[Tintin_Triton (OP)]

Courtsey, The Scientific American, April 2010, I found out that the seas [O8)] are a little basic. I think their pH varies from 8.0 to 8.4. However, this is contradictory to many other processes, such as absorption of CO₂ causes a slight formation of Carbonic acid, in equilibrium.

So, my question is, again [], why are oceans (an seas) slightly basic? 

[Bored chemist]

If they were acid the white cliffs of Dover would dissolve to neutralise them.
http://en.wikipedia.org/wiki/White_Cliffs_of_Dover
There are other big carbonate deposits too.

[Tintin_Triton (OP)]

Well, true, but just why are they basic, and not acidic, so that we do not see the white cliffs of dover dissolving to produce a fantastic display of bubbles?

[LeeE]

It would seem that the oceans are becoming more acidic:

http://en.wikipedia.org/wiki/Ocean_acidification

but I don't know why/how they initially became basic in the first place.  However, I would guess that the ambient levels of atmospheric CO2 were too low to reach saturation with the result that most of the CO2 that was absorbed combined with calcium to form the carbonate exploited by shellfish.  Just a guess though.