A Weizmann Institute of Science scientist has revealed that groundwater reservoirs near the coast affect the ability of seawater to regulate climate change.
To understand Earth's climate—and climate change—we have to dive into the ocean. The chemical composition of seawater plays a key role in regulating climate, and to find out, scientists have delved as far as the ocean floor and into the vents of active underwater volcanoes. Now it turns out that the journey has missed out on subsurface currents that have always been right under our feet.New research published in the scientific journal Nature Communications., presented by Dr. Yael Kiro, a geochemist from the Department of Earth and Planetary Sciences at the Weizmann Institute of Science, has made surprising findings about the influence of groundwater in coastal areas on the chemical composition of seawater, showing that this is a powerful influence on a scale similar to that of river flows into the sea and volcanic eruptions in its depths.
“For years I wondered if anyone had measured the flow of chemicals from coastal aquifers to the oceans and back,” says Dr. Kiro. “I assumed someone had already done it, but I couldn’t find any explicit evidence. It took me years to work up the courage to investigate it myself.”
Dr. Kiro's interest in groundwater began at the Dead Sea. As a doctoral student in hydrology, she studied the dramatic phenomenon of sinkholes that form in the area due to the dissolution of salt layers beneath the surface by groundwater flows.
""We have uncovered a hidden climate system that we did not know about. To understand the influence of seawater on climate, we must take into account this important component that was missing until now""
Contrary to popular belief, that study discovered that in addition to freshwater groundwater, salty, mineral-rich groundwater may promote chemical processes below the surface of the earth. Dr. Kiro then realized that what was discovered in the Dead Sea might be relevant in various ways to groundwater reservoirs around the world. “One day I would like to study the effect of groundwater on seawater, I thought to myself at the time,” she recalls.
The missing piece in the climate puzzle
The dramatic impact of seawater on climate is reflected, among other things, in the fact that the ocean absorbs huge amounts of CO₂ from the air and helps regulate the Earth's temperature. How much carbon can it absorb and how quickly? The answer to this depends on the chemical composition of the water.
"When we try to understand how the oceans respond to the increase in atmospheric CO₂ levels as a result of climate change, we must first understand what controls the chemical balance of ocean water," says Dr. Kiro.
Until now, researchers have focused on rivers as the main source of influence on the chemical composition of seawater. Another source, also widely studied, is found on the seafloor: very hot, chemical-rich streams of water created by volcanic activity. Groundwater reservoirs known as coastal aquifers have been almost unstudied in this context, and no one has tried to quantify their influence on ocean chemistry.
To understand the extent of the impact of those aquifers, Dr. Kiro came up with a creative idea. She decided to compare two types of groundwater samples collected by other researchers in different parts of the world: samples taken from drilling sites deep underground, several hundred meters inland from the shoreline, and samples taken close to the shore – just below the waterline.
This revealed a surprising difference: the groundwater in the samples close to the coast was relatively little affected by seawater that penetrated the aquifer by tides and waves – a short-term process lasting no more than a few months. In contrast, the deeper samples showed a strong chemical signature of seawater, which had seeped into the aquifer due to density differences between seawater and freshwater in a long-term process of tens or even hundreds of years. Dr. Kiro concluded from this that slow and continuous contact of the groundwater with seawater and sediments changed the composition of the water in the depths of the aquifer over time.
In the next step, Dr. Kiro calculated the amount of chemical elements such as calcium, magnesium, sodium, and potassium exchanged between the coastal aquifers and the ocean. She also calculated the concentrations of these chemicals in different regions, and from this she deduced the scope of the phenomenon on a global scale. This revealed a clear pattern: certain elements flowed steadily into the ocean, while other elements enriched the aquifers.
One of the substances that flowed into seawater in abundance is calcium – an element that plays an indirect but crucial role in the carbon cycle on Earth. When CO₂ breaks down in ocean water, one of the breakdown products is carbon (carbonate), which binds to calcium in a series of biochemical and geochemical reactions to form calcium carbonate – the mineral from which the exoskeletons of marine creatures are built. When these creatures die, their skeletons are buried in the seabed, locking the carbon in the soil for thousands and even millions of years. In other words, calcium affects the ocean’s ability to capture CO₂ from the atmosphere in a stable and sustainable manner, thereby serving as one of the natural control mechanisms for Earth’s climate.
Dr. Kiro's calculations showed that coastal aquifers contribute about 5 teramoles of calcium to the ocean annually. For comparison, rivers contribute 13 teramoles, while underwater volcanic springs contribute 1.6 teramoles. In other words, the study revealed that groundwater contributes a significant portion of the calcium flowing into the ocean and therefore has a real impact – which was previously unknown – on the global carbon cycle on our planet.
While calcium flows into the ocean from groundwater, other elements, such as sodium and potassium, flow from the ocean into groundwater – and stay there. These previously undocumented processes add a whole new layer to our understanding of ocean chemistry – one that is of extraordinary significance in an era of climate change.
As sea levels rise due to global warming and melting glaciers, more seawater is pushed into coastal aquifers, affecting the flow of chemicals in and out of the ocean in ways that could increase the ocean’s carbon uptake. That’s the good news. But there’s also the bad news: rising sea levels could contaminate the aquifer’s freshwater with salt, putting our drinking water supplies at risk.
"Aquifer salinization may occur faster than current models predict, and this must be taken into account in the management of groundwater reservoirs," says Dr. Kiro.
The Earth's coastlines stretch for hundreds of thousands of kilometers, so the new findings have far-reaching global implications. "We have uncovered a hidden climate system that we didn't know about," concludes Dr. Kiro. "To understand ocean chemistry and the role of the oceans in the long-term carbon cycle, we must take into account this important component that has been missing until now."
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