cutler@ihlpa.ATT.COM (Herber) (03/14/88)
Scientists, or those who profess to know such things, indicate that the earth has reversed it's magnetic polarity many time is the past. Has anyone given thought on the resulting effect this might have on animals, migratory birds in particular, that use this field for whatever purposes? Will Northern hemisphere birds migrate north in the winter? Are there any indications of specie disappearance during these times?
dmark@sunybcs.uucp (David Mark) (03/15/88)
In article <7387@ihlpa.ATT.COM> cutler@ihlpa.ATT.COM (Herber) writes: > >Scientists, or those who profess to know such things, indicate that >the earth has reversed it's magnetic polarity many time is the past. > Has anyone given thought on the resulting effect this might have on >animals, migratory birds in particular, that use this field for >whatever purposes? Will Northern hemisphere birds migrate north >in the winter? Are there any indications of specie disappearance during >these times? It turns out that migration cage studies in Europe have shown that (at least some) birds use the *slope-direction* of the Earth's magnetic field, rather than the polarity, to orient during migration. The lines of magnetic force plunge downward in the direction toward each pole. The experiment went something like this: 1) capture a southbound migrating warbler in the autumn and put it in a cage; the cage has a floor which slopes toward the cage center (funnel shaped), and an ink-pad in the center. The bird leaves footprints on the funnel which can be analyzed statistically to determine the direction that the bird is trying to fly. 2) surround the cage with a coil in which a magnetic field, considerably stronger than the Earth's, can be generated. 3) - when the coil tilts downward toward true north, and the polarity agrees with the Earth's natural direction, the bird continues to try to fly south; - when the polarity is reversed but the tilt is not changed, the bird keeps flying south (toward the 'north' polarity of the artificial field); - however, when the polarity is kept the same as the Earth's but the tilt is tipped southward, the birds turn around and head northward (toward the direction that the lines of magnetic force head upward from the horizontal). If reversals of the Earth's magnetic field took place in a short time (less than a year), there would be a very strong selective pressure in favor of this system and against against a polarity-based navigation scheme. I don't recall how many species have been tested in this way. I could look up the reference if anyone is interested. It was described in a small but good monograph on bird navigation; my recollection is that the author's name was Schmidt-Koenig. David Mark dmark@joey.cs.buffalo.edu geodmm@ubvms.BITNET
hdunne@amethyst.ma.arizona.edu (|-|ugh) (03/15/88)
In article <7387@ihlpa.ATT.COM> cutler@ihlpa.ATT.COM (Herber) writes: } Will Northern hemisphere birds migrate north }in the winter? Are there any indications of specie disappearance during }these times? Geomagnetic reversals take place at random intervals on the order of several million years. Each individual reversal takes about ten thousand years for the process to be completed. Hugh Dunne | UUCP: ..{cmcl2,ihnp4,seismo!noao}!arizona!amethyst!hdunne Dept. of Math. | Phone: | ARPA: hdunne@amethyst.ma.arizona.edu Univ. of Arizona | +1 602 621 4766 | Bitnet: hdunne@arizrvax Tucson AZ 85721 | +1 602 621 6893 | Internet: hdunne@rvax.ccit.arizona.edu
karish@denali.UUCP (karish) (03/16/88)
In article <460@amethyst.UUCP> hdunne@amethyst.ma.arizona.edu (|-|ugh) writes: >In article <7387@ihlpa.ATT.COM> cutler@ihlpa.ATT.COM (Herber) writes: > >} Will Northern hemisphere birds migrate north >}in the winter? Are there any indications of specie disappearance during >}these times? > >Geomagnetic reversals take place at random intervals on the order of several >million years. Each individual reversal takes about ten thousand years for the >process to be completed. > During polarity reversals, which probably take ten thousand years or more, the earth's magnetic field is probably weak and irregular, and generally useless for navigating. The dates of ancient extinctions are not known precisely enough to allow convincing correlations with magnetic reversals; for that matter, the timing of reversals longer ago than ten million years is not known too precisely, either. Chuck
guy@slu70.UUCP (Guy M. Smith) (03/17/88)
In article <7387@ihlpa.ATT.COM>, cutler@ihlpa.ATT.COM (Herber) writes: > > the earth has reversed it's magnetic polarity many time is the past. > Has anyone given thought on the resulting effect this might have on > animals, migratory birds in particular, that use this field for The reversals of the field are well documented. They are not instantaneous by the way, best estimates are that reversals take something like five to ten thousand years during which the field is much more complex than at present (in short, no simply defined north and south poles). See "The Earth's Magnetic Field" (Merrill and McElhinny. I haven't followed the magnetic navigation of animals lately but my recollection is that even those animals that are field sensitive only use the field as a secondary mechanism (e.g., during cloudy weather). As such, reversals probably wouldn't have much effect. Attempts to correlate reversals with mass extinctions have not been successful. For what it's worth, I've got a PhD in this field and have been an active researcher for several years.
