[meteorite-list] December Geology Media Highlights

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Nov 30 15:45:36 2004
Message-ID: <200411302045.MAA28842_at_zagami.jpl.nasa.gov>

http://www.eurekalert.org/pub_releases/2004-11/gsoa-dgm113004.php

Public release date: 30-Nov-2004

Contact: Ann Cairns
acairns_at_geosociety.org
303-357-1056
Geological Society of America

December Geology media highlights

Boulder, Colo. - The December issue of GEOLOGY covers a wide variety of
potentially newsworthy subjects. Topics include: impact of shifts in the
North Atlantic current on European climate; new method for estimating
elevations of Earth's ancient land surfaces; evidence of terrestrial
causes of the Permian-Triassic mass extinction; evidence of a major
Precambrian asteroid impact in northwestern Australia; the relationship
of intensified hydrologic cycles and global heat transfer during
greenhouse phases of Earth's history; and insights into Martian
mineralogy based on experiments involving weathering of iron phases in
the Martian atmosphere.

Highlights are provided below. Representatives of the media may obtain
complimentary copies of articles by contacting Ann Cairns at
acairns_at_geosociety.org <mailto:acairns@geosociety.org>. Please discuss
articles of interest with the authors before publishing stories on their
work, and please make reference to GEOLOGY in articles published.
Contact Ann Cairns for additional information or other assistance.
Non-media requests for articles may be directed to GSA Sales and
Service, gsaservice_at_geosociety.org <mailto:gsaservice@geosociety.org>

[snip]

Iridium anomalies and shocked quartz in a Late Archean spherule layer
from the Pilbara craton: New evidence for a major asteroid impact at 2.63 Ga
Birger Rasmussen, University of Western Australia, School of Earth and
Geographical Sciences, Perth, Western Australia 6009, Australia; and
Christian Koeberl, University of Vienna, Department of Geological
Sciences, Vienna A-1090, Austria. Pages 1029-1032.

Several spherule layers in South Africa and Australia, with ages of ca.
3.4 to 2.6 Ga, have been interpreted as the result of large asteroid or
comet impacts onto the early Earth. Some of these spherule layers show
extreme enrichments in the PGEs, unlike modern ejecta deposits, which
caused some questions regarding the initial impact interpretation. On
the other hand, until now, no shocked minerals-the hallmark for all
confirmed impact structures and ejecta-have been found in any of these
spherule layers. Even rocks from the 2 Ga Vredefort impact structure
contain abundant shocked minerals, so it is unlikely that Archean
impacts would, for some reason, not produce shocked minerals. In the
present work, Rasmussen and Koeberl document, for the first time, the
presence of shocked quartz in a sample of the ~2.63 Ga spherule layer
from the Jeerinah Formation (Pilbara craton, northwestern Australia).
The survival of shocked quartz in ~2.63 Ga rocks, which have undergone
multiple metamorphic events, suggests that their absence in other impact
ejecta layers may not only be a question of preservation. The presence
of shocked quartz in a layer containing melt spherules provides
compelling evidence for an extraterrestrial impact with a target area
that was at least partly silicic, favoring a continental impact site. In
addition, enrichments in Ir and other siderrophile elements in the
spherule layer indicate that they contain as much as 2-3 wt% of a
chondritic meteorite component. If proposed correlations between the
Australian spherule layer and similar South African layers are correct,
then the combined ejecta blanket represents fallout from a single major
impact with an areal distribution of >32,000 km2, which is among the
largest yet documented in the Precambrian rock record. Thus so far the
impact record on Earth is quite limited: nothing for the first billion
years, then some spherule layers until about 2.5 Ga, and then some
impact craters. Nevertheless, the discovery of these spherule layers
aids in the discussion of the importance of impact events in the early
parts of Earth's history. Clearly the "early" impact record on Earth,
which spans more than half of the age of our planet, is still a
wide-open field of research.

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Weathering of iron rich phases in simulated Martian atmospheres
Vincent Chevrier, Univ. Aix-Marseille 3, Aix en Provence - 13545,
France; et al. Pages 1033-1036.

