You don't actually sequester C02 in gaseous form when you do geologic carbon sequestration. First off, even the simplest methods wind up with it dissolved in w

You don't actually sequester C02 in gaseous form when you do geologic carbon sequestration. First off, even the simplest methods wind up with it dissolved in water. You basically can't just inject gaseous C02 into the ground and have it remain a gas. The rocks you're injecting it into are filled with water and it's highly soluble in water. Under certain conditions, you can inject liquid C02 and actually displace the water already there. However, that's mostly not what's being proposed. Geologic carbon sequestration can take a handful of different approaches. The two you'll hear talked about the most are: 1) Inject it into a reservoir where you expect it to react with something and eventually form carbonate minerals (in-situ mineral sequestration, or mineral trapping). 2) Inject it into a very deep reservoir where it will dissolve into the brine already there and/or is unable to find a way back out again due to the geometry of the reservoir. (Solubility trapping and/or residual trapping) The first method is preferable, but actually hard to guarantee in practice. The second may sound more risky, but we're actually pretty good at identifying regions that can store a buoyant phase over geologic time -- it's exactly what we do in hydrocarbon exploration. Both are being actively explored along with several others. There's a lot of geologic research being done on carbon sequestration right now (and has been for quite awhile, actually, though the government funded it well before industry started to). The risk of the "seed the oceans" approaches (or the olivine beach proposals as well) are the environmental side effects. Altering ocean chemistry _will_ have significant effects. Similarly, putting olivine on beaches will result in a lot of heavy metal contamination (you can't get pure olivine -- you're going to have a lot of chromite and fun other odds and ends in ultramafic rocks). I'd argue we're better off impacting the deep underground where more or less nothing lives than we are altering the oceans.