photographic tour I've linked a number of times now. However, page 6 of that same tour also mentions another cause: loss of dissolved CO2. When CO2 goes into aqueous solution, it makes carbonic acid, which dissolves calcite. When the CO2 leaves solution, the solution becomes over saturated with calcium, which then precipitates out as calcite. The common presumption is that water coming into the cave has received most of its CO2 load as rain falling through the air- and indeed, when it hits the ground, rainwater is slightly acid, typically with a pH of about 5.5. What's rarely pointed out is that soil CO2 levels are extraordinarily high compared to freely-mixing atmospheric air: tens to over a hundred times higher. From here (and the following is only a third of the full diagram, which has lots more info):
Now all this is not to say water evaporation plays no role in the creation of speleothems. In some situations it may indeed be the major player. My experience, though, is that the role of soil CO2 gets much less attention than it probably should. I don't think most people, even geoscientists, realize how high concentrations of that gas can get in natural conditions just under their feet.
Photo unmodified. May 9, 2013. FlashEarth Location. (Since we're underground, I have only a vague idea where this is with respect to the surface.)
Is This Your Hat?
2 years ago