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Groundwater has a larger impact on ocean chemistry than expected

In a new study from UC Santa Cruz, experts have discovered that groundwater has a much greater impact on ocean chemistry than what was expected. Even though groundwater flows into seawater along coastlines all over the world, its contribution to the chemical properties of the ocean has been largely overlooked. 

The findings have implications for global models of biogeochemical cycles and for the interpretation of isotope records of Earth’s climate history.

“It’s really hard to characterize groundwater discharge, so it has been a source of uncertainty in the modeling of global cycles,” said study first author Kimberley Mayfield. “It took a large effort by researchers around the world who came together to make this happen.”

The investigation was focused on five key elements including lithium, magnesium, calcium, strontium, and barium. The researchers measured the concentrations and isotope ratios in coastal groundwater at 20 sites around the world, and used previously published data from additional sites.

“Those elements are important because they come from the weathering of rocks, and weathering of silicate rocks accounts for a huge uptake of carbon dioxide from the atmosphere over long time scales,” explained Mayfield.

According to study co-author Adina Paytan, a research professor in UCSC’s Institute of Marine Sciences, groundwater is an important source of inputs to the oceans, but has been easy to ignore because it is unseen and hard to measure.

“This is the first global assessment of groundwater discharge for most of these elements,” said Professor Paytan. “This information is useful for our understanding of how weathering of rock is related to climate, not only in the present but also in the past.”

The researchers estimated that up to 16 percent of these elements are entering the sea through groundwater. The results also showed that the isotopic composition of groundwater discharge can be different from that of rivers.

“The composition of groundwater discharge is very dependent on coastal geology, whereas river water is more influenced by the interiors of continents,” said Mayfield. “It’s important to recognize that groundwater makes a difference globally, and now that we have this large data set, people can keep improving it with more sampling and develop better models of global groundwater discharge.”

The study is published in the journal Nature Communications.

By Chrissy Sexton, Staff Writer

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