Introduction: There is much convincing palaeobiological testament to life in the Palaeoarchaean, but most such evidence is based on morphological or isotopic signals [1], which are rarely assimilated into calibrated, holistic models of Earth-Life co-evolution. Consequently, the ecosystem-level appraisal of early life remains an open question. We here present a multi-scalar (from stratigraphic interval to micron-scale fossil), trace and rare earth element assessment of the environments colonised by early life that may provide indications of the metabolic networks of ancient microbial communities.

I: Trace element biosignatures: Despite their scarcity in modern environments, trace elements are key proteometallomic components and enzymatic cofactors. It has been suggested that this unusual biological chemistry of elements is the result of the richness of elements, now occurring in trace amounts, in the environments of early life and, thus, that palaeoenvironment was the gross control on early microbial evolution [2]. Using high-resolution elemental mapping (particle-induced X-ray emission spectroscopy), we show that common elemental associations in certain morphologies of Palaeoarchaean carbonaceous material match the expected metallomes of anaerobic, thermophilic micro-organisms, including Fe, Ni, Cu, V and Co [cf. 3]. Measurements on recently degraded microbial material showing that trace element signatures have the potential to be preserved over many decades demonstrate the validity of such elemental signatures – when recurrent in fossilized carbonaceous material – as a powerful, micro-scale, agnostic biosignature in the absence of obvious cellular preservation and a means of qualitatively estimating environmental parameters [4]. Relative concentrations of trace elements in controversial fossil material may be used to resolve debates regarding their biogenicity.

II: Biomes of Palaeoarchaean life:At millimetric and centimetric scales within the stratigraphy of fossiliferous horizons, the normalised rare earth element plus yttrium (REE+Y) patterns of Precambrian sediments can be considered equivalent to those of younger sediments [5] and may be used to infer the aqueous geochemistry of the palaeoenvironment [6]. Studying four fossiliferous horizons spanning 150 Ma of the Palaeoarchaean of South Africa (the 3.472 Ga Middle Marker horizon, 3.45 Ga Hooggenoeg chert H5c, 3.334 Ga Footbridge Chert and 3.33 Ga Josefsdal Chert), we show that major photosynthetic microbial biomes of this time existed in localised, restricted epicontinental basins strongly influenced by major continental inputs, marine recharge and high-temperature hydrothermal influence [7]. This underlines the hitherto unrecognised importance of terrigenous inputs for the flourishing of life early in Earth history [7]. The importance of both continental inputs and hydrothermally influenced marine inputs are evident in that it is within horizons showing a strong combined signature (mixing/palaeoenvironmental disequilibria) that the most well-developed microbial biosignatures occur.

Palaeoarchaean Earth-Life co-evolution:The highly complementary approaches of in situtrace and rare earth elements provide a fossil-calibrated framework within which one can appraise the early evolution of life. A time series approach to deducing the relative importance of biomes in fossiliferous horizons through time could, if accounting for the preservation potentials of individual palaeoenvironments, aid our understanding of microbial evolutionary trajectories through the lens of environmental reconstruction.

References: Hickman-Lewis, K., et al. (2018) In Earth's Oldest Rocks(Eds. Van Kranendonk, M.J., et al.); [2] Fraústo da Silva, J.J.R., Williams, R.J.P. (2001) OUP; [3] Zerkle, A.L., et al., Am. J. Sci.305, 467–502; [4] Hickman-Lewis, K., et al., in revision, Nature Geoscience; [5] Shields, G., Webb, G. (2004), Chem. Geol.204, 103–107; [6] Gourcerol, B., et al. (2016), Precam. Res.281, 47–79; [7] Hickman-Lewis, K., et al., submitted, Geochimica et Cosmochimica Acta.