Tracing the source and transformations of organic matter in a Mars analogue lava tube using Py-CSIA

Volcanic speleothems are promising archives of organic matter (OM) in planetary analogue environments, offering valuable insights into past or extant microbial life and environmental conditions. In this study, we applied elemental analysis–isotope ratio mass spectrometry (EA/IRMS), analytical pyrolysis (Py-GC/MS), and pyrolysis-compound-specific isotope analysis (Py-CSIA) to characterize the molecular and isotope composition of four samples collected from the Corona Lava Tube system in Lanzarote (Canary Islands, Spain). This site, explored during the ESA PANGAEA-X astronaut training campaign, serves as a natural laboratory for Mars subsurface analogues. We investigated two distinct cave samples (a black, organic-rich microbial mat and a white, mineral-dominated deposit), along with overlying topsoil and Euphorbia balsamifera vegetation, to trace the origin, transformation, and preservation of OM in this extreme subsurface environment. Stable isotope data and compound-specific signatures revealed marked differences in carbon and hydrogen isotope compositions between surface- and cave-derived organics, indicating divergent biogeochemical pathways and microbial activity. Notably, ¹³C-enrichment in sterols and lignin-derived compounds within the cave matrix pointed to intense microbial processing, while δ ²H data reflected the incorporation of past meteoric water and diagenetic alteration. Our findings demonstrate the power of Py-CSIA for resolving biosignatures at the molecular level and underscore the diagnostic potential of isotope tools in volcanic subsurface systems. This approach provides a critical framework for future astrobiological exploration and life-detection strategies in Martian lava tubes, where subtle organic traces may offer the clearest evidence of habitability or life beyond Earth. • Py-CSIA enables unprecedented resolution in tracing organic matter in lava tubes • Microbial transformation of plant-derived organic matter in a nutrient-poor cave ecosystem • δ²H signatures suggest diagenetic alteration linked to ancient meteoric water • Pyrolysis-coupled isotope analysis offers high-resolution biosignature detection valuable for astrobiology