The ice – hidden some 2,700 meters deep in the Antarctic ice sheet – could be up to 1.2 million years old and solve a big mystery about our climate.

About 15 years ago, scientists drilled a deep hole in the Antarctic ice sheet to eventually unearth a drill core that harbored ice up to 800,000 years old. An analysis of that ice confirmed that there was a close connection between the CO2 concentration in the atmosphere and the Earth’s climate over all those years. But not only that, the ice core also revealed a rather mysterious phenomenon known as the Middle Pleistocene Transition.

Middle Pleistocene Transition

What those 800,000 years of ice revealed was that the climate fluctuated quite a bit over all those years: warm periods (interglacials) alternated with cold periods (ice ages). And what struck me was that each of those periods lasted about 100,000 years. And that is the case today, says Roderik van de Wal, climate researcher at the UU. “The last cold period started about 120,000 years ago and we are now about 11,700 years in the warmer Holocene.”

40,000 years

But where the ice core that scientists drilled at the beginning of this century revealed that ice ages had to give way to interglacials about every 100,000 years, significantly older sediment cores tell a different story. “About 1 million years ago, the ice ages and interglacials followed each other much faster; on average every 40,000 years.” And that naturally raised the question of why the behavior of the ice cycles fundamentally changed somewhere between 800,000 and 1 million years?

Milanković theory

Climate researchers can only partially answer this question at the moment, says Van de Wal. “The Milanković theory plays a role. According to this theory, the gravitational pull of other planets in the solar system affects the Earth’s orbit and position, and thus the amount of solar radiation it receives. But that theory in itself is not sufficient to explain the major change in climate cycles, and it does not change around that period.” And that gnaws. And that’s why researchers decided to hunt for ice that is older than 800,000 years and thus dates back to the period when the glacial cycles changed. It should result in the drilling of the oldest ice in the world. “It is hoped that ice that is about 1.2 million years old will be recovered, which will allow us to reconstruct the temperature and CO2 concentration around that Middle Pleistocene Transition and determine what exactly happened.”

Little Dome C

The exciting research project was preceded by a long period of preparation. The first question, of course, was where to look for ice cream of such an age. This has been carefully researched, Van de Wal knows (see box). And eventually a location was rolled out there: Little Dome C, in Antarctica.

Where is the best place to drill for the oldest ice in the world? That depends on several factors. For example, it is important that the ground heat is limited. At the heart of our earth are radioactive elements that give off heat as they decay. This causes the soil to warm up slightly. This phenomenon is worldwide, but the degree of warming varies from place to place. If you want to bring out the oldest – and therefore deep-lying – ice, you have to find a place where the bottom heat is limited to such an extent that the lower layers of ice do not melt. In addition, it is important that precipitation falls in such a place: not too much, but also not too little. Because it is this precipitation that eventually forms the thin annual layers of ice (actually nothing but compacted snow) that can give the researchers more insight into how the climate changes over the years. Finally, it is also important that the ice sheet formed by all those layers does not move too much (sideways). Because then the chronology of the layers can be affected and it becomes difficult to date those layers – and the temperature and CO2 concentration to which they testify.

And now preparations for drilling that 1.2-million-year-old ice are in full swing. For example, the drilling rig is already in place and has already been drilled about 130 meters deep. Ice up to 3000 years old has been extracted. It does indicate that the researchers will have to drill much deeper: the expectation is even to about 2700 meters deep.

And that is – for several reasons – an enormous challenge. Little Dome C, for example, is quite remote and that makes it logistically difficult to get people and equipment there. “And then you can only work during the Antarctic summer. That slows down enormously.” The drilling itself is not easy either. For example, care must be taken that the carefully made drill hole does not freeze up again or that the drill bit gets stuck. In addition, the drilling rig can only extract 4.5 meters of ice each time. “As a result, the drill has to be brought up and down endlessly before you reach a depth of 2700 meters.” And then the drilled-out ice also has to be stored carefully. “That will be done locally for the time being, but eventually it will also have to be brought to Europe for analysis.”

Analysis

During that analysis, the focus will be on air bubbles that are trapped in the ice layers. “Those layers of ice are compacted snow. This means that a large part of the air – because later precipitation came on top of it – has been pushed out. But there are always some small air bubbles left and the air in them comes from the time when that ice layer was formed. We can remove those air bubbles from that ice and use them to determine the CO2 concentration from the time in which that air bubble was formed.” In addition, researchers can also read from the ice layers at which temperatures they were formed. “To do this, we look at the ratio between two oxygen isotopes – oxygen-18 and oxygen-16.”

Interest

The measurements should provide more insight into how the climate changed during the Middle Pleistocene Transition. “If – as the Milanković theory assumes – the irradiance changed, did that lead to a change in temperature first, or did the CO2 concentration change first? There is a lot of discussion about that.” The oldest ice can help settle those. “And if we know the order of those processes, we can also put the processes in the correct order in our climate models and then also better predict, for example, how large the ice sheets became in the past. And that is important, because the ice sheets have a major influence on the climate system.”

But the researchers’ study isn’t just about better understanding past climates; the findings that ultimately emerge from the research project may also help to understand our current and future climate. “To better understand how the current climate system functions, we often look at analogies: historical periods in which similar climate changes occurred as now.” Those historical periods can reveal the impact of climate change like today’s. “But to find out whether current climate change has the same impacts as comparable climate changes in the past, we need to better understand the climate system in general.” And that requires that as many climate mysteries as possible – such as the Middle Pleistocene Transition – be clarified.

Patience is in order here; it could be a few more years before scientists reach the oldest ice. That is, if it waits for us at the predicted place. “We expect to find it in Little Dome C,” says Van de Wal. “But there are no guarantees.” It will therefore remain exciting in Antarctica in the coming years. “If we eventually manage to uncover 1.2 million-year-old ice, the research project will already be a success,” says Van de Wal. He is not concerned about the later analyses. “It will be alright.” And so it seems only a matter of time before scientists – armed with the oldest ice on Earth – fathom the Middle Pleistocene Transition.