Biogeochemistry of Holocene intertidal microbial mats of Qatar: implications for petroleum source rock formation in carbonate-evaporite systems

Słowakiewicz, M, Thomas, L, Tucker, M.E, Zheng, Y, Whitaker, F and Pancost, R.D (2014) Biogeochemistry of Holocene intertidal microbial mats of Qatar: implications for petroleum source rock formation in carbonate-evaporite systems. In: 19th International Sedimentological Congress: Sedimentology at the Crossroads of New Frontiers, Geneva, Switzerland, 18 - 22 August 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.

Item Type: Conference or Workshop Item (Paper)
Divisions: School of Sciences
Date Deposited: 21 Mar 2018 12:11
Last Modified: 15 Aug 2021 09:49
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