Antarctica: The Antarctic Current was completed millions of years after the continent froze

The planet’s main marine current, which today thermally separates Antarctica and protects all the ice it houses, closed much later than previously believed. That is the main conclusion of a work that has studied the skeleton of fish and the size of sand grains from the past to affirm that the Antarctic circumpolar current was completed, at the earliest, 14 million years ago. This immense water front, the driving force of all ocean circulation, largely determines the global climate and is feared to be currently weakening.

Antarctica was a garden where there were even palm trees until about 34 million years ago. But at that time there was a sharp drop in CO₂ levels in the atmosphere that cooled the entire planet and the Antarctic continent began to accumulate ice and more ice. During the last half century it has been maintained that the separation first of Australia (the so-called Tasmanian Gate) and then of America (through the Drake Passage) from the new continent, until it was located at the South Pole, opened the way to oceanic waters. , which surrounded and isolated Antarctica. That current, the Antarctic Circumpolar (CCA), became a wall of cold waters that prevented the arrival of warmer waters from the tropics. This is how the largest accumulation of ice that exists on the planet was raised.

For Carlota Escutia, researcher at the Andalusian Institute of Earth Sciences (CSIC-University of Granada), “it is a very nice explanation, but our work shows that it was not like that.” And it wasn’t because the clues point in another direction. In her research, published in the scientific journal Nature Geosciencea group of scientists have reached this conclusion after combining the analysis of two types of sediments from different points through which the CEC passes.

One of these tests is the size of sand grains sedimented over millions of years. Escutia explains it: “You have a sediment at the bottom of the ocean and a current. If it is too loose, it will only be able to drag very fine materials or even nothing. But as you increase the intensity, the speed of that current will be able to drag much thicker materials. It is the same process that occurs in deserts.” From the size of the grain of sand they were able to estimate that the transfer of water occurred at a rate of about 10 centimeters per second. That’s half the current pace, which is 20 centimeters per second. That doesn’t fit with the brutal strength exhibited by the current circumpolar current. “Near the surface it is even stronger, with speeds between 25 and 40 centimeters per second,” details the senior author of the research.

The Circumpolar Current, with a front of up to 2,000 kilometers, connects the Atlantic, Indian and Pacific oceans, distributing the planet’s heat.Copernicus

The first author of the work, Dimitris Evangelinos, was a student of Escutia and has become an expert on the other indicator that they have used to contradict the dominant theory about CCA. The teeth and heads of dead fish accumulated in the fossil record preserve a certain ratio of two isotopes of the chemical element neodymium. By comparing the fossils from the two points on the seabed, they could know if the passage of the current had made them uniform. “The ratio between neodymium isotopes that we have measured in fish teeth and bones can give us information about the water masses that, in the past, acted in our study area and thus reconstruct the changes in ocean circulation,” he explains. Evangelinos.

What they saw is that the current passing through both points was not the same and was much less intense, at least until about 14-12 million years ago, that is, long after the time in which it was believed to have formed. the CEC. It is from a time when the speed of the waters at the bottom accelerates until it approaches current values and when the neodymium signal is the same between both excavated sites: everything indicates that it is in that period, and not before, when the flow of waters around Antarctica is completed and similar to the current one.

“The Antarctic Circumpolar Current is the strongest in the world and connects all the oceans. “This has global implications for the climate of our planet.”

Dimitris Evangelinos, researcher at Imperial College London and the University of Barcelona

“Today, the CEC is the strongest current in the world and connects all oceans. “This has global implications for the climate of our planet,” recalls Evangelinos, who now works at Imperial College London and the University of Barcelona. One of these implications is that its current configuration results in an enormous flow. Its dimensions almost escape the human scale. It is 23,000 km long, around Antarctica, and its front is up to 2,000 km wide. Furthermore, unlike other large currents, it continues from the surface to the bottom. According to data provided by the authors of the study, water transport is 100 to 150 million cubic meters per second, several times greater than other strong currents in the ocean (such as the Gulf or Kuroshio currents.) Through the Passage of Drake, which separates South America from Antarctica, the water flow is 135 million cubic meters per second; something that is equivalent to the flow of all the rivers in the world combined.

Such a strong current acts as a barrier, preventing warmer waters from approaching Antarctica, generating thermal insulation of Antarctica. This mechanism plays a very important role in preserving the Antarctic ice cap and would explain why the South Pole was escaping the action of global warming. “A much weaker current like the one our data propose and a lower presence of sea ice allow warmer waters to reach closer to Antarctica. This implies a very different oceanographic configuration in the past from what we have today,” maintains Evangelinos.

To put an end to the dominant theory until now, Escutia and Evangelinos’ thesis must cover the weakest flank they have: their current study is based on the analysis of sediments extracted from two ends of the Pacific Ocean, but they lack information about the Atlantic part of the current, the one that receives the waters through the Drake Passage (the ACC runs from west to east, in the same direction as the Earth’s rotation). Evangelinos is now on board the Hesperides, the oceanographic research vessel of the Spanish Navy. One of his missions will be to recover from the bottom a sediment tube with the teeth and heads of fish from millions of years ago to support his idea that the engine of ocean circulation, and, therefore, of global climate, was put in motion. It is going much later than previously thought.

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