Dr Joshua Davies, Geotop-UQAM
12 avril 2023
The Paleoproterozoic world was very different from the one we live in now. The oceans and atmosphere were mostly anoxic although bacterial mats were starting to produce oxygen, potentially resulting in local oxygen oasis. One feature of this time, just after the end of the Archean is the development of huge deposits of banded iron formation (BIF) which indicate the change from an ocean which contained soluble reduced ferrous iron, to a more oxygenated condition with ferric iron which is insoluble. Banded iron formations therefore have long been used as a redox proxy for conditions in the ancient ocean, however very little attention has been recently placed on the mineralogy and detailed stratigraphy of these important units. Here we show based on chemical and field analysis that the banded iron formations have cyclical variations which represent Milankovitch (climate) forcing. Using our unique datasets we go further than just identifying the cyclicity, we document the preservation of a signal from orbital precession which can be used to calculate various solar system parameters (e.g. the distance between the earth and the moon, the length of the day) at the time the rocks were deposited. We also use the chemical and climatic signals from these rocks to suggest a mechanism for their formation. Finally, we present potential future avenues of research which use Milankovitch cycles in deep-time to study processes occurring in the lower mantle.