We report about experimental weathering of iron phases (metal, sulfide,
oxide) in carbon dioxide and water vapor, i.e., present day Martian
atmosphere. Such experiments have never been conducted. We evidence that
the neoformed mineral phases are similar to the ones invoked in the
Martian regolith according to spatial probes data (including the MER
rovers). Therefore the occurrence of such phases (carbonate, sulfate,
oxyhydroxides) can no longer be taken as an evidence for formation of
these minerals in presence of liquid water and oxidants not present
today (oxygen, acids, etc.). Moreover these results emphasize the key
contribution of "extramartian" materials (metal and sulfide due to
meteorite bombardment) in the mineralogy of the Martian regolith. This
contribution should be of wide interest for the present debate on the
processes having affected the Martian surface, and thus on the
interpretation of present rover data (Spirit and Opportunity) and on the
question of life on Mars.

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Evidence for sulfidic deepwater during the Late Permian in the East
Greenland Basin
Jesper K. Nielsen, University of Troms?, Department of Geology, Troms?,
Norway; and Yanan Shen, Harvard University, Botanical Museum, Cambridge,
MA 02138, USA. Pages 1037-1040.

The most severe biological disaster over Earth's history occurred around
the Permian-Triassic (P-Tr.) boundary of 251 million years ago. The
causes of this biological crisis have been greatly debated. In order to
find the killers of the P-Tr life, we went to the East Greenland Basin
where the sedimentary rocks across the P-Tr. boundary are perfectly
preserved. In the laboratory, we measured diameters of framboidal
(strawberry-shaped) pyrite crystals from the Late Permian black shales.
We discovered a remarkable size difference of framboidal pyrite between
bioturbated shales and laminated organic-rich black shales. In
bioturbated shales, the larger (>5 ?m) and wider distribution of
framboidal pyrites clearly indicates oxic bottom water conditions. By
contrast in laminated shales, the smaller (<5 ?m) and less variable
distribution of framboidal pyrites provides compelling evidence for
sulfidic deepwater during the Late Permian in the East Greenland Basin.
In combination with S-isotope data, our geological and geochemical study
indicates that the Late Permian oceans were quite similar to the modern
Black Sea whose deepwater is enriched in malodorous H2S gas. We believe
that high levels of noxious H2S gas in deepwater of Late Permian oceans
severely restricted the ecological space available for animal survival.
This oceanic H2S gas could have caused the P-Tr mass extinction if
H2S-enriched waters had reached shallow water regions triggered by
oceanographic processes and if H2S gas outgassed in to the atmosphere,
to affect the marine and terrestrial environments.

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[snip]

Geochemistry of the end-Permian extinction event in Austria and Italy:
No evidence for an extraterrestrial component
Christian Koeberl, University of Vienna, Department of Geological
Sciences, Vienna A-1090, Austria; et al. Pages 1053-1056.

The Permian-Triassic (P-Tr) boundary is associated with the largest mass
extinction known in Earth history. Following the association of the K-T
boundary mass extinction with a large impact event, speculations bloomed
that other major mass extinctions might also be related to impact
events. However, so far the evidence in favor of such a proposal is
controversial. Siderophile element anomalies (e.g., enhanced Ir
contents) were found at some P-Tr boundary locations; their presence was
also confirmed in the present work from elemental and isotopic analysis
(using platinum group element abundances and osmium isotopes), but
Koeberl et al. clearly showed that purely terrestrial processes were at
work in concentrating these rare metals, and that there is no evidence
for an extraterrestrial component. The present work also indicates that
there are no traces the extraterrestrial helium-3 isotope, the alleged
presence of which in the so-called fullerenes (large carbon molecules)
has been the subject of a lot of debate. The sparse evidence for impact
has recently resulted in suggestions of a possible buried underwater
impact structure near Australia (Becker et al., Science May 2004), but
it became rapidly clear that the evidence for the existence of such an
impact structure (and any age information) is tenuous at best. On the
other hand, recent research (e.g., Mundil et al., Science 305, 1760,
2004) succeeded in demonstrating that the P-Tr boundary event is exactly
synchronous with the Siberian flood volcanism, indicating a causal link.
This was also supported by sulfur isotope data for P-Tr samples,
indicating a volcanic source (Maruoka et al., EPSL 206, 101, 2003). The
work by Koeberl and coworkers provides, for the first time, clear
evidence that most of the alleged indicators for an extraterrestrial
signature are, in fact, of purely terrestrial origin.

[snip]

###

To view the complete table of contents for the December issue of
GEOLOGY, go to
http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.


Geological Society of America
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Boulder, CO 80301-9140, USA
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Received on Tue 30 Nov 2004 03:45:29 PM PST