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minerals from groundwater brines, similar to some modern
sabkha evaporites. Samples MOU19, 22 and 23 (Fig.
5.2
) are
strongly affected by evaporite brines leading to dolo-
microsparitization of the microenterolithes and sample
MOU26 is characterized by desiccation (Figs.
5.2
and
5.3e,
f
). These samples containing
cavities related to fungal colonization of the original mineral
substrate.
The colonizing microbes display various morphologies,
more irregular than the ones of the cyanobacteria seen in the
matrix, with non-septate (Fig.
5.4c
) or septate-like (Fig.
5.4f
)
thin filaments similar to hyphae, and diameters
(samples MOU19, 23
and 26) come from the upper part of such a diagenetic
salinity cycle overprinted on a stromatolitic layer (cycle A,
MOU19) and cycle B, MOU22, 23, 26 Fig.
5.2
) (Pr
´
at et al.
2010
,
2011a
,
b
).
“
fungi
”
m.
Other filaments are more regular and have diameters varying
between 0.25 to 1.0
1.0
μ
<
m (Fig.
5.4c
). The filaments are
associated and engulfed with what we presume to be
fossilized EPS material (Figs.
5.4c-f
and
5.5a,b
). Some
spherical bodies (diameter around 1.0
μ
m) are visibly adher-
ing to the fossilized EPS (Fig.
5.5c-d
). These spheres are
richly encrusted with sub-micron sized rounded crystals that
collectively yield the framboidal shapes characteristic for
authigenic pyrite. However, microprobe analyses reveal
that the spheres are entirely dolomite in composition. These
spheres could represent fungal spores encrusted with mineral
crystals. A similar observation has also been made under
laboratory conditions (Kolo and Claeys
2005
). The EPS is
systematically desiccated (Figs.
5.4c-e
and
5.5a-c
) yielding
strands reminiscent of actual microbial filaments. Very fine
(
μ
5.3
Detailed Petrography of Nsc3
5.3.1 Diagenetic Alteration
Both dolomicrite and dolomicrosparite replace (at an infra-
millimeter-scale) the microbial laminae and developed pro-
gressively from the cyanobacteria which are partly or totally
mineral-enveloped and still recognizable: they form a 3D-
network, some are dichotomic,
the average diameter is
m) aggregates of minerals or clusters adhering to the
strands are frequently observed in the cavities (Fig.
5.4c-f
).
<
1
μ
between 2-5
m
(Fig.
5.4a
). Dolomite crystals are greyish (abundant micritic
inclusions), xenototopic to hypidiotopic and approach sizes
up to 50
μ
m and their minimal
length is 20
μ
μ
m. Larger whitish hypidiotopic
crystals
5.3.2 Evidence of Fungal Colonization
(50-100
m) are associated with the replacement of former
sulfate crystals and irregular fenestrae. The association of
fine-grained dolomite with mudcracks and sheet-cracks
(disrupted flat laminar lamination), together with their very
fine grain-size and the presence of former sulfates, suggest
that dolomite is a secondary mineral phase, most likely
precipitated from hypersaline waters during the dry season.
Petrographic and SEM study reveal abundant subrounded
(circular to oval-shaped) and irregular pits (quasi-
rectangular) in the first stromatolitic level (samples
MOU19 and MOU26, Figs.
5.2
and
5.4b
). They range
between 5 and 50
μ
The probable
spores, the dolomitic prismatic
quadratic and tetragonal crystals, the clusters of very fine
crystals along the filaments and on the former larger crystals
inside the cavities and the abundance of the thin filaments
(
'
dolomitic
'
m in diameters) in the pits suggest that the pits were
formed through the activity of ancient fungi. Figures
5.6a-d
show completely mineralized and well embedded forms in
the rock matrix that can be attributed to fungal vegetative
parts such as sporangia, sporangiophores and hyphae. Care-
ful study of these images reveals a dense fungal colonization
of the sediments, especially as shown in Fig.
5.6c-e
.In
Fig.
5.6e, f
several individual fungal sporangia and
sporangiophores can be clearly seen. The black color of
fungal parts is attributed to organic content. The low contrast
in the backscattered image (Fig.
5.6f
) emphasizes the embed-
ding of the fungal forms within the matrix and the uniform
elemental composition. On close examination, the blackish
sporangial area in the lower-mid part of Fig.
5.6e
reveals that
it is actually harboring three superimposed sporangial bodies
that make intersecting circles. The lower one of these circles
shows very fine and continous zig-zag wavy ornamentation
likely representing the ancient ornamental spines or ridges at
the perimeter of the sporangial wall. These are comparable to
present day spine ornamentation on the sporangia of some
Mucorales fungi (Alexopoulos and Mims
1979
; Moore-
Landecker
1991
).
1.0
μ
<
μ
m and are spatially arranged into either
single pits or a network pattern. The latter is formed from
boundary-connected subrounded-rounded pits that form a
honeycomb or alveolar structure containing both colonizing
fungal material and neominerals (see Sect.
4.1
below). Gen-
erally, single pits show three major features. The first is the
presence of an elevated mineral
or ring (originally
probably Ca- or Mg-oxalates) around the pits
“
collar
”
circumference
that is composed of authigenic minerals. The second is the
deposition of authigenic minerals inside the pits in a process
which we are tempted to call
'
. The minerals are
dolomite in the form of rhombohedra (Fig.
5.4c,d
), but also
as quadratic crystals possibly related to former oxalates
(Fig.
5.4e
). The third, as discussed in the section below, is
the colonization of the pits by invading fungal hyphae.
Indeed, it is our contention that the pits are dissolution
“
nesting
”
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