Robert Waldmann points out a neglected potential source of carbon credits...

European carbon emissions credits cost over 20 Euros per ton of CO2.
Hardwood pulp wood costs about $8 an English ton in Mississippi so about $8.80 per metric ton. Carbon could be sequestered by burying the wood in the desert in permafrost or sinking it deep in the ocean. How much would this process have to cost to make it a less attractive option than reducing carbon emissions in Europe ?

A few points come to mind: first of all, sinking it in the ocean probably isn't too practical, as a) wood floats and b) the ocean floor is not a dead zone; even if you tethered baulks wood to big concrete blocks, say, it would rapidly get eaten or rotted away, and the carbon would return to the atmosphere.
This is also a problem, to an extent, with other means of sequestration. Buried wood rots. Wood dumped in the desert may not last long either - certainly, if it's left out in the open, decay organisms will get to it. The desert isn't as dead as it appears. Buried in permafrost, wood will last a long time - as long as the permafrost lasts, that is. But do you really want to construct a sort of immense reverse mining industry in the Canadian Arctic? Because the polar regions are really the only place you can leave wood lying around and not expect it to rot.

I've taken a look at his maths. Wood is roughly 50% carbon by dry weight. So a tonne of wood give you 0.33t of water, 0.33t of carbon and 0.33t of everything else - for $24. Meanwhile, one tonne of CO2 emissions - for €20 - represents 270 kg of carbon (the rest is oxygen), which you could buy in the form of 0.81 tonnes of wood, costing you €4.76.

The numbers look good, but I suspect the storage issue is the problem. Not insoluble - but not easily brushed aside either.