The ice encountered on site prior the start of MOSAiC was severely weathered. Observations indicated that a large fraction was melted completely during the summer months. Residual ice that survived was exceptionally thin and characterised by frozen-over melt ponds. Credit: Mario Hoppmann/ Alfred Wegener Institute. CC-BY 4.0.

Pioneering research has revealed that Arctic matter pathways are poised for major shifts amidst ongoing climate change.

A new study has shed unprecedented light on the highly variable and climate-sensitive routes that substances from Siberian rivers use to travel across the Arctic Ocean. The findings raise fresh concerns about the increasing spread of pollutants and the potential consequences for fragile polar ecosystems as climate change accelerates. The international research provides the clearest ever picture of how the underlying transport system, known as the Transpolar Drift, operates.

ScienceVega spoke to Professor Kate Hendry, of the British Antarctic Survey, to better understand what is happening in the Arctic ecosystem given the increasing influence of global warming and climate change…

ScienceVega: When was the research first proposed and how long has the actual work taken (both in the field and in the lab)? 

KH: Our research was part of a large international expedition called ‘Multidisciplinary drifting Observatory for the Study of Arctic Climate’ or MOSAiC for short, which happened from September 2019 to October 2020 and was led by the Alfred Wegener Institute in Germany. The actual planning for MOSAiC started many years before the expedition – it takes a long time to plan something on this scale! The German research vessel, Polarstern (meaning ‘Polar Star’ or ‘North Star’ in English), was the central platform for the scientists, but they were supported by icebreakers from other countries.

The Polarstern was frozen into sea ice, and drifted for months on end, whilst scientists (over 600 researchers in total!) took measurements of the ocean, the ice and the atmosphere covering physics, chemistry and biology. The expedition fell right over when the Covid 19 pandemic happened, which meant that some scientists had to stay on board for longer than planned, and caused a lot of delays, including to the laboratory work. It took another few years to get all the analyses done in the lab, before we could interpret and publish the data.

MOSAiC wasn’t the first expedition to study the Central Arctic Ocean in this way. Fridtjof Nansen led the Fram Expedition from 1893 to 1896, also on a ship frozen into ice, but they didn’t have the modern research tools that we have at our disposal to fully understand all the complex processes involved.

Image shows the German research icebreaker Polarstern moored to an ice floe during the polar night. Credit: Steffen Graupner/ Alfred Wegener Institute.

ScienceVega: How does geochemical fingerprinting work?

KH: There are two different types of geochemical fingerprinting at work in our study, both of which rely on measurements of isotopes. Isotopes are different forms of atoms of the same element: they have the same number of electrons and protons in their atoms, but different numbers of neutrons. This results in two isotopes of the same element have different masses and, so, different properties. Some isotopes are stable, and so stay as they are without changing. However, when a physical, chemical or biological reaction occurs, it might be that lighter or heavier isotopes react for favourably, resulting in a very slightly lighter or heavier product.

In this study, we use stable oxygen isotopes in the seawater and sea ice to trace meltwaters and river waters. This is because evaporation ‘prefers’ lighter oxygen and so rain (or snow) is light… and so river water is light! This light isotopic ‘fingerprint’ of river water can be traced all the way out into the ocean, so we can tell how much of the seawater has been mixing with water from the large, Arctic rivers of Siberia and northern Europe.

Some isotopes are not stable and slowly break down over time through radioactive decay. If we know how fast this happens (called the ‘half-life’), we can figure out how long it’s been since the isotope was formed. In our study, we measured isotopes of an element called neodymium, along with other rare-earth metals. Neodymium itself isn’t radioactive, but one of its isotopes, neodymium 143, is formed when a radioactive element called samarium breaks down very slowly over millions of years. Because of this, neodymium isotopes are great for telling how old a rock is. In the Arctic, different rocks have different ages, so they leave behind different neodymium ‘fingerprints’. We can even find these fingerprints in seawater, which helps us figure out which Arctic river the water originally came from.

