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I recently stumbled on the images of the Cosquer cave, located in France, famous for being a comparatively recently discovered as a Paleolithic decorated cave.

The entrance to cave now is more than 30m below waterline, and, according to Wikipedia, has presumably been underwater since Holocene.

In the photo attached, we can see the diver breathing the cave air.

I started to wonder, what would the initial air composition be, when the cave was entered by humans, if we assume the cave was in rocky strata, and the only source of gas for a long period of time was the seawater column.

Would it be correct to say that the air composition should be similar to what it was when cave started being underwater, or are there processes which would ensure the air in it would be similar to the one in the outside atmosphere nearby?

I understand there can be several ways in which gas exchange could happen

enter image description here

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  • $\begingroup$ note the cave was not occupied during the Holocene it was occupied during the late Pleistocene, when the entrance was above water. when the cave was entered by humans it was well above sea level. or are you talking about when it was entered by modern divers? $\endgroup$
    – John
    Feb 20 at 22:48
  • $\begingroup$ @John yes, about modern divers. $\endgroup$
    – Gnudiff
    Mar 25 at 12:15

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Considering the presence of speleothems, indicating the infiltration of water along fractures (i.e. fracture permeability), and the range of daily tides in the vicinity (noted as presently about 0.8 ft, or 0.24 m), one would suspect that there is at present an exchange of air with the exterior atmosphere. Air would be gently pulled into the cave along permeable fractures during the transition to low tide, and be similarly exhausted during the transition to high tide. Consequently, the composition of the air in the cave would be similar to, or exactly the same as, the exterior air outside of the cave.

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