Conrad Hodson wrote in message ...
On 15 Dec 2002, Jaak Suurpere wrote:

I remembered that the evaporites had been seen by seismic reflections,
but this would not tell much about the precise conditions of deep or
shallow brine.

So, a lot of evaporites have actually been brought to surface?

Drilled cores, going back at least to one of the GLOMAR ships in the '70's
IIRC. The evaporites--thick beds of water-soluble stuff like halite on
what is, after all, the floor of a sea was unexpected, to say the
least. Then the seismic mapping you mention showed just how extensive the
evaporite deposits are. Putting this together with ground-based work that
showed how valleys such as the Nile and Rhone are sediment-filled cuts in
bedrock that go far deeper than modern sea level is what led to the
realization that Gibraltar has been intermittently closed (and the Med
dried up) ever since Africa got close enough to Spain to apply the
squeeze. The Med is a Tethys Sea relict of course--but it's since it got
pinched off at both ends that things have gotten really interesting!


Compare it with the Gangetic Plain!

You could expect cold air to build up over the large plateaux and
basins of Tibet and then spill into the gorges of the numerous rivers
passing through the Himalayas.

And the effects? None that I know of. Gangetic Plain in winter has
quiet, clear weather, except for feeble cyclones arriving from the
west.

Good point. And "winter" is the hot dry season there, at least away from
the coasts. There may be a difference, though, in that the Med is a
winter rainy season area--at least today, and the extensive and
distinctive ecosystem suggests that that's been true for quite a while.

Siberian air isn't any kind of moisture source--mP air spilling over
Europe from the North Atlantic is.

I am not sure if all the air in Tibet comes from Siberia.
Part of the air comes from the west, Atlanic Ocean, and arrives the
western edge of Pamir, Hindu Kush and Karakoram as source of
snowfalls.

But if it tries to continue east across Pamirs and Tibet, apparently
it suffers adiabatic heating. And if it attempts to turn south towards
the Gngetic Plain, it apparently is even more unable to descend.

A possible difference? Now _I'm_
speculating beyond my knowledge... :-)


Apparently the descent of the 4 or so kilometres from Tibet to India
precludes any cold air from getting through. The Mediterranean could
have had similar relief.

Or take the Colorado Plateau. Again, do cold air outbursts often
trouble Phoenix?

Don't know about Phoenix, but I've been in the Los Angeles Basin while
katabatic winds from the high deserts were blowing the roofs off houses in
Glendale. Adiabatic heating warms the air up, and a good thing, too, if
your roof is gone.


Yes.

And the barrier between Los Angeles and high desert has several gaps
around 1000 metres.

My impression is that strong katabatic winds exist downwind of gaps
that are either on the same level or include barriers of up to one
kilometre.

But descents of over two kilometres, like the western slope of Sierra
Nevada, southern slopes of Colorado Plateau and Alps et cetera
apparently force any cold air not only to heat up but also to break
off the surface before reaching the bottom.


Adiabatic heating of the cold air mass is only part of
the story, because the cT air in the temporarily humid basin is
solar-heated as well, powerfully.

But its temperature would have upper constraints set by wet convective
instability, whereas the cold air mass has to undergo dry adiabatic
heating.

Agree. Aren't we talking about something akin to the difference between
local and frontal thunderstorms in a place like the Mississippi Valley,
though? The "upper contraints" you talk about serve to recycle moisture
locally, in afternoon thunderstorms of essentially local origin. But it's
when a cold air mass blows in from outside the area that all hell breaks
loose. Instead of being diffused over a large area, that moist-air
potential energy gets concentrated and organized into a violent contact
zone. That's the sort of thing I wonder about happening along the
air-drainage routes into the basins.

Again remember that Mississippi Valley has no high barriers to cold
air and thus no adiabatic heating.

Even without adiabatic heating, take the modern mistral. It follows
the Rhone gap and crosses relief of at most 300 metres in Burgundy.

But mistral is usually accompanied by clear weather and sunshine -
however cold and strong the wind is.

Yo could expect the wintry Mediterranean to have a lot of moisture
from warm water surface. But where are the violent rainfalls mistral
migh cause?

The cold front remains a cold front all
the way to the bottom of the basin. There's also more going on than air
responding instantly and passively to the gas laws. Moving air has
momentum, and violent katabatic winds in the real world go for long
distances even after adiabatic heating has done its bit. If that air mass
hits the unyielding basin floor and its momentum goes to work lifting
already unstable humid air--well, it would be fun to watch, especially
from the mouth of a cave in the side of a hill. In a canyon bottom, or
lighting-riddled hilltop, or on a plain where tornadoes came to dance, it
might be less fun.

So this still seems plausible to me, but I've called you on the subject of
ground truth, and here _I_ am speculating. Does anyone here know of
geological evidence that's relevant? Flash-flood deposits from
now-submarine canyons, perhaps? Fossils or pollen from plants (if
any) growing in the basins? If your steamy supertropical lakes and swamps
of fresh water really existed, stuff probably lived there. If my "Venus
Lite" suggestion is more accurate, there might not have been much down
there but thermophilic bacteria digesting flood debris when they could
come up with the moisture to do so.

Now let us think of the physiological constraints on plant life.

The hottest places on Earth are Azizija and Death Valley.

Both have reasonably obvious and diverse plant life. Part of it is
perennial.

Which is no wonder. Death Valley receives over 40 mm of rain per year.
Azizija gets over 200 mm.

But we must conclude that many of the plants who grow at Azizija, or
their ancestors, endured heat of 58 degrees one day.

Given that most of the desert plants, even perennial ones, do not have
constant plentiful water supply for evaporative cooling, the plants
must have heated to 58 degrees - or more, if they themselves were
heated by sunlight and hot ground.

They lived.

Now, plants do not like drying. We can conclude that if these plants
endured 58 degrees with low relative humidity, they would also have
endured 58 degrees with high humidity.

From what I have heard, the Red Sea coasts don't have particularly
high temperature extremes.

So, an area could have absolute maximum temperature of 57 degrees over
the average of 34 degrees with low humidity, (Death Valley) or
absolute maximum of 58 degrees over the average of 28 degrees with low
humidity (Azizija) and be settled by plants. But what if there is an
area that has absolute maxima of 58 degrees over a July average of 45
degrees and high humidity? Would plants endure?

Not all, of course. There are plants that have trouble occasionally
enduring 30 degrees.

I'd like to crosspost to sci.bio.botany for opinions and drop
sci.geo.oceanography for that goal.

Microfossil studies might be really relevant here, if cores have hit the
right places and researchers have asked the right questions of the cores.

Conrad Hodson