New review article: Glacier biogeochemical cycling and downstream impacts

A new review article synthesises recent research that shows glaciers to be biogeochemical reactors and regulators, upending the older view of glaciers as inert landscapes devoid of life.

In plain terms, that means glaciers can act like chemical factories. They can make, transform, store and release materials that matter for life and for the climate. These materials travel downstream into streams, rivers, fjords and oceans via meltwater and icebergs.

Poorly understood glacial biochemical processes highlighted. Credit: Nature Reviews Earth & Environment

From glacier surface to glacier bed

The authors highlight two broad sources of this material.

First, the glacier surface. Snow and ice can host active microbial communities that are more active during the summer months when meltwater is present. These microbes can produce organic carbon that is easy for other organisms to use. The surface also accumulates up and transports nutrients and carbon that arrive from the atmosphere as dust and aerosols. 

Second, the glacier bed. Surface meltwater can drain into cracks and shafts and reach the base of the glacier. There it mixes with water that has been in contact with crushed rock and sediments. This subglacial zone can be a hotspot for chemical reactions and microbial activity.

Glacier plumbing controls glacier chemistry

How long water spends inside a glacier changes the composition of the water.

If water drains through the glacier quickly, it has less time to react with rock and sediments and so usually starts to dissolve only the most reactive components of the rock. If it stays trapped for longer, it can pick up more dissolved elements, and microbes have more time to change the balance of gases and nutrients.

Many mountain glaciers and parts of the Greenland Ice Sheet often have seasonal melt and water residence times ranging from hours to weeks. In contrast, the subglacial environment of the Antarctic Ice Sheet is dominated by basal melt inputs and water residence times that can last years to decades or beyond.

“That longer isolation can lead to more advanced chemical weathering,” says iC3 researcher Jon Hawkings, lead author of the review. 

These kinds of comparisons matter because they hint at how different polar regions may respond differently to warming.

It also showcases why iC3 values cross-disciplinary teams. Understanding meltwater chemistry and impact means linking glaciology, hydrology, oceanography, geochemistry and microbiology in the pursuit of new knowledge.

Greenhouse gas impacts: a key uncertainty

The review also tackles a question that comes up often in climate discussions: do glaciers add greenhouse gases to the atmosphere, or remove them?

The answer is that glaciers can do both. Under ice, mineral reactions can draw down carbon dioxide in some settings, while other reactions can release it. Microbes can also either produce or consume gases such as methane. 

“Microbial processes and physical-chemical weathering can both sequester or emit greenhouse gases, but the net effect in glacierized environments remains unknown,” Jon comments, adding that:

“The science is moving fast, but the most policy-relevant conclusion is not yet settled. Closing that gap will need better measurements in hard-to-reach places and stronger links between field data and Earth System Models.”

“The answer will also tell us more about how glaciers have impacted climate through Earth’s history.”

Pairing multiple tracers is especially powerful

The review synthesises existing knowledge, bringing together results from many glacier settings and scales, from small glaciers to major ice sheets. It highlights how researchers can use chemical and isotopic ‘fingerprints’ to trace where elements come from and what happens to them.

For example, carbon isotopes can help reveal the age and source of organic carbon in meltwater. Other isotope systems can help track types of chemical weathering, redox reactions, and the fate of glacial material downstream.

The review argues that pairing multiple tracers is especially powerful to disentangle the sources and processes occurs within glacial systems and, ultimately, their impact on ecosystems and climate.

Downstream impacts of glacial biogeochemistry

“Glacier melt can prime glacier-fed streams, lakes, fjords, and oceans with reactive rock flour and nutrients,” Jon says.

Such priming can have different effects in different places. In some coastal waters, glacial particles and micronutrients such as iron can support marine productivity. In other cases, turbid meltwater can limit light penetration and alter ecosystem function, potentially shifting where the most productive regions in coastal areas occur.

The review stresses that the full picture is nuanced and still being debated.

There are also potential water quality concerns during deglaciation. The authors describe evidence linking retreat to elevated concentrations of potentially harmful metals in some meltwater streams and lakes, especially where certain rock types are exposed, potentially generating substantial risks for downstream communities.

Find out more

The review article “Glacier biogeochemical cycling and downstream impacts” has been published in Nature Reviews Earth & Environment.

Lead author Jon Hawkings works on how meltwater moves carbon, nutrients and sediments from ice to downstream ecosystems. Read this interview to find out more about his research. Co-author Jemma Wadham is the director of iC3.