Geology Reference
In-Depth Information
Fig. 13. (a) Profile of the flume barrage (hachures) with biofilm growth (stipple) and cemented tufa (black). This sketch
was made after 8 months of continuous development under the same temperature, calcium ion supply and flow regime
(autumn experiment, fast-flow). The profile of the tufa illustrates the tendency of the biofilm to prograde down-flow at
the barrage top. Note that all sketches in Figure 13 are drawn from hydrated, living material taken directly from the
flume experiments. (b) Profile of the biofilm immediately down-flow of the barrage (stippling represents the calcite
crystal precipitates within the EPS. Note the large alveolus developed on the right side which is lined by EPS but filled
with flume water. (c) Profile at the spill-over point of the barrage where the deposit is thickest. The upper 4 mm consists
of living green coloured EPS containing several zones of fine calcite crystals dispersed within the EPS. The living
biofilm transitionally overlies 5 mm of well cemented crystalline tufa with a thrombolitic texture. Note that the tufa
layer is coarsely granular and totally without biofilm. The interconnected alveoli are filled with flume water. (d) Profile
from the fast-flow flume down-flow of the barrage. Note the extensive development of water filled alveoli within the
biofilm. This gives a flexible sheet which accommodated flow variability with time. It develops by the addition of extra
laminae which prograde down-flow (one is shown on the right side of the sketch). The stippled areas represent calcite
crystal precipitates within the EPS which is well developed in the older layers but generally is absent from the surface
biofilm layer. The alveoli are elongated parallel to flow and are filled with flume water.
tufas. Alternatively, the cavities within the thrombo-
lytic tufa fabric may develop from water filled
alveoli within successive biofilm layers. These are
best seen in the cushion-like biofilms developed
under fast flow regimes (see Fig. 13d) where they
irregularly separate successive biofilm layers.
Calcite crystals develop both within the fenestral
biofilm layers (coarse stipple in Fig. 13d) and can
grow on the surface of
diurnal or 'seasonal' changes are not necessary for
their development. The only introduced variable
was the addition of nutrients on five occasions. At
the present time it would be speculative to correlate
these with the 5 or more basal calcite layers present
(see Fig. 13b) though it is quite likely that nutrient
addition may change the overall microbial domi-
nance within the biofilm. However, the only evi-
dence at present is an apparent 'greening' of the
biofilm over a few days subsequent to nutrients
being added.
Considerable precipitation is also recorded
within the unlit areas (sump) in association with het-
erotroph dominated EPS. Such aphotic communities
the alveoli
in deeper
buried sites.
Significantly, these multilayered stromatolite
fabrics developed under constant conditions of
temperature, illumination conductivity and flow
regime during the experimental
runs. Clearly,
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