PLEASE NOTE:
*
JACK SEPKOSKI (1948-1999): SCHOLAR WHO TURNED THE STUDY OF
MASS
EXTINCTION INTO A HARD SCIENCE
From Steven N. Koppes <s-koppes@uchicago.edu>
Benny,
I suspect that many members of the CCNet list knew Jack Sepkoski
or
his extraordinary work. It was my sad duty today to issue this
news
release regarding this death.
Steve
May 4, 1999
For immediate release
Contact: Steve Koppes
(773) 702-8366
s-koppes@uchicago.edu
Paleontologist J. John Sepkoski Jr., 1948-1999
University of Chicago paleontologist J. John Sepkoski Jr.,
whose work has had innovative and far-reaching impact on the
scientific understanding of the fossil record and the
diversification
of animal life throughout much of Earth's history, died Saturday,
May
1, of sudden heart failure related to high blood pressure. He was
50.
"Paleobiology is a small profession, so when we lose one of
our very greatest, it's really a tremedously painful
experience,"
said Harvard University paleontologist Stephen Jay Gould, with
whom
Sepkoski studied as a graduate student. "Jack was one of the
leading
lights of the profession."
One of Sepkoski's major contributions was quantifying the
nature of life's diversity through time, said Douglas Erwin, a
research paleontologist at the Smithsonian Institution in
Washington,
D.C., and editor of the journal Paleobiology. "He
exhaustively
documented the ups and downs of life through the last 600 million
years. By collecting the data and developing a series of
statistical
methods to study it, he gave us a new way of understanding the
history of life in the oceans.."
During the 1980s and 1990s, there were many meetings of
paleontologists during which literally every speaker used a
figure
from Sepkoski's work, Erwin said. "His contributions were
fundamental
to everything that people were doing," he said.
Sepkoski's death was a huge loss for paleontology, said
University of Chicago paleontologist David Jablonski. "Jack
was
extremely generous with his ideas and for that matter with the
huge
database that he spent decades compiling. His combined approach
of
mathematical modelling, paleoecology and massive data arrays
really
changed the way we do paleontology," Jablonski said.
His work is discussed in the book Mystery of Mysteries: Is
Evolution a Social Construction? by Michael Ruse, published this
year
by Harvard University Press. "One of the first chapters is
on Charles
Darwin and one of the last chapters is on Jack," Jablonski
said.
During the 1980s, using rigorous statistical analyses of the
fossil record, Sepkoski and his Chicago colleague David Raup put
forth the controversial theory that catastrophic extinctions of
marine animals may have occurred approximately every 26 million
years
during the past 250 million years of Earth's history. These
periodic
events also included the extinction of the dinosaurs 65 million
years
ago. The extinctions previously were thought to have been random
events.
The theory helped open the possibility that mass extinctions
both on land and in the oceans were caused by some force external
to
Earth, such as catastrophic comet and asteroid impacts of the
type
that inspired the films "Deep Impact" and
"Armageddon" and inspired
the popular Shriekback song "Nemesis." These findings
prompted a
major interdisciplinry research effort on extinction events in
the
fossil record.
The exact cause of periodic extinctions remains a mystery.
"The theory has been under attack, but I don't hink anyone's
been
able to disprove it," said Sepkoski's wife, Christine M.
Janis, a
Brown University paleontologist.
Sepkoski's taste in music was as unconventional as some of
his scientific theories. His favorite musical group was the punk
rock
Sex Pistols. "He thought that the Velvet Underground album
was the
best album of all time," Janis said.
Sepkoski was born July 26, 1948, in Presque Isle, Maine. "He
started collecting dinosaur bones and fossils in New Jersey when
he
was 10 and had wanted to become a paleontologist since that
time,"
said his former wife, Maureen Meter.
He earned his B.S. degree, magna cum laude, from the
University of Notre Dame in 1970 and his Ph.D. in geological
sciences
from Harvard University in 1977. His Ph.D. research was on the
field
geology and paleontology of South Dakota's Black Hills.
He taught at the University of Rochester from 1974 to 1978
first as an instructor, then as an assistant professor. He was
appointed Assistant Professor in the Department of Geophysical
Sciences at the University of Chicago in 1978. Sepkoski attained
the
rank of Associate Professor in 1982 and Professor in 1986. He
also
had been a research associate at the Field Museum of Natural
History
in Chicago since 1980.
