PLEASE NOTE:


*
Date sent:        Tue, 14 Oct 1997 11:14:27 -0400 (EDT)
From:             " Benny J Peiser" <B.J.PEISER@livjm.ac.uk
Subject:          IMPACT CRATERING
To:               cambridge-conference@livjm.ac.uk
Priority:         NORMAL

NEW PAPERS ON IMPACT CRATERING

(1) M. R. Rampino, B. M. Haggerty & T. C. Pagano: A unified theory of
impact crises and mass extinctions: Quantitative tests.

(2) G. Schmidt, H. Palme & K. L. Kratz: Highly siderophile elements (Re, Os,
Ir, Ru, Rh, Pd, An) in impact melts from three European impact craters
(Saaksjarvi, Mien, and Dellen): Clues to the nature of the impacting bodies.
 

=================================================================

M. R. Rampino, B. M. Haggerty & T. C. Pagano: A unified theory of
impact crises and mass extinctions: Quantitative tests. ANNALS OF THE
NEW YORK ACADEMY OF SCIENCES, 1997, Vol.822, pp.403-431

Several quantitative tests of a general hypothesis linking impacts of
large asteroids and comets with mass extinctions of life are possible
based on astronomical data, impact dynamics, and geological
information. The waiting times of large-body impacts on the Earth
derived from the flux of Earth-crossing asteroids and comets, and the
estimated size of impacts capable of causing large-scale environmental
disasters, predict that impacts of objects greater than or equal to 5 km in
diameter (greater than or equal to 10(7) Mt TNT equivalent) could be
sufficient to explain the record of similar to 25 extinction pulses in the
last 540 Myr, with the 5 recorded major mass extinctions related to impacts
of the largest objects of greater than or equal to 10 km in diameter
(greater than or equal to 10(8) Mt events). Smaller impacts (similar to
10(6) Mt), with significant regional environmental effects, could be
responsible for the lesser boundaries in the geologic record. Tests of the
''kill curve'' relationship for impact-induced extinctions based on new data
on extinction intensities, and several well-dated large impact craters, also
suggest that major mass extinctions require large impacts, and that a step
in the kill curve may exist at impacts that produce craters of similar to
100 km diameter, smaller impacts being capable of only relatively weak
extinction pulses. Single impact craters less than similar to 60 km in
diameter should not be associated with detectable global extinction pulses
(although they may explain stage and zone boundaries marked by lesser faunal
turnover), but multiple impacts in that size range may produce significant
stepped extinction pulses. Statistical tests of the last occurrences of
species at mass-extinction boundaries are generally consistent with
predictions for abrupt or stepped extinctions, and several boundaries are
known to show ''catastrophic'' signatures of environmental disasters and
biomass crash, impoverished postextinction fauna and flora dominated by
stress-tolerant and opportunistic species, and gradual ecological recovery
and radiation of new taxa. Isotopic and other geochemical signatures are
also generally consistent with the expected after-effects of catastrophic
impacts. Seven of the recognized extinction pulses seem to be associated
with concurrent (in some cases multiple) stratigraphic impact markers (e.g.,
layers with high iridium, shocked minerals, microtektites), and/or large,
dated impact craters. Other less well-studied crisis intervals show elevated
iridium, but well below that of the K/T spike, which might be explained by
low-Ir impactors, ejecta blowoff, or sedimentary reworking and dilution of
impact signatures. The best explanation for a possible periodic component of
similar to 30 Myr in mass extinctions and clusters of impacts is the
pulselike modulation of the comet flux associated with the solar system's
periodic passage through the plane of the Milky Way Galaxy. The quantitative
agreement between paleontologic and astronomical data suggests an important
underlying unification of the processes involved.

========================

G. Schmidt, H. Palme & K. L. Kratz: Highly siderophile elements (Re,
Os, Ir, Ru, Rh, Pd, An) in impact melts from three European impact
craters (Saaksjarvi, Mien, and Dellen): Clues to the nature of the
impacting bodies. GEOCHIMICA ET COSMOCHIMICA ACTA, 1997, Vol.61, No.14,
pp.2977-2987

