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We've gone to the bottom of the ocean to study how its chemistry shapes our planet's climate, even chasing lava-spewing underwater volcanoes to do it. But it turns out we may have missed something far closer to home: the water beneath our feet.

In a study in Nature Communications, Dr. Yael Kiro, a geochemist in the Earth and Planetary Sciences Department at the Weizmann Institute of Science, presents surprising new insights into the exchange of water and chemicals between the ocean and coastal aquifers, groundwater reservoirs beneath coastal regions. Her research shows 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.

"For years I wondered whether the amount of chemicals flowing between coastal aquifers and the ocean had been measured," Kiro says. "I assumed someone must have already done it but found no studies addressing it in detail. It took me years to work up the courage to look into it myself."

Her interest in aquifers began in the salty, sun-scorched basin of the Dead Sea. As a Ph.D. student studying hydrology, Kiro investigated the region's sinkholes, a dramatic phenomenon caused by groundwater dissolving salt layers below the land surface. Contrary to common assumptions, the local aquifer contained not only freshwater but also mineral-rich saltwater, hinting at chemical processes it might be triggering beneath the surface.

Kiro realized that the Dead Sea findings might shed light on processes occurring in aquifers worldwide, even if this would require an entirely different approach. "That's when I started thinking—I want to study this one day in the oceans," she says.

Climate, chemistry and carbon

Ocean chemistry is crucial to our understanding of climate. The ocean absorbs massive amounts of carbon dioxide from the atmosphere, helping regulate global temperatures. But how much it can absorb, and how fast, depends on the chemistry of its water.

"When we try to understand how the ocean responds to rising COâ‚‚ levels that come with climate change, we need to figure out what controls its chemical balance," Kiro says.

Until now, scientists focused mostly on rivers as a major 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 mountain ridges are formed by the movement of tectonic plates. But coastal aquifers were rarely studied in this context, and no one had tried to quantify their effect on ocean chemistry.

To understand that effect, Kiro had an original, creative idea. She compared two types of aquifer water samples collected by other researchers: those taken from deep drilling sites several hundred meters inland from the shore, and those obtained closer to shore, right beneath the coastline itself.

She discovered a surprising difference. In the nearshore samples, the chemistry of the aquifer water was only mildly affected by mixing with the seawater pushed into the aquifer by tides and waves—a short-term process taking a year at most. But the deeper samples containing seawater that seeps into the aquifer due to differences in water density—a long-term process taking decades or even centuries—showed a much stronger seawater signature. Kiro concluded that a slow but steady interaction with seawater through the sediments had transformed the water's composition in the depths of the aquifers over time.

Going a step further, Kiro calculated the amounts of chemical elements such as calcium, magnesium, sodium and potassium that moved between the aquifers and the ocean. She determined their concentrations in different regions and scaled those figures up to the global level. A clear pattern emerged: Certain elements were consistently flowing into the ocean, while others were being removed from it.

One of the elements was calcium, which plays an indirect but crucial role in Earth's carbon cycle. When CO₂ disintegrates in ocean water, one of its breakdown products is 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 get 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.

Kiro's calculations showed that coastal aquifers contribute to ocean water around 5 teramoles of calcium per year, compared to about 13 teramoles from rivers and 1.6 from seafloor vents. That's a substantial share, and it means these aquifers play a real—and overlooked—role in the global carbon cycle.

At the same time, other elements like sodium and potassium are being removed from the ocean into the aquifers: Because they tend to stay in the aquifer, the water leaving the aquifer had noticeably less of these elements than what had gone in.

A missing piece of the climate puzzle

These previously undocumented processes add an entire layer to our understanding of ocean chemistry. And it matters even more in the face of climate change.

As sea levels rise, more seawater is pushed into coastal aquifers, which changes the flow of chemicals into and from the ocean in a way that could potentially enhance carbon capture by ocean water. That's the good news. But there's a downside: More seawater can also contaminate the freshwater aquifers by making them saltier and endangering our freshwater supplies.

"Salinization of aquifers might be happening faster than current models predict, which needs to be taken into account in the management of coastal water resources," Kiro says.

With hundreds of thousands of kilometers of coastline around the world, the implications of her study are global. "We've shown that there's a whole hidden system beneath the shoreline that we didn't know about," Kiro 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."