Factors controlling growth of modern tufa: results of a field experiment - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 176

filaments are micritic rather than sparitic in size.
Nonetheless, the filament moulds have smooth
walls, regular cross section and constant diameters.
This strongly suggests that the moulds in question
are a product of calcification around and not
within cyanobacterial sheaths, that is of encrusta-
tion, not impregnation type (cf. Merz 1992).
Merz-Preiß & Riding (1999) stated, based on the
study of recent tufa in Germany and on previous
observation carried out by Merz (1992) in the Ever-
glades, that encrustation takes place under SI calc.
.0.8 while impregnation proceeds under SI calc.
,0.3. Therefore, the present data imply that the
former process may take place in SI calc. range
slightly lower than previously postulated.
The sparitic crystals in the discussed tufa are
interpreted as precipitated from solution. They do
not seem to be formed by agradational neomporh-
ism as described by Love & Chafetz (1988). This
interpretation is strongly supported by the competi-
tive crystal growth fabrics displayed by sparitic
crystals in the studied samples (Fig. 22a - e). The
crystal fans in some samples are laterally separated
by micrite with algal remnants, which suggests that
both types of tufa grew simultaneously (Fig. 18d, e;
cf. Jones & Renaut 1994).
SI calc. values exceeding 0.8 make the growth of
sparitic crystals plausible, as it is uncommon
under lower SI calc. . The elongation of sparry crystals
changes along with the changes in SI values. Higher
SI values favour the origin of highly elongated crys-
tals. The tufa sample that grew on a limestone tablet
at the L ´ ˇ ky E point between June and October 2003
provides a good example (Figs 9b & 22d, e). It is
built of highly elongated crystals arranged in
closely coalescent fans. This tablet was installed
when SI calc. reached 1.27, that is the highest value
recorded during the experiment (Table 2). Other
samples grown at the same point under lower
SI calc. between 0.82 and 0.94 display crystals of
shorter elongation also grouped in tightly packed
fans (Fig. 22a - c). This is in line with the opinion
of Given & Wilkinson (1985) that growth rate of
crystals governs the crystal nature. The molar
Mg/Ca ratio appears to be unimportant because
this parameter is constantly low. It varied during
the exposition of the tablets within a small range:
between 0.29 and 0.42 at station L3. Gonz ´ lez
et al. (1992) claimed that changing flow velocity
determined speleothem textures and that fast flow
conditions caused formation of fibrous fabrics of
highly elongated crystals. However, this rule does
not explain the textural differences between particu-
lar samples from the L ´ ˇ ky E point, because all the
tablets in this point were in comparable conditions
of constant and intense water flushing.
The

0.35 - 0.88 and 0.7 - 1.08, respectively (Fig. 23a, b).
Tufa composed of these crystals has high intercrys-
talline porosity. Thus, the tendency that higher
SI calc. values support grow of compact tufa with
lower intercystalline porosity (Kano et al. 2003)
seems to be confirmed.
The extremely high SI calc. , probably around 1 or
more, enables nucleation of calcite crystals directly
on diatom frustules, which is a rare phenomenon
(Fig. 15d; cf. Merz-Preiß & Riding 1999; Lu et al.
2000). One can suppose that such a high supersa-
turation allowed to break the barrier resulting from
structural differences between the silica nucleation
surface and the carbonate precipitate.
The growth of calcite tubes of Oocardium
stratum and formation of sparitic bush texture took
place under relatively low SI calc. values, which
only once - in June 2003 - reached 0.78, but
otherwise did not exceed 0.55 (Table 2). In
spite of rather widespread occurrence of this alga
in European hard-water streams, the data on
chemistry of its growth environment are scarce.
Pentecost (1991) quoted the values of SI calc.
between 0.28 - 0.81, which correspond well to the
present data.

Environmental factors controlling
tufa textures

The origin of tufa textures is controlled by a number
of factors interacting in a complex way. The princi-
pal factors and their interactions are characterized
below.

Water energy. The distribution of tufa facies points
to a clear relationship between facies and local
environmental conditions. The most important of
them is energy of flow, which not only governs
the rate of tufa growth, as it is discussed above,
but also exerts a fundamental control on the tufa
texture.
Sparry crystals are the most common texture on
the tablets exposed in extreme energy on the vertical
face of the L´ˇky waterfall, regardless of the type of
tablet (Figs 9b, c, 11d, 12a, 22 & 23d). By contrast,
some samples of tufa deposited at the same time on
horizontal streambed, close to the waterfall head,
display miciritic texture with abundant algal rem-
nants (Fig. 18a, b). Similarly, the tufa growing at
the waterfalls in H´j displays abundant sparry tex-
tures. In a high-energy setting the growth of micro-
organisms inhabiting the streams is impeded, which
enables formation of sparry calcite. This relation-
ship was earlier pointed out by Pedley (2000). High-
velocity flow causes shear stress at the bottom which
prevents colonization by micro-organisms. The
experimental work by Horner et al. (1990) implied
that at flow velocity over 80 cm/s algal periphyton

elongated sparry crystals grew at the

Karw ´ w H ´ j

and sites under SI calc. values of