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Fig. 15. Mean irradiance near tufa biofilm surfaces in German karstwater streams at different seasons during
2006 - 2008. Filled circles: Westerh
¨
fer Bach, open circles: Deinschwanger Bach.
macroenvironmental conditions at the stream sites,
not microenvironmental conditions within biofilms
partially decoupled from the bulk hydrochemical
system. While the tufa stromatolite-forming bio-
films do not change oxygen isotope ratios, the
carbon isotopic composition of the calcite laminae
is potentially strongly affected by photosynthesis,
which even drives their initial formation. In
addition, heterotrophic CO
2
release, as detected
during night conditions, potentially alters or blurs
carbon
Continuous and fast growth with low diagenetic
overprint of tufa stromatolites is certainly a further
prerequisite for accurate palaeoclimate reconstruc-
tions at annual scale. In this regard, Westerh¨fer
and Deinschwanger Bach tufa stromatolites are
disappointing study objects, because the small-scale
flow paths commonly change. On the other hand,
the petrographic observations at samples from
these streams indicate that aggrading neomorphism
appears to be more significant in slowly growing
tufa stromatolites and at growth interruptions, than
in the few continuously and comparatively fast
growing tufa stromatolites. Such diagenetically
alterated laminae consisting of palisade crystals,
formed by crystal growth and recrystallisation in
supersaturated water and lack of inhibiting exopoly-
mers, therefore likely contain mixed isotope signals.
Lateral groundwater flow along porous laminae may
contribute to this effect as well. Fast and continu-
ously growing tufa crust such as that of Japan
(Kano et al. 2003, 2004) therefore appear to be par-
ticularly suitable for palaeoclimate reconstructions,
while the palaeoclimate record of tufa stromatolites
at the Westerh ¨fer and Deinschwanger Bach rarely
exceeds several years.
isotope
values
of
the
tufa
stromatolite
laminae via solid state diffusion.
The stable isotope analysis of stream waters
and tufa carbonates at the Westerh ¨fer Bach how-
ever shows that, despite photosynthesis-induced
precipitation within the biofilms, the effect on
d
13
C in tufa stromatolite carbonate is not detectable
(Shiraishi et al. 2008b). This observation might
be explained by spatially too small gradients (i.e.
isotope exchange still fast enough at these small
distances) and/or isotope exchange between
12
C-
depleted daytime precipitates and
12
C-enriched
nighttime biofilm water phase. Earlier studies on
the stable isotopic composition of tufa carbonate
also detected only a minor
12
C-depletion, but
considered this observation as indicative of a
largely inorganically forced precipitation (Spiro &
Pentecost 1991).
In any case, the results underline the potential of
tufa stromatolites for palaeoclimate reconstructions,
with d
13
C reflecting the bulk water carbon system
(i.e. influx of
12
C-enriched fluids from soil versus
photosynthesis effect of the bulk stream system;
Rayleigh fractionation). This also implies that
12
C-depletion (if present at all) in fossil stromatolite
laminae is rather the result of a photosynthesis-
effect on the whole water body, and not of the
stromatolite-forming biofilm itself (e.g. in Norian
lacustrine stromatolites; Arp et al. 2005).
Microbialite fabrics and dissolved inorganic
carbon concentrations in fresh- and seawater
In addition to Quaternary palaeoclimate studies,
tufa stromatolites and other non-marine micro-
bialites provide insights for the interpretation of
marine microbialite fabrics as related to seawater
composition, which varied during the Phanerozoic
and Precambrian (e.g. Arp et al. 2001a).
Microbialite
fabrics
form
dependent
on
the
microbial
community
involved
and
the
mecha-
nisms
of
CaCO
3
precipitation,
both
reflecting
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