rwb@scicom.UUCP (03/26/87)
I have become very interested lately in the subject of mass extinctions
in earth's past. Apparently, with remarkable regularity, about every
26 million years a large percentage of the life forms on our planet
die out.
I know that there are many proposed explanations to this phenomenon.
Perhaps an object of a fairly large mass out on the fringes of our
solar system interferes with the Oort Cloud, sending a barrage of comets
at the earth. I've also heard of many other explanations: everything
from near-by supernovas to aliens.
I would appreciate any information on this phenomenon as well as
current hypotheses explaining it. Thanks.
Robert Brumley
POST: 4661 S. Vivian St.
Morrison, CO 80465
(303) 978-1838
UUCP: (isis,hao)!scicom!rwbdplatt@teknowledge-vaxc.UUCP (03/27/87)
Robert Brumley asked for information about the 26-million-year
periodicy in mass extinctions that has been reported, and the
mechanisms that have been proposed that might account for it.
There's a lot of controversy about this issue in the scientific
community. Quite a few scientists feel that the apparent periodicy of
mass extinctions may not really be there... that it may be a
statistical fluke resulting from the mathematical model used to plot
the species-extinction rates. Even if there is a periodicy, this
doesn't necessarily imply that extraterrestrial influences (e.g. comet
showers) are the sole, or even the primary cause.
The "Nemesis" (a.k.a. "death star") hypotheses has fallen rather far
out of favor recently, for a number of reasons. For one, nobody has
located Nemesis; for another, celestial mechanics seem to predict
that if our sun had a companion with a 26-million-year orbit, it would
be orbiting so far away that gravitational influences from other stars
would have disrupted its orbit (and possibly stolen it away) long ago.
In any case, if Nemesis is actually out there, the chances are that
the IRAS satellite probably recorded its presence, and it'll be
identified when the IRAS data is eventually analyzed.
One of the things that has influenced the ET-influence-on-extinction
issue is the report by Alvarez et al concerning anomalously high
levels of iridium in the clay layer corresponding to the time of one
of the major extinctions. Alvarez hypothesized that the iridium
probably came from an extraterrestrial object (a comet or asteroid)
which collided with Earth and raised a very large dust cloud. [I've
seen suggestions that the point of collision may have been in the
northern Atlantic ocean, on the mid-oceanic ridge... the levels of
iridium seem to be higher in that area. It was suggested that the
asteroid punched through the thin crust at that point, triggering an
upwelling of magma, and voila! Iceland is born!] There's some
evidence that other extinction episodes occurred at about the same
time as other [possible] asteroid or large-meteoroid impacts [there's
a suggestion that the Okefenokee Basin in Florida is the site of one
of those impacts].
On the other hand... there have been a large number of extinction
episodes throughout history, and the high-levels-of-iridium signature
doesn't show up on all of them (or even many of them, I gather). It
has also been suggested that the raised levels or iridium could arise
from major outburst of volcanic activity, which could bring up large
amounts of iridium-enriched magma from the Earth's mantle, and could
also lead to ecological disturbances (dust, ash, sulphur dioxide
outgassing) just as distruptive as those resulting from an asteroid or
comet impact.
It's also worth noting that the dinosaurs had been undergoing a long
and fairly gradual fall-off (in the number of species, at least) prior
to the [apparently] sudden extinction event. There is some reason to
believe that while an impact event may have dealt the "coup de grace"
to the dinosaurs, they may already have been under substantial
survival pressure from some other cause (or causes).
Other mechanisms have been proposed to account for some of the
non-iridium-related extinctions. In particular, climatic change
triggered by the motion of continental plates across the surface of
the globe can have a major effect on animal and plant populations.
Several of the major extinction episodes have occurred when one or
another major continent passed across the south polar region... it has
been suggested that this results in massive continental glaciation,
leading to a reduction in global temperatures and a change in ocean
water levels and circulation patterns, global weather, and such
second-order effects. Interestingly, when mass extinctions occur in
the ocean, the species most affected seem to be tropical species that
can't survive in cooler waters; temperate-zone species tend to survive
more readily.