Sepkoski held visiting professorships at the California
Institute of Technology in 1986 and at Harvard Uniersity in 1990
and
1991. In 1988, Sepkoski visited the University of California at
Los
Angeles as a senior fellow and lectured at the Polish Academy of
Sciences, where he was elected a foreign member.
The Paleontological Society bestowed its Charles Schuchert
Award upon Sepkoski in 1983. He served a term as the society's
president from 1995 to 1996 and founded the Paleontological
Society
International Research Program, or PalSIRP, the society's program
for
assisting paleontologists in the countires of the former Soviet
Union
through small competitive grants.
He was co-editor of the journal Paleobiology, regarded as the
major journal of his field, from 1983 to 1986 and a member of its
editorial board from 1987 to 1989.
"Jack was admired by his family, friends and colleagues not
only for the brilliance of his research, but also for his
untiring
devotion to scholarship and teaching," said Mike Foote, a
University
of Chicago paleontologist. He taught popular, large-enrollment
courses in the University's Core curriculum, advanced
undergradaute
courses in paleontology, and demanding, advanced graduate
courses. He
took great satisfaction in mentoring and advising students and
colleagues, as well, Foote said.
When Sepkoski led paleontological field trips for
undergraduate and graduate students over the years, his son,
David,
was a frequent companion. David Sepkoski went on to receive an
M.A.
degree from the University of Chicago and is a doctoral student
in
the history of science at the University of Minnesota.
He is survived by his wife, Christine M. Janis of Providence,
R.I.; his son, David Sepkoski of Minneapolis, Minn.; 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
former
wife, Maureen Meter of Chicago. Sepkoski also had a dog, Ronnie.
Arrangements for a memorial service are pending. In lieu of
flowers, donations may be made to PalSIRP, c/o Dr. Thomas W.
Kammer,
Treasurer, Paleontological Society, Dept. of Geology and
Geography,
West Virginia University, P.O. Box 6300, Morgantown, WV
26506-6300.
####
Radio stations: The University of Chicago has an ISDN line.
Please
call for information.
For more news from the University of Chicago, visit our Web site
at
http://www-news.uchicago.edu.
sk/99-69
Steve Koppes
University of Chicago News Office
5801 South Ellis Ave. Room 200
Chicago, IL 60637-1473
773-702-8366
773-702-8324 (fax)
=======================
PAPERS BY JACK SEPKOSKI (1948-1999): SCHOLAR WHO TURNED THE STUDY
OF
MASS EXTINCTION INTO A HARD SCIENCE
M. Foote & J.J. Sepkoski: Absolute measures of the
completeness of the
fossil record. NATURE, 1999, Vol.398, No.6726, pp.415-417
Measuring the completeness of the fossil record is essential to
understanding evolution over long timescales, particularly when
comparing evolutionary patterns among biological groups with
different
preservational properties. Completeness measures have been
presented
for various groups based on gaps in the stratigraphic ranges of
fossil
taxa and on hypothetical lineages implied by estimated
evolutionary
trees. Here we present and compare quantitative, widely
applicable
absolute measures of completeness at two taxonomic levels for a
broader
sample of higher taxa of marine animals than has previously been
available. We provide an estimate of the probability of genus
preservation per stratigraphic interval, and determine the
proportion
of living families with some fossil records. The two completeness
measures use very different data and calculations. The
probability of
genus preservation depends almost entirely on the Palaeozoic and
Mesozoic records, whereas the proportion of living: families with
a
fossil record is influenced largely by Cenozoic data. These
measurements are nonetheless highly correlated, with outliers
quite
explicable, and we find that completeness is rather high for many
animal groups. Copyright 1999, Institute for Scientific
Information
Inc.
