Local paleoenvironmental controls on the carbon-isotope record defining the Bitter Springs Anomaly

Carbon isotope evidence for large methane emissions to the Proterozoic atmosphere

The Proterozoic Era records two periods of abundant positive carbon isotope excursions (CIEs), conventionally interpreted as resulting from increased organic carbon burial and leading to Earth’s surface oxygenation. As strong spatial variations in the amplitude and duration of these excursions are uncovered, this interpretation is challenged. Here, by studying the carbon cycle in the Dziani Dzaha Lake, we propose that they could be due to regionally variable methane emissions to the atmosphere. This lake presents carbon isotope signatures deviated by ~  + 12‰ compared to the modern ocean and shares a unique combination of analogies with putative Proterozoic lakes, interior seas or restricted epireic seas. A simple box model of its Carbon cycle demonstrates that its current isotopic signatures are due to high primary productivity, efficiently mineralized by methanogenesis, and to subsequent methane emissions to the atmosphere. By analogy, these results might allow the reinterpretation of some positive CIEs as at least partly due to regionally large methane emissions. This supports the view that methane may have been a major greenhouse gas during the Proterozoic Era, keeping the Earth from major glaciations, especially during periods of positive CIEs, when increased organic carbon burial would have drowned down atmospheric CO₂.

Chromium Isotope Systematics in Modern and Ancient Microbialites

Changes in stable chromium isotopes (denoted as δ53Cr) in ancient carbonate sediments are increasingly used to reconstruct the oxygenation history in Earth’s atmosphere and oceans through time. As a significant proportion of marine carbonate older than the Cambrian is microbially-mediated, the utility of δ53Cr values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and δ53Cr values of modern marginal marine and non-marine microbial carbonates. These data are supported by stable C and O isotope compositions, as well as rare earth elements and yttrium (REY) concentrations. In addition, we present data on ancient analogs from Precambrian strata. Microbial carbonates from Marion Lake (Australia, δ53Cr ≈ 0.99‰) and Mono Lake (USA, ≈0.78‰) display significantly higher δ53Cr values compared with ancient microbialites from the Andrée Land Group in Greenland (720 Ma, ≈0.36‰) and the Bitter Springs Formation in Australia (800 Ma, ≈−0.12‰). The δ53Cr values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable δ53Cr values in skeletal carbonates, do not induce variability in δ53Cr values in microbialites. Together with stable C and O isotope compositions and REY patterns, δ53Cr values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and δ53Cr results of ancient microbial carbonates agree well with data of abiotically precipitated carbonates of the Proterozoic. If detrital contamination is carefully assessed, microbialites have the potential to record the δ53Cr values of the waters from which they precipitated. However, it remains unclear if these δ53Cr values record (paleo-) redox conditions or rather result from other physico-chemical parameters.

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

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.