Eh Environmental Parameters

Figure 1. Theoretical representation of preservation windows (A-C) displayed in a three-dimensional space defined by essential environmental parameters for preservation of biological information such as temperature, Eh and ion concentration (named as S in mg L-1). (A) represents ion-enriched medium temperature and high Eh conditions compatible for acidic to neutral environments where paleobiological structures are preserved under a fast mineralization. Same S and Eh records changing to high temperature conditions (B) would be a window for hydrothermal areas where high mineralization rates are also accounted. (C) corresponds to medium-temperature and low-concentrated fluids under reducing conditions that favor a net preservation of organics. Note that some taphonomic gradients emerge according to changes in environmental parameters as observed in Río Tinto for Eh, pH and ion concentration (see Fig. 2).

Figure 1. Theoretical representation of preservation windows (A-C) displayed in a three-dimensional space defined by essential environmental parameters for preservation of biological information such as temperature, Eh and ion concentration (named as S in mg L-1). (A) represents ion-enriched medium temperature and high Eh conditions compatible for acidic to neutral environments where paleobiological structures are preserved under a fast mineralization. Same S and Eh records changing to high temperature conditions (B) would be a window for hydrothermal areas where high mineralization rates are also accounted. (C) corresponds to medium-temperature and low-concentrated fluids under reducing conditions that favor a net preservation of organics. Note that some taphonomic gradients emerge according to changes in environmental parameters as observed in Río Tinto for Eh, pH and ion concentration (see Fig. 2).

encloses strong changes in several parameters that are drivers in selecting some paleobiological entities. In this sense, oxidant replenishment in sediment fluids produces changes in the hydrochemistry exposed to meteoric waters that produces the complete oxidation of organic matter (Fernández-Remolar and Knoll, 2008) to heavy carbon-bearing compounds of complicated determination.

3. Preservation in Terrestrial Analogs

Some terrestrial habitats dated from Archean to modern ages have been claimed as feasible analogs of different Mars potential habitats that have emerged over time (Benison and LaClair, 2003; Benison, 2006). Such a statement is based on a methodological background that considers the terrestrial life inhabiting Earth regions as the unique reference to detect life in other regions of the Universe. Moreover, given that there is not a conceptual base to define Life, the terrestrial nature has currently provided the source to infer what particular life inhabites a defined area considered as a Mars analog. On the other hand, as one of the main connections between life and habitable region as water, it can be deduced that water is the main factor that characterizes a Mars potential habitat. As a result, water, as the exchange matrix for matter and energy used by life, and life itself are intrinsically linked in a search for extinct or extanct non-terrestrial living beings on Mars.

Many other environmental parameters such as pysichochemical or hydro-geochemical produce differentiation between environments, which will also cause characteristic imprints on preserving paleobiological traces. Although some terrestrial analogs are inferred through the geological record, they bear information to unlock some essential questions to uncover the paleoenvironmental conditions that could have been linked to the emergence of life on Mars, assuming both planetary sytems had an analogous volatile inventory during early stages of evolution (Grady and Wright, 2006). Therefore, very early geohistorical terrestrial analogs such as Isua metasedimentary sequences older than 3.7 billions of years (Rosing et al., 1996; Fedo, 2000) are of great importance to trace back those driving forces that might have impeled the possible emergence of life on Mars. Later on, the geological record of subsequent Archean environments (Pilbara and Barberton Archean deposits younger than 3.7 billion of years) are reference systems to determine those mechanisms driving the planetary divergence between Mars and Earth (Grady and Wright, 2006). Table 1 displays an equivalence between some Mars potential habitats and their terrestrial analogs, including environmental parameters involved in preservation.

As showed in Table 1, combined studies of ancient and modern environments will improve acquiring knowledge not only about that processes involved in recording paleobiological information, but also those involved in long-term preservation. Therefore, to unlock the aging processes that favor preservation of some paleobiological over other entities it is necessary to appeal to analogous modern environment. In a few cases the geological record of ancient and its corresponding modern equivalent coexist within the same area as seen in the Río Tinto fluvial basin (Fernández-Remolar et al., 2005; Fernández-Remolar and Knoll, 2008).

