Global assessments may overestimate carbon accumulation in fjords: new study

Norwegian fjords may look like relatively small features compared with the open ocean, but their sediments play an important role in the marine carbon cycle. Fjords are estimated to bury around 18 millions tonnes of organic carbon each year globally, with more than half of this carbon originating from land. But how reliable are these large-scale estimates?

A new study, led by Markus Diesing and including Jochen Knies from the iC3 Polar Research Hub, takes a detailed look at the seabed of fjords around Stavanger, Norway, and finds that the answer depends strongly on where you look.

The research team mapped seabed sediments and sedimentary environments across approximately 500 km², combining geological mapping with sediment samples, sediment cores and geochemical analyses. Their results reveal a highly heterogeneous environment: while fine-grained sediments accumulate in around half of the mapped area, the rest is dominated by coarser sediments associated with erosion and sediment transport.

Figure: Regionalisation with four clusters. Boxplots of the variables used for clustering show the properties of the clusters. The thick horizontal line of the boxplots is the median, the ends of the box are the upper (Q3) and lower (Q1) quartiles, defining the interquartile range (IQR), the whiskers show the range of values within Q3 + 1.5 × IQR to Q1 − 1.5 × IQR, representing the highest and lowest values, excluding outliers and outliers are the dots beyond the whiskers. CRI=carbon reactivity index. Credit: Diesing et al. (2026), Biogeosciences.

Carbon varies dramatically over short distances

The differences are not only visible in the seabed itself. Organic carbon accumulation rates in depositional areas range from 18.7 to 82.6 grams of carbon per square metre per year, while organic carbon stocks range from 0.1 to 1.37 kilograms per square metre. The sources of that carbon also vary substantially, from predominantly terrestrial to predominantly marine.

Importantly, these contrasts can occur over just a few kilometres. In Talgjefjorden, for example, three sediment cores captured almost the entire observed range of organic carbon accumulation rates. This demonstrates why selecting only convenient or particularly carbon-rich locations can produce estimates that are difficult to scale up to an entire fjord.

Rethinking how we scale up

The study suggests that some global assessments may overestimate carbon accumulation in fjords because they implicitly assume that seabeds are relatively homogeneous and that measurements from individual cores represent much larger areas.

“The seabed is not a uniform carbon sink. Where we take a sample matters enormously, because deposition and erosion can occur side by side,” says Jochen.

The authors propose a more robust approach: first determine the actual area of fjords, then identify where fine-grained sediment accumulates, collect representative samples, and explicitly account for uncertainty. Applying this framework to mainland Norway produces a tentative estimate of 0.41–3.68 millions tonnes of organic carbon accumulating annually in fjord surface sediments.

From local fjords to the global carbon cycle

The findings highlight a broader challenge in understanding the ocean carbon cycle. Local measurements can provide detailed information, but translating those observations into regional or global numbers requires careful consideration of environmental variability.

“For carbon-cycle estimates to be meaningful at larger scales, we need to understand the landscape in which individual measurements are made,” Jochen says. “Detailed mapping and representative sampling can help us distinguish where carbon is actually accumulating from areas where sediments are being transported or eroded.”

The study therefore offers more than a new estimate for Norwegian fjords. It provides a framework for improving how scientists scale up observations from individual sediment cores—and ultimately for building a more realistic picture of the role fjords play in the global carbon cycle.

Find out more

The study “Spatial heterogeneity of sedimentary organic carbon in fjords around Stavanger, Norway – implications for upscaling” is available open access in Biogeosciences.

Lead author Markus Diesing is a researcher at the Geological Survey of Norway.

Co-author Jochen Knies is a researcher in the Department of Geosciences at UiT The Arctic University of Norway and at The Geological Survey of Norway. He is part of the iC3 Polar Research Hub where he leads a research unit that aims to reliably estimate how carbon cycles and ocean ecosystems altered during past ice sheet retreat. He additionally co-leads the Into the Blue project. His research focuses on reconstructing the past of the Arctic Ocean.