The evidence is captured in a 1,034-foot core sample the team drew from the bottom of Lake El'gygytgyn, known informally as Lake E. It formed 3.6 billion years ago after a meteor punched a crater in Russia's northeastern Arctic. The crater filled to form a lake 7 miles across and some 560 feet deep. The area around the lake managed to remain ice free during the ebb and flow of continental ice sheets during the Pleistocene ice ages, allowing sediment layers – and the pollen and other climate indicators they contain – to build up uninterrupted for the last 3.56 million years.
The team published initial results from the Lake E core last year, focusing on the Pleistocene. This latest effort focuses on the climate from the earlier, mid Pliocene through the onset of the glacial cycles.
“There are sites all over the Arctic that have little pieces of information” about the climate at different dates during this time, Brigham-Grette says of other places where teams have taken sediment samples from lakes and ponds. “All these little pieces of information tell us that the Arctic had tremendous forest cover in the past.”
Between 3.56 and 3.4 million years ago, the region would have looked quite exotic compared with today's Arctic, she says. Pollen and other indicators point to a region cloaked in Douglas fir and hemlock all the way to the shores of the Arctic Ocean, which the team posits was ice free in the summer.
Today, “you've got to go pretty far south and west in Russia to get those kinds of forest types,” she says. The vegetation and other indicators speak to summer temperatures in the Middle Pliocene at the site that reached the high 50s to low 60s Fahrenheit, roughly 14 degrees warmer than today. The region saw precipitation amounts of perhaps 23 inches a year, compared with 15 inches today.
The warmth and moisture, along with the vegetation present, are consistent with atmospheric CO2 levels of about 400 parts per million, a level the atmosphere is nudging today – a result of the buildup of CO2 from burning fossil fuels.