Paleoecological Significance of the Banded Iron-Formation

Abstract

Banded iron-formation (BIF) is rare in rocks younger than about 2 aeons (years X 10 9 ). It records a major episode of chemical iron sedimentation, however, at about 2 to 2.1 aeons, just before the deposition of the oldest oxidized terrestrial sediments. 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.In the absence of biological silica secretion, the contemporaneous hydrosphere was saturated with H 4 SiO 4 , whose continuing precipitation was favored by near neutral pH. Ferric oxides were intermittently added. The episodicity of the iron-rich bands suggests cyclical changes in procaryotic populations, or rates of supply of ferrous iron, or both. Mechanical differences between geochemically similar contemporaneous BIFs imply variations in turbulence and depth of water. Geochemically varying facies of the same age suggest variations in availability of O 2 . With advanced biological oxygen mediation, continuing segregation of metabolic carbon, and increasing supplies of oxygen, the oceans were depleted of iron to become saturated with O 2 , which thereafter evaded to the atmosphere, giving rise to continental red beds. Rare Paleozoic BIF seems to be related to local euxinic conditions and volcanic sources of iron, reminiscent of Archean deposits. The apparently late Proterozoic deposits of the Rapitan (north-western Canada) and Jacadigo (southwestern Brazil) beds remain enigmas.