Saturday, June 6, 2009

Banded Iron Formations

Geological Processes and Evolution - 3

Science News has a well written and informative summary on the origins of Banded Iron Formations (BIF). I thought the writer Sid Perkins has done a good job describing the various theories of BIF genesis and early atmosphere and sea water chemical evolution and how that may have constrained biological evolution.

The long causal chain centers around the levels of atmospheric oxygen. Early photosynthesizers released oxygen but that could not escape into the atmosphere in appreciable amounts because it was being used up by the precipitation of dissolved iron. When that leveled off, meaning when the bulk of BIF's had formed by early Proterozoic (2.4 - 2.0 billion years ago) the level of atmospheric oxygen increased. This was likely aided by a decrease in methane production. Nickel fixing microbes release methane and levels of nickel show a decrease in BIF's over time as source processes like volcanism declined in activity reducing the supply of nickel. Methane reacts with oxygen (forming CO2) preventing its buildup in the atmosphere. With less methane, atmospheric oxygen increased leading to the formation of a protective ozone shell in the atmosphere (and cooling the earth as CO2 levels decreased). This enabled photosynthesizers to inhabit the shallower more sunlight part of the sea column boosting their numbers and therefore enabling more oxygen production. Higher oxygen levels then may have played a role in the evolution of eukaryotes who consume oxygen in several metabolic reactions.

Its good to see such a comprehensive review article about geology from the media.

In India BIF's occur in the Archaean and Early Proterozoic terrains of south India and eastern India.

The article is worth reading.

See: Geological Processes and Evolution

1 comment:

  1. Banded iron formations are a type of rock often found in ancient sedimentary rocks, consisting of alternating thin layers of iron oxides and bands of shale and chert. Am I right?