M. Foote, J.P. Hunter, C.M. Janis, J.J. Sepkoski: Evolutionary
and
preservational constraints on origins of biologic groups:
Divergence times of eutherian mammals. SCIENCE, 1999, Vol.283,
No.5406, pp.1310-1314
Some molecular clack estimates of divergence times of taxonomic
groups undergoing evolutionary radiation are much older than the
groups' first observed fossil record. Mathematical models of
branching
evolution are used to estimate the maximal rate of fossil
preservation
consistent with a postulated missing history, given the sum of
species
durations implied by early origins under a range of species
origination
and extinction rates. The plausibility of postulated divergence
times
depends on origination, extinction, and preservation Fates
estimated
from the fossil record. For eutherian mammals, this approach
suggests
that it is unlikely that many modern orders arose much earlier
than
their oldest fossil records. Copyright 1999, Institute for
Scientific
Information Inc.
F.K. McKinney, S. Lidgard, J.J. Sepkoski, P.D. Taylor: Decoupled
temporal patterns of evolution and ecology in two
post-Paleozoic clades. SCIENCE, 1998, Vol.281, No.5378,
pp.807-809
Counts of taxonomic diversity are the prevailing standards for
documenting Large-scall patterns of evolution in the fossil
record.
However, the secular pattern of relative ecological importance
between
the bryozoan clades Cyclostomata and Cheilostomata is not
reflected
fully in compilations of generic diversity or within-fauna
species
richness, and the delayed ecological recovery of the
Cheilostomata
after the mass extinction at the Cretaceous-Tertiary boundary is
missed
entirely. These observations demonstrate that evolutionary
success and
ecological dominance can be decoupled and profoundly different,
even
over tons of millions of years. Copyright 1999, Institute for
Scientific Information Inc.
J.J. Sepkoski & A.I. Miller: Analysing diversification
through time.
TRENDS IN ECOLOGY & EVOLUTION, 1998, Vol.13, No.4, pp.158-159
J.J. Sepkoski: Rates of speciation in the fossil record.
PHILOSOPHICAL
TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL
SCIENCES, 1998, Vol.353, No.1366, pp.315-326
Data from palaeontology and biodiversity suggest that the global
biota
should produce an average of three new species per year. However,
the
fossil record shows large variation around this mean. Rates of
origination have declined through the Phanerozoic. This appears
to have
been largely a function of sorting among higher taxa (especially
classes), which exhibit characteristic rates of speciation (and
extinction) that differ among them by nearly an order of
magnitude.
Secular decline of origination rates is hardly constant, however;
many
positive deviations reflect accelerated speciation during
rebounds from
mass extinctions. There has also been general decline in rates of
speciation within major taxa through their histories, although
rates
have tended to remain higher among members in tropical regions.
Finally, pulses of speciation appear sometimes to be associated
with
climate change, although moderate oscillations of climate do not
necessarily promote speciation despite forcing changes in
species'
geographical ranges. Copyright 1999, Institute for Scientific
Information Inc.
J.J. Sepkoski: Biodiversity: Past, present, and future
JOURNAL OF PALEONTOLOGY, 1997, Vol.71, No.4, pp.533-539
D. Jablonski & J.J. Sepkoski: Paleobiology, community
ecology, and
scales of ecological pattern. ECOLOGY, 1996, Vol.77, No.5,
pp.1367-1378
The fossil record provides a wealth of data on the role of
regional processes and historical events in shaping biological
communities over a variety of time scales. The Quaternary record
with
its evidence of repeated climatic change shows that both
terrestrial
and marine species shifted independently rather than as cohesive
assemblages over scales of thousands of years. Larger scale
patterns
also show a strong individualistic component to taxon dynamics;
assemblage stability, when it occurs, is difficult to separate
from
shared responses to low rates of environmental change.
Nevertheless,
the fossil record does suggest that some biotic interactions
influence
large-scale ecological and evolutionary patterns, albeit in more
diffuse and protracted fashions than those generally studied by
community ecologists. These include: (1) the resistence by
incumbents
to the establishment of new or invading taxa, with episodes of
explosive diversification often appearing contingent on the
removal of
incumbents at extinction events; (2) steady states of
within-habitat
and global diversity at longer time scales (10(7)-10(8) yr),
despite
enormous turnover of taxa; and (3) morphological and
biogeographic
responses to increased intensities of predation and substratum
disturbance over similarly long time scales. The behavior of
species
and communities over the array of temporal and spatial scales in
the
fossil record takes on additional significance for framing
conservation
strategies, and for understanding recovery of species, lineages,
and
communities from environmental changes. Copyright 1999, Institute
for
Scientific Information Inc.
