Geology Reference
In-Depth Information
5
Probable Fungal Colonization and Carbonate
Diagenesis of Neoproterozoic Stromatolites
from South Gabon, Western Congo Basin
Kamal Kolo, Kurt Konhauser, Jean-Pierre Prian, and Alain Pr
´
at
5.1
Introduction
result in the slow alteration of the primary mineral surfaces,
but frequently they induce the formation of secondary min-
eral phases, such as Ca- and Mg-oxalate and calcite (Gadd
1999
; Verrecchia
2000
; Chen et al.
2000
; Burford et al.
2003a
; Hoffland et al.
2004
) or the so-called desert varnish
comprising Fe- and Mn-oxides (Krumbein and Jens
1981
;
Grote and Krumbein
1992
). Finally, extreme bioweathering
can even form a new diagenetic
Fungi are able to colonize any number of rock surfaces in
their efforts to extract nutrients and trace metals for their
metabolism. Their filaments, called hyphae, physically
exploit grain boundaries, cleavages and cracks to gain access
to new mineral surfaces, and in the process, they cause
several alteration features, ranging from simple surface
roughing by etching and pitting to selective mineral dissolu-
tion and cavity formation to extensive physical disintegra-
tion of the minerals (see Konhauser
2007
for details).
Simultaneously, all exposed mineral surfaces become cov-
ered in EPS, which serves to retain water and fuel hydrolysis
reactions (Welch et al.
1999
). Through the release of organic
acids, such as oxalic acid or citric acid (Richter et al.
2007
),
mineral dissolution is accelerated because the acids dissoci-
ate and release protons that can attack minerals directly
by complexing with ligands at the minerals surface.
Deprotonated organic anions (e.g., oxalate, citrate) indi-
rectly affect dissolution rates by complexing with metal
cations in solution, thereby lowering the mineral
“
mycogenic rock fabric
”
(Burford et al.
2003b
).
As weathering agents, fungi have played a particularly
important role in the alteration of carbonate rocks: the
biodeterioration of carbonate monuments and buildings
(Hoffland et al.
2004
; Sterfinger and Krumbein
1997
),
bioerosion of corals and sediment particles (Vogel et al.
2000
; Golubic et al.
2005
), and the accumulation of
carbonate-sourced metals (Sterflinger
2000
; Gadd
2007
),
are just a few examples. The large quantities of oxalic acid
produced by fungi can also react with carbonate host rocks to
yield Ca-oxalates crystals or re-precipitation of Ca-minerals
in the form of calcretes (Verrecchia
2000
). Indeed, it has
been suggested that fungi are probably at the origin of much
the calcium carbonate accumulation in paleosols and CaCO
3
enrichment of surficial sediments throughout the Phanero-
zoic (Verrecchia et al.
2003
; Cardon and Whitbeck
2007
).
For instance, paleosols in the Lower Carboniferous of South
Wales contain needle-fibre calcite as coatings on sediment
grains and rhizocretions (Wright
1986
,). The fibres were
probably formed by the calcification of fungal hyphae.
Esteban and Klappa (
1983
) illustrated fungal hyphae in a
Pleistocene caliche hardpan from Spain. A well-developed
biogenic structure (sparmicritization), related to the activity
of fungi and algae is reported by Kahle (
1977
) on the
Pleistocene Miami Limestone which has been transformed
into calcareous crusts. Part of the spar-micritization was
caused by boring of sparry calcite cement by fungi, followed
by in situ calcification. Fossilized fungal hyphae and spores
have also been observed in the Upper Devonian of the Rocky
Mountains (Canada), in the Lower Carboniferous of north-
ern France and in the Cretaceous of Central Italy by Pr
´
at
s saturation
state (e.g., Bennett et al.
1988
). These interactions not only
'
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