Twenty-two large (10 g) impact melt samples from three Scandinavian
craters (i.e., Saaksjarvi, Finland; and Mien and Dellen, Sweden) were
analyzed for highly siderophile elements (HSE: platinum group elements, Rh,
and Au) by the nickel sulfide technique in combination with neutron
activation. The ten impact melt samples from Saaksjarvi are enriched in Ir
and other highly siderophile elements (Ir = 2.48 +/- 0.73 ng/g) relative to
average upper crust concentrations (0.03 +/- 0.02 ng/g Ir). The twelve
Dellen and Mien samples are marginally enriched in Ir (0.48 +/- 0.23 ng/g
for Dellen, and 0.37 +/- 0.23 ng/g for Mien). The amount of meteoritic
component corresponds to 0.5% of a nominal CI component for Saaksjarvi, and
about 0.1% for Mien and Dellen. The Saaksjarvi pattern is fractionated
relative to CI-chondrites. Normalized abundances increase from the
refractory to the more volatile siderophile elements (Ir < Ni). The trend is
qualitatively similar to magmatic iron meteorites and corresponds to 0.4% of
a nominal magmatic iron meteorite component (Tamarugal, IIIAB). For Mien and
Dellen no projectile assignment can be made. All samples from the three
impact craters have low Os/Ir ratios compared to chondritic ratios. Either
the projectile had low Os/Ir ratios or Os was lost during the impact as
volatile OsO4. Based on the results of this study and on a compilation of
literature data, average upper crustal abundances of the highly siderophile
elements in the target area (Baltic shield) are estimated as 0.03 ng/g for
Lr and Os, 1.1 ng/g for Ru, 0.38 ng/g for Rh, 2.0 ng/g for Pd, 8 mu g/O for
Co, and 37 mu g/g for Cr. These data are representative of the upper crust.
They allow, for the first time, reliable estimates of crustal abundances for
Rh and Ru. Upper crustal abundances also show that Ir and Os are the most
favourable elements for the identification of meteoritic signatures.
Fractionated Os/Ir ratios in magmatic iron meteorites or potential losses of
Os during the impact leaves Ir as the best indicator element for meteoritic
contamination. With Ir, extraterrestrial components as low as 2 x 10(4) x CI
may be identified.



*
Date sent:        Tue, 14 Oct 1997 11:01:41 -0400 (EDT)
From:             " Benny J Peiser" <B.J.PEISER@livjm.ac.uk
Subject:          THERA CATASTROPHE
To:               cambridge-conference@livjm.ac.uk
Priority:         NORMAL

NEW PAPERS ON BRONZE AGE CATASTROPHES
 

(1) M. B. Cita & B. Rimoldi: Geological and geophysical evidence for a
Holocene tsunami deposit in the eastern Mediterranean deep-sea record.

(2) P. C. Buckland, A. J. Dugmore & K. J. Edwards: Bronze Age
myths? Volcanic activity and human response in the Mediterranean
and North Atlantic regions.

======================================================================

M. B. Cita & B. Rimoldi: Geological and geophysical evidence for a
Holocene tsunami deposit in the eastern Mediterranean deep-sea
record. JOURNAL OF GEODYNAMICS, 1997, Vol.24, No.1-4, pp.293-304

Extended geological and geophysical exploration of basinal settings
in different areas of the eastern Mediterranean demonstrate the
existence of a Holocene mud layer several metres in thickness (up
to more than 20 m) and typically showing a graded basal part. The
event producing this peculiar deposit is correlated with the
gigantic 'Bronze Age' or Minoan eruption of the Santorini volcano
(3500 years BP), which resulted in caldera collapse and supposedly
produced a strong seismic sea-wave, that is a tsunami. Order of
magnitude calculations demonstrate that the wave speed was
sufficient to induce erosion and liquefaction of the soft
unconsolidated sediments draping the deep-sea floor. The event is
recorded in over 50 deep-sea cores recovered in the last 20 years
which contain the fine grained 'Homogenite' layer  starting with a
fining-upwards sandy base and having a thickness of more than 24 m
in the Sirte Abyssal Plain area. Several depositional models
related to setting and source areas and based on thickness,
composition, carbonate content and sedimentary structures of the
deposits have been proposed.

========================

P. C. Buckland, A. J. Dugmore & K. J. Edwards: Bronze Age myths?
Volcanic activity and human response in the Mediterranean and
North Atlantic regions. In: ANTIQUITY 273 (1997), pp. 88-105

A first rule of statistics is that the existence of a correlation
does not itself prove a causal connection. This is the heart of
the recurrent question in later European prehistory whether in the
Mediterranean or in the Atlantic northwest about volcanic
eruptions, their impact on climate, and then of the climatic
impact on human populations. The burial under tephra of the Late
Bronze Age settlement of Santorini is proof of a particular
catastrophe: but is there the evidence to prove wider European
calamity?

