Geology Reference
In-Depth Information
Box 4.2. Microbial Controls on Carbonates.
Interest in the bacterial fossil record is expanding owing to the recognition of the fundamental microbial controls on
sedimentation and Earth's biogeochemical evolution (Neuweiler et al. 1997, Ehrlich 1999). Bacterial sulfate reduction
plays a paramount role in the formation and decomposition of minerals, and the control of early diagenetic processes in
carbonates (Machel 2001).
Bacteria and other microbes are able to form carbonate rocks (limestones: Castanier et al. 1999; dolomite: Wright
1999) and minerals on all scales. The significant role of microbes in the formation and degradation of shallow- and
deep-marine as well as non-marine carbonates becomes increasingly more evident (Riding and Awramik 2000).
Microbes
(cyanobacteria, photosynthetic and non-photosynthetic bacteria, green and red algae, fungi) and 'microbi-
alites' (Burne and Moore 1987) play a crucial role in the genetic interpretation of micrite, carbonate cements and bio-
mineralization processes.
Microbialites are laminated or nonlaminated organosedimentary deposits formed under the control of benthic micro-
bial biofilms, organic matter degrading microorganisms, benthic metazoan communities, free organic matter, and sedi-
ment trapping. The formation and construction of inter- and subtidal microbial mats, intertidal and subtidal stromato-
lites, mud mounds and reefs, and vent- and seep carbonates is highly controlled by microbes. Marine microbialites are
common structures in cryptic niches of modern and ancient reefs (see Facies, vol. 29, 1993).
Mud mounds are regarded as a specific carbonate factory that differs fundamentally from the tropical and from the
cool-water carbonate factories (Schlager 2000). The geologic record of the microbial contribution to framebuilding in
deep-water mud mounds is inferred from (1) problematic microfossils, (2) laminated structures, and (3) clotted and
fenestral micrite (thromboids). Bacterial calcification occurs in pedogenic (e.g. caliche: Loisy et al. 1999), sabkha
environments, cave carbonates and lacustrine carbonates, and in hot-water travertines and beachrocks.
Microbial activity has a high impact on diagenesis (Sedimentary Geology, vol. 126/1-4, 1999), controls lithification
of tidal deposits and reefs and the formation of cements (Gonzales-Munoz et al. 2000), and contributes to the formation
of manganese nodules, concretions (Coleman 1993) and iron sulfide and phosphorite deposits. Fe-encrusted bacteria
and fungi produced the red color of Devonian mud mounds (Boulvain et al. 2001) and hematite staining of carbonate
grains and microstromatolites in Jurassic carbonate ramps (Préat et al. 2000).
Most heterotrophic bacteria are able to induce calcium carbonate precipitation as a by-product of physiological
activity in laboratory experiments (Buszynski and Chavetz 1993). Bacteria act as nucleation sites and may become
embedded in growing carbonate particles (Castanier et al. 2000). The calcification in procaryotic cyanobacteria depends
on water chemistry and photosynthetic bicarbonate uptake, and the existence of suitable sheets. Physicochemical pre-
cipitation at high levels of supersaturation leads to a dense encrustation of the filaments, forming solid micritic tubes
(Pl. 53/2). Photosynthetic bicarbonate take-up in less saturated waters can lead to micrite precipitation within the sheets
(Merz-Preiss 2000). Globular grains and peloids occurring in modern microbial mats and ancient stromatolites (Pl. 123/
3) are formed by bacterially induced calcium carbonate precipitation around dead cyanobacterial threads as proved by
laboratory experiments.
Microbial mats are vertically laminated organosedimentary structures, which develop on solid surfaces and have
steep gradients of oxygen and sulfur (Van Gemerden 1993). They are dominated by cyanobacteria, colorless and purple
sulfur bacteria, and sulfate-reducing bacteria. The driving force of most microbial mats is photosynthesis by cyanobac-
teria and algae.
Microbial signatures in thin sections are in-situ formed benthic peloids and ooids (Chavetz 1986, Gerdes et al. 1994)
within mats, thrombolitic, clotted and fine-peloidal fabrics, laminated and stromatolitic fabrics, fossils interpreted as
cyanobacteria (see Pl. 53) and microborings (Pl. 52/1-4, 6).
Thrombolites (Kennard and James 1986; Pl. 6/3, Pl. 50/5, Pl. 131/5) characterized by clotted microtexture and the
absence of lamination occur in a variety of settings ranging from freshwater environments to deep shelf, and were
important in the formation of framework reefs from the Cambrian to the Cretaceous (Leinfelder and Schmid 2000;
Pl. 50/5). SEM observations may exhibit nannobacteria, and organic biomarker and isotope data assist greatly in the
evaluation of microbial contributions to the formation of carbonate rocks.
Nannobacteria in the 0.03 to 0.2 m range occur in modern and ancient carbonates. Folk and Chavetz (2000) stress
the importance of these microbes for carbonate precipitation in non-marine and marine environments, but also say that
'one needs faith and optimism' to see these extremely tiny grains. Southam and Donald (1999) argue that these very
small spherical or rod-shaped grains may represent solid, inorganic precipitates.The geological record of calcified bac-
terial mats and microbial carbonates starts in the Archaean.
Microbial mats are the oldest structured ecosystems on earth, known already from 3.5 Ga old carbonates (Sumner
2000). The predominance of microbes in Precambrian ecosystems has no counterpart in the Phanerozoic.
