Retreating Glaciers May Send Fewer Nutrients to the Ocean

The cloudy, sediment-laden meltwater from glaciers 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. 

Led by scientists at UC San Diego’s Scripps Institution of Oceanography, the study finds that meltwater from a rapidly retreating Alaskan glacier contained significantly lower concentrations of the types of iron and manganese that can be readily taken up by marine organisms compared to a nearby stable glacier. These metals are scarce in many parts of the ocean including the highly productive Gulf of Alaska, and they are also essential micronutrients for phytoplankton, the microorganisms that form the base of most marine food webs.

The findings, published today in Nature Communications and funded by the National Science Foundation (NSF), are limited to just two glaciers in

 Alaska, but they suggest that climate change-driven glacial retreat could alter the role glaciers play in delivering nutrients to the ocean. 

“If we can duplicate these findings elsewhere, the impacts go beyond our scientific understanding of glaciers,” said Sarah Aarons, a geochemist at Scripps who co-authored the study. “This could impact the productivity of really significant marine ecosystems, which could have long term implications for the health of major fisheries.” 

As glaciers grind across bedrock some of the pulverized rock and sediment they create flows into the ocean via glacial runoff. The sediments contained in glacial runoff are an important source of trace metal micronutrients like iron and manganese for coastal marine ecosystems in Alaska, Antarctica, Greenland and other high-latitude regions. These nutrients fuel phytoplankton growth, which forms the base of the marine food web and absorbs many tons of planet-warming carbon dioxide.

The world’s glaciers are threatened by climate change, which is causing most to lose ice and shrink. The researchers behind the study wanted to investigate whether all this rapid ice loss and retreat changed the nutrient content of glacial meltwater.

To investigate, the researchers traveled to two adjacent fjords on Alaska's Kenai Peninsula in May 2022. Each fjord contained a glacier, but one was stable and the other had retreated approximately 15 kilometers (nine miles) since 1950. Crucially, because the two glaciers were so close together, they were each grinding over the same bedrock. This meant the source material for the sediment carried by the glaciers’ meltwater was nearly identical, creating a natural experiment that allowed the team to isolate the influence of glacial retreat on nutrient content. 

The team collected surface water samples, suspended sediments and iceberg material from the stable glacier, named Aialik Glacier, and the retreating glacier, named Northwestern Glacier.

The researchers analyzed the chemical composition of their samples with a particular focus 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 were present in chemical forms that made them bioavailable or able to be absorbed and utilized by living organisms.  

Despite both glaciers eroding the same underlying bedrock, the team found striking differences between their sediment plumes. The stable Aialik Glacier produced sediments where approximately 18% of iron and 26% of manganese existed in bioavailable forms. In contrast, Northwestern Glacier's sediments contained lower fractions of bioavailable iron (13%) and manganese (14-15%). 

The retreating glacier's sediments showed signs of extensive chemical weathering and depletion of reactive metals as well as other evidence of prolonged interactions between water and rock. 

The researchers said their findings suggest that for the glacier that has retreated inland, meltwater and sediments take

 longer to reach the ocean, providing more opportunities for chemical interactions that could transform any iron and manganese into less bioavailable states.

“The longer you have water in contact with rock or sediments the more chemical breakdown or weathering takes place,” said Aarons. “So a retreating glacier might be sending more sediment to the ocean but with lower concentrations of bioavailable nutrients like iron because more weathering is occurring.” 

In this view, the eroded bedrock being sent into the ocean by the 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 worldwide are losing ice as climate change progresses, so if the patterns observed at these Alaskan fjords prove consistent across glaciers the implications could be significant — particularly for regions like the Gulf of Alaska and the Southern Ocean which support productive fisheries and where iron is a scarce nutrient.

"We see very clear geochemical differences between these two glacier systems that we link to their state of retreat," said Kiefer Forsch, the study’s lead author 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 one region. Understanding whether these patterns hold across glaciers elsewhere in the world with different bedrock types and stages of retreat will require more research."

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

“This research would not have been possible without funding from the National Science Foundation and cooperation with the National Park Service,” said Aarons. “Funding from NSF allows us to understand how this landscape is responding to a warming planet, and has a direct impact upon the many people who subsist on these lands and visit these glacial fjords for their abundant and diverse wildlife.”

The researchers suggest that future work should analyze meltwater sediment for multiple glacier systems at different stages of retreat to clarify whether the results from these two Alaskan fjords can inform predictions about ecosystem responses to continued glacier retreat worldwide.

In addition to Aarons and Forsch, Angel Ruacho of the US Environmental Protection Agency co-authored the study. Ruacho conducted the research while completing a postdoctoral fellowship at the University of Washington.