A warm climate with CO2 levels similar to today delayed ice sheets from forming over land in the Arctic until less than 2 million years ago. That’s the latest instalment in a climate history scientists are building using sediment from a lake created by a giant meteorite impact around 3.6 million years ago. The international team has found that 3-3.2 million years ago, summer temperatures in the region were about 8°C warmer than they are today.
Julie Brigham-Grette from the University of Massachusetts, Amherst, explained that other scientists have estimated CO2 levels in the Pliocene period from 5.3 to 2.6 million years ago. “Though the estimates are quite broad, most scientists suggest that 2-3 million years ago CO2 levels may have been similar to today,” she told me. “Our data are consistent with that – the world today could be headed toward a Pliocene-like world.” And as well as pointing to the warmer future, these findings could also help unpick climate puzzles from our past.
These insights are the prize Julie and her team-mates sought on an epic trek to North-East Russia’s frozen wilderness in 2009. She was chief scientist for the US side of the team, leading the expedition alongside Martin Melles and Pavel Minyuk, chief scientists for the German and Russian sides. Their goal lay at the bottom of Lake El’gygytgyn, or Lake E. A 13 km wide crater blasted by a meteorite up to a kilometre in diameter that filled with water, Lake E has slowly collected sediment ever since. It’s unusual because it largely escaped damage from the creep of ice sheets, meaning scientists can use its sediment to rebuild conditions further back in time.
And to get there, Julie, Martin and Pavel had to pave political, financial, logistical, and actual physical paths, Julie explained. “This lake sits in an area that has no roads,” she said. “It was an amazing logistical feat to gather the drillers and equipment and get there, without damaging the environment. It was the most difficult scientific project I’ve ever undertaken.”
Once at Lake E, the scientists drilled 318m of sediment, the longest continuous Arctic land record yet, which they flew and trucked back to Melles’ lab in Cologne. Since 2009, they have been carefully scanning and processing their three sediment ‘cores’, from top to bottom, reaching ever further into the past. “I’ve worked in the Arctic for over 30 years,” Julie said. “This is the first time I’ve been able to work on a continuous record of this entire time period. It’s been a fabulous experience for all of us on the project.”
The scientists split the cores into thousands of samples to date them. They then tracked slight changes in which way magnetic particles in the samples line up, which match changes in Earth’s magnetic field. By scanning the samples with light and X-rays, the researchers could work out their chemical composition and from that calculate the amount of algae growing in the lake. They could then compare both magnetic particle direction and algae growth against other well-dated records.
To build a picture of the past climate, the scientists relied largely on counting pollen under a microscope. That shows which plants lived around the lake, and knowing the temperature and rainfall those plants need to survive lets the researchers work out past conditions. Last year the team published their results on the first 2.8m years from the upper part of the cores, showing surprisingly warm periods between ice ages.
But now, having reached the end of the core, they’ve found intriguing insights about the Pliocene. “The Arctic was a forested environment with no ice sheets 3.6 million years ago, and then over some millions of years developed large ice sheets that come and go periodically,” Julie explained. “There has been a lot of controversy in trying to understand how.”
Hold the ice
Understandably, CO2 levels in the air play an important role. Temperatures had been gradually declining from 65 million years ago until the last century, as weathering and eroding rocks slowly uses up CO2. “Eventually the CO2 level gets low enough, and the climate responds by a gradual building up of ice sheets,” Julie said. In recent years scientists have published sediment cores from the Arctic Ocean suggesting these were large amounts of sea ice as early as 40 million years ago. But studies had already suggested that ice didn’t build up on land at this point, Julie noted. “I’d argue that what some geologists were reporting was not consistent with what some of us were seeing on land,” she said.
The Lake E core now provides further evidence that ice sheets built up on land more recently, held back by high temperatures in a world with similar CO2 levels to today. In a paper just published in top research journal Science, they reveal their record from 2.2 million to 3.6 million years ago. It shows that until around 2.2 million years ago temperatures were higher than they have been in the past 10,000 years. “We’re seeing that up to 2.5-2 million years ago Arctic summers were still warm enough that they were maybe not able to grow really large ice sheets until sometime later,” Julie said. “This provides a new way to evaluate what people have learned from the Arctic Ocean, and we need to re-evaluate what both sets of data are telling us.”
When the ice sheets did appear, they did so in a series of steps, a fact that has been seen elsewhere, but never in the Arctic before. And together with the warm periods between ice ages they revealed last year, these latest findings are raising more important questions about Pliocene climate. “We’re having trouble explaining all of that data – it provides us with new questions,” Julie said. “That’s more or less the scientific method – once you have some answers, you also get more questions. It’s to be expected.”
- 20 more papers on the Lake E core are due out in the open-access journal Climate of the Past in August.
Melles, M., Brigham-Grette, J., Minyuk, P., Nowaczyk, N., Wennrich, V., DeConto, R., Anderson, P., Andreev, A., Coletti, A., Cook, T., Haltia-Hovi, E., Kukkonen, M., Lozhkin, A., Rosen, P., Tarasov, P., Vogel, H., & Wagner, B. (2012). 2.8 Million Years of Arctic Climate Change from Lake El’gygytgyn, NE Russia Science, 337 (6092), 315-320 DOI: 10.1126/science.1222135
Julie Brigham-Grette, Martin Melles, Pavel Minyuk, Andrei Andreev, Pavel Tarasov, Robert DeConto, Sebastian Koenig, Norbert Nowaczyk, Volker Wennrich, Peter Rosén, Eeva Haltia, Tim Cook, Catalina Gebhardt, Carsten Meyer-Jacob, Jeff Snyder, Ulrike Herzsch (2013). Pliocene Warmth, Polar Amplification, and Stepped Pleistocene Cooling Recorded in NE Arctic Russia Science : 10.1126/science.1233137