Factors controlling growth of modern tufa: results of a field experiment - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 175

even high-energy settings. The essence of this
biologically driven mechanism is explained in the
following sections.
Apart from energy, the mode of water supply
seems to be relevant to the rate of tufa growth.
This is evident at the L ´ ˇ ky waterfall. The tablets
placed in the south-facing wall of the waterfall
(L´ ˇ ky W point) obtained slowly seeping water
and periodically intense spray. The tufa growth
rate on these tablets was slower by a factor of 2
than on the tablets exposed on the vertical face in
the eastern part of a waterfall (L´ ˇ ky E point),
constantly flushed by water falling freely down.
Drysdale & Gillieson (1997), while studying Loui
Creek tufa in Australia, observed a similar tendency
to the strong dependence of the tufa growth rate on
water-flow regime.
Differences in textures were also observed
between tufas deposited in close vicinity, between
the tufa deposited on tablets exposed throughout
the whole period of experiment and on tablets
exposed seasonally. The L ´ ˇ ky E point provides
an example. The younger part of the tufa deposited
on the limestone tablet exposed there between
August 2002 and October 2003 is built of laminated
tufa of fibrous texture with highly encrusted cyano-
bacterial filaments (Fig. 9c). In contrast, the lime-
stone tablet exposed at the same point from June
2003 to October 2003 is covered exclusively with
sparry calcite (Figs 9b & 22d, e). Both tablets
were exposed close to each other in almost the
same conditions during more than three months.
Despite the fact that the distance between their
locations was only 35 cm, the tufas differ in
texture (compare Fig. 9b, c). A similar difference
was noted at the Karw´w cascade site. Tufa with
fibrous texture was deposited in spring and probably
in the beginning of summer of 2003 on the tablet
exposed there throughout the whole experiment.
During the same time, tufa with hemispherically
layered micrite and with sparry fringe cement with
entombed algal filaments was formed on the season-
ally exposed tablets located in close vicinity. Hence,
the difference in texture may be explained by phys-
ical conditions differing over a very small distance
or by reduced capacity of algae to colonize a fresh
tablet because of the relatively fast growth of crys-
tals.

Fig. 26. Relationship between tufa growth rate and
SI calc. value of feeding water, used SI calc. value
is calculated as a mean values of the tablet exposure
time.

tablet location. Stream environment is very unstable
from the chemical point of view, hence the
measured chemical parameters may be slightly
different from those at the point where tufa
samples grew. This reason seems to be essential,
though one can also take into consideration that
chemical analyses of water were carried out only
four or five times a year. Thus, all the above par-
ameters are discrete values, extrapolated to the
whole period of experiment, while the real par-
ameters are unknown. Nonetheless, it seems that
any departures from real values are not significant.
Where traces of filamentous algae are visible in
any of the described textures, always perfectly pre-
served moulds of filaments are present. This implies
that calcification proceeded outside the cyanobac-
terial sheaths. It was thus, external surficial precipi-
tation sensu Riding (1991), typical of freshwater
environment with SI calc. higher than 0.8 (Merz-Preiß
& Riding 1999). A similar value is accepted as the
threshold for effective precipitation of calcium
carbonate in freshwater stream environments
(Michaelis et al. 1985; Herman & Lorah 1987)
and in soda lakes (Kempe & Ka´mierczak 1990,
1993). In the studied sites SI calc. values around 0.8
and higher favoured the origin of sparry crystals or
fibrous texture. However, the experiment shows
that the values lower than 0.8 also promote external
surficial precipitation of calcite around cyanobacter-
ial filaments. This was the case at the Karw ´ w site in
the summer and autumn of 2002, when SI calc. sys-
tematically decreased from 0.45 to 0.35 (Table 2).
In

The latter possibility is considered to be

more probable.

Water chemistry, tufa growth rate and
algal calcification

There is a general tendency that the higher SI calc.
value is, the faster tufa grows. However, correlation
between these two factors is weak (Fig. 26) for two
reasons. Firstly, SI calc. values were calculated for
stations located at some distance upstream of the this case, crystals precipitated around the