Breaking the ice — Climate systems during Snowball Earth

Featuring image: modern sea ice at Antarctica. Denis Luyten (Wikimedia Commons), public domain (CC0).

Paper: Orbital forcing of ice sheets during snowball Earth

Authors: R. N. Mitchell, T. M. Gernon, G. M. Cox, A. R. Nordsvan, U. Kirscher, C. Xuan, Y. Liu, X. Liu, X. He

When you think about the Earth, you might imagine a blue and green globe orbiting the Sun. But the face of Earth has changed significantly over its life time and in the past, there were times when the Earth resembled more to a frozen, white snowball. Geologists, studying the climate during these cold epochs, found a connection between climate conditions in frozen oceans and variations of Earth’s orbit.

Mitchell and his colleagues looked into old sediments in Australia, which were deposited there during the Cryogenian, 635 – 720 million years ago. There is global evidence, that the world was entirely covered by ice and snow during this time. This condition is known as Snowball Earth. The sediments show signs of climate variations, which seem to be connected to the variation of the orbit in which our planet circuits the Sun. These variations might have helped to sustain non-frozen oasises. Our understanding of the climate during the Cryogenian period is difficult and researchers are debating if the world was completely frozen or not. This new results would point towards a more dynamical climate system.

The axial tilt of Earth’s rotational axis with respect to the sun is called obliquity. Additionally, the rotational axis shifts slowly over time. The technical term for this movement is precession. Mysid (NASA, Wikimedia Commons), public domain (CC0).

The shape of the orbit of the Earth around the Sun is defined by its radius and four additional parameters: Eccentricity describes how oval the orbit is (as the orbits of celestial bodies are usually not perfect circles); the obliquity is the deviation of Earth’s rotational axis and the precession is the wobbling of this rotational axis. Variations in orbital parameters cause a change in the amount of solar irradiation that reaches the surface of the Earth and occur in periodic cycles due to the gravitational interactions between the planets of our solar system. These variations are named Milanković cycles after the Serbian mathematician and geoscientist Milutin Milanković. Scientists knew already that these cycles are related to recurring changes in the global climate system, but their effects during the times of worldwide glaciation is still poorly understood.

How can we know about these variations in the past from sediments? The group of Mitchell and his coworkers looked into two outcrops in Australia, where sediments from the Cryogenian epoch were preserved. Among those sediments, iron rich formations named BIFs (banded iron formations) occurred. BIFs are not uncommon and usually indicate a change in the concentration of oxygen in seawater, when water soluble Fe2+ is oxidised to insoluble Fe3+ and deposits on the seafloor. To analyse the iron content, the scientists measured the magnetic susceptibility of the sediments, which means how easy the sediments can be magnetised. The more iron the sediments contain, the higher is their magnetic susceptibility. When the authors tried to look for variations of the iron content inside the sediments, they found a remarkable agreement between the variation in iron and the Milanković cycles. In times of higher solar irradiation, more iron was deposited inside the sediments.

How can that be? Somehow the higher solar influx had to cause an increase in the oxygen concentration of Earth’s sea water, which then led to the sedimentation of more iron-rich sediments. So, the geologists looked deeper into the composition of the iron to understand where it is coming from, using isotopes. Isotopes are often used in geology to study the interactions between ocean, land, and atmosphere. The isotopic composition of the iron bearing minerals in the very iron-rich parts of the sediments imply an extensive exchange between ocean and atmosphere. This exchange can only occur, if the oceans were at least partly ice-free. The increase of irradiation caused by the Milanković cycles was apparently strong enough to melt parts of the frozen oceans. It seems that Earth was maybe not a hard, frozen snowball, but more slushy ice water mixture and the climate system was maybe much more complex than previously thought.

‘Breaking the ice — Climate systems during Snowball Earth’ by Max Winkler is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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