Dales Gorge FormationThese redbeds are found in Western Australia near the northwestern corner of th

Dales Gorge FormationThese redbeds are found in Western Australia near the northwestern corner of that continent, in the Pilbara region near the iron mining community of Pannawonica. They are all part of the Dales Gorge formation, one of the major sources of iron for mining in Australia and one of the classic Banded Iron Formations associated with the rise of oxygen on the planet.When the planet first formed, its atmosphere was nearly oxygen-free. Instead, it contained components such as methane, carbon dioxide, and sulfur-species. It took life nearly a billion years to figure out a process to take energy from sunlight, and that process that we now call photosynthesis began releasing oxygen as a byproduct.When oxygen levels in the atmosphere were low, iron and several other metals existed in reduced oxidation states. Iron, for example, was found with a +2 charge, compared to the rusted iron with a +3 charge that you see in the hematite of these rocks. When iron has a +2 charge, it is able to dissolve in water, so prior to the arrival of oxygen in the atmosphere, the oceans likely had a whole lot of iron dissolved in them.Once those little bacteria started putting out extra oxygen, a chemical reaction was triggered – the iron oxidized to a +3 charge, and as a consequence it could no longer dissolve in the ocean. The iron precipitated, creating ooze layers that accumulated like gel at the bottom of the ocean. Over time, layer after layer of sediments were stacked on top and the pressure squeezed out the water, eventually compacting them into solid layers. These rocks, formed 2.48 billion years ago, are thought to have been one of the last layers to form before oxygen began fully building up in the atmosphere.Banded Iron Formations, like this one, also have bands of siliceous material – minerals with the same chemistry as quartz, but in microcrystalline formats that we call chert. These cherty layers formed when something about the conditions in this basin changed – either the water chemistry, sediment supply, or maybe even the temperature. If more siliceous material entered the basin, the sediments were more cherty, and iron was deposited when there was no silica to get in the way.As you see in the close up view, these layers also come in packages – alternating between siliceous and iron rich. When we see packages like those today, we often suspect that they are driven in part by climate changes associated with wobbles in Earth’s orbit and release of carbon dioxide to the atmosphere. Recent work studied the packages of layers in these rocks and found that, if they are driven by wobbles in the Earth’s orbit, the timing was very different from that found today – meaning these packages may tell us a story of how Earth’s orbit has been stabilized over time by the influence of the Moon’s gravity.-JBBImage credits:https://commons.wikimedia.org/wiki/File:Pilbara_dales_gorge_formation.jpghttps://commons.wikimedia.org/wiki/File:Banded_iron_formation_Dales_Gorge.jpghttps://flic.kr/p/24RxcJ2References: https://doi.org/10.1016/j.precamres.2019.04.007https://bit.ly/3gWAXdohttps://doi.org/10.1038/s41561-019-0372-0 -- source link
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