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

For the first time, a science team directly documented and extensively sampled a freshened water system beneath the ocean floor. This major discovery comes from the initial analyses of sediment cores recovered during an international scientific expedition led by Co-Chief Scientists Professor Brandon Dugan (Colorado School of Mines, Golden, USA) and Professor Rebecca Robinson (Graduate School of Oceanography, University of Rhode Island, USA).

The cores, retrieved from deep below the seafloor, are now being opened, analysed and sampled by the science team during almost a month of intensive collaborative work at the University of Bremen. In January and February 2026, 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.

Among those involved is Dr Thomas Harald Müller, geochemist at GEOMAR Helmholtz Centre for Ocean Research Kiel. During the offshore phase of the expedition last year, he was responsible for collecting pore water samples from the sediment cores. While a few analyses were performed on board, the majority of the samples were stored for further investigations and are now being analysed during the onshore operations in Bremen.

The goal of the expedition went far 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 science team has successfully documented and sampled freshened water within a zone nearly 200 metres thick below the seafloor.

Brandon Dugan: “We were excited to see that freshened water exists in multiple kinds of sediments – both marine and terrestrial. Freshened water in such different materials will help us understand the conditions that emplaced the water.” Further analyses that are conducted by the science team will help to find out where and especially when the water was placed here.

Rebecca Robinson: “It was surprising to see sediment, not rocks, throughout the section. The sediment has not yet transformed into rock – I did not expect to see that. Also, the cores comprise four lithologic units, from younger and older ages.” In order to understand when and how the sediments were deposited, the science team is developing age models.

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. These goals align closely with the 2025 Science Framework for Ocean Research Drilling – one of the foundations of the IODP³ scientific programme. 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.

“The researchers will continue to work on and with the samples to decipher more – for example, to date the groundwater more accurately which is critical to advancing our knowledge,” adds Rebecca Robinson.

The expedition is a joint collaboration between the International Ocean Drilling Programme (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 (Germany).

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.

About: IODP³‑NSF Expedition 501 “New England Shelf Hydrogeology”

40 science team members from 13 nations (Australia, China, France, Germany, India, Italy, Japan, Netherlands, Portugal, Sweden, Switzerland, United Kingdom, USA) take part in this Mission Specific Platform expedition that consists of two phases: offshore and onshore operations. Offshore operations took place between May and early August 2025.

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).