During the Cryogenic Period 720–635 million years ago, Earth experienced one of its most severe ice ages. It is believed that during this period the climate system practically “stopped”: ice covered the planet all the way to the tropics, and the exchange between the atmosphere and the ocean was blocked for millions of years. However, a new study by scientists from the University of Southampton has demonstrated that even at the peak of global glaciation, the climate continued to fluctuate. The work was published in the journal Earth and Planetary Science Letters (EPSL).
The study is based on analysis of thin layers of sediment on the Garwellach Islands off the west coast of Scotland. These rocks were formed during the Sturtian Ice Age, Earth's longest “frozen” period, lasting about 57 million years. Scientists studied 2,600 consecutive layers of the Port Askaig Formation, each layer representing a year of sedimentation.
Co-author of the work, Professor Thomas Gernon, said: “These rocks preserve the full range of climate rhythms we are familiar with today – annual seasons, solar cycles and interannual variations – all during global ice ages. This is astonishing.”
Lead author Dr Chloe Griffin said the layers formed in calm, deep-sea conditions beneath the ice sheet and reflected seasonal cycles of freezing and thawing. Statistical analysis of layer thickness shows repeated fluctuations on the scales of several years, decades and centuries.
“We found distinct climate cycles, some of which resemble modern oscillations, including processes similar to El Niño and solar cycles,” Griffin said.
To understand how such fluctuations could occur in an icy world, researchers conducted climate modeling. Models show that if the ocean were completely frozen, those fluctuations would actually be suppressed. However, with about 15% open water in the tropics, the atmosphere and ocean can interact to support climate change.
“We don't need large, ice-free oceans. Even small open waters can produce climate regimes similar to modern regimes,” explains Dr. Minmin Fu, who led the simulation.
The authors stress that the observed fluctuations are likely just a short-term period – given that the climate is generally extremely cold and stable. However, the results cast doubt on the idea of a completely “frozen” planet and support the so-called “Mudgy Earth” hypothesis, where open ocean areas may have existed during ice ages.
