CXXII Evolution of the most powerful ocean current on Earth

CXXII Evolution of the most powerful ocean current on Earth
CXXII Evolution of the most powerful ocean current on Earth

US drillship JOIDES Resolution © 2024 by Bill Crawford, IODP

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The Antarctic Circumpolar Current (CCA), the most powerful on Earth, plays an important role in the global overturning circulation, the exchange of heat and CO2 between the ocean and the atmosphere, as well as the stability of the Antarctic ice masses.

An international team of scientists led by the Alfred Wegener Institute and the Lamont-Doherty Earth Observatory has now reconstructed the velocity of the ACC over the past 5.3 million years, using climate archives in South Pacific sediments. The drill cores carried out in the pole of inaccessibility Point Nemo reveals climate-induced fluctuations in the Antarctic Circumpolar Current in earlier times

Their data show that the current lost momentum during cold periods and accelerated during warm ones. If it gained momentum in the future due to current global warming, the Southern Ocean could store less CO2 and more heat reach Antarctica, researchers conclude in a study published in the scientific journal Nature.

Fluctuations

The CEC carries more than 100 times more water than all the rivers on Earth combined, is up to 2,000 kilometers wide and reaches the deep sea. In the past, this current system was subject to considerable natural fluctuations, as revealed by current research on the aforementioned sediment cores.

The coldest phases of Pliocene and the Pleistocene later, in which the ACC slowed, correlate with the advance of the West Antarctic ice sheet. In the warmer phases, the retreat of the ice masses accelerated and continued.

The doctor Frank Lamyresearcher in the Marine Geology section of the Helmholtz Center for Polar and Marine Research (AWI) and lead author of the study states that:

The retreat of the ice can be explained by the increase in heat transport towards the south. A stronger CCA ensures more warm, deep water reaches the edge of the Antarctic ice shelf. The CEC has a major influence on heat distribution and CO2 storage in the ocean. However, until now it was unclear how the CEC reacts to climate fluctuations and whether changes in the CEC slow down or aggravate the consequences of warming. To better predict the future climate and stability of the Antarctic ice sheet using computer models, we therefore need paleodata that tells us something about the strength of the ACC in earlier warm phases of Earth’s history.

Drilling

To obtain this data, an international expedition led by Frank Lamy and geochemist Professor Dra Gisela Wincklerfrom the Lamont-Doherty Earth Observatory at Columbia University (USA), traveled to the central South Pacific on the drillship JOIDES Resolution in 2019.

There, in the subantarctic, the research team was able to obtain two long drill samples of the seabed at a depth of 3,600 meters, each composed of several sediment cores.

Professor Dr Helge Arzmarine geologist Leibniz Institute for Baltic Sea Research in Warnemünde and one of the main authors of the work, points out that:

The drilling was carried out near Point Nemo (pole of inaccessibility), the farthest place on the planet from land masses or islands where the CEC flows unaffected by continental land masses. Its past mean flow velocity can be reconstructed from sediment deposits in this region.

Ocean discoveries

The 145 and 213 meter deep surveys in the South Pacific were part of the International Ocean Discovery Program (IODP), which deciphers the history of the Earth from geochemical traces in marine sediments and rocks beneath the seabed. They were preceded by extensive exploration work on several expeditions with the research vessel Polarstern. The drill core sediments date back 5.3 million years and span three full eras:

  1. the Pliocene, during which it was up to three degrees warmer than today and the concentration of CO2 in the atmosphere reached levels similar to today’s, with more than 400 ppm;
  2. the Pleistocene, which began 2.6 million years ago and was characterized by alternating glaciations and warm periods (interglacials)
  3. and the Holocenea warm period that began with the end of the last ice age about 12,000 years ago and continues to the present day.

Evolution

The researchers analyzed the size distribution of sedimentary particles, which are deposited on the seabed depending on the speed of the current, along the layers of drill cores assigned to the different eras. In this way, they were able to determine the evolution of the CCA since the beginning of the Pliocene, when a prolonged cooling of the climate began. Sediment cores from previous trips with the Polarstern in the South Pacific provided additional information on the dynamics of the ACC in recent geological history.

