Ancient life at the extremes : molecular fossils and paleoenvironmental contexts of Neoproterozoic and Cambrian hypersaline settings

Abstract

This thesis investigates the molecular remains of microorganisms from Late Precambrian and Early Cambrian hypersaline settings. In particular, it assesses the composition and antiquity of halophilic microbial communities, which survive and flourish under highly saline conditions. While numerous studies have examined modern hypersaline ecosystems, their biological composition in the geologic past, particularly in the Precambrian, is poorly understood. All deposits in this study consisted of evaporites from the Gillen Member of the ~800 Ma Neoproterozoic Bitter Springs Formation and the Early Cambrian Chandler Formation. These evaporites originated from the Amadeus Basin in central Australia and were deposited in inland seas at those times. Due to the shallow nature of these seas and tenuous connections with the contemporaneous oceans, the waters were characterized by elevated salinity levels that resulted in the deposition of dolomite, gypsum and halite.

The biological composition of these ancient hypersaline settings have been assessed by analysing the hydrocarbon remains of lipids, which act as molecular fossils, or biomarkers. However, since all evaporites were collected from a drill core (Mt Charlotte 1) that was contaminated during drilling and storage (e.g. through sampling bags), the syngeneity of the molecules needed to be tested. Such tests were conducted by removing the outer surfaces of the evaporites and quantifying and comparing the amount of hydrocarbons in both the exterior and interior rock portions. This work allowed for the detection of various contaminants, which masked or overprinted indigenous biological signals.

While no indigenous hydrocarbons were detected in the Early Cambrian evaporites, those from the Neoproterozoic yielded a diverse range of syngenetic biomarkers. A detailed analysis of the biomarkers and the enclosing sedimentary textures yielded the oldest current evidence of microorganisms from hypersaline conditions. In particular, the saturate fractions of these samples revealed high ratios of mono- and dimethylalkanes relative to n-alkanes. Such a pattern is typical of Precambrian and Cambrian samples and observed in a number of facies settings. An outstanding characteristic of these evaporites are the presence of several pseudohomologous series of both regular (to C₂₅) and irregular (to C₄₀) acyclic isoprenoids. The relative concentrations of these molecules vary and depend on the mineralogy and textural characteristics of the sedimentary host rock. These isoprenoids are interpreted as the oldest evidence of haloarchaea in the geological record.

Apart from haloarchaea, evidence for cyanobacterial mats was also observed. Molecular evidence for cyanobacteria included elevated concentrations of n-heptadecane (n-C₁₇), and mono- and dimethylalkanes. Such biomarkers were present in anhydrite (altered from gypsum) as well as in layers of dolomite. The carbonate layers can be interpreted as fossil microbial mats based on: 1) evidence for cohesive dolomitized layers that resemble modern mat structures; 2) characteristics of low-temperature dolomite precipitation; 3) concentric framboidal pyrite inside the dolomite; 4) shape, distribution and association of clay laminae with dolomite crystals and 5) inorganic carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotope values. These results also indicate that the dolomite was most likely formed through microbial activity, making this the oldest evidence for biologically-induced dolomite precipitation.

Such dolomite-precipitating mats would have also harbored a community of other microorganisms including sulfide-reducers (as evidenced by framboidal pyrite precipitation) and methanogens, possibly methylotrophic, through the detection of 2,6,11,15-tetramethylhexadecane (crocetane) and 2,6,10,15,19-pentamethylicosane (PMI). The detection of these two compounds in the Neoproterozoic makes this their oldest occurrence to date and yields insights into the antiquity of methanogens in hypersaline settings.

Through comparisons with published data from modern hypersaline settings, it can be concluded that similar microbial communities as that of today were present on Earth by at least ~800 Ma. Therefore, such results provide new insights into ancient halophilic ecosystems. Show more