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

The goal of this 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 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, 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 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: “The cores contain sediment with a wide range of composition and ages. 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 and it will be an interesting component of our future work.” 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 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,” 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). “We greatly appreciate being able to conduct this advanced research at MARUM, supported by its world-class laboratories, exceptional facilities, and dedicated staff,” adds Brandon Dugan

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.

International approach

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 has taken 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).

More Information:

• About the expedition – https://www.ecord.org/expedition501/
• Scientific Prospectus – https://iodp3.org/documents/expedition-501-scientific-prospectus/
• About the international research collaboration – https://iodp3.org/about/
• About the European part of the program – https://www.ecord.org/
  and Mission-Specific Platform expeditions – https://www.ecord.org/expeditions/msp/concept/
• Frequently Asked Questions – https://expedition501.wordpress.com/2025/02/19/faq1/Offshore
• More pictures are available on https://www.ecord.org/expedition501/

IODP³ Mission Statement

To advance scientific understanding of Earth's processes and history by exploring and monitoring the subseafloor, unlocking insights into climate change, geohazards, the deep biosphere and Earth system evolution through international collaboration, cutting-edge technology, and open data sharing.

The International Ocean Drilling Programme (IODP³) is committed to fostering global scientific cooperation, building capacity across nations, and promoting responsible stewardship of our planet's oceanic resources for the benefit of future generations.