For a good summary of the currently-recognized extinction episodes,
and an analysis of them (by a scientist who clearly favors the
climatic-change hypothesis), I recommend "Extinction", a book recently
published by the Scientific American Library.
Apologies to all for any mis-statements of fact that I've made in this
posting; I'm strictly an enthusiastic amateur. Corrections,
clarifications, and opposing points of view are encouraged.
Dave Platt
Internet: dplatt@teknowledge-vaxc.arpa
UUCP: {hplabs|sun|ucbvax|seismo|uw-beaver|decwrl}!teknowledge-vaxc.arpa!dplatt
Voice: (415) 424-0500
USnail: Teknowledge, Inc.
1850 Embarcadero Road
Palo Alto, CA 94303werme@alliant.UUCP (03/27/87)
Subscribe to Science News. They've had dozens of articles on the various hypotheses over the last few years. With few exceptions (like SN1987A as sci.astro readers know), there's just about no single source of better information (Omni, Discover) or faster (Scientific American, NY Times, radio, TV). It's even readable! -- The Art of Programming Ric Werme needs to be tempered with uucp: decvax!linus!alliant the Structure of Engineering Phone: 603-673-3993
g-rh@cca.UUCP (03/29/87)
In article <11235@teknowledge-vaxc.ARPA> dplatt@teknowledge-vaxc.ARPA (Dave Platt) writes: >... It >has also been suggested that the raised levels or iridium could arise >from major outburst of volcanic activity, which could bring up large >amounts of iridium-enriched magma from the Earth's mantle, and could >also lead to ecological disturbances (dust, ash, sulphur dioxide >outgassing) just as distruptive as those resulting from an asteroid or >comet impact. > The volcanic origin theory for the iridium layer had some popularity. However I believe it is now pretty much ruled out. The Alvarez layer is anomalously rich not only in iridium but also in a number of related metals, Osmium among them. Osmium has two stable isotopes, 186 and 187. In Earth's lithosphere the ratio of isotope 187 to 186 is about 10:1; in meteorites it is about 1:1. In abyssal ooze it is between 6 and 8.5 (abyssal sediments contain signifigant cosmic contamination.) In the Alverez layer the ratio is ~1.5 (the deviation being due to terrestrial contaminants.) Confirmation of nonterrestrial isotope mixtures have been found for other metals. Also worth noting is that an iridium spike has been reported for the precambrian/cambrian extinction. Ref: The Great Dying, Kenneth Hsu, 1986, Harcourt Brace Jovanovich -- Richard Harter, SMDS Inc. [Disclaimers not permitted by company policy.]
