Cull 4. Unlike the others, 4 fails at thermodynamics. At mid-latitudes dry ice simply cannot form on Earth. Even if Earth didn't have any atmosphere to offer any kind of greenhouse effect, its average temperature would still be
-18 ?C. Underground temperatures could conceivably reach below -78 ?C near the poles in a climate that is much colder than that of today (in our time the
lowest recorded underground temperatures on Earth are in the -57/-56 ?C range, under 45 meters of ice near the Antarctic Pole of inaccesibility; below and above that depth it only gets
warmer).
The problem is, however, that most or all of the
Mesozoic had a warmer climate than today. But let's assume the plot takes place in an ice age in the Mesozoic. There is no evidence of ice ages in the Mesozoic, but for at least some periods, like the early Triassic, ice ages cannot be conclusively ruled out.
So, we have a desert on the northern or southern extreme of Pangaea during an ice age. We can expect an extremely cold and arid climate. We must make it like Antarctica. Solar radiation is low, and planetary carbon dioxide levels are extremely low as well.
How do the plot's protagonists, if they are anything more complex than a fungus, survive in such an environment, or in a neighboring environment? The atmosphere brings oxygen from the productive regions in the mid-latitudes, and nothing else. There's no precipitation, and not enough sun and carbon dioxide for plants to grow. How do the protagonists end up in such a place? You don't get lost and end up in such a place by accident. Going there can only occur as a deliberate attempt at suicide.
But OK, let's assume that summer in such a place actually isn't unbearable. You can walk outside, and there will be frozen carbon dioxide beneath the ground left over from winter. For some reason the protagonists are here. The only native lifeforms are communities of fungi and algae that survive against all odds.
Here's the ultimate problem. How much biomass is there per square meter? How much acetone can the fungi produce per square meter in a year?
If we assume the fungi are in continuous contact with the dry ice, comparing with biota living in similar environments, like the Ross desert on Antarctica, we can expect a
net ecosystem productivity of carbon of 3 milligrams per square meter per year or less. That would mean about 5 milligrams of acetone per square meter per year. This is such a small amount as to be inconsequential. This can't freeze anything.
However, if we assume that during the summer the fungi are shielded from the dry ice by soil, and start churning out rapidly acetone, and as the acetone contacts the dry ice and freezes they enter a dormant state, then we could reach the productivity of southern Atacama (the wetter part of the desert). Something like
10 grams of organic matter per square meter per year. Even if all of it is acetone, it is still very far from producing a nasty burst of cold to a victim.
As for the other possibilities you named, Loyfe, I can't find any obvious flaws with them.
Well, 6 is iffy, but I'm not in the mood to check it right now.
