Geology Reference
In-Depth Information
eventually, makes the basic life processes imposs-
ible (Golubi
´
1969; Kano & Fuji 2000). However,
the filamentous organisms due to their apical
growth may effectively emerge from a calcified
zone. This process is of crucial importance for the
origin of fibrous texture. Vaucheria appears to be
especially prone to constructing such a texture.
This alga can colonize fast-flowing water where cal-
cification proceeds at a high rate. Moreover, it is
particularly prone to being forced to faster growth
and 'escaping' because of its susceptibility to
decreased irradiance (see Ensminger et al. 2005).
Such a texture built of filamentous cyanobacteria
was detected in samples characterized by a high
rate of tufa growth from the L ´ˇky site. The
texture may be attributed to higher photsynthetic
plasticity of cyanobacteria (Pentecost 1978; Merz
1992) which require a relatively thick calcite
cover to be forced to grow. Tufa with fibrous
texture includes only scarce diatom frustules,
while coccoid cyanobacteria are absent. For these
algae, entombing with calcite may be critical as
they do not show the ability to 'escape' when
being covered with calcite crystals. The traces of
diatoms are probably only the effect of abortive
colonization of growing crystals by these algae as
the first colonizers (Fisher et al. 1982) that did not
contribute to the formation of the texture itself.
It is noteworthy that algal growth is particularly
vigorous in such settings where a reduction of diffu-
sion boundary layer facilitates access to dissolved
nutrients for algae (Horner et al. 1990; Raven
1992). At the same places inorganic precipitation
effectively proceeds (Liu et al. 1995; Zhang et al.
2001). This implies that a kind of specific positive
feedback exists between the growth rate of algae
and the rate of calcification. The proceeding calcifi-
cation encrusts the algae and forces them to 'escape'
by faster growth (Kano et al. 2003). In turn, the
faster growth of algae promotes the faster growth
of tufa, which not only influences its texture and
thickness, but also affects its mass increase by
several coexisting mechanisms mentioned below.
The appearance of new parts of algal filaments
solely augments the surface of the new substrate
for calcite nucleation. A pierced algal construction
constantly flushed by water disturbs the water
flow, accelerates outgassing of CO
2
and in some
cases also leads to evaporation and to reduced
diffusion boundary layer. Each of these processes
stimulates calcite precipitation. Moreover, many
authors claim that algae and their EPS may in
several ways passively stimulate calcite growth
(Emeis et al. 1987; Riding 2000; Turner & Jones
2005). Finally, recent experimental studies by
Shiraishi et al. (2008) and Pedley et al. (2009)
demonstrate that algae actively influence calcite
growth.
mechanism on the growth rate of tufa is indirectly
confirmed by the unexpected growth rate -
1.93 mg/cm
2
/day - of the tufa sample with
fibrous texture that formed at the Karw
´
w cascade
site during the whole period of the experiment.
Similarly, the samples from the L ´
ˇ
ky site, which
show fibrous texture, grew at the rate between
0.87 - 6.43 mg/cm
2
/day.
Hence, the experiment invites an interpretation
that the tufa exhibiting fibrous texture, which is a
product of the discussed feedback mechanism, was
formed mainly under high SI
calc.
values. Such a
tufa did not originate either in Z
´
zriv
´
or in H
´
j
sites, where SI
calc.
values exceeding 0.78 and 1.08
were never detected, but it grew at the Karw
´
w
and L´
ˇ
ky sites under SI
calc.
values of 0.88 and
.1.2. Hence, conditions favourable for algae to
grow are also needed, that is light intensity and
temperature. In other cases, under the same SI
calc.
values tufa showing other texture grows.
Tufa with fibrous texture was formed pre-
dominantly on the tablets exposed all year around,
for instance at the Karw
´
w and L ´
ˇ
ky sites. At the
same sites the tufa formed at the same points on sea-
sonally exposed tablets displays crystal texture con-
taining only some entombed algal filaments. This
suggests that the high SI
calc.
values, causing high
crystal growth rate, which favour formation of
fibrous texture, preclude algal colonization on a
newly installed tablet. Hence, the feedback mechan-
ism presumably demands earlier colonization of a
given area by algae when conditions are not so
severe and crystal growth is a little slower.
The feedback mechanism works particularly
efficiently as long as the rate of calcite nucleation
and crystallization is equal or slightly lower than
the rate of algal growth. When calcite growth accel-
erates, algal filaments become entombed com-
pletely, and sparry calcite without biogenic texture
grows on them (see Pentecost 1998). Deposition
may switch to pure sparite growth by increase in
values of SI
calc.
caused by some independent
factors, such as higher temperature and higher
flow rate, which stimulate degassing, or by various
factors impeding algal growth. Moss tufa grows in
an analogous manner, as postulated by Pentecost
(1998). The feedback mechanism probably resulted
in exceptionally fast growth rate of moss tufa of
140 mm a
21
noted by Weijermars et al. (1986)
from Spain.
The above described feedback mechanism
results in the formation of fibrous tufa facies, with
encrusted calcitized algal filaments, known from
several modern and fossil localities (Fig. 20c; e.g.
Irion & M ¨ller 1968; Szulc 1983). This facies, com-
posed of long and friable fibres, which in some cases
are not completely cemented together, evinces the
tendency to be easily eroded. Thus, it may provide
The
influence
of
the
above
feedback
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