gallagher@husc4.UUCP (03/30/87)
In article <784@scicom.AlphaCDC.COM> rwb@scicom.AlphaCDC.COM (Robert Brumley) writes: > >I have become very interested lately in the subject of mass extinctions >in earth's past. Apparently, with remarkable regularity, about every >26 million years a large percentage of the life forms on our planet >die out. > >I would appreciate any information on this phenomenon as well as >current hypotheses explaining it. Thanks. > This information is from "Periodicity in Marine Extinction Events" by J.J. Sepkoski, Jr., and David M. Raup, in Dynamics of Extinction, ed. by D.K. Elliott: The article presents a statistical analysis of extnction rates over the past quarter billion years and strongly supports the hypothesis that mass extinctions have occurred regularly every 26.2 million years, +/- 1 million years. Since they occur at regular intervals, it is very likely that they result from some single ultimate cause, but there are "no obvious terrestrial or solar processes operating cyclically on time scales of 10^6 to 10^7 years", except for the unproved hypothesis of periodic magnetic reversals. So the ultimate cause may be extraterrestrial. This is supported by: impact debris of apparent extraterrestrial origin at two of the extinction horizons (the Maestrichtian and Late Eocene); meteoric debris and geochemical anomalies within 7 million years of where an extinction event should have occured in the 26 m. yr. cycle but did not; and the observation that "terrestrial cratering may display a periodicity that is in phase with that of extinction events". There have been 4 proposals about the astronomical cause of these events, but they remain basically guesses: "1. Transit of the solar system through the spiral arms of the Galaxy. 2. Vertical oscillation of the solar system about the plane of the Galaxy. 3. Precession of an undetected tenth planet. 4. Orbital dynamics of an unobserved solar companion." In more detail: 1. Napier and Clube (Nature (1979), Vol, 282, pp. 455-459.) pointed out that the solar system passes through the spiral arms of the galaxy every 50 million years and that during this time the solar system may capture interstellar "planetismals" which would increase asteroid bombardment of the earth. Also, the tidal forces from the spiral arms might perturb the cometary cloud surrounding the solar system and in this way increase asteroid bombardment of the earth. Unfortunately, this is a cycle of 50 million years, which is twice the observed cycle of 26 million years. It's possible that 1/2 the extinction events are cause by the passage through the spiral arms and 1/2 are random, but Sepkoski and Raup reject this explanation because the mass extinctions are simply too regular to permit more than one random event. 2. The solar system oscillates vertically through the galactic plane every 52 to 74 millon years. So, it reaches an oscillatory extreme every 33 million years. Schwartz and James (Nature (1984), Vol. 308, pp. 712-713) suggest that the Earth might be subjected to extremes of soft x-rays and hard UV radiation from the galactic center at these extremes. This radiation might disturb the upper atmosphere and thus cause mass extinctions. Alternatively. Rampino and Stothers (Nature, Vol. 308, pp. 709-712, and Science, (1984), Vol. 226, pp. 1427-1431) argued that tidal forces from molecular clouds concentrated near the galactic plane "might perturb the Oort Cloud and inner cometary reservoir as the solar system approached the plane", which could produce comet showers of several million years duration, such that several comets would strike the Earth. However, once again the mass extinction events occur in a 26 million year cycle, not a 33 million year cycle. Also, Thaddeus and Chanan (Nature (1985), Vol. 314, pp. 73-75) argued that the concentration of matter near the galactic plane is not so concentrated as previously assumed and probably would not affect the Oort Cloud significantly. 3. Whitmore and Matese (Nature (1985), Vol. 313, pp. 36-38) argued that "Planet X", the possible tenth planet beyond Pluto, "might be able to produce periodic comet showers if its orbit were highly inclined and if the inner edge of the comet cloud extended as a thin disk almost to the orbit of Neptune." This would cause Planet X to precess through the planetary plane. If the precession period of the planet was 56 million years, the planet, with 1 to 5x the mass of Earth, "would sweep comets out of the inner disk as it passed through the plane every half-period", causing maybe 10^5 comets to cross Earth's orbit and possibly impact. It is questionable whether the approach of the planet would be rapid enough and its mass great enough (it must be small since it has never been observed) to cause a large number of comets to scatter. In any case, the precession period of this possibly non-existent planet is bases entirely on the periodicity of the mass extinction events, so it is "very speculative" to say the least. 