The BIFs formed when layers of sea ice prevented the exchange of oxygen between the atmosphere and the ocean, allowing iron from underwater volcanic eruptions to build up in the seawater. However, the periodic layers of silica are evidence that pulses of oxygen must have been involved as well.
“Layers like this don’t form when the ocean is completely sealed off with ice,” says Mitchell.
To examine how oxygen could have entered the ocean during Snowball Earth, he and his colleagues measured the extent to which the BIFs became magnetized when exposed to a magnetic field. They discovered that the timing of changes in Earth’s orbit (known as Milankovitch cycles) aligned well with the timing of variations in the rocky layers.
BIFs and Milankovitch cycles
Milankovitch cycles refer to the periodic changes in the shape of Earth’s orbit, along with its tilt and wobble, over tens of thousands of years. The changes affect Earth’s position relative to the Sun and the amount of solar radiation that reaches its surface, in turn influencing Earth’s climate.
One possible interpretation of the newest finding is that Milankovitch cycles caused ice sheets to advance and retreat in rhythm with the changes in incoming solar radiation. This would explain why rocks, such as those at the Flinders Ranges, alternate between the sedimentary layers of BIFs and glacial deposits.
The researchers report that the BIFs were deposited during hundreds of glacial advances and retreats over a period of four million years. “Our new data suggests that the ice retreat opened ice-free areas, allowing oxygen to mix into the oceans and enabling life to persist during Snowball Earth,” says Mitchell.
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