A closed-basin alkaline lake on the basaltic plateau of Tigray, Ethiopia, Lake Ashenge hosts living and fossil stromatolites composed primarily of magnesium-bearing calcite. In the present study, the morphogenetic and preservational processes that underlie stromatolite formation in such extreme environments were investigated, with implications for biosignature preservation. Using a combination of petrographic, spectroscopic, and microscopic techniques, we identified fossilized biomass, including microbial mat remnants, filamentous cyanobacteria, microfossil-like structures, and amorphous organic matter. Micritic and microsparitic Mg-calcite layers were found to preserve abundant sheaths, cyanobacterial filaments, and extracellular polymeric substances (EPS), which suggest a high fossilization potential. EPS-rich sheaths frequently contained stevensite (a Mg-silicate), which contributed to mold preservation. Subaqueous precipitation of Mg-calcite in the presence of organic biomass enhanced the entombment of microbial material and facilitated biosignature retention. Organic geochemical analyses revealed algaenan-like aliphatic structures associated with cyanobacterial cell walls, which are known for their resistance to degradation. These findings position Lake Ashenge as a relevant planetary field analog for ancient terrestrial and martian (habitable) lacustrine systems and advance our understanding of microbial fossilization pathways in alkaline settings. Since Mg-bearing phases, particularly Mg-calcite and stevensite, were found to be critical for biosignature preservation, such findings should guide sample collection and geological analyses on Mars, particularly in the framework of the Mars Sample Return mission.
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