A search for precision beyond that currently available is a frequent
aspect of archaeological interpretation. Tensions exist as a
result of the need to resolve events on a human time-scale using
techniques often incapable of producing such accuracy or
precision. Dendrochronology, ice-core analysis and
tephrochronology, where data-resolution can be constrained either
by annual to sub-annual banding or precise isochrones, can make
important contributions to tackling the persistent chronological
problems in archaeology. In these interdisciplinary transfers
there is always the danger that the necessary caution about the
ways in which the data are used may be lost. This problem is
particularly acute when the events being studied are real, or
imagined catastrophes (cf.. White & Humphreys 1994). Catastrophes
be they the destruction of Bronze Age Thera, the modern island of
Santorini in the Aegean, or the apparent collapse of Middle Bronze
Age settlement in upland Britain are headline news; of such things
myths and reputations are born and enter the literature as if
proven fact. This paper examines some of the available evidence
for these two Bronze Age 'catastrophes', the one real and in need
of a calendar date, the other hypothesized on archaeological
grounds and dated by a tenuous link through tree rings to an
Icelandic volcano. Since Marinatos (1939) connected a major
eruption on Santorini, which destroyed the extensive Late Bronze
Age town at Akrotiri, with the end of Minoan Crete, the date of
this eruption has generated more discussion and controversy than
perhaps any other cataclysmic event in prehistory. Initial
archaeological considerations favoured a date close to c. 1500 BC
(Renfrew 1990a), whilst later, calibrated radiocarbon dates tended
towards the 17th century BC (Kuniholm 1990). In 1977, Hammer noted
a correlation between acidity, measured by electrical conductivity
in the Crête ice core from central Greenland, and the timing of
volcanic events on a world scale. It was further suggested that
one particular acidity peak lay sufficiently close to the
archaeological evidence for the date of the eruption to be that
generated by Santorini (Hammer et al. 1980). By the counting of
annual layers of ice accumulation in the core, this provided a
date of 1390&plusmn;50 BC. On the additional evidence of the Dye 3 core
from southern Greenland, this was subsequently revised to 1645&plusmn;7
BC (Hammer et al. 1987). LaMarche & Hirschboeck (1984), working on
tree rings from the American Southwest, had noted 'frost rings'
(lines of severely retarded growth) which they associated with
unseasonally cold conditions and correlated with major eruptions;
on this basis, they suggested a date of 1628&shy;1626 BC for the
Santorini eruption. This was taken up by researchers at the
Queen's University Palaeoecology Centre in Belfast, who sought
volcanic impact in the extended oak chronology from Ireland.
Baillie & Munro (1988) located a particularly narrow series of
rings beginning in 1628 BC; influenced by Lamb's (1970) discussion
of the impact of volcanoes on climate, they equated these with a
stratospheric dust veil from Santorini. Despite several cautionary
comments from both archaeologists (Manning 1988; Warren 1988) and
geologists (Pyle 1989; 1990), the 1628 BC date, or one close to
it, continues to be accepted (e.g. Michael & Betancourt 1988),
without questioning why the effects of the Santorini eruption
should be especially recognizable in the ice-core and tree-ring
sequences. Large-scale explosive volcanic activity is common on a
global scale (Zielinski et al. 1996), and so before accepting the
possibility that the Santorini eruption can be recognized by
unusual perturbations in the regional records of ice-cores or
tree-rings, the case for its distinctive character must be proved.
Despite the lack of critical assessment of these basic
assumptions, Renfrew (1990b), in his summing up to the Third
International Congress on Santorini in 1988, went so far as to
suggest that, by the time of the next Congress, the date of
the eruption would be unequivocally known to within one year. If
the Aegean and Anatolian tree ring sequences (cf. Kuniholm 1995;
Kuniholm et al. 1996) can be tied in from trees lying directly in
the paths of the fall-out cloud, this may well be the case. The
correlations from California to Greenland, Ireland and the Aegean,
however, rely upon suppositions which find questionable support in
the basic scientific evidence.



*
From:             Bob Kobres <bkobres@uga.cc.uga.edu
Organization:     University of Georgia Libraries
To:               Benny J Peiser <B.J.PEISER@livjm.ac.uk
Date sent:        Tue, 14 Oct 1997 13:26:04 EST
Subject:          1828 Essay On Comets
Copies to:        cambridge-conference@livjm.ac.uk
Priority:         normal
 
 

To provide a feel for the academic atmosphere regarding the past and
potential influence of comets on the Earth, at a time period just prior to
the first publication of Lyell's Principles of Geology, I have made
available on my web server section III of David Milne's Essay On Comets.
This paper was awarded the first of the Dr. Fellowes's Prizes from the
University of Edinburgh and was published by that institution in 1828.

bobk

http://abob.libs.uga.edu/bobk/peocomet.html
 

Bob Kobres

email= <bkobres@uga.cc.uga.edu
url= http://abob.libs.uga.edu/bobk
phone= 706-542-0583



CCCMENU CCC for 1997

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of

The content and opinions expressed on this Web page do not necessarily reflect the views of nor are they endorsed by the University of Georgia or the University System of Georgia.