Mathematics – Logic
Scientific paper
Sep 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987orli...17..229h&link_type=abstract
Origins of Life and Evolution of the Biosphere, Volume 17, Issue 3-4, pp. 229-250
Mathematics
Logic
3
Scientific paper
A combined hydrothermal/biogenic model is suggested for the origin of banded Fe-formation. The model is mainly based on the distribution of trace elements such as Mn, Co, Cu, Zn, and P in modern biogenic metalliferous sediments and Proterozoic banded iron-formations, but also on carbon isotope data of both carbonates and reduced elemental carbon. One of the characteristics of banded Fe-formations is that they are poor in trace elements. An argument in favour of a combined hydrothermal/biogenic model would therefore be that some of the modern analogues, too, are associated with very little trace elements. Such sediments are microbiologically precipitated in areas with restricted water circulation and limited amounts of free oxygen, which favours the oxidation of reduced Fe by microaerophilic chemolithotrophic bacteria relative to inorganic oxidation. The biological enzymatic pathway for the oxidation excludes the participation and co-precipitation of transition metals other than iron until the original amorphous Fe oxide hydroxide precipitate is transformed to the crystalline polymorphs. The wide lateral microband continuity of many banded Fe-formations could be explained by injection, interleaving and horizontal spreading of chemically reduced hydrothermal solutions in an early ocean with limited amounts of free O2. Due to the low O2 concentrations Fe would be precipitated slowly as an effect of mainly microbiological oxidation. Periods of Fe precipitation from hot hydrothermal solutions would alternate with periods of precipitation of silica from cool hydrothermal solutions.
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