Geology Reference
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(3000 - 6000 ppmv) falling to around 500 ppmv in
the summer. Continuous monitoring of CO
2
levels
at seven locations has been carried out since
October 2006 and shows that CO
2
concentrations
can exceed 10 000 ppmv for brief periods and are
subject to rapid fluctuations which are closely
related to wind strength and direction rather than
barometric pressure (our unpublished data). These
data also clearly show that the seasonal switch
from high to low CO
2
modes is closely correlated
with the temperature difference between the exterior
and interior of the cave which seasonally reverses
chimney ventilation driven by density contrast of
cave air. Thus the high winter levels of CO
2
in
cave air are diluted in the summer by penetration
of outside atmosphere.
The switches between winter and summer modes
take place in mid-April and mid-November and cor-
respond to the external temperature respectively
rising above and falling below the MAT of the
deep cave (17.9 8C). The former allows dilution
by the atmosphere and the latter permits CO
2
levels to rise by flux from the interior of the Rock.
This chimney ventilation is clearly revealed by sea-
sonal temperature and humidity variations in OSM,
but ventilation dilution of CO
2
is observed in even
the most distal regions of NSM despite having no
known natural entrances and only linked to the
show cave via a small hatchway. Furthermore, the
transition from high winter levels to low summer
levels is very rapid and occurs throughout the cave
system within hours. Sealing of the hatchway
between OSM and NSM for a month in August
2007 had no effect on the CO
2
behavior in NSM
and dilution of cave air must take place via natural
pathways to other caves or directly to the surface.
These observations, supported by data for CH
4
and
d
13
C data for both CO
2
(see below) and CH
4
(to
be presented elsewhere), provide compelling evi-
dence for seasonal advective transport of CO
2
-rich
'ground air' through macroporous Gibraltar lime-
stone via enlarged bedding planes and joints.
Whether this process is local to the elevated cave
systems located near the summit of the Rock or
part of a larger scale advective system affecting
cave systems at other levels in the rock is not
yet known.
The carbon isotopic composition of cave and soil
air CO
2
is shown on a Keeling plot of 1/CO
2
versus
d
13
C in Figure 8. The d
13
C of cave air CO
2
shows
wide variation between 210‰ to 223‰ which
is a result of mixing between local atmosphere
(d
13
C ¼ 29.6‰) and an isotopically light CO
2
-rich
component, a processes also seen in Obir cave
Fig. 8. Abundance and carbon isotopic composition of CO
2
in local atmosphere, cave air (all sites) and soil air. Line is
a best fit mixing vector between atmospheric CO
2
and a 'ground air' CO
2
endmember with a d
13
Cof222‰. See
text for discussion.
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