Planetary habitability: volcano-sedimentological settings for biosignatures on Earth and Mars

Introduction and summary

Habitable environments presently under study on Mars are principally in shallow water, volcanic settings and influenced by later sabkha to fluvial and/or pedogenic conditions. Both Gale (MSL) and Jezero (Mars2020) craters host such sediments in which potential biosignatures have been detected [1,2]. Oxia Planum (ExoMars 2028) appears to be a mainly lacustrine-pedogenic setting [3]. Similar shallow water, volcanoclastic environments were common on the early Earth and they host abundant traces of past life. As such, they are good analogues for understanding potential traces of life on Mars, within specific sedimentary contexts. One such example is the 3.33 Ga Josefsdal Chert in the Barberton greenstone belt, South Africa [5,6]. This deposit records sediments formed in shallow water to littoral environments in an ultramafic volcanic setting. The overall geological context is that of an infilling, shallow sedimentary basin evolving into a fluvial environment, influenced by periodic volcanic episodes with which hydrothermal activity was associated. Geochemistry documents the fluctuating influences of marine and terrestrial (fluvial) influences. Within this sedimentological context, traces of both phototrophic and chemotrophic life abound. Their styles of preservation and the resulting biosignatures varied depending upon the local diagenetic conditions. The variety of sedimentological environments and types of biosignatures found in the Josefsdal Chert provide useful information in the search for life on Mars.

Josefsdal Chert sedimentology and geochemistry

The Josefsdal Chert in the Barberton greenstone belt (Fig. 1) is, with the Pilbara in Australia, one of the two, oldest and best preserved, Palaeoarchaean volcano-sedimentary environments on Earth. It is close in age (3.33 Ga) to the sedimentary deposits being explored on Mars at the present time. The area was subjected to later greenschist facies metamorphism and tectonically folded; the sedimentary sequences now dip at angles between 80-90°.

The sediments were deposited in a shallow basin, originally possibly similar to a collapse basin [6]. The sedimentary sequence varies in thickness from about 8m to more than 30m owing to an uneven, faulted pillow basalt base and to tectonic cutting out of the upper portions of the sequence (Fig. 1). The sediment composition varies depending upon volcanic activity. Graded, volcanoclastic sediments represent volcanic ash outfall, either proximal (coarser) or more distal (finer)(Fig. 2A). They fell into upper offshore to foreshore settings that were affected by periodic storms and were episodically exposed as in sabkha settings. The ultramafic sediments were first rapidly altered to phyllosilicates (initially probably smectite, now muscovite) and anatase. They were then rapidly silicified during early diagenesis owing to high silica saturation levels in the sea water, hydrothermal influx, and also through in situ devitrification of the volcanic ashes. The volcanoclastic facies record mixed marine/fluvial geochemical influences (Fig.2)[5].

In between episodes of volcanism, the sedimentary basin presented a relatively stable, long-lived (some 10⁵ years up to a million years) very shallow water to sabkha-type (volcanic particle free) environment, as indicated by exposure, erosion, and evidence of desiccation. Sediments formed in this setting comprise Fe-rich, biogenic mats and chemically-deposited silica gel (jaspilite facies)(Fig. 2B). Rare volcanic particles represent wind-blown elements. Bulk REY compositions indicate overall marine conditions (HREY), although there is a hint of increased fluvial input (LREY) during deposition of the chemical silica layers [5].

Josefsdal biosignatures

The biosignatures preserved in the Josefsdal Chert sediments are of two main types. Phototrophic biofilms and mats are the only traces of life occurring during volcanically quiet periods (Fig. 3C). The biogenic films were completely oxidised (by the activity of heterotropic microbes) and replaced by microcrystalline siderite before silicification. They exhibit evidence of frequent exposure and desiccation (cracks, tearing of the soft laminae, erosion), suggesting a sabkha-like environment. These mats were periodically but frequently interspersed with chemically-precipitated silica gel, possibly related to seasonal climatic events [4,5]. These non-carbonaceous phototrophic mats contrast with carbonaceous biofilms that formed during brief interludes of eruption during the volcanic periods (Figs. 3A). Generally poorly preserved in the sandy sediments (similar to MISS, microbially induced sedimentary structures, formed on top of bedding planes), there is nevertheless evidence of ephemeral subaerial exposure of some of the phototrophic biofilms as documented by desiccation, cracking and the formation of evaporite crystal layers in between biofilm laminae (Fig. 3A)[7,8]. However, in hydrothermal silica facies, phototrophic biofilms were well-preserved due to very early diagenetic silicification (less time for further diagenetic degradation)(Fig. 3B).

Chemotrophic biosignatures are of two kinds. There is organo-geochemical evidence of chemotrophic degradation of the phototrophic microbial mats: the organic matter of the siderite-replaced mats in the jasperlite sediments was completely oxidised, while in situ aragonite and sulphate precipitation in a carbonaceous biofilm suggests SRB (sulphur-reducing bacteria) activity (W 2011). However, particularly in the parts of the volcanic facies that were strongly influenced by hydrothermal fluids, chemotrophs formed colonies around volcanic particles and within volcanic dusty silica gel (Fig. 3D). Their identification is based on in situ geochemical signatures, as well as morphological features, all associated with the sedimentary context [4, 9].

Conclusion

The Mars-analogue, Josefsdal sediments comprise volanoclastic facies, as well as hydrothermal, chemical and biogenic components, whose differential contributions changed with time and position, depending upon volcanic activity. The nature and preservation of phototrophic and chemotrophic biosignatures in this high water/rock ratio, early terrestrial, shallow water basin were controlled by the local sedimentary and volcanic environment.

[1] Freissinet + 2025, PNAS 122, e2420580122 ; [2] Hurowitz + 2025 Nature, 64, 332 ; [3] Fawdon + 2024 J Maps, 17, 621-637. [4] Westall + 2015 Geology 43, 615; [5] Westall+ 2026, in prep; [6] Nijman + 2017. J Geol Soc, 174, 1090; [7] Westall + 2006 Phil. Trans. Roy. Soc Series B., 361, 1857; [8] Westall + EPSL 310, 468; [9] Hickman-Lewis + Sci Rep 10, 4965.