Geology Reference
In-Depth Information
Microbes versus metazoans as dominant reef builders: insights
from modern marine environments in the Exuma Cays, Bahamas
MIRIAM S. ANDRES
*1
, R. PAMELA REID
*
, EMILY BOWLIN
*
,
A. PATRICIA GASPAR
*
and ANTON EISENHAUER
†
*
Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science,
University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA (E-mail: miriam.andres@chevron.com)
†
IFM-GEOMAR, Research Center for Marine Geosciences, Department for Marine Environmental Sciences,
Wischhofstraße 13, 24118 Kiel, Germany
ABSTRACT
Mixed microbial-metazoan reefs, with distinct lateral zonation of microbialites,
coralline algae and coral, fringe the windward-facing margin of two islands in the
Exuma Cays, Bahamas: Stocking Island and Highborne Cay. These reefs offer a unique
opportunity to identify and characterize controls governing the spatial distribution
of microbial versus metazoan reef builders in a modern setting. Distribution patterns
characterized by prolifi c stromatolite development in back-reef lagoons, prominent
coralline algal growth along reef crests, and rare coral development on seaward reef
edges are a direct response to intensity and frequency of sediment stress, sand abra-
sion and burial of reef biota. The recent discovery and dating of an outcropping coral
reef underlying stromatolites in the southern Highborne Cay reef serves as a basis for
interpreting the late Holocene evolution of this reef and addressing the question: what
led to the demise of a metazoan and rise of a microbial reef? Comparison of growth
histories of Highborne and Stocking Island reefs allows us to explore processes that
have governed microbial versus metazoan reef building from mid-Holocene to present,
and to speculate on future reef development at these sites. Antecedent topography
and late Holocene sea-level transgression determined the timing of reef nucleation at
both sites. Initial reef development was relatively unaffected by sedimentation due
to a lag in sediment production relative to rising sea level. Once produced, sediment
was deposited on the Pleistocene terrace, which due to rapid sea-level rise was below
average wave base and sediment suspension. Slowing of sea-level rise combined with
increasing sediment production decreased accommodation space. Mobile sediment
progressively impinged upon the reef environment and, following the emergence of an
algal ridge, sediment was trapped in the back-reef area. Although corals can tolerate
some sediment stress, they cannot cope with extended periods of burial. Increasing
sediment stress and more importantly increasing amplitude and frequency of sediment
burial are interpreted as dominant factors leading to the demise of metazoan reef build-
ers and the prolifi c growth of microbialites in the back-reef lagoon. Microbial buildups
are surprisingly young (<1000 years) and a recent addition to the fringing reef systems
along the Exuma margin. The study of modern analogues is important for our process-
orientated understanding, which in turn provides concepts and hypothesis to apply
and validate in the rock record.
Keywords
Microbialite, microbial reef, Holocene, Bahamas, U/Th dating.
INTRODUCTION
microbial deposits dominated the shallows of
the Precambrian oceans for over 3 billion years,
forming massive reefs, rivalling or exceeding
those of the Phanerozoic (Grotzinger & Knoll,
1999). We use the term 'microbialites' following the
defi nition by Burne & Moore (1987): 'Microbialites
are organosedimentary deposits that have accreted
as a result of a benthic microbial community
Microbialites, i.e. microbial buildups such as
stromatolites and thrombolites, are rare in today's
metazoan-dominated reef systems. In contrast,
1
Present address: Chevron Energy Technology Company,
6001 Bollinger Canyon Rd, San Ramon, CA 94583, USA.
