Geology Reference
In-Depth Information
9 Limestones are Biological Sediments
Most limestones are directly or indirectly influenced
and controlled by biological processes. The present
chapter deals with the formation of carbonates by mi-
crobes and benthic encrusting organisms, and with the
destructive role of micro- and macroborers. Knowledge
of constructive and degrading processes is essential in
evaluating carbonate budgets.
algae, (b) extracellular polymeric substances (EPS; ac-
cumulating outside the cells to form a protective and
adhesive matrix that attaches microbes to the substrate),
(c) biofilms (submillimetric veneers of bacterial com-
munities in an EPS matrix), (d) microbial mats (milli-
meter-sized complex layers composed of filamentous
cyanobacterian algae and diatoms, and able to trap sedi-
ment), and (e) organomineralization (precipitation of
CaCO
3
in association with nonliving macromolecules
independent of organic activity). See Riding (1991,
2000), Van Gemerden (1993), and Riding and Awramik
(2000) for further explanations.
The eminent role of bacteria and other microbes in
the formation of carbonate rocks was summarized in
Sect. 4.1.2 and is discussed in the following paragraphs
which also cover controls and the terminology of mi-
crobial carbonates, and examine the importance of stro-
matolites.
9.1 Microbial Carbonates and
Stromatolites
Microbes encompass bacteria, fungi, small algae and
protozoans. Bacteria comprise two major groups -
Archaebacteria and Eubacteria, now respectively called
Archaea and Bacteria (including Cyanobacteria).
The sedimentologically important
Cyanobacteria
(Sect. 10.2.1.1) are
aerobic phototrophs
, living in shal-
low-water and using sunlight as energy. Cyanobacterial
calcification is associated with the photosynthetic up-
take of CO
2
and/or HCO
3
-
that raises alkalinity (Pente-
cost and Riding 1986; Merz-Preiss 1999) and leads to
calcification of the mucilaginous sheaths. Present day,
intense cyanobacteria calcification appears to be essen-
tially a freshwater phenomenon and is rare in modern
subtidal environments, by contrast to ancient cyano-
bacteria which occupied tidal and subtidal environ-
ments.
Many
other bacteria
are
anaerobic heterotrophs
and
take their energy from the decomposition of organic
material to inorganic components by redox processes.
These bacteria can occupy lighted and shallow as well
as dark and deep-water settings, and are responsible
for ammonification, denitrification, sulfate reduction,
anaerobic sulphide reduction and methanogenesis pro-
cesses. These processes can lead to HCO
3
-
concentra-
tion and increasing alkalinity favoring fine-grained
CaCO
3
precipitation (Knorre and Krumbein 2000) in
the form of micrite.
9.1.1 Bacterial Contribution to Carbonate
Precipitation
Microbial precipitation of calcium carbonates played a
vital role in the development of Proterozoic and Phan-
erozoic carbonate platforms and reefs. The importance
of bacteria and cyanobacteria in the formation of fine-
grained carbonates in natural aquatic environments has
long been a matter of discussion, starting with the re-
search of Drew (1911, 1913) and Black (1933) on the
action of denitrifying bacteria in tropical and in tem-
perate seas, followed by observations on calcium car-
bonate precipitation in seawater and freshwater envi-
ronments (Pentecost 1985). Reviews discussing this
topic have been published by Cohen et al. (1984) and
Jones (1985). Many researchers have demonstrated that
life processes of marine bacteria and the decomposi-
tion of organic matter by bacteria cause physicochemi-
cal changes in the microenvironment that can result in
calcium carbonate precipitation. This has been proved
in laboratory experiments and observed in various mod-
ern carbonate-producing environments (soils, freshwa-
Microbial CaCO
3
precipitation is triggered by vari-
ous processes associated with (a) bacteria and small
