PLEASE NOTE:
*
CCNet DIGEST, 7 May 1999
------------------------
(1) PRIMORDIAL DEPLETION OF THE STABLE REGION BETWEEN THE EARTH
AND MARS
A. Morbidelli and Jean-Marc PETIT <petit@obs-nice.fr>
(2) RESPONSE to MORBIDELLI AND PETIT
Wyn Evans <w.evans1@physics.ox.ac.uk>
and
Serge Tabachnik <serge@thphys.ox.ac.uk>
(3) CRATERS SUGGEST HOW VENUS LOST HER YOUTH
INSCiGHT, 5 May 1999
(4) OERSTED AND POSSIBLE DETECTION OF NEW IMPACT CRATERS
Holger Pedersen <holger@astro.ku.dk>
(5) THE AGE OF ASTRONOMY-RELATED ORGANISATIONS
A. Heck, ASTRONOMICAL OBSERVATORY STRASBOURG
(6) PALEOECOLOGY OF CAMBRIAN MASS EXTINCTION
S.R. Westrop & M.B. Cuggy, UNIVERSITY OF
OKLAHOMA
(7) PATTERNS OF MASS ORIGINATIONS AND MASS EXTINCTIONS
D. Hewzulla et al., UNIVERSITY OF EAST LONDON
(8) COSMIC INFLUENCE ON EARTH'S CLIMATE AND THE ORIGIN OF THE
OCEANS
D. Deming, UNIVERSITY OF OKLAHOMA
(9) THE ROLE OF CLIMATE IN PALEOECOLOGY AND ITS IMPLICATIONS FOR
THE
K/T MASS EXTINCTION
P.J. Markwick, ROBERTSON RESEARCH INT LTD
(10) THE MAN WHO CHANGED THE FIELD OF PALEONTOLOGY
The New York Times, May 6, 1999
(11) J. JOHN SEPKOSKI JR., 50, PALEONTOLOGIST AT U. OF C.
Chicago Tribune, May 6, 1999
====================
(1) PRIMORDIAL DEPLETION OF THE STABLE REGION BETWEEN THE EARTH
AND
MARS
From Jean-Marc PETIT <petit@obs-nice.fr>
A. Morbidelli and J-M. Petit
CNRS/OCA, Nice, France
Concerning the paper POSSIBLE LONG-LIVED ASTEROID BELTS IN THE
SOLAR
SYSTEM by N. Wyn Evans and Serge Tabachnik published in NATURE (6
May
1999), we remark that there are other regions of stable orbits
with
small eccentricity and small inclination throughout the solar
system
which are depleted of bodies (in the outer asteroid belt for
instance).
All the known populations of small bodies of the solar system
have been
(somehow) dynamically excited during the primordial evolution.
Such
excitation explains why stable niches at low ecentricity and low
inclination have been depleted of their potential initial
populations.
We therefore believe that it is implausible that bodies could
have
survived in the stable 'belt' found between the Earth and Mars.
The
stability result by Evans and Tabachnik is valid in the framework
of
the present Solar System, but not in the framework of its
primordial
violent evolution.
We remark also that in the case of the Vulcanoids, the region of
space
is so small and the dynamical time scale is so short that any
sizeable
population of bodies would have been grinded to dust by mutual
collisions as shown by Leake et al., Icarus 71, 350 (1987).
====================
(2) RESPONSE to MORBIDELLI AND PETIT
From Wyn Evans <w.evans1@physics.ox.ac.uk>
and
Serge Tabachnik <serge@thphys.ox.ac.uk>
Our Nature paper is carefully worded -- all it aims to do is
provide
the results of a lengthy calculation and an analysis of such
asteroidal
data that is available to us. It is not a proof, just a
suggestion. The
paper will have served its purpose if it focuses attention and
observational activity on these interesting areas of the Solar
System.
It is the observational evidence that will settle this matter, of
course.
The assumptions underlying our calculation are naturally open to
question (as is true of all scientific work) -- we merely remark
that
they are much the same as the assumptions used by Holman in his
seminal
calculation suggesting a belt between Uranus and Neptune as well
as by
Mikkola, Innanen, Duncan, Gladman and numerous others in their
studies
of test particle evolution in the inner and outer Solar System
(the
references to the individual papers are given in our Nature
article,
although most of the papers will be familiar to
subscribers). While
being far from conclusive, these kinds of calculations seem to us
to be
worth pursuing.
