Benjamin, Thanks for elaboration at CWAN. I agree with you. The same thing is happening at Honey Creek. As you understand, but for those readers on the list who are less familiar with carbonate geochemistry, it isn't really the airflow per se that is causing the calcite precipitation. The airflow is instead the mechanism of keeping the partial pressure of carbon dioxide in the air below that of the water, allowing the water to degas some of its carbon dioxide and precipitate the calcite rafts. Without airflow to replace the carbon dioxide-rich air with fresher air, the carbon dioxide partial pressures in air and water would effectively equalize so no degassing and thus no calcite precipitation would take place.
During my research at CWAN I also saw great rafts form and disappear, especially around my stage recorder stilling well at the dam. I measured atmospheric carbon dioxide monthly in the cave and those who have a copy of my dissertation (apparently still available from the TSS at: http://www.utexas.edu/tmm/sponsored_sites/tss/publications/tsspubmono.htm) can see a plot of the seasonal changes. Of course I expect Evelynn's data to be far more detailed than mine and I'd love to see her results someday. George *************************** George Veni, Ph.D. Executive Director National Cave and Karst Research Institute 400-1 Cascades Avenue Carlsbad, New Mexico 88220-6215 USA Office: 575-887-5517 Mobile: 210-863-5919 Fax: 575-887-5523 gv...@nckri.org www.nckri.org -----Original Message----- From: Benjamin Schwartz [mailto:b...@txstate.edu] Sent: Tuesday, February 14, 2012 07:51 To: texascavers@texascavers.com Subject: [Texascavers] Re: Honey Creek sand observation 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