J.J. Sepkoski: Paleontologists lend a helping hand. GEOTIMES,
1996,
Vol.41, No.10, pp.9-10
J.J. SEPKOSKI: EXTINCTION AND THE FOSSIL RECORD. GEOTIMES, 1994,
Vol.39, No.3, pp.15-17
J.J. SEPKOSKI & D.C. KENDRICK: NUMERICAL EXPERIMENTS WITH
MODEL
MONOPHYLETIC AND PARAPHYLETIC TAXA. PALEOBIOLOGY, 1993, Vol.19,
No.2, pp.168-184
The problem of how accurately paraphyletic taxa versus
monophyletic
(i.e., holophyletic) groups (clades) capture underlying species
patterns of diversity and extinction is explored with Monte Carlo
simulations. Phylogenies are modeled as stochastic trees.
Paraphyletic
taxa are defined in an arbitrary manner by randomly choosing
progenitors and clustering all descendants not belonging to other
taxa.
These taxa are then examined to determine which are clades, arid
the
remaining paraphyletic groups are dissected to discover
monophyletic
subgroups. Comparisons of diversity patterns and extinction rates
between modeled taxa and lineages indicate that paraphyletic
groups can
adequately capture lineage information under a variety of
conditions of
diversification and mass extinction. This suggests that these
groups
constitute more than mere 'taxonomic noise' in this context. But,
strictly monophyletic groups perform somewhat better, especially
with
regard to mass extinctions. However, when low levels of
paleontologic
sampling are simulated, the veracity of clades deteriorates,
especially
with respect to diversity, and modeled paraphyletic taxa often
capture
more information about underlying lineages. Thus, for studies of
diversity and taxic evolution in the fossil record, traditional
paleontologic genera and families need not be rejected in favor
of
cladistically-defined taxa. Copyright 1999, Institute for
Scientific
Information Inc.
C.C. LABANDEIRA & J.J. SEPKOSKI: INSECT DIVERSITY IN THE
FOSSIL RECORD
SCIENCE, 1993, Vol.261, No.5119, pp.310-315
Insects possess a surprisingly extensive fossil record.
Compilation of
the geochronologic ranges of insect families demonstrates that
their
diversity exceeds that of preserved vertebrate tetrapods through
91
percent of their evolutionary history. The great diversity of
insects
was achieved not by high origination rates but rather by low
extinction
rates comparable to the low rates of slowly evolving marine
invertebrate groups. The great radiation of modern insects began
245
million years ago and was not accelerated by the expansion of
angiosperms during the Cretaceous period. The basic trophic
machinery
of insects was in place nearly 100 million years before
angiosperms
appeared in the fossil record. Copyright 1999, Institute for
Scientific
Information Inc.
J.J. SEPKOSKI: 10 YEARS IN THE LIBRARY - NEW DATA CONFIRM
PALEONTOLOGICAL PATTERNS. PALEOBIOLOGY, 1993, Vol.19, No.1,
pp.43-51
A comparison is made between compilations of times of origination
and
extinction of fossil marine animal families published in 1982 and
1992.
As a result of ten years of library research, half of the
information
in the compendia has changed: families have been added and
deleted,
low-resolution stratigraphic data have been improved, and
intervals of
origination and extinction have been altered. Despite these
changes,
apparent macroevolutionary patterns for the entire marine fauna
have
remained constant. Diversity curves compiled from the two data
bases
are very similar, with a goodness-of-fit of 99%; the principal
difference is that the 1992 curve averages 13% higher than the
older
curve. Both numbers and percentages of origination and extinction
also
match well, with fits ranging from 83% to 95%. All major events
of
radiation and extinction are identical. Therefore, errors in
large
paleontological data bases and arbitrariness of included taxa are
not
necessarily impediments to the analysis of pattern in the fossil
record, so long as the data are sufficiently numerous. Copyright
1999,
Institute for Scientific Information Inc.
J.J. Sepkoski, 1992. A compendium of fossil marine animal
families, 2nd
edition. Milwaukee Public Museum Contributions in Biology and
Geology, No. 83. 156 pp.
