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RE: Antarctic Impact
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Asteroid impacts in Antarctica

Gravity anomalies in Antarctica might mark spots where hundreds of thousands of years ago the frozen continent was struck by giant meteorites or fragments of a comet.
These, scientists suggest, would have punched deep into the crust, shattering rock and producing zones of slightly lower gravity waiting to be found by modern instruments.
Such instruments, carried aboard satellites, aeroplanes, and snow machines, can measure the Earth's gravity field with remarkable precision.

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Title: Microtektites from Victoria Land Transantarctic Mountains
Authors: L. Folco, P. Rochette, N. Perchiazzi, M. D'Orazio, M.A. Laurenzi, and M. Tiepolo

We report on the discovery of a microtektite (microscopic impact glass particles) strewn field from the Victoria Land Transantarctic Mountains, Antarctica. Microtektites were found trapped in the local detritus accumulated in weathering pits and in joints of several glacially eroded summits (~2600 m above sea level [asl]) distributed latitudinally for 520 km. Their physical and chemical properties define a coherent population with a geochemical affinity to Australasian microtektites and compatible Quaternary 40Ar-39Ar formation age. We therefore suggest that Transantarctic Mountain microtektites (TAMM) define the southern extension of the Australasian strewn field. The margin of the Australasian strewn field is thus shifted southward by ~3000 km and the maximum distance from the putative parent impact site in Indochina by ~2000 km. This emphasises the paradox of the missing parent crater of the largest (>10% of the Earth's surface) and youngest tektite strewn field discovered on Earth. Furthermore, TAMM are depleted in volatile elements (i.e., Pb, Na, K, Rb, Sr, Rb, and Cs) relative to Australasian ones, suggesting a possible relationship between high-temperaturetime regimes in the microtektite-forming process and high-angle trajectories in the ejecta plume.

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Ancient meteoritic events
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Antarctic ice cores record ancient meteoritic events
Antarctica represents the best site to collect small meteoritic particles because windblown terrestrial dust is scarce and extreme environmental conditions prevent chemical weathering. Although many investigations have searched for micrometeorites deposited within modern times, few have looked for them within ancient ice. As part of a new micrometorite collection project launched at the permanent French-Italian station of Concordia, on the East Antarctic Plateau, Narcisi et al. study the Dome C ice core, a core collected by the European Project for Ice Coring in Antarctica. They find two distinct dust layers which, through comparisons with extraterrestrial debris found in deep-sea sediments and polar caps, they determined to be of meteoritic origin. Closer inspection revealed that these layers represented individual meteoritic events, with the first occurring about 481,000 years ago and the second occurring 434,000 years ago, as indicated by layers in the ice core. The authors note that, similar to ashfall from a known volcanic explosion, such meteoritic events have the potential to serve as time markers in other less detailed stratigraphic records.

Title: First discovery of meteoritic events in deep Antarctic (EPICA-Dome C) ice cores
Authors: Biancamaria Narcisi,  Jean Robert Petit,  Cécile Engrand

Two distinct dust layers in the EPICA-Dome C ice core (75°06S, 123°21E, East Antarctic Plateau) have been shown to relate to individual meteoritic events. Particles forming these layers, investigated by electron microprobe, show peculiar textural, mineralogical and geochemical features and closely resemble extraterrestrial debris in deep-sea sediments and polar caps. Preliminary estimates of cosmic debris input at the studied layers, obtained from Coulter Counter measurements, are 45 orders of magnitude greater than the yearly micrometeorite flux in East Antarctic snow and ice. The cosmic events are accurately dated through glaciological models at 434 ± 6 and 481 ± 6 ka, respectively and are located in the core climatic stratigraphy near the Mid-Brunhes Event. This is the first report of well-dated cosmic horizons in deep Antarctic ice cores. It significantly improves the extraterrestrial record of Antarctica and opens new correlation perspectives between long climatic records of the South polar region.

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Wilkes Land crater
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In June 2006, researchers at Ohio State University announced that NASA’s Gravity Recovery and Climate Experiment (GRACE) had found a mass concentration under the ice in East Antarctica.

antarcticagravity
Expand (128kb, 560 x 603)
Credit Ohio State University

This image shows GRACE’s measurements of gravity, indicated in galileos. Areas with more intense gravity appear in red, and areas with less intense gravity appear in blue. Yellow and green show levels in between these extremes. Not far from the coast is a 320-kilometer-wide mass concentration, outlined in white. This spot attracted the researcher's attention.
The mass concentration GRACE detected indicates an area containing unusually dense material. Such mass concentrations can result from more than one cause. One possible explanation is an upwelling of volcanic rock from deep within Earth’s crust. Ralph von Frese, a geology professor at Ohio State University, proposed that the mass concentration may have resulted from an asteroid impact.