bpdickson@trillium.waterloo.edu (Brian P. Dickson) (03/18/88)
In article <20@denali.UUCP> crkarish@stanford.edu (Chuck Karish) writes: >In article <460@amethyst.UUCP> hdunne@amethyst.ma.arizona.edu (|-|ugh) writes: >>}Are there any indications of specie disappearance during >>}these times? >> >>Geomagnetic reversals take place at random intervals on the order of several >>million years. Each individual reversal takes about ten thousand years for the >>process to be completed. >> > >During polarity reversals, which probably take ten thousand years or more, ^^^^^^^^ >the earth's magnetic field is probably weak and irregular, and generally ^^^^^^^^ ^^^^^^^^^ >useless for navigating. The dates of ancient extinctions are not known >precisely enough to allow convincing correlations with magnetic reversals; ^^^^^^^^^^ >for that matter, the timing of reversals longer ago than ten million years >is not known too precisely, either. > >Chuck ***FLAME ON: AAAAAARGHH!! The 'net is supposed to be used for the dissemination of useful information, not freshmen's humble opinions. Not only is your posting conjecture, but it is most probably *wrong*! ***FLAME OFF In a recent issue of {Popular Science | Scientific American | Nat'l Geographic}, some scientists from several disciplines had worked together to produce a very convincing arguement for their theory. It links several *very* *important* elements of observed information in different areas, and could possibly give us a better understanding of several areas in science. Theorem in brief: Every 30 million years or so, another star (our sun's evil twin brother, as it were) passes very close, by cosmic distances, to our sun, close enough to disturb the Van Oort comet cloud. For the next while, a lot of comets make close passes to our sun in highly elliptical orbits. Every now and then, on the average of every 10000 years, a larger-sized comet strikes the earth's atmosphere. If it is big enough, the equivalent of a nuclear winter occurs: debris scattered into the upper atmosphere, for long enough that the earth's albedo increases significantly, and an ice age occurs. As more water freezes, the earth's rate of rotation increases. This in turn screws up the core of the planet in such a way that the magnetic field (which is weak to start with) reverses. When the air clears, the ice melts, the earth slows, and the field reverses again. For *very* big comets, it gets *real* cold, and mass extinctions occur. These scientists were geophysicists, paleontologists, and astronomers, and they plotted known data: extinctions, ice ages, magnetic reversals, increases in rare-earth metals (which are common in comets) being deposited, and so on. The coincidences are all over the time line, far too close and far too often to be taken lightly. Incidentally, the evil-twin sun could account for our planets being formed as well; the 30 million year period with high eccentricity makes finding the bugger very difficult. -- Brian Dickson, closet scientist
msb@sq.uucp (Mark Brader) (03/18/88)
> During polarity reversals, which probably take ten thousand years or more, > the earth's magnetic field is probably weak and irregular, and generally > useless for navigating. The dates of ancient extinctions are not known > precisely enough to allow convincing correlations with magnetic reversals; > for that matter, the timing of reversals longer ago than ten million years > is not known too precisely, either. I happen to have at hand a copy of "The Nemesis Affair", a book* by David M. Raup, one of the proponents of the theory of periodic extinctions. He writes: : Unlike the paleontological record, the [magnetic] reversal record is : fairly clean, at least for the past 165 million years of geologic time. : About 300 reversals have been found in this interval, most well dated. This does not necessarily disagree with the previous posting; Raup admits that the magnetic/extinction correlation is not convincing, and his idea (as a paleontologist) or "well dated" may be looser than that of someone in another field. The book is well worth reading, by the way, no matter what you think of the theory. * W. W. Norton & Co., 1986, ISBN 0-393-30409-4 paperback, $10 in Canada. Mark Brader, SoftQuad Inc., Toronto, utzoo!sq!msb, msb@sq.com Until 3,000 million years ago we can say not a lot happened although further study would not come amiss. Then signs of life appeared, including some large reptiles and, very recently, bipeds. It is too soon to say whether these bipeds will play an important part in the world's story. -- Colin Morris in "History Today"
