Retreating glaciers may send fewer nutrients to the ocean, study finds

Turbid, sediment-laden glacial meltwater is a key source of nutrients for ocean life, but a new study suggests that as climate change causes many glaciers to shrink and retreat, their meltwater may become less nutritious.

A study conducted by scientists at the Scripps Institution of Oceanography at the University of California, San Diego, found that meltwater from a rapidly receding Alaska glacier contains significantly lower concentrations of types of iron and manganese that are easily absorbed by the body. marine organisms compared to a nearby stable glacier.

These metals are in short supply in many parts of the ocean, including the highly productive Gulf of Alaska, and they are also important trace elements for phytoplankton, the microorganisms that form the basis of most marine food webs.

Conclusions published V Natural communications, limited to just two glaciers in Alaska, but they assume that climate changeGlacial retreat may change the role of glaciers in delivering nutrients to the ocean.

“If we can replicate these results elsewhere, the implications go beyond our scientific understanding of glaciers,” said Sarah Aarons, a Scripps geochemist who co-authored the study.

“This could impact the productivity of really important marine ecosystems, which could have long-term consequences for the health of large fisheries.”

As glaciers grind against bedrock, some of the crushed rock and sediment they create flows into the ocean through glacial runoff. Sediments contained in glacial runoff are an important source of micronutrients such as iron and manganese for coastal marine ecosystems in Alaska, Antarctica, Greenland, and other high-latitude regions.

These nutrients promote the growth of phytoplankton, which form the basis of the marine food web and absorb many tons of planet-warming carbon dioxide.

The world's glaciers are threatened by climate change, which is responsible for most lose ice and shrink. The researchers behind the study wanted to find out if all this rapid ice loss and retreat changed the nutrient content of glacial meltwater.

For the study, researchers traveled to two adjacent fjords on Alaska's Kenai Peninsula in May 2022. Each fjord had a glacier, but one was stable and the other had retreated about 15 kilometers (nine miles) since 1950.

It's important to note that because the two glaciers were so close to each other, they were each grinding down the same rock. This meant that the source material of the sediment carried by glacial meltwater was almost identical, creating a natural experiment that allowed the team to isolate the influence of glacial meltwater. glacial retreat by nutrient content.

The team collected samples of surface water, suspended sediment and iceberg material from a stable glacier called Aialik Glacier and a retreating glacier called the Northwest Glacier.

The researchers analyzed the chemical composition of their samples, focusing on metals including manganese and iron, as well as the element phosphorus, which is also a key nutrient. The analysis also revealed whether these elements are present in chemical forms that make them bioavailable or capable of being absorbed and used by living organisms.

Even though both glaciers erode the same bedrock, the team found striking differences between their sediment plumes. The stable Ayalik Glacier deposited deposits in which approximately 18% of the iron and 26% of the manganese were in bioavailable forms. In contrast, Northwest Glacier sediments contained lower amounts of bioavailable iron (13%) and manganese (14–15%).

Sediments from the retreating glacier showed signs of extensive chemical weathering and reactive metal depletion, as well as other evidence of long-term interactions between water and rocks.

The researchers said their findings show that for a glacier that has retreated inland, meltwater and sediment take longer to reach the ocean, providing more opportunity for chemical interactions that can convert any iron and manganese to less bioavailable states.

“The longer water is in contact with rock or sediment, the more chemical breakdown or weathering occurs,” Aarons said. “Thus, a retreating glacier may send more sediment to the ocean, but with lower concentrations of bioavailable nutrients such as iron because more weathering occurs.”

In this view, eroded bedrock sent into the ocean by a stable glacier is “fresher” and contains more bioavailable nutrients because it has spent less time interacting with water and other materials.

Most ocean-terminating glaciers around the world are losing ice as climate change progresses, so if the patterns seen in these Alaskan fjords turn out to be the same across glaciers, the impacts could be significant, especially in regions like the Gulf of Alaska and the Southern Ocean that support productive fisheries and where iron is a scarce nutrient.

“We see very clear geochemical differences between these two glacial systems that we attribute to their states of retreat,” said Kiefer Forsch, lead author of the study, who conducted the research as a postdoctoral fellow at Scripps and is now at the University of Southern California.

“However, this is a snapshot of two glaciers in the same region. More research will be required to understand whether these patterns persist in glaciers in other parts of the world with different bedrock types and stages of retreat.”

Aarons also emphasized the importance of government support that made this research possible.

The researchers suggest that future work should analyze meltwater deposits for several glacial systems at different stages of retreat to see if the results from these two Alaska fjords can help make predictions about ecosystem responses to ongoing glacier retreat around the world.

In addition to Aaron and Forsch, the study's co-author was Angel Ruacho of the U.S. Environmental Protection Agency. Ruacho conducted the research while completing a postdoctoral fellowship at the University of Washington.