Biology
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p21a0140c&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #P21A-0140
Biology
0406 Astrobiology And Extraterrestrial Materials, 1041 Stable Isotope Geochemistry (0454, 4870), 5200 Planetary Sciences: Astrobiology
Scientific paper
Meridiani Planum sediments formed in an acid environment and include jarosite and other evaporitic sulfate minerals. Nodular spheroidal concretions appear to have grown in situ and are predominantly hematite. The source of the Rio Tinto, S. Spain, drains an area of extensive sulfide mineralization and is dominated by acid mine drainage processes. The system is not a Mars analog but potentially similar processes of sulfide oxidation produce sulfate rich waters which feed into the river and precipitate a large range of evaporitic sulfates including jarosite. Iron oxide minerals associated with the evaporites are either dispersed or bedded but not nodular. The water compositions appear to be mixtures of a few discreet end-members: the two most significant occur in undiluted form as inputs to the river and are relevant to many such systems. They both have all sulfur totally oxidized as sulfate. The first is a bright red water, pH ~1.5, Fe/S 0.5 and 23 g/L iron which is greater than 95% Fe3+. Its sulfate oxygen isotope composition is +2‰SMOW and about +7‰, relative to the water O isotope composition. These data indicate pyrite oxidation by Fe3+ with O in sulfate coming mainly from water. The second end-member is a pale green water, pH ~0.7, Fe/S 0.7, 50 g/L iron present mainly as Fe2+ and O isotope composition of sulfate about +6‰SMOW , about +12.5‰ relative to the water O value. Oxygen in sulfate comes mainly from atmospheric oxygen resulting from pyrite oxidation by molecular oxygen dissolved in water. Although the Rio Tinto system reactions probably are microbiologically mediated (relevant genera have been identified there) similar processes could occur abiotically but more slowly. Meridiani Planum sediments and nodules can be described by a plausible set of similar end-member processes. The primary source of sulfate is oxidation of sulfides present in basalt (pyrite, FeS2 or pyrrhotite, FeS) and weathering would have produced oxidized sulfate rich solutions at low pH. Ground water migration could produce evaporitic ponds where various bedded sulfate mineral sediments could form. The intergranular pore-spaces would be water filled. Most terrestrial spheroidal nodular concretions form by radial diffusion in pore-water of a chemical component of a very different oxidation state from that of the surrounding water. A nodular concretion is most usually formed by the reaction of the diffusive component with others in the pore-water. There are two main possible reaction sets for formation of the Blueberries that are consistent with all current data. 1. Local concentrations of organic matter (pre-biotic or biotic) formed reduction spots in which a small amount of Fe3+ either in solution or from evaporite mineral salts, was reduced to Fe2+ and then diffused radially to form an iron oxide nodule by reaction with inwardly diffusing dissolved oxygen. 2. Similar local concentrations of organic matter could also have engendered sulfate reduction and consequent outward diffusion of dissolved sulfide reacted with iron in solution to produce an iron sulfide nodule, subsequently oxidized in situ to hematite (maybe via goethite). Our current work is successfully identifying chemical and stable isotopic characteristics for both microbial and abiotic modes of all relevant reactions.
Black Samson
Coleman Max L.
Hubbard Christopher G.
Mielke Randall E.
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