Paleoecology of Neoproterozoic hypersaline environments: Biomarker evidence for haloarchaea, methanogens, and cyanobacteria.

Abstract

While numerous studies have examined modern hypersaline ecosystems, their equivalents in the geologic past, particularly in the Precambrian, are poorly understood. In this study, biomarkers from ~820 million year (Ma)-old evaporites from the Gillen Formation of the mid-Neoproterozoic Bitter Springs Group, central Australia, are investigated to elucidate the antiquity and paleoecology of halophiles. The sediments were composed of alternating laminae of dolomitized microbial mats and up to 90% anhydrite. Solvent extraction of these samples yielded thermally well-preserved hydrocarbon biomarkers. The regularly branched C25 isoprenoid 2,6,10,14,18-pentamethylicosane, the tail-to-tail linked C30 isoprenoid squalane, and breakdown products of the head-to-head linked C40 isoprenoid biphytane, were particularly abundant in the most anhydrite-rich sediments and mark the oldest current evidence for halophilic archaea. Linear correlations between isoprenoid concentrations (normalized to n-alkanes) and the anhydrite/dolomite ratio reveal microbial consortia that fluctuated with changing salinity levels. Halophilic archaea were the dominant organisms during periods of high salinity and gypsum precipitation, while bacteria were prevalent during stages of carbonate formation. The irregularly branched C25 isoprenoid 2,6,10,15,19-pentamethylicosane (PMI), with a central tail-to-tail link, was also abundant during periods of elevated salinity, highlighting the activity of methanogens. By contrast, the irregularly branched C20 isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane) was more common in dolomite-rich facies, revealing that an alternate group of archaea was active during less saline periods. Elevated concentrations of isotopically depleted heptadecane (n-C17 ) revealed the presence of cyanobacteria under all salinity regimes. The combination of biomarkers in the mid-Neoproterozoic Gillen Formation resembles lipid compositions from modern hypersaline cyanobacterial mats, pointing to a community composition that remained broadly constant since at least the Neoproterozoic. However, as a major contrast to most modern hypersaline environments, the Gillen evaporites did not yield any evidence for algae or other eukaryotes.