GRACE’s discovery is just the first step in understanding this mass concentration. More clues can come from a visit to the site, as well as chemical analyses that provide a geologic age for the rocks in that area. If the rocks date to the same time as the Permo-Triassic extinction, GRACE may have made an important discovery about the history of life (and death) on Earth.

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Antarctic Meteorites
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U.S. Regulations Governing Antarctic Meteorites

NSF is issuing a final rule that authorises the collection of meteorites in Antarctica for scientific research purposes only. In addition, the regulations provide requirements for appropriate collection, handling, and curation of Antarctic meteorites to preserve their scientific value. These regulations implement Article 7 of the Protocol on Environmental Protection to the Antarctic Treaty and are issued pursuant to Section 6 of the Antarctic Conservation Act, as amended by the Antarctic Science, Tourism and Conservation Act of 1996.

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The researchers have put rough bounds on the date of the crater. The crater is cut by the rift which opened to form the Indian Ocean, meaning it must be older than 100 million years.
And impact craters older than the start of the Cambrian geological period – 543 million years ago – no longer have gravity anomalies. The crust has eroded and crept in to fill in the depression
In between those two dates, 250 million years ago, the end of the Permian period marked the greatest die-off of species ever seen.

However the gravity anomaly could well be a volcanic feature - a large caldera.

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Wilkes Land crater
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An large crater has been found in Antarctica. Scientists think it was carved by a space rock that caused the greatest mass extinction on Earth, 250 million years ago.

The crater, buried beneath 1 kilometre of ice and discovered by some serious airborne and satellite sleuthing, is more than twice as big as the one involved in the demise of the dinosaurs.
The crater's location, in the Wilkes Land region of East Antarctica, south of Australia, suggests it might have instigated the break-up of the so-called Gondwanaland supercontinent, which pushed Australia northward, the researchers said.

"This Wilkes Land impact is much bigger than the impact that killed the dinosaurs, and probably would have caused catastrophic damage at the time" - Ralph von Frese, a professor of geological sciences at Ohio State University.

060601_crater_radar_02

Airborne radar image of land elevation in East Antarctica. Higher elevations appear red, purple, and white; the location of the Wilkes Land crater is circled (above center). Image courtesy of Ohio State University . An inset of the Chicxulub crater is included for comparison.
Credit: Ohio State University


The crater is about 500 kilometres wide. It was found by looking at differences in density that show up in gravity measurements taken with NASA's GRACE satellites. Researchers spotted a mass concentration, which they call a mascon — dense stuff that welled up from the mantle, likely in an impact.

"If I saw this same mascon signal on the moon, I'd expect to see a crater around it. And when we looked at the ice-probing airborne radar, there it was" - Ralph von Frese. (The moon, with no atmosphere, retains a record of ancient impacts in the visible craters there.)

060601_crater_gravity_02

This colour-coded map shows gravity fluctuations beneath East Antarctica, as measured by the GRACE satellite. Denser regions are redder; the location of the Wilkes Land crater is circled in yellow.
So Frese and colleagues overlaid data from airborne radar images that showed a 300-mile-wide subsurface, circular ridge. The mascon fit neatly inside the circle.
Credit Ohio State University


The Permian-Triassic extinction, as it is known, wiped out most life on land and in the oceans. Researchers have long suspected a space rock might have been involved. Some scientists have blamed volcanic activity or other culprits.
The die-off set up conditions that eventually allowed dinosaurs to rule the planet.
The newfound crater is more than twice the size of the Chicxulub crater in the Yucatan peninsula, which marks the impact that may have ultimately killed the dinosaurs 65 million years ago. The Chicxulub space rock is thought to have been 10 kilometres wide, while the Wilkes Land meteor could have been up to 50 kilometres wide, the researchers said.

Postdoctoral researcher Laramie Potts assisted in the discovery.

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See recent meteor article

http://researchnews.osu.edu/archive/erthboom.htm

-- Edited by Blobrana at 15:42, 2006-06-02

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RE: Antarctic Impact
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Evidence for a Possible Late Pliocene Impact in the Ross Sea, Antarctica.
Gerard-Little, P., D. Abbott, D. Breger, and L. Burckle.



