Characterization of the geochemistry and organic content of Martian analog environments (reactive gossans) in the Canadian Arctic to assess their astrobiological potential.

Characterization of the geochemistry and organic content of Martian analog environments (reactive gossans) in the Canadian Arctic to assess their astrobiological potential.

1. Context

The past existence of potentially habitable environments on Mars [1-4] raises the question of whether any organic and mineralogical biosignatures have been preserved within the weathered Martian rock formations. However, the extreme physicochemical conditions prevailing on the surface of Mars, combined with oxidation and weathering processes, considerably complicate the identification and interpretation of such signatures [5-7].

In this context, the study of Mars analog environments is an essential approach for better understanding the interactions between geochemistry, mineralogy, and the preservation of organic matter under Mars-like conditions [5,8-11]. Among these, reactive gossans of the Canadian Arctic are of particular interest. These formations, resulting from the oxidation of sulfide deposits and subject to seasonal chemical weathering in a permafrost environment, constitute geochemically active environments capable of supporting sustained microbial activity [5,8,9,12-15]. Since similar conditions may have existed on Mars, these systems represent relevant analogs for studying Martian astrobiological potential and identifying preserved biosignatures.

This work aims to characterize the geochemistry and organic content of reactive gossans to assess their astrobiological potential and their relevance for interpreting future in situ observations of Mars.

2. Method

2.1 Field sampling

Two field campaigns were conducted on Axel Heiberg Island, Nunavut, to investigate six reactive gossans sites [12,13]. We focus on three of them: Color Ridge (CR-G3), White Glacier (WG-G1), and White Glacier Vein Array (WGVA-G1). Sampling was conducted at the surface and at depth to characterize the geochemical and organic evolution of the various strata. Six strata were sampled at CR-G3, four at WG-G1, and 12 at WGVA-G1. Sterile and non-sterile sampling protocols were applied depending on the planned laboratory analyses. Samples were stored at subzero temperatures to minimize the alteration of organic carbon.

2.2 Laboratory analyses

Quantitative and semi-quantitative characterizations included elemental analyses of sulfur, carbon, and nitrogen using an elemental analyzer; loss-on-ignition measurements to determine total organic carbon, total inorganic carbon, and total carbon; and pH

measurements in aqueous and saline solutions. Mineralogical and geochemical analyses were performed using WDXRF, XRD, and SEM-EDX. SEM-EDX analyses enabled the characterization of the elemental composition and mineralogy of structures or anomalies observed at the microscopic scale.

Qualitative characterizations were based on Raman spectroscopy, XRD, WDXRF, and SEM-EDX. Raman spectroscopy was used to identify mineral phases and to detect the D and G bands associated with the presence of organic carbonaceous material. An application was developed in-house to automate the interpretation of Raman spectra.

Some samples were metallized for observation under an SEM to identify any fossilized microstructures that might be associated with ancient microbial activity. This exploratory approach also aimed to assess the potential of this instrumentation for detecting morphological biosignatures in this type of Martian-analog environment.

3. Results

Preliminary results reveal significant geochemical, mineralogical, and organic heterogeneity among the three gossans studied. pH measurements show a marked contrast between the sites, with highly acidic conditions for CR-G3 (pH 2.2-2.7), acidic to near-neutral conditions for WG-G1, and neutral to slightly alkaline conditions for WGVA-G1. This variability reflects different degrees of oxidation and weathering within the profiles, linked to the reactivity of the sulfide phases.

Elemental analyses of sulfur, carbon, and nitrogen also reveal significant differences between sites and strata. CR-G3 exhibits the highest and most variable sulfur contents, suggesting the persistence of residual sulfides and/or the formation of secondary sulfates. Carbon contents measured by elemental analysis remain generally low, while nitrogen remains near detection limits, indicating a low contribution of nitrogenous organic matter. At the same time, loss-on-ignition measurements show mass losses that are sometimes more significant, which can be interpreted as a contribution from total organic carbon or carbonates depending on the temperature ranges considered. However, a discrepancy is observed between the results from elemental analysis and those obtained by loss on ignition. This difference indicates that the losses measured during heating do not exclusively reflect organic matter but may also be related to the dehydration of hydrated phases, the transformation of sulfates, or the decomposition of certain carbonate phases. These results highlight the need for a combined interpretation of elemental, mineralogical, and thermal data to better constrain the origin of mass losses and the actual preservation of organic carbon in these environments.

The WDXRF data indicate a composition dominated by SiO₂, Al₂O₃, and Fe₂O₃, with local enrichments in CaO, Na₂O, MgO, and SO₃ depending on the location. CR-G3 is characterized by high variability in Fe₂O₃, consistent with the oxidation of iron-rich sulfide phases. XRD analyses confirm this contrasting mineralogy, with a matrix dominated by

albite, with residual pyrite and secondary phases such as bassanite/gypsum, barite, and jarosite. The presence of jarosite is particularly significant, as it indicates acidic and oxidizing conditions relevant to altered Martian environments.

Raman and SEM-EDX analyses complement these observations by revealing fine mineral phases, Fe-S-rich structures, and potential carbon signatures, notably through the presence of D and G bands around 1350 and 1600 cm⁻¹, as well as light, carbon-rich structures observed in BSE. Filamentous or curved morphologies remain exploratory but constitute interesting targets for the evaluation of potential morphological biosignatures.

These results highlight the value of reactive gossans as Martian analogs for studying the relationships between sulfide weathering, pH gradients, the preservation of organic carbon, and the detection of potential biosignatures.

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