Vulcanoids have been suggested by others before (e.g.,
Weidenschilling
et al). The possiblity of their existence has been raised again
this
year, not just by ourselves, but also by Namouni, Christou and
Murray.
Morbidelli and Petit are right to point to short dynamical times
as
being a matter of serious concern. How damaging this really is
depends
on the number of objects, their distribution of sizes and so on
-- so
that a proper calculation of the collisional evolution is needed.
Our
understanding is that these kinds of simulations are being
performed by
Stern and collaborators at the SWRI at the moment.
Morbidelli and Petit refer to caculations peformed by Leake et
al.,
Icarus 71, 350 (1987). We urge the subscribers to look at the
paper
directly. At the end of their thoughtful investigation, Leake et
al.
conclude "We cannot say definitely whether any vulcanoids in
fact
existed, although they might have". We agree entirely!
====================
(3) CRATERS SUGGEST HOW VENUS LOST HER YOUTH
From INSCiGHT, 5 May 1999
[http://www.academicpress.com/inscight/05051999/grapha.htm]
Craters Suggest How Venus Lost Her Youth
By Richard A. Kerr
After the Magellan spacecraft flew by Venus in the early 1990s,
some
researchers came to an astonishing conclusion: The planet's
volcanoes
had shut down in a geological blink of an eye. Now an analysis of
Magellan radar images suggests that Venus's fiery youth probably
ended
much more gradually, as geophysicists would expect.
The ancient lava flows of Venus's plains are pocked by half a
billion
years' worth of impact craters. Earlier analyses of the Magellan
images
seemed to show that only 5% to 10% of craters had been flooded by
lava,
suggesting that the venusian volcanoes had shut off like a
faucet.
Entire planets weren't supposed to do that, but geophysicists
soon
managed to come up with any number of theories to explain it,
from a
sudden freezing of the surface to cyclic sinking of crustal
plates
(Science, 5 March 1993, p. 1400).
Yet many craters did look as if lava had smoothed the crater
floors,
making them dark in radar images. That lava was usually presumed
to be
rock melted on impact. In work presented at the recent Lunar and
Planetary Science Conference in Houston, two researchers --
planetary
geologist Robert Herrick of the Lunar and Planetary Institute in
Houston and Virgil Sharpton of the University of Alaska,
Fairbanks --
decided to test that assumption by measuring how deeply lavas
might
have flooded outside the craters as well as inside them. On a
limited
number of orbits, the Magellan radar imaged the same crater from
two
different angles, so the pair of images could create a
three-dimensional stereo image. That allowed the researchers to
measure, for 70 craters, the relative heights of each crater's
rim,
floor, and surrounding terrain.
With this 3D perspective, Herrick and Sharpton say they can see
lava
not only filling crater interiors but also flooding around
seemingly
pristine craters. On average, both the crater floors and
surrounding
terrain are higher, relative to crater rims, than the floors or
surroundings of bright-floored craters. "That must mean
these things
are not only being filled on the inside, but they're being
surrounded
on the outside too," says Herrick, implying that Venus's
volcanoes did
indeed ooze lava during the past 500 million years.
The idea that global volcanic activity shut off in just 10
million or
even 100 million years "is clearly wrong," says
Herrick. Most
researchers aren't so convinced. However, says planetary radar
specialist Ellen Stofan of University College London and the Jet
Propulsion Laboratory in Pasadena, California, "I think
they're on the
right track."
© 1999 The American Association for the Advancement of Science
====================
(4) OERSTED AND POSSIBLE DETECTION OF NEW IMPACT CRATERS
From Holger Pedersen <holger@astro.ku.dk>
Dear Dr.Peiser,
It may interest you to see that early results from the Danish
OERSTED
satellite give indication for a surprisingly detailed structure
of the
Earth magnetic field strength, caused by iron in the crust. One
feature
at Kiruna, Sweden, is obviously due to the late deposit of iron
ore
there. Another structure, at Bangui, Central African Republic, is
hypothesized to be an ancient meteorite impact.
My only source is the Danish-language home page NYT FRA RUMMET,
i.e.
'news from space' (here may well be other sources of information,
including ones in English):
http://www.rummet.dk/2_nyheder/2_SidsteNyt/ny_jernmalm/body_ny_jernmalm.html
It appears that the OERSTED results communicated on the Danish
language
home page largely confirm earlier results (from other geomagnetic
satellies) on http://denali.gsfc.nasa.gov/research/crustal_mag/prep/
(I am indepted to Nils Olsen, Danish Space Research Institute for
this information).