4. Davis, Hut, and Muller (Nature, Vol. 308, pp. 715-717) and Whitmire and Jackson (Nature, Vol. 308, pp. 713-715) proposed that the Sun may be part of a binary star system with an undetected companion of low mass and density. If you assume it has a periodicity of 26 million years, it should have a major axis of about 3 light years and a perihelion distance of about 0.3 to 0.5 light years from the Sun, which is within the inner comet reservoir. Its mass is estimated to be 1/10 to 1/1000 of the sun's mass. Thus, this star, called "Nemesis", would perturb about 10^9 comets into the inner solar system at perihelion, of which about 10 to 200 should impact the Earth over a 10^5 to 10^6 year period. Nemesis' orbital periodicity should be irregular, varying 10 to 20%, and the orbital configuration would have a "half-life" of about a billion years. The problem with this hypothesis is, of course, that it's just speculation. In any case, observing anything as small and dark as Nemesis would be very difficult. The mass extinction events over tha past quarter billion years are as follows: 1. Guadalupian. This is the Late Permian mass extinction. More than 50% of marine families and 95% of marine species become extinct. The trilobites, rostroconchs, and blastoids disappeared entirely. Brachiopods, crinoids, cephalopods, and corals lost more than 20 families each. 248-258 million years ago. 2. Rheatian. The Late Triassic mass extinction. Conodonts and conulariids disappeared, and cephalopods, brachiopods, bivalves, gastropods, and marine reptiles suffer major losses. 213-225 million years ago. 3. Pliensbachian. Early Jurassic. Many cephalopod extinctions, and scattered extinctions in other groups. 194-200 million years ago. 4. Tithonian. End of Jurassic. Many cephalopod, bivalve, and dinoflagellate extinctions. 144-147 million years ago. 5. Cenomanian. Extinctions among cephalopods, echinoids, osteichthyan fishes, sponges, dinoflagellates, and globegerinids. 90 to 91 million years ago. 6. Maestrichtian. End of Cretaceous. All ammonoids, rudistids, stromatoporoids, pleisiosaurs disappear. Also, many families of other cephalopods, bivalves, gastropods, bryozoans, echinoids, sponges, osteichthyan fishes, marine reptiles, and microplankton become extinct. Meanwhile on land, the dinosaurs disappear. 64-65 million years ago. 7. Late Eocene (Priabonian). Decline in diversity among many groups, especially microplankton. 38-40 million years ago. 8. Middle Miocene (Kanghian-Serravallian). High rates of extinction among various groups, especially microplankton, but again no taxonomic group is inordinately affected. 2-14.4 million years ago. These do not form a 26 million years cycle, since certain mass extinctions don't occur as expected. There are small extinction peaks in the Olenakian in the Early Triassic and in the Bajocian in the Middle Jurassic, but there are none in the Callovian (Middle Jurassic) or the Hauterivian (Early Cretaceous) as would be expected. However, using statistics Sepkoski and Raup show that the data nonetheless show a cyclic pattern. They treat the "measures of extinction intensity as continuous time series and search for temporal regularities in the fluctuations" using standard Fourier analysis and also "treat the identified extinction peaks as discrete events and test for regularity in their timings" using a nonparametric randomization procedure. (I don't know enough about statistics to evaluate the math, but since this theory has been well-exposed I think you can trust it.) Paul Gallagher
guy@slu70.UUCP (03/30/87)
In article <784@scicom.AlphaCDC.COM>, rwb@scicom.AlphaCDC.COM (Robert Brumley) writes: > > I have become very interested lately in the subject of mass extinctions > in earth's past. Apparently, with remarkable regularity, about every > 26 million years a large percentage of the life forms on our planet > die out. > Note that the periodicity proposed by Raup and Sepkowski is still rather contraversial and is disputed by many scientists, especially paleontologists. Most of the argument hinges on the definition of an extinction (species go extinct all the time and defining a *mass* extinction can be somewhat arbitrary). A second problem is that dating mass extinctions is difficult as the usual radiometric techniques do not work for sediments (you end up dating the source rock which may be far removed from the sediment). I've heard one comment that human civilization may qualify as a mass extinction agent given the number of species that we have or are in the process of eliminating. Your friendly net geophysicist, Guy M. Smith
gwe@cbosgd.UUCP (03/31/87)