J.J. Sepkoski, 1992. Phylogenetic and ecologic patterns in the
Phanerozoic history of marine biodiversity. Pp. 77-100. In: N.
Eldredge (ed.) Systematics, Ecology, and the Biodiversity
Crisis. Columbia University Press; New York.
J.J. SEPKOSKI: A MODEL OF ONSHORE-OFFSHORE CHANGE IN FAUNAL
DIVERSITY
PALEOBIOLOGY, 1991, Vol.17, No.1, pp.58-77
Onshore-offshore patterns of faunal change occurred at many
taxonomic
scales during the Paleozoic Era, ranging from replacement of the
Cambrian evolutionary fauna by the Paleozoic fauna to the
environmental
expansion of many orders and classes. A simple mathematical model
is
constructed to investigate such change. The environmental
gradient
across the marine shelf-slope is treated as a linear array of
discrete
habitats, each of which holds a set number of species, as
observed in
the fossil record. During any interval of time, some portion of
the
species in each habitat becomes extinct by background processes,
with
rates of extinction varying among both clades and habitats, as
also
observed in the record. After extinction, species are replaced
from
within the habitat and from immediately adjacent habitats, with
proportions dependent on surviving species. This model leads to
the
prediction that extinction-resistant clades will always diversify
at
the expense of extinction-prone clades. But if extinction
intensity is
highest in nearshore habitats, extinction-resistant clades will
expand
preferentially in the onshore direction, build up diversity
there, and
then diversify outward toward the offshore. Thus,
onshore-offshore
patterns of diversification may be the expectation for faunal
change
quite independently of whether or not clades originate onshore.
When
the model is parameterized for Paleozoic trilobites and
brachiopods,
numerical solutions exhibit both a pattern of faunal change and a
time
span for diversification similar to that seen in the fossil
record.
They also generate structure similar to that seen in global
diversification, including logistic patterns of growth, declining
origination but constant extinction within clades through time,
and
declining overall extinction across clades through time.
Copyright
1999, Institute for Scientific Information Inc.
J.J. Sepkoski, 1991. Diversity in the Phanerozoic oceans: a
partisan
review. Pp. 210-236. In: E. Dudley, (ed.) The Unity of
Evolutionary
Biology. Proceedings of the Fourth International Congress of
Systematic
and Evolutionary Biology. Dioscorides Press; Portland, Oregon.
J.J. Sepkoski, 1990. The taxonomic structure of periodic
extinction.
Pp. 33-44. In: V. Sharpton and P. Warda (eds.), Global
Catastrophes in Earth History. Geological Society of America
Special Paper 247.
J.J. SEPKOSKI: PERIODICITY IN EXTINCTION AND THE PROBLEM OF
CATASTROPHISM IN THE HISTORY OF LIFE. JOURNAL OF THE GEOLOGICAL
SOCIETY, 1989, Vol.146, No.Pt1, pp.7-19
A.I. MILLER & J.J. SEPKOSKI: MODELING BIVALVE DIVERSIFICATION
- THE
EFFECT OF INTERACTION ON A MACROEVOLUTIONARY SYSTEM.
PALEOBIOLOGY, 1988, Vol.14, No.4, pp.364-369
J.J. SEPKOSKI: ALPHA, BETA, OR GAMMA - WHERE DOES ALL THE
DIVERSITY GO.
PALEOBIOLOGY, 1988, Vol.14, No.3, pp.221-234
D.M. RAUP & J.J. SEPKOSKI: TESTING FOR PERIODICITY OF
EXTINCTION
SCIENCE, 1988, Vol.241, No.4861, pp.94-96
J.J. SEPKOSKI: IS THE PERIODICITY OF EXTINCTIONS A TAXONOMIC
ARTIFACT -
REPLY. NATURE, 1987, Vol.330, No.6145, pp.251-252
D.H. ERWIN, J.W. VALENTINE, J.J. SEPKOSKI: A COMPARATIVE-STUDY OF
DIVERSIFICATION EVENTS - THE EARLY PALEOZOIC VERSUS THE
MESOZOIC. EVOLUTION, 1987, Vol.41, No.6, pp.1177-1186
J.J. SEPKOSKI: ENVIRONMENTAL TRENDS IN EXTINCTION DURING THE
PALEOZOIC.