Please note that I am not a spokesperson for the OERSTED
satellite - I
just happened to stumble over this report.
Sincerely,
Holger Pedersen
================
(5) THE AGE OF ASTRONOMY-RELATED ORGANISATIONS
A. Heck: The age of astronomy-related organizations. ASTRONOMY
&
ASTROPHYSICS SUPPLEMENT SERIES, 1999, Vol.135, No.3, pp.467-475
ASTRONOMICAL OBSERVATORY,11 RUE UNIV,F-67000 STRASBOURG,FRANCE
The age of currently active astronomy-related organizations is
investigated from comprehensive and up-to-date samples. Results
for
professional institutions, associations, planetariums, and public
observatories are commented, as well as specific distributions
for
astronomy-related publishers and software producers. Some events
had a
clear impact on the rate of foundation of astronomy-related
organizations, such as World War I and II, the beginning of space
exploration and the landing of man on the Moon, but not all of
them
affected in the same way Western Europe and North America. It is
still
premature to assess the impact of the end of the Cold War. A
category
such as the software producers would of course not exist nor
prosper
without the advent of the computer age and the subsequent
electronic
networking of the planet. Other aspects are discussed in the
paper.
Copyright 1999, Institute for Scientific Information Inc.
============
(6) PALEOECOLOGY OF CAMBRIAN MASS EXTINCTION
S.R. Westrop & M.B. Cuggy: Comparative paleoecology of
Cambrian
trilobite extinctions. JOURNAL OF PALEONTOLOGY, 1999, Vol.73,
No.2,
pp.337-354
UNIVERSITY OF OKLAHOMA,OKLAHOMA MUSEUM NAT HIST,NORMAN,OK,73019
Analysis of 164 collections from shelf facies of Laurentian North
America indicates that three successive trilobite mass
extinctions at
Late Cambrian stage boundary intervals ('biomere' boundaries) are
characterized by a common pattern of change in distributional
paleoecology and species diversity. In all cases, the extinction
intervals are marked by a shift to biofacies that have broader
environmental distributions than those prior to the onset of
extinctions, implying a reduction in between-habitat (beta)
diversity.
Significant declines in within-habitat (alpha) diversity also
characterize each extinction and the compositions of shelf
biofacies
record extensive immigration of taxa from off-shelf and
shelf-margin
sites. The nature and extent of ecologic disruption of the shelf
appears to be comparable to changes associated with major mass
extinctions, such as those at the end of the Ordovician and
Permian.
Unlike major mass extinctions, the Cambrian events are followed
by a
complete recovery of diversity and biofacies structure within a
few
million years. Copyright 1999, Institute for Scientific
Information
Inc.
=============
(7) PATTERNS OF MASS ORIGINATIONS AND MASS EXTINCTIONS
D. Hewzulla*), M.C. Boulter, M.J. Benton, J.M. Halley:
Evolutionary
patterns from mass originations and mass extinctions.
PHILOSOPHICAL
TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL
SCIENCES, 1999, Vol.354, No.1382, pp.463-469
*) UNIVERSITY OF EAST LONDON,PALAEOBIOL RES UNIT,LONDON E15
4LZ,ENGLAND
The Fossil Record 2 database gives a stratigraphic range of most
known
animal and plant families. We have used it to plot the number of
families extant through time and argue for an exponential fit,
rather
than a logistic one, on the basis of power spectra of the
residuals
from the exponential. The times of origins and extinctions, when
plotted for all families of marine and terrestrial organisms over
the
last 600 Myr, reveal different origination and extinction peaks.
This
suggests that patterns of biological evolution are driven by its
own
internal dynamics as well as responding to upsets from external
causes.
Spectral analysis shows that the residuals from the exponential
model
of the marine system are more consistent with 1/f noise
suggesting that
self-organized criticality phenomena may be involved. Copyright
1999,
Institute for Scientific Information Inc.
==============
(8) COSMIC INFLUENCE ON EARTH'S CLIMATE AND THE ORIGIN OF THE
OCEANS
D. Deming: On the possible influence of extraterrestrial
volatiles on
Earth's climate and the origin of the oceans. PALAEOGEOGRAPHY
PALAEOCLIMATOLOGY PALAEOECOLOGY, 1999, Vol.146, No.1-4, pp.33-51
UNIVERSITY OF OKLAHOMA,SCH GEOL & GEOPHYS,NORMAN,OK,73019
A consideration of observational and circumstantial evidence
suggests
that Earth may be subject to high influx rates (10(11)-10(12)
kg/yr) of
extraterrestrial-sourced volatile elements (carbon, hydrogen,
oxygen,
nitrogen) derived from comets or other primitive solar-system
material.
