Microfossils from the late Mesoproterozoic - early Neoproterozoic Atar/El Mreiti Group, Taoudeni Basin, Mauritania, northwestern Africa

The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/El Mreïti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreïti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of organic-rich laminae interpreted as benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a circular opening interpreted as a sophisticated circular excystment structure (a pylome), and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well-dated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreïti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicates that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy.Research support from BELSPO IAP PLANET TOPERS to J. Beghin (PhD scholarship) and E.J. Javaux (PI), and European Research Council (ERC) Stg ELiTE FP7/308074 to J.-Y. Storme (postdoc fellowship) and E.J. Javaux (PI) are gratefully acknowledged. J.J. Brocks acknowledges support from the Australian Research Council (DP1095247)

A palaeoecological model for the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa

Reconstructing the spatial distribution of early eukaryotes in palaeoenvironments through Proterozoic sedimentary basins provides important information about their palaeocology and taphonomic conditions. Here, we combine the geological context and a reconstruction of palaeoenvironmental redox conditions (using iron speciation) with quantitative analysis of microfossil assemblages (eukaryotes and incertae sedis), to provide the first palaeoecological model for the Atar/El Mreïti Group of the Taoudeni Basin. Our model suggests that in the late Mesoproterozoic – early Neoproterozoic, the availability of both molecular oxygen and nutrients controlled eukaryotic diversity, higher in oxic shallow marginal marine environments, while coccoidal colonies and benthic microbial mats dominated respectively in anoxic iron-rich and euxinic waters during marine highstands or away from shore where eukaryotes are lower or absent

Microfossils from the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa

The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/El Mreïti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreïti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a pylome interpreted as a sophisticated circular excystment structure, and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in well-dated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreïti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicate that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy

Late Mesoproterozoic Microbial Communities

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

1.1 billion years old porphyrins and their isotopic composition establish a marine ecosystem dominated by bacterial primary producers

The average cell size of marine phytoplankton is critical for the flow of energy and nutrients from the base of the food web to higher trophic levels. Thus, the evolutionary succession of primary producers through Earth’s history is important for our understanding of the radiation of modern protists ∼800 million years ago and the emergence of eumetazoan animals ∼200 million years later. Currently, it is difficult to establish connections between primary production and the proliferation of large and complex organisms because the mid-Proterozoic (∼1,800–800 million years ago) rock record is nearly devoid of recognizable phytoplankton fossils. We report the discovery of intact porphyrins, the molecular fossils of chlorophylls, from 1,100-million-year-old marine black shales of the Taoudeni Basin (Mauritania), 600 million years older than previous findings. The porphyrin nitrogen isotopes (δ15Npor = 5.6–10.2‰) are heavier than in younger sedimentary sequences, and the isotopic offset between sedimentary bulk nitrogen and porphyrins (εpor = −5.1 to −0.5‰) points to cyanobacteria as dominant primary producers. Based on fossil carotenoids, anoxygenic green (Chlorobiacea) and purple sulfur bacteria (Chromatiaceae) also contributed to photosynthate. The low εpor values, in combination with a lack of diagnostic eukaryotic steranes in the time interval of 1,600–1,000 million years ago, demonstrate that algae played an insignificant role in mid-Proterozoic oceans. The paucity of algae and the small cell size of bacterial phytoplankton may have curtailed the flow of energy to higher trophic levels, potentially contributing to a diminished evolutionary pace toward complex eukaryotic ecosystems and large and active organisms

1.1-billion-year-old porphyrins establish a marine ecosystem dominated by bacterial primary producers

The average cell size of marine phytoplankton is critical for the flow of energy and nutrients from the base of the food web to higher trophic levels. Thus, the evolutionary succession of primary producers through Earth’s history is important for our understanding of the radiation of modern protists ∼800 million years ago and the emergence of eumetazoan animals ∼200 million years later. Currently, it is difficult to establish connections between primary production and the proliferation of large and complex organisms because the mid-Proterozoic (∼1,800–800 million years ago) rock record is nearly devoid of recognizable phytoplankton fossils. We report the discovery of intact porphyrins, the molecular fossils of chlorophylls, from 1,100-million-year-old marine black shales of the Taoudeni Basin (Mauritania), 600 million years older than previous findings. The porphyrin nitrogen isotopes (δ15Npor = 5.6–10.2) are heavier than in younger sedimentary sequences, and the isotopic offset between sedimentary bulk nitrogen and porphyrins (epor = −5.1 to −0.5) points to cyanobacteria as dominant primary producers. Based on fossil carotenoids, anoxygenic green (Chlorobiacea) and purple sulfur bacteria (Chromatiaceae) also contributed to photosynthate. The low epor values, in combination with a lack of diagnostic eukaryotic steranes in the time interval of 1,600–1,000 million years ago, demonstrate that algae played an insignificant role in mid-Proterozoic oceans. The paucity of algae and the small cell size of bacterial phytoplankton may have curtailed the flow of energy to higher trophic levels, potentially contributing to a diminished evolutionary pace toward complex eukaryotic ecosystems and large and active organisms.This work was supported in part by Australian Research Council Grants DP1095247 and DP160100607 (to J.J.B.) and by Belgian Science Policy Interuniversity Attraction Pole “PLANET TOPERS” and Euro- pean Research Council Starting Grant ELiTE FP7/308074 (to E.J.J. and J.B.). A portion of the work was performed at the National High Magnetic Field Laboratory at Florida State University, which is supported by the National Science Foundation through Grant DMR 11-57490 and the State of Florida. C.J.B. publishes with the permission of the chief executive officer of Geoscience Australia