The early atmosphere contained a large amount of carbon dioxide. When the oceans formed, carbon dioxide from the air dissolved in the water. Carbon dioxide is soluble in water, so the oceans took a lot of it out of the atmosphere.
Because carbon dioxide moved from the air into the water, the amount of carbon dioxide in the atmosphere decreased. Some of the dissolved carbon dioxide went on to form carbonate compounds, which built up as sediments on the ocean floor.
So the oceans formed first, when water vapour condensed, and then they acted as a store that removed carbon dioxide from the atmosphere. If a large amount of carbon dioxide dissolves in the oceans, the amount left in the atmosphere will decrease.
