Proglacial Freshwaters Found to be Carbon Sinks

Researchers in Canada have discovered that proglacial freshwaters are important carbon sinks. Glacier retreat has often been considered a negative consequence of climate change, but this finding suggests there may be benefits as well.  

The study published on September 3 in the journal Proceedings of the National Academy of Sciences, was conducted in Nunavut, Canada, home to Lake Hazen, the world’s largest Arctic lake. Lake Hazen is fed by rivers, which flow from glaciers within the Northern Ellesmere Icefield. The icefield is located on Ellesmere Island, the world’s tenth largest island. Ellesmere Island is home to polar bears, muskoxen, caribou, and most interestingly, thirteen species of spiders.

Ellesmere Island, Northern Canada (Source: Wiki Commons/NASA/MODIS Rapid Response System)

The results of the study revealed that glacier-fed rivers consume more carbon dioxide than they release into the atmosphere. The carbon dioxide concentrations of all seven glacial rivers sampled were found to be below atmospheric equilibrium at all but one sample site. This result indicates that the rivers are storing carbon from the atmosphere.

Kyra St Pierre is a research associate at the Institute for the Oceans and Fisheries at the University of British Columbia, and lead author of the study. “Glacier-fed rivers differ quite substantially from other rivers because they’re very cold, are located in landscapes without a lot of plant material (i.e., organic carbon), but with a lot of finely ground sediments created by the successive cycles of glacier advance and retreat,” St Pierre told GlacierHub. “Because the landscapes lack things that would otherwise constrain the flow of a river, like trees or well-developed soils, glacier-fed rivers reorganize themselves regularly and in so doing, entrain a lot of finely ground sediment in the rivers.” It is because of these unique characteristics of proglacial rivers and the chemical weathering process associated with them that they can act as carbon sinks.

Researchers collected water samples from Lake Hazen and seven glacial rivers within the Lake Hazen watershed. With these samples, researchers measured changes in river chemistry and carbon dioxide fluxes during the summers of 2015 and 2016. Samples were collected in summer when glaciers were melting rapidly.

Kyra St Pierre conducts field work on the Blister River (Source: Kyra St Pierre)
Jessica Serbu collects water samples downstream from the Gilman Glacier (Source: Kyra St Pierre)

The researchers discovered that the weathering process initiated reactions between minerals within the sediment. The process that dominated in the sampled rivers was one that consumed carbon dioxide. Researchers noted that carbon dioxide concentrations decreased with distance from the glaciers, while dissolved inorganic carbon concentrations increased. Dissolved inorganic carbon is a main component of inland waters and influences organic productivity.

“The concentration of carbon dioxide within the rivers declines as these reactions proceed,” St Pierre said. “The difference between the carbon dioxide concentration in the waters (low carbon dioxide) and the atmosphere (high carbon dioxide) increases, such that more carbon dioxide from the atmosphere moves into the rivers and is then consumed. Chemical substances will always try to move from areas of high concentrations to areas of low concentration in an attempt to find a ‘“balance’” or equilibrium.”

St Pierre also told GlacierHub that most rivers across the globe are sources of atmospheric carbon dioxide, which is released through photosynthesis and decomposition of aquatic plants. For this reason, the discovery of proglacial freshwaters as carbon sinks was initially a surprise to researchers.

Gilman Glacier and its associated proglacial river, Ellesmere Island (Source: Kyra St Pierre)

Researcher Martin Sharp first proposed the idea of proglacial freshwater carbon sinks in an article in 1995. Sharp was a member of the research team and co-authored the journal article with St Pierre. St Pierre and her colleagues have worked in the Lake Hazen watershed for nearly 15 years. They were inspired to develop this study when they began testing water quality parameters of the glacier-fed rivers connected to Lake Hazen. It was by testing the river water temperature and chemical composition, that they first noticed that carbon dioxide concentrations were not what they expected.

Before their study, carbon dioxide concentrations had not been directly measured within glacier-fed systems, making this study ground-breaking. However, there is evidence from related research that supports the findings of this study. Data from study sites in Greenland, Svalbard, central Europe, and western Canada suggest carbon dioxide consumption by proglacial freshwaters is not isolated to the Lake Hazen watershed, but might also be relevant in other regions of the globe. St Pierre said, “In the past, we’ve always thought of freshwater systems as being sources of CO2 to the atmosphere, but these findings suggest that there’s a lot more nuance that needs to be accounted for.”

This research is somewhat of a conundrum for climate scientists. Carbon is a major driver of global climate change and carbon sequestration is viewed as significant for climate mitigation. However, the environmental benefits of glacier-fed rivers consuming more carbon dioxide than they emit to the atmosphere come at the cost of melting glaciers. Climate change and melting glaciers result in sea level rise and change the temperature and salinity of oceans. Ultimately, more research must be done to fully weigh the positive and negative effects of this process.

A river flows from Henrietta Nesmith Glacier, Ellesmere Island (Source: Kyra St Pierre)

Carbon cycling between proglacial freshwaters and the atmosphere also influences aquatic ecosystems. Sediments transported by glacial rivers are an important contribution to freshwater and marine ecosystems. St Pierre explained, “In lake environments, these sediment-laden rivers can also form turbidity currents, which transport lots of sediment and terrestrial materials, but also waters with low concentrations of carbon dioxide and high concentrations of oxygen directly to the bottom of the lake. Lake bottoms typically have low oxygen and high carbon dioxide concentrations, so this is an important downstream impact for organisms at the bottoms of glacial melt-affected lakes.”

There is much more to learn about proglacial freshwater carbon sinks and this study is just the beginning for this research subject. St Pierre’s colleagues from the University of Alberta are continuing this work with a study that began in the summer of 2019 that will examine proglacial rivers and lakes in Banff and Jasper National Parks in the Canadian Rockies. 

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