25th International Meeting on Organic Geochemistry IMOG 2011

P-167
Microbial deposits from Shark Bay and their geologic
significance - a multidisciplinary re-visit
Tobias Ertefai 1,2 , Ricardo Jahnert 3 , Grzegorz Skrzypek 4 , John Dodson 2 , Kliti Grice 1 ,
Lindsay Collins 3
1 WA Organic & Isotope Geochemistry Centre, Curtin Univsersity, Perth, Australia, 2 Insitute for
Environmental Research, ANSTO, Sydney, Australia, 3 Department of Applied Geology, Curtin University,
Perth, Australia, 4 West Australian Biogeochemistry Centre, University of Western Australia, Perth, Australia
(corresponding author:t.ertefai@curtin.edu.au)
Accumulations of modern microbial mats resemble
carbonate systems of the Neoproterozoic to Mesozoic
from which significant commercial oil and gas
reservoirs have been identified (e.g. Oman, Brazil).
Cyanobacterial mats and living stromatolites
represent life forms similar to those preserved in
rocks of some 3.5 billion years in age. At Shark Bay
(Western Australia), a variety of distinct
cyanobacterial mat morphotypes grow within distinct
zones of the intertidal plain, while the reasons for this
striking succession are not resolved. Mat
morphologies are likely linked to the interplay of
different microbial consortia and their metabolic
pathways that influence mat geochemistry and
lithification via the ―alkalinity engine‖ and extracellular
polymeric substances as potential nucleation sites for
carbonates [1].
The geological significance of modern photosynthetic
mats inspired us to investigate these features within a
multidisciplinary approach combining
sedimentological, organic and inorganic geochemical
and molecular techniques. Using LC-MS techniques,
we observe profiles of intact polar lipids (IPLs) that
are typical of photosynthetic microbial mats [2]. The
complex microbial community is represented by 9 IPL
classes including phospho- and glycolipids, N- and Scontaining
IPLs. Systematic differences in the
abundance of betaine lipids vs. ornithine lipids
suggest a different contribution of S-cycling bacteria
to the microbial community within smoothly layered
mats as opposed to other morphotypes. The analysis
of phospholipid fatty acids (PLFAs) is in good
accordance with the profiles of IPL acyl side chains
(range: C14:0 to C19:1 fatty acids) and allow the
discrimination of different biochemical pathways, as
isotopic signatures of the PLFAs range from -11.3 to
-29.6 ‰ vs. VPDB. δ 13 C values of C17-n-alkanes
attributed to cyanobacteria support differences
between mat types (range: -18.6 to -31.2 ‰),
whereas discrete mat morphotypes (e.g. smooth
mats) reflect isotopic similarities across the
embayments (-18.6 to -23.5 ‰). Isotopic values of
bulk organic matter (-10.6 to -30.0 ‰, Fig. 1) and
dissolved organic carbon in the porewater of the mats
(-9 to -19 ‰) are consistent with rising salinity levels
between the embayments (40,000-80,000). Dissolved
inorganic carbon isotope values are relatively
depleted (-1.4 to -12.9 ‰) compared to δ 13 C values of
carbonates (update: 2.2 to 4.4 ‰).
Fig. 1: δ 13 C of bulk organic matter (OM) liberated from
intertidal microbial mats (tufted, pustular, smooth, colloform)
at four locations (Hutchinson and Nilemah are within
Hamelin Pool, Rocky and Garden Point outside of it, Shark
Bay, Western Australia).
References
[1] Dupraz, C., Reid, P., Braissant, O., Decho, A.,
Norman, S., Visscher, P. (2009) Earth. Sci. Rev. 96, 141-
162.
[2] Hoelzl, G., Doermann, P. (2007) Prog. Lip. Res.
46, 225–243.
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