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Let is snow, let it snow, let it snow...
Michael Lovejoy wrote:
>>> Is there any evidence, for or against, snow falls in the Mesozoic? has
>>> anything been published on this?
Can any specific areas / periods when snow might be a possibility be identified?
Hope someone can help! <<<
Mike... I hope this helps.
While doing research on the Pangean Megamonsoon for a Tropical Dynamics course
a year or so back, I pretty much found what you are asking about...
Basically, just like today, you can count on the presence of snow at high
elevations during all intervals of the Mesozoic. That's a given. But, there's
Something along the lines of being specific would be work done with conceptual
climate models. (For an example, check out "Climate of the Supercontinent
Pangea" by Parrish, The Journal of Geology, 1993, v. 101, p. 215-233.) Take
Laurasia during the Jurassic. During the Late Triassic, Laurasia was relatively
dry. By the Early Jurassic, contrary to the conditions on the Colorado Plateau
at that time, eastern Laurasia appears to have become wetter as indicated by
widespread coal deposition. This type of pattern seems to have persisted into
the Middle Jurassic. One possible explanation is that evaporation was reduced
due to cooler temperatures brought about by the more northerly position of
Laurasia during the Early Jurassic. Also, if global temps were dropping, the
shift would be explained by changing the precipitation-evaporation balance with
a change in temperature and no changes in the northward transport of moisture
by the monsoon. CO2 curves suggest that the climate was a!
ually warming at this time... but... evidence does exist for cooling in the
higher latitudes in the form of the presence of Boreal Realm ammonites in the
Northern Hemisphere. This fauna is commonly interpreted to represent cooler
water. There is also the issue of glendonites in higher latitudes. These are
pseudomorphs of radiating crystals of calcite which require near-freezing water
temperatures in order to form. They are also commonly associated with glacial
This cooling could have been regional due to the northward movement of
northeastern Pangea as the entire continent rotated counterclockwise. But, this
fails to adequately explain coal distribution in Gondwana, meaning whether or
not Pangea was still a coherent mass at the time, and it doesn't explain the
southward movement of the Boreal Realm ammonites. Obviously, this area needs
There's also the dramatic shift in climate at the Middle-Late Jurassic boundary
that took place in southern Laurasia. In short, things changed to be indicative
of a zonal circulation, showing the breakdown of the northern monsoon. Laurasia
broke away from Gondwana, effectively removing the principle forcing mechanism
of the monsoon, that being Pangean geography. Once the monsoon broke down, the
means to transport moisture to higher latitudes no longer existed. For example,
those coal beds in eastern Laurasia were replaced by evaporitic deposition
between 20 - 40 degrees north. Also, the Boreal Realm ammonites reached their
maximum southern extension at the end of the Middle Jurassic, meaning that the
drying wasn't due to global warming.
The coldest temps during the Jurassic seemed to have occurred in the Middle
Jurassic, as indicated by glacial-marine sediments, diamictes (pebbly
argillites), and glendonites.
In effect... Yes... I'd have to conclude that it was snowing.
Then there are the numerical Global Climate Models, or GCMs. The few that have
been done pretty much show the same sort of trend. General cooling of northern
climates from earlier Mesozoic time (roughly 200 mya) to present can be
accounted for by the increasing amount of land at higher latitudes. Various
workers have produced results of below freezing temperatures and a very large
seasonal range of temperatures for the continental interiors of Pangean middle
and high latitudes. Simulations of Cretaceous climates have also produced
freezing conditions in winter and a large seasonal range of temperatures. In
fact, work done some 20 years ago about ice-rafted deposits throughout much of
the Phanerozoic points towards supporting cold polar climates. Of course,
there's diverse fossil flora as far north as 75 degrees north, and as far south
as 63 degrees north, along with warm water corals that are 10 degrees poleward
from their present day position that like to conflict with this!
cenario. But, these are from COASTAL locations where seasonality would have
been at a minimum, especially if SSTs were warmer than present. Though GCMs
like to give high latitude continental interiors a seasonality comparable to
that seen in Siberia today, available data seems to support a reduction in the
seasonal temperature range over the continents for much of the Mesozoic and
into the early Cenozoic. Always keep in mind that these are numerical models
with boundary conditions that are well beyond being simply called "uncertain".
And it's as I've stressed in my past few posts... The effects of the ocean on
climate cannot be underestimated. GCMs often use parameterizations for moisture
and heat transport from the ocean to the atmosphere based on sea surface
temperature (SST). The error inherent in supplying the SST field represents a
serious limitation to paleoclimate modeling for the pre-Pleistocene.
One GCM that's worth mentioning was doneby Chandler, Rind, and Ruedy
(Geological Society of America Bulletin, 1992, v. 104, p. 543-559). Unlike most
numerical models before it, the simulations included more realistic
interpretations of paleocontinental distribution, topography, epeiric seas, sea
surface temperatures (SSTs), and vegetation, in order to get better agreement
between model results and paleoclimate data for the Early Jurassic. Many
results, such as summer temps, precipitation and evaporation patterns, and
large seasonal temperature fluctuations over mid and high latitude continental
interiors, were produced just as in other GCMs, and were also in good agreement
with the geologic record with few identifiable discrepencies. But... The thing
that is interesting for you is that although the simulations did not begin with
snow cover, snow was formed by the GCM and accumulated over portions of Pangea
during the winter months.
And I might as well mention that a GCM simulation done by Sellwood, Valdes, and
Price ("Geological evaluation of multiple general circulation model simulations
of Late Jurassic paleoclimate", Paleogeography, Paleoclimatology, Paleoecology,
2000, v. 156, p. 147-160), produced the possibility of small permanent ice
sheets at high latitudes for both hemispheres that came and went on a
Milankovitch time-scale, resulting in meter-scale sea level variations.
Basically, GCM outputs make the presence of ice at high elevations and at high
latitudes look favorable.
With completely different GCMs producing results that agree respectively well
to each other... results that happen to also compare very well with the known
geologic data (though arguably, the presence of either permanent or periodic
ice sheets that came and went with either the seasons or the Milankovitch
cycle, is still inclonclusive), I'd have to conclude... The yes... it was