The total extraterrestrial influx rate may be four to five orders
of
magnitude greater than previously thought, large enough to
account for
today's total near-surface inventories of water and carbon. The
possibility of high rates of extraterrestrial volatile-accretion
suggests a new climatic paradigm wherein Earth's surface
temperature is
influenced by conflicting internal and external processes. A
variable
influx of volatile elements tends to warm the Earth, while
terrestrial
processes cool the planet by absorbing these gasses at a more
uniform
rate. Variations in extraterrestrial influx rates may explain the
variation of sea level and mean global temperature over geologic
time,
as well as some types of climate change, the occurrence of the
Pleistocene ice ages, and the asymmetry of the Phanerozoic
climate
record (sudden warmings, slow coolings). The extraterrestrial
influx
rate may also act as the pacemaker of terrestrial evolution, at
times
leading to mass extinctions through climatic shifts induced by
changes
in accretion rates with concomitant disruptions of the carbon and
nitrogen cycles. Life on Earth may be balanced precariously
between
cosmic processes which deliver an intermittent stream of
life-sustaining volatiles from the outer solar system or beyond,
and
biological and tectonic processes which remove these same
volatiles
from the atmosphere by sequestering water and carbon in the crust
and
mantle. (C) 1999 Elsevier Science B.V. All rights reserved.
=============
(9) THE ROLE OF CLIMATE IN PALEOECOLOGY AND ITS IMPLICATIONS FOR
THE
K/T MASS EXTINCTION
P.J. Markwick: Crocodilian diversity in space and time: the role
of
climate in paleoecology and its implication for understanding K/T
extinctions. PALEOBIOLOGY, 1998, Vol.24, No.4, pp.470-497
ROBERTSON RESEARCH INT LTD,LLANDUDNO LL30 1SA,WALES
The taxonomic diversity of crocodilians (Crocodylia) through the
last
100 million years shows a general decline in the number of genera
and
species to the present day. But this masks a more complex
pattern. This
is investigated here using a comprehensive database of fossil
crocodilians that provides the opportunity to examine spatial and
temporal trends, the influence of sampling, and the role of
climate in
regulating biodiversity. Crown-group crocodilians, comprising the
extant families Alligatoridae, Crocodylidae and Gavialidae, show
the
following trend: an initial exponential diversification through
the
Late Cretaceous and Paleocene that is restricted to the Northern
Hemisphere until after the K/T boundary; relatively constant
diversity
from the Paleocene into the middle Eocene that may be an artifact
of
sampling, which might mask an actual decline in numbers; low
diversity
during the late Eocene and Oligocene; a second exponential
diversification during the Miocene and leveling off in the late
Miocene
and Pliocene; and a precipitous drop in the Pleistocene and
Recent. The
coincidence of drops in diversity with global cooling is
suggestive of
a causal link-during the initial glaciation of Antarctica in the
Eocene
and Oligocene and the Northern Hemisphere glaciation at the end
of the
Pliocene. However, matters are complicated in the Northern
Hemisphere
by the climatic effects of regional uplift. Although the global
trend
of diversification is unperturbed at the K/T boundary, this is
largely
due to the exceptionally high rate of origination in the early
Paleocene. Nonetheless, the survival of such a demonstrably
climate-sensitive group strongly suggests that a climatic
explanation
for the K/T mass extinctions, especially the demise of the
dinosaurs,
must be reconsidered. Copyright 1999, Institute for Scientific
Information Inc.
===============
(10) THE MAN WHO CHANGED THE FIELD OF PALEONTOLOGY
From The New York Times, May 6, 1999
http://www.nytimes.com/yr/mo/day/news/national/obit-sepkowski.html
J. John Sepkoski Jr., 50; Changed Field of Paleontology
By CAROL KAESUK YOON
Dr. J. John Sepkoski Jr., a professor and paleontologist at the
University of Chicago, who conducted important research
illuminating
large-scale patterns of evolution in the fossil record, died May
1 at
his home in Chicago. He was 50.
The cause was heart failure related to high blood pressure, said
Steve
Koppes, news officer at the University of Chicago.
