Geology Reference
In-Depth Information
(Sp¨tl et al. 2005). Sources of the CO
2
-rich 'ground
air' component may include CO
2
degassed from
groundwater, CO
2
from the soil zone that has pene-
trated the epikarst as a gas phase, CO
2
respired from
plant roots that may penetrate deeply into fractures,
and CO
2
generated from decomposition of colloidal
or dissolved organic matter in infiltrating water
(Atkinson 1977; Wood & Petraitis 1984; Wood
1985). The tight mixing array on Figure 8 constrains
the ground air d
13
Cas222.0 + 1.5‰ with no evi-
dence of seasonal variation. Soil CO
2
levels
measured from a site vertically above the cave
(Fig. 2) show seasonal variation with lowest
values in the dry summer months and maximum
values reaching 7300 ppmv in the winter when
soil moisture is greatest and bioproductivity is still
active. The isotopic composition of soil air is
shown on the Keeling plot on Figure 8 and forms
a more scattered array that shown by cave air. The
soil air CO
2
data are also a resulting of mixing,
this time between a respired CO
2
end member and
ambient atmospheric CO
2
. The d
13
C of soil respired
CO
2
is much more variable, ranging from 212 to
222‰ but does not show evidence of regular sea-
sonality. The heavier values are most likely a
result of diffusive loss of CO
2
to the atmosphere
which is greatest during dry periods. Thus the light-
est end member values approach the true value of
respired soil CO
2
which, at220‰ (Fig. 8), is slightly
different (i.e. heavier) than the composition of
CO
2
-rich ground air inferred as the end-member of
the mixed air observed in caves.
Calculated pCO
2
in equilibrium with undegassed
cave drip waters (.10
21
or 7% by volume, see
above) are much higher than measured soil CO
2
concentrations and it is speculated that sites of
karstic CO
2
production may lie below the soil
zone in pockets where organic material has accumu-
lated (cf. Atkinson 1977; Wood & Petraitis 1984;
Wood 1985). In this environment CO
2
generation
may take place at a more constant rate throughout
the seasonal year to become the main source of
'ground air' percolating into the epikarst.
summer. This seasonal variation is synchronized
with changes in cave air CO
2
levels (Fig. 7)
related to reversing chimney ventilation patterns.
Covariance among non-conservative parameters
such as pH, total alkalinity, d
13
C, Mg/Ca and cave
air pCO
2
are consistent with variable degrees of
calcite precipitation coupled to CO
2
degassing
which is externally controlled by the switch from
winter 'high' to summer 'low' levels of cave air
pCO
2
. Modern speleothem carbonates forming in
this environment are characterized by annual
laminae composed of paired columnar and dark
compact calcite bands which preserve well devel-
oped cycles in trace elements and stable isotopes
(Mattey et al. 2008).
Evolution of calcite and drip water
compositions in a seasonally ventilated cave
The composition and carbon isotopic evolution of
drip water undergoing coupled degassing - calcite
precipitation has been modeled as a Rayleigh
fractionation process using PHREEQC and
code adapted from Appelo & Postma (2007) in
Figures 9a, b. The monthly compositions of coexist-
ing cave air and drip water sampled between 2004
and 2009 are plotted on Figure 9a as cave air
ppmv CO
2
versus [Ca]
drip water
and on Figure 9b
as cave air ppmv CO
2
versus the d
13
C of cave air
CO
2
and drip water HCO
3
2
. Figure 9a also shows
the curve for the compositions of solutions in equi-
librium with calcite and CO
2
(g) at 18 8C and curves
showing the compositions of supersaturated sol-
utions with calcite saturation indices (log of the
ratio of ionic activity product to calcite solubility
product) of 0.5, 1.0 and 1.5. Drip water compo-
sitions at both the flowstone and Gib04a sites
define trends which are a result of variable
amounts of coupled degassing - calcite precipitation
from groundwater that is initially in equilibrium
with water in very CO
2
-rich environments. On
emerging into ventilated cave spaces these waters
are then subjected to degassing cycles driven by
the winter - summer switches in cave air CO
2
levels. The flowstone drip waters carry the highest
solute load (Figs 6 & 7) and the elevated Ca levels
require that the groundwater originally equilibrated
with carbonate in a very CO
2
-rich environment
where pCO
2
was in the order of 7% (Fig. 9a). This
far exceeds the observed CO
2
levels in the soil
zone suggesting that the site of CO
2
accumulation
may be deeper, perhaps within voids along the
inclined bedding planes forming the feeder
network for the drip (Fig. 3). The Gib04a drip
water, fed vertically downwards, equilibrated in an
environment with a maximum pCO
2
of around 1%
which is closer to the actual measured pCO
2
levels
in the soil zone directly above.
Discussion
Shifts in drip water chemistry may be a result of a
variety of processes including changes in the water
source composition, dilution of groundwater by
rain, calcite precipitation elsewhere in the aquifer
or changes in the degree of CO
2
degassing
(Baldini et al. 2006; Banner et al. 2007; Sp¨tl
et al. 2005; Tooth & Fairchild 2003). In Gibraltar,
the drip water compositions are largely independent
of discharge rates and simultaneously have lowest
pH (around 7) lowest d
13
C and highest Ca in winter
and lowest Ca with highest pH and d
13
C DIC in
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