What is the total mass of organic carbon on Earth?
You could never have an estimate which was anything more than a wild guess masquerading as an estimate. To estimate the carbon in living vegetable matter alone would be hard enough, and result in a very inaccurate approximation, but to include the quantity in the Earth's crust as well could only be a wild and very inaccurate guess. The exact quantity of all the Earth's oil reserves, coal, peat, limestone and other carbonates isn't known with any precision, then there are oil shales,tar sands, asphalt lakes,clathrates and God knows what else,but the great thing about a sufficiently large, wild guess is that no one could ever prove you wrong, especially as on a few occasions and by sheer chance, guesses turn out to be right!  But it is an estimate, presumably an accurate one that you asked for, and that is impossible.