A key stage at the start of the great glaciations has been described for the first time by a new study in which the Andalusian Earth Sciences Institute (IACT) (CSIC-UGR) was involved. The study suggests that it can occur again to Earth in the future.
The results of the study, which were published recently in the scientific journal Nature, claim to have identified a new link that could offer insights into the start of the ice ages on Earth.
Melting of icebergs in Antarctica could be the key to triggering a sequence of mechanisms that make the Earth experience extended periods of global cooling, says Francisco J. Jiménez-Espejo, a researcher at the Andalusian Earth Sciences Institute (CSIC-UGR), whose findings were recently published in the renowned journal Nature.
For a long time, it has been known that changes in the Earth’s orbit, while moving around the Sun, activate the start or end of glacial periods by influencing the amount of solar radiation reaching the surface of the planet. But, to date, how slight changes in the solar energy reaching Earth can cause such drastic shifts in Earth’s climate is still a mystery.
As part of the new study, a multinational group of scientists put forward, when the orbit of the Earth around the sun is just apt, the Antarctic icebergs start melting further and further away from the continent, thus shifting enormous volumes of freshwater from the Antarctic Ocean into the Atlantic.
This process makes the Antarctic Ocean highly salty, whereas the Atlantic Ocean turns fresher, which influences the overall ocean circulation patterns, thus drawing CO2 from the atmosphere and decreasing what is called the greenhouse effect. These are the first stages marking the start of an ice age on Earth.
During this study, the researchers employed various methods to rebuild oceanic conditions of the past, such as by finding small fragments of rock broken away from Antarctic icebergs while they melted into the ocean.
Such deposits were acquired from marine sediment cores retrieved by the International Ocean Discovery Program (IODP) as part of Expedition 361 off the sea-margins of South Africa.
With the help of the sediment cores, the researchers could rebuild the history of the icebergs that arrived at these latitudes in the last million and a half years, since this is one of the most continuous records ever known.
The research illustrates how such rocky deposits seem to be consistently related to changes in deep ocean circulation, rebuilt from chemical changes in tiny deep-sea fossils called foraminifera.
The researchers employed new climate simulations to examine the hypotheses put forward. They found that icebergs carry enormous volumes of freshwater northward.
The researchers are surprised to have discovered that this teleconnection is present in each of the different ice ages of the last 1.6 million years. This indicates that the Antarctic Ocean plays a major role in the global climate, something that scientists have long sensed, but that we have now clearly demonstrated.
Aidan Starr, Study First Author and PhD Student, Cardiff University
During the IODP 361 expedition aboard the JOIDES Resolution research vessel, Francisco J. Jiménez Espejo, a researcher at the IACT, took part as a specialist in inorganic geochemistry and physical properties. Between January and March 2016, for two months, the researchers sailed between Mauritius and Cape Town to collect deep-sea sediment cores.
Jiménez Espejo’s key contribution to the study was to find the geochemical changes related to glacial and interglacial periods, which enabled the age of the sediment and its sensitivity to the various environmental changes related to those periods to be estimated with higher accuracy.
During the last three million years, the Earth started to undergo periodic glacial cooling. Nearly 20,000 years ago, during the latest episode, icebergs continuously arrived at the Atlantic coasts of the Iberian Peninsula from the Arctic. At present, the Earth is in a warm interglacial period called the Holocene.
But the steady increase in global temperature due to CO2 emissions from industrial activities could have an impact on the natural rhythm of glacial cycles.
Eventually, the Antarctic Ocean could turn too warm for Antarctic icebergs to be able to shift freshwater north, and thus, it would be hard for a fundamental stage to the start of the ice ages—the changes in thermohaline circulation—to occur.
According to Ian Hall, co-director of the scientific expedition who is also from Cardiff University, suggests that the findings may help understand how the Earth’s climate might respond to anthropic changes.
Last year, during an expedition aboard Hespérides, the Spanish Navy research vessel, we were able to observe the immense A-68 iceberg that had just broken into several pieces next to the islands of South Georgia. Ocean warming may cause the trajectories and the melt patterns of these large icebergs to alter in the future, affecting the currents and, therefore, our climate and the validity of the models that scientists use to predict it.
Francisco J. Jiménez-Espejo, Researcher, Andalusian Earth Sciences Institute
Starr, A., et al. (2021) Antarctic icebergs reorganize ocean circulation during Pleistocene glacials. Nature. doi.org/10.1038/s41586-020-03094-7.