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Another source of old carbon is the outgassing from volcanoes: in locations where this is a significant source of CO, plants growing in the area will appear older than they actually are.Even participation in the terrestrial carbon cycle does not quite guarantee the date: we could, for example, imagine termites eating their way through the wood of a 200 year old house; these termites would date to 200 years old or more (depending on the age of the tree).After about 60,000 years the quantity will be too small for our instruments to measure accurately, and the best we'll be able to say is that the sample is about 60,000 years old or more.For this reason radiocarbon dating is of more interest to archaeologists than to geologists.Fourthly, the carbon in the organic remains does have to originate with the terrestrial carbon cycle and with plants performing photosynthesis.If this is not the case, it is sometimes possible to correct for the fact; in other cases it makes dating impossible.
Such dates typically agree to within 1 or 2 per cent.
One of the nice things about this method is that we don't have to worry about carbon being lost from the sample.
Because we are measuring the abundance of two isotopes of carbon, and because isotopes of the same element will be chemically identical, no ordinary process can preferentially remove C is going to be small enough to begin with, being only 0.0000000001% of atmospheric carbon, and then as the decay process progresses it's going to get smaller and smaller.
C (carbon-14) in the upper atmosphere as a result of bombardment by neutrons in so-called cosmic rays: high-energy particles bombarding the Earth's atmosphere from outer space. On formation, the newly-born carbon atom quickly oxidizes to form a molecule of carbon dioxide (COC being produced annually is more or less constant, whereas the quantity being destroyed is proportional to the quantity that exists, it can be shown that the quantity in the atmosphere at any given time will be more or less constant: the processes of production and decay of C, which need not concern us in this article.
The terrestrial carbon cycle is fairly simple: plants get their carbon from the atmosphere via the process of photosynthesis; herbivores get their carbon from plants, and carnivores from the herbivores.
This increases the apparent age of the sample by about 400 years, depending on where in the ocean the organism lived and died.