Geology Reference
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early evening (roughly 20.00) and minimum values
in the early morning (04.00 to 06.00). The timing
and amplitude of this variability is consistent with
a small variability in temperature driven by the
temperature of the room in which the facility is
housed. Colonized flumes show larger amplitudes
of diurnal variability (c.5mScm
21
) and a different
pattern, with maximum values in the afternoon
(roughly 16.00) and minimum values in the mid-
morning (roughly 08.00). As it is different to the
temperature cycle, this cyclicity has to be driven
by repeatable chemical changes, and the peak in
conductivity during the day, when pH is high,
suggest that conversion of HCO
3(aq)
Reducing the length of the day has a significant
effect on pH in the colonized systems (Figs 7
& 8), though the fundamental character of the
diurnal pH curve (i.e. two opposed asymptotes)
remains the same. In addition to the expected exten-
sion of the low pH conditions throughout the dark
period, the daytime pH curve in Flume 1 changes
its shape slightly. During the 'mid-summer' and
'cold summer' experiments, approximate equili-
brium is achieved in ,5 hours in this system.
However, under both 'warm winter' and 'mid-
winter' conditions a similar equilibrium is not
achieved during the entire 6-hour daylight period,
which lies in contrast to Flume 2, which achieves
equilibrium in around 3 hours of 'dawn' regardless
of experimental conditions. The failure of Flume 1
to achieve equilibrium may reflect an inability of
photosynthesis to remove the respired CO
2(aq)
accu-
mulated during the night. As with the day-night
amplitude differences noted above, this apparently
enhanced buffering of the Flume 1 chemical
system reflects its longer overturning time (i.e.
slower flow rate). In terms of comparison between
pH values during the day and during the night,
reducing temperature or photoperiod in isolation
or simultaneously either has no effect on colonized
Flumes (1 and 2) or tends to cause a small increase.
Only during night-time in the 'cold summer' exper-
iment in Flume 1 is pHmarginally lower than during
the 'mid-summer' run. Moreover, night-time pH
during the 'mid-winter' experiment is roughly
0.05 higher than during the 'warm winter' exper-
iment in both flumes. The tendency of macroen-
vironmental pH in colonized flumes to increase
with cooling stands in contrast to the sterile
systems, and indicates that temperature control on
photosynthesis is smaller than that of respiration.
A significant consequence of this is that where
biofilm is present, changes in microbial metabolism
dominate over the impact of CO
2
solubility.
22
is
playing a dominant role. However, this would
suggest that conductivity should rise synchronously
with pH, and this is not the case. The conductivity
minimum at the night-day transition must therefore
reflect loss of ions from solution, most likely a
reduction in IAP
(Ca,HCO
3
)
, and provides very strong
confirmation for the photosynthesis-driven precipi-
tation of calcite inferred from the pHmeasurements.
As a change of 5 mScm
21
is the equivalent of
4.56 10
24
in IAP
(Ca,HCO
3
)
this change is subtle,
c. 1% of background values, but this reflects a rela-
tively undersaturated solution. We anticipate that
for a system maintained at higher IAP, the diurnal
effect on conductivity may well be higher than in
our Flumes, which is consistent with the data pre-
sented by Liu et al. (2008).
2
to CO
3(aq)
Light and temperature conditions
and their effect on pH
The sterilized Flumes (3 and 4) (Figs 5 & 6) show
a decrease in pH from summer to winter condi-
tions (0.15 for Flume 3 and 0.4 for Flume 4) and
from 'mid-summer' to 'cold summer' conditions
(0.15 for Flume 3 and 0.05 for Flume 4). The
response of Flume 3 is consistent between these
two experiments and also with expectations for
the solubility of CO
2
, which will decrease by
2.54 10
23
M 8C
21
for simple aqueous solutions
in this temperature range (Duan & Sun 2003).
Flume 4, though displaying lower pH under lower
temperature, does not show the same response
between these experiments, and this is largely due
to an unexpected response in this system to
reduced light conditions. The 'warm winter' exper-
iment produces minimal change in pH in
Flume 3, but a significant change (20.15) in
Flume 4. The cause of this may be either be evi-
dence of compromised sterility in Flume 4, or alter-
natively be reflecting a change in an unknown
photochemical reaction not present in Flume 3. In
either case, data from Flume 4 must be interpreted
with caution.
Geological and geochemical implications
Microbial influence on precipitation
Field-based research has given much emphasis on
the role of bulk water saturation index in regulating
the occurrence of precipitation (Chen et al. 2004),
despite the apparent failure of this parameter to
effectively predict the spatial occurrence of tufa
deposits (Pentecost 1992). The discovery that pre-
cipitation of tufa-like precipitates within laboratory
systems requires the presence of biofilm, that pre-
cipitates derive from biofilm interstitial water
rather than directly from bulk ambient water
(Pedley et al. 2009; Rogerson et al. 2008), and
that biofilms organisms exert a dominant control
on the chemistry of
this fluid (Bissett et al.
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