In article <14443@cca.CCA.COM> g-rh@CCA.UUCP (Richard Harter) writes: >In article <11235@teknowledge-vaxc.ARPA> dplatt@teknowledge-vaxc.ARPA (Dave Platt) writes: >>... It >>has also been suggested that the raised levels or iridium could arise >>from major outburst of volcanic activity (...) >> > The volcanic origin theory for the iridium layer had some >popularity. However I believe it is now pretty much ruled out. The >Alvarez layer is anomalously rich not only in iridium but also in a >number of related metals, Osmium among them. Osmium has two stable >isotopes, 186 and 187. In Earth's lithosphere the ratio of isotope 187 >to 186 is about 10:1; in meteorites it is about 1:1. In abyssal ooze >it is between 6 and 8.5 (abyssal sediments contain signifigant cosmic >contamination.) In the Alverez layer the ratio is ~1.5 (the deviation >being due to terrestrial contaminants.) Confirmation of nonterrestrial >isotope mixtures have been found for other metals. Just how certain are we of the composition of the earth's magma ? Given the conditions existing beneath the crust, it seems reasonable that significant elemental segregation would occur; i.e., there would be local variations in the composition. I believe such variations are credited for the creation of gold veins, etc. Further, it seems not unlikely that osmium might preferentially segregate to the melt, rather than the lithosphere, and that the ratio of isotopes might be different, as well. Perhaps the (comparatively) small amount of volcanic eruption in "recent" history is sufficiently low as to draw magma only from the region nearest the crust; this magma would more closely approximate the crust's composition. Under periods of greater volcanic activity, magma would be tapped from deeper within the earth, and might have heavy metal contents and isotopic ratios consistent with the Alverez layer. I dunno; this is just off the top of my head. Critiques are welcome. ------------------------------clip and save---------------------------------- Bill Thacker cbatt!cbosgd!gwe DISCLAIMER: Farg 'em if they can't take a joke ! "The two most common things in the Universe are hydrogen and stupidity" -----------------------------valuable coupon---------------------------------
guy@slu70.UUCP (03/31/87)
In article <1522@husc6.UUCP>, gallagher@husc4.HARVARD.EDU (paul gallagher) writes: > some single ultimate cause, but there are "no obvious terrestrial or solar > processes operating cyclically on time scales of 10^6 to 10^7 years", except > for the unproved hypothesis of periodic magnetic reversals. So the ultimate Not only is this hypothesis unproved, there are excellent reasons for believing it to be untrue. Careful statistical analysis of the reversal record shows no periodicity whatsoever. The probability of reversal is best modeled as a completely stochastic process (although there are gradual changes in the reversal rate). See "The Earth's Magnetic Field" (Merrill and McElhinny) for more details and citations. Your friendly geophysicist, Guy M. Smith
guy@slu70.UUCP (04/02/87)
In article <3497@cbosgd.ATT.COM>, gwe@cbosgd.ATT.COM (George Erhart) writes: > Just how certain are we of the composition of the earth's magma ? Given the > conditions existing beneath the crust, it seems reasonable that significant > elemental segregation would occur; i.e., there would be local variations in > the composition. I believe such variations are credited for the creation of The variations are not that large. the segregation of gold is due to processes occurring after the creation of magmas (mainly hydrothermal) not variations in magma source. We can measure compositions of lots of volcanic rocks, both recent and older, so statements about the composition and homogeneity of the mantle are tolerably well supported. > gold veins, etc. Further, it seems not unlikely that osmium might preferentially > segregate to the melt, rather than the lithosphere, and that the ratio of > isotopes might be different, as well. This is probably the case (none of my references discuss osmium specifically but I suspect that it is a large ion lithophile). Most of the ratios used involve ratios of isotopes which are chemically identical and segregate in the same fashion. They also reflect the ratio (not the absolute abundance) in the source region. > > Perhaps the (comparatively) small amount of volcanic eruption in "recent" > history is sufficiently low as to draw magma only from the region nearest Magmas which have to pass through continental crust often have a crustal component. Isotope ratios often allow this to be determined, at least qualititatively. Certain kinds of rocks (e.g., basalts) can only have a mantle source. There is simply no way to produce them from crustal rocks without completely melting the rock which is highly unlikely. There is also seismic evidence (e.g., for hawaii) which places the source well below the base of the crust. > the crust; this magma would more closely approximate the crust's composition. > Under periods of greater volcanic activity, magma would be tapped from deeper > within the earth, and might have heavy metal contents and isotopic ratios > consistent with the Alverez layer. There is no evidence for this happening. There is evidence for different geochemical reservoirs in the mantle but they involve different kinds of volcanoes (e.g., oceanic island vs mid-ocean ridge) rather than changes in volcanic activity with time. This should be discussed in any decent book on igneous petrology as well as a number of papers (some quite accessible to non-geochemists) by De Paolo and Wasserburg. I can provide exact references if you like. > I dunno; this is just off the top of my head. Critiques are welcome. Check out the article by Officer et al. in the Mar. 12 issue of Nature for the most recent exchange in this ongoing battle. I haven't read it yet so I don't know if I believe them or not.
