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Desert reservoirs found to trap organic carbon in sediment

In 2021, while revelers across America celebrated the fourth of July, three researchers waded through a shallow river delta in the New Mexican desert. Abby Eckland, Irina Overeem and Brandee Carlson stood in what remained of the Rio Grande—years of drought had shrunk the river to a few small channels. Just downstream, the channels entered the Elephant Butte Reservoir—New Mexico's largest.

Then, all of a sudden, the water started rising. First, to the scientists' calves. Then above their knees.

"Wow, it's really coming up," Overeem remarked.

The river became muddy and turbid. Debris—tamarisk leaves, pine needles and trash—floated down the widening channel. Dead fish rose to the surface and came to rest in the weeds on the riverbank. It was a flash flood.

At this point, a normal sightseer would probably head for the hills, but the scientists, instead, responded with excitement. This was an opportunity for inquiry into an ephemeral natural phenomenon. Eckland bottled up water samples while Overeem and Carlson checked on the sensing equipment they had placed in the river that morning.

A carbon sink in the desert

This month, the team a new study, led by Eckland, in Water Resources Research. The analysis draws on their 2021 field season and parses information about sediment and organic carbon in river water for a surprising result.

Reservoirs like Elephant Butte may sequester organic carbon within layers of sediment, especially during periods of drought and flash floods. Essentially, the reservoir acts as a carbon sink—trapping organic material that would otherwise emit carbon dioxide through natural decay.

The explanation lies in physics. Normally, when water flows into a reservoir, it spreads out over the surface. But, if the river picks up enough sediment, the process flips upside down. Instead of the river water fanning out on top, an underwater current plunges it downward. Scientists call this a "hyperpycnal plume."

"We saw this plume developing based on the data from instruments we cast near the mouth of the Rio Grande," Eckland said. "That means that it's likely that whatever sediment, carbon and other materials are being carried will flow to the bottom of the reservoir and get deposited."

Armed with this evidence, the researchers next turned their attention to the samples they had gathered in the field. It was time to, literally, dig through the muck.

Droughts and floods

In the laboratory, an array of tests characterized the contents of samples from the river water, reservoir water, and underlying delta and reservoir beds. Once they had these values mapped out, the researchers compared their results to a repository of historical data from the U.S. Bureau of Reclamation—a process aided by Eckland's familiarity with the system after years of interning with the Bureau. This allowed them to extrapolate their findings back in time.

"Abby had a lot of connections to the scientists there and knew what to look for," Overeem said. "It was really good that we had this, sort of, liaison to the federal government system. It led to a unique partnership."

Finally, the team had all of the information they needed to compare carbon sequestration in the river delta and reservoir over seasons, years and even decades.

This second set of analysis provided the study's most striking result. Not only was organic carbon getting buried beneath layers of sediment, but this process was actually amplified during drought. Because the overall footprint of the reservoir was smaller during these periods, sediment piled on faster.

"There's less of a footprint when the water level is low," Eckland said. "There's just less space for it to go, so you get more carbon buried per area."

The serendipitous timing of their field excursion produced another insight—carbon burial rates are also elevated during flash floods. It makes sense, of course. Flash floods tear through the landscape, picking up loose soil, leaves and whatever else is lying around. By the time they reach the reservoir, they are full of sediment, which creates a hyperpycnal plume, and full of organic material, which is subsequently buried.

Though carbon burial in reservoir sediment has been observed in the past, the new paper is the first to identify exactly how it happens.

"The key link is the role of the hyperpycnal plume in delivering carbon to the bottom of the reservoir," Eckland said.

Next steps

As with any novel scientific finding, the next step is to confirm the discovery and gather more information. This is already underway. Overeem recently returned to the site with former INSTAAR postdoc and current University of New Mexico assistant professor Marisa Repasch to gather samples from the reservoir bed. Repasch is an expert in organic carbon storage in the landscape, and her lab is hard at work digging deeper into the chemical characteristics of the sediment. So far, the preliminary results are promising.

"We found even higher numbers than what Abby estimated," Overeem said.

The researchers are hopeful that these results might help water managers make more informed decisions in the future. Essentially, they have highlighted a unique benefit of dryland reservoirs—that they might capture and store the source material of Earth's most ubiquitous greenhouse gas. This insight could become important in weighing the potential costs and benefits of infrastructure on the landscape.

"There is renewed interest in carbon sequestration, especially because there might be a market for stored carbon at some point in the future," Overeem said. "It's a futuristic vision, but we will need this kind of information to get there."