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TOPIC: Antarctica
Blobrana


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RE: Antarctica
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A 30-mile maze canyons in Antarctica was carved out of bedrock by the catastrophic draining of subglacial lakes during global warming between 12 million and 14 million years ago, according to university researchers who warn a similar event today could have serious environmental consequences.

Although scientists have previously theorized that the Labyrinth region in southern Victoria Land was created by water released from lakes that had formed under glaciers, researchers at Syracuse University and Boston University say they found geological evidence to bracket the timing of the last major flooding and link it to a global warming trend at the time.
The scientists pinpointed the timing of the last subglacial flood by dating volcanic ash preserved on bedrock surfaces, said Laura Webb, a professor of earth sciences at Syracuse who took part in the study.
The Labyrinth is a network of ice-free bedrock channels and scoured terrain emerging from beneath the East Antarctic Ice Sheet. It is one of a series of large channel networks that cross the Transantarctic Mountains. Some of the chasms are up to 800 feet deep and thousands of feet wide. Scientists have long speculated that the volume of water required to create the channels was far more than that produced by melting glaciers.
Webb said it appeared the subglacial flooding was not continuous but episodic, and likely lasted days or months at a time.
In an article on the study that appeared last month in the journal "Geology," Webb and her colleagues estimated the flood raged with approximately 1,000 times the volume of water flowing over Niagara Falls. At that rate, it would take Lake Ontario, for instance, about a month to drain, she said.

Title: The age and origin of the Labyrinth, western Dry Valleys, Antarctica: Evidence for extensive middle Miocene subglacial floods and freshwater discharge to the Southern Ocean
Authors: Adam R. Lewis1, David R. Marchant1, Douglas E. Kowalewski1, Suzanne L. Baldwin2 and Laura E. Webb2

1 Department of Earth Sciences, Boston University, Boston, Massachusetts 02215, USA
2 Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA

A 50+-km-long network of bedrock channels and scoured terrain occupies the ice-free portion of a major trough that crosses the Transantarctic Mountains in southern Victoria Land. The channels, collectively termed the Labyrinth, emerge from beneath the margin of the East Antarctic Ice Sheet (Wright Upper Glacier) and are incised into a 300-m-thick sill of Ferrar Dolerite at the head of Wright Valley. Upper- and intermediate-elevation erosion surfaces of the Labyrinth exhibit striations and molding characteristic of glacial erosion. Channels and canyons on the lower surface are as much as 600 m wide and 250 m deep, have longitudinal profiles with many reverse gradients, and contain potholes <35 m deep at tributary junctions. These characteristics are most consistent with incision from fast-flowing subglacial mel****er; estimated discharge is on the order of 1.6–2.2 x 106 m3s–1. Our 40Ar/39Ar analyses of volcanic tephra from the Labyrinth show that the channels are relict, that major channel incision predates 12.4 Ma, and that the last major subglacial flood occurred sometime between 14.4 Ma and 12.4 Ma. The most plausible origin for the Labyrinth is erosion associated with episodic drainage of subglacial lakes in East Antarctica. One compelling possibility is that discharge of large volumes of subglacial mel****er to the Southern Ocean, and to the Ross Sea in particular, may have coincided with, and contributed to, oscillations in regional and/or global climate during the middle Miocene.

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Blobrana


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Antarctic Snowfall
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The most precise record of Antarctic snowfall ever generated shows there has been no real increase in precipitation over the southernmost continent in the past half-century, even though most computer models assessing global climate change call for an increase in Antarctic precipitation as atmospheric temperatures rise.

"The year-to-year and decadal variability of the snowfall is so large that it makes it nearly impossible to distinguish trends that might be related to climate change from even a 50-year record. There were no statistically significant trends in snowfall accumulation over the past five decades, including recent years for which global mean temperatures have been warmest" - Andrew Monaghan, a research associate with Ohio State University's Byrd Polar Research Centre and lead author of an article on the topic published in the August 10 edition of Science magazine.

The findings also suggest thickening of Antarctica's massive ice sheets haven't reduced the slow-but-steady rise in global sea levels, as some climate-change critics have argued.
The study looked at both the West Antarctic Ice Sheet (WAIS), a marine ice sheet with a base below sea level, and the much thicker East Antarctic Ice Sheet (EAIS) that sits atop dry land. In recent years, large volumes of ice along the WAIS coast have melted at a faster rate than previously seen. Some observers have blamed global warming for the melting and for the increased calving of icebergs along the continent's margin.
The 16 researchers from nine institutions in seven countries wanted to assemble a half-century-long record of snowfall back to the International Geophysical Year, or IGY. Work during the IGY began the first real modern study of the Antarctic continent and substantive research has continued there ever since. That work will continue in 2007-2008 during the upcoming International Polar Year (IPY), including NSF-funded studies of ice-sheet dynamics.
The research team coupled data from existing ice cores in the region, snow pits and networks of snow stakes, and meteorological observations. To these they added numerous new ice core records obtained by the International Transantarctic Scientific Expedition (ITASE), a 12-nation research program begun in 1990 and tasked with reconstructing the continent's climate history. NSF funded the U.S. component of ITASE.