Colleagues described Sepkoski as pivotal in changing paleontology
from
a science often focused on the minutiae of describing fossil
species to
one that employed quantitative studies to understand evolutionary
change over the course of the history of life.
He was also well known for the way he worked. In what 20 years
ago was
a radically new style of research, Sepkoski identified patterns
of how
life evolved on earth using huge compilations of data gathered
from
numerous studies of fossils by many researchers from around the
world.
In these databases, he recorded when groups of organisms first
appeared
in the fossil record and when they disappeared or became extinct.
He
gathered these data by searching the paleontological literature,
poring
over obscure foreign language journals and trying to turn a
disparate
collection of studies into a uniform body of information.
Just one study took a decade, as he noted in the title of a 1993
paper
in the journal Paleobiology, "Ten Years in the Library: New
Data
Confirm Paleontological Patterns." He spent decades
continuing to build
the database.
"No one had ever done it before," said Dr. Stephen Jay
Gould, a
paleontologist at Harvard University, who was Sepkoski's graduate
school adviser. "He compiled a completely consistent data
set on all
groups, terrestrial, marine, single-celled, multicelled, animals
and
plants, everything."
Sepkoski's most important finding, researchers say, was the
discovery
of what appears to have been three distinct faunas, each
dominating
hundreds of millions of years in the fossil record, patterns
never before
documented or even proposed by others.
"Jack was the first one to recognize those," said Dr.
Douglas Erwin, a
paleobiologist at the National Museum of Natural History at the
Smithsonian Institution. The effort, Erwin added, prompted
considerable
follow-up research by others.
"In the '80s, when I was in graduate school," he said,
"his work
influenced what most people did, what most people were thinking
about."
In one of Sepkoski's most famous papers, done with a close
colleague,
Dr. David Raup, professor emeritus at the University of Chicago,
Sepkoski also reported evidence that mass extinctions appeared to
strike with regularity every 26 million years or so. The
findings,
still the subject of heated debate, have provided years of work
for
statisticians still studying the patterns of extinctions and for
paleontologists seeking possible causes for such cycles
Colleagues say Sepkoski helped develop not only new ways of
thinking
about the fossil record but also new statistical methods to deal
with
the enormous and unwieldy data required to answer the field's
new,
broad-scale questions about the evolution of life.
"Jack had remarkable mathematical skills," Gould said.
Sepkoski is also credited with pushing the field of paleontology
toward
greater quantitative rigor in what some describe as "the
quantitative
revolution." Because of Sepkoski and others at the
University of
Chicago, in particular Raup, this new number-crunching style of
paleontology, also called analytical paleontology, became known
as the
Chicago School.
While other paleontologists earned reputations searching for data
with
rock hammers in hand, Sepkoski was known for his countless hours
in the
library, which he referred to as his "field site." But
close colleagues
said most researchers did not recognize that Sepkoski was also an
experienced field geologist.
His son, David Sepkoski, a graduate student in the history of
science
at University of Minnesota, often accompanied his father in the
field.
He said his father's love of field geology was lifelong, as
Sepkoski
had been a rock and fossil enthusiast as a child and kept large
collections of rocks he had amassed as a boy.
But Sepkoski also was described by colleagues as a man with a
quirky
sense of humor working in an office filled with little plastic
dinosaurs and the blaring sounds of musical groups like the Sex
Pistols
and Spinal Tap.
And in addition to his rock collections, he also favored more
unusual
collections, including memorabilia of Elizabeth, the Queen
Mother, said
his wife, Dr. Christine Janis, a paleobiologist at Brown
University.
Sepkoski studied geology as an undergraduate at the University of
Notre
Dame and received his doctorate at Harvard in geology and
paleontology,
studying with Gould.
Sepkoski then taught at the University of Rochester for four
years
before moving to the University of Chicago in 1978. While there,
he was
appointed a research associate at the Field Museum of Natural
History,
and, at different times, held visiting professorships at
California Institute
of Technology and Harvard.
Sepkoski was co-editor of the journal Paleobiology from 1983 to
1986.
He was the Paleontological Society's president in 1995 and 1996.
In
1983, the society recognized his contributions with the Charles
Schuchert Award.
In addition to his wife and son, he is survived by his father,
Joseph
J. Sepkoski of Sparta, N.J.; two sisters, Carol Sepkoski of
Cambridge,
Mass., and Diane Karl of Cedar Brook, N.J., and his first wife,
Maureen
Meter of Chicago.
