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
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