Sterol preservation in hypersaline microbial mats

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 analysed freely extractable and carbonate-bound sterols 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 both 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 sterols concentrations, up to 98 %, in the deeper layers, and a concomitant decrease in total organic carbon. Carbonate-bound sterols were generally low in abundance, suggesting that incorporation into the mineral matrix does not play a major role for the preservation of eukaryotic sterols in this mat. Likewise, pyrolysis revealed that steroids (i.e., including sterenes, steranes and sterols), in contrast to hopanoids, were not sequestered into insoluble organic matter which may give rise to a further bias in the preservation of steroids vs. hopanoids, particularly in the later stages of burial. While these findings argue for a strong 'mat-seal effect' in the mat studied, they markedly differ from recent findings made for another microbial mat growing in the near-by 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 sterols distributions in modern and ancient microbial mat settings.