All environments described in Table 1, as others not mentioned here, deserve a detailed analysis in order to have a perspective for the preservation evolutionary changes occurred on Mars since its earliest evolutionary stages. This would demand indeed an extensive work dealing on characterization of the taphonomic processes driving preservation of any paleobiological entities that can be potentially produced in each analog. On the contrary, some interesting analogs having a Mars real counterpart are herein described in order to provide an idea how the research on the preservation in Earth analogs is essential to drive the exploration for extinct life on Mars.

Some Mars potential habitats

Terrestrial analogs

Environmental analogs

Parameters for preservation

Fluvial to lacustrine/marine Noachian environments in geo-morphic and sedimentary units as fluvial to deltaic sedimentary systems as observed in Holden Crater and others (Cabrol and Grin. 2001; Grant et al., 2007)

Fluvial to deltaic environments of Archean sedimentary systems which substrate is frequently weathered to be exposed to the atmosphere (i.e. Moodies Group in South Africa )

Fast decreasing of redox potential in the water column and high silica and iron availability from mildly acidic conditions buifered by a CO-rich and aggressive atmosphere, which chemical attack would decrease over time as carbon budget was sequestered in form of carbonates. Lack of atmospheric oxygen would produce strong redox gradients to reducing conditions in the water masses, but shallow oxidizing conditions cannot be discarded maintained by the oxidant production in surface

Late Noachian to Early Hesperian acidic environments recorded as sulfate and oxide-iron rich deposits (Squyres et al., 2004)

Modern and ancient Acidic extreme environment-sand its underground conterparts (Rio Tinto, ephemeral acidic saline lakes. Permian deposits) (Benison, 2006; FernandezRemolar et al, 2005 )

Oversaturation in ferric ionic complexes through leaching under low pH and high redox changing to lowredox and quasi-neutral pH subsurface conditions (Fernández-Remolar et al., 2008b), which will create strong a phonomic gradients

Hydrothermal and related-sedimentary systems associated to tectono magmatic complexes, shield volcanoes impactors. Some silica-rich deposits recently discovered in Gusevare supposed to be mineral evidences for hydrothermal activity (Prof. R. Arvidson, personal communication, October 2007). Tharsis, Elysium, Nili Fossae or Terra Tyrrhena (Varnes et al., 2003; Schulze-Makuch et al., 2007). Sedimentary-like environments associated to subaqueous hydrothermal complexes (finegrained sulfidic to silica-rich)

Acidic to neutral silica enriched hot springs and hydrothermal fluids (i.e. Kilauea and Manua Kea volcanic region, Yellowstone hydrothermal system. Iberian Tectono Magmatic Complex) (Lewis et al., 1997; Leistel et al., 1998). Study of Archean hydrothermal deposits are also essential to understand long-term preservation processes to be present on Mars (Kiyokawa et al., 2006)

Fast mineralization rates under moderate to high temperature fluids enriched in Si, S, Fe and other ions leached from the rock substrate and from magmatic sources (S sources). In origin, material leaching will be destructive but mineral re-precipitation will preserve paleobiological traces

Rock coatings and varnishesoriginated as thin films on weathered rocks exposed to fluids, volcanic fog and atmosphere(atmospheric sprays)

Fine water lamina sourced in aqueous environments (Rio Tinto terrace deposits, upper Permian to lower Triassic fluvial and lake deposits), coating of boulders alfected by subaqueous volcanism (Moore and Clague, 2004), but extposed to fog, dew or snow in deserts (Death Valley, Atacama desert ) (Kuhlman and McKay, 2007) and volcanic centers (Kilaueavolcano) (Schiifman et al., 2006)

High mineralization rates induced specifically in briny lacustrine and fluvial systems (i.e. Rio Tinto acidic brines) but also favored under weathering rates (acidicfog)

3.1. PRESERVATION IN FLUVIAL AND DELTAIC TO MARINE

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