Introduction to tufas and speleothems - Tufas and Speleothems: Unravelling the Microbial and Physical Controls - page 4

Sibler, present an experiment-based study of EPS
associated with three strains of picocyanobacteria.
The authors conclude that cyanobacterial extra-
cellular polysaccharides have a strong potential to
exchange protons with the environment. Addition-
ally, the strains studied were capable of calcium car-
bonate precipitation. Rogerson et al. showed that
the precipitation of calcium carbonate occurs prefer-
entially under conditions of rising pH at the night-
day transition by working with a flowing mesocosm
laboratory experiment. The observations indicate
that daylight length does not control overall precipi-
tation rate and confirms suggestions that seasonal
laminations require either strong variability in
ambient physico-chemical activity or else an
ecological/compositional change in the microbial
community. However, Arp et al. present a very
comprehensive study of natural stream-grown stro-
matolitic tufa together with details of the biofilm
community which include cyanobacteria, a non-
phototrophic prokaryote community and diatoms.
This study concludes that annual laminae were the
result of seasonal changes in temperature and
light. Furthermore, the stable carbon isotope com-
position of the laminae was not affected by
photosynthesis-induced microgradients but mir-
rored bulk water chemistry. Arenas et al. monitored
physical, hydrochemical parameters and sedimen-
tation rates over a 3.5-year period in an active
stream. They demonstrate that sedimentation rates
have a strong seasonal pattern with higher values
in warm periods. Conversely, the sediment d 18 O
composition shows a rhythmic variation, with
higher values in cool periods. This was probably
caused by fractionation due to seasonal temperature
variations. The calculated temperatures, for a theor-
etical equilibrium precipitation, accord with the
measured temperatures and display the same seaso-
nal trend. In conclusion they suggest that: (a) tufa
d 18 O values can be used to estimate relative palaeo-
temperature variations in fluvial carbonate deposits;
and (b) laminated tufa sediments can be useful as
high-resolution records of temperature change.
Gradzi´ ski monitored four active tufa field sites
during a 14-month period and recorded the highest
tufa growth rates in the fastest flow conditions.
These precipitates are dominated by a crystalline
texture or a highly encrusting mode. In contrast
the areas of slow tufa growth are dominated by
micritic textures. These observations are confirmed
by Pedley & Rogerson from independent in vitro
laboratory experiments. Furthermore, this study
also demonstrates the physical structure of the fresh-
water biofilm. SEM studies reveal that heterotrophic
bacteria are dominant in zones within the EPS where
calcite precipitation occurs. The intimate relation-
ships between nanofabrics and calcite precipitates
are well demonstrated in this study. Benzerara

et al. provide an illuminating study, aided by
microscopy and microspectroscopy, of the extent
to which biological processes control stromatolite
structure, and of the respective contributions of
biological and abiotic processes in their formation.
The chemical composition of the calcium carbonate
precipitates is outlined and their developed within
the stromatolite is shown. Some carbonate fabrics,
however, continue to remain enigmatic and none
more so than pedogenic calcitic nanofibres.
Bindschedler et al. conclude that they are probably
the product of the organic decay of fungal hyphae
into cell wall microfibrils. These could then be
replaced by calcite pseudomorphosis, perhaps also
acting as templates for direct calcite precipitation.
The article emphasises the importance of organic
matter and fungi in carbonatogenesis generally.
Gonz´lez-Mu˜oz et al. (earlier in the volume) indi-
cate how bacteria are capable of precipitating a
range of chemistries and Pentecost illustrates this
with respect to the development of up to 19% phos-
phorous in the carbonate fraction of some active tufa
sites. The percentage increase in some fossil
examples to over 48% is attributed to further miner-
alization of contained organic matter. ¨ zkul et al.
provide an exceptionally detailed account of a
partly active perched springline tufa. The spring
water hydrochemistry and trace element and stable
isotopic composition of the tufa precipitates are
well documented and provide a useful comparison
between the composition of aqueous supply and
tufa product. Radiocarbon dating of the older depos-
its demonstrates them to have developed between
2000-5800 yr BP. The final article within this
section is by Capezzuoli et al. Here, the regional
study of a Quaternary succession containing tufas
demonstrates convincingly that tufa developments
appear to be controlled by water availability rather
than by ambient temperature. More precisely they
consider that climate controls water table levels
and indirectly the volume of water available to the
system. Four depositional carbonate (tufa), events
each separated by active down cutting events, are
recognized in the Valdelsa succession. These
depositional carbonate events are radiocarbon
dated and correlate with the Last Glacial Interstadial
through the Younger Dryas to the Atlantic
'Optimum Climatic', and Sub-Boreal episodes
respectively. It would appear therefore that tufas
are clear indicators of high palaeo-water tables.
In developing the subterranean theme introduced
by the opening article by Jones, Baldini explores
the effects of soil derived atmospheric pCO 2
within caves. He concludes that soil temperature is
a major control on cave air pCO 2 and that variations
in pCO 2 can trigger either precipitation of dissol-
ution, thereby controlling stalagmite growth rates
generally and holds the potential to skew the