Submarine glacial troughs are hotspots of organic carbon accumulation on the Norwegian continental margin

Glacial troughs, deeply incised submarine valleys once carved out by glaciers during ice ages, accumulate and store a considerable amount of organic carbon in their sediments. Protection from human disturbance might provide an opportunity to counteract human-induced climate change.
Published in Earth & Environment
Submarine glacial troughs are hotspots of organic carbon accumulation on the Norwegian continental margin

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To keep the human-induced global average temperature rise well below 2°C in line with the Paris Agreement requires drastic emission reductions and a removal of carbon dioxide from the atmosphere. Part of the carbon dioxide removal could be achieved by nature itself, if ecosystems that remove substantial amounts of carbon from the atmosphere are protected, managed, or restored. In the marine environment, the focus has so far been placed on coastal ecosystems with rooted vegetation, as they remove carbon at high rates, are threatened by human activities and are amenable to management. Collectively, these are called Blue Carbon ecosystems. More recently, marine sediments have been proposed as a promising natural climate solution given their ability to lock away large amounts of carbon over long-term geological timescales. However,  knowledge gaps are currently too large to judge to what extent marine sediments could help mitigate human-made climate change.

To close some of these knowledge gaps, we mapped the amount of organic carbon stored in sediments on the Norwegian continental margin and the rates at which it is accumulated. We achieved this by first compiling 20 years of data collected as part of the Norwegian seafloor mapping programme Mareano. These data were then integrated in the Modern Ocean Sediment Archive and Inventory of Carbon (MOSAIC), a curated and harmonised database of marine sediment data centred on carbon. Finally, we used an extract from the MOSAIC database that included our own Mareano data as well as data from other sources in a harmonised format. To make sense of this large volume of data, we applied state-of-the-art machine learning techniques. This allowed us to predict the spatial distribution of organic carbon stocks and accumulation rates.

An image of a multi-corer containing sediment collected from the seafloor
A multi-corer, the work horse for collecting samples from the seafloor in the Mareano programme. The sediment collected in the tubes is sliced into 1-cm disks. These subsamples are later analysed in the laboratory. Photo: Mareano

We found that the upper ten centimetres of seafloor sediments alone store 814 million metric tonnes of organic carbon in Norway, much more than Blue Carbon ecosystems. On average, seafloor sediments accumulate six million metric tons of organic carbon per year, equivalent to 45% of Norway’s greenhouse gas emissions in 2022. Nearly half of this accumulation happens in glacial troughs, seaward continuations of fjords that are deeply incised into the continental shelf. Due to the much greater water depths in comparison to their surroundings, glacial troughs act as efficient sinks for sediments and organic carbon. Glacial troughs are found worldwide on high-latitude continental shelves and cover an area ten times larger than that in Norway. Extrapolating our results globally indicates that glacial troughs might be environments with the potential to accumulate organic carbon on a scale comparable to Blue Carbon ecosystems.

A bubble plot showing how much organic carbon is accumulated in salt marshes, mangroves, seagrass meadows, fjords and glacial troughs globally.
This bubble plot shows how much organic carbon likely accumulates in salt marshes, mangroves, seagrass meadows, fjords and glacial troughs globally.  Low and high estimates are indicated by different circle sizes. Numbers are in teragrams of carbon per year. One teragram equals one million metric tonnes. Estimates for ecosystems other than glacial troughs are taken from other studies. See the paper for references.

Protecting submarine environments, which accumulate and store large amounts of organic carbon, against human activities such as bottom trawling might provide benefits in the fight against human-induced climate change. This could be achieved by avoiding emissions of carbon dioxide caused by the enhanced remineralisation of accumulated organic carbon. Beyond that, it will be important to increase the amount of organic carbon that is sequestered in seafloor sediments through increasing the flux of carbon to the seafloor or decreasing the amount of carbon that gets remineralised. Exactly how this could be achieved is open to debate, as we are only beginning to understand the complex interactions of human activities with the carbon cycle. More research is needed to robustly quantify the effects of human activities on organic carbon sequestration. This will ultimately lead to a better management of the seafloor and its climate-regulating functions.

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Carbon Cycle
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Biogeosciences > Biogeochemistry > Carbon Cycle
Climate Sciences
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Climate Sciences
Ocean Sciences
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Ocean Sciences

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