In its stable form, carbon consists of the isotopes C-12 and C-13. During photosynthesis, plants preferentially fix the lighter C-12. Since fuels consist predominantly of fossil plant material, they contain comparatively little of the heavy carbon isotope C-13. As a result of their combustion, the ratio of C-13 to C-12 in the atmosphere decreases. Through exchange with the atmosphere, surface water increasingly takes on this altered isotopic signature. Through vertical mixing and the formation of deep water, anthropogenic carbon can be transported deep into the ocean.
New measurements now show how far this anthropogenic carbon signal has already penetrated into the deep-water masses of the North Atlantic. For their study, published in the journal Geophysical Research Letters, the team led by Emma Bavoux analyzed carbon isotope ratios in water samples collected during two expeditions aboard the research vessel MARIA S. MERIAN in 2017 and 2018 along a transect at approximately 48° North across the entire North Atlantic. In addition, the researchers used earlier measurements of the trace gas sulfur hexafluoride (SF₆)—which is also produced by human activities—to estimate the human-induced component in the carbon isotope ratio.
“We were surprised by how significant the Suess effect already is in the deep-water masses of the North Atlantic,” said Emma Bavoux. The Suess effect is now detectable in nearly all of the North Atlantic water masses studied. Young, subsurface water masses in the North Atlantic—or those that had most recently been in contact with the atmosphere—exhibited the strongest Suess effect. The Suess effect was extremely weak or undetectable in deep, ancient water masses such as the Northeast Atlantic Deep Water, which likely have had no contact with the atmosphere for several hundred years. Samples from the western North Atlantic, on the other hand—the Labrador Sea water and the water masses flowing through the Denmark Strait and subsequently sinking to greater depths—have exhibited a Suess effect of approximately 0.3 to 0.6 per mill since pre-industrial times. The researchers also succeeded in observing a large oceanic eddy in the Labrador Sea that carries a distinct Suess effect signal to water depths of up to 2,000 meters.
The publication shows that traces of industrial activities are increasingly detectable in the ocean. However, the researchers also point out that the Suess effect is already detectable in the shells of microfossils. These include, for example, so-called foraminifera, which have been deposited on the ocean floor over the past few decades. “Through the Suess effect, the ocean floor preserves signals of human origin that now mark the beginning of the Anthropocene in these long-term geological archives,” says Stefan Mulitza, co-author of the study. “How clearly this stratigraphic boundary will be defined depends primarily on the future trajectory of carbon dioxide emissions.” As part of the Cluster of Excellence ‘The Ocean Floor – Earth’s Uncharted Interface’, the scientists are working to further unravel the role of the ocean and its ecosystems in the global carbon cycle.
Original Publication:
Bavoux, E., Mulitza, S., Steinfeldt, R., Kuhnert, H., Martinez-Mendez, G., & Pälike, H.: Anthropogenic carbon isotope signals in North Atlantic water masses at 48°N. Geophysical Research Letters, 53, e2025GL121339. https://doi.org/10.1029/2025GL121339
Infobox: What Is the Suess Effect?
The Suess effect is evident when looking at the evolution of the ratio of 13C to 12C in seawater inorganic carbon, especially in surface waters. Stable carbon isotope ratios in global surface waters typically decrease within the range of 0.15 ‰ per decade (Sonnerup et al., 1999). Carbon released during the burning of fossil fuels is richer in the lighter carbon isotope 12C, as this is what the plants that form the fossil fuels preferentially absorbed during their lifetime, thus inducing a decrease in the ratio.
