Båndet jernformation

For alternative betydninger, se BIF.

Båndet jernformation (også båndet jernstensformation, båndede jernaflejringer, båndet jernmalm, forkortet til BIF fra engelsk banded ironstone formations eller banded iron formations) er distinkte enheder af sedimentære bjergarter, som næsten altid er fra superæonen Prækambrium. En typisk BIF består af repeterende, tynde lag af jernoxider, enten magnetit (Fe₃O₄) eller hæmatit (Fe₂O₃), vekslende med bånd af jernfattige skiferler and kiseljord(en.chert?). Nogle af disse ældste klippeformationer, dannedes for over 3,7 milliarder år siden, omfatter båndet jernformation. ^[1] Båndede lag rige på jern var en almindelig karakteristisk sedimentær aflejringer i det meste af jordens tidlige historie men er nu sjældne. Post-Prækambrium jernsten har generelt en anden herkomst.

Relation til atmosfærisk iltning redigér

Nærbillede af båndet jernformation eksemplar fra øvre Michigan. Skalalinjen er 5,0 mm.

Båndet jernformationer er rigelige omkring iltkatastrofen,^[2] 2,4 milliarder år siden, og blev mindre hyppige omkring 1,8 milliarder år siden (mindre ilt).^[3] Betingelser for opløst jern (mindre ilt) tog fart igen ved 1,9 milliarder år siden,^[3] og BIF tog fart igen i forbindelse med den formodede "sneboldjord" for 750 millioner år siden,^[4]^[3] og her er BIF svær at forklare.

Båndede jernformation senge er vigtige kommercielle kilder af jernmalm, såsom Pilbara området i vestaustralien og Animikie Group i Minnesota.

Oprindelse redigér

Den konventionelle opfattelse er at båndede jernaflejringer blev dannet i havvand grundet frit ilt (O₂) dannet af fotosyntetiserende cyanobakterier, kombineret med opløst jern i Jordens oceaner danner vanduopløselige jernoxider, hvilket "regner" ud, og danner tynde lag på underlaget, hvilket kan have været anoxisk mudder (dannende skiferler og kiseljord(en.chert?)). Hvert bånd kan minde om en "årring", forstået på den måde at båndene skyldes cykliske variationer i tilgængelig ilt.

Det er uklart om disse båndede jernstensformationer var årstidsafhængige, fulgte en tilbagekoblende oscillation i oceanernes komplekse system eller fulgte en anden cykel.^[5] Det formodes at jorden i starten startede med enorme mængder opløst jernforbindelser i verdens sure have.

Efterhånden som fotosyntetiserende organismer dannede frit ilt, dryssede vanduopløselige jernoxider ud i samme takt.^{[kilde mangler]}

Kilder/referencer redigér

^ Minik T. Rosing, et. al., Earliest part of Earth's stratigraphic record: A reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence, Geology, 1996, v. 24 no. 1 p. 43-46
^ doi:10.2113/gsecongeo.68.7.1135, November 01, 1973, Preston Cloud. Economic Geology (1973) 68 (7): 1135–1143.: Paleoecological Significance of the Banded Iron-Formation Citat: "...The great iron-formations of this age reflect dramatic events. Ferrous iron residual in stagnant oceans beneath an anoxygenous atmosphere may then have moved into the photic zone on a vast scale because of overturning instigated by climatic change. A previously limited, photosynthetic, procaryotic microbiota, dependent on ferrous iron to maintain ambient oxygen at tolerable levels, then expanded and diversified, converting the iron to insoluble ferric oxides..."
^ ^a ^b ^c doi:10.1038/461179aCitat: "...argue that the return of an iron ocean, peaking about 1.9 billion years ago, was triggered by a dramatic decline in oxygen, perhaps to values approaching those in the atmosphere before the GOE...The story culminates in the largest observed shift in the chromium-isotope data, about 750 million years ago, corresponding to a big leap in atmospheric oxygen to levels that welcomed the first animals..."
^ doi:10.1126/science.281.5381.1342
^ Good discussions for the layman are in Cesare Emiliani, Plant Earth 1992:407f, and Tjeerd van Andel, New Views on an Old Planet 2nd ed. 1994:303-05.

Jelte P. Harnmeijer, 2003, Banded Iron-Formation: A Continuing Enigma of Geology, University of Washington Doc format Arkiveret 8. september 2006 hos Wayback Machine
Klein, Cornelis, 2005, Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins, American Mineralogist; October 2005; v. 90; no. 10; p. 1473–1499; doi:10.2138/am.2005.1871 http://ammin.geoscienceworld.org/cgi/content/short/90/10/1473 abstract.
Andreas Kappler, et al., 2005, Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria, Geology; November 2005; v. 33; no. 11; p. 865–868; doi:10.1130/G21658.1 http://www.gps.caltech.edu/~claudia/papers/kappleretal_GEO2005.pdf Arkiveret 16. december 2008 hos Wayback Machine

Eksterne henvisninger redigér

Wikimedia Commons har flere filer relateret til Båndet jernformation
University of British Columbia. (2015, September 9). Metal-eating microbes in African lake could solve mystery of the planet's iron deposits. ScienceDaily Citat: "...While iron-respiring bacteria were discovered in 1993, the new Scientific Reports study provides evidence that microorganisms could have been directly involved in depositing Earth's oldest iron formations..."

[1] Minik T. Rosing, et. al., Earliest part of Earth's stratigraphic record: A reappraisal of the >3.7 Ga Isua (Greenland) supracrustal sequence, Geology, 1996, v. 24 no. 1 p. 43-46

[Cloud-2] :10.2113/gsecongeo.68.7.1135, November 01, 1973, Preston Cloud. Economic Geology (1973) 68 (7): 1135–1143.: Paleoecological Significance of the Banded Iron-Formation Citat: "...The great iron-formations of this age reflect dramatic events. Ferrous iron residual in stagnant oceans beneath an anoxygenous atmosphere may then have moved into the photic zone on a vast scale because of overturning instigated by climatic change. A previously limited, photosynthetic, procaryotic microbiota, dependent on ferrous iron to maintain ambient oxygen at tolerable levels, then expanded and diversified, converting the iron to insoluble ferric oxides..."

[Lyons2009-3] :10.1038/461179aCitat: "...argue that the return of an iron ocean, peaking about 1.9 billion years ago, was triggered by a dramatic decline in oxygen, perhaps to values approaching those in the atmosphere before the GOE...The story culminates in the largest observed shift in the chromium-isotope data, about 750 million years ago, corresponding to a big leap in atmospheric oxygen to levels that welcomed the first animals..."

[Hoffman1999-4] :10.1126/science.281.5381.1342

[5] Good discussions for the layman are in Cesare Emiliani, Plant Earth 1992:407f, and Tjeerd van Andel, New Views on an Old Planet 2nd ed. 1994:303-05.

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