Insights into the ancient history of the Greenland Ice Sheet (GIS) are provided in two separate papers published in this week’s Nature. The studies report the presence of cosmic-ray-produced isotopes known as cosmogenic nuclides in bedrock from underneath Greenland and in marine sediments deposited offshore, respectively, but support different scenarios about the ice sheet’s past behaviour.
The GIS is a major contributor to present-day sea-level rise and understanding its growth and stability during past warm periods could potentially help to predict future mass loss. However, the ice sheet’s behaviour during the Pleistocene period (2.6 million to 11,700 years ago) has remained uncertain.
Joerg Schaefer and colleagues analysed isotope evidence from a bedrock core extracted from beneath the central GIS. Their findings suggest that, contrary to existing model simulations, Greenland was nearly ice-free at least once during the Pleistocene. Although several scenarios exist, the ice-free period must have lasted at least 280,000 years. The authors cannot rule out the possibility that remnant ice sheets existed during the Pleistocene, but their analysis challenges the idea of a large, continuously present ice sheet during the past several million years.
In a second paper, Paul Bierman and colleagues studied marine records containing Greenland bedrock that has been weathered and transported off the coast. Their results, which are consistent with previous model simulations, point to a fluctuating - but continuously present - regional ice sheet in East Greenland over the past 7.5 million years. These isotope data are continuous but are sourced only from East Greenland, which means the authors cannot determine whether the isotopes originate from a remnant ice sheet in East Greenland or are indicative of a diminished continent-wide ice sheet.
Although the two papers reach largely contrasting results, they both allow for the possibility of a remnant ice sheet in the East Greenland highlands, and are thus not entirely incompatible. Still, as Neil Glasser writes in an accompanying News & Views article, the papers highlight the need to “understand the dynamical processes of the ice sheet that make possible the required huge and rapid variations in the size and volume of the GrIS” and to “assess whether such variations could happen again in the near future.”