Geology Reference
In-Depth Information
The two variables that exert greatest control over
the solute and isotopic evolution of carbonate satu-
rated solutions during coupled degassing - calcite
precipitation are: (1) the contrast between the
initial equilibrium pCO
2
and that of the new lower
pCO
2
environment; and (2) the rate of CO
2
degas-
sing relative to calcite precipitation. The contrast
between the initial and final pCO
2
is well con-
strained by field measurements but the kinetics
and rate of degassing and calcite precipitation may
depend on local factors such as the discharge rate,
drip size, water film thickness (for degassing) and
the nucleation and crystal growth mechanisms
during calcite precipitation. Figure 9a shows
modeled degassing curves for the flowstone drip
water compositions using an initial solution in equi-
librium with pCO
2
set at 7% containing 4.5 mmol/l
Ca. If the pCO
2
contrast remains small and CO
2
degassing and calcite precipitation keep pace with
each other the solution will evolve following the
equilibrium calcite solubility curve as pCO
2
decreases to a new lower level. For CO
2
saturated
drip water emerging into a well ventilated space
the CO
2
degassing rate will be much greater than
the calcite precipitation rate rapidly forming
calcite supersaturated solutions as shown by the
drip water compositions at both the flowstone and
Gib04a sites in Figure 9a.
The drip water data plotted in Figure 9a represent
three complete ventilation cycles and are the end
product of degassing under two regimes: winter
and summer. To illustrate the effects of different
degassing - calcite precipitation rates, four degas-
sing curves are calculated for 'summer' degassing
where final equilibrium would be attained under
low cave air levels of 500ppmv CO
2
. The curves
represent degassing rates 10, 10
2
, 10
3
and
10
4
greater than the rate of calcite precipitation
and show in all cases that solutions rapidly
become strongly supersaturated until delayed
calcite precipitation restores equilibrium at the
new lower pCO
2
. As the degassing rate increases
the degree of maximum supersaturation also rises
and the form of these curves mirror the observed
trends in drip water evolution (Fig. 9a). Flowstone
drip water compositions suggest that degassing
from at this site was around 10
3
times faster than
calcite precipitation, and this is also the case for
Gib04a drip water although at lower overall
degrees of calcite supersaturation (Fig. 9a, open
circles). For degassing under winter conditions the
pCO
2
contrast is lower and degassing vectors will
return to the equilibrium curve more quickly. This
is illustrated by the dashed curve on Figure 9a
which is the 10
3
rate curve (solid line) recalculated
for winter degassing to a cave air pCO
2
of 5000
ppmv and embrace data at relatively low Ca levels
sampled under high pCO
2
conditions on Figure 9a.
Thus the spread of measured drip water compo-
sitions fit within a dynamic degassing model
where degassing vectors are continuously respond-
ing not only to seasonal ventilation but possibly
also to the rapid (day-week) synoptic time scale
fluctuations revealed by continuous monitoring of
cave air pCO
2
.
The evolution of d
13
C in bicarbonate, cave air
and calcite has been calculated as a Rayleigh
model using PHREEQ as a complete system that
accounts for apportionment of
13
C among all coex-
isting carbon species as a function of changing pH
during degassing (Appelo & Postma 2007).
Monthly values for d
13
C of coexisting dissolved
inorganic carbon in drip water from the flowstone
and Gib04a sites, and for coexisting cave air CO
2
(calculated as the end-member composition 'added'
as ground air, see Fig. 8) can be compared with the
modeled isotopic evolution of the composition of
least degassed drip water (sampled from within
roof straws at 216‰) under the same summer con-
ditions as the solid degassing curve in Figure 9a,
where the rate of CO
2
degassing is set 10
3
times
faster than calcite precipitation.
Starting with an initial solution having a d
13
C
of 216‰ (representing the bulk composition of
all carbon components, measured as total DIC)
measured and calculated data d
13
C
DIC
are closely
comparable and d
13
C
DIC
values rise as a result of
degassing of isotopically light carbon under
decreasing cave air pCO
2
. The modeled bulk isoto-
pic composition of carbonate is initially remains
fairly constant at around 212.5‰ and rises to
around 210‰ during the final stages of degassing.
Although the instantaneous isotopic composition
of calcite will track that of dissolved bicarbonate
and would rise to isotopically heavy values during
the final stages of degassing, the modeled range in
Fig. 9. (Continued) comparison. The dashed curve represents the case where degassing takes place under winter high
pCO
2
conditions. (b) Monthly values for d
13
C of dissolved inorganic carbon in drip water from the flowstone (and
Gib04a) sites coexisting with end-member cave air CO
2
compared with the isotopic evolution of dripwater total DIC,
dripwater HCO
3
2
degassed CO
2
and precipitated calcite. The parent drip water has an initial d
13
C
DIC
of 216‰ and
degasses under the same summer conditions plotted in Figure 9a as the solid curve. The amplitude of calcite d
13
C annual
cycles in Gib04a are also shown (from Fig. 10 and Mattey et al. 2008) which closely match the modeled evolution of
calcite d
13
C responding to degassing. See text for discussion. [Degassing curves are calculated using PHREEQC
and code adapted from Appelo & Postma (2007) with calcite precipitation rate constants from Plummer et al. (1978) and
relevant isotope fractionation factors from Clark & Fritz (1997).]
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