The Western Interior Seaway: Riding out the storms

[MariusRomanus@aol.com]

Time to play everyone's favorite game, "Speculation!"...

If you look at Kump and Slingerland 1999, their overall conclusion that there 
was a cyclonic surface circulation (gyre) looks sound given that it was 
consistently reproduced regardless of the forcing mechanisms; southern waters 
moved 
north along the eastern coast, while northern waters moved south along the 
western coast. (This is saying a lot since as many of you know, I'm not a fan 
of 
computer models, especially those dealing with climate and the weather.) I 
wonder if you can gleam supporting info for this from the rock record?  The 
authors could be right when they stated that it appears that large storms 
temporarily modified or completely destroyed this flow... If this frequently 
happened 
during the course of the year, then there's a possibility that there wouldn't 
be much in the way of a clear circulation recorded in the rocks.

Keeping these winter storms in mind.....

Kump and Slingerland state that if the KWIS was vertically stratified, likely 
candidates to induce the stratified layers were strong seasonal contrasts in 
temperature forcing producing a seasonal thermocline, and in wind stress and 
water balance. (The last two would depend greatly upon the local geology.)

So, what about it??? A stratified water column which allowed for anoxic 
bottom conditions had to come from somewhere. If you look at oceans and seas 
today, 
they are composed of layers. The surface mixed layer is where heat and 
freshwater transfer takes place between the atmosphere and sea, and you find it 
within the uppermost 50 - 150 m. In the summer, mixing does not reach very deep 
and is achieved only by the action of wind and the waves they produce. In the 
winter, on the other hand, cooling at the sea surface produces convective 
overturning, causing the mixing to extend deeper. For an obvious reason, (that 
being 
air temperature and sea depth) when it comes to the KWIS, it's hard to say 
just how deep the mixing layer would have extended (especially during the 
northern hemisphere winter). Directly below the mixing layer is your transition 
layer where water temperatures usually decrease rapidly with depth... This is 
your 
seasonal thermocline. During the course of the year, the seasonal thermocline 
varies in size... It is shallow in the spring and summer, deep in the fall, 
and completely disappears in the winter.

Taking into account a warmer atmosphere and the shallow depth of the KWIS... 
Let's assume that there was just enough of a seasonal contrast in temperatures 
for a seasonal thermocline to exist. If the air temperature didn't get cold 
enough in the northern hemisphere winter to completely dissolve the KWIS's 
seasonal thermocline (a safe assumption), then I am willing to bet that the 
KWIS 
possessed a seasonal thermocline which was much like that of the tropics today. 
In the tropics, winter cooling is not strong enough to erase the seasonal 
thermocline. Instead, a very shallow feature called the tropical thermocline is 
maintained throughout the year.

With all of that mess in mind, it could very well be that Kump and 
Slingerland are partly right in their conclusion of a KWIS which was 
turbulently mixed, 
top to bottom, eliminating any sort of stratification of the water column. 
But, this was the condition only for part of the year. Maybe, there was just 
enough of a seasonal temperature contrast to induce a shallow seasonal 
thermocline 
like that of the modern tropics, and the right wind stress and water balance 
supported stratified layers. But!!! This was only the case during the northern 
hemisphere summer. These stratified conditions were completely destroyed with 
the coming of the midlatitude winter storms, which could have had storm 
tracks that carried them as far south as 35 degrees North (over Texas). The 
well-documented intervals of basinwinde anoxia and resulting extinctions could 
actually be indicating an expansive range of time where the winter storms 
simply 
were not powerful enough to mix the entire water column. Given enough years of 
weak, or even little to no mixing, one can imagine a build up of anoxic waters 
which lead to extinctions.

It's interesting that Kump and Slingerland's mean annual winds are very weak, 
on the order of 1 to 2 m/s. If decent seasonl forcing was in place to support 
stratified layes, these weak winds might not have been able to induce 
turbulent mixing throughout the entire water column on their own... hence the 
need 
for storms, the strongest of which would have occurred in winter.

Then again, I still kinda favor the KWIS as a dilution basin. The KIWS 
obviously experienced some serious freshening of the surface waters from rain 
and 
river input (20 percent more input over evaporation according to Kump and 
Slingerland), which would have reduced surface density that prevented the 
sinking of 
surface waters and therefore mixing of deep layers. Surface mixing via wind 
would obviously take place, resulting in your surface mixed layer. Even with 
the mixing, you end up with a fresher, less dense upper layer and a strong 
halocline. Water below the halocline is renewed very slowly through mixing 
across 
the halocline and from inflow of ocean water. Oxygen content below the 
halocline is therefore very low, and can be altogether anoxic. If the exchange 
of 
ocean and seaway waters of the KWIS were inhibited at the northern and southern 
entrances by sills (as some have suggested), then the dilution basin model 
looks 
even better. You can even retain the surface gyre, though decent seasonal 
contrasts in temperature, wind, and water balance are probably still required 
to 
produce stratified layers in such a shallow seaway. I'm not sure about Kump 
and Slingerland's conclusion of a complete lack of a fresher "lid". (Their 
modeled gyre is the element that inhibits the lid's formation, a gyre that was 
centered at different positions depending upon which forcing mechanism was 
being 
evaluated. This indicates a good deal of inherent variability and uncertainty.) 
However, if the sea was stratified and there was no deep water current, then 
a complete lid might not have even been necessary for anoxic bottom conditions 
to begin with.

If you apply the winter storm scenario to this dilution basin KWIS, the 
results are the same... You get mixing within the entire column only during the 
winter months as strong storms kicked up the sea. When the winter storms didn't 
come, or didn't pack enough of a punch, then the bottom remained anoxic. If 
this type of situation persisted, then the result was the building up of anoxic 
conditions which eventually lead to basinwide extinctions. Only after the 
winter storms returned in force did the KWIS once again mix and anoxic 
conditions 
subside.

Kris
http://hometown.aol.com/saurierlagen/Paleo-Photography.html