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I recently stumbled upon images of the Cosquer cave in France, famous for being a recently discovered Paleolithic decorated cave.

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

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

I wondered what the initial air would be when humans entered the cave if we assumed the cave was in rocky strata. The source of gas for an extended period was the seawater column.

Would it be correct to say that the air composition should be similar to when the cave started being underwater, or are there processes that would ensure the air in it would be identical 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
    Commented Feb 20 at 22:48
  • $\begingroup$ @John yes, about modern divers. $\endgroup$
    – Gnudiff
    Commented 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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It's extremely likely that the cave was on dry land a few tens of thousands of years ago. Early humans would not have had any diving equipment, so 30+ meters below the waves was downright inaccessible for our early ancestors. Also it's safe to assume the cave was once part of the mainland, as boats had probably yet to be invented. That said, it is possible that early humans did have boats of some kind, like hollowed out logs.

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