"Most of us aren't going to be remembered in a hundred
years' time,"
said Erwin, the Smithsonian paleobiologist, "but Jack will,
because he
changed the way we think about the fossil record and the history
of
life."
Copyright 1999, The New York Times Newspapers Ltd.
================
(11) J. JOHN SEPKOSKI JR., 50, PALEONTOLOGIST AT U. OF C.
From the Chicago Tribune, May 6, 1999
http://chicagotribune.com/news/obituaries/article/0,1051,SAV-9905060199,00.html
By Megan O'Matz
Tribune Staff Writer
Wearing a cowboy hat and carrying a rock hammer, University of
Chicago
paleontologist J. John Sepkoski Jr. loved being in the quarry,
unearthing prehistoric fossils.
But it was in the library and at the computer that he had the
greatest
impact on the scientific understanding of evolution and
extinction. For
decades, Mr. Sepkoski scoured scientific journals dating back to
1860,
noting the last known occurrence of sea-dwelling creatures and
entering
the information in a massive computerized database to provide
paleontologists with a valuable new tool in uncovering patterns
in
nature.
On Saturday, Mr. Sepkoski, 50, died of heart failure at his home
in
Hyde Park.
It was in 1984 that he garnered national attention for uncovering
a
startling pattern in the data. Working with colleague David Raup,
Mr.
Sepkoski found that in the past 250 million years catastrophic
extinctions of marine animals seem to occur every 26 million
years
instead of at random, a controversial and still inexplicable
finding.
"There are a lot of people who don't believe it, but no one
has been
able to disprove it either," said Douglas Erwin, research
paleontologist at the Smithsonian Institution in Washington,
D.C..
The discovery led scientists and astronomers to theorize that
mass
extinctions on land and at sea were caused by some force external
to
Earth, such as a comet or asteroid.
"Many people think of science as a dull, boring discipline.
But science
can be a fantastically exciting world," said Mr. Sepkoski's
wife,
Christine M. Janis, also a paleontologist.
Mr. Sepkoski earned a bachelor's degree from the University of
Notre
Dame in 1970 and a doctorate in geological sciences from Harvard
University in 1977.
He taught at the University of Rochester from 1974 to 1978 before
coming to the U. of C. in 1978.
While knowledgeable about the Precambrian, Permian and Triassic
periods, Mr. Sepkoski was also an authority on the punk rock era.
"He really liked the Sex Pistols, and he liked the
Ramones," said his
son, David.
Besides his wife and son, Mr. Sepkoski is survived by his father,
Joseph, and two sisters, Carol Sepkoski and Diane Karl.
Plans for a memorial service were pending.
Copyright 1999, Chicago Tribune
----------------------------------------
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*
EVIDENCE OF ARCHAEAN (3.26-3.24 BILLION YEARS-OLD) OCEANIC
IMPACT BASINS:
TERRESTRIAL ANALOGUES OF LUNAR MARE.
From Andrew Glikson <andrew.glikson@anu.edu.au>
In 1986 Don Lowe (sedimentologist) and Gary Byerly (petrologist)
- both
from the US - reported spherulitic units - similar to those
observed
along the K-T impact boundary - from 3.26-3.24 * 10^9 years-old
Archaean sediments in the Barberton greenstone belt (eastern
Transvaal), and from the Pilbara Craton (Western Australia)
[1,2]. A
meteoritic fallout origin by condensation of impact-produced
silicate
vapor was supported by platinum group element (PGE) patterns
studied by
Frank Kyte and colleagues [2] and by the occurrence of relic
Ni-rich
chromites of chondritic trace metal affinities [3] More recently
a
meteoritic origin has been confirmed by Chromium isotopic studies
by A.
Shukolayukov, F.T. Lugmair and colleagues [4]. For spherule unit
S4 -
mass balance calculations based on Cr and Ir abundances suggest a
chondritic projectile larger than 30 km in diameter, and possibly
as
large as 40-50 km in diameter, implying cratering on the scale
larger
than 600 km in diameter. A probable oceanic setting of the
crater/s is
suggested by the lack of shocked quartz fragments in the
spherulitic
sediments [2-4]. It is likely that the craters themselves were
subsequently detroyed by subduction of the oceanic crust.
