This is one interesting conundrum (/me is laughing at the word). But how do we know the metal used for the Conastoga (spelling?) wasn't an alloy with a lot of carbon in it? Seeing as we have the ability to artificially make diamonds using pencil lead, I don't see why they couldn't find a way by that time period to extract the carbon from the modules that came down and turn it into coal. Once they have all that started, you also have to remember that there is more CO2 in the air than is need for the plants, so how do we know they can't extract some Carbon from that? The possibilites are endless.
(BTW, knux, because Carbon is an element, couldn't it have another existant form under different conditions that we just do not know about because it isn't possible for it to go into because of the conditions on Earth?)