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. 2016 Jan 25;11(1):e0147629.
doi: 10.1371/journal.pone.0147629. eCollection 2016.

A Rare Glimpse of Paleoarchean Life: Geobiology of an Exceptionally Preserved Microbial Mat Facies from the 3.4 Ga Strelley Pool Formation, Western Australia

Jan-Peter Duda  1   2 , Martin J Van Kranendonk  3 , Volker Thiel  1 , Danny Ionescu  4 , Harald Strauss  5 , Nadine Schäfer  1 , Joachim Reitner  1   2
Affiliations

A Rare Glimpse of Paleoarchean Life: Geobiology of an Exceptionally Preserved Microbial Mat Facies from the 3.4 Ga Strelley Pool Formation, Western Australia

Jan-Peter Duda et al. PLoS One. .

Abstract

Paleoarchean rocks from the Pilbara Craton of Western Australia provide a variety of clues to the existence of early life on Earth, such as stromatolites, putative microfossils and geochemical signatures of microbial activity. However, some of these features have also been explained by non-biological processes. Further lines of evidence are therefore required to convincingly argue for the presence of microbial life. Here we describe a new type of microbial mat facies from the 3.4 Ga Strelley Pool Formation, which directly overlies well known stromatolitic carbonates from the same formation. This microbial mat facies consists of laminated, very fine-grained black cherts with discontinuous white quartz layers and lenses, and contains small domical stromatolites and wind-blown crescentic ripples. Light- and cathodoluminescence microscopy, Raman spectroscopy, and time of flight-secondary ion mass spectrometry (ToF-SIMS) reveal a spatial association of carbonates, organic material, and highly abundant framboidal pyrite within the black cherts. Nano secondary ion mass spectrometry (NanoSIMS) confirmed the presence of distinct spheroidal carbonate bodies up to several tens of μm that are surrounded by organic material and pyrite. These aggregates are interpreted as biogenic. Comparison with Phanerozoic analogues indicates that the facies represents microbial mats formed in a shallow marine environment. Carbonate precipitation and silicification by hydrothermal fluids occurred during sedimentation and earliest diagenesis. The deciphered environment, as well as the δ13C signature of bulk organic matter (-35.3‰), are in accord with the presence of photoautotrophs. At the same time, highly abundant framboidal pyrite exhibits a sulfur isotopic signature (δ34S = +3.05‰; Δ33S = 0.268‰; and Δ36S = -0.282‰) that is consistent with microbial sulfate reduction. Taken together, our results strongly support a microbial mat origin of the black chert facies, thus providing another line of evidence for life in the 3.4 Ga Strelley Pool Formation.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Study area (modified after [90], with permission from the copyright holder, the Geological Survey of Western Australia).
The analyzed black chert facies crops out at the Trendall locality in the northern part of Western Australia.
Fig 2
Fig 2. Stratigraphical context of the Strelley Pool Formation (representative sections; modified after [8], [35], [36], [43], [90]).
The black chert facies corresponds to Member III.
Fig 3
Fig 3. Field observations of the black chert facies.
(A) Laminated and stromatolitic carbonates (Member II of the Strelley Pool Formation) below the black chert facies. (B-E) Characteristics of the black chert facies include fenestral fabrics (B), intercalated cm-high stromatolitic layers (C) that locally show ductile deformation (dashed line in D), as well as small-scaled cm-sized cross lamination (E). (F) Conglomerate (Member IV of the Strelley Pool Formation) above the black chert facies.
Fig 4
Fig 4. Petrographic observations on the black chert facies.
(A-D) Thin sections (A, B, D: transmitted light; C: reflected light). (A) Layers are laterally not continuous in thickness but wavy. (B, C) Dark chert layers consist of a fine grained matrix that is enriched in organic matter and pyrite. Note that the organic material is laterally interwoven (B) and closely associated with pyrite (C). (B, D) Fenestrae are filled by blocky cements. (E, F) SEM photographs of the pyrite crystals. Note that the pyrite crystals are enriched in layers (E) and framboidal in shape (F). (G, H) Clusters of silicified acicular crystals, probably representing chert pseudomorphs after aragonite.
Fig 5
Fig 5. ToF-SIMS ion images of a 300μm × 300μm area of the back chert facies with an organic matter layer in the center.
The pixel brightness reflects the signal intensity of (from the left): Si = [Si]+, representing the chert matrix; Ca = sum of [Ca]+, [CaO]+, and [CaOH]+, representing CaCO3; Corg = sum of major hydrocarbon ions [C2H3]+, [C2H5]+, [C3H]+, [C3H2]+, and [C3H3]+, representing organic matter. Pixel brightness in Ca and color coded overlay of Si (red) and Corg (green) is enhanced to increase image contrast.
Fig 6
Fig 6. Thin sections (A, C) and corresponding Raman spectra (B, D; point measurements) documenting local occurrences of organic material (A, B) and dolomite (C, D) in some fenestrae.
Fig 7
Fig 7. CL overlay photographs of the black chert facies.
Note that the calcite aggregates (red luminescence) appear to be linked to organic material (dark colors). (A) Finely laminated layers of organic matter containing CaCO3 spheroids (red luminescence). (B) Close up view of (A). (C) Aggregated CaCO3 spheroids (red luminescence) associated with organic matter. (D) Close up view of (C).
Fig 8
Fig 8. NanoSIMS isotope enrichment maps of organic layers shown in Fig 7A, revealing the presence of spheroids (TSI: Total secondary ions).
Note that these bodies are generally enriched in C (12C) compared to the surrounding areas. Organic matter (12C14N) and sulfur (32S) are preferentially enriched at the edges, whereas the centers represent carbonate phases. This spatial arrangement of the isotopes is further illustrated in the color-coded overlay maps (12C & 14N; 12C & 32S).
Fig 9
Fig 9. NanoSIMS isotope enrichment profile across a spheroid (see Fig 8 for orientation of the section).
Organic matter (12C14N) and sulfur (32S) are closely associated and preferentially enriched at the edges of the body, whereas carbon (12C) is also enriched in intermediate spaces due to the presence of carbonate phases.
Fig 10
Fig 10. Fenestral carbonate facies from the Triassic Dachstein Limestone of the Northern Calcareous Alps.
This facies was formed by microbial mats in peritidal- to shallow lagoonal environments.
See this image and copyright information in PMC

References

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