Posted: Feb 06, 2017 12:13 am
bert wrote:Hi Itsdemtitans
I always love your posts. Thanks for this one too!
Question: Why does the ratio vary? I'd expect it to be constant: We have a supply of carbon on earth, and that is it. Is it a temperature related thing (and if so, what is the mechanism)?
Bert
Hey Bert! Great question.
You're right we only have one supply of total carbon, but the ratio of 13C and 12C can vary depending on if one is being added or removed faster than the other. The biggest mechanisms for this is the burial of organic carbon (carbon in the form of living tissue, like whale remains, algae, etc.) and the formation of limestone.
Organic carbon, by itself, tends to have a larger amount of 12C to 13C. This is because 12C, otherwise called "light" carbon, is because photosynthetic organisms that form the base of the food chain have an easier time absorbing 12C from CO2 due to it's lighter mass.
So, lets say all of a sudden, there's an extinction event, and lots of organisms in the oceans die and get buried under marine sediment. All their carbon is now trapped, isolated from the ocean waters. Suddenly, a large portion of 12C has been removed, and so the ratio of 13C/12C will shift. If you look at major extinction events in the fossil record, you'll see they're almost all accompanied by radical shifts in carbon ratio. That's no coincidence!
The formation limestone actually has the opposite preference, in that it likes to take more 13C as opposed to 12C. Because limestone formation is slow, and most of the time animals aren't dying in huge numbers, the ratio stays somewhat constant, giving everything enough time to recycle. But if you have, say, a shift in ocean currents which suddenly makes the oceans warmer and have a better mixing of nutrients that causes an increase in algae production (and thus burial of carbon), or changes in climate make the formation of limestone easier, it becomes fairly easy for the oceans 13C/12C ratio to tip in either direction.
Hope that makes sense!