One idea to mitigate the effects of burning fossil fuels on atmospheric co2 concentrations is to pipe liquid co2 into the ocean at depths of 2,500 feet or greater. at the high pressures at such depths, co2 is heavier than water. what potential effects might result from implementing such a scheme?

Although the deep ocean is not well studied, the sequestration of CO2 in that ocean depth (2,500 feet or grater) could finish in a havoc for marine ecosystems.  The principal issues are the ocean acidification, and the reduction of carbonate ion concentration and saturation states of biologically important calcium carbonate minerals.  

Explanation:

When carbon dioxide (CO2) is absorbed by seawater, carbonic acid (H2CO3) is produced as shown in the next equilibrium reaction:

CO2(ac) + H2O(l) ⇆ H2CO3(ac)

As the concentration of CO2 increase, the system of equilibrium is disturbed and then the system will shift toward the product formation. Increasng concentration of H2CO3(ac), the pH of the seawater is reduced.

Besides, the carbonate ion (CO3)2- depletes, which is very important as calcium carbonate (CaCO3) mineral which is the base of skeletons and shells of many marine organisms.

CO2 +  H2O + (CO3)2- → 2(HCO3)-

The recution of (CO3)2- and the pH, reduce, for instance the ability of the corals rebuild itself.

It is important to point out the fact that the depth ocean is not well studied, so this answer is based in the scenario that the CO2 dissolves in water at that depth.