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sample no pH or Ca
2þ
microgradients were detected
(Fig. 12). Additional microsensor measurements at
moss surfaces as well as at spring site endolithic
biofilms revealed only microgradients too low to
induce CaCO
3
precipitation (Shiraishi et al. 2008b).
Further experimental studies (Bissett et al.
2008b), manipulating pH and temperature, indi-
cated that biofilms exhibited control over chemical
conditions within the microenvironment of the
tufa surface and that precipitation continued under
photosynthesis over a wide temperature and pH
range (4 - 17
o
C and 7.8 - 8.9). Further, this work
demonstrated that pH was maintained between 7.8
(dark) and 9.5 (light) regardless of the pH of the
overlying water.
The interpretation of these results is that: (1)
under illumination calcite precipitation is driven
by the photosynthetic activity of the cyanobacterial
biofilm; and (2) in darkness, biofilm respiration
decreases the oversaturation so much that calcite
precipitation stops. The minor Ca
2þ
release under
dark conditions possibly resulted from exopolymer
degradation releasing complexed Ca
2þ
, as calcite
dissolution appears unlikely because the calcite sat-
uration state calculated from the microsensor data
remains positive at the biofilm surface even in dark-
ness. In conclusion, tufa stromatolites are formed
biogenically by photosynthesis-induced precipi-
tation, not by externally forced permineraliza-
tion due to physicochemical CO
2
degassing
responsible for the high ambient calcite supersatura-
tion (Bissett et al. 2008a; Shiraishi et al. 2008a, b).
At first glance, this conclusion appears to be in
conflict with the results from macroenvironmental
hydrochemical analysis, which suggest that phys-
icochemical CO
2
degassing is the major factor in
driving calcite precipitation in the investigated
streams (see previous section on the 'Hydrochemis-
try of stream waters'). In order to solve this discre-
pancy, mass balance calculations were carried out
for the Westerh¨fer Bach, based on: (1) annual
average flux of photosynthesis-induced CaCO
3
deposition (c.2 10
26
mol m
22
s
21
) estimated
from the mean annual depositional rate of tufa stro-
matolite (c. 3900 g m
22
year
21
obtained by weigh-
ing at downstream site WB5); (2) the biofilm surface
area within stream (94 m
2
recorded by mapping in
field); (3) water flow rate at site WB5 (c. 2.0 L
s
21
); and (4) the Ca
2þ
loss from bulk water during
the course of the stream (Shiraishi et al. 2008a, b).
2.7 - 4.2 10
3
mol year
21
. This means that only
about 10 - 20% of Ca
2þ
lost from the bulk stream
water is bound via biofilm photosynthesis to form
tufa stromatolite calcium carbonate. The remaining
80 - 90% Ca
2þ
lost from the bulk stream waters,
in turn, can only be explained by physicochemical
precipitation (spar cement) on branches, leaves,
tufa debris and as fine-grained calcite precipitated
directly in the water column (Shiraishi et al.
2008b). This interpretation may help to resolve the
long standing controversy on biogenic v. inorganic
origin of tufa stromatolites.
Biofilm structure, calcification pattern
and annual lamination
Biofilm structure and calcification pattern
The structure of tufa-forming biofilms and the corre-
sponding calcification pattern were investigated
in detail at the Deinschwanger and Westerh¨fer
Bach using formol-fixed, resin-embedded hard-
part sections (for methods see Arp et al. 1999)
and tape-stabilized cryosections (Shiraishi et al.
2008c). Biofilms of the: (1) non-calcifying spring
sites; (2) moss plant surfaces; and (3) tufa stromato-
lites at downstream stream sites show characteristic
compositions:
(1) Biofilms on hard substrates at spring sites are
characterized by endolithic cyanobacteria, locally
overgrown by epilithic coccoid cyanobacteria
(e.g. Pleurocapsa minor morphotypes) and filamen-
tous
green
algae
(e.g.
Gongrosira
sp.).
Spring
waters
show
only
minor
calcite
supersaturation
(SI
Cc
¼ 0.0
to
0.1)
and
no
calcification
was
observed in the corresponding biofilms.
(2) Farther downstream, moss plants dominate at
margins of the stream and at cascades. Once the
stream waters have surpassed a calcite supersatura-
tion of at least SI
Cc
¼ 0.8, mosses show initial
calcite spar cements on their leaf surfaces. These
crystals are commonly idiomorphic rhombs or
palisade-like. In depressions of the moss plant
surfaces and between the crystals, scattered fila-
mentous cyanobacteria and diatoms occur. Here,
endolithic filamentous cyanobacteria (e.g. mor-
photype Schizothrix perforans) and epilithic
cyanobacteria such as Chamaesiphon subglobosus
morphotype locally flourish.
(3) Central flow paths of the streams in middle
and lower stream sections are covered by tufa
stromatolite-forming biofilms, except for sections
of loose tufa gravel. In tufa stromatolite-forming
biofilms, primary producers are cyanobacteria,
most of them filamentous, and pennate diatoms
(see section Cyanobacteria and diatoms). Both
groups of microorganisms concentrate at the top
Mass balance
The mass balance (see Appendix for calcula-
tion) showed that the total Ca
2þ
loss in the stream
is 2.2 10
4
mol year
21
, but photosynthesis-
induced precipitation via the tufa stromatolite's
biofilms
Ca
2þ
loss
accounts
for
a
of
only
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