Although convincing evidence for life extends to almost 3.5 Ga, much of the Archaean fossil record is poorly preserved, largely due to pervasive diagenesis and high-pressure high-temperature metamorphism. Exceptional preservation of biogeochemical complexity in the Precambrian is largely limited to cherts, phosphates and shales; however, ancient fossils, including microbial mats and microbially induced sedimentary structures, also occur, more rarely, in poorly sorted, coarse-grained siliciclastic sedimentary rocks. Regrettably, the precise micromechanics by which exceptional retention of organic microbial traces occur within such rocks over billion-year geological timescales remain poorly understood.

Herein, using a combination of optical and electron microscopy (including high-resolution TEM), Raman and infrared microspectroscopy, electron energy loss spectroscopy, synchrotron-based scanning transmission X-ray microscopy and NanoSIMS, we explore the micro–nano-scale characteristics of microbial mats preserved in ~2.9 billion-year-old sandstones from the Mosquito Creek Formation (Pilbara, Australia). By using a suite of advanced spatially correlated microscopy and geochemistry techniques, we demonstrate that sedimentary horizons rich in K–Al-phyllosilicates exhibit exceptional and unexpected preservation of biogeochemical complexity in organic materials despite the age and metamorphic grade of the sequence.

We propose that authigenic phyllosilicates intimately intercalated with microbial kerogen at the nanoscale promote the preservation of nanoscopic domains of poorly ordered amorphous and turbostratic carbonaceous materials. These nanoscopic domains, though comprising a very small fraction of the volume of the rocks, archive an exceptional level of primary biogeochemical information. The unexpected preservation of these organic materials can be interpreted through pressure compensation associated with the kaolinite–illite transition during burial diagenesis and metamorphism, leading to the development of pressure shadows that impede the maturation of organic materials during diagenesis and metamorphism. Clay minerals also likely obstruct the flow of oxidising fluids through the rock, thereby protecting organic materials from dynamic degradative influences.

Elucidating organic preservation in coarse-grained siliciclastics opens new avenues for biosignature searches in ancient Earth sequences, suggesting that siliciclastic sequences represent non-traditional biosignature repositories with unrealised biosignature preservation potential. One can also consider identifying localities on Mars that might preserve organic materials through similar mechanisms. For example, phyllosilicate-bearing sandstones have been collected from the Jezero delta front by the Mars 2020 Perseverance rover for near-future sample return. Although the amount of organic material preserved in these Martian sedimentary rocks appears to be very low, potential nanodomains of organic carbon preserved within could yield a large diversity of data concerning the Martian carbon cycle.

Archaean,microbial mat,biogeochemistry,nanogeoscience,clay minerals