Geology Reference
In-Depth Information
the past. In general, cave air P
CO
2
levels are con-
trolled by the relative strengths of sources and
sinks characteristic of each individual cave site
(Fig. 1). Carbon dioxide in caves is sourced from:
(1) vertical movement of gaseous CO
2
derived
from root respiration and organic matter decay
through bedrock permeability; (2) degassing of soil-
derived CO
2
from percolation waters and streams;
(3) decomposition of organic material within the
cave; (4) respiration of microbes and animals in
the cave; and (5) geothermal activity. The mechan-
isms responsible for the first three sources are all
temperature dependent, and therefore suggest that
caves in warmer climates should have higher cave
air P
CO
2
values than caves in colder climates, assum-
ing no ventilation. These high cave air P
CO
2
values
should result in lower growth rates in warmer
caves; however, the opposite is often the case, and
it is clear that soil P
CO
2
and subsequent bedrock
dissolution are more important controls on calcite
deposition for stalagmites fed by drips with a
certain hydrology (discussed in more detail below).
The source also controls the vertical distribution
of CO
2
in a cave passage. Slow degassing of CO
2
from numerous drip sites produces localised high
concentrations near the roof of a cave (Atkinson
1977), while more rapidly dripping water may
degas preferentially nearer the floor. Vertical
movement of soil gas through fissures produces
higher concentrations near the roof if the flow is
more diffuse through many small pores. Conver-
sely, because CO
2
is heavier than 'average' air,
theoretically if the flow is more concentrated the
gas will tend to behave as a fluid and flow down-
wards spilling over the floor of the passage
(Berger 1988). Whether the gas diffuses at the
ceiling or falls to the floor is a complex function
of diffusion rate, fall height, diameter of the fall
column, and the density of the gas (Berger 1988),
though more field research is needed to confirm
these theoretical predictions. Carbon dioxide
derived from either the decomposition of organic
matter within the cave or geothermal activity will
tend to be found at higher concentrations near the
floor. However, regardless of the CO
2
source, if
the CO
2
influx ceases the vibrational energy of
the gas molecules will eventually result in diffusion
and molecular-mass independent mixing of the
gases present, resulting in a homogenous gas
mixture. Therefore occasionally elevated P
CO
2
levels in deeper passages may not be due to the
high molecular mass of the CO
2
compared to
'average' cave air, but rather may be due to a deep-
seated source or reduced advection in a sheltered
location (Fig. 1). The latter is well illustrated in
Grotte de Lascaux, where the Mondmilch Gallery
has a P
CO
2
of between 1.0-1.5% atm while the
slightly shallower but more sheltered Shaft of the
Dead Man has very high values of between 3-7%
atm, although these elevated values could also be
due to localised micro-organism metabolism in
sediments (Denis et al. 2005). Production of CO
2
through the process of aerobic respiration can be
identified by a mole-to-mole replacement of O
2
by CO
2
(e.g. if CO
2
increases by 1% and O
2
decreases by 1%).
By far the most important CO
2
sink in caves
is the transfer of air out of the cave system by
advection. This is usually readily identifiable as a
seasonal (or permanent) reduction in cave air P
CO
2
to values approximating outside atmospheric
values (currently c. 0.038% atm), and is commonly
observed at many sites (Ek &Gewelt 1985; Baker &
Genty 1998; Baldini et al. 2008). Ventilation is
often driven by temperature or pressure differences
between entrances. A good example of this is
the dynamically ventilated Rassl-Bumslucke-O2J
System in Austria, where cool cave air exits the cave
during the warm season, but this airflow reverses
when outside air is colder than cave air (Sp¨ tl
et al. 2005). While ventilation is more common in
caves with multiple entrances, particularly when
these are at different altitudes, caves with one
entrance can still be ventilated. Ballynamintra Cave
(southern Ireland) appears to have active air circula-
tion during the summer, which shuts off during the
winter, though the trigger for the ventilation
remains unknown (Baldini et al. 2008). Conversely,
several caves in Texas appear to ventilate during the
winter, when cold winter air displaces warmer cave
air, initiating air circulation (Banner et al. 2007).
Some cave sites may display very static cave air
P
CO
2
levels, suggesting limited ventilation (Banner
et al. 2007). In most caves with no ventilation
cave air P
CO
2
levels can only increase as high as
soil air P
CO
2
. Higher levels could theoretically be
reached through the consumption of atmospheric
O
2
and production of CO
2
through micro-organism
metabolism (Smith 1999), or through the addition
of high levels of geothermally-derived CO
2
.
Another possible sink for cave air CO
2
is the
absorption and dissolution of CO
2
into cave streams,
percolation water, or water condensed onto cave
walls (i.e. cave waters that are not equilibrated with
cave air P
CO
2
). While potentially locally important,
particularly in the case of a large cave stream com-
posed of water with a very short mean residence
time in the aquifer, in general these processes are
of secondary importance to ventilation.
Estimating cave air P
CO
2
Baldini et al. (2008) used long time-series datasets
from two cave sites to link cave air P
CO
2
to
primary production and ventilation. Equation 1 is
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