Imaging of Ancient Microbial Biomarkers within Miocene Dolomite (Kuwait) Using Time-of-Flight Secondary Ion Mass Spectrometry

Abstract

Time-of-Flight–Secondary Ion Mass Spectrometry (ToF-SIMS) using a bismuth liquid metal ion source was utilized to characterize and image microbial biomarkers within dolomite from early-middle Miocene coastal mud volcano outcrops in Kuwait. ToF-SIMS analysis revealed biomarkers of ancient microbial consortia of sulfate reducers and methane oxidizers participating in the anaerobic oxidation of methane. The identified lipid biomarkers comprised 17α(H),21β(H)-Norhopane, Hop-17(21)-ene or Hop-22(29)-ene (diploptene), non-isoprenoidal dialkyl glycerol diethers (DAGEs), and Diacylglycerol esters (DGs). The ion µ-scale images of carbonate rocks showed two characteristic styles: (1) high signal intensity of dolomite, halite, and biomarkers, where the biomarkers demonstrate a distinctive co-localization pattern with both dolomite and halite; and (2) a lack of dolomite, halite low signal intensity, and an absence of biomarker co-localization patterns. Our results highlight three remarkable observations. Firstly, the concomitance of dolomite and halite suggests a common source of magnesium and sodium supply, likely from the hypersaline Al-Subiya sabkha. This emphasizes the importance of hypersaline seawater for dolomite formation. Secondly, microbial biomarkers correspond to methane- and sulfate-rich conditions under which dolomite was formed. Lastly, the high intensity of biomarker signals and their association with dolomite and halite indicate that the consortia involved in dolomite formation have preferences for high-salinity conditions. The three observations align with previous studies that have highlighted the importance of methane-sulfate redox, high salinity, and halophilic microbes for dolomite formation. This work is the first to acknowledge ancient microbial biomarkers within Miocene dolomite in Kuwait, which aims to broaden the understanding of the biogeochemical processes triggering dolomite formation in similar environments and ancient geologic settings.

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“…More rarely, dolomite appeared as large aggregates, consisting of co-directional columnar crystals with a high content of magnesium (17.6% mass fraction of Mg and 2.5% for Ca). In such cases, non-mineralized carbon-rich biogenic microfilms were observed on the surface of the aggregate and between individual columnar crystals, which is a commonly described pathway of Mg-rich carbonates precipitation in aquatic environments [42,43].…”