New study shows that glacial retreat reversed an ancient Arctic groundwater system

New research from the continental shelf off northern Norway shows that retreating glaciers can trigger a long-lasting reversal in groundwater flow, with profound consequences for coastal ecosystems and carbon cycling.

Scientists studying submarine canyons near the Lofoten archipelago have found that as the Fennoscandian Ice Sheet retreated at the end of the last Ice Age, seawater began infiltrating the seabed and displacing ancient freshwater. 

Image credit: Nature Geoscience

This process slowed the flow of groundwater into the ocean, reducing the supply of nutrients, trace metals and other solutes that help support life in coastal waters.

It also triggered changes in methane storage in marine sediments, increasing the burial of methane as carbonate minerals.

The research, published in Nature Geoscience, provides the first observational evidence of this kind of offshore groundwater transition, and reveals how polar coastal systems may evolve as glaciers retreat in the future. 

The study was led by Sophie ten Hietbrink from Stockholm University. iC3 collaborator Henry Patton provided constraints on the ice sheet’s history, while iC3’s Jochen Knies was responsible for conducting the research cruise. 

Powerful subsurface flows  

The team studied groundwater discharge sites in deep submarine canyons carved into the continental shelf west of the Lofoten Islands. Here, features such as microbial mats and carbonate crusts hint at chemical-rich fluids emerging from below the seabed.

Using radiocarbon dating of dissolved carbon in sediment porewaters, they were able to determine when the groundwater last exchanged carbon with the atmosphere.

The oldest groundwater was recharged between 11,500 and 8,800 years ago—just after the ice sheet had pulled back from the shelf edge. 

This marks the beginning of a new era when rising sea levels allowed salty ocean water to infiltrate the sediment and push out fresher groundwater left behind by the glaciers. 

"What's striking is that we can see this switch so clearly in the chemistry of the groundwater. It tells us that the ice sheet wasn’t just shaping the surface landscape—it was driving powerful subsurface flows that lasted for thousands of years,” Jochen said.

Detective work in marine sediments

To unpick the groundwater history, the researchers measured carbon isotopes in the sediments.

These isotopes revealed three distinct carbon sources: old methane from deep below, seawater-derived carbon, and ancient freshwater recharged under the ice sheet. 

The team also used a transport-reaction model to simulate how groundwater and carbon moved through the sediment over time. They found that the slowing of groundwater flow after the ice sheet retreated allowed methane to be oxidised more efficiently in the seabed. 

This methane was then trapped in the form of carbonate minerals—a natural form of carbon sequestration.

"By studying the isotopic signals and mineral deposits, we can reconstruct past changes in fluid flow, and link them to glacial dynamics and sea-level rise," explains Henry.

Broader implications for a warming world

As climate change continues to shrink glaciers around the world, similar processes are likely unfolding in other polar coastal regions.

 The study suggests that we should expect a long-term slowdown in submarine groundwater discharge wherever marine-based glaciers are retreating. This could alter the delivery of nutrients and trace elements to the ocean, with knock-on effects for marine ecosystems and carbon cycling.

It also highlights the role of sediments as natural archives of past environmental change, and as sites where carbon can be locked away for millennia.

“This research changes how we think about groundwater at sea,” said Jochen. “It’s not just a passive reservoir—it responds to ice sheet changes in dramatic ways.”

About the paper and the authors

The study, “Deglaciation drove seawater infiltration and slowed submarine groundwater discharge” has been published in Nature Geoscience. 

Lead author Sophie ten Hietbrink is a PhD student at Stockholm University. Two researchers from the iC3 Polar Research Hub, Henry Patton and Jochen Knies, contributed to the study. Both work at Tromsø University’s Department of Geosciences and are also members of the Into the Blue project.

Henry Patton’s work within iC3 focuses on the carbon stores found beneath ice sheets, with his research seeking to expand our understanding of how glacial systems develop and impact their environment.

Jochen Knies leads iC3’s research unit focusing on how past changes in ice sheets affect the global carbon cycle and marine ecosystems. He co-leads the Into the Blue project.