Re: carbonic púddels and caves

Apart from being a geologist,I was once a caver. Of course it is well known
of the effects of methane in mines, and of asphyxiation accidents
associated with methane, due to shortage of oxgen. 
In 1959 Neil Moss died of asphyxiation due to CO2 build up in Peak Cavern,
Castleton in Derbyshire,UK (ref: http://www.grantham.karoo.net/paul/graves/neilmoss.htm).
This was mainly due to his own breathing. 
In a cave stream water will introduce oxygen to the area, and mostly caves
have a free air circulation and breathe in and out depending in outside
temperatures and pressure. In the UK quite often this is inwards during
cold spells and out in the summer. There isn't any reason why CO2 should
not accumulate in a blind chamber with limited air circulation, where 
heavier than air gases like CO2 can puddle or accumulate. I think this sort
of event would be very rarely found. Most deaths underground would arise
from exposure, starvation, rock falls, drowning, and in the case of humans,
caving accidents. Here is an account of accidents in caves. Web searches
for underground deaths due to asphyxiation show these are rare (http://www.sat.dundee.ac.uk/arb/cpc/century.html).

And from http://www.agius.com/hew/resource/lung.htm:

"Life depends on an adequate supply of oxygen reaching the tissues of
the body. Oxygen present in the air breathed into the lungs passes into the
blood and is carried to the tissues. Simple asphyxiants may interfere with
this process either by displacing oxygen from the air breathed in.
Examples: Methane, Nitrogen. This happens usually in enclosed, poorly
ventilated spaces particularly underground where methane can be produced by
naturally occurring processes or where natural oxygen has been depleted.
Symptoms include breathlessness due to lack of oxygen. Carbon dioxide also
causes rapid breathing, headache and sweating. Eventually, loss of
consciousness and death can result."