The cloudy, sediment-laden meltwater from glaciers is a key source of nutrients for ocean life. However, a new study suggests that as climate change causes many glaciers to shrink and retreat, their meltwater may become less nutritious. This revelation, led by scientists at UC San Diego’s Scripps Institution of Oceanography, highlights a potential disruption in the nutrient supply crucial for marine ecosystems.
The study, published in Nature Communications and funded by the National Science Foundation (NSF), reveals that meltwater from a rapidly retreating Alaskan glacier contained significantly lower concentrations of bioavailable iron and manganese compared to a nearby stable glacier. These metals are scarce in many parts of the ocean, including the highly productive Gulf of Alaska, and are essential micronutrients for phytoplankton, which form the base of most marine food webs.
Understanding the Impact of Glacial Retreat
The findings, although limited to just two glaciers in Alaska, 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 and co-author of 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. These sediments 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.
Field Research in Alaskan Fjords
The researchers conducted their study in May 2022, traveling to two adjacent fjords on Alaska’s Kenai Peninsula. Each fjord contained a glacier, but one was stable while the other had retreated approximately 15 kilometers (nine miles) since 1950. This natural experiment allowed the team to isolate the influence of glacial retreat on nutrient content, as both glaciers were grinding over the same bedrock.
The team collected surface water samples, suspended sediments, and iceberg material from the stable Aialik Glacier and the retreating Northwestern Glacier. They analyzed the chemical composition of their samples, focusing on metals like manganese and iron, as well as phosphorus, to determine their bioavailability.
“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, suggesting that 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.
Global Implications and Future Research
Most ocean-terminating glaciers worldwide are losing ice as climate change progresses. If the patterns observed at these Alaskan fjords prove consistent across other glaciers, the implications could be significant, particularly for regions like the Gulf of Alaska and the Southern Ocean, which support productive fisheries 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. “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 emphasized the importance of 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,” she noted. “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, having conducted the research while completing a postdoctoral fellowship at the University of Washington.
