something we’ll be keeping a close eye on.”

Professor Peck and his colleagues compared records of coastal glacial retreat with records of the amount of chlorophyll (green plant pigment essential for photosynthesis) in the ocean. They found that over the past 50 years, melting ice has opened up at least 24,000 km2 of new open water (an area similar to the size of Wales) – and this has been colonised by carbon-absorbing phytoplankton. According to the authors this new bloom is the second largest factor acting against climate change so far discovered on Earth (the largest is new forest growth on land in the Arctic).

Professor Peck continues,
“Elsewhere in the world human activity is undermining the ability of oceans and marine ecosystems to capture and store carbon. At present, there is little change in ice shelves and coastal glaciers away from the Antarctic Peninsula, but if more Antarctic ice is lost as a result of climate change then these new blooms have the potential to be a significant biological sink for carbon.

Alberta’s hidden valleys offer both resources and danger

Alberta is crisscrossed with hidden glacial valleys that hold both resource treasures and potential danger. University of Alberta researcher Doug Schmitt discovered a 300 metre deep, valley hidden beneath the surface of the ground near the community of Rainbow Lake in northwestern Alberta.

The valley was created by glaciers and over time filled with loose rock gradually disappearing from the landscape.

There had already been extensive underground mapping of the area, but Schmitt went beyond the standard practices to locate the valley. He combined a variety of the existing seismic and electrical mapping data and found the valley. It ranges between two and three kilometers in width.

If these hidden alleys go undetected by standard underground mapping practices there could be serious consequences. Schmitt says if a tailings pond were unknowingly put in an area like this and the liner failed, the effluent could spread far and wide, underground via the aquifer.

And then there’s the issue of pockets of natural gas lying in the porous rock just metres beneath the surface. An energy exploration crew could trigger an explosion and fire. Schmitt says it’s happened more than once in Alberta.

Schmitt says there are hidden valleys like his find near Rainbow Lake, all over Alberta.