Abstract EANA2024-65

Jezero crater is a Early Hesperian (~3.5 Ga) palaeolacustrine system on Mars. It is a closed basin with mafic/ultramafic orbital spectral signatures and lake margin magnesium carbonate-bearing horizons that could potentially preserve evidence of ancient life. The NASA Mars 2020 Perseverance rover is currently exploring Jezero, collecting geological samples, including crater margin carbonate-bearing materials, for Mars Sample Return (MSR). Here we present a study of modern stromatolite from three highly alkaline, basalt-hosted lakes: Lake Ashenge (pH ~8.8), Ethiopia, Lake Abbe (pH ~9.9), Djibouti, and the Carri Laufquen system (pH ~7.2–9.9), Argentina. These ecosystems may have formed in analogous environmental conditions to Jezero, and may therefore inform us about potential Martian microbial ecosystems and their fossilisation processes.

A multi-technique approach was used to characterise the stromatolites. The biomineralised remnants of stromatolitic biomass, including microbial mats, filamentous cyanobacterial microfossils, and extracellular polymeric substances (EPS), were observed using optical and scanning electron microscopy, Raman and FTIR microspectroscopy, and solid-state 13C NMR.

Stromatolites from all studied sites are dominated by Mg-calcite, and organic-rich layers within are also associated with microscale silicate minerals. Molds of cyanobacteria were preserved as voids, or partially filled with EPS or Mg-silicates. The fossilisation potential of microbial remnants is correlated with Si-rich phases, which could be related to weathering of the volcanic bedrock, since all lakes formed within basaltic terranes, to the dissolution of diatoms as a source of silica in Lake Ashenge and Carri Laufquen, or to hydrothermal influence in Lake Abbe. The pH values of the sites (8.8–9.9) provide an ideal environment for microbially mediated silicate formation. Authigenic microbially induced silicates, such as stevensite, have been reported in alkaline lakes as a syngenetic phase in association with carbonates in Lake Clifton, Australia. However, different clay minerals are associated with the filamentous microorganisms in our materials: the samples from Africa are dominated by Mg-silicates, whereas the Argentinian samples also show an Al-rich silicate phase. Amorphous Mg-silicates have been reported in Great Salt Lake microbialites where increased alkalinity facilitates their microbially mediated precipitation. The formation of Al-rich silicate phases, such as kaolinite, has also been experimentally proposed to occur through the precipitation of aluminosilicate gels associated with EPS; the subsequent crystallisation of kaolinite within the gel occurs due to changes in microenvironment induced by metabolic activity, suggesting syndepositional mineralisation of cyanobacterial filaments at Carri Laufquen due to carbonate–silica interplay.

Organic materials, including aromatic and aliphatic moieties, have also been identified in the stromatolites using a range of geochemical approaches; these organics are interpreted to originate from the primary stromatolite-building ecosystem. Similar biosignatures may be preserved in Jezero margin carbonate-bearing sequences if a similar microbial biosphere emerged on Mars. Such biosignatures may be detectable during rover exploration or subsequent analysis after MSR. The range of techniques applied herein includes approaches similar to rover capabilities and approaches only possible after MSR. Overall, the studied sites can provide guidance for in situ searches for biosignatures on Mars and allow the development of models of fossilisation that can be tested after MSR.