25th International Meeting on Organic Geochemistry IMOG 2011

P-169
Lipid biomarkers in ooids from different locations and ages
provide evidence for a common bacterial flora
Aimee Gillespie 1 , Laurence Bird 2 , Sara Pruss 3 , Alex Sessions 4 , Mark Roberts 5 , Roger
Summons 1
1 Massachusetts Institute of Technology, Cambridge, MA, United States of America, 2 Pennsylvania State
University, University Park, PA, United States of America, 3 Smith College, Northampton, MA, United States
of America, 4 California Institute of Technology, Pasadena, CA, United States of America, 5 Woods Hole
Oceanographic Institution, Woods Hole, MA, United States of America (corresponding
author:aimeeg@mit.edu)
Ooids are an important but enigmatic part of
the sedimentary carbonate system. They form in low
latitude, agitated, shallow water environments that are
supersaturated with respect to calcium carbonate. At
present,ooids are actively precipitating in only a few
places, notably the tropical Atlantic and Indian
Oceans. However oolitic limestones have been found
in many sedimentary sections throughout Earth
history. Ooids are useful as stratigraphic markers and
as indicators of paleoenvironment; they are also a key
component of the carbonate that is being trapped and
bound in modern and ancient stromatolites.
Despite their importance in the carbonate
system and usefulness in stratigraphic reconstruction,
the ooid formation process is not well understood. A
particularly contentious aspect is the role of
microorganisms in carbonate precipitation,
cementation (in the case of oolites), and
remineralization. Early research on the topic
suggested that organic matter, and perhaps biofilms,
plays a key role in the concentric layering that
characterizes ooids.
Modern and Holocene ooid samples,
collected from outcrops and beaches in the Bahamas
and in Shark Bay in Western Australia, were
examined for their contents of lipid biomarkers.
Modern samples from Cat and Andros islands in the
Bahamas and from Carbla Beach in Hamelin Pool,
Western Australia, showed abundant and surprisingly
similar distributions of hydrocarbons, fatty acids, and
alcohols. A large fraction of these lipids were bound
into the carbonate matrix and only released on acid
dissolution, which suggests that these lipids were
being incorporated continuously during ooid grow.
This is further evidenced by older 14 C ages of the
carbonate-bound lipids.
The distributions of hydrocarbons, and their
varied � 13 C values, were consistent with mixed inputs
from cyanobacteria (C17-C19; � 13 C = -15 to -24
‰VPDB) together with small and variable amounts of
vascular plant leaf wax (C27-C35; � 13 C = -25 to -32
‰VPDB). The fatty acids (FA) comprised a complex
mixture of C12-C18 normal and branched short chain
isomers with the predominant straight-chained
components attributable to bacteria and/or
cyanobacteria.
Branched FA isomers, especially 10-MeC16
and 10-MeC18, together with the prevalence of
elemental sulfur in the extracts, indicate an origin from
sulfate reducing bacteria. The iso- and anteiso- FA
were quite variable in their 13 C contents suggesting
that they come from organisms with diverse
physiologies. Hydrogen isotopic compositions
provide further insights into this issue, with branched
fatty acids having relative D-enrichments
characteristic of heterotrophic bacteria.
The most enigmatic lipid assemblage is a
homologous series of long-chain (C24-C32) FA with
pronounced even carbon number preference.
Typically such long-chain FA are thought to come
from land plant leaf wax but their relative 13 C
enrichments (C24-C32; � 13 C = -18 to -22 ‰VPDB),
indicate a microbial origin in this case.
The distributions of lipids isolated from
Holocene oolites from the Rice Bay Formation of Cat
Island, Bahamas were very similar to the beach ooids
described above and, in total, these biomarker data
lead us to hypothesize that ooid formation in tropical
marine environments is mediated by a defined
microbial community.
References:
Duguid et al.,80 (3): 236. Journal of Sedimentary
Research (2010).
Taylor and Parkes 129 (11): 3303. Journal of General
Microbiology (1983).
Zhang et al., 106: 12580. Proceedings of the National
Academy of Science (2009).
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