Interpretation of Phanerozoic CO2 levels graph

Question

I used to talk with a family friend who often sent me links to Nobel Prize winners, claiming that climate change is a myth.

Last week, we had a disagreement over a link referencing a Nobel Prize recipient (I believe it was Dr. John Clauser). This link displayed a graph of CO₂ levels spanning the past 140 million years, indicating a linear decreasing trend. The article posited that we are depleting our CO₂ reserves, a vital gas for plants and, by extension, the biosphere.

I responded by saying:

"Jorge, that graph is misleading. CO₂ levels during the Phanerozoic era are influenced by the Wilson Cycle, and they correlate with the formation and fragmentation of Pangaea. We are not depleting CO₂ as the subduction process is initiating in the Atlantic. In the forthcoming million years, CO₂ stored by organisms will be released."

_{Source: Wikimedia Commons.}

I heard this argument at the University of Zaragoza. However, I am uncertain if both my teacher and I have a comprehensive understanding of this matter. One of my concerns is that CO₂ dissolves below the Calcite Compensation Depth. I'm unsure if all sediments dissolve or just the top layers. This uncertainty leaves me pondering whether my argument could be refuted on this basis. I couldn't find any clarifications on Science Direct either.

Could someone elucidate this matter for both my friend (who will read this) and me?

Are the Phanerozoic CO₂ levels indeed linked with the Wilson Cycle? If so, why?

Bonus Question:

If humans wouldn't exist, would we run out of CO₂? (To me, this seems like an absurd query, especially in the context of that Nobel Prize article, because we'll likely gain control over Earth's geochemistry and climate long before then.) To rephrase: if we exclude human influence, would shell organisms eventually consume all the CO₂ by the end of the Phanerozoic era, leading to a mass extinction and the emergence of a distinct form of life? Would this scenario transpire in this cycle or the next?

Answer 1

On geological timescales, yes, the Wilson cycle (opening and closing of ocean basins) is bound to have an effect on atmospheric CO2 levels, if only because it will affect rainfall patterns, which in turn will affect chemical weathering of rocks, which is one of the things that removes CO2 from the air on very long timescales.

This obviously doesn't explain the post-industrial rise in atmospheric CO2 though, which has occurred on the scale of a century or so, rather than tens to hundreds of millions of years. So it seems like a grain of truth, but the argument is a non-sequitur.

"In the forthcoming million years, CO2 stored by organisms will be released."

Citation required.

As well as weathering (which is regarded as a thermostat that has been regulating planetary temperatures - see "The Global Carbon Cycle" by David Archer, Princeton University Press), they have also failed to mention that the sun's brightness has decreased (as it is a main sequence star) by about 6% over that timescale. CO2 and temperature are part of a feedback loop, so the gradual dimming of the sun may be expected to reduce the amount of CO2 in the atmosphere, all things being otherwise equal. It also ignores the evolution of land plants, and refinement of their photosynthetic machinery.

So while it is reasonable to expect some linkage with the Wilson cycle, the claim seems to be highlighting only a cyclic component while ignoring other factors, without giving evidence of their relative importance.

"if we exclude human influence, would shell organisms eventually consume all the CO2 by the end of the Phanerozoic era"

I very much doubt it, for a start CO2 is constantly being cycled from the sea floor via subduction and released by volcanos on the crust above. This is an ongoing process.

Would this scenario transpire in this cycle or the next?

It hasn't been established that there IS a cycle, rather than the combination of the effects of multiple factors affecting CO2 levels on geological timescales. It is very easy to look at a graph and see something that looks like a cycle, what you need to demonstrate it is not spurious is some physics that explains not only the cycle but the magnitude of the effect.