Factors controlling growth of modern tufa: results of a field experiment - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 169

Fig. 21. (a) Neighbouring tubes of Oocardium stratum cemented together forming one sparry crystal accompanied by
uncalcified diatom frustules; (b) upper surface of sparite bush, all tubes form one monocrystal, the cells are still situated
at tube tips; (c) almost uniform extinction of sparite bushes built of Oocardium stratum tubes; (d) different optic
orientation of crystals within one Oocardium stratum bush, the same sample as in c; all samples from Z ´ zriv ´ site,
a, b - October 2003, SEM images, lyophilized samples, c, d - tufa formed on copper tablet between August 2002 and
October 2003, thin sections, crossed nicols.

(Gradzi ´ ski

and the thicker ones display granular fabrics. Calcite
crystals do not change their optic orientation cross-
cutting many of the thin laminae, probably because
of the low content of detritic grains within the
laminae. Thicker laminae hindered calcite crystal
growth, which is reflected in competitive growth
fabrics of sparry crystals constituting the younger
sparry lamina. Laminae of this type were formed
especially at the L ´ˇ ky site during autumn and
spring seasons. Less distinctive and thin laminae
of this type were found at the Karw ´ w and H ´ j
sites (Fig. 23a). Such laminae were previously
known from crystalline tufa and are related to the
concentration of organic matter (Irion & M ¨ller
1968; Pedley 1992). The enrichment in fine-grained
clastics was thought to cause similar lamination in

speleothems et al. 1997; see also

Baker et al. 2008 and references herein).

Discussion
Influence of micro-organisms on the rate of
tufa growth

The field experiments conducted in the course of
this study allow the semi-quantitative estimation
of the difference between the rate of microbially
and inorganically driven tufa growth in various
natural settings. The interpretation is based on
differences between the rates of tufa growth on
two different substrates (limestone and copper),
in various settings and during various seasons, and