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.
Highlights
Detecting early life on Earth is challenging, as many potential biosignatures may be explained by abiotic processes
Lenses and layers of white quartz within the black chert facies vary in thickness and are wavy (Fig 4A)
Our findings support previous interpretations that the Strelley Pool Formation was deposited in a shallow marine microbial mat environment, and that carbonate precipitation and silicification occurred during sedimentation and earliest diagenesis prior to the deposition of the overlying Member IV (e.g., [5], [7]-[9], [11], [18]-[20], [22], [35])
Summary
Detecting early life on Earth is challenging, as many potential biosignatures may be explained by abiotic processes. It has been suggested that potential biosignatures in Paleoarchean rocks should only be considered valid if all possible pathways of abiological formation are ruled out (i.e. the “null hypothesis”; [1]–[4]). This approach is problematic, as single rock characteristics can commonly be explained by several processes, making the identification of any unambiguous traces of early life on Earth almost impossible. It has been shown that abiotically formed stromatolite-like structures in natural environments (e.g., geyserites; [29]) and in laboratory experiments (e.g., from the precipitation of synthetic colloids; [33]) can exhibit complex characteristics such as columnar and branched growth forms, non-isopachous laminae and wrinkle structures that are commonly regarded as hallmarks of biogenicity
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