RE: NS reversals

[David Dockstader <DRDOCK00@UKCC.UKY.EDU>]

While the correlation of mag reversals with major extinctions or the formation
of new species seems attractive it has not stood up to close scrutiny and the
idea has been discarded by mainstream geologists.  I believe David Raup and
his group at Chicago have done most of the research in this area.

Ice ages and mag reversals don't correlate at all.  Ice ages have recognizable
periodicity which can be correlated with astronomical events (like the wobble
of the earth's axis).  Periods are on the order of 10s and 100s of thousands of
years.  Mag reversals occur on the order of several millions or 10s of millions
of years and no recognizable periodicity has been found.

Only the extinction 65 million years ago (that took out dino and about 90% of
all species of life) has been linked to a meteor impact.  Models of the impact
at Ch.... in the Yucatan suggest that the meteor was not big enough to create
such an effect just form inert dust thrown into the air (I've also heard that
early models of the nuclear winter greatly exaggerated the effect).  The
answer appears to be that the rocks in the area contain a lot of sulfur and
that along with the cold and the dark there was a severe case of acid rain.
In fact the acid may have been the main killer.  This fits nicely with the
observation that land species were hurt the worst, deep water species hurt the
least and shallow water species experienced intermediate losses.

The difference in rotation rate between crust and core is a recent discovery
and at this point correlation with ice ages, meteor impacts, etc. is pretty
much speculation.  While a record of the paleo magnetic field is preserved in
igneous rocks, I can think of no way that this spin mismatch would be recorded
and thus no way to test or verify the speculation.

Conventional wisdom (most geology texts) states that during a reversal the
earth's mag. field is reduced to a fraction of its normal strength and then
rebuilds in a reversed polarity, with the reversal taking place over a thousand
years or so.  As mentioned Understanding Earth is an excellent intro text and
general reference for things geological.  For geophysics P.V. Sharma writes
a book that I like.  It is more earth physics and less oil and mineral
exploration than most geophysics texts.

On Wed, 06 May 1998 17:13:41 EDT David Bowman said:
> Regarding Joel's interesting question:
> > Does anybody if there has been any attempts to correlate the reversing
> > events, when there must be a period of "randomized" field, with speciation?
> > I suspect that the time scales may be grossly different, just curious.
>
> This is a very interesting speculation.  I have not heard of such an
> attempted correlation (this certainly is not to say that I don't think
> there has been any--just that I haven't heard of it; not being a
> paleontologist it would be reckless of me to think otherwise).  But I
> think I remember reading a Scientific American article a number of
> years ago that reported on a supposed correlation between major
> *extinctions* and magnetic field reversals/fluctuations.  As I recall, the
> scenario went something as follows:
>
> Every million-score years or so a large celestial object (asteroid or
> comet) strikes the Earth--the aftermath of which causes an extinction
> event.  One of the effects is that as a result of the extra greenhouse
> gases thrown up into the atmosphere (both as a direct consequence of the
> collision event and from the subsequent extra volcanism caused by the
> disruption of the crustal plates from the seismic shock of the event) the
> Earth's surface heats up (after the temporary winter ends due to the
> settling of the world-wide pall of suspended soot and dust particulates).
>
> The higher temperatures causes increased precipitation in the temperate and
> especially polar regions as a greater amount of heat (er, thermal energy)
> is transmitted poleward by the increased storms spawned by the greater
> polar-to-tropic temperature gradient.  All this extra precipitation in the
> polar regions causes ice caps to grow inducing a major glaciation.  Taking
> so much water out of the tropics and placing it closer to the Earth's spin
> axis near the poles lowers the sea level and reduces the Earth's moment of
> inertia about its spin axis (in spite of the warmer tropical water
> expanding in the ocean basins near the equator).  The Earth then spins
> faster as it conserves its spin angular momentum.  But the increased spin
> rate appears mostly in the hard crust and the fairly stiff mantle.  The
> liquid outer core is more decoupled from the rest of the planet so that
> the outer layers of the Earth spin faster leaving the core to rotate at
> its former speed.  The extra shearing effect at the mantle-outer core
> boundary disrupts the convective cells in the outer core leading to a
> collapse of the cells and the magnetic field.
>
> After a while dissipation causes the core and the rest of the planet to
> spin at a common rate once again.  New stable convective patterns are
> produced in the outer core which then results in the establishment of a new
> stable (possibly reversed) dynamo-generated magnetic field configuration.
> Presumably, once the surface climate goes back to normal the ice caps melt
> back to their former size and the water returns to the oceans and the
> Earth's moment of inertia changes again.  I don't know if this is also
> supposed to result in a magnetic field fluctuation (maybe reversal) as well.
>
> I don't recall how much, if any, evidence there actually is for this
> scenario, but I think that there are supposed to be many more field
> reversals than there are extinction events.  So there ought to be some
> other mechanism for most of the field fluctuations/reversals besides
> the one described here.
>
> David Bowman
> dbowman@georgetowncollege.edu