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Re: Brrr, bone chilling paleopolar summers(Polar dinosaur growth and other new papers)



A little late (and a little long), but I'm going to put my "weather
man/climatologist" hat on and chime in...

I'm glad someone finally mentioned the Spicer and Herman 2010 article.
Thank you Greg.  The study does an excellent job of integrating the
latest techniques/knowledge about paleobotany and
climate/ocean/atmospheric modeling to produce a significantly more
robust picture of the possible state of the Late Cretaceous Arctic
climate. And all of it is mildly frightening because it actually makes
meteorological sense! It piles on support for the proposed cold
Pacific Gyre that limited the incursion of atmospheric heat from lower
latitudes, eliminates any remaining doubts (if there were any) of the
critical function of the Western Interior Seaway as the highway for
heat transport and moisture, and provides a good explanation for the
sediment record via persistent cloud cover/precip.  It was also a
relief to see their ocean heat results casting doubt on the 59F annual
mean sea surface temp that often floats around the literature,
bringing it down to a more reasonable 42 - 50F; keeps the coastal
margins above freezing in winter, but definitely not balmy (which I
always found to be silly). Furthermore, it allows for the occurrence
of significantly colder coastal, and definitely inland, temps overall.
In fact, the mean summer SST of the Eastern Shelf of the Bering Sea
today falls within the middle of that temp range.  Noodle that one for
a while...

Their climate reconstructions also tie together many other climate
proxies... such as possible frosts from Campanian-Maastrichtian tree
rings from Ellesmere Island, estimates of polar temps down to 14F from
late Campanian-Maastrichtian Alaskan vertebrate enamels, the general
agreements of other model work, and a number of others I can't recall
off the top of my head. You might say at this point, "Wait a minute!
The results of their study indicate Late Upper Cretaceous summer mean
temps of around 60F, winter mean low temps of around 30F, a mean
annual temp of about 40F, and an above freezing ocean surface!".  But,
remember... these values are MEANS. They say nothing about extremes,
let alone transient anomalies, climate cycles/modes, or
variability/oscillations (think ENSO, PDO, IPO, NAO, AO, NAM, SAM,
etc). As we cannot resolve oscillations, let alone things like
regional effects, with any real fidelity, from a meteorological
perspective you must leave the freezer door open; it obviously got
freakn' cold in the Late Cretaceous Arctic, and these cold outbreaks,
just like cold outbreaks today, probably lasted for lengthy periods.

This goes to another point... Spicer and Herman doubt it, but I still
insist that given the geography and topography, sea ice existed in the
Late Upper Cretaceous Arctic. Now, keep the time frame in mind... It's
well agreed upon that the Late Upper Cretaceous experienced a cooling
trend. Prior to that, let's say during the Mid Upper Cretaceous
(Turonian), that inland seaway was wide open, and as the wonderful
heat transport mechanism that it was, brought warm waters and moist
air from the lower lats to the High North. To that effect, there is a
cash of plant fossils from the Russian high Arctic (82-85N) that seem
to indicate that the climate at that time was humid, with warm summers
and generally frost-free winters. Mean annual temp of about 48F,
warm-month mean temperature of about 63F, and cold-month mean temp of
about 34F. And lots of precip.  Basically, the climate was similar to
modern temperate or even warm-temperate zones today... but differed in
a big way, of course, due to having a pronounced high-latitude
sunlight seasonality. Were conditions favourable for the formation of
sea ice that that time?  I'm certain snow fell in the uplands, and you
definitely had, at the very least, mixed precip/snow along the coast
during the winter if the profile was right, but the ocean probably
never got cold enough to generate persistent sea ice, at least nothing
even remotely solid. Shallow lakes/ponds probably did freeze over
during prolonged cold air outbreaks or if they existed in valleys...
Greg's drawing of the tyrannosaur cracking the ice to get a cold drink
wouldn't have been far from reality. (Not that I don't think there
were climate cycles, lasting a few thousand years, or even a million
or two, where ice sheets didn't come and go... I think there's enough
evidence and reason that points to intermittent periods where
conditions fostered portions of the Arctic to be frozen over, if at
least only seasonally.)

But... by the Late Upper Cretaceous, Campanian-Maastrichtian, the
situation changed, and somewhat drastically, with the closing/draining
of the Western Interior Seaway, which meant heat transport to the
Arctic was diminished. The mechanism to keep the threat of freezing at
bay was lost. Various model outputs and ocean sediment analysis from
that time indicate there was seasonal ocean variability that probably
lead to the formation of solid-ish sea ice.  Did it happen every
single year? Probably not... but if the climate during a winter was
stuck in a cold mode... To that end, seasonal diatom production and
flux from that time indicate sea ice-free summers, with thin
accumulations of terrigenous sediments within the diatom sediments
indicating intermittent sea ice in the winter. This sea ice doesn't
appear to have been tidal glacial ice as the sediments lack the coarse
material usually associated with that type, nor does its character
indicate that it was anchor ice... which has harmony with the results
of Spicer and Herman. Instead, the sediment has the characteristic of
the type generated by turbid ice, which is unconsolidated and mobile,
facilitating long-range ice rafting. (It's interesting that turbid ice
forms during a turbulent freeze-up as free-floating frazil ice
crystals form, collide with, and aggregate to, sediment in
suspension... which makes sense for the Cretaceous Arctic ocean as it
would have been cooled further as its surface was whipped up by winter
storms, producing better conditions for ice formation). In other
words, the ocean didn't freeze over solid, but turned slushy here and
there with semi-solid spots... Hence, "warm" water was always exposed
to "cold" air, warming that air, infusing it with moisture, and
producing something else... Clouds.

Spicer and Herman's conclusion of a persistent cloud deck is very
interesting with far-reaching ramifications (as Greg pointed out).
Besides acting as a thermal blanket in the winter (warming) and a
reflector in the summer (albedo effect cooling) this cloud deck would
have been accompanied by lots of persistent, annoying, drizzly precip
in the winter, and downpours in the summer as solar forcing lifted
those clouds into storms... which would have kept things even more
cool and damp (sedimentation analysis appears to support, especially
coals). Furthermore, it was great that Spicer and Herman pointed out
the pronounced thermal gradient that would have existed as one moved
inland from the coast... Liquid/mixed precip in the lowlands,
frozen/mixed precip in the uplands. As Spicer and Herman note, that
cloud deck would have been reduced by the Late Upper Cretaceous with
the closing of the inland seaway, so the winters would have been even
colder (feedback too... loss of warm water makes things colder and
also reduces clouds which makes enables the ocean/ground to radiate
out, making things colder still). Summer would have seen a decrease in
this cloud cover as well, but the resolution doesn't exist to say
whether it was a decrease in area or thickness or both (depends on a
number of factors, but my meteorological inclination is to say a
thickness decrease... enhanced sensible heat fluxes and reduced latent
heat fluxes meant drier and warmer air that probably resulted in the
formation of elevated thinner clouds. So, it still would have been a
cloud-filled sky).

It comes down to what I always say when the subject comes up... Bottom
line is that given the latitude and geography and topography, "cold"
temps and the occurrence of sea ice and frozen precip in one form or
another, at one time or another, simply makes meteorological and
oceanographic sense for the Cretaceous Arctic.  The Spicer and Herman
paper added new refinement to the possible environmental conditions
that this set up would have produced, and Greg extended this to
implications for biology. It's great when multiple disciplines come
together...

Good stuff.

Kris