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Fig. 24. (a) Compact and porous laminae formed by highly encrusted cyanobacterial filaments, tufa formed on
limestone tablet exposed between August 2002 and October 2003; (b) crystal sizes asymmetry in compact laminae, tufa
formed on Cu tablet exposed between August 2002 and October 2003; (c) Alternating compact and porous laminae, tufa
formed on limestone tablet between August 2002 and October 2003; (d) compact laminae comprising cemented
cyanobacterial filaments, the same sample as in c; all samples from L´ ˇky E point, a, b - thin sections; c, d - SEM
images.
November 2002 and March 2003 exhibits exactly
the same texture, even in terms of sequence of
laminae (Fig. 22a, b). In this case the difference in
growth rates is relatively small.
Pentecost (2005, 198), commenting on the
earlier experiments on tufa and travertine growth,
argued that in some conditions copper ions can
inhibit inorganic calcite precipitation. Indeed, such
a phenomenon was experimentally confirmed
(Parsiegla & Katz 1999, 2000). However, one can
rule out the importance of Cu ions in the conducted
experiments because if any Cu ions were liberated
during a possible dissolution of copper tablets,
they could be instantly removed by flowing water.
The higher concentration is possible only within a
diffusion boundary layer adjacent to the copper
surface. In the stream conditions, the thickness of
such a layer may be estimated at 0.01 cm (Liu
et al. 1995). The thickness of this layer decreases
or the layer even disappears over elevations of the
substrate. Thus, any freshly precipitated carbonate
patches protruding from the copper surface reduce
the thickness of the diffusion boundary layer, and
become a favourable place for algae to settle. As a
consequence, algae are able to colonize small
patches of calcite previously crystallized on a
copper tablet, even if calcite does not form a con-
tinuous layer insulating entirely the copper sub-
stratum. Hence, algae can grow in close vicinity to
bare copper surface. It suggests that, in the studied
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