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Fig. 8. (a) Typical diatom dominated summer slow-flow biofilm (glass slide substrate) associated with micropeloids
and well formed rhombic calcite. SEM view. (b) Summer fast-flow biofilm and precipitates (glass slide substrate)
submerged in the same water and under the same sunlight conditions as (a). Note the relative abundance of calcite
precipitate and the larger crystal size produced under fast-flow conditions [cf. (a)]. Air dried SEM sample.
calcification, might be expected at higher tempera-
tures, these are balanced by the buffering capacity
of the biofilm itself, causing precipitation rates to
remain fairly constant within a wide temperature
range.
In contrast, however, flow rate proved to be
consistently the dominant influence in determining
the calcite precipitate yield. This confirms results
from field sites studies by Zaihua et al. (1995) and
Lu et al. (2000), also accommodating the data
from Kano et al. (2003), which recorded fastest
growth in the summer-autumn period when river
flow rates were highest. These observations are
also endorsed by recent theoretical insights
(Hammer 2008; Hammer et al. 2008; Veysey &
Goldenfeld 2008).
Clearly, if ion flux, moderated by flow rate, is
the overriding control of precipitation rate, then a
number of long-held assumptions about the relative
roles played by turbulence and degassing (Zhang
et al. 2001; Chen et al. 2004) and microbial bio-
mediation (Bissett et al. 2008; Shiraishi et al.
2008) need to be reassessed. However, our exper-
iments indicate there may be a threshold in this
flow rate regulation system, as in the fast-flow
flume experiment biofilm tended to partly slough
off and erode; we anticipate that had the flume vel-
ocities been much higher, biofilm colonization
would have been severely restricted. True tufa pre-
cipitation (in terms of microfabrics) appears only
to occur in association with biofilm EPS (Pedley
et al. 2009) therefore, at the limiting shear stress
for biofilm development a significant change in
depositional fabric should occur with microbialites
giving way to dense, massive developments of
calcite. This was seen in Figure 7a, b where large
spar crystals rapidly developed, apparently external
to the EPS, under laminar flow conditions. This also
appears to explain a common characteristic of fossil
tufa barrages, in that the spillover point of the
barrage is generally characterized by horizontally
laminated, massive calcite with less dominance by
microbial fabrics (also seen on the stoss side of
erect semi-aquatic vegetation, fig. 21 in Pedley
1994). This observation raises the critical question
of whether aggradation by the microbial colony
(largely by calcite biomediation) is more important
than physico-chemical precipitation (largely con-
trolled by ion flux) in tufa growth at spillover
points. Answering this question is likely the key
step in understanding why cool-humid and semi-
arid barrages have different architectures.
Biofilm structure
The freshwater biofilm structure generated during
experiments was initiated as a single EPS sheet
which rapidly developed a basal calcite layer.
However, during the autumn fast-flow experiments
the biofilm also developed a multilayered structure
involving alternations of crystalline calcite layers
and cellular (polygonal) EPS zones. Figure 9
shows the basal calcite layer of inter-grown crystals
and the overlying polygonal zone EPS (see enlarge-
ment of this part in Fig. 4a). In addition, a second
layer of more loosely associated calcite crystals
occurs above the EPS zone and is succeeded by a
second EPS zone also with a cellular structure.
This is succeeded by a further calcite layer followed
by a less well defined layer of cellular EPS. This
multi-layered structure appears to have much in
common with marine stromatolite biofilms
described by Reid et al. (2000); K¨hl et al. (2003);
and Decho et al. (2005). In addition, the polygonal
structures within the freshwater EPS appear to
fulfil a similar role to the micro-polygonal fabrics
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