Geology Reference
In-Depth Information
bags using a small hand-held pump for analysis of
mixing ratios and the d
13
CofCO
2
. Soil gas
samples were collected initially using a steel tube
inserted into the soil and from June 2007 using
porous PTFE gas sampling cells permanently
buried at 25 cm and 50 cm depth. Mixing-ratio
analysis of CO
2
was carried out using a LiCor
6252 NDIR analyser calibrated against NOAA
standards. Isotope analyses of CO
2
were determined
using a GV Instruments Trace Gas - Isoprime
system and precisions (1s) for 10 consecutive ana-
lyses of the secondary standard tank were between
0.03 - 0.05‰ for carbon dioxide d
13
C analysis.
are much more variable in the Hospital area
(Fig. 4b). Here temperatures are significantly
lower than the MAT of 17.5 8C all year round and
fail to reach MAT even by the end of the summer
(Fig. 4d). Humidity in the Hospital area is also
lowest in winter when chimney ventilation is vigor-
ously pulling cold outside air through the show cave
(Fig. 4b). The permanently below-MAT tempera-
tures of the showcave indicate that winter
ventilation removes sufficient heat from cave wall
rock such that the mean annual temperature of the
Hospital area (16.3 8C) remains over 1 8C cooler
than the annual outside MAT (17.5 8C) (cf. Atkin-
son et al. 1983). Conversely, the slightly higher
MAT of the deeper NSM (17.9 8C) may be indica-
tive of advected warmer air rising from lower
levels in the rock, a hypothesis developed in more
detail below to explain the cave air CO
2
data.
Spot measurements taken over the four-year
period are more variable than data returned by temp-
erature logging recording spot measurements at
30-minute intervals and suggest that there may
some local short term variation of up to +0.5 8C
at the Gib04a and other deeper sites in NSM.
Although the 0.4 8C resolution of the early logging
devices would not have fully resolved such small
variations in air temperature, logging since June
2008 at an improved 0.1 8C resolution fails to
record regular seasonality (Fig. 4) indicating that
small fluctuations in air temperature may occur
even in the more distal regions of OSM and are
too rapid to be clearly resolved by temperature
loggers recording at 30-minute intervals.
Results
Cave temperature and humidity
The variation of temperature and humidity within
the OSM and NSM cave systems generally decrease
as a function of distance from entrances, and temp-
eratures approach the local mean annual values in
the deeper parts of NSM. In detail, though, these
variations are quite complex both in terms of
spatial position within the cave, and as a function
of the time of year. Variations in cave air tempera-
ture and humidity at two of the monitoring sites,
the Hospital (the lowest part of the show cave,
Fig. 2) and at the GIb04a site (representative of
the distal areas of the NSM system, Fig. 2) measured
by continuous logging and monthly using hand held
instruments are summarized in Figure 4. Figure 4a
compares three NSM cave air temperatures
measured in 1948, 1954 and 1958 (Shaw 1955;
Tratman 1971), the mean annual temperature
(MAT) at the cave entrance measured in this study
between 2004 - 2008 and the local MAT recorded
by the Meteorological Office since 1940. The
MAT plotted on Figure 4 are sea level data corrected
to the corresponding temperature at 325 m asl using
a lapse rate calculated from over 1300 measure-
ments of the difference in mean daily temperature
measured at the Hospital tunnel entrance (Fig. 2)
and at sea level by the Meteorological Office
station (1.22 + 0.25 8C). The three historic spot
measurements show a 1.6 8C range and cannot be
distinguished from modern temperatures which are
within error of the MAT corrected for altitude.
Unfortunately there are insufficient data to assess
whether cave temperatures tracked the decreasing
and then rising trend in MAT observed since 1948
(Fig. 4).
Temperatures monitored by continuous logging
appear to be relatively constant in the deep cave
(, +0.4 8C) but show marked seasonal variation
of around 3 8C in the Hospital area of the show
cave (Fig. 4c, d). Relative humidity measurements
average 95 + 3% in the deep cave but are again
Drip-rate variations and recharge pathways
Following the release of tracers near the end of the
period of winter recharge Photine CU was detected
at sites dispersed along the length of the cave
(Fig. 3). The drip falling onto the Gib04a stalagmite
itself was not monitored but the Photine dye was
detected at seepages nearby. The position of the
two Photine injection sites almost directly above
the cave requires that the detected dye must have
followed a nearly vertical pathway through 68 m
of rock to reach these. Dye was also detected in see-
pages up to 60 m horizontally NW of the nearest
injection, and 50 m away S - SE. These pathways
have overall deviations of 418 and 368 from vertical,
and indicate that percolation through a network of
interlinked voids spread the tracer laterally
between paths that diverge vertically by up to 778.
The second dye, DY96, was detected at the flow-
stone site, which lies almost directly down dip
from the injection point (Fig. 3). This dye was
found at only a few of the other sites monitored,
and the fluorescence at these was much weaker
and
less
prolonged
than
at
the
flowstone
site.
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