Well having had absolutely zero feedback from my requests for topics and from my typical cavers trip report in last issue I am forced to fall back onto my proposed topics. Please somebody send me some original and interesting questions (which are not about cave coral, moonmilk and/or helictites) and we will have an ANDYSEZ of interest.

Because nobody has asked for a subject I am forced into the dull .... which doesn't mean that it is not useful and/or interesting. We are going to discuss gases - at least one in particular. Carbon dioxide is the stuff that makes you breath - it stimulates the breathing reflex and you would die in its absence. It also gives you a lift according to Coca Cola or someone similar. This ANDYSEZ is a prerequisite for the next which will discuss the dynamics of the limestone-carbon dioxide-water system. You all know that carbon dioxide dissolves in water to form a weak acid because you have been telling the tourists that for years. Stop reading and go and buy a bottle of soda water .... don't open it but start reading again. The earth's atmosphere is made up of a large number of gases in relatively fixed proportions as follows:

Nitrogen	78.03%	gas inert for our purposes
Oxygen	20.99%	gas we need
Argon	0.94%	inert gas
CARBON DIOXIDE	0.031%	our wonder gas
Hydrogen	0.01%	gas for balloons
Neon	0.0018%	inert gas
Ozone	0.00006%	rising and hole-developing gas
Helium	0.00005%	gas for up-market balloons
Krypton	0.00001%	see Clark Kent Henderson for details (your mild-mannered [?] reporter!)
Xenon	0.000009%	gas to fill your electronic flash tubes

Note well that the amount of water vapour in the air (normally around 2-3%) will alter these proportions: many other gases are present and may be locally high (e.g. methane around cattle feed lots and rice paddies). Each of these gases contributes to the total pressure of the atmosphere; the proportion of each is known as the "partial pressure". Thus nitrogen exerts a partial pressure around 80% of the total pressure and carbon dioxide a very small 0.03% (pCO2  =  0.03% in shorthand).

Now lets look at your soda water. Don't open it yet. Gaze at it lovingly and you will see a) clear water with no bubbles unless you are rich enough to have bought Perrier water in which case it will be green and taste of benzene) and b) a space above the liquid which we presume contains a gas. The contents of the bottle are under pressure as we will hear as you now remove the lid as the gas which occupied the space above the liquid escapes from the bottle. Bubbles also form in the liquid portion but not instantaneously - they then vent to the atmosphere. Screw the lid back on and after a short time we are back to the situation before the bottle was opened - i.e. a clear liquid and a clear space above.

What has happened here? Before we opened the bottle there were gases present in both the liquid and the space above the liquid (let's just call this the "space"). In the liquid the gases were in solution. In the space there is the same composition of gases - in equilibrium with those dissolved in the liquid. Once we open the bottle the gases in the space escape as they attempt to come into the equilibrium with the atmospheric pressure. Gases also escape from the liquid as bubbles (come out of solution) but more slowly as the liquid comes into equilibrium with the atmosphere.

When we screw the lid down again the space comes back into equilibrium with the liquid and the bubbles vanish as the gases go back into solution. If your soda water came in a plastic bottle you can hasten the process by squeezing the bottle thus increasing the pressure in the space and enhancing the re-solution process. The hiss will be less next time you open the bottle indicating that we lost gas from the system (the bottle) the first time around.

We must know ask what relevance all this has to do with caves and karst. I assure that it does. Soda water is carbon dioxide dissolved in water; it occurs naturally as all water in equilibrium with the atmosphere (uninterestingly bubbleless in this case) but fizzy sometimes as it rises through springs, artesian and other bores. When we have carbon dioxide in water we have a weak acid and limestones dissolve in acids. If the solution (gas plus water) has not been in contact with limestone it will be "aggressive" (capable of dissolving limestone). If it has been in contact long enough to achieve equilibrium between the acid and the rock (again a rate-controlled reaction slower than liquid degassing) the liquid will be saturated, hard or non-aggressive (all mean much the same thing).

Where does carbon dioxide come from to make our acid waters? Firstly, as discussed above, we have the atmosphere. It contributes a pCO2 of about 0.03% as rainwater comes into equilibrium with the atmosphere - which doesn't make a very acid solution. This isn't going to make very good caves.

However, as Arthur Farrowfield says, "the onswer loies in t'zoil" (younger readers should send a stamped, addressed envelope for further explanation). Here in the soil we have beasties fornicating and breathing heavily, other beasties just breathing and plant roots respiring (the above ground parts of plants take up carbon dioxide and produce oxygen having used the carbon to produce tissue: the non-photosynthesing roots produce carbon dioxide and consume oxygen). Because the soil (does not have completely free interchange with the atmosphere (a little like the liquid in our soda water bottle only more so because the routes through the soil are far more tortuous) the partial pressure of carbon dioxide (pCO2) in the soil atmosphere is much higher than in the outside atmosphere.

pCO2 [I only did this so I could start a sentence with a small "p"] in the soil is commonly as high as 2-3% but may be very much higher if biological activity is more intense. Water in the soil is in equilibrium with the soil atmosphere and thus more CO2 will go into solution in the soil water. It will therefore become more acid and capable of dissolving more limestone.

The next ANDYSEZ will look at what all of this means in terms of dissolving and redepositing limestone - i.e. making caves and stalactites. Readers wishing to receive reimbursement for the purchase of their now flat soda water should address all correspondence to Peter Gesling at Wellington (NSW not NZ) as he can supply extra-virgin carbonated water at discount prices.