The 4.5 billion years story of our planet Earth is filled with dramatic twists and turns. Earth went hotter and colder multiple times throughout these years, continents formed and moved, and more complex life emerged. One such dramatic turn happened around 700 million years ago.
The entire planet got covered in thick ice, like a snowball. From poles to the equator, oceans and land surfaces were all covered in thick ice. American geologist, J.L. Kirschvink, coined the term ‘Snowball Earth’ referring to this extreme glaciation.
Challenges to the Snowball Earth
Along with the evidence supporting Snowball Earth, scientists also discovered evidence challenging this theory. “One of the most fundamental challenges to the Snowball Earth theory is that life seems to have survived. So, either it didn’t happen, or life somehow avoided a bottleneck during the severe glaciation” said Dr Thomas Gernon, Associate Professor in Earth Science at the University of Southampton.
The other challenge is the layers in the glacial rocks deposited in the ice-covered oceans. The rock layers show cycles similar to that of the climate cycles, resembling the advance and retreat of ice sheets. For the frozen ocean, entirely cut off from the atmosphere, without the normal hydrological cycle, climate variations may not normally occur in the snowball earth. This is called the sedimentary challenge.
A recent study by researchers from the University of Southampton, the Chinese Academy of Sciences, Curtin University, the University of Hong Kong, and the University of Tübingen, unfolds some new possibilities through their findings published in Nature Communications journal. These new possibilities have pointed towards Earth’s orbit as a potential answer to the challenges mentioned above.
Understanding the rocks
The team studied the magnetic characteristics of glacial rocks formed about 700 million years ago in the South Australian outback. During that period, Australia was located towards the equator. These rocks are made up of layers of iron and silica-rich materials and the team targeted the “Banded Iron Formation” to find their answer. “Banded Iron Formation” is caused due to alternating layers of iron-rich and silica-rich material, in these sedimentary rocks. This formation provided compelling evidence to the snowball earth, as the ocean covered in ice allowed iron to accumulate, resulting in the Banded Iron Formation.
The team used magnetic susceptibility, a measure to find the extent to which a material can be magnetized by a magnetic field. They discovered that the layered rocks have evidence for nearly all orbital cycles. The magnetism of these rocks depends on the changes in earth’s orbital shape, tilt of earth’s spinning axis, and orientation of the earth’s axis. In the 1920s, astronomer Miluntin Milankovitch hypothesized that these astronomical cycles change the amount of solar radiation reaching the earth’s surface, thereby controlling the climate.
Lead author and professor at the Chinese Academy of Science, Professor Ross Mitchell states, “Even though Earth’s climate system behaved very differently during the Snowball, earth’s orbital variations would have been blissfully unaware and just continued to do their thing.”
“This observation is important, because complex multicellular life is now known to have originated during this period of climate crisis, but previously we could not explain why”, says Dr Thomas Gernon, Associate Professor at the University of Southampton, and a co-authors of the study.
The findings of this study explained the reason behind the presence of sedimentary rocks during snowball earth as well as the evidence of flowing water, giving answers to how life survived and evolved into more complex life forms during that severe glaciation period.
Journal Reference: Nature Communications, https://doi.org/10.1038/s41467-021-24439-4
Written by Moumita Mazumdar & Raghul M R