[Rockhounds] 'Snowball Earth': The Best Evidence Yet May Have Just Been Found

[Kreigh Tomaszewski]

For nearly 60 million years, our home planet was likely frozen into a big
snowball.

Now, scientists have discovered evidence of Earth's transition from a
tropical underwater world, writhing with photosynthetic bacteria, to a
frozen wasteland – all preserved within the layers of giant rocks in a
chain of Scottish and Irish islands.

The team, led by researchers from University College London (UCL), examined
more than 2,000 grains of zircon from 11 sandstone samples, taken from up
to 200 meters within the 1.1 km-thick (0.7 miles) Port Askaig formation,
and the older, underlying Garbh Eileach formation, which is 70 meters thick.

These formations are part of the Dalradian Supergroup
<https://atrium.lib.uoguelph.ca/server/api/core/bitstreams/864d58f7-fac7-4ff7-b578-a9092bd02815/content#:~:text=The%20Port%20Askaig%20Formation%20is,(Lossit)%20and%20Bonahaven%20Formations.>
of
Scotland and Ireland, a chain of geological formations spanning from
Donegal in Ireland in a north-easterly line
<https://www.geolsoc.org.uk/~/media/shared/images/groups/commissions/stratigraphy/Straigraphy%20webpage%20Resource%20images/Dalradian_map.gif?la=en>
up
through the center of Scotland, exposed to the surface in places like the
Scottish island Garbh Eileach, where the researchers found their evidence.

Grains of zircon deposited in sedimentary layers can be used to determine
the age of a rock layer. As zircon forms, it rejects any lead
<https://www.gsoc.org/news/2020/12/07/zircon> from nestling within its
structure. But it always contains a degree of uranium, which eventually
decays into lead at a constant rate over time, even if it's nestled within
lead-hating zircon.

So any lead found within zircon indicates decay from uranium, which
provides an excellent record of time passing.

This technique revealed the rocks in the Port Askaig and Garbh Eileach
formations were laid between 720 and 662 million years ago, a bracket of
time during which Earth underwent drastic climatic change, the Sturtian
glaciation.

This was the first of two worldwide 'freezes' that may have kickstarted
multicellular life on Earth
<https://royalsocietypublishing.org/doi/10.1098/rspb.2023.2767>, so finding
such a well-preserved geological archive of this time so close to the
surface is pretty exciting (not to mention convenient).

https://www.sciencealert.com/snowball-earth-the-best-evidence-yet-may-have-just-been-found