Re: Large animal extinctions caused by early man

Scott Barboza (
Fri, 5 Jul 1996 14:49:35 -0700

On Thu, 4 Jul 1996, Richard C. Schmidt wrote:

> In my view, you have two problems.
> 1) Did, or did not the climatic changes that took place
> during that time period cause the large scale die-out
> of the larger animals?
> 2) What was the cause of the climatic changes during
> this time period?
> --------
> I feel that this entire discussion is confusing the two issues.
> The fact is, the was a change in the climate 10-12,000 years ago.
> We know this! The ice age ENDED!!!! Whether this was quick, in a
> period of 1-3 years or less, or somewhat longer, is essentially
> immaterial. IT HAPPENED!

Just a reminder to be careful of terminology here. The ice age is, of
course, the Quarternary ice age and the Earth is currently in the midst of
it. It did not end ~10,000 years ago, that age only marks the end of the
last glacial advance of the Pleistocene. There have been numerous such
advances and retreats throughout the Pleistocene epoch. The 18,000-10,000
b.p. advance was only the latest. IMO the geologic time scale is a little
misleading. The Holocene is taken to begin about the time of the last
glacial retreat. However, what is different about this retreat from the
numerous retreats throughout the Pleistocene? I think that having a
different epoch for the last 10,000 yrs overemphasizes the last retreat
too much and often leads to confusion about the present ice age.

> Did this change in climate cause a die-off? i'm not sure, nor
> is anyone else. But, these climatic changes ussually change
> the habitate of the areas affected, and thus, the survivalability
> of the inhabitants. Humans are, generally, better able to cope with
> this than animals, and the large animals, have, traditionally,
> been the ones that suffer the most! Humans had not done the
> job in the previous 100,000 years in Eur-Asia, so there is no
> reason to suspect that humans, in a relatively short time would
> suddenly do so now. They may have helpped push the marginally
> able over the edge though.

I'm not an paleontologist, but the question that comes immediately to my
mind is this. Why did all of these extinctions happen only after the last
ice advance to species which wheathered all of the previous advances and
retreats without any problem at all? In the context of the Quarternary
ice age, what is the evidence that large animals "traditionally" suffered
except at the end of the Pleistocene. I think that the answer must be
that there was _something_ different (presence of humans?) about this
latest retreat. I also think that, in general, the fossil record is
incredibly biased in favor of large animals. So, it shouldn't be a
suprise that we know of the extinctions of large animals the best as a
statistical artifact alone.

> As to why the ice age ended, the climate changed, we, at present,
> have NO DIRECT evidence! We open to suthe great guessing game.
> A moderately sized metorite impact in the middle of the Atlantic
> is as good a reason as any I've seen so far.If anyone else
> has a theory, please put them forward. I've seen writings
> about the effects, BUT this is the first guess as to the
> cause!!!!!!!!!!!
> ~~ Richard C. Schmidt

Didn't Milutin Milankovitch propose a celestial mechanics mechanism in the
late 1920's or early 1930's? I assume that most readers here are
familiar with Milankovitch cycles. If not, they are explained in any
introductory geology text. Many geologists still accept this explanation
for glacial advances and retreats during an ice age. However, there have
been many ice ages and the mechanism for what initiates one is a bit more
enigmatic. The oldest known is the Precambrian ice age which occured
about 2.2 ga. There was probably another during the Paleozoic (~500 ma)
and an extensive one during the Permian (250-300ma). The Quarternary ice
age began about 1.6 ma. There have been many mechanisms that have been
proposed - in truth, probably all of the factors played a role. The
relative rarity if ice ages suggests that some combination of factors must
be at work. Some of the proposed mechanisms for the initiation of the
Quarternary ice age:

1) Positioning of Antarctica over the pole (~50ma)
2) Separation of the South America land bridge from Antarctica. This
allowed circum-polar currents to flow and is probably responsible for the
flow of Anarctic Bottom Water which began shortly thereafter.
3) Iceland volcanism blocked cold water from the vicinity of the North
Pole from mixing with equatorial waters in the Gulf Stream in the
4) Subduction-related volcanism along the Aleutian arc blocked cold water
from the Bering Sea from mixing with equatorial waters in the Pacific.

I favor a combination of these factors (and probably some that I have
ommitted). I have heared 1) and 2) put forward the most often and I
assume that there is some sort of consensus about their role.

Scott Barboza
University of Washington
Department of Geological Sciences