Geology Reference
In-Depth Information
based on the assumption that cave air P
CO
2
is
derived from the soil, where production is modu-
lated by temperature, and that cave air P
CO
2
(in %
atm) is influenced by ventilation:
Belgium; Caverns of Sonora, Texas). The model
overestimates cave air P
CO
2
for the well-ventilated
site GB Cave by 41% (deviation from maximum
measured values), but the P
CO
2
dataset is limited
(n ¼ 3) and intervals with higher P
CO
2
values
could have been missed. The model also slightly
overestimates cave air P
CO
2
values for Trou Joney
Cave and St. Anne Cave by 22% and 9%, respect-
ively. The greatest deviation between the modelled
and actual values occurs for the Caverns of
Sonora, where modelled values are 82% higher
than the maximum observed value (Banner et al.
2007). This suggests that maximum soil CO
2
pro-
duction at the site is limited not only by temperature,
but also by moisture. The Caverns of Sonora are
located in the geographic area with the lowest
average annual precipitation (c.41cmyr
21
)of
any of the 20 sites investigated here, supporting
this interpretation.
Equation 1 appears to predict cave air P
CO
2
values well if temperature and cave ventilation
strength can be estimated. The equation was
derived from empirical data collected at two tem-
perate sites where soil CO
2
production was never
moisture limited; consequently the equation
appears to breaks down when soil-CO
2
production
is limited by a factor besides temperature (e.g.
moisture limited CO
2
production in soils above the
Caverns of Sonora). Nonetheless, the cave air P
CO
2
values predicted using Equation 1 are within the
range of previously observed values for 16 out of
20 cave sites, suggesting that with further work
these relationships could be used to estimate palaeo-
cave P
CO
2
values from estimated temperatures.
P
CO
2
¼ (b) 0:318
b
0
þb
1
Tþb
2
T
2
8:314
(1)
ð
Þ
1
283:15
T
0:03
where bis a ventilation-dependent scaling factor, T
is temperature (K), and b
0
, b
1
and b
2
are estimated
parameters of the quadratic function (1, 2101.1
and 0.1, respectively). The scaling factor (b)
accounts for reduced P
CO
2
in well-ventilated caves
or in locations within individual caves that are
better ventilated. Based on linear regression
between the minimum cave air P
CO
2
values and b
values presented in Baldini et al. (2008), bcan be
estimated using:
b¼ 10 (P
CO
2min
) 0:1
(2)
This equation assumes that minimum cave air P
CO
2
value (in % atm) is a measure of total ventilation.
Cave air P
CO
2
measurements from various cave
systems that were the focus of various previously
published investigations demonstrate that minimum
values can approach atmospheric regardless of the
total range in values measured (Fig. 2), suggesting
that ventilation is actively occurring and producing
a minimum b¼ 0.28. Maximum values generally
appear to be controlled by a combination of soil pro-
duction and ventilation; in many records maxima
occur during times of maximum soil CO
2
pro-
duction (usually controlled by temperature) and
reduced ventilation. Some sites (e.g. Uamh an
Tartair, Scotland) appear to be well ventilated
(Fuller 2006), preventing CO
2
accumulation and
maintaining low cave air P
CO
2
values. Conversely,
poorly ventilated sites (e.g. the Caverns of Sonora)
maintain elevated values year round (Banner et al.
2007). The greatest range in cave air P
CO
2
is found
in caves that experience seasonal ventilation com-
bined with high soil temperatures, thereby increas-
ing the soil-to-cave CO
2
flux through either drip
water degassing or by physical connections
between the soil and the cave (Fig. 2). Equations 1
and 2 can be used to estimate cave air P
CO
2
for a
number of sites with published time-series P
CO
2
data in order to compare predicted to actual
values. Overall, Equation 1 predicts values very
well, but certain limitations in the model are also
apparent (Fig. 2). Of the 20 sites investigated, the
model-derived estimates are above the range of
observed values for four sites (GB Cave, England;
Trou Joney Cave, Belgium; St. Anne Cave,
Cave air control on cave calcite growth
Quantifying how cave air P
CO
2
behaves is important
for understanding the systematics of calcite depo-
sition on stalagmites. Drip rate, temperature, drip
water [Ca
2þ
], cave atmosphere P
CO
2
, and the thick-
ness of the thin film of water covering the stalagmite
all control calcite deposition rates (Buhmann &
Dreybrodt 1985a; Dreybrodt 1999; Genty et al.
2001). The following equation theoretically
describes stalagmite growth rates (Baker et al.
1998; Dreybrodt 1999; Baldini et al. 2008):
R
o
¼1:17410
3
(CaCa
eq
)
(dDT
1
)b1e
(aDTd
1
)
c
(3)
where: R
o
is the extension rate (mm yr
21
)
1.174 10
3
is a conversion constant used
to
change molecular
accumulation
(mmol mm
22
s
21
)
rates
into growth
rates (mm yr
21
)
Search WWH ::
Custom Search