This map shows the locations of the Eltanin cores within the Ross Sea, Antarctica.
The larger dots: cores in which impact glass was found.
Smaller, lighter dots within the larger dots: cores in which microtektites were discovered. Based on biostratigraphic work on diatoms, the inferred age of the impact layer is late Pliocene. The cores themselves have basal ages in the Gauss

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Evidence for what may be a large and relatively recent impact crater has been found off the coast of Antarctica.

Scientists say the evidence, if correct, points to a space rock some 5km across having crashed into the Ross Sea about three million years ago.
This could have generated a huge tsunami, according to a member of the team investigating the collision.
Details were reported at the Lunar and Planetary Science Conference in Houston, Texas.

Researchers from the Lamont-Doherty Earth Observatory in New York have been studying a 100km-wide depression, known as Bowers Crater, under the Ross Sea.
Team members examined cores drilled from around the area to look for evidence of an impact.
In the cores, they found microscopic glassy grains shaped like teardrops, spheres and dumbbells which are collectively known as tektites.
Some scientists believe these are created when rock fragments are hurled high up into the atmosphere by the impact of a large meteoroid or asteroid, and then partially re-melt as they fall back to the ground.

Other glasses were also found. These are thought to have been formed by cooling of the melted rock and sediment. Similar glasses can be formed through volcanism, but the Ross Sea specimens seem to have a distinct structure under the microscope.

The findings alone do not prove there was an impact in the area a few million years ago, but team member Dallas Abbott says she hopes to search the core material further for a mineral called shocked quartz.
This type of quartz can be distinguished from normal quartz by characteristic lines visible under the microscope which are thought to be formed by the intense pressure of an impact.
The presence of this mineral is considered most diagnostic of a space collision.

Source BBC.

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An international scientific-mountaineering team discovered in 2004 what appears to be one of the world’s largest meteoroid impact craters.
Prof Van der Hoeven first realised that there may have been a giant asteroid strike in the Antarctic while on an expedition across the continent in 1960 when he noticed severe anomalies in the gravity from the rocks below, indicating a crater. Located in Victoria Land, Antarctica beneath more than two thousand meters of ice, the subglacial topographic feature, with a minimum of two hundred and forty kilometres in diameter, was plotted using field gravity survey techniques, by the Champs GPS satellite.


The gravity anomaly amplitudes range between -30 mGal (magenta) and +30 mGal (red), and are "illuminated " by a light source from 90E. Because short wavelength (less than 200 km) gravity anomalies are highly correlated with seafloor topography, fine-scale features of the ocean floor are revealed.
The anomalies for the Antarctic continent range from -57 to 65 mGal

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The Champs program uses a single satellite to measure gravity field changes. The satellite's orbit fluctuates as it orbits the earth, moving higher in regions where earth's gravity is weak and lower where it's stronger. The satellite can locate its position relative to the earth to within a few centimetres by acquiring GPS signals from 12 GPS satellites simultaneously.
The gravity anomaly data revealed a continent-wide negative gravity anomaly field of numerous very large negative gravity anomalies in the Ross Sea in the vicinity of the Pacific Ocean, in central East Antarctica, and extending to the Weddell Sea adjacent to the Atlantic Ocean. While most of these gravity anomaly sites lie beneath the East Antarctic continental ice sheet, and are not presently accessible to direct observation, those offshore beneath the Ross and Weddell Seas are likely to be accessible to future direct observation from oceanographic research platforms.


Weddell Sea

An asteroid between 5 - 11 km across had broken up in the atmosphere and five large pieces had hit the Earth, creating multiple craters over an scatter ellipse area.
Scatter ellipses such as this accompany all such multiple impact sites, except that the Antarctic ellipse is the largest known on Earth. Of the five new impact craters, three of them are on the continental land mass and two more are in the Weddell Sea. The largest of these craters is about 322km by 322km.
The Antarctic scatter ellipse is of enormous size by Earth standards, measuring some 2,092 kilometres by 3,862 kilometres. Melted rocky debris, blasted from such meteoroid craters upon impact and explosion, and known as tektites, may have been carried thousands of kilometres from the impact site. Such tektites, called australites, are found in large strewn fields in Australia some 5600 kilometres from the largest proposed impact sites in the Ross Sea region.