karish@denali.UUCP (karish) (03/19/88)
In article <5792@watdragon.waterloo.edu> bpdickson@trillium.waterloo.edu (Brian P. Dickson) writes: >>During polarity reversals, which probably take ten thousand years or more, > ^^^^^^^^ >>the earth's magnetic field is probably weak and irregular, and generally > ^^^^^^^^ ^^^^^^^^^ >>useless for navigating. The dates of ancient extinctions are not known >>precisely enough to allow convincing correlations with magnetic reversals; > ^^^^^^^^^^ >>for that matter, the timing of reversals longer ago than ten million years >>is not known too precisely, either. >> >>Chuck > >***FLAME ON: AAAAAARGHH!! The 'net is supposed to be used for the dissemination >of useful information, not freshmen's humble opinions. Not only is your posting >conjecture, but it is most probably *wrong*! >***FLAME OFF > [summary of the Nemesis hypothesis follows] >-- >Brian Dickson, >closet scientist My statements were summaries of my readings of carefully-done scientific studies, as published in refereed journals. The hedging in my wording was meant to convey the analytical imprecision of the scientific data under consideration, not my personal uncertainty. If there's a demand for them, I'll post citations to support what I said; I thought that this group was interested in wildlife, not geophysics. The periodicity of mass extinctions that forms the basis for the Nemesis theory is based on data with questionable (and much-questioned) statistical significance. It's still a sexy story, though, and makes good copy in the popular press. Far from being eclipsed by the paradigm Mr. Dickson refers to, the information about the timing of magnetic reversals and extinctions are critical information which must be used to evaluate that paradigm. Mr. Dickson, if you'd like to come out of your closet, the first step might be to browse through back copies of _Science_ and/or _Nature_ at your public library, instead of quoting _Popular Science_ to me. Those magazines might give you a hint of how scientific research is done, in contrast to the approach that says "Gee whiz, listen to the trippy things that all these VERY IMPORTANT EXPERTS are saying now!". Chuck Karish
guy@slu70.UUCP (Guy M. Smith) (03/22/88)
In article <5792@watdragon.waterloo.edu>, bpdickson@trillium.waterloo.edu (Brian P. Dickson) writes: > some scientists from several disciplines had worked together to produce a very > Theorem in brief: Every 30 million years or so, another star (our sun's evil I don't know a single researcher in geomagnetism who believes this theory. As a researcher in the field myself, I either know most of the major players or have heard them speak or read their papers. The people who originally proposed it were not geophysicists and were rather hazy about a lot of relevant data. It is not possible to conclusively rule the idea out at present but most of us are *very* skeptical. There is, for instance, no evidence of any periodicity in the reversal record (see several recent papers by McFadden, Merrill, and co-authors).
bpdickson@trillium.waterloo.edu (Brian P. Dickson) (03/22/88)
In article <22@denali.UUCP> karish@denali.UUCP (Chuck Karish) writes: >... The hedging in my wording was meant to convey the analytical imprecision >of the scientific data under consideration, not my personal uncertainty. I stand corrected. However, references would be appreciated, and can tend to prevent knee-jerk flames like mine. >...I thought that this group was interested in wildlife, not geophysics. Well, certainly, but the Nemesis theory is cross-disciplinary. I would think it inappropriate to summarise the biological implications without their geophysical causes. >The periodicity of mass extinctions that forms the basis for the Nemesis >theory is based on data with questionable (and much-questioned) statistical >significance. The strongest point about the Nemesis theory is that it provide a good explanation for the irregularity of the reversals, since cometary impacts are probabilistic, whereas a single-body type intereaction would be periodic as anything. But I restate the obvious. Brian Dickson