Other
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....4958w&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #4958
Other
Scientific paper
3.45 b.y.-old cherts from the Kitty's Gap locality in the Pilbara contain superbly preserved microbial biofilms that formed on an early Earth characterised by extreme conditions. They therefore represent good analogues for potential early Martian life. Volcaniclastic sediments influenced by hydrothermal activity were deposited in very shallow water to exposed conditions, as evidenced by interstratified cross-bedded and evaporite units. Two microbial layers with different characteristics, occurring within 2 cm of each other, have been closely studied in the field, by optical microscopy, and by high resolution SEM (+ light element EDS) of carefully etched, cut surfaces. Both layers can be traced laterally for at least a couple of meters before being broken up by a chert vein. The lower microbialite is a 2--3 mm thick stromatolite-thrombolite association. The stromatolites consist of tiny columns, 250 μm wide and about 2 mm high, whereas the thrombolitic zones present a simple, clotted texture. They were formed by microbial colonies consisting of consortia of mostly coccoidal bacteria (two species, one <0.5 μm and one ˜1 μm in diameter). Most of the organisms present a turgid condition and large numbers of them exhibit cell division (fission and budding). Filamentous bacteria occur but are rare. The deflated and lysed filaments are small, 4--5 μm and about 100 nm in width. Small amounts of EPS are associated with the colonies. The second microbialite is a fine microbial mat (tabular stromatolite) that occurs at the surface of an evaporite horizon 2 cm above the first microbialite. The 0.5 cm thick evaporite horizon consists of ghosts of carbonate and possibly halite crystals. It is coated by a <50 μm thick microbial mat, formed by consortia of 0.5 μm coccoidal bacteria and filaments, 0.2--0.4 μm wide and 10--20 μm long. Some EPS is associated with the bacteria and the sedimentary particle surfaces are coated with a nm-thick polymer layer. The filaments are characterised by a striated surface. Both coccoids and filaments exhibit some deflation due to lysis before fossilisation. The organisms in both layers have been silicified but still contain significant amounts of carbon. Silicification of these deposits appeared to be continuous, since there was also secondary replacement by silica of the evaporites and silica cementation of the volcaniclastic and evaporite particles. Shallow water to evaporitic environments influenced by hydrothermal activity would have been common on Mars during the Noachian period and possibly also later. If life ever arose on Mars, it could have left similar, macroscopic to microscopic traces that could be investigated both in situ and in returned samples.
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