Victoria Land

The scientists told that the impacts occurred roughly 780,000 years ago during an ice age. When the impacts hit, they would have melted through the ice and through the crust below. The mapping showed that the holes in the rock created by the strike had refilled with a mixture of ice, rock and other debris far less dense. This material, called breccia, shows where and how deep the craters are.
The impact would have melted about 1% of the ice sheet, raising water levels worldwide by 60cm.
Early humans would have been living in Africa and other parts of the Old World at the time of the strikes. The early Pleistocene extends to the Brunhes-Matuyama paleomagnetic boundary.

"The extraordinary thing about this meteor strike is that it appeared to do so little damage. Unlike the dinosaur strike there is no telltale layer of dust that demonstrates the history of the event. It may have damaged things and wiped out species but there is no sign of it" - Prof Van der Hoeven

One thing that happen at exactly the same time was the reversing of the Earth's magnetic field. This may just have been a coincidence, though Prof Van der Hoeven believes it was caused by the impact.

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Abstract
Morphological and mineralogical information indicates that there are two structures on the Ross Sea shelf which may be of a meteoritic origin. The crater has a diameter of about 100 km and the wall rises as much as 800 m above the bottom. The Bauers annular structure corresponds to a concentric system of gravity anomalies with positive delta g values for horsts and negative values for grabens, as is typical for meteoritic structures. The clear expression of the crater in the relief, the fact that it is superposed on the present-day continental slope and is filling with thick strata of recent sediments are indicative of its youth. A meteorite explosion logically explains a number of contradictory facts concerning the geological history of the Ross Sea, especially the 300 km northward jump of the boundary of iceberg drift and cold-loving fauna in the ocean and the simultaneous interglacial and lush development of fauna in the Ross Sea 5 to 3 million years ago. After formation of the crater the waters of the Ross Sea for some time became warmer and the surrounding ocean became colder due to the thawing of ice transported from the Ross Sea ice shelf.



Abstract
In general, sedimentary basins are characterised by negative free-air and Bouguer gravity anomalies. However, the extensional basins of the Ross Sea are paradoxical in that positive gravity anomalies overlay the Victoria Land Basin, Northern Basin, Central Trough and Northern Central Trough while basement highs are associated with negative gravity anomalies. Measured basement densities from DSDP basement cores give values between 2600–2800 kg/m3 while bulk sediment densities range from 1210–2200 kg/m3, indicating a normal density relationship between basement and sediment infill. In contrast, the relatively young and narrow Terror Rift is associated with negative free-air and Bouguer gravity anomalies, but has a different geological history as compared to the larger Ross Sea basins. Process-oriented gravity modelling indicates that magmatic underplating and crustal intrusions are inconsistent with the observed gravity and basement geometry of the Ross Sea basins. The magma volume necessary to account for the distribution and amplitude of the positive gravity anomaly of the Central Basin and be isostatically balanced would need to be comparable to the tholeiitic flood basalt volume of the Columbia River province—it is thus unlikely that the volume of Neogene volcanics of the Ross Sea region is sufficient to explain the observed gravity relationship by modifying the bulk density of the crust.

We demonstrate that positive free-air and Bouguer gravity anomalies over extensional basins are the consequence of a relatively low flexural strength of the lithosphere during rifting being contrasted by higher flexural strengths later during sedimentation. As the difference between the rigidity of the lithosphere during sedimentation increases relative to the rigidity of the rifted lithosphere, the gravity over the basin becomes progressively more positive but only for a limited range of wavelengths. The narrow width of the Terror Rift precludes it from having a positive gravity anomaly while the opposite is true for the large Ross Sea basins. For the Ross Sea region, such a loading scenario requires a significant delay between extension and the timing of sediment infilling of the basins, consistent with the late Cretaceous extension of the Ross Sea region and the sedimentary succession being dominated by large-scale late Eocene–Neogene glaciogenic progradational sequences. Sediment source was presumably from the denudation of the Transantarctic Mountains, which commenced in the late Palaeogene. The time delay between the late Cretaceous formation of the Transantarctic Mountains, late Palaeogene exhumation, and the generation of significant Palaeogene paleobathymetry requires either the Ross Sea region to be sub-aerial and sediment starved for most of the Palaeogene and/or the Palaeogene climate was ineffective in producing clastics until the onset of glaciation in the late Eocene–early Oligocene.

Source


Dr Weihaupt and Dr. Van der Hoeven

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