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

For the first time, scientists have 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 Brandon Dugan, professor and associate department head of geophysics at Colorado School of Mines, and Rebecca Robinson, professor of oceanography at the University of Rhode Island.

The cores, retrieved from deep below the sea floor, are now being opened, analyzed and sampled by the science team, during almost a month of intensive collaborative work at the University of Bremen. 

During January and February, 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.

“We were excited to see that freshened water exists in multiple kinds of sediments – both marine and terrestrial,” Dugan said. “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.”

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 clay layers that usually keep the water in place beneath the seafloor, known as aquitards. 

Although roughly 70 percent 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, slightly briny water beneath the ocean floor. Scientists have known these offshore systems existed since 1976, 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.

Shedding light on similar water aquifers around the world

The approach used during IODP³-NSF Expedition 501 will not only deepen 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 such as nitrogen 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 2050 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,” Robinson said.

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 has met at the Bremen Core Repository at MARUM – Center for Marine Environmental Sciences of the University of Bremen (Germany).

 “We greatly appreciate being able to conduct this advanced research at MARUM, supported by its world-class laboratories, exceptional facilities, and dedicated staff,” Dugan said.

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.

A total of 40 science team members from 13 nations – Australia, China, France, Germany, India, Italy, Japan, Netherlands, Portugal, Sweden, Switzerland, United Kingdom and U.S. – have taken part in the expedition’s off and on-shore phases.