Factors controlling growth of modern tufa: results of a field experiment - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 150

continually the increase of the surface available
for the growth of subsequent crystals (cf. Liu et al.
1995). The tablets were colonized by algae,
mosses and housing producing larvae, all of which
enlarge the effective nucleation surface. Algae
may form freely hanging three-dimensional gar-
lands. They are subsequently incorporated into
tufa which overgrows a tablet (Fig. 9b, c). More-
over, many different foreign objects are incorpor-
ated into growing tufa. They include detrital
mineral grains and plant debris, especially leaves.
A substantial part of their surface also acts as a sub-
strate for nucleation. Taking into account that all the
above processes are ubiquitous in natural tufa
depositing system the initial surface of the tablet
may be employed while calculating the tufa rate of
growth, though the obtained value must be regarded
as semi-quantitative only.
The foreign objects mentioned above are
included into the tufa increasing its mass, hence
the mass of tufa employed into the calculation of
the growth rate is somewhat overestimated.
Table 3 lists the mass increments and the calcu-
lated rates of tufa growth on all tablets. The obtained
data set displays some regularity. The highest
growth rate was at the L ´ˇky site and the smallest
at the Z´zriv´ site and at Karw´w dam point. Tufa
growth rate was almost always quicker on the lime-
stone tablets than on respective copper ones. Tufa
growth was also more rapid at settings with rapid
flow, as on waterfalls.
A comparison between the tufa growth in mass
and in thickness clearly shows that there is no
simple relation. A good example is the comparison
of tufa growing throughout the whole experiment
at the Karw´w cascade point and at the L ´ˇky E
point. Although at Karw ´w tufa grew to the
maximum thickness of 20.04 mm, its mass is only
771.8 mg/cm 2 , the tufa at L ´ ˇ ky attained a similar
thickness of 24 mm, but a mass of 2 739.5 mg/
cm 2 (Table 3). This discrepancy is due to the tex-
tural difference between them, reflected in their
different porosity (cf. Fig. 9a, c).

Fig. 4. Seasonal temperature, HCO 3 , Ca and SI calc. data
for Z´zriv´ and L ´ˇky sites.

Tufa textures

surface; and (iii) incorporation of foreign allochtho-
nous components.
Tufa asymmetrically overgrew the tablets
(Fig. 9). Much more tufa was deposited on the air-
facing surface. Since the total area of the tablets
was applied to the calculations, the obtained rate
of growth is underestimated.
The total area of the tablet was assumed as the
surface available for nucleation and deposition. In
fact, the surface that served as substrate for nuclea-
tion is much larger and the increase is hard to be esti-
mated. Even the constantly growing crystals cause

The studied tufa deposited on the tablets during
the experiment is built of various components and
exhibits a great spectrum of textures (Table 3).
X-ray diffraction analyses confirmed that the only
autochthonous carbonate mineral phase in all the
studied tufa samples is calcite.

Biogenic components and their affinity

Filaments. Filaments are the most common biogenic
components. On the surface of growing tufa and in
its shallow subsurface they are commonly built of