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bert wrote:Thanks for your reply Itsdemtitans! Call me slow but let me try to digest that.

Let me start with something I do understand (a bit; I used to be a biochemist in a previous life) which is an interesting thing to know and perhaps relevant for the discussion. Scientists can distinguish sugar from sugar cane and from sugar beets, even though it is chemically 100% the same. Interestingly, the isotopic carbon ratio differs. There is a group of plants that catch CO2 using a so-called C3 mechanism (the relevant molecule has 3 carbon atoms), while another group uses a different route, the C4 mechanism (you guessed it; the relevant molecule has 4 carbon atoms). As it happens, the enzymes involved in each group have a slightly different preference for the CO2 involved (the catalytic cavity is so well adapted that the minute difference in bond length has a tiny but measurable effect; or something).

Now, back to what you tried to explain to me. AFAIK the amount of carbon in the atmosphere is minute compared to the amount available differently. I can image that there is some kind of preference of one type over the other. So, in the atmosphere (but nowhere else) one type builds up by a tiny bit. If there is a release of CO2 (say, lots of decay), it is mostly the other type that is released, so we do get a change. OK. If it is locked away from the cycle (e.g. frozen in limestone), then it no longer plays a role and we have a relatively higher ratio. Got it, I think!

Yep, you've got the gist of it!

bert wrote: However, I can't help thinking that different (climate) situations favor different groups of plants, and that they play a role here too. AFAIK sugar cane is a C4 plant and well, as you know, it favors high temperatures. Let's not rely one my memory (learned this stuff 30 years ago) and see what Google turns up. And there it is:
https://en.wikipedia.org/wiki/C4_carbon_fixation

Yes, can't remember new stuff but can recall old stuff. I"m officially old now! Wait, I digress.

So, I posit that there is a correlation between the graph that you showed and the temperature in geological time (which hopefully could be deduced through other manners). Can you expand on that?

This is a good point, Bert! However, I'm not so sure how much of an affect this would have.

As far as we can tell from the fossil record, per your own link, plants using the C4 pathway evolved about 35 million years ago. Before that, all the plants we see seem to belong to genera that utilize the C3 pathway. Given that the graph I posted spans the time period of 541-65 mya, I'm not so sure that temperature would have had that big of an effect in the way you describe. Of course, I could be missing something. Perhaps plants using the CAM pathway could have some effect, I'm not sure though.

But anyways, I took another look at the paper I cited earlier, and found something interesting.

We can actually use the ratio of Oxygen 16 and Oxygen 18 as a proxy for temperature. This is because Oxygen 16 evaporates more easily than Oxygen 18, and during cold periods, that means more Oxygen 18 rich water will be frozen by glaciers. Now, Oxygen chemostratigraphy is a little more tricky, as it's more prone to contamination. But if carefully done, it's very useful.

That same paper I got the graph from also measured the ratio of O16/O18 in their shells and whole rocks. If temperature was a factor for the carbon cycle, I'd expect that the Oxygen ratio would roughly follow the same trend as the carbon ratio (though with more variance, as it's sensitive to more factors). Turns out, they found just that. There was a general increase in both Oxygen and Carbon ratio's to a more positive number over the Phanerozoic, though they did fluctuate with ice ages and extinction events respectively.

So your idea may have some merit! I'm just not to certain what the cause of that would be, given there's no evidence for the existence of C4 pathway plants in that time period.