water conditions existed during OMaccumulation. The presence of β -carotane, gammacerane and traces of arylisoprenoids could indicate a stratified water column. OMin a few samples with lower than average hydrogen index (HI) is mainly of terrestrial origin with a higher proportion of C steranes, diterpanes and oleanoids and lower concentrations of both saturated and aromatic 29 hopanes. Higher Pr/Ph, a higher C isohopane ratio (IHR34) and a lower homohopane index (HHI) indicate 34 that this low-HI OMwas originally deposited under more oxic conditions and subsequently transported to deeper water. Low sterane and hopane isomerization ratios show that no samples have reached the oil-window. Some novel maturity parameters for immature lacustrine samples are proposed.

O'Neill, C., Marchi, S., Bottke, W., Fu, R., 2019. The role of impacts on Archaean tectonics. Geology 48, 174-178.

https://doi.org/10.1130/G46533.1 Field evidence from the Pilbara craton (Australia) and Kaapvaal craton (South Africa) indicate that modern tectonic processes may have been operating at ca. 3.2 Ga, a time also associated with a high density of preserved Archaean impact indicators. Recent work has suggested a causative association between large impacts and tectonic processes for the Hadean. However, impact flux estimates and spherule bed characteristics suggest impactor diameters of <100 km at ca. 3.5 Ga, and it is unclear whether such impacts could perturb the global tectonic system. In this work, we develop numerical simulations of global tectonism with impacting effects, and simulate the evolution of these models throughout the Archaean for given impact fluxes. We ∼ demonstrate that moderate-size ( 70 km diameter) impactors are capable of initiating short-lived subduction, and that the system response is sensitive to impactor size, proximity to other impacts, and also lithospheric thickness gradients. Large lithospheric thickness gradients may have first appeared at ca. 3.5 - 3.2 Ga as cratonic roots, and we postulate an association between Earth's thermal maturation, cratonic root stability, and the onset of widespread sporadic tectonism driven by the impact flux at this time. GeoRef Subject

Orsi, W.D., Vuillemin, A., Rodriguez, P., Coskun, Ö.K., Gomez-Saez, G.V., Lavik, G., Morholz, V., Ferdelman, T.G., 2020. Metabolic activity analyses demonstrate that Lokiarchaeon exhibits homoacetogenesis in sulfidic marine sediments. Nature Microbiology 5, 248-255.

https://doi.org/10.1038/s41564-019-0630-3 The genomes of the Asgard superphylum of Archaea hold clues pertaining to the nature of the host cell that acquired the mitochondrion at the origin of eukaryotes. Representatives of the Asgard candidate phylum Candidatus Lokiarchaeota (Lokiarchaeon) have the capacity for acetogenesis and fermentation, but how their metabolic activity responds to environmental conditions is poorly understood. Here, we show that in anoxic Namibian shelf sediments, Lokiarchaeon gene expression levels are higher than those of bacterial phyla and increase with depth below the seafloor. Lokiarchaeon gene expression was significantly different across a hypoxic - sulfidic redox gradient, whereby genes involved in growth, fermentation and H -dependent carbon 2 fixation had the highest expression under the most reducing (sulfidic) conditions. Quantitative stable isotope probing revealed that anaerobic utilization of CO and diatomaceous extracellular polymeric substances by 2 Lokiarchaeon was higher than the bacterial average, consistent with higher expression of Lokiarchaeon genes, including those involved in transport and fermentation of sugars and amino acids. The quantitative stable Candidatus isotope probing and gene expression data demonstrate homoacetogenic activity of Lokiarchaeota, whereby fermentative H production from organic substrates is coupled with the Wood - Ljungdahl carbon 2 8 fixation pathway. The high energetic efficiency provided by homoacetogenesis helps to explain the elevated metabolic activity of Lokiarchaeon in this anoxic, energy-limited setting.