Geology Reference
In-Depth Information
Fig. 5.8
SEM photomicrographs from samples MOU26 (Fig.
5.8a
mou26-2/2006/ap) and sample carb246A, showing comparative
honeycomb-alveolar structures produced by fungi on dolomitic
substrates. Bar scale as indicated. (
a
) An ancient and naturally pro-
duced one from the Neoproterozoic of Mouila quarry, Gabon and (
b
)an
experimentally produced structure from the Carboniferous of the
Bocahut quarry in France. Both figures share common features, such
as quadratic-rhombic pore space after the dissolution of dolomite
crystals and the formation of a filamentous-EPS mat cover on old
crystal boundaries associated with new crystal deposition on those
boundaries, suggesting a similar colonization pattern and diagenetic
process. In Fig.
5.8a
the original fungal material (filaments and EPS)
are all permineralized but the old honeycomb-alveolar structure is still
well preserved
“
”
dolomite crystals, an elevated mineral
composed of
mineral authigenesis (here Ca-oxalates—weddellite and
whewellite), fungal hyphae and EPS material lining the
pits were experimentally produced by fungal interaction
with a dolomite substrate, indicating these diagenetic
features are a characteristic of fungal interaction with car-
bonate substrata and generally also a part of the honeycomb
structures demonstrated above. Figures
5.9c, d
reveal more
details on the above-mentioned similarities. In both cases,
the filaments are occasionally encrusted with fine crystals
(Fig.
5.9c
) and usually form a lining on the pit wall. Fungal
hyphae are known to form envelopes of Ca-oxalates crystals
that partially or completely engulf the hyphae especially
under Ca-rich conditions (Gadd
1999
; Verrecchia
2000
;
Kolo and Claeys
2005
). This crystal adherence to fungal
hyphae is also visible in both cases.
“
collar
5.5
Discussion
5.5.1 Neoproterozoic-Aged Colonization
and Weathering
The Neoproterozoic carbonate stromatolites of the Mouila
series were originally composed of a magnesian micritic
mud colonized by benthic cyanobacterial mats in a shallow
tidal depositional system. Then, through a combination of
microbially-induced biomineralization of fine- to medium-
grained dolomicrospar, coupled with copious EPS excretion,
the original muddy sediment was transformed into domal
stromatolites, similar to those occurring today in evaporitic
carbonate sabkha-like environments (Walter
1976
;
Grotzinger and Knoll
1999
). These microbialites seemed to
have resisted erosion as evident by the preservation of the
overall morphology and internal features of the stromatolites
(see below). As suggested by the typical shallowing-upward
sequences, this semi-lithified to progressively well-lithified
sediment experienced periodic or episodic desiccation (as
revealed by a
is a term we use here to describe the deposi-
tion of fungally produced biominerals, mainly Ca-Mg-
oxalates or even calcite, in the partially to totally dissolved
crystals of dolomite substrata (Fig.
5.9e, f
) following fun-
gal colonization and growth. In our experiments,
Nesting
”
these
biominerals are much finer (1-3
m) than the original
substrate crystal size and typically are represented by vari-
ous forms of Ca-oxalates: prismatic, tetragonal bi-
pyramidal, and rhombic (Fig.
5.9f
). Such fine crystals are
also observed in the Neoproterozoic pits (Fig.
5.9e
). The
formation of these metal-oxalates is largely attributed to
the reaction of oxalic acid, excreted by fungi, and the high
availability of Ca
2+
and Mg
2+
μ
of the mud in thin
sections and the EPS under the SEM) coupled with evapo-
ritic salty brine invasion leading to gypsum and other
evaporitic minerals being interstratified within the mats.
Cyanobacteria were progressively destroyed and cementa-
tion was the dominant process. During this period of subaer-
ial conditions, fungi were able to colonize the substrate and
drive carbonate diagenesis. The most striking result of their
activity was the formation of the circular-oval-shaped pits in
'
polygonal pattern of cracks
'
in the growth environment
(Gadd
1999
).
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