Reconstructing Palaeoarchaean microbial biomes flourishing in the presence of emergent landmasses using trace and rare earth element systematics

Palaeoarchaean cherts preserve some of the most ancient traces of life, but such palaeobiological testament is rarely assimilated into fossil-calibrated ecosystem or biome models. Trace and rare earth element plus yttrium (REE+Y) compositions reliably decode the palaeodepositional settings of these cherts, and thus constrain the environments within which early microbial life flourished. Herein, we present a systematic study comparing bulk inductively coupled plasma mass spectrometry (ICP-MS) of four cherts from the Barberton greenstone belt, South Africa (the 3.472 Ga Middle Maker horizon, 3.45 Ga Hooggenoeg H5c chert, 3.334 Ga Footbridge Chert and ~3.33 Ga Josefsdal Chert), with in situ laser ablation (LA) ICP-MS transects through multiple microbial laminations therein. In situ analyses can be interpreted in the framework of bulk analyses, correlating microbial biomes to their regional and local geochemical environments. Bulk ICP-MS analyses typically exhibit fractionated REE+Y patterns typical of anoxic hydrogenous sedimentation when normalised to Mud from Queensland (MUQ), supporting previous assertions that the Palaeoarchaean habitable realm was a hydrothermally influenced ocean. Suppressed La, Eu and Y anomalies, together with largely supra-chondritic Y/Ho ratios, however, indicate some degree of restriction from the open ocean and the influence of non-marine waters. In situ LA ICP-MS transects through microbial mat horizons are characterised by flat, LREE-enriched MUQ-normalised REE+Y patterns indicating strong non-marine influences from terrigenous, riverine fluids, although recurrent resurgences of marine REE+Y chemistry (increased Y/Ho ratios, La and Y anomalies) occur within the microbial laminations themselves. These findings show that widespread microbial life developed in semi-restricted basins with strong terrigenous, continental influences from mixed mafic and felsic erosion sources (estimated by trace element compositions and Cr/V versus Y/Ni trace element plots). Periodic seawater recharge into these basins generated disequilibrium conditions under which microbial life flourished. This systematic approach highlights both the development of emergent, volcanic, continental landmasses in the Palaeoarchaean, and their influence as loci for early microbial biomes, more than 100 Ma before the earliest evidence for large-scale terrestrial ecosystems. Throughout the Palaeoarchaean, semi-restricted, epicontinental basins represent an important, hitherto unrecognised, microbial habitat for some of the earliest photosynthetic life and may have been a precursor and contemporary to terrestrial microbial ecosystems, such as the lacustrine and riverine biomes identified in Mesoarchaean and Neoarchaean sequences. Understanding the distribution of Archaean life in terms of biomes should form a keystone of the strategy for Precambrian palaeoecology. The relative importance of biomes through time could, if accounting for the preservation potentials of individual palaeoenvironments, illuminate microbial evolutionary trajectories through the lens of environmental reconstruction.