"Now the sinking heavy and salty water starts a journey of several thousand years close to the ocean bottom,"
How do we know this? We have seen how the tanker spill of yellow rubber ducks migrated, but what about movements deep under the ocean?
"Consequently, after hundreds and thousands of years you would expect a distinct isotope differentiation between ocean bottom water and surface water."
Would you? The bottom of the ocean is not a desert. There is a lot of biological activity going on. And is saltwater a static thing? Does the salt affect isotopic behavior?