Inception of the Antarctic Ice Sheet slowed by the erosion of coastal carbon stores

A study published in Nature Communications demonstrated that the establishment of the Antarctic Ice Sheet started 300,000 years earlier than previously thought but that its growth was slowed by the erosion and release of carbon from coastal environments as sea level fell
Inception of the Antarctic Ice Sheet slowed by the erosion of coastal carbon stores

The team made up of scientists from six countries (Brazil, United Kingdom, Malaysia, Japan, Switzerland and the United States) examined the chemical composition and microfossil content of sedimentary rocks drilled in the central Mississippi, Gulf Coastal Plain, in the United States. Until today, it was believed that the emergence of an ice sheet in Antarctica took about 400,000 years, starting 34.1 million years ago, at the end of the Eocene epoch. This climate cooling was the end of the “Greenhouse” climate state that had persisted for more than 250 million years, including throughout the Mesozoic “age of the dinosaurs’. Since this first growth of ice sheets on Antarctica 34 million years ago they have been a persistent feature of our “modern” climate world. Understanding the climate conditions that promoted Antarctic ice sheet growth and allowed them to remain is a major focus of research. The new study revealed that the growth of the Antarctic Ice Sheet had already begun 34.4 million years ago, that is, 300,000 years earlier, but that it caused sea level fall and the erosion of coastal pools of organic carbon. This carbon was then likely broken down and released carbon dioxide to the atmosphere, preventing further global cooling and acting as a brake on rapid ice sheet growth. It was only once this organic carbon had been eroded and ‘used up’ that a rapid transition to the modern cold climate state took place.

But why did an ice sheet form over Antarctica? Two main theories have been proposed. On the one hand, there are those who argue that the movement of tectonic plates gradually “detached” Antarctica from the South American and Australian continents, leaving it isolated at the South Pole, surrounded by the cold Southern Ocean and separated from warmer ocean waters to the north. On the other hand, a growing number of scientists suggest that the burial of organic carbon in sediments, ultimately coming from the uptake of atmospheric carbon dioxide by plants and algae during photosynthesis, had already been reducing atmospheric carbon dioxide (CO2) levels for many millions of years. With less CO2 in the atmosphere, the Earth's surface becomes cooler, with snow on Antarctic highlands less likely to melt completely from one year to the next. Instead, snow accumulates into ice, and the layers of ice eventually form ice sheets, kilometers thick.

In the late Eocene, 34.4 million years ago, all of the water now locked-up in the Antarctic ice sheets was in the ocean, making global sea levels between 50 and 70 meters higher than today. So high, in fact, that much of today’s American state of Mississippi was beneath a shallow sea. And, just like today, it was near to the mouth of the mighty Mississippi River. In this study, researchers analyzed the sediments that formed at the bottom of this ancient sea. They measured the amounts of plant and algal material washed in from land down the Mississippi River and compared this to the quantity of the remains of organisms that lived in the sea. This comparison between marine- and land-derived organic material gives a measure of how close the study site was to the outflow of the Mississippi River, and how this distance changed through time. What they discovered was a significant shift towards land-derived material – interpreted as a ~40-meter sea level fall and with it the nearing of the Mississippi Delta towards the study site – 300,000 years before the main phase of Antarctic ice sheet is thought to begin. With this fall in sea level and advance of the coastline towards the ocean, river waters began to modify the chemical and biological composition of the region's sea waters, which was reflected in the sediments and preserved for millions of years. But what mechanism would be capable of inducing a drop in sea level at this speed and magnitude? The team of scientists concluded that the sea level fall was due to the significant growth of ice sheets on the Antarctic continent to an extent that had not been previously observed. But this was not the group's only discovery.

The study went further and investigated the consequences of falling sea levels on the Earth's climate and why this early stage event is not recognized as a major phase in the growth of Antarctic ice sheets. The key lies, once more, in organic carbon – mostly the remains of plant and algal matter, some of it buried in soils and sediments – and atmospheric carbon dioxide. Just as the long-term burial of organic carbon in marine sediments was the likely driver of the falling atmospheric carbon dioxide levels and global cooling that allowed Antarctic ice sheets to start to grow, falling sea levels do something else. As the seas retreat, they expose coastal regions and once submerged marine sediments to intense erosion by wind, rain and rivers. Organic carbon, such as plant material, that was once bound up in these sediments and environments – think of today’s tropical mangrove swamps – is now exposed to oxygen in the air and is available for bacteria to eat and convert back into carbon dioxide that can be released to the atmosphere. Increased carbon dioxide levels in the atmosphere stops global cooling, and halts or reverses the ice sheet growth that started the whole process. Such a mechanism is a negative feedback, limiting the rate of climate cooling and slowing the transition into our modern cold climate. But, there was a limit on this feedback. Once this coastal organic carbon reservoir had been eroded, or “used up”, and with nutrients flowing into the oceans, the photosynthesis of ocean algae and their capture of carbon dioxide rebalanced the system back to falling atmospheric carbon dioxide levels and global cooling. Now with no strong negative feedback left from the erosion of coastal organic carbon, the planet transitioned into the cold, ‘icehouse’ climate of the past 34 million years, which have seen Antarctic ice sheets as a permanent feature of our climate system. The events across the late Eocene show the intimate links between global carbon reservoirs – including our soils, biosphere and coastal systems – atmospheric carbon dioxide, global climate, Polar ice sheets and sea levels. We would do well to take heed.

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