What was the Arctic like six to 10 million years ago? Drilling on the Lomonosov Ridge has revealed that summers in the North Pole were once completely ice-free.
Led by the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), an international team of scientists collected unique sediment samples during a 2014 expedition.
Until recently, much of the Arctic's past climate and sea ice cover has been limited, as only a few sediment cores have been sampled from the region.
The researchers not only discovered that summers were ice-free six to 10 million years ago, but also that sea surface temperatures peaked at nine degrees Celsius. In the spring, autumn and winter, however, the ocean was covered by sea ice of variable extent.
This, the team says, provides a new benchmark for future studies on the current global warming event.
"The Arctic sea ice is a very critical and sensitive component in the global climate system. It is therefore important to better understand the processes controlling present and past changes in sea ice," explained Prof. Ruediger Stein, AWI geologist, expedition leader and lead author of the study. "In this context, one of our expedition's aims was to recover long sediment cores from the central Arctic, that can be used to reconstruct the history of the ocean's sea ice cover throughout the past 50 million years."
Specifically, the researchers collected 18 sediment core samples from the western slope of the Lomonosov Ridge, a large undersea mountain range in the central Arctic that was believed to have experienced massive, recurring landslides in the past. One core sample, in particular, stood out to the team, providing answers that they had long been searching for.
"With the help of certain microfossils, so-called dinoflagellates, we were able to unambiguously establish that the lower part of this core consists of approximately six to eight million-year-old sediments, thereby tracing its geological history back to the late Miocene," Stein added. "With the help of so-called 'climate indicators or proxies', this gave us the unique opportunity to reconstruct the climate conditions in the central Arctic Ocean for a time period for which only very vague and contradictory information was available."
Previously, scientist thought that the Arctic Ocean was already covered in dense sea ice year-round six to 10 millions years ago - much like it is today.
"Our data clearly indicate that six to ten million years ago, the North Pole and the entire central Arctic Ocean must in fact have been ice-free in the summer," Stein said.
One of the organic compounds, or biomarkers, preserved in the cores was a carbonaceous alga that lives in surface water, meaning that it needs open water and sunlight to live.
"Since in the central Arctic Ocean sunlight is only available during the spring and summer months and is pitch-dark at all other times, the data derived from these carbonaceous algae provide us with information about the surface water conditions during the summer period," Stein added.
Based on their findings, the researchers estimate that the surface water temperature of the Arctic Ocean was between four and nine degrees Celsius in the late Miocene. This, therefore, indicates that the ocean was ice-free in the summer, as the temperatures would have been well above zero.
"By combining our data records on surface water temperature and on sea ice, we are now able to prove for the first time that six to ten million years ago, the central Arctic Ocean was ice-free in the summer," Stein concluded. "In the spring and the preceding winter, on the other hand, the ocean was covered by sea ice. The seasonal ice cover around the North Pole must have been similar to that in the Arctic marginal seas today."
The study was published in the April 4 issue of the journal Nature Communications.
