Geology Reference
In-Depth Information
In specifi c modern hypersaline environments,
sulphate-reducing bacteria apparently induce
dolomite precipitation (Vasconcelos & McKenzie,
1997; van Lith
et al
., 2002, 2003; Wright & Wacey,
2005). These studies demonstrated the impor-
tance of anaerobic processes in carbonate mineral
formation, but many modern geological settings,
such as the Andros Island, Bahamas, supratidal
environment, indicate that aerobic microbial pro-
cesses may also play an important role in dolomite
precipitation. A diverse microbial population is
generally involved in carbonate precipitation in
natural environments, including photoautotrophic
cyanobacteria and bacteria-producing ammonia
from the degradation of nitrogen-rich organic
matter (Thompson & Ferris, 1990; Rivadeneyra
et al
., 2000; Sánchez-Román
et al
., 2007).
Rivadeneyra
et al
. (1993) and more recently,
Sánchez-Román (2008) have demonstrated that
halophilic aerobic bacteria mediate primary pre-
cipitation of dolomite at room temperatures.
These results indicate that heterotrophic micro-
organisms, using molecular oxygen as an electron
acceptor, may be able to precipitate dolomite at
low temperatures and may be important play-
ers in modern environments, as well as in the
geological past.
Riding (2000) has suggested that morphology
(microfabric) studies can provide insight on
biomineralization processes, as well as on the
environmental conditions where dolomite
precipitates. This combined information can be
useful for the identifi cation of microbial fossils
and/or precipitates, such as biomarkers in carbon-
ate sedimentary rocks (Buczynski & Chafetz, 1991;
Allen
et al
., 2000; Reid
et al
., 2000; Riding,
2000). Regardless of bacterial type (autotrophic
or heterotrophic; marine or non-marine), carbon-
ate morphologies mediated by microorganisms
are distinguishable from inorganic precipitation
(Krumbein, 1979; Buczynski & Chafetz, 1991;
Chafetz & Buczynski, 1992; Knorre & Krumbein,
2000; Braissant
et al
., 2003).
This study focuses on the role of aerobic
bacteria in dolomite formation. A comparative
analysis of modern dolomite formation in Brejo do
Espinho (Brazil), with generally oxic conditions
at the sediment-water interface, and dolomite
precipitation in laboratory experiments, using two
moderately halophilic aerobic bacteria isolated
from Brejo do Espinho, links microbial metabol-
ism with the natural observed conditions, such as
pH, salinity, oxygen and carbonate mineralogy.
In addition, the role of these two previously unde-
scribed bacterial strains in dolomite precipitation
are evaluated and a mechanism for microbially
mediated dolomite formation under aerobic con-
ditions is proposed.
GEOLOGICAL SETTING
Brejo do Espinho is a very shallow (< 0.5 m)
hypersaline coastal lagoon located east of Lagoa
Vermelha about 100 km east of Rio de Janeiro city
(Brazil) (Fig. 1). Situated within a Pleistocene dune
system, the hydrology of Brejo do Espinho is infl u-
enced by seepage through the sand dunes from the
hypersaline lagoon Lagoa Araruama on the conti-
nental side and seawater from the Atlantic Ocean.
20
o
S
22
o
Rio de Janeiro
24
o
50
o
W
48
o
46
o
44
o
42
o
40
o
6km
100km
Fig. 1.
Satellite image showing Brejo do Espinho (22°56
18 W) located between the Atlantic coast and Lagoa
Araruama in the State of Rio de Janeiro, Brazil. Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA
Johnson Space Center.
08 S, 42°14
