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In vitro investigations of the impact of different temperature and
flow velocity conditions on tufa microfabric
H. MARTYN PEDLEY* & MIKE ROGERSON
Department of Geography, University of Hull, Cottingham Road, Hull, HU6 7RX, UK
*Corresponding author (e-mail: h.m.pedley@hull.ac.uk)
Abstract: A series of experiments on freshwater carbonates (tufas) involving biofilm colonization
in both fast-flow and slow-flowmesocosms was carried out in order to assess the changing nature of
biofilm and associated precipitates under contrasting conditions. A thin biofilm developed over 14
weeks during the 'summer' experimental run contained a basal calcite layer overlain by small
calcite crystals suspended within the Extracellular Polymeric Substances (EPS). The 'autumn'
biofilm, however, showed the development of multi-laminated calcite precipitates within the
EPS despite constant environmental conditions throughout the run. The experiments also
showed that the largest volume of calcite precipitate developed in the fast-flow flumes regardless
of temperature control. Development of an extensive calcite layer at the base of EPS in conditions
of complete darkness within the sump was also observed. This study provides increased weight
for the concepts: (1) that fresh- and saltwater stromatolites appear to be highly comparable
multi-laminated systems with precipitation strongly influenced by both phototrophic and hetero-
trophic microbes; and (2) that microbial precipitation may be more common within aphotic
(including cave, lake bottom and soil) systems than has previously been considered.
Field investigations show that freshwater carbonate
(tufa) deposition is affected by climate. Pedley et al.
(1996) showed that barrage tufas in the UK differ
from those developed under semi-arid conditions
in Spain in the relative rates of aggradation and pro-
gradation in their barrages. Cool-humid barrages
well seen in Derbyshire (UK), but typical of
present day north western Europe, show strong
progradation but only weak aggradation, resulting
in the development of shallow pools upstream of
the barrages which may be maintained for millennia
in some cases (e.g. Taylor et al. 1994). Ultimately,
however, these 1-3 m deep pools became infilled
with lime muds and marginal paludal creep-out
facies. In contrast, the barrage tufa developments
at Ruidera Natural Park (Spain), have developed
under a semi-arid climate (Pedley et al. 1996).
This appears to have encouraged almost vertical
barrage aggradation but little or no progradation
(Fig. 1). This mode of barrage accumulation
involved progressive deepening of the upstream
pools which today can be over 20 m deep and over
one kilometer in length (e.g. Pedley et al. 1996).
An extreme example of this mode of arid climate
vertical barrage wall build-up is seen in the
Band-e-Emir tufas (Afghanistan) (Brett 1970;
Bourrouilh-Le et al. 2007). Similar climatic influ-
ences can be seen when comparing paludal tufa
systems at Ddol in North Wales (UK) with those
from semi-arid areas of Central Spain (Pedley
et al. 2003). The climatic effect on accumulation
rates provides a potential means of extracting
qualitative palaeoclimate information from tufa
systems on the basis of their architecture alone.
However, the precise climatic factors forcing the
cool-humid and semi-arid models are currently too
poorly constrained to unlock this important source
of information about the past. The variables poten-
tially affecting precipitation are many and include
physico-chemical (including degassing rate of
carbon dioxide and flow rate) and biochemical
(temperature, nutrient availability, water flow rate,
daylight hours) probably in complex, variable
combination.
Progressively, the approaches to solving these
problems have shifted from field based outcrop
modelling (Pedley 1990), via sampling for the
geochemical (Franˇiˇkovi´-Bilinski et al. 2004;
Andrews & Brasier 2005) and hydrochemical
characterization of tufas (Zaihua et al. 1995; Lu
et al. 2000; Kano et al. 2003), to an increasing
awareness that the living biofilm plays a major
role in freshwater carbonate precipitation (Pedley
1994; Freytet & Verrecchia 1998; Caudwell et al.
2001; Bisset et al. 2008; Shiraishi et al. 2008;
Pedley et al. 2009). Many recent studies have
involved placing artificial substrates in static water
(Pedley 1994; Benzerara et al. 2006) or collecting
living river bed material (Kano et al. 2003) in
order to sample the biofilm products. Although
most in situ biofilm studies have been carried out
on marine stromatolites (Reid et al. 2000; Decho
et al. 2005), efforts on freshwater systems are
currently advancing at considerable pace. Shiraishi
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