SCIENCE, 1987, Vol.235, No.4784, pp.64-66
J.J. SEPKOSKI & D.M. RAUP: WAS THERE 26-MYR PERIODICITY OF
EXTINCTIONS?
NATURE, 1986, Vol.321, No.6069, p.533
D.M. RAUP & J.J. SEPKOSKI: PERIODIC EXTINCTION OF FAMILIES
AND GENERA.
SCIENCE, 1986, Vol.231, No.4740, pp.833-836
J.J. SEPKOSKI: A KINETIC-MODEL OF PHANEROZOIC TAXONOMIC DIVERSITY
.3.
POST-PALEOZOIC FAMILIES AND MASS EXTINCTIONS
PALEOBIOLOGY, 1984, Vol.10, No.2, pp.246-267
B.D. WOODBURY & J.J. SEPKOSKI: DIVERSITY AND FAUNAL PATTERNS
OF
ENDOBIONT AND EPIBIONT COMMUNITIES THROUGH THE PHANEROZOIC
AMERICAN ZOOLOGIST, 1984, Vol.24, No.3, p.A145
D..M. RAUP & J.J. SEPKOSKI: PUBLISHING CHRONOLOGY. NATURE,
1984,
Vol.309, No.5966, p.300
D. JABLONSKI, J.J. SEPKOSKI, D.J. BOTTJER, P.M. SHEEHAN:
BIOLOGICAL
DIVERSITY. SCIENCE, 1984, Vol.224, No.4655, p.1294
J.J. SEPKOSKI, A.H. KNOLL: PRECAMBRIAN-CAMBRIAN BOUNDARY - THE
SPIKE IS
DRIVEN AND THE MONOLITH CRUMBLES. PALEOBIOLOGY, 1983, Vol.9,
No.3, pp.199-206
D.M. RAUP, J.J. SEPKOSKI: PERIODICITY OF EXTINCTIONS IN THE
GEOLOGIC
PAST. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE
UNITED STATES OF AMERICA-BIOLOGICAL SCIENCES, 1984, Vol.81,
No.3, pp.801-805
D. JABLONSKI, J.J. SEPKOSKI, D.J. BOTTJER, P.M. SHEEHAN:
ONSHORE-OFFSHORE PATTERNS IN THE EVOLUTION OF PHANEROZOIC SHELF
COMMUNITIES. SCIENCE, 1983, Vol.222, No.4628, pp.1123-1125
D.M. RAUP, J.J. SEPKOSKI, S.M. STIGLER: MASS EXTINCTIONS IN THE
FOSSIL
RECORD. SCIENCE, 1983, Vol.219, No.4589, pp.1240-1241
J.J. SEPKOSKI: MAMMALIAN EVOLUTION AND THE GREAT AMERICAN
INTERCHANGE
SCIENCE, 1982, Vol.215, No.4538, pp.1351-1357
D.M. RAUP & J.J. SEPKOSKI: MASS EXTINCTIONS IN THE MARINE
FOSSIL RECORD
SCIENCE, 1982, Vol.215, No.4539, pp.1501-1503
R.E. CHAPMAN, P.M. GALTON, J.J. SEPKOSKI, W.P. WALL: A
MORPHOMETRIC
STUDY OF THE CRANIUM OF THE PACHYCEPHALOSAURID DINOSAUR
STEGOCERAS. JOURNAL OF PALEONTOLOGY, 1981, Vol.55, No.3,
pp.608-618
J.J. SEPKOSKI, R.K. BAMBACH, D.M. RAUP, J.W. VALENTINE:
PHANEROZOIC
MARINE DIVERSITY AND THE FOSSIL RECORD. NATURE, 1981, Vol.293,
No.5832, pp.435-437
J.J. SEPKOSKI: A FACTOR ANALYTIC DESCRIPTION OF THE PHANEROZOIC
MARINE
FOSSIL RECORD. PALEOBIOLOGY, 1981, Vol.7, No.1, pp.36-53
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*
LETTERS TO THE MODERATOR, 5 May 1999
------------------------------------
(1) WHERE HAVE ALL THE CRATERS GONE?
