Beginning a sentence with an abbreviation?

I use the word "carbon" about 150 times a page when writing proposals, so naturally I abbreviate it as "C" (after of course letting the reader know I am doing such). But what happens when I want to begin my sentence with this abbreviation? It surely does not look/sound correct.

"C being stored in lacustrine sediments is...."

I have not found a hard and fast rule on this in the blogosphere. Most posts seem to say something to the effect, "If it sounds wrong, don't write it", or "Do what makes your professor/teacher happy".

Therefore, I am making my own judgement call. NO. That is to say, Tommy, you cannot start a sentence with an abbreviation.

There, I made up my mind.