Extensive freshened water beneath the ocean floor confirmed for the first time

The expedition IODP³-NSF Expedition 501 New England Shelf Hydrogeology brings together ocean drilling specialists with hydrogeologists, experts who study groundwater movement and composition. During the summer of 2025 the expedition drilled in the ocean floor off the north eastern coast of the United States.

The sediment cores, retrieved from deep below the sea floor, are now being opened, analysed and sampled by the science team, during almost a month of intensive collaborative work at the University of Bremen. The expedition’s scientists are working side by side to uncover new insights into the formation, evolution, and significance of this newly documented subseafloor freshwater system.

Freshened water found in 200 metres thick zone

The goal of this expedition went beyond collecting sediment cores. Scientists also set out to sample the water stored within the sediments, including from sandy layers that act as aquifers and from clayey layers that usually keep the water in place beneath the seafloor, known as aquitards. Although roughly 70 per cent of Earth’s surface is covered by water, significant volumes of water also move and are stored below ground. Many coastal communities depend on land-based aquifers for their freshwater supply. What fewer people realize is that, in many parts of the world these aquifers continue offshore, containing zones of freshened water beneath the ocean floor. Scientists have known these offshore systems existed for decades, but they have remained virtually unexplored until now. During this expedition, the scientists have successfully documented and sampled freshened water within a zone nearly 200 metres thick below the seafloor.

"These offshore freshened groundwaters could become an important source of freshwater for coastal regions facing water shortages", says Sophie ten Hietbrink, PhD student at the Department of Geological Sciences, Stockholm University and member of the expedition.

And she continues:
"By determining the age of the freshened groundwater, we can tell whether it is being replenished today, making it a potentially sustainable freshwater source, or whether it is paleogroundwater left behind in the past when different hydrological conditions prevailed."

Stockholm researcher part of the expedition

Sophie ten Hietbrink was on board the lift-boat platform used to drill into the seafloor last summer. She is writing her PhD thesis about offshore groundwater freshening driven by ice sheets, glaciers, and permafrost. Onboard she worked as a geochemist. She split and prepared the water samples from the sediment cores and from the groundwater that was pumped up. She also ran some measurements already on the ship.

The last month Sophie ten Hietbrink has been in Bremen for the onshore phase of the expedition. She and the other researchers have a working schedule of 65 hours per week to split all the cores that they sampled offshore, process all the samples and data, and perform analyses both on the sediments as well the porewaters that have been extracted. Similar to the offshore phase, she also works as a geochemist for the onshore phase. This implies for example that she is preparing samples for different measurements to get information about elemental and mineral composition of the sediments.

The results will be published in the expedition proceedings report which is part of the IODP³ legacy. During her stay in Bremen at the research institute MARUM (link) she works on the data compilation, writing, and figure plotting for this report.

“In addition, as science team members we also receive our personal samples, which we will continue working on after the offshore and onshore phases are finished. I am co-leading the radiocarbon dating study, where we aim to constrain the age of the offshore freshened groundwater to determine when and how it was emplaced. Moreover, we will work on quantifying the seawater infiltration rates into the continental margin to determine salinization rates of this extensive offshore freshened groundwater body”, says Sophie ten Hietbrink.

Shedding light on similar water aquifers around the world

The approach used during IODP³-NSF Expedition 501 will not only deepen our understanding of offshore freshened groundwater systems off the coast of New England, but will also shed light on similar hidden water aquifers around the world. Because many coastal regions rely on groundwater for their freshwater supply, the expedition’s initial findings are highly relevant to society. The research will also reveal how nutrients cycle through continental shelf sediments and how these processes influence the abundance and diversity of microbes living in these environments. Ultimately, the expedition’s research will help to decipher how sediments and fluids cycle through the Earth system and improve our knowledge about sea level changes and freshwater flow beneath the seabed along our coastal shelves.

Stockholm hosting the ESSAC office

Sophie ten Hietbrink is not the only scientist from Stockholm University involved in the IODP3 activities. Through the years, Swedish researchers have been involved on numerous f IODP expeditions, inlcuding those conducted in the Southern Ocean, Tropical Pacific, Baltic Sea and the Arctic Ocean. The Science Support and Advisory Committee (ESSAC) for the European Consortium for Ocean Research Drilling is responsible for the scientific planning and coordination of ECORD’s contribution to IODP. It is composed of scientists from the 14 ECORD member countries.

The ESSAC office has recently moved to the Department of Geological Sciences at Stockholm University and is being run by professor Matt O'Regan (chair) and Hanno Kinkel (science coordinator) from 2026 through 2028. ESSAC advises the European funding organisations on IODP-related matters and coordinates European participation in IODP activities. It oversees expedition applications and scientist nominations, training and outreach programmes, and representation on IODP boards and advisory panels. It also stimulates and supports the development of drilling proposals aligned with the IODP Science Plan, initiates scientific workshops, and assists ECORD partners with public outreach.

The involvement of Swedish scientists in IODP³ is supported by the Swedish Research Council (Vetenskapsrådet), since IODP³ is a regarded as a national Swedish infrastructure.

More about the expedition

The expedition is conducted by the European Consortium for Ocean Research Drilling (ECORD) as part of the International Ocean Drilling Programme (IODP³), funded by IODP³ and the US National Science Foundation (NSF). The cores were retrieved during offshore operations between May and August 2025. For onshore operations the science team have met at the Bremen Core Repository, at MARUM – Center for Marine Environmental Sciences of the University of Bremen. The cores will be archived and made accessible for further scientific research for the scientific community after a one year-moratorium period. All expedition data will be open access in the IODP³ Mission Specific Platform (MSP) data portal in PANGAEA, and resulting outcomes will be published.

Further reading: Chasing freshened groundwater beneath the seafloor

IODP³-NSF Expedition 501 New England Shelf Hydrogeology

This news article was originally published by SU's Communications Office.