Heavy Dry ice
Hi Nick
It's funny how a non-polar solid will behave. Apparently there's CO2 ice on Triton and because of the cold - just 38 K - its density is up to ~ 1.7. An icy crust of H2O/CH4 clathrate could overlay quite a substantial thickness of the stuff if the two differentiated instead of clathratising. The condensing and sinking bubble effect could be very handy for sequestration. Paul Birch wanted to terraform Venus by liquefying the CO2 via a shade soletta, then sequestering it under an ocean. He always thinks big.
Adam