Source: National Science Foundation

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Blobrana


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The gateway between the Atlantic and Pacific at the bottom of the globe opened up 41 million years ago, according to a study of old fish teeth.

The research in Science pushes back the date of the forging of Drake Passage to twice as long ago as once thought.
US geologists believe it kick-started the ocean current that swirls around Antarctica, helping to bring about a dramatic cooling effect.
The continent was transformed from lush forest to the icy landscape of today.

"We're saying we now have a date for the opening of the Drake Passage that looks like it's early enough that it may have contributed to the cooling. It illustrates that ocean circulation may play a very important role in climate change" - Dr Ellen Eckels Martin of the University of Florida, US.

Millions of years later, the current still plays a major role in keeping the Antarctic cool.

"We are warming the world through greenhouse gases and this is leading to the decay of the West Antarctic Ice Sheet and the cool current around Antarctica is probably helping slow that process" - Dr Ellen Eckels Martin .



Drake's Passage is named after Sir Francis Drake, the English sea captain who circumnavigated the globe in the 16th Century.
The stretch of water between the southern tip of South America at Cape Horn and the South Shetland Islands of Antarctica formed when the "arm" that once connected South America and Antarctica fell away.
The world was a very different place then. Levels of carbon dioxide were three to four times today's levels and it was so warm that alligators sunned themselves in the high Arctic.
But some 30 million years ago, there was a dramatic shift in climate from "greenhouse" to "icehouse".
The rapid cooling swept over the Antarctic and, over the course of several million years, its pine trees were replaced by glaciers.
Scientists have long speculated about how this happened and suspect the opening of the Drake Passage played a key role.
But until now, the date has been fuzzy, with estimates ranging from 15 million to 49 million years ago.

The University of Florida team came up with a reliable date by studying fish teeth recovered from rocks lying more than 300 metres beneath the bottom of the Atlantic Ocean.
A chemical called neodymium accumulates in fish teeth, as they settle on the ocean floor.
The chemical signatures for the Atlantic and the Pacific are different, allowing the scientists to determine when water from the Pacific Ocean began seeping into the Atlantic Ocean.

The research shows that the rift in the plates of the Earth that caused the gap to open up happened about 41 million years ago, which fits in with the build-up of ice sheets on Antarctica a few million years later.
Scientists believe the formation of the ocean current that circulates around Antarctica played a key role in the cooling as it deflects warm streams of water coming from the equator.
Decreasing levels of carbon dioxide in the atmosphere may have also contributed to the cooling effect, but their relative roles are still a matter of debate.

Source BBC

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Ohio State University scientists used tiny gravity changes to produce the best map yet of ocean tides flowing beneath two large Antarctic ice shelves.



GRACE-measured gravity fluctuations from twice-daily tides caused by the moon, above, and a daily tide caused by the sun, below.



Credit Ohio State University.


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Blobrana


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Scientists using satellite data have now created the most detailed maps ever produced of the vast snow-covered Antarctic continent. The maps reveal unprecedented views of surface features that provide clues to how and why the continent's massive ice sheets and glaciers are changing.

Researchers can now decipher the intricate history of ice movements in the just-released "Mosaic of Antarctica," which uses images from the Moderate Resolution Imaging Spectrometer onboard NASA's Terra and Aqua satellites. The map is the result of a partnership between NASA's Goddard Space Flight Centre, Greenbelt, Md.; the University of Colorado's National Snow and Ice Data Centre (NSIDC), Boulder; and the University of New Hampshire, Durham.



A second map to be released early next year will provide the most complete and accurate topographical survey of the continent ever undertaken, with more than 65 million points surveyed from space by the Geoscience Laser Altimeter System orbiting on NASA's Ice, Cloud and Land Elevation Satellite (ICESat). This "digital elevation model" produced at Goddard will be distributed by NSIDC in a format compatible with the Mosaic map.

"The Antarctic Mosaic shows a lot of very subtle changes in the slope of the terrain that you cannot see from the ground. These subtle variations are important because they tell us the direction the ice is flowing now and they indicate where it has gone in the past. The surface roughness also tells us about the bed underneath the ice and whether the ice is sliding over the bed or frozen to it" - Robert Bindschadler, chief scientist of Goddard's Hydrospheric and Biospheric Sciences Laboratory.

The map will very likely reveal unseen features and new opportunities for exploration.