The spherulitic condensate units include: unit S2 - 3243±4 *
10^6 years;
units S3 and S4 - 3227±4 to 3243±4 * 10^6 years. The Barberton
spherule
units are marked by PGE anomalies showing chondrite-normalized
profiles
which are depleted in the volatile species (Pd, Au) [2] - which
is the
reverse trend from terrestrial PGE patterns (excepting those of
residual
mantle peridotite). The spherules include relic quench-textured
and
resorbed Ni-rich chromites with high siderophile and chalcophile
element
abundances (Co, Zn, V) [3], Iridium nano-nuggets in sulphides
[2], and
53Cr/52Cr ratios which are diagnostic of C1 chondrites [4].
Lowe and Byerly [1] remarked on the potential significance of
these impact
records for the change from a simatic (oceanic) volcanic
assemblage
(komatiite and basalt-dominated Onverwacht Group - 3.55-3.3 *
10^9
years-old) to a turbidite, felsic volcanic and
conglomerate-dominated
sequence (Fig Tree Group, Moodies Group - less than 3.24 * 10^9
years-old).
The period 3.26-3.24 * 10^9 years is well represented in the
Pilbara Craton
of Western Australia [5,6]. In the central Pilbara Craton it is
represented
by a volcanic and sedimentary sequence (the Sulphur Springs
Group) [7],
which includes high-Mg komatiite volcanics, andesite and dacite,
dated by
U-Pb zircon as about 3.24 * 10^9 years-old [6]. These volcanics
are capped
by felsic volcanic lenses and olistostromes, unconformably
overlain by a
yet-undated sequence of siltstone-banded ironstone sequence
(Gorge Creek
Group), and is intruded by the comagmatic Strelley Granite. In
both the
Barberton and the Pilbara the first occurrence of granite-derived
detrital
sediments above the 3.26-3.24 * 10^9 years-old units signifies
the onset of
uplift and erosion of granitic batholiths associated with
differential
vertical movements.
The temporal juxtaposition between major impacts and the onset of
magmatic
and rifting events which involve the uplift and exposure of
granite
batholiths provides the first test case for potential
relationships between
mega-impacts and Precambrian magmatic/rifting episodes, suggested
by
Glikson (1993, 1996) [8,9]. Modelling of the effects of
very large impacts
on thin thermally active oceanic crust overlying shallow
asthenosphere
predict regional to global tectonic and magmatic effects,
consistent with
observations in the Barberton Mountain Land and the Pilbara
Craton.
References: [1] Lowe, D.R., Byerly, G.R., Asaro, F. and
Kyte, F.T., 1989,
Geological and geochemical record of 3400 m.y.-old terrestrial
meteorite
impacts. Science 245:959-962; [2] Kyte, F.T., Zhou, L., and
Lowe, D.R.,
1992, Noble metal abundances in an early Archaean impact deposit.
Geochimica et Cosmochimica Acta, 56:1365-1372; [3] Byerly, G. R.,
and Lowe,
D. R., 1994, Spinels from Archean impact spherules: Geochimica et
Cosmochimica Acta, 58:3469-3486; [4] Shukolayukov, A., Kyte,
F.T., Lugmair,
G.W. and Lowe, D.R., The oldest impact deposits on Earth - first
confirmation of an extraterrestrial component, European Science
Foundation
Impact Project, Cambridge Meeting on Impacts and the Early Earth,
1998; [5]
Sun, S.S. and Hickman, A.H., 1998, New Nd isotopic and
geochemical data
from the West Pilbara - implications for Archaean crustal
accretion and
shear zone movement: Australian Geological Survey Organization
Research
Newsletter, 28:25-28; [6] Vearncombe, E.S., Barley, M.E., Groves,
D.I.,
McNaughton, N.J., Micucki, E.J., and Vearncombe, J.R., 1995, 3.24
Ga black
smoker-type mineralization in the Strelley belt, Pilbara Craton,
Western
Australia: J. Geological Society of London, 152:587-590; [7] Van
Kranendonk, M.J. and Morant, P.,1998, Revised Archaean
stratigraphy of the
North Shaw 1:100 000 sheet, Pilbara Craton: Geological Survey
Western
Australia Annual Review 1997-98, 55-62; [8] Glikson, A.Y., 1993,
Asteroids
and early crustal evolution. Earth Science Reviews, 35: 285-319;
[9]
Glikson, A.Y., 1996, Mega-impacts and mantle melting episodes:
tests of
possible correlations. Australian Geological Survey Organisation
Journal,
16/4:587-608.
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