
Originally Posted by
Badjelly
There's been a decline in atmospheric C13/C12 isotope ratios, as you'd expect if it came from fossil fuels.

Originally Posted by
Winston001
Just for clarification, the "normal" ratio of carbon isotopes is presumably known from geological strata? And when volcanoes vent, that ratio is maintained? Can't remember the isotope stuff except for the usefulness of half-lifes.
Disclaimer: I am not an expert on this stuff.
My original statement oversimplified things a bit. It would have been more accurate to say:
There's been a decline in atmospheric C13/C12 isotope ratios, as you'd expect if it came from fossil fuels or plants.
And I think what the observation of a decline in atmospheric C13/C12 tells you is that the additional CO2 did not come from the ocean, which has a similar C13/C12 ratio to the atmosphere.
The C13/C12 ratio is lower in plants than in atmospheric CO2 because plants take up C12 (in the form of CO2) in preference to the heavier C13.
Fossil fuels were originally plants so fossil fuels also have a low C13/C12, preserved from when they were formed, given that C13 and C12 are both stable. (Again, this is obviously oversimplifying things a bit, because fossil fuels were formed a long time ago. Was the atmospheric C13/C12 ratio different then? I don't know, though doubtless somebody has sorted all this out. Anyway, whatever the details of how it came to be, when you get coal or oil out of the ground now and burn it, the CO2 produced has a C13/C12 ratio that's lower than current atmospheric CO2.)
What about C13/C12 in CO2 emitted by volcanoes? I don't know. I'll look into it.
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