Preservation Windows For Paleobiological Traces In The Mars Geological Record


1Centro de Astrobiología, INTA-CSIC, Ctra Ajalvir km. Torrejón de Ardoz, Spain

2Área de Geología, Universidad Rey Juan Carlos, C/Tulipán s/n, Madrid, Spain

3Unidad de Microbiología, Centro de Biología Molecular, Universidad Autónoma de Madrid, Spain

Keywords Mars, astrobiology, preservation windows, paleobiological traces

1. Introduction: A New Perspective on the Mars Sedimentary Record

For years, the Mars robotic missions have provided different evidences that Mars had an active hydrologic past which involved the emergence of distinctive sedimentary systems and its corresponding weathering sources. Minor geomorphic features to regional-scaled structures have been used to inferthat sedimentary systems such as deltaic, fluvial, lacustrine or marine-like environments (Malin and Edgett, 2000) to have occurred sometimes in Mar's history (Carr, 2006). In this context, the information obtained by geomorphological interpretations have inferred those physical conditions - e.g. hydrological activity, water energy or climatic evolution- that were in equilibrium with the landforms (Baker, 2001). In recent times, new instrumentation aboard the different planetary missions to Mars (e.g. IR specs in the Mars Oddyssey, Mars Express and MRO, or APXR and Mossbauer specs of MERs) have shed light in the mineralogical and geo-chemical composition of some ancient materials.

Both Orbiter and Rover explorers have recognized that the two main age-differentiated Mars terrains have differential mineralogy and geochemistry (Poulet et al., 2005; Bibring et al., 2006). In the oldest Noachian areas (age older than 3.8 Ga), the Mars Express orbiter has detected phyllosilicates concentrated in layered terrains (Michalski and Noe Dobrea, 2007), currently covered by younger deposits of lavas and other sediments. On the other side, Late Noachian to Hesperian younger terrains (3.8-3.0 Ga) are composed of sulfates, some of them bearing iron (Squyres et al., 2004; Morris et al., 2004; Fernández-Remolar et al., 2005). These mineralogies have been used as indirect evidences in inferring the hydrogeochemical processes occurring on the Mars surface, which have resulted of the interaction between climate, hydrosphere and geosphere of Mars. On Earth, phyllosilicates are formed during diagenesis and metamorphism in a diverse range of marine and subaerial environments. Thus, the clays found on Mars, if sedimentary (Michalski and Noe Dobrea, 2007), denote high rates of weathering that could be easily driven by CO2-saturated meteoric waters (Francois and Walker, 1992; Franck et al., 1999) under warm conditions. Analogous aqueous acidification through CO2 hydration to H2CO3 (Orr et al., 2005) has been induced for early Earth water masses by CO2 saturation (Corcoran and Mueller, 2004). Such an scenario would fit a early development of potential habitats on Mars characterized by atmospheric enrichment on carbon dioxide, carbonate lacking and formation of phyllosilicates under acidic conditions though weather-ing.On the other hand, the hydrated sulfates bearing ferric iron precipitate under strong acidic brines (pH < 3) are sourced in the sulfide weathering by oxygen-rich meteoric waters and/or anaerobic iron oxidizers (Amils et al., 2007; Fernández-Remolar et al., 2008b). As a result, an association of different iron-bearing sulfates as copiapite, jarosite and schwertmannite co-occurs with any other sulfates like gypsum or epsomite formed by cations sourced in the silicate dissolution.

Later on, the occurrence of Hesperian to Amazonian outflow channels and great catastrophic landforms suggest that Mars had some post-Noachian planetary events of thermal reactivation and transient water masses (Carr, 2006) when great iced terrains - clathrate-rich deposits permafrost and glacier-like systems -were warmed up. Such episodic massive release of water took place under a declining atmosphere that was transitionally recovered through the volatile replenishment (Baker, 2001). As a consequence, different highland lacustrine and lowland sea-like regions were created or reactivated during one or several episodes where the climatic conditions were warmed up.

If life arose on Mars, it should have adapted and evolved to the long-term climate evolution that is driven by the inner planetary activity. Moreover, preservation of biological information produced by the Mars life into the Mars geological record must have occurred according to fossilization processes that depend on the crust diagenetic geochemistry, which also emerges from atmosphere, crust composition, hydrological activity and heatflow. According to the Mars geological record uncovered along different planetary missions, we propose some preservation windows in which primary biological information may have been recorded in the form of one or several fossil states, ranging from pure organic compounds to resistant mineralized remains. The following preservation windows are considered to have preserved paleobiological entities:

• Early Noachian phyllosillicate deposits (e.g. Mawrth Vallis)

• Fluvial to lacustrine or marine deltaic-like Early Noachian materials (e.g.

