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 - 32434 * 10^6 years;
units S3 and S4 - 32274 to 32434 * 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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