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Re: Ice Sheets Caused Massive Sea Level Change During Late Cretaceous
Dann Pigdon wrote:
>> These could have been glacial dropstones. Glaciers don't just form in polar
>> icesheets - you can find them in many other parts of the world. <<
Tommy Tyrberg wrote:
>> However glacial dropstones in marine deposits requires that the glaciers
>> reach sea level, otherwise they couldn't calve into the sea. No glaciers do
>> this outside the subarctic (southern Alaska, Iceland) even today. <<
Righto.... Glaciers form and calve icebergs without the need for vast polar ice
sheets, and glacial dropstones need to be carried by ice as it makes its way
down off a mountain and to the sea. Just come up to Alaska and we'll take less
than a 45 minute drive out of Anchorage, which is at about 61N, so you can
touch one. Furthermore, 3/4 of Alaska is classified as being in a Northern
Temperate Zone, not Subarctic. The Northern boundary with the Arctic Climate
Zone is defined as being the 50F isotherm during the warmest month of June.
This line happens to correspond with the northern limit of tree growth as well.
Basically, the Subarctic/Arctic is north of the Brooks Range, which lies almost
right along 68N. There, you transition rapidly from trees to shrubs.
What's interesting about all of this, is that counter to the belief of many a
person, the glaciers are not found along the Arctic Coastal Region (North
Slope). In reality, they are down south in the Temperate Zone and are located
in an area encompassing the South Central Region along the Gulf of Alaska
Coast. The Arctic Coast is simply too damn cold... It's under the influence of
an Arctic Air mass all year round... and as such, not enough precipitation
falls in order to form glaciers on the mountains. In fact, there really isn't
much snow up north at all. What does fall is so dry, and thus light, that it's
literally blown away by winds or sublimates due to extremely cold temps. What
I'm saying is that there is no snow pack in the Subarctic/Arctic of Alaska, and
hence no glaciers.
The South Central Region is under a Maritime Air influence all year round, with
west to east storm tracks that, in winter, steer storms right up into the Gulf
(4-5 a month). There, they can't make it over the Alaskan and Chugach Mountain
Ranges which boarder the coast, so all their precipitation is dumped along the
windward side of the mountains, owning up to the locations of the glaciers.
Vladez actually gets an average of 50 FEET of snow a year, but it quickly
melts. In fact, most of what the Gulf Coast gets in the winter is rain below
1,000 feet. Thanks to the relatively warm Alaskan Current that comes right up
the Aleutians and goes right into the Gulf, the winter temperatures along the
Gulf Coast range from 35F-45F, keeping the waters ice free (except for long,
deep inlets and streams/rivers). The summer temps are also mild, averaging
50F-55F. More inland areas like the Anchorage Bowl will see the 80s on
occasion during the summer months given topographical influence a!
as incoming insolation time increases and radiative heat loss time decreases.
As for what happens in the Interior, temps will exhibit dramatic swings over
the course of the year. Interior temps regularly climb into the 90's during the
summer due to long days as it falls under the spell of Continental Air.
Topography also plays a part in this dramitic warming, as the Interior is
completely surrounded by mountain ranges.... You effectively end up with a
desert effect. In the winter, insolation is dramatically cut and the Interior
will see Continental and Arctic Air mass influence producing outbreaks of the
coldest temps found in ALL of Alaska (-50F and lower, the record being around
-72F). Arctic doesn't necessarily mean where the coldest of the cold is found.
Your coldest temps are not found over the ice pack in the Arctic Ocean (which
is permanent north of 70N) due to leads in the ice bringing up warmer temps
from the water below. This actually keeps the Arctic Coast north !
the Brooks Range under a maritime influence, so temps th!
ere can b
e in the -35s while the Interior is 20 degrees colder.
I'm getting carried away... What to get out of all of that? How about this...
Topography and large bodies of water obviously play decisive roles in shaping
Alaska's climate, and as such, are instrumental when it comes to the locations
of glaciers. Generalizing when it comes to climate, at least in this case,
isn't a feasible option. One can look at Alaska as being a lesson of caution
when attempting to construct ancient climate, especially when scaling down to