Reading the isotopic fingerprints of the Artic water

Looking at the sea from the deck, it is impossible to know where the water beneath us has come from. Some of it has travelled northward from the Atlantic; some has arrived from the Arctic, while another fraction may ultimately derive from precipitation, melting sea ice or the Greenland Ice Sheet. Yet these different histories leave chemical fingerprints in the water.

In a way, the global water cycle behaves like a gigantic natural still. Water evaporates from the ocean and, as atmospheric moisture is transported towards colder and higher latitudes, it repeatedly condenses and precipitates along the way. At each step, the

heavier water molecules containing ¹⁸O and ²H are preferentially removed (δ values are more negative), leaving the remaining moisture progressively depleted in these heavy isotopes (Fig. 1). By the time snow falls over the interior of Greenland, its isotopic composition is therefore very different from that of the ocean from which the water originally evaporated.

Figure 1. Schematic illustration of the effect of continental ice storage on the oxygen-isotope composition of seawater. On the present-day isotopic scale, mean ocean water has a δ¹⁸O value close to 0‰. During glacial maxima (sea ​​level about 120 m lower than the current level), large amounts of isotopically light snow and ice accumulate at high latitudes, removing water strongly depleted in ¹⁸O from the ocean and increasing global mean seawater δ¹⁸O by approximately +1‰. Conversely, melting of continental ice returns this isotopically light water to the ocean, lowering seawater δ¹⁸O (schematically represented here by values approaching −1‰ for a largely ice-free world). Under present-day conditions, Greenland ice is strongly depleted in ¹⁸O relative to seawater. Meltwater released from the Greenland Ice Sheet therefore carries a distinctive isotopic fingerprint, allowing even relatively small freshwater contributions to be traced with high sensitivity in the surrounding ocean.

Figure 2. Iceberg observed during the IMAGE expedition (11 August 2026; Javier Arístegui). Greenland-derived freshwater is strongly depleted in ¹⁸O and ²H compared with seawater, providing a distinctive natural tracer of freshwater inputs to the North Atlantic.

Over hundreds of thousands of years, part of this isotopically light water has been stored in the Greenland Ice Sheet. Today, when ice melts and freshwater reaches the surrounding ocean, it carries this distinctive isotopic fingerprint with it (Fig. 2).

This makes δ¹⁸O and δ²H powerful natural tracers, allowing us to identify freshwater inputs and better understand how Arctic, Atlantic and Greenland-derived waters mix at this remarkable oceanographic crossroads.

Salinity, together with δ¹⁸O and δ²H values, is key to investigating these water-mass mixtures (Fig. 3).

This matters because freshwater is not just another component of seawater. By lowering salinity, it changes seawater density and stratification, properties that influence how readily surface waters mix and participate in the large-scale circulation of the North Atlantic.

Figure 3. CTD salinity section along the completed IMAGE transect across the Denmark Strait, based on Cast 1–33. The horizontal axis has been plotted so that the stations closest to Iceland are shown on the right-hand side, whereas those closest to Greenland are shown on the left. The section already reveals a distinct low-salinity surface layer towards Greenland. Once the isotope analyses are completed, we will be able to investigate whether this freshwater signal reflects Greenland-derived meltwater, Arctic meteoric water, sea-ice melt, or a mixture of these sources.

At each station of the IMAGE transect, we therefore collect more than just bottles of seawater. We are sampling different pieces of the history connecting the Arctic, Greenland and the North Atlantic.

This freshwater input has implications that extend well beyond the waters surrounding Greenland. The Atlantic Meridional Overturning Circulation (AMOC), which transports heat northward and returns cold, dense waters southward at depth, is projected to weaken as the climate warms, although the magnitude and timing of this change (and particularly the possibility of an abrupt collapse) remain actively debated. Freshwater entering the North Atlantic can alter salinity, density and stratification, potentially affecting the processes involved in deep-water formation and circulation. Understanding where this freshwater comes from, how much is present and where it goes is therefore an important piece of a much larger climate puzzle.

Our measurements do not monitor the AMOC itself, but they help characterize freshwater inputs and mixing processes that can influence this complex circulation system.

Isotopes provide one piece of this broader picture. Combined with physical oceanography and biological observations made by the other teams on board, they help us understand how water masses, biogeochemical cycles and marine ecosystems interact at this Arctic-Atlantic crossroads.