Continuous Surface Water Isotopes Reveal Fram Strait Water Mass Interactions during the Onset of Sea Ice Melt

The Fram Strait is a key region for Arctic freshening and Atlantification, shaped by the interaction between warm, saline Atlantic Water inflow and the export of cold, fresh Polar Surface Waters, with Atlantic heat transport exerting a strong control on regional sea ice conditions. While the Fram Strait has been extensively studied using hydrographic transects and deep water profiles via mooring arrays, open surface waters and those beneath sea ice and non-summer periods, when sea-ice cover limits accessibility, are comparatively under sampled. We present one of the first Fram Strait dedicated, ultra high resolution surface water geochemistry datasets from the I/B Oden as part of the ARTofMELT 2023 expedition (May–June). We have collected continuous measurements of temperature, salinity, δ¹⁸O, and d-excess in surface waters at 8 m depth along the expedition cruise track during the transition from winter to spring. Our stable water isotope measurements provide a powerful tool to distinguish surface water provenance and freshwater modification in this region of the Arctic, where temperature–salinity (T-S) properties often converge under sea ice. We observed pronounced gradients across the Fram Strait, with fresher and isotopically depleted surface waters in the west, consistent with influence from the East Greenland Current, and more saline, isotopically enriched waters in the east. Conventional T-S frameworks would classify most observations as Polar Surface Water, inherently indicating surface waters being derived and exported from the Arctic. However, the isotopic composition suggests that waters with an Atlantic provenance intrude substantially farther beneath the sea ice in the eastern Fram Strait than previously appreciated. By combining continuous surface water isotope measurements with isotopic observations from sea ice and precipitation, we further show that the contribution of local sea ice melt to surface waters increases during the second half of the expedition, preceding the observed surface melt onset by approximately two weeks. Our results demonstrate that T-S relationships alone are insufficient to resolve surface water provenance and freshwater modification, whereas seawater isotopes provide critical constraints on the sources and evolution of surface water masses and freshwater in the seasonally ice-covered Fram Strait.