Almost right. The CO2 levels and airflow velocities are essentially barometrically driven (not chimney effect which requires substantial elevation differences). In the summer barometric pressure changes are usually small so little air exchange occurs (and CO2 levels can build). In the winter, frequent high pressure systems (cold fronts) cause rapid increases in pressure driving lots of outside air in, dropping CO2 levels by large amounts. The bigger the cave volume the better for this to happen. A low pressure system can do the opposite, drawing out CO2 laden air from the depths of the system.
The idea of supersaturated water forming the raft sounds very plausible and fits with the observations. Note that CO2 levels can vary by a factor of 10 or more in only a few hours when pressures are changing, so single observations can be deceiving as to what is going on. As for the origin of the CO2, I think that is still an open question. I'm not so sure about soils since there isn't much in central Texas. In small caves it could be organics. But in larger caves, I tend to favor out gassing from the limestone as it is dissolved by water - especially in caves that are connected to an underlying aquifer such as the Edwards where there is lots of water in continual contact with rock. Joe Sent from my iPhone Joe Sent from my iPhone On Feb 14, 2012, at 8:51 AM, Benjamin Schwartz <b...@txstate.edu> wrote: > While temperature, moisture content, and seasonality probably do have some > effects on raft deposition in the stream passage, and certainly effect CO2 > generation in the soil horizon, my observations in CWAN are that these > effects on the surface are actually driving airflow changes in the cave, > which is the main engine driving the raft precipitation. > > During the warm months in CWAN, no rafts form, and previously formed and/or > sunken rafts (after going over a rimstone dam, or under a drip site, for > example) will re-dissolve and disappear. This is because airflow slows down > during the hot months and is coming in from many of the (relatively) higher, > small, and peripheral fissure and fracture 'entrances' to the system, as well > as through the shallow soils, all of which will have a lot higher than > atmospheric CO2. This causes generally higher concentrations of CO2 in the > cave atmosphere, and relatively equilibrium conditions. I'm not sure what the > actual concentrations are in CWAN, but the air is definitely a lot less > 'fresh' in the summer. Evelynn Mitchell has some CO2 data for the tourist > part of the cave, so perhaps she can give us some relative idea of what > summer vs. winter is. > > During the winter, this chimney-effect airflow reverses and strong airflow is > pulling cool, low-CO2 atmospheric air in from the main entrance and > transporting it across the (now super-saturated with respect to the cave air) > water in the stream. As the air moves upstream, CO2 de-gasses and calcite > rafts will precipitate in just a few hours. We've sunk them going upstream > and found them reformed on the way back downstream. At the bottom of rimstone > dams and at our gaging weir, we sometimes see large spectacular drifts of > snow white sunken rafts - but these all go away in the summer or after a > large storm event. I can send a picture of our weir with beautiful raft > drifts below it, if anyone would like to see it. > > At any time of year, the far upstream reaches of the cave, where there is > little airflow, rarely have any rafts at all. The rafts (and rimstone dams) > pretty much stop when you pass the points where the main airflow leaves. And > those shall remain unattractive, gnarly, low-air secrets until we map the > passages. ;-) > > So, while I can't say this is what is happening in Honey Creek, I am very > confident this is what is driving calcite raft precipitation and dissolution > in CWAN. > > Best, > > Benjamin Schwartz > > --------------------------------------------------------------------- > Visit our website: http://texascavers.com > To unsubscribe, e-mail: texascavers-unsubscr...@texascavers.com > For additional commands, e-mail: texascavers-h...@texascavers.com > --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: texascavers-unsubscr...@texascavers.com For additional commands, e-mail: texascavers-h...@texascavers.com