nather@ut-sally.UUCP (04/02/87)
In article <10@slu70.UUCP>, guy@slu70.UUCP (Guy M. Smith) writes: > Note that the periodicity proposed by Raup and Sepkowski is still rather > controversial and is disputed by many scientists, especially paleontologists. > > Your friendly net geophysicist, > > Guy M. Smith ...and anyone else who deals with time-series analysis of noisy data. When astronomers get results as uncertain, they go back to the telescope to get more data. The basic problem lies in the tranformation of noise (as well as signal) in the power spectrum analysis process. The resulting noise is far from friendly (Gaussian) and is, I'm told, a Chi-squared distribution with n-1 degrees of freedom, where n is the number of data points in the spectrum. I *do* know it is VERY easy to get "peaks" in the power spectrum due to happenstance noise buildup that have no basis in reality (whatever *that* is). This is not to say the claimed periodicity is not real --- just not proven. I'd suggest more data, with less noise. How to do that is left as an exercise for the geophysicists ... Your friendly curbstone astronomer, -- Ed Nather Astronomy Dept, U of Texas @ Austin {allegra,ihnp4}!{noao,ut-sally}!utastro!nather nather@astro.AS.UTEXAS.EDU
gallagher@husc4.UUCP (04/06/87)
In article <7697@ut-sally.UUCP> nather@ut-sally.UUCP (Ed Nather) writes: >...and anyone else who deals with time-series analysis of noisy data. When >astronomers get results as uncertain, they go back to the telescope to get >more data. The basic problem lies in the tranformation of noise (as well as >signal) in the power spectrum analysis process. The resulting noise is far >from friendly (Gaussian) and is, I'm told, a Chi-squared distribution with >n-1 degrees of freedom, where n is the number of data points in the spectrum. >I *do* know it is VERY easy to get "peaks" in the power spectrum due to >happenstance noise buildup that have no basis in reality (whatever *that* is). >This is not to say the claimed periodicity is not real --- just not proven. >I'd suggest more data, with less noise. >-- >Ed Nather Here is Raup and Sepkoski's reply (SciENcE, 2/21/86, Volume 231, pp.833-836): The claims of periodicity have produced considerable controversy. Many of the negative criticisms can be summarized by two fundamental arguments: first, periodicity is just an artifact of uncertainties in the geologic time scale or in the identification of extinction events; second, periodicity is the natural consequence of many complex causes of extinction operating independently. The first argument says that inclusion of random noise in the form of spurious data could create the appearance of periodicity where none actually exists. The analyses in question start by asking whether extinction events are randomly distributed in time. This is the fundamental null hypothesis for formal statistical testing as well as the conventional wisdom in paleontology. Only if this hypothesis of randomness can be rejected with high confidence can a search for nonrandom patterns begin. Inaccurate geologic dates or nonexistent extinction events will degrade the sample in a direction toward randomness and away from any regular signal. Thus, to include uncertain ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ data is to make statistical testing more conservative. To argue that uncertainty in data explains the observed periodicity is illogical. The second argument is based on a misconception of randomness. If extinction events (as opposed to individual species extinctions) are caused by a complex of time-independent processes, they should exhibit a random (Poisson) distribution in time, typified by irregular clusters of closely spaced events separated by gaps of widely varying length. Even if the individual events in a cluster cannot be distinguished because of poor time resolution, the clusters themselves will be irregularly spaced. The surprisingly uniform spacing of extinction events in Mesozoic-Cenozoic time is thus distinctly atypical of