Archean Deposits

Early terrestrial habitats are likely the most suitable approaches to the early Mars environments (Figs. 2A, B). During the Archean, the upper part of the Terrestrial crust, lacking terrestrial flora, was directly exposed to C02-driven atmospheric weathering on basaltic-like primary rocks, which induced high physicochemical alteration (Hessler and Lowe, 2006). As a result, detrital sediments were massively transported to marine environments through transition sedimentary systems like deltaic.Moreover, the aggressive chemical weathering introduced silica into solutions (Hamade et al., 2003) in greater proportion than the subaqueous geo-thermal systems. On the other hand, some oxidative processes might have occurred (Ohmoto, 2004) impeled by photochemical pathways affecting the ferrous iron in the form of aqueous complexes that were sourced in hydrothermal-ism. Such oxidation likely resulted in the production of ferric deposits as banded iron formations and red beds. Interestingly, there is no evidence of carbonates before 3.5 Ga (Grotzinger, 1994) despite a high concentration of C02 in the early atmosphere on Earth, which suggests some acidification mechanisms prevented the massive production of these minerals.

Strong redox gradients, as well as fast silicification and ferruginization are mechanisms expected to favor preservation of organics and paleobiological morphologies, respectively (Walsh and Westall, 2003). An exceptional case to be mentioned is the preserved microbial mats occurring in deltaic tidal flat deposits of the 3.2 billion-of-year Mesoarchean Moodies Group in Barberton (Noffke et al., 2006), South Africa (Figs. 2C, D). In this case, high sedimentation rates added to strong redox gradients along the water column might have favored reducing conditions in shallow areas inhabited by the microbial mats. Moreover, abundance of silica in sediment porewater likely increased the preservation potential of microbial remains by fast mineralization of the deposits. Although organics would have prevailed in these conditions, late metamorphic processes related to a combining increase of sediment pressure and temperature (Brocks and Summons, 2005) have favored the organic destruction.

Analogous Mars environments can be found in shallow deltaic deposits that occur elsewhere infilling crater lacustrine basins and different-sized impact craters of Noachian age (Fasset and Head, 2005).

3.2. PRESERVATION IN ENVIRONMENTS LINKED

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