Fire, Water, Iron, Air, & Ice Stripes of smooth gray and fiery orange give banded iron forma
Fire, Water, Iron, Air, & IceStripes of smooth gray and fiery orange give banded iron formations a striking appearance. When deformed, this special sedimentary rock can even evoke the image of a flickering flame in its bands. Despite their burning appearance, banded iron formations reveal information about air, water, and ice of the past, rather than fire.Banded iron formations are composed of alternating layers of dark-colored iron oxide minerals like hematite or magnetite, and reddish cherts or shales. Cyclicity between these two types of sediments was due to intermittent oxygen availability in the oceans where they were deposited. When oxygen levels were high, dissolved iron in the seawater would bond with the oxygen, creating insoluble oxide minerals that fell to the seafloor. When oxygen levels were low, the iron did not precipitate and sediment deposition resulted in iron-poor cherts and shales.Many of the world’s banded iron formations were formed between 2.5 and 1.9 billion years ago. This time period contains the Great Oxygenation Event, when oxygen became more prevalent in Earth’s atmosphere and oceans. This oxygen was created by cyanobacteria, an early type of photosynthetic life. The oxygen levels of the ocean varied during the overall rise in oxygenation, as ocean currents moved water of different oxygen levels around and as cyanobacteria produced variable amounts of oxygen with seasons and populations. The shifting oxygen levels during this period created the perfect conditions to deposit banded iron formations on the seafloor.Curiously, banded iron formations have also been found that date from 750 to 580 million years ago, long after the Great Oxygenation Event. The cyclic deoxygenation of the oceans in this case was caused by ice. This period of the past was a “Snowball Earth” era, when extensive glaciations covered a great deal of the planet’s surface. Sea ice would have limited ocean circulation, creating variably anoxic conditions conducive to banded iron formation deposition.-CeMore information: Western Australia Museum,https://bit.ly/1NOOT5aImage: Flickr user James St. John, CC license,https://bit.ly/2RUjTrz -- source link
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