Holden Crater)

• Hydrothermal-associated deposits (e.g. Holden Crater)

• Late Noachian to Hesperian sulfate to hematitic basaltic sands (e.g. Meridiani Burns Fm)

To these five preservation windows two more can be added in relation to subsurface regions and rock coatings. The main reason is whatever surface conditions reigned on the Mars surface, its subsurface counterpart may have been more stable, concerning temperature and shielding against radiation. Moreover, environmental conditions are easier to control by microbes in the subsurface as it has been shown concerning to temperature and pH (Gómez et al., 2004; Fernández-Remolar et al., 2008a). Under these favorable circumstances, biogeochemical cycling can operate during the long and cold episodes that followed the benign climate inferred for the Early Noachian age. Subsurface areas associated to volcanic centers with hydrothermal activity are exposed to high mineralization rates that are an essential parameter for morphological conservation. Finally, Mars has developed along its long history fluvial and desertic systems in which boulders of sedimentary bars or pavemented soils are covered by complex thin films composed by oxides, sulfates, carbonates and weathering silicates (Potter and Rossman, 1977; Giorgetti and Baroni, 2007). Microbial endoliths and endolithic structures (Golubic and Schneider, 2003), not discussed in this work, should be added to these geobiological entities of great importance for searching for life on Mars.

2. Taphonomic and Organic Chemistry-Related Processes Involved in Preservation

Paleobiological remains and fossils are currently concerned as real biological entities, but are not life entities. On the contrary, they result from the interplay and succession of several geo-biological processes that occur before, during and after burial, and which are recorded additionally to the primary remain (Fernández-López, 1991, 1995; Brocks and Summons, 2005). As a consequence, fossil entities record not only some information concerning to its biological origin, but also all those processes that have played any role in generating the preserved entities. A good example is the organics obtained in sedimentary rocks that come from the multi-way degradation of primary biomolecules under different thermal and compositional conditions along the rock diagenesis (Brocks and Summons, 2005). In this sense, exposition of biomolecules to iron- and sulfur-rich environments has a strong imprint in the final geopolymers that are associated with iron and sulfur (Sinninghe Damsté and de Leeuw, 1990). Therefore, the fossilization process can follow complex pathways that involve preservation before and after definitive burial, fossilization phases known and biostratonomy and fossildiagenesis, respectively (Fernández-López, 1991, 1995). Obviously, the final fossil state will be the result of all these stages and can be as simple as a fast and in-situ burial, which is the best case for the preservation of chemical fossils.

The parameters involved in the formation of the preserved entities (fossils and/or any paleobiological trace) are countless (Farmer and Des Marais, 1999). They range from molecular processes (Banfield et al., 2005) currently driven by microbes to planetary-scaled events such as sea level global changes (Fernandez-Lopez, 2007). These planetary events are ruled by macrotectonic to long-term climatic changes affecting global biogeochemical cycles (Brasier, 1992) that are drivers of preservation in simple parameters as e.g. redox potential to preserve organics.

Whatever processes drive fossil preservation, all micro and macro mechanisms are sustained on some physicochemical parameters which are essential in the final record of the biological information. These parameters are hydrodynamics (diffusion vs. advection), temperature (biogeochemical reaction, mineralization and recrystallization rates), redox potential (oxidation processes), and ion concentration (mineralization), which dominance or co-occurrence can diversify the paleobiological record into different preservation windows. Obviously, exceptional fossil preservation - e.g. the so called conservation deposit fossil-lagerstatten as Burguess Shale (Conway Morris, 1990; Seilacher, 1990) - result from the positive concurrence of all these parameters; but from the interplay of all parameters will emerge the diversity of preservation windows that enrich the geological record on Earth. In the next section some ancient and modern terrestrial analogs will be considered to exemplify different kinds of preservation windows that can be expected to occur in the extensive geological record of Mars (Fig. 1).

Oxygen availability and redox potential are two elements that rule the molecular preservation of life. However, it should be noted that low oxygen fugac-ity does not mean low redox potential, which can be essential to understand preservation pathways on Mars. Although biotic and abiotic oxidation destroys most of low resistant biomolecules (e.g. sugars, proteins and nucleic acids), a minor part can be transformed to macromolecular humic complexes and geopol-ymers precursing kerogen and bitumen. On the other hand, more resistant fatty acids and lipids are transformed to geolipids and hydrolyzed hydrocarbons, but maintain the original structure that enable an easier identification concerning to its biological origin (Brocks and Summons, 2005). In any case, it has to be noted that not only redox, and other primary factors, but temperature during diagenesis is an essential factor to maintain preserved the molecular traces of life. Indeed, organic matter that can suffer thermal destruction under metamorphism, show a distinctive preservation under high-temperature extreme areas that are close to hydrothermal centers (Brocks and Summons, 2005). On the contrary, when the thermal and redox history of the preserving remains converge in a positive way, exceptional conservation of biopolymers occurs and some of them can be amplified using current molecular biology techniques (Logan et al., 1993).

A final thing to consider as essential to understand the processes involved in preserving biology is time. Indeed, rock aging, currently known as diagenesis,

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