Biosignature Investigation at Lake Ashenge, Ethiopia: Implications for Martian Paleolacustrine Environments

Jezero crater is a paleolacustrine system on Mars, active until the Early Hesperian era (~3.5 Ga). It features a closed-basin structure with distinct mafic and ultramafic spectral signatures and an elevated zone containing unique lake margin magnesium carbonates that could potentially hold evidence of ancient life. Terrestrial field analogue sites are key for investigating the environmental and mineralogical factors affecting potential biosignature preservation on Mars, but such sites are rare. Lake Ashenge is an alkaline closed-basin lake, developed on the basaltic plateau of Tigray, NE Ethiopia. Living and fossil stromatolites composed of magnesium carbonates occur on its shores. Our aim is to characterize Lake Ashenge stromatolites, in terms of their morphogenetic and preservational processes, in order to understand stromatolite development in extreme alkaline environments that are potentially analogous to Martian paleolake systems. Our primary focus was analyzing field samples to image entombed biomass mineralized within carbonate phases. This entombed biomass includes microbial mat remnants, organic structures resembling microbes (microfossils), and amorphous organics encapsulated by minerals, which were observed using optical and scanning electron microscopy (SEM). Carbonaceous materials, including biomolecules, in the stromatolites were characterized using Raman microspectroscopy and solid 13C nuclear magnetic resonance (NMR). Thin sections revealed abundant filamentous structures resembling cyanobacteria within layers of micritic and microsparitic Mg-calcite, suggesting a high potential for fossilization. SEM examination detected amorphous organics resembling extracellular polymeric substances (EPS) and organic tubes, interpreted as filamentous cyanobacterial sheaths, commonly filled with a Mg-silicate phase, contributing to the preservation of sheath molds. We identified Mg-calcite as the main phase forming the stromatolites using X-ray diffraction and Raman spectroscopy. Mg-calcite precipitates subaqueously in biomass-rich environments, significantly increasing the likelihood of entombment of organic materials and enhancing biosignature preservation. Abundant EPS in the studied stromatolites further contributes to a higher probability of biosignature preservation, as more microbes could potentially be concentrated, entombed and preserved. NMR spectra showed aliphatic structures, corresponding to the algaenan biomacromolecule, which is present in the cell walls of cyanobacteria and known to be resistant to degradation. Lake Ashenge offers a potential modern analogue for habitable environments on ancient Mars. The importance of Mg-calcite, other calcium carbonates, and minor Mg-silicate phases in these systems should be considered when selecting rock formations and outcrops for sample collection at the margins of Jezero crater.