Beneath the shoreline, an unseen exchange is shaping the chemistry of our oceans. In a study published in Nature Communications, Dr. Yael Kiro presents surprising new insights into the exchange of water and chemicals between the ocean and coastal aquifers, groundwater reservoirs beneath coastal regions. Her research reveals that these hidden water flows can have a powerful effect on the ocean’s chemistry―one that may rival the impact of rivers and deep-sea volcanic vents.
Until now, scientists have focused mostly on rivers as a source of chemicals entering the ocean. Another well-studied source lies at the bottom of the sea: extremely hot, chemical-rich water flows generated by volcanic activity on the ocean floor, where the movement of tectonic plates forms mountain ridges. Coastal aquifers, however, were rarely studied in this context, and no one had attempted to quantify their effect on ocean chemistry.
To understand that effect, Dr. Kiro compared two types of aquifer water samples: those taken from deep drilling sites hundreds of meters inland from the shore, and those obtained closer to shore, directly beneath the coastline.
While nearshore samples exhibited mild effects from mixing with seawater―pushed into the aquifer by tides and waves―the deeper samples, containing seawater that seeped in due to differences in water density, exhibited a much stronger seawater signature. Dr. Kiro concluded that a slow but steady interaction with seawater through the sediments had transformed the water’s composition over time.
She calculated the amounts of elements such as calcium, magnesium, sodium, and potassium that moved between the aquifers and the ocean, finding that certain elements were consistently flowing into the ocean, while others were being removed from it.
One of these elements is calcium. When CO₂ disintegrates in ocean water, it produces carbonate which, in a series of biochemical and geochemical reactions, bonds with calcium to form calcium carbonate―the mineral that makes up the shells of marine organisms. When these organisms die, their shells are buried in the seafloor, locking away carbon for thousands or even millions of years. In other words, calcium affects the ocean’s ability to trap atmospheric CO₂ in a solid, stable form, serving as one of the planet’s natural climate regulators.
Dr. Kiro’s calculations showed that coastal aquifers contribute a substantial amount of calcium to ocean water each year, compared to rivers and seafloor vents, suggesting these aquifers play an underappreciated role in the global carbon cycle.
As sea levels rise, more seawater is pushed into coastal aquifers, altering the flow of chemicals into and from the ocean in ways that could enhance carbon capture by ocean water. Unfortunately, the same process can also contaminate freshwater aquifers by increasing their salinity, endangering our freshwater supplies.
With hundreds of thousands of kilometers of coastline worldwide, Dr. Kiro’s work has global implications. “We’ve shown that there’s a whole hidden system beneath the shoreline that we didn’t know about,” she says. “For scientists trying to understand ocean chemistry and the role of the oceans in the planet’s long-term carbon cycle, this system adds an important missing piece.”
Dr. Yael Kiro is the incumbent of the Rowland and Sylvia Schaefer Career Development Chair in Perpetuity.
