Microbial dolomite precipitation under aerobic conditions: results from Brejo do Espinho Lagoon (Brazil) and culture experiments - Perspectives in Carbonate Geology - page 164

Microbial dolomite precipitation under aerobic conditions: results
from Brejo do Espinho Lagoon (Brazil) and culture experiments

MÓNICA SÁNCHEZ-ROMÁN * , CRISÓGONO VASCONCELOS * , ROLF WARTHMANN * ,
MARIAN RIVADENEYRA † and JUDITH A. MCKENZIE *
* ETH, Geological Institute, 8092 Zürich, Switzerland (E-mail: [e-mail])
† Department of Microbiology, Faculty of Pharmacy, University of Granada, Spain

ABSTRACT

Microbially mediated high-Mg calcite and dolomite precipitation occurs under
oxic conditions in Brejo do Espinho lagoon, Brazil, within the upper 5 cm below
the sediment-water interface. With burial to < 25 cm in the sediment sequence, early
diagenesis associated with sulphate-reducing bacterial activity transforms the mixed
carbonate mineralogy to 100% dolomite, as the pore-water becomes undersaturated with
respect to calcite, while remaining supersaturated with respect to dolomite. Laboratory
culture experiments using moderately halophilic aerobic bacteria ( Virgibacillus
marismortui and Marinobacter sp.) isolated from the uppermost part of the microbial
mat in Brejo do Espinho demonstrate that microbially mediated dolomite precipita-
tion can occur under ambient Earth's surface conditions in the presence of oxygen.
These results add an additional metabolic process, aerobic respiration, to bacterial
sulphate reduction and methanogenesis, which have previously been identifi ed with
dolomite formation. Furthermore, the formation of carbonate minerals with spherulitic
structures in both the natural environment and laboratory culture experiments points
to microbial involvement, as recognized in numerous other modern environments
and ancient systems. This study suggests that previously recognized modern dolo-
mite-forming environments, such as the supratidal areas of Andros Island, Bahamas,
with recent dolomite crusts should be revisited to evaluate the importance of aerobic
respiration in dolomite precipitation.

Keywords Moderately halophilic aerobic bacteria, microbial dolomite, hypersaline
environment, Brejo do Espinho, Brazil, spherulites.

INTRODUCTION environments presented the possibility to study

the geochemical parameters promoting dolomite
precipitation, attempts at laboratory experiments
based on these actualistic studies proved to have
limited or no success in precipitating dolomite
under Earth's surface conditions (Land, 1985,
1998). Thus, the rarity of modern environments
relative to the rock record and the lack of success
with experimental studies led to a continuing
enigma, often called the Dolomite Problem.
In recent years, however, a new approach
using microbial experiments has provided funda-
mentally new data to understand the mechanisms
that may be involved in dolomite precipitation
under Earth's surface conditions (Vasconcelos
et al ., 1995; Warthmann et al ., 2000) and has
inspired the development of the microbial dolo-
mite model based on a study of a modern dolo-
mite-forming hypersaline coastal lagoon, Lagoa
Vermehla, Brazil (Vasconcelos & McKenzie, 1997).

Dolomite is a common carbonate mineral in
sedimentary rocks throughout the geological
record, especially in Precambrian carbonate rocks
where it is abundant and often found associated
with microbial structures, e.g. stromatolites, but
it is rarely found precipitating in modern envir-
onments. In the 1960s, the discovery of dolomite
forming in specifi c environments, such as beneath
the Abu Dhabi sabkhas, UAE and in the ephemeral
lakes of the South Australian Coorong Lagoon,
provided new insights into the physico-chemical
controls on the process. Apropos to the theme of
this special publication, Robert N. Ginsburg and
his colleagues contributed to this renaissance in
dolomite research in the 1960s with their discov-
ery of modern dolomite precipitation in exten-
sive supratidal crusts on Andros Island, Bahamas
(Shinn et al ., 1965, 1969). Although these modern