Neodymium Cycling and Water Mass Structure in the Indian Sector of the Southern Ocean

The Southern Ocean plays a central role in the global climate system by regulating large-scale circulation, facilitating interbasin exchange, absorbing large amounts of anthropogenic heat and carbon and influencing Antarctic Ice Sheet stability. Yet, observational data, including rare earth element measurements such as neodymium (Nd) isotopes and samarium (Sm), have so far remained sparse in the Indian sector and along the East Antarctic continental margin, thereby limiting our understanding of circulation, water mass transformation, and sediment-ocean interactions in a changing climate.

Water masses in the Southern Ocean are traditionally characterized using hydrographic parameters (e.g., potential temperature, salinity, neutral density). During physico-chemical weathering along the East Antarctic margin, old continental crust supplies a distinctly unradiogenic neodymium isotope signature (low εNd) to regional shelf waters that interact with more radiogenic Antarctic Circumpolar Current waters (higher εNd) further north. This isotopic difference makes neodymium isotopes an especially powerful tracer of regional circulation and mixing along the East Antarctic continental margin. We present the first high-resolution dataset of dissolved εNd, together with Nd and Sm concentrations, from the Indian sector of the Southern Ocean. Gridded water column samples in conjunction with bottom water samples extracted from multicorer sediment supernatants, collected during expedition EASI-2 onboard RV Polarstern (Dec 2023–Feb 2024), provide a meridional transect from the Denman Glacier front (~66°S) to ~45°S along 100°E. Combined with hydrographic observations, this dataset provides a detailed framework for examining water mass structure, mixing, and regional boundary fluxes along the transect.

Away from direct Antarctic continental influences, the εNd distributions show largely conservative behavior in intermediate to deep waters and allow clear identification of major Southern Ocean water masses. A striking feature is the persistence of a remnant North Atlantic Deep Water εNd signal within lower Circumpolar Deep Water, highlighting long-range interbasin connectivity. Near the Denman Glacier, warm and radiogenic modified Circumpolar Deep Water (mCDW) intrudes onto the continental shelf, evident in both physical properties and εNd signatures below ~400 meters water depth. As seen in earlier studies, a pronounced mCDW tongue was observed to reach close to the Denman Glacier front, with associated high basal melt rates evident from potential temperature and salinity in sampled local East Antarctic shelf waters.

Nd and Sm concentrations increase linearly with depth north of the Polar Front, but exhibit substantial enrichment south of the front, reflecting deep-water upwelling, biogenic scavenging, and a latitudinally gradual boundary exchange. Pronounced variations in εNd and rare earth element concentrations in bottom waters point to substantial benthic additions in the southern reaches of the transect driven by weathering inputs from ambient terrigenous sediments, whereas particle-related scavenging appears to dominate offshore.

This study closes a critical observational gap in the Indian sector of the Southern Ocean and provides new constraints on the present-day circulation, water mass structure, and the influence of Antarctic crustal and benthic Nd additions, while demonstrating the value of εNd as a tracer in modern and paleoceanographic contexts.