Science

Hunt begins for ancient Antarctic ice—and clues to Earth’s response to rising temperatures

In East Antarctica, ice has piled up, year after year, to more than 3 kilometers thick. It preserves many secrets: meteorites from outer space, dust from ancient volcanic eruptions, and mysterious subglacial lakes. But the most prized scientific treasures from this realm are the tiniest: trapped air bubbles that record shifts in Earth’s climate. By drilling deep cores from the ice, scientists have read this record as far back as 800,000 years.

Now, six international drilling teams are attempting to double that history—reaching back 1.5 million years. “It’s kind of become a trend,” says Dorthe Dahl-Jensen, a glaciologist at the University of Copenhagen. “Everyone has to drill for the oldest ice.” It’s not just for bragging rights: The goal is to probe a time when Earth was warmer and ice ages were shorter than today, in search of clues to how Earth will react as the climate warms again because of greenhouse gas emissions.

One leading effort, an €11 million European-funded project, will begin drilling this year. The scientists flew to New Zealand this month, where they will isolate to avoid carrying COVID-19 on their journey south to their target drill site, Little Dome C, high on the Antarctic plateau. They will have company there: About 6 kilometers away, scientists from Australia will also begin drilling this year.

China began its own drilling in 2012, at Kunlun Station on Dome A, near the highest point on the Antarctic Ice Sheet. But progress has been slow, because reaching the remote location leaves only a few weeks to drill each year. Japan and Russia plan to begin to drill similar cores later this decade at Dome Fuji and Dome B. And last month, the United States funded a $25 million Center for Oldest Ice Exploration (COLDEX), led by Oregon State University (OSU), to hunt for its own drill site.

The “domes” the teams are targeting are stable regions in East Antarctica where the ice stands high. Snow falls slowly but steadily at those sites, trapping air bubbles that survive as the snow is compacted into ice.

The teams hope to extract a perfect layer-cake record of climate changes stretching back 1.5 million years. But success is far from certain. Shifting ice can fold young ice layers beneath older ones. Unexpected melting from subsurface heat can create gaps. “The oldest ice project is inherently high risk,” says Kenji Kawamura, a glaciologist at Japan’s National Institute of Polar Research, who is leading the Japanese effort. The redundant drilling efforts, he says, better the odds that at least a couple will succeed, allowing the cores to corroborate each other.

Frozen in time

In search of clues to Earth’s ancient climate, six teams will drill cores containing ice as old as 1.5 million years. The new cores would surpass the age of one from Dome C, and could rival ancient but discontinuous samples from Allan Hills.

C. BICKEL AND N. DESAI/SCIENCE

Geopolitical ambitions are also driving the multiple drilling efforts. With its $33 million campaign at Little Dome C, Australia is hoping to reestablish its ability to travel deep into East Antarctica from its oceanside bases. Similarly, Russia will erect a new base at its historic Vostok drilling site so it can attempt to drill for old ice 300 kilometers away at Dome B, says Vladimir Lipenkov, an ice core scientist at Russia’s Arctic and Antarctic Research Institute.

Once the precious cores are returned to laboratories, geochemists plan to analyze the ancient air inside the bubbles. They can directly measure ancient carbon dioxide (CO2) levels, and from the mix of carbon isotopes can infer how much of the gas came from biological sources such as decaying plants, versus geological ones like volcanoes. Meanwhile, the ratio of oxygen isotopes from the melted ice can point to temperatures and storm frequency. Together, those data can shed light on what Dahl-Jensen, a leader of the European Beyond EPICA drilling project, calls a “powerful argument in our climate debate”: how much of a role CO2 played in past temperature shifts.

Dahl-Jensen’s project is a successor to the European Project for Ice Coring in Antarctica (EPICA), which in 2004 finished drilling an 800,000-year-old core, the current record holder, at Dome C. But the ancient climate it revealed was much the same as the modern climate, at least before human-driven global warming set in: a comparatively cold period when the world went in and out of deep 100,000-year-long ice ages, prodded by tiny swings in Earth’s orbit.

The new cores would open a window on a warmer climate, more than 1 million years ago, when ice ages occurred every 40,000 years. They might also reveal whether changes in atmospheric carbon were behind Earth’s jump to the 100,000-year ice age cycle. In one theory, carbon had little role: It holds that until 1 million years ago, the northern ice sheets formed on top of a thick layer of soil that, like grease, kept them unstable and relatively short-lived. Once the ice sheets scraped away the soil and hit bedrock, however, they grew taller and more persistent, perhaps leading to enhanced reflection of sunlight and other feedbacks that drove cooling and extended their life span. In another view, however, the trigger for longer ice ages was a small drop in CO2 levels, perhaps driven by dust-fertilized plankton in the Southern Ocean that absorbed the gas from the atmosphere.

In the past few years, ice even older than the drillers will reach has offered tantalizing clues. In shallow “blue ice” regions in the Allan Hills of Antarctica, ice flows across buried rocky ridges, driving layers as old as 2.7 million years upward while harsh winds strip away younger ice. Although blue ice samples lack the easy dating and high resolution of the continuous ice cores, they revealed that CO2 levels in the earlier, warmer period were on average quite similar to levels seen in the past million years, a hint that a CO2 drop might not have triggered the climate shift. “We feel like we’ve just scratched the surface of these blue ice regions,” says Ed Brook, COLDEX’s principal investigator at OSU Corvallis.

The COLDEX team will continue to explore the blue ice even as the researchers hunt for a continuous core at a deep drill site somewhere between the South Pole and Dome A. They will scout cautiously, deploying a heated probe that melts its way through the ice all the way to the bedrock, measuring dust as it descends. Dust levels can be compared with known records as a quick and dirty way to gauge the age of the ice. “If we don’t get older than 800,000 years, we’ll say OK, that’s not a good place,” Brook says.

Wherever the U.S. researchers end up, they will be able to build on the experience of the European and Australian teams, which hope to hit bedrock by the middle of this decade. The teams have already discussed how best to make use of their cozy proximity, by Antarctic standards, and are even talking about getting together for an outdoor soccer match, says Joel Pedro, a paleoclimatologist at the Australian Antarctic Division who leads the project. “Though we’d probably struggle” against the Europeans, he says. “It might have to be a cricket match—so we’ll win.”

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