I'm 99% sure the answer you're looking for is increased pollution.
Answer and Explanation:
We realize that the new plate made at the MOR, i.e., Mid Oceanic Ridge which is called divergent plate limit or constructive plate edge and consequently that oceanic plate is devoured or destructed at the destructive plate limit.
Sub-duction happen at the subduction zone where a high thick and inflexible plate going down relative to the another less thick plate.
As the outermost layer of the ocean get more seasoned it is vulnerable to subduction.
The period of subduction potential is 125 to 200 million years. Since when an oceanic plate gets more seasoned it becomes colder, progressively unbending and increasingly denser as a result of mafic constituents and submerged yet in the case of continental plate, they are constituted of felsic or granitic piece which is comparatively lighter.
So constantly denser plate i.e oceanic plate moves down relative to continental plate or plate which is less dense.
I think it was the North Sea.
