Actually from the perspective of preserving artifacts this is a good thing right? It's saying there is even less oxygen in the water which is good. In terms of life in the sea it's probably bad.
Wouldn't all the preserved stuff be directly at the bottom, though? So the only change to preserving would be in places where the bottom is 90-150m, and the only stuff it would apply to is whatever sank after the level changed to 90m, so it can't be a lot or very interesting.