Biogeochemistry of Holocene intertidal microbial mats of Qatar: implications for petroleum source rockformation in carbonate-evaporite systems
2014
Organic-rich mesohaline microbial mats occur in the intertidal zone of a lagoonal area developed to the lee of a
coastal spit in Mesaieed, eastern Qatar. The mats grow on a substrate of seagrass-rich dolomitic lime mud with
cerithid and monachid gastropods and other small bioclasts, reaching a thickness >20 cm. The mats are well
laminated with different microbial communities, from cyanobacteria to sulphur bacteria, reflected in the distinct
colour changes from green to pink to brown. The lipids reflect the biomass of the principal mat-building
phototrophic and heterotrophic microorganisms. A variety of hydrocarbons, including n-alkanes, diploptene,
and isoprenoids such as phytane, phytene, phytadiene and squalene were detected, in varying concentrations
within the various mat layers. In particular, n-heptadecane, likely derived from cyanobacteria, dominated the nalkane
distribution at a depth of 0-0.1 cm. The concentration and abundance of n-alkanes increase with depth
through the mat, likely representing the early diagenetic initiation of hydrocarbon generation.
The most representative compounds of sterols and stanols are 5α-cholestan-3β-ol or cholest-5-en-3β-ol but
hopanol distribution with trimethyl-5α-cholest-22E-en-3β-ol (dinosterol) is predominant of all neutral fraction
compounds. The significant contribution of dinoflagellates to the total biomass is also confirmed by the
presence of spheroidal (2-10 μm in diameter) dinocysts. Abundance of C27-29 sterols and stanols decreases with
depth whereas concentration of dinosterol (0.4-13 μg g-1TOC) is similar at all depths within the mat. It is very
likely that marine organisms (algal phytoplankton) are the major contributor of sterols and stanols to the mats.
High concentrations of dinosterol can be related to periodically increased productivity of marine algae during
red tides which are common in the Arabian Gulf. Hopanoids are represented in the apolar fraction solely by
diploptene and in the polar fraction by hopanols (C30-32) and C27 hopanoid ketone which is most abundant of all
compounds eluting in the polar fraction. Hopanols have similar concentrations at almost all depths within the
mat. As the C27 ketone is most abundant in dark layers of the mat it is very plausible that it is associated with
anoxic conditions. Fatty acids are dominated by saturated, unsaturated and branched fatty acids and are most
abundant in the top surface layer of the mat. The distribution of fatty acids provides evidence for the presence
of different bacteria including sulphate-reducing bacteria. ‘Phospholipid’ and ‘glycolipid’ fractions reveal the
presence of glycerol dialkyl glycerol tetraethers (GDGTs). They are solely dominated by methyl branched (Ia,
IIa, IIIa) GDGTs (total 442 μg g-1TOC) and are most abundant in the middle part of the mat (0.9 cm depth).
Isoprenoid GDGTs are mostly represented by archaeol and crenarchaeol whose concentrations increase with
depth; this is consistent with dark anoxic layers of the mat.
In summary, unravelling the early diagenetic alteration of organic matter and its preservation in marine
carbonate-evaporite systems, as well as other associated processes such as CaCO3 and dolomite precipitation,
could improve our understanding of the hydrocarbon potential of such systems. This will help considerably in
the prediction of hydrocarbon occurrence in frontier, as well as mature, petroleum carbonate-evaporite basins.
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