Fig. 7: Isotopic source contributions to the Southern Ocean.

| A schematic introduces the various sources influencing the seawater oxygen isotope (δ¹⁸O_sw) composition of the Southern Ocean. δ¹⁸O_sw in the Southern Ocean depends on the fluxes of several freshwater inputs (such as sea ice meltwater, run-off, precipitation, and land-ice meltwater), along with their respective δ¹⁸O values. In association with Rayleigh distillation, lighter isotopes are preferentially fixed into the precipitation (~−8‰) and the Antarctic ice-sheet (−32‰) at the higher latitudes^{30,31,32,33,34}. In contrast, during the freezing process, sea ice takes heavier isotopes (+1.4‰) from seawater^35,36,37, resulting in a less pronounced impact on δ¹⁸O_sw (−0.7‰). In iCESM sea-ice also includes accumulated snowfall. When sea-ice melts, the snowfall contribution usually overcompensates the positive isotopic fractionation effect in sea-ice – even in the annual mean. Therefore, to monitor ice-sheet melting, only two end-members need to be considered: δ¹⁸O_sw and δ¹⁸O_ice.