Looking for biosignatures in a pristine Mars analogue environment on Earth

The Selvagens Islands (Madeira, Portugal), located in the North Atlantic, are a small archipelago of volcanic origin formed by two main islands, which emerged in the Oligocene (25-29 Ma). These oceanic islands are recognized as a unique example of marine and terrestrial biodiversity, characterized by many endemic species. Still unblemished by civilization, this isolated and undisturbed ecosystem makes the volcanic caves from Selvagens a promising model system for investigating biosignatures preserved in the rock record valuable for astrobiology. 

The Inferno Cave, one of the three main terrestrial caves of the Selvagens Islands, has copious amounts of fine white powder crusts scattered throughout the cave. From the mineralogical point of view, the whitish powdery deposits are mainly composed of gypsum and minor amounts of clay minerals. Thermal analyses (TG/DTG-DsC) revealed the presence of labile and stable organic matter (OM), with contrasting relative abundances among the gypsum samples. The stable isotope composition of carbon was determined by elemental analysis coupled to isotope ratio mass spectrometry (EA/IRMS) to decipher the origin of the organic fraction preserved in the gypsum deposits. Two well-differentiated organic matter pools were distinguished: one comprising 𝛿¹³C values > –19 ‰ related to microbial activity (i.e., microbial degradation of fresh organic matter), and another with 𝛿¹³C < –20 ‰), which may suggest the preservation of recalcitrant biomarkers from aboveground vegetation. U-series results from the gypsum deposits were used to produce isochron ages needed to generate an age-depth model for the 1-m thick gypsum deposit.

A detailed study of the organic fraction preserved in the gypsum deposits was conducted by pyrolysis gas chromatography/ mass spectrometry (Py-GC/MS) and by GC-MS after extraction of total lipids. Preliminary results show the presence of n-alkanes of low molecular weight (<C₂₁). In addition, long-chain n-alkanes (C>₂₁) were also observed, which indicates a direct contribution of plant biomass from the overlying surface. Interestingly, the Py-GC/MS analysis also shows relatively large contents of mid-chain branched alkanes associated with a direct biological source focused on microorganisms since they are known to biosynthesize such alkanes. A predominance of chemolithotrophic microbial communities was found based on the 16S rRNA gene analysis, which is consistent with the biological origin of the mid-chain branched alkanes. 

The mineralogical, biogeochemical, and microbiological characterization of the gypsum samples from the pristine Selvagens Islands is thus a reliable way to infer the biogenicity of cave mineral deposits, recognize biosignatures, and determine paleoenvironmental changes from a natural environment, free from anthropogenic influence. Considering the analogies with Mars, the Selvagens Islands are also suitable for space research, including astronaut training and rover testing for future planetary missions.

Acknowledgements: This work received support from the Portuguese Foundation for Science and Technology (FCT) under the MICROCENO project (PTDC/CTA-AMB/0608/2020). The financial support from the Spanish Ministry of Science and Innovation (MCIN) under the research project TUBOLAN PID2019-108672RJ-I00 funded by MCIN/AEI/ 10.13039/501100011033 is also acknowledged. A.Z.M. was supported by the CEECIND/01147/2017 contract from FCT, and the Ramón y Cajal contract (RYC2019-026885-I) from the MCIN.