So, it’s all about bringing together as much evidence as possible, using isotope fingerprints to trace where the river water in the Arctic originated. This information helps us understand how changes to river flow in the future, due to climate change, might impact what’s going on downstream in the ocean.

ScienceVega: Why has the research been described as ‘pioneering’?

KH: The MOSAiC expedition was a pioneering feat, bringing together people, equipment, knowledge and skills from all around the world. The expedition also lasted a whole year, meaning that we were able to get measurements and observations in winter and spring, which are very rare and very valuable for understanding Arctic climate. In this study, we were able to measure our geochemical fingerprints not only in seawater but also in sea ice (which was very difficult to do, especially in the case of neodymium isotopes… but we did it!).

Image shows scientists retrieving seawater from different depths using a carousel water sampler deployed through a hole in the sea ice. Josefa Verdugo.

ScienceVega: What is the Transpolar Drift?

KH: The Transpolar Drift is a big ocean current that cuts right across the middle of the Arctic Ocean, past Greenland and Iceland, and into the North Atlantic. We’re interested in it for lots of reasons, but importantly because it takes a lot of the freshwater from the big Arctic rivers from the Arctic into the Atlantic, where it plays an important role in controlling how the rest of the global ocean moves around (and, so, how the ocean moves heat and carbon around the planet).

ScienceVega: What pollutants are being carried from Siberian rivers to the central Arctic and north Atlantic oceans?

KH: We didn’t specifically look at the pollutants ourselves in this study, but we know from other researchers that there are all sorts of industrial, agricultural and domestic pollutants in Arctic rivers. These include heavy metals, persistent organic compounds (or POPs) including pesticides, waste from oil and gas production, nuclear waste, and microplastics. These pollutants can be solid particles swept along by river water, or dissolved in the water itself. Pollutants can also be released from melting ice and permafrost.

ScienceVega: Why is sea ice no longer considered a passive transport medium, but ‘an active agent in shaping dispersal patterns’?

KH: Rather than following a simple flow from the river to the ocean, and then along ocean currents and in sea ice, our results suggest that the movement that Arctic matter is influenced by shifts in river discharge and processes going on in the shallow shelf seas. This means that the traditional view – that there is a steady flow of water, ice and matter – might be wrong and that the Arctic is a lot more variable and dynamic than we thought.

Russian research icebreaker Akademik Tryoshnikov and Polarstern meet in the Arctic Ocean to exchange personnel, cargo and fuel. Credit: Steffen Graupner/ Alfred Wegener Institute. CC-BY 4.0.

ScienceVega: To what extent is global warming having an impact on Arctic matter pathways?

KH: This is a good question, and one that we’re still trying to fully understand. There will be impacts from global warming on river flow, ice and permafrost melting, which will determine how much Arctic matter is getting into the ocean (and from where). Warming will also influence sea ice extent as well as ocean circulation, and so how the matter then moves around (and potentially out of) the Arctic once it gets there. We’ve also shown that the processes going on in the shallow shelf seas are critical, and these will also be sensitive to climate change. All of this means that there is likely to be a change in how Arctic matter gets into the ocean and moves around, and – especially – these processes are likely to get more variable with more extremes. Scientists are working on models to try and make good predictions of these changes into the near future.

ScienceVega: How will this impact the central Arctic and north Atlantic oceans? 

KH: Changes in the amount of Arctic matter getting into the ocean will also impact life in the oceans. This matter might include nutrients, which algae in seawater need to make the food that sustains marine ecosystems, but will likely also include all of those pollutants, which could potentially do harm. If these pollutants are in the Transpolar Drift, they could well be carried out into the North Atlantic, impacting ecosystems there as well as in the Arctic.

Edge of MOSAiC Floe with ROV hut. Credit: Luisa Von Albedyll/ Alfred Wegener Institute. CC-BY 4.0.

Professor Kate Hendry

Polar Oceans Team

British Antarctic Survey

The link to the original paper can be found here.