Between Melt and Memory – The Research Benjamin Keisling There is enough ice in Greenland that if it were to melt completely, global sea levels would rise by more than 20 feet. As Greenland’s melting accelerates, there is a growing urgency to understand when the ice sheet has waned in the past in response to changes in climate. However, clues to when Greenland was last ice-free are notoriously difficult to obtain, because they are hidden today beneath the ice sheet’s vast body. New and historical efforts to access these records are revolutionizing our understanding of the ice sheet’s stability. By drilling through the ice and accessing the sediments and rocks beneath, and applying cutting-edge geochemical tools, we can unlock knowledge of when certain areas were last deglaciated. This knowledge allows us to refine the ice-sheet models that are used to predict how quickly the ice sheet will melt in the future, better equipping global society to prepare for rising sea levels; and perhaps motivating action that can slow the tide of the ice sheet’s demise. Here, we chronicle three locations where subglacial materials have yielded novel insights to the ice’s past. At Prudhoe Dome (78ºN, -71ºW), near the northwestern corner of Greenland, scientists in 2023 drilled through 600 meters of ice to retrieve seven meters of core. The first three and a half meters are frozen permafrost, underlain by a similar amount of solid bedrock. Optically-stimulated luminescence methods reveal that the sand grains in the upper-most sediments last saw sunlight seven thousand years ago. Ice-sheet models indicate that for Prudhoe Dome to be ice-free, the Greenland ice sheet must have been about 10% smaller than it is today – suggesting that ice loss at this time led to ~2 feet of sea level rise across the globe. Following the spine of the ice sheet three hundred kilometers further inland, we arrive at Camp Century (77ºN, -61ºW). Here, scientists drilling an ice core in 1967 accidentally recovered 4 meters of sediment laying beneath the ice. At the time, the sediments were incidental to the project, and they were packed away in cookie tins and forgotten about for fifty years. After rediscovering the samples in a Copenhagen freezer in 2019, the same methods were used to show that this location was ice-free 400,000 years ago, during a period of Earth’s history that was slightly warmer than today for thousands of years because of our orbit around the sun. The Camp Century sediments are rich with fossils that record a lush tundra ecosystem, abundant with flowing meltwater and thriving plants in a region that is today covered by a mile of ice. Ice-sheet models demonstrate that these conditions require global sea levels to rise by at least 4 feet – but because of Camp Century’s inland location, there is much greater uncertainty in how small the ice sheet could have been when these sediments were deposited. Without further data to constrain the shape of the ice sheet during this time, it is possible that Greenland contributed closer to 15 feet to global sea level. Further up the ice sheet’s profile, eight hundred kilometers southeast at the ice sheet’s thickest part, subglacial rocks were retrieved at the GISP2 site (72.5ºN, -38.5ºW ) in 1991 beneath two miles of ice. Twenty-five years later, cosmogenic nuclide techniques revealed that these rocks had been uncovered much more frequently over the last few million years than had been thought possible. For this location to be ice-free, ice sheet models indicate that only the highest peaks in southeastern Greenland could have maintained ice – meaning Greenland alone would have caused global sea level to rise 20 feet. However, the cosmogenic techniques alone could not say precisely when the ice-sheet last disappeared. Now, two new advances – one measuring the isotopes of Chlorine in the bedrock, and another measuring isotopes of Krypton in the sediment-striped ice just above the bedrock – have shown that this site has been covered by ice for the last million years. Prior to that, the ice sheet must have been much smaller – and it must have been smaller for a long time. Although the recent stability is encouraging, and speaks to a resilient core of Greenland’s ice, it is also troubling – what changed a million years ago that allowed the ice sheet to persist? If we do not understand this, we cannot say what may allow the ice sheet to regrow in the future.
