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trapping and binding detrital sediment and/or
forming the locus of mineral precipitation'. The
term encompasses stromatolites, characterized
by a laminated internal structure (Awramik &
Margulis, 1974; Walter, 1976b) and thrombolites
distinguished by a mesoscopic clotted internal
fabric (Aitken, 1967; Kennard & James, 1986).
Although microbial reefs have been in decline
since the Late Proterozoic, both stromatolites and
thrombolites continued to be important reef-build-
ers during various phases of the Phanerozoic (see
Riding (2006) for latest review). The long-term
decline of these microbial structures is attributed
to eukaryotic competition and interference, com-
petition for space and substrate, changes in the
physical environmental conditions (Fischer, 1965;
Awramik, 1971, 1990, 1992; Fischer & Arthur,
1977; Vermeij, 1987), and specifi cally to the rise
of grazing and burrowing animals (Garrett, 1970).
Based on the sporadic resurgence of microbialites
following mass extinctions, they have been referred
to as 'disaster forms' (Schubert & Bottjer, 1992).
When stromatolite resurgences do not coincide
with a mass extinction, authors such as Soja (1994)
infer the 'localized eradication of benthic marine
communities'. The question of whether metazoan
competition is the primary factor governing micro-
bial carbonate abundance was recently addressed
in a comprehensive study comparing microbial
carbonate abundance to the fl uctuation in metazoan
diversity through the geological record (Riding,
2006). By comparing the response of microbial
carbonates in the aftermath of mass extinctions,
i.e. their resurgence or failure to recover, Riding
(2006) '. . . raises doubts that metazoan competition
can be invoked as a general explanation for fl uc-
tuations in microbial carbonate abundance' and
further that '. . . a variety of factors, each changing
through time, must have operated'.
Few studies have addressed the issue of fac-
tors controlling microbial versus metazoan reef
builders in modern settings, which in part may
be explained by the scarcity of suitable local-
ities. Originally thought extinct, modern stroma-
tolites were discovered in Shark Bay, Australia
(Logan, 1961) and Bahamas (Dravis, 1983; Dill
et al. , 1986; Dill, 1991). Since then, Holocene
microbialites and in particular stromatolites of
all sorts and variety have been reported. Most
modern microbialite occurrences are confi ned to
lacustrine (Casanova, 1994; Andrews et al. , 1997)
or extreme environments with respect to the tol-
erance limits of most eukaryotes, i.e. temperature
(Walter, 1976a), salinity (Dupraz et al. , 2004;
Vasconcelos et al. , 2006) and alkalinity (Kempe
et al. , 1991).
With respect to true open-marine environ-
ments (Shark Bay is restricted marine to hyper-
saline), microbialites from French Polynesia are
considered signifi cant contributors of Holocene
reef accretion, i.e. Tahiti (Camoin & Montaggioni,
1994; Camoin et al. , 1999) and Tikehau atoll
(Sprachta et al. , 2001). The Tahitian examples are,
however, limited in extent to open cavities
within reef framework and lagoonal settings.
Bahamian stromatolites, termed 'uncommonly
common' after extensive mapping in the Exuma
Cays (Reid et al. , 1995), are the only known exam-
ples of open-marine stromatolites developing as
isolated columnar structures, such as the often
cited examples from Lee Stocking Island (Dill
et al. , 1986; Dill, 1991; Feldmann & McKenzie,
1998). In two Bahamian locations, stromatolites
'. . . occur with other reef-building organisms
as integral parts of laterally extensive fringing
reefs'; on Stocking Island (Reid & Browne, 1991;
Macintyre et al. , 1996) and Highborne Cay (Reid
et al. , 1999).
Development of mixed microbialite-metazoan
reefs at Stocking Island and Highborne Cay offer a
unique opportunity to gain insights into the phys-
ical environmental factors controlling microbes
versus metazoans as dominant reef builders. The
Holocene history of the Stocking Island reef com-
plex has been described (Reid & Browne, 1991;
Macintyre et al. , 1996) and ecological controls
on the stromatolite development in this reef have
been assessed (Steneck et al. , 1998). Results from
Stocking Island, together with the recent discov-
ery and dating of an outcropping basal coral sur-
face underlying stromatolites in the Highborne
Cay complex, serve as a basis for interpreting reef
history at Highborne Cay: what led to the demise
of a metazoan and rise of a microbial reef? Results
from these two examples allow discussion on pro-
cesses and environmental factors determining the
evolution and spatial distribution of microbial
versus metazoan reef builders in Exuma margin
build-ups during the late Holocene.
MIXED MICROBIAL-CORALLINE
ALGAE-CORAL REEFS IN THE
EXUMAS, BAHAMAS
Fringing reefs composed of mixed microbial-
coralline algae-coral reef-building assemblages
are reported from two locations in the Exuma
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