Geology Reference
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influences may be larger. In these locations, the pet-
rographic character of seasonal laminae have been
shown to be extremely variable (Pentecost 1978,
1987; Freytet & Verrecchia 1998; Janssen et al.
1999; Arp et al. 2001; Carthew et al. 2006), with
no universal rule governing the character of summer
or winter layers (Kano et al. 2003). Clearly, there is
much complexity in these systems that remains to
be unravelled.
Physical modelling provides a means to assess
the importance of individual processes without the
complexity inherent to field sites. The literature con-
cerning in vitro and/or ex situ work on tufa systems
is small, but is growing rapidly (Merz 1992; Pedley
1994; Dittrich et al. 2003; Bissett et al. 2008b;
Rogerson et al. 2008; Shiraishi et al. 2008a, b, c;
Pedley et al. 2009) and provide a crucial testing
grounds for hypotheses regarding the origin of phys-
ical and chemical characteristics of tufa carbonate.
These studies confirm that photosynthesis drives
a significant (10-20%) increase in calcium flux
to the water-calcite interface relative to non-
colonized carbonate surfaces under artificial sun-
light, and export of calcium from the surface in
dark conditions (Shiraishi et al. 2008a). Perhaps
surprisingly, however, they also indicate little
change in calcium flux under macroenvironmental
conditions between 4-17 8C or between pH 7.8-
8.4, indicating an unexpected ability of microbial
mats to maintain constant internal (microenviron-
ment) conditions (Bissett et al. 2008b).
This study builds on these ground-breaking
experiments by exploring diurnal pH and conduc-
tivity variability within a karst water system close
to equilibrium with respect to calcite precipitation.
The daily cycle of precipitation and dissolution
is investigated, and the impact of varying day
length and ambient temperature discussed. In the
light of these experiments, we consider the impli-
cations for the origin of annual laminae in tufa
stromatolites.
environment. Water flow was driven by a 'Hozelock
Cascade Cyprio 1000' variable rate submersible
pump (Hozelock Cyprio Ltd., Aylesbury, UK).
This system represents a recirculating flume, and
therefore is not a precise analog for natural tufa
systems which are essentially flow-through. Conse-
quently, the magnitude of hydrochemical changes
may be somewhat amplified in our data as a result
of so-called 'additive' effects. However, relative
changes within a system and between similar
systems are unlikely to be altered by these passive
influences and can be compared to natural systems
with a high degree of confidence.
Three identical Flumes (1, 2 and 3) were
employed and a fourth system (Flume 4) which con-
tained no transverse barriers, thereby reducing tur-
bulence to a minimum. Each flume was sealed
within a purpose-built transparent perspex tank
with entry and exit connectors for the flexible pipe
work and access for sampling. To ensure good air
circulation, a 'Hiblow HP40' diaphragm-type air
pump (Hiblow, Saline, MI, USA) was connected
to the flume with a pre-flume filter and an open
(slightly back pressured) exhaust port. The lighting
for the experiments was provided by a timer-
controlled Thorn-Lopak 250 W, HPS-T sodium
lamp (Thorn Lighting, London, UK) for each meso-
cosm; this provided the fullest practicable light
spectrum for photosynthesis. Experimental temp-
eratures were maintained by means of in-line
'Titan 150 minicooler' chiller units (Aqua Medic,
Bissendorf, Germany) in combination with refriger-
ation units placed around the sump. The ambient
room temperature was buffered at 16 8C throughout
the experiments by means of an 'Airforce Climate
Control' air conditioning unit (10 000 BTU h
21
;
cooling capacity 2.9 kW; Airconwarehouse, Stock-
port, UK). Flume 1 had about half the flow rate
(0.04 L s
21
) of the other flumes (0.09, 0.08 and
0.07 L s
21
respectively).
Flumes 1 and 2 were colonized with biofilm
recovered from the River Lathkill (Derbyshire).
Initial communities were imported attached to
plastic mesh pads that were placed on the river
bed and attached to the bottom of the flume by
wire springs. Subsequent to import of this commu-
nity, the biofilm is allowed a colonisation period
of 12 weeks to establish throughout the flume liner
after which experiments were initiated. After this
period,
Methods
The experiments described in this paper were per-
formed within the mesocosm facility at the Univer-
sity of Hull described in previous publications
(Rogerson et al. 2008; Pedley et al. 2009). The
mesocosm is designed to re-circulate 40 L of
water contained in a 60 L sump (clamp top barrel).
The flume consisted of a 1 m length of 112 mm
wide polycarbonate gutter, into which three low bar-
riers were fixed to trap small water pools in an other-
wise recycling, direct through-flow system. Water
depth varies between 1 mm in riffles to a 5-6 cm
in pools, allowing maximum interaction between
the water and the biofilm and compares well with
small
biofilm coverings
of
surfaces were
between 1 mm and 5 cm.
Throughout the experiments described in this
manuscript, pH and conductivity were monitored
by means of submerged probes (2-3 cm depth)
linked to 'Pinpoint' digital metres (American
Marine Inc., Ridgefield, CT, USA). All data were
relayed (every 10 min on a 24/7 basis) via a
webcam, to a PC for archiving. Certain experiments
tufa producing systems
in the natural
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