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RE: NS reversals



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