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Antarctica’s Descent into the Cold

June 22, 2021

Atmosphere, Climate Change, Daily, Earth Science, Environment, Hydrosphere, Repost, Science

When and why the continent became covered in ice

Today, on the east side of Antarctica, the deepest continental canyon on our planet plunges 3,500 meters (11,000 feet) below sea level. But you can’t stand on the edge of Denman Canyon and peer into its depths because the entire canyon is filled with ice from the Denman Glacier, which measures 16 kilometers across and runs for 110 kilometers along its length. The volume of ice contained in the canyon is stupendous. Yet, it is just a fraction of the ice in Antarctica. This continent’s glaciers and ice sheets account for a full 90 percent of earth’s ice and 70 percent of its freshwater. But Antarctica wasn’t always a frozen wasteland. So when and why did Antarctica become the planet’s deep freezer?

A Princeton-led team announced in 2017 that an ice core taken from near McMurdo station in East Antarctica contained 2.7 million-year-old ice. The dating method relied on an analysis of trace amounts of argon and potassium gases in the ice, and the results are considered accurate to plus or minus 100,000 years. This information provides a good starting point — Antarctica must have been icy 2.7 million years ago.

The International Commission on Stratigraphy places the age of this McMurdo ice core as late Pliocene. But the age dating is simply evidence that there was ice in Antarctica in the late Pliocene (Piacenzian). It’s not evidence of when the continent first started icing over.

We need to step back and take a larger view of geological history. When we do, we find the earth entered into its current glacial age about 34 million years ago near the Eocene-Oligocene boundary. Prior to this latest cooling trend, the earth spent about 100 million years under hot-house, ice-free conditions. Cores of ancient seafloor sediments, recovered from drilling projects in Tanzania, contain ancient seafloor sediments that record the history of the late Eocene. There, in the fossil record, we find evidence of the planet’s descent into the cold. Our question of “when?” is now bracketed between 34 and 2.7 million years ago.

Deep below the present-day ice surface is another unseen Antarctica consisting of rugged mountains and valleys. Some of these mountain ranges rival the European Alps, and high mountain glaciers were undoubtedly the first large ice features to develop in Antarctica. These first ice events probably did occur at the end of the Eocene, but high mountain glaciers are a far cry from a continent wholly covered in ice.

Fossil evidence of tundra vegetation from the McMurdo Dry Valleys indicates that about 14 million years ago, that part of Antarctica was similar in climate to the tip of South America. Since then, the continent has further cooled, with the East Antarctica Ice Sheet reaching full size about 14 million years ago, but the West Antarctica Ice Sheet not reaching its present size until about 6 million years ago.

All evidence indicates that the development of the cold Antarctica we know today has been a gradual process starting about 34 million years ago and continuing into the late Miocene and Early Pliocene. The thick ice cap at our southern pole probably did not fully reach its current state until relatively recently in geologic history during the late Miocene. The long descent of Antarctica into a land of ice was driven by plate tectonics and the opening of the Southern Ocean.

Geography and Plate Tectonics

Some 200 million years ago, Antarctica was part of the Gondwana supercontinent, wedged between Africa, India, and Australia, and close to the tip of South America. As the supercontinent broke apart, Antarctica started migrating south, but the tips of South America and Antarctica remained attached. It was well into the Eocene, some 40 million years ago, when Antarctica settled into its current position over the South Pole.

Once Antarctica took up residence at the southern extremes of the globe, the stage was set for ice and snow. But a final push was needed to seal the deal. This critical event was the split between South America and Antarctica, and the subsequent opening of Drake’s Passage. The timing of this split is heavily debated, but it may have occurred as late as 17 million years ago.

The split from South America was an important event because it marked the formation of the last great ocean on our planet, the Southern Ocean. The only place in the world where an ocean current can circumnavigate the globe and return upon itself like a mythical ouroboros is in the southern hemisphere. There, unimpeded by continents and driven by westerly winds, the Antarctic Circumpolar Current (ACC) endlessly circles the globe flowing from west to east.

The ACC extends from the sea surface to depths of 4000 meters, and the water flow measures approximately 175 million cubic meters per second. For scale, this is about 100 times greater than the combined flow of all the rivers on the planet. The circumpolar current travels at speeds over 3 km per hour, and water temperatures range from -1 to 5 degrees Celsius.

A nickname for the ACC is the “giant mixmaster” because it connects the Atlantic, Pacific, and Indian Oceans. When water from one of these oceans entrains within the ACC, it can be transported to either of the other two oceans.

The primary forces creating the ACC are strong westerly winds and changing ocean surface temperatures. Winds provide the power that moves the current along, but the current itself is constrained by physical barriers, temperature differentials, and salinity changes. Its narrowest constriction point is the 800 km stretch between South America and Antarctica. This point is where the ACC flows through Drake’s Passage.

The ACC encircles Antarctica in the same way atmospheric currents, like the jet stream, circle the globe around a polar vortex. This deep, cold mass of circulating water provides a barrier to heat transfer, keeping the warmer northern waters at bay. The Southern Ocean seals in Antarctica’s cold, which is a critical component of why Antarctica remains a frozen world today. But while the circumpolar currents offer some mitigation to polar warming, they cannot ward off the future of a melting continent.

Investigations of Anthropocene climate change tell us sea levels are rising, and significant portions of that rise are fueled by melting ice. So as we look to the future and seek to tackle the daunting challenges of climate change, it is appropriate to also reflect on the past and first understand how we got to where we are today.