Andrew Glikson <andrew.glikson@anu.edu.au>
(2) ETA AQUARIDS & THE LYRIDS
Mark Kidger <mrk@ll.iac.es>
(3) XF11 & AN10: LEGITIMATE SCARES
Jens Kieffer-Olsen <JKO@dst.dk>
===========
(1) WHERE HAVE ALL THE CRATERS GONE?
From Andrew Glikson <andrew.glikson@anu.edu.au>
Dear Benny,
A couple of comments in response to the queries raised by Timo
Niroma
(CCNet 5.5.99):
(1) For craters larger than 20 km-diameter - lunar mare crater
counts,
terrestrial crater counts in long-lived stable cratons, and
astronomical observations are consistent with an impact flux of
4-6 *
10^-15 km^-2yr^-1 since the end of the Late Heavy Bombardment.
Coupled
with the observed crater size vs cumulative crater size frequency
relationship of N µ Dc^-1.8 (N = cumulative number of craters of
diameter > Dc), these rates imply formation on Earth of more
than 450
craters larger than 100 km-diameter and more than 50 craters
larger
than Dc³ 300 km-diameter since ~3.8 b.y. ago.
(2) Dominant crater removal mechanisms (erosion, burial,
metamorphism,
subduction) fully explain the fact that less than 180 craters
have been
identified to date. In Australia about 1-2 new craters are
discocvered
each year - mostly through geophysical surveys and drilling in
sedimentary
basins. As larger craters have a better chance of
preservation, the
fact that only 4 craters larger than 150 km in diameter
(Vredefort,
Sudbury, Morokweng, Chicxulub) were found to date - out of the
predicted number of about 100 or so - provides a measure of the
degree
of crater preservation.
(3) The late Eugene Shoemaker suggested an increase in the rate
of
cratering during the Phanerzoic. From the 3He/4He flux we
live in a
period of peak bombardment, although considerations regarding the
mobility of Helium in sediments may modify such a
conclusion. As
suggested by many, the tendency for medium to large impacts to
occur in
clusters, typically at late stages of geological eras (late
Ordovician,
late Devonian, late Triassic, Jurassic-Cretaceous boundary, late
Eocene), suggests significant correlations with major biological
extinctions, as in the classic case of the K-T boundary.
Andrew Glikson
andrew.glikson@anu.edu.au
==============
(2) ETA AQUARIDS & THE LYRIDS
From Mark Kidger <mrk@ll.iac.es>
Dear Benny:
Re: Meteors down under/NASA News
I should point out that, although the Eta Aquarids are a very
active
shower (here in the Canary Islands we get to observe them very
well),
NASA is being a bit misleading with its headline story. In fact,
the
bright Moon which is approaching the Eta Aquarid radiant in the
dawn
sky will make observing this shower practically impossible this
year.
NASA also was a bit fast off the mark writing off the Lyrid
shower as
"visually disappointing" given that the first IMO
shower report,
supported by observers in the Canary Islands and, most recently,
by new
Cuban data, show that the shower was a factor of 2-3 ABOVE its
normal
level for several hours on the morning of maximum. Far from being
disappointing, it was probably the finest show from the Lyrids
for a
number of years.
Mark
=============
(3) XF11 & AN10: LEGITIMATE SCARES
From Jens Kieffer-Olsen <JKO@dst.dk>
Re: RELATIVE RISKS & THE PROVISIONAL NATURE OF TRUTH IN
SCIENCE
From Michael Martin-Smith <martin@miff.demon.co.uk>
Indeed the long-term expectation among ordinary folks will be
that
anything other than than a trivial impactor, say up to 10m, is
predicted or subject to an advance warning.
We know that this level of forewarning takes a long time to
establish.
In the meantime I find it perfectly legitimate that scares such
as XF11
and AN10 stimulate the universal desire to come to grips with the
threat from outer space.
Neither Brian Marsden nor Andrea Milani deserve any criticism for
their
choice of method for communicating their observations to the
public.
The only ones to blame are those scientists, who might keep mum
about
similar discoveries still unknown to the rest us.
--
Jens Kieffer-Olsen
-----------------
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