As water moves from high in the sky to deep in the ground, it picks up chemical signatures that can tell researchers secrets about Earth—from the density of air pollution to global ice cover to what kind of rock the raindrops seeped through. Scientists can look at specific radioactive isotopes and reconstruct past climates and weather patterns recorded in stored groundwater.
In a new paper in the Proceedings of the National Academy of Sciences of the United States of America, researchers leveraged an isotope of krypton (Kr) that can reach more than a million years back in time to discover the age of groundwater in the Negev desert in Israel.
Pairing krypton with other isotopes in groundwater, the researchers found two distinct ages and sources within the Nubian Sandstone Aquifer. They note that their method can be used to reconstruct paleoclimates from aquifers around the globe.
Groundwater is an important resource for life on the planet, especially in arid regions. Replenishing, or recharging, groundwater requires that rainfall soak into the soil, traveling downward for storage in aquifers—a process that requires recurring wet periods.
“Groundwater can serve as a paleo-humidity, -precipitation proxy,” said Reika Yokochi, a geochemist at the University of Chicago and lead author of the paper. By using groundwater as a record of past climate conditions, researchers can use that information to better predict how precipitation might change in the future.
In the Negev, Yokochi said there is currently no groundwater recharge occurring. But in the geologic past, the area went through climate changes, including wetter conditions that renewed the aquifers.
Yokochi said that previous work in the Negev used carbon-14 (14C) to date groundwater reserves to about 30,000 years. The problem with using 14C is that the isotope extends back in time only 30,000–40,000 years, meaning the age of older water remained a mystery.
Using the noble gas isotope 81Kr solved this problem. Krypton-81 is found throughout the troposphere, where it mixes with raindrops.
“Once that water is isolated from the atmosphere [in the ground], there’s no more supply, and it decays,” Yokochi explained. “That’s when the clock starts.”
Yokochi said that 81Kr, with its half-life of about 230,000 years, allowed the team to measure the age of aquifers that are up to 1.3 million years old—a huge extension from 14C.
“Krypton isotope dating of groundwater is exciting, as it opens a new window into past climatic conditions that was previously unavailable, or cloudy, with existing dating techniques,” said Jennifer McIntosh, a hydrogeochemist at the University of Arizona. She was not involved in the research.
“This is one of the few studies to use groundwater as a record of paleoclimate beyond the Last Glacial Maximum,” said McIntosh.
Aging an Aquifer
In the past, Yokochi said that amassing enough krypton gas in a water sample required sampling about 200–300 liters. Bringing that much water back to a lab for analysis is “a lot to carry,” so Yokochi said she and her team used a field gas extraction device.
The team used atom trap trace analysis (ATTA) methods to measure krypton isotopes in the lab. The combination of field gas extraction and ATTA bolstered the team’s ability to collect water from 22 wells in the eastern Negev.
“Everything became more efficient and fast,” said Yokochi.
The researchers found that 81Kr data revealed two distinct groundwater recharge dates: one less than 38,000 years ago and a much older wet period about 360,000 years ago.
Pairing this information with hydrogen and oxygen isotope data from the samples, the team found that the younger aquifer was recharged with water sources from the Mediterranean, whereas the older wet period brought precipitation from the tropical Atlantic.
“Knowing the source of the water helps scientists understand past climatic conditions that led to groundwater recharge, and how vulnerable the aquifer is to future climate change,” said McIntosh. “This information can be used to inform paleoclimate models and aid in water management decisions.”
There have been other studies using 81Kr as a tracer, but Yokochi said that “this is the first case where we take a lot of data from a very small area, do the spatial distribution analysis, and talk about climate.”
McIntosh said she anticipates—and is excited for—krypton isotopes to be widely used in future studies.
“Knowing how ‘old’ groundwater is helps water managers evaluate how much water they can extract without significantly depleting the aquifer,” said McIntosh. “It can also help them determine how vulnerable the aquifer is to anthropogenic sources of contamination.”
—Sarah Derouin (@Sarah_Derouin), Freelance Journalist