Late Mesoproterozoic Microbial Communities

Abstract

The first eukaryotes are found in the geological record at ~1.6 Ga, a further 800 million years later they became more abundant and diverse, and only during the Ediacaran did they start shaping ecosystems. This work studies a marine and a lacustrine aquatic ecosystem at the edge of the Mesoproterozoic (~1.1 Ga) to gauge the role of eukaryotes and to investigate the environmental conditions that may have prohibited their proliferation. The evolutionary state of the earliest eukaryotic fossils remains unresolved. The first unambiguous stem group eukaryote appears at 1.2 Ga, but modern ferns occur around 0.8 Ga. Extreme bias on fossil preservation prevents estimation on how ecologically abundant early eukaryotes were. Here we use biomarkers to close this gap. They have low taxonomic resolution but afford a quantitative view of relative organism abundances. We combine biomarkers with inorganic, isotope geochemical and microscope analysis to investigate successions of the marine Taoudeni Basin and lacustrine Nonesuch Formation. Further, we include an analysis of the Cretaceous Maracaibo Basin to obtain a clear point of contrast from a period of time where redox environments were similar but eukaryotes were abundant. The extraordinary black shales of the Taoudeni Basin have high TOC (< 31 %), lack eukaryotic steranes despite present eukaryotic microfossils, contain aromatic steroids, and are mostly deposited under ferruginous and euxinic conditions. This implies at first sight a stagnant deep water environment. Yet, clear crinkly mats are preserved, invoking a non-uniformitarian ecosystem. Low atmospheric oxygen levels facilitate to explain clear, anoxic, shallow (<20 m) waters above phototrophic microbial mats. Biomarker data imply that the microbial community was composed of cyanobacteria, anoxygenic purple and green sulfur bacteria, and microaerophilic methanotrophs. It is likely that cyanobacteria switched between oxic and anoxic photosynthesis and dominated the photosynthetic community. The latter is supported by nitrogen isotopic composition of individual porphyrins, which range between 5.6 and 10.2 per mil and yield epsilon-porphyrin values of 0.5 to - 5.1 per mil. This study is the first unambiguous report of Mesoproterozoic geoporphyrins. The dominant species contain Ni and their structures relate to chl a, chl b/chl c3 and a chl c-like molecule. The biomarker and iron speciation results of Nonesuch shales qualitatively resemble the ones of the marine Taoudeni Basin including a mainly ferruginous depositional setting, absence of diagnostic eukaryotic biomarkers despite eukaryotic microfossils and biomarkers specific for cyanobacteria, anoxygenic purple and green sulfur bacteria, and microaerophilic methanotrophs. The bitumens of the Phanerozoic Maracaibo Basin were composed of degradation products of marine algae, green sulfur bacteria and archaea as well as terrestrial higher plants and lacustrine algae. The mixing of two components, marine and terrestrial organic matter, can explain the distribution of biomarkers. The data describe the restricted Maracaibo Basin as a stable, stratified sea influenced by upwelling waters near a shallow shelf. The results exemplify that biomarkers of primary producers such as algae are in fact preserved in similar environments as in the Mesoproterozoic and that the absence in ~1 Ga samples is not a preservation artefact. Show more