Their results show that the CEC initially accelerated in the Pliocene, up to three million years ago, as the Earth slowly cooled. This was due to the increasing temperature gradient from the equator to Antarctica, which caused an intensification of westerly winds, the main driver of the ACC. Despite continued cooling, it later lost strength.

Dr Frank Lamy explains that:

The change in trend coincides with a time in which the climate and atmospheric and oceanic circulation changed radically. 2.7 million years ago, at the end of the Pliocene, large areas of the northern hemisphere froze and the Antarctic ice sheets expanded. This was due to changes in ocean currents triggered by tectonic processes in combination with long-term ocean cooling and declining atmospheric CO2 levels.

Behavior of carbon dioxide (CO2)

During the last almost 800,000 years, in which the CO2 content in the atmosphere fluctuated between 170 and 300 ppm, researchers were able to demonstrate a close correlation between the strength of the CEC and glacial cycles in sediment cores: In the periods warm, when the CO2 content in the atmosphere increased, the flow speed increased up to 80% compared to today, and in glaciations it decreased up to 50%.

At the same time, the CEC and thus the upwelling of nutrient-rich deep waters in the Southern Ocean changed between warm and glacial times, as revealed by geochemical analyzes of sediments. These analyzes show that the silicate shells of diatoms – the most important phytoplankton in the Southern Ocean – were deposited on the seafloor further north during ice ages than during warm periods.

Professor Dr Gisela Winckler states that:

The weakening of the CEC and the lower CO2 content in the atmosphere during the Pleistocene glaciations indicate weaker upwelling and stronger stratification of the Southern Ocean, i.e. greater CO2 storage.

The study concludes that human-caused climate change could cause the CEC to gain strength in the future. As a result, the CO2 balance of the Southern Ocean could deteriorate and Antarctic ice could melt more rapidly.

Background: the Antarctic Circumpolar Current

As an annular current that flows clockwise around Antarctica, the Antarctic Circumpolar Current (ACC) connects the Atlantic, Pacific and Indian oceans. It therefore plays a key role in global ocean circulation and ultimately also influences Europe’s climate via the Atlantic conveyor belt. It is driven by stormy westerly winds from the subantarctic and differences in temperature and salinity between the subtropics and the Southern Ocean.

The CEC forms a barrier to warm surface waters from the subtropics on their way to the Antarctic. At the same time, it feeds on comparatively warm deep waters from the Atlantic and Pacific. Large ocean eddies that form in the CEC and migrate southward, as well as deep-water upwelling, transport heat to the ice shelves of the continental margin, especially in the Pacific sector of Antarctica. Upwelling caused by CEC also brings nutrients to the surface, which stimulate algae growth and thus increase the biological export of carbon to the deep sea, but it also transports CO2, which escapes into the atmosphere.

Original post

Frank Lamy, Gisela Winckler, Helge W. Arz, Jesse R. Farmer, Julia Gottschalk, Lester Lembke-Jene, Jennifer L. Middleton, Michèlle van der Does, Ralf Tiedemann, Carlos Alvarez Zarikian, Chandranath Basak, Anieke Brombacher, Levin Dumm, Oliver M. Esper, Lisa C Herbert, Shinya Iwasaki, Gaston Kreps, Vera J. Lawson, Li Lo, Elisa Malinverno, Alfredo Martinez-Garcia, Elisabeth Michel, Simone Moretti, Christopher M. Moy, Ana Christina Ravelo, Christina R. Riesselman , Mariem Saavedra-Pellitero, Henrik Sadatzki, Inah Seo, Raj K. Singh, Rebecca A. Smith, Alexandre L. Souza, Joseph S. Stoner, Maria Toyos, Igor M. Venancio P. de Oliveira, Sui Wan, Shuzhuang Wu, Xiangyu Zhao: “Five million years of Antarctic Circumpolar Current strength variability”, Nature (2024). DOI: 10.1038/s41586-024-07143-3.

 
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