phenomena driven by complexes of independent processes. Raup and Sepkoski perform their time-series analysis using a method developed by Stothers,"in which the goodness of fit of the timings of the extinction events to a set of periodic impulse functions with differing wavelengths is assessed. Goodness of fit is measured by the standard deviation of the differences between observed and expected times of extinction events for a given period length (in its best fit position). The lower the standard deviation, the better the fit. For each individual period length, the probability that the observed fit could have occurred by chance was computed by comparing that fit with fits obtained by a large number of randomized versions of the same data. " The reference given is R.B.Stothers, Astron. Astrophys. Vol. 77, 121 (1979). Their conclusion is that the eight major extinction events of marine families over the past 250 million years "stand significantly above local background (P < 0.05)"; the events are more pronounced when when analyzed at the level of genus. Time series analysis strongly suggest a 26-million year periodicity; "when the time series is limited to the four best-dated events..., the hypothesis of randomness is also rejected for the 26-million year period (P < 0.0002)." Raup is very persuasive, but, unfortunately, I'm in no position to evaluate his use of statistics, so I'll leave to others on the net to do so. I'm well aware of the way biologists and others have misused statistics, and I'm also aware that the paleontological record may be a very biased sample of previously existing life. For example, paleontologists do most of their research close to home. That means most of the fossil record comes from those areas with a day's bus ride from major universities! Paul Gallagher
thomas@bnl.UUCP (04/10/87)
> In article <1522@husc6.UUCP>, gallagher@husc4.HARVARD.EDU (paul gallagher) writes: > > some single ultimate cause, but there are "no obvious terrestrial or solar > > processes operating cyclically on time scales of 10^6 to 10^7 years", except > > for the unproved hypothesis of periodic magnetic reversals. So the ultimate > Not only is this hypothesis unproved, there are excellent reasons for believing > it to be untrue. Careful statistical analysis of the reversal record shows > no periodicity whatsoever. The probability of reversal is best modeled as > a completely stochastic process (although there are gradual changes in the > reversal rate). See "The Earth's Magnetic Field" (Merrill and McElhinny) > for more details and citations. > > Your friendly geophysicist, > Guy M. Smith In Physics Today, February 1987, p. 17, there is a report on "Do asteroid impacts trigger geomagnetic reversals?" which is based on an article by Richard Muller and Donald Morris in Geophysical Research Letters, November, 1986, p. 1177. Their coupling of extraterrestial impact to geomagnetic reversals predicts that abrupt climatic change should occur at the same time. Supporting evidence has recently become available from discoveries by R. V. Krishna- murthy and coworkers at the Ahmedabad Physical Research Laboratrory in India. (Nature 323, p. 150, 1986.) Quoting: The Karewa plateau in the Vale of Kashmir was once a vast lake. The geomagnetic reversals of the last few million years are precisely documented in the magnetostratigraphy of its deep sediment, now conveniently exposed. Examining the carbon-nitrogen ratio of the organic component of this same sediment, Krishna- murthy and company found striking, narrow peaks rising an order of magnitude above background at precisely the times (depths) of three geomagnetic reversals, including the most recent. The nitrogen concentration is taken to be a measure of the protein-rich plankton and algae population of the lake water. The Physics Today article is a "news" report, so it is quite readable even by those who are not experts in this area. It also quotes some skeptics near the end of the article. Richard A. Thomas Brookhaven National Lab