Geology Reference
In-Depth Information
Fig. 1. Profiles of barrage tufa constructions from two dissimilar climatic regimes. Upper sketch represents a tufa
barrage at Caerwys, NorthWales, UK and associated pools. This is typical of deposits in cool-humid climates. Note that
barrage progradation and aggradation rates are similar leading to relatively shallow upstream pool development prone to
lateral infill by paludal facies. Stipple symbol indicates well-cemented areas at the barrage crest; black areas are sapropel
deposits no symbol is lime mud. Lower sketch represents a tufa barrage in Ruidera Natural Park, Spain. This is typical of
deposits in warm, semi-arid climates. Note the development of the barrage mainly by aggradation of stromatolites
(laminated symbol). In consequence, a deep lake is developed immediately upstream of the barrage. Accumulation style
of barrage development is shown in the top left inset of both sketches.
et al. (2008) have successfully studied the effects of
photosynthesis on in-situ fluvial biofilms and Bissett
et al. (2008) have studied precipitation products in
ex-situ fluvial biofilms. Collectively, these works
indicate tufa precipitation is not the straightforward
process of physico-chemical CO
2
degassing at
waterfalls and riffles but is much complicated by
the precipitating activities of microbial biofilms in
both freshwater and marine static and flowing
water situations.
In order to circumvent many of the problems
inherent to field work, a controllable system
capable of supporting fluvial biofilms and tufa
precipitation has been constructed in the laboratory
during the present study. This laboratory-based
experimental approach has the benefit over field
site studies of sampling accessibility and indepen-
dent control. Single environmental variables can
be changed progressively through a series of exper-
iments run under the same underlying conditions.
Previously reported flowing mesocosm experiments
(Rogerson et al. 2008; Pedley et al. 2009) have
demonstrated the feasibility of such work and the
close comparison between laboratory and natural
calcite precipitates (cf. Turner & Jones 2005). The
present study extends this experimental work into
investigating the effects of variable day length,
ambient temperature and flow rate on biofilm devel-
opment and calcium carbonate precipitation. It also
considers the partitioning of carbonate products
within the communal EPS and presents a new
model for the freshwater tufa biofilm.
Mesocosm and experimental apparatus
The mesocosm apparatus used in this set of exper-
iments is closely comparable to that used in
Pedley et al. (2009). However, in the current exper-
iments two, 1 m long flumes were run simul-
taneously from a continuous flow of recycled
water held in a closed sump. The system was
driven by a 4000 l/h Blagdon Hydratech Torrent
HTT 4000 submersible pump which lifted the
water 0.5 m from the 40 litre sump to the flumes.
The flow rate to the first flume (slow-flow) was set
at c. 100 litres/hour while the second flume (fast-
flow) received c. 1000 litres/hour. Illumination
was provided by a 250 watt Thorn-Lopack
sodium lamp sited 1.5 m above the flumes. Temp-
erature was controlled by an in-line Aquamedic
'minicooler' chiller unit directly linked to the
pump outflow pipe. Water was diverted to each of
the flumes immediately down-flow of the chiller
unit. Natural spring water for the system obtained
from a Cretaceous Chalk spring at Welton, East
Yorkshire.
In order to adequately compensate for a pro-
gressive fall
in calcium ion supply during the
Search WWH ::
Custom Search