"Antarctica is a big place, and there is still an awful lot of the ice sheet that hasn't been explored" - Robert Bindschadler.

The new map will be used by researchers to identify interesting areas and plan expeditions to investigate them.
The Mosaic removes the terrain distortion and produces a more accurate and natural-looking view of the continent and its very subtle surface features.

"Using the Mosaic map together with the Canadian satellite, RADARSAT, is a real breakthrough. The Mosaic shows the snow and rock surface almost perfectly, and RADARSAT reveals some of the features below the snow. It's very informative" - Ted Scambos, one of the creators of the Mosaic at NSIDC.

The ICESat topographic map complements the Mosaic's detailed views of the surface with elevation measurements over more of the continent than has ever been surveyed before. Although the very centre of Antarctica remains unmapped because the satellite does not fly directly over the pole, more of the interior of the continent was mapped and in unprecedented detail.

"This is the most accurate elevation map of the ice sheet ever produced. And it will get even better as ICESat continues to acquire more elevation data for studying changes in the ice-sheet volume" - Jay Zwally, ICESat project scientist at Goddard.

Download (6.5Mb)
Mpeg movie showing detailed topographic features on the Antarctic Ross Ice Shelf, using both Mosaic Of Antarctica (MOA) and ICESat Digital Elevation Model data. + View Ross Ice Shelf without ICESat elevation information.
Credit: NASA

The precision of the ICESat map is more than 10 times better than previous satellite surveys due to the very narrow beam of the laser altimeter instrument compared to the broader beam of radar instruments flown before. The improved mapping of the height of the ice sheet, particularly in the interior of the continent, yields new information about how the topography of this remote area drives the flow of interior ice streams. Key areas such as the major ice streams feeding the Ross Ice Shelf are seen in detail for the first time.
Both maps will be distributed by NSIDC, which serves as one of eight Distributed Active Archive Centres funded by NASA to archive and distribute data from NASA's past and current satellites and field measurement programs. The Mosaic map is available through a user-friendly zoom-in Web interface that brings together previous maps, such as those from RADARSAT, with new data in different contrast settings that draw out hard-to-see features. The NASA-funded interface was developed at the University of New Hampshire by Mark Fahnestock and Norman Vine.

The digitally smoothed red-light images are available via FTP at two spatial grid scales: 750 m (112 MB) and 125 m (4 GB), and via a Web-based map server capable of creating manually-selected JPEG images...

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Blobrana


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Ohio State University scientists used tiny gravity changes to produce the best map yet of ocean tides flowing beneath two large Antarctic ice shelves.

Large tides flow along the Antarctic ocean floor beneath the Larsen and Filchner-Ronne Ice Shelves, but scientists have been unable to model the tides that affect melting or freezing and, therefore, global sea levels.
The ice is one and a half kilometres thick in parts, and the tides are so large they can lift the shelves -- with a combined area bigger than the state of California -- by as much as 4.5 metres. Large portions of the two ice shelves float on the water, so the rise and fall of the ice with the tides prevents precise measuring of ice thickness.
Using the twin satellites of the Gravity Recovery and Climate Experiment, a joint NASA-German Aerospace Centre project, the Ohio State University researchers produced what they believe to be extremely accurate data.

Shin-Chan Han, a research scientist in the Ohio State University School of Earth Sciences presented the study this week during the American Geophysical Union meeting in San Francisco.

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Blobrana


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New results shed light on how Antarctica became the icy, barren continent that we know today. British Antarctic Survey (BAS) scientists have discovered that 30-50 million years ago, South America and Antarctica split apart very rapidly. This formed the Drake Passage and resulted in a major global cooling. The findings are published in the latest issue of Earth and Planetary Science Letters.



"we deciphered the remarkable ‘herringbone’ pattern of ridges that were etched into the Earth’s crust beneath the remote Weddell Sea when South America moved away from Antarctica. This revealed that the two continents separated extremely quickly in geological time forming a shallow ‘gateway’ between the Pacific and Atlantic oceans. We estimate that this happened some ten to twenty million years earlier than the previous oldest estimate. Even a shallow (less than 1000 metres) gateway would have had a profound effect on Southern Ocean circulation and subsequently climate" - Dr Roy Livermore, lead author.

Such a gateway, by completing a circuit of water around Antarctica, eventually led to the formation of the Antarctic Circumpolar Current, the world’s largest deep current which now transports some 130 million cubic metres of water through the Drake Passage every second. The effect was to cut Antarctica off from warm southward flowing currents leaving it frozen and desolate.

This new research reinforces findings from deep-sea sediments cores taken from the Southern Ocean and supports the theory that the opening of the Drake Passage could have triggered the abrupt global cooling event and extensive growth of the Antarctic ice sheet 33-34 million years ago.

Source

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