[edsheppa]

Isn't it possible that the layer of rock was inhabited later, possibly much later, than its original formation and that this later habitation produced the carbon isotope differential?

[Robert A Cook PE]

Isn't it possible that the layer of rock was inhabited later, possibly much later, than its original formation and that this later habitation produced the carbon isotope differential?

Not likely.

You have to make the assumption that an undisturbed layer of rock (found under more recent rocks) could not get even layers (deposits of carbon) inserted into it with disruptions or other detectable events. And of course, most insertion events (Lava, upthrusts, faults, etc.) would be observable. And most "insertion" events would destroy the (apparently smooth) layers of carbon they have observed.

So if the rock is xxx million years old, then what's in the rock should be that many million years old.