A study by an international team of scientists offers important new insights into the hidden role of ancient groundwater beneath the ocean floor – and how it may have interacted with ice sheets and rising sea levels during past climate changes.

Groundwater is a vital source of fresh water, supplying nearly half of global domestic needs, but a large portion of this water – between 42-85% is actually fossil groundwater. This water flowed into the ground more than 11,700 years ago, before the start of the current geological period known as the Holocene.

This ancient water is not only difficult to replenish, but also increasingly vulnerable to modern pollution or salinisation through seawater mixing. To protect this vital resource, it is important to understand how long it stays underground and how it moves and changes over time.

“In regions once covered by vast ice sheets and glaciers, fossil groundwater may have been shaped by dramatic changes in sea level and the movement of massive ice bodies,” says Wei-Li Hong, department of geological sciences at Stockholm University and one of the researchers behind the study.

“There are some studies that suggest that this deep groundwater flow could help destabilise ice sheets or accelerate their melting. However, these areas are difficult to reach, so there has been very little direct evidence from the field to support these ideas – until now,” he continues.

In a new study led by PhD student Sophie ten Hietbrink with Hong as her supervisor, researchers used a new method to investigate this question. Instead of drilling directly beneath glaciers, they looked at how ancient groundwater flows into the ocean – an indication that the water likely came from glacial melt thousands of years ago.

Working with scientists from Norway, Poland and Germany, the team collected fluid samples from the seafloor off the northern Norwegian coast. At a depth of 760m below sea level, the researchers documented freshened groundwater emerging from the seabed – a strong indication that it originated from glacial processes potentially dating back thousands of years.

“By analysing radiocarbon content of the fossil groundwater, a temporal marker for its last atmospheric contact, we were able to provide precise time constraints on the groundwater flow,” says Hietbrink. “We found that the fossil groundwater changed its composition after the retreat of the Fennoscandian ice sheet.

"When this region was covered by a thick, 1km-high glacier, meltwater from the ice filled underground spaces. After the ice sheet collapsed and sea levels rose, this fresh groundwater was gradually replaced by seawater”, she explains.

The study is published in Nature Geoscience.

The study provides, for the first time, a detailed timeline showing how fossil groundwater flowed into the ocean and how it was influenced by glacial changes – even tens of kilometres offshore. The findings not only confirm when the groundwater composition changed, but also show that once the glacier stopped supplying fresh meltwater, the remaining groundwater quickly became vulnerable to mixing with seawater.

These results have wide-reaching implications, for understanding glacier stability, the researchers say, as well as understanding the supply of nutrients, the health of marine ecosystems, and how much carbon the coastal ocean can absorb.

“This work is especially important for today’s warming climate, as many glaciers in Greenland, Antarctica, and Svalbard are already retreating. Continued research into submarine groundwater systems in these regions will help scientists better understand how ice sheets and groundwater interact in the future”, says Wei-Li Hong.