During the late Pliocene, Arctic sea ice began to expand rapidly. The new ice created changes to sea level, albedo, the thermohaline circulation, and a host of other factors that still drive the planet’s climate today. But piecing together what caused the ice to expand rapidly has remained an elusive goal for scientists.
Now, a new study by Ma et al. shows that the sea expansion coincided with the formation of Siberia’s Irtysh River 2.77 million years ago. Previous work has shown that the Irtysh River was once a series of inland rivers that drained into a large paleolake in the Junggar Basin, located in northwestern China. But at some point, the basin burst, and the Irtysh began to flow northward toward the sea.
By analyzing neon-21 isotopes along with aluminum-26/beryllium-10, the researchers determined the timing of this critical event. Isotopes like these can be used to date rock and sediment samples because they are cosmogenic in nature and decay at different rates, meaning that if a sample is exposed to cosmic rays at the surface, the isotopes will be created. Then, if the sample is buried, the different nuclides will decay at different rates, providing insight into how long the sample has been sequestered from cosmic rays. With this technique, the scientists reconstructed much of the Junggar Basin’s geologic history and imply when the Siberian-Arctic river system began supplying fresh water to the Arctic Ocean.
The new water provided by the Irtysh created a layer of fresh water roughly 9 meters thick in the Kara Sea, which lies off of western Siberia. The scientists say this sudden influx of fresh water would’ve disrupted the vertical stability of the water and reinforced the stratification of vertical circulation. In combination, these changes created more sea ice in the Arctic, which then drove a series of albedo-based feedbacks, creating colder temperatures and yet more ice. The results show what an incredible impact even a single freshwater input can have in driving sea ice formation and the planet’s climate at large. (Geophysical Research Letters, https://doi.org/10.1029/2021GL093217, 2021)
—David Shultz, Science Writer