Abstract. Microbial mats are self-sustaining benthic ecosystems
composed of highly diverse microbial communities. It has been proposed that
microbial mats were widespread in Proterozoic marine environments, prior to
the emergence of bioturbating organisms at the Precambrian–Cambrian
transition. One characteristic feature of Precambrian biomarker records is
that steranes are typically absent or occur in very low concentrations. This
has been explained by low eukaryotic source inputs, or degradation of
primary produced sterols in benthic microbial mats (“mat-seal effect”). To
better understand the preservational pathways of sterols in microbial mats,
we analyzed freely extractable and carbonate-bound lipid fractions as well
as decalcified extraction residues in different layers of a recent
calcifying mat (∼1500 years) from the hypersaline Lake 2 on
the island of Kiritimati, central Pacific. A variety of C27–C29
sterols and distinctive C31 4α-methylsterols (4α-methylgorgosterol and 4α-methylgorgostanol, biomarkers for
dinoflagellates) were detected in freely extractable and carbonate-bound
lipid pools. These sterols most likely originated from organisms living in
the water column and the upper mat layers. This autochthonous biomass
experienced progressive microbial transformation and degradation in the
microbial mat, as reflected by a significant drop in total sterol
concentrations, up to 98 %, in the deeper layers, and a concomitant
decrease in total organic carbon. Carbonate-bound sterols were generally low
in abundance compared to the freely extractable portion, suggesting that
incorporation into the mineral matrix does not play a major role in the
preservation of eukaryotic sterols in this mat. Likewise, pyrolysis of
extraction residues suggested that sequestration of steroid carbon skeletons
into insoluble organic matter was low compared to hopanoids. Taken together,
our findings argue for a major mat-seal effect affecting the distribution
and preservation of steroids in the mat studied. This result markedly
differs from recent findings made for another microbial mat growing in the
nearby hypersaline Lake 22 on the same island, where sterols showed no
systematic decrease with depth. The observed discrepancies in the taphonomic
pathways of sterols in microbial mats from Kiritimati may be linked to
multiple biotic and abiotic factors including salinity and periods of
subaerial exposure, implying that caution has to be exercised in the
interpretation of sterol distributions in modern and ancient microbial mat
settings.
Chemical component depositions in the microbial mats and travertine in the Khoito-Gol thermal spring (Eastern Sayan)
