The End of Earthly Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 207

Ocean Remnants
The end of the oceans is not likely to be simple. A once-watery planet
does not become a uniformly dry, dusty planet overnight—not even
geologically overnight—unless, that is, another kind of greenhouse
comes into play, and we will come to that shortly.
The Earth, even in its hotter future, will still have areas that are
relatively colder at the poles, even while the equatorial regions bake.
Even as the water is siphoned off from the oceans, there is still at
least a further ocean's worth of water deep underground, dissolved
into the minerals of the mantle. This water will slowly be released to
the surface as the Earth's volcanism, within its newly emerging pat-
tern, continues. It will help preserve the Earth's ocean remnants a
little longer.
These remnants will shelter what have been called swansong bio-
spheres. 120 The evocative term is new, but the concept is not. The
swansong biospheres will probably be microbial, as the increasingly
harsh conditions will likely have put paid to all species of multicellu-
lar animals and plants some time before. They will be in the polar
regions. 121 Buried underground, out of reach of the fierce rays of the
Sun, there may be caves, or partially collapsed lava tubes. Any air that
is relatively colder and denser may gather in such places. With suffi-
cient cooling, the water vapour may condense into underground
pools and puddles, to give some of the last patches of more or less
pure water on Earth, a last gift of the oceans to life on Earth, shelter-
ing some of the last microbial communities.
Elsewhere, there may be the final puddles left in the remnants of
the oceans, highly saline and nestling within hollows in thick salt
deposits. The microbial communities surviving in these last ocean
patches would need to be hardy, capable of tolerating not just the high
salinities, but also swings in pH from highly acid to very alkaline,