Geo 1095: March 3, Day 792: Gassy Flowstone

One often sees descriptions of speleothems implying that water evaporation is the single important process causing calcium carbonate (CaCO3- calcite or aragonite) to precipitate. See, for example, page 26 of the 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):

So when rain hits the ground, it's in equilibrium with an atmospheric partial pressure of CO2 of (as of present day) 0.04%. Then it seeps into soil with a partial pressure of CO2 of 2%-5%, or 50 to 125 times that of the atmosphere! In a very real sense, the waters' passage through soil supercharges its acidity, allowing to be a much more powerful dissolving and erosive agent. Cave air, on the other hand, mixes more freely with the atmosphere, so tends to be much closer to "normal" atmospheric conditions. When ground water reemerges into the cave, it fairly quickly re-equilibrates to the conditions therein. Agitation, such as falling as water droplets, accelerates the degassing of CO2.

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.)

Geo 1095: March 2, Day 791: Pointy Grotto

As in yesterday's photo, there appears to be a correlation between tone and age in these stalactites and drapes: older ones look darker, younger ones, brighter. Also, it appears that the younger deposits are being laid down faster than in the past.

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.)

Geo 1095: March 1, Day 790: Drapery Regenesis

This spot is apparently called Angel Falls (page 22 of this PDF). The photographic tour doesn't mention it, so I'm speculating here, but that broken drape near the center, with the large, regrown, bright white stalactite, looks as if it might be more souvenir collecting, with the regrowth occurring in the past century or so. As we saw earlier, though, natural processes can also break these speleothems in the absence of human defacement, so that must be considered as another possibility. We're also getting close to the artificial addit of the exit tunnel, which means air flow patterns have changed in historical times. Sometime in the near future, I need to get on it and explain why airflow (among other issues) is so important. However, it feels like this entire year so far (that is, the last couple of months) has been a struggle to keep up: a Red Queen's race, where I have to run as fast as I can just to stay in place. It does not bode well for online stuff as the field season approaches...

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.)

Geo 1095: February 28, Day 789: Aquaclude Versus Aquatard

Last Tuesday, I posted a photo of a feature I described as an "intrusive aquaclude." On Twitter, I was informed that the correct word was "aquatard." It turns out, both are terms in use, but an aquatard slows groundwater flow, while an aquaclude blocks its flow entirely. As this source points out (PDF), that distinction is difficult to make in the real world. I'm pretty sure this is the same feature as that in the Tuesday post, and once again, if you enlarge the image to full size, you'll see numerous small stalactites to the right of the dike, but none to the left. However it may be farther along and another feature that's similar. I'm in the habit of posting these photos in chronological order, and this photo was taken after the three previous pictures from Paradise Lost. However, my recollection (which may not be accurate) is that Paradise Lost is a side branch, and up from, that same room- that is, we backtracked from there to the same place again.

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.)

Geo 1095: February 27, Day 788: Two Tickets to Paradise

I am not a fan of Eddie Money, but I do enjoy working rock song puns into my posts on... well, rocks. Pretty much every surface on the inside of this spire at Paradise Lost is covered with flowstone draperies. It's a breathtaking spot, and I'm awfully glad I made the effort to climb up into it, despite my misgivings.

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.)