The problem here is that we are pumping more CO2 into the atmosphere than would be there through natural processes. Sequestration of CO2 happens naturally, but we have, and continue to throw natural cycles out of balance with our ever increasing volume of CO2. We have added more C02 than the ocean can handle.
We don't even fully understand how the ocean sequesters CO2:
Ocean's 'Twilight Zone' Plays Important Role In Climate Change
Science Daily — A major study has shed new light on the dim layer of the ocean called the "twilight zone"--where mysterious processes affect the ocean's ability to absorb and store carbon dioxide accumulating in our atmosphere.