Characterizing the stable oxygen isotopic composition of the southeast Indian Ocean

New seawater stable oxygen isotope (δ¹⁸O) samples were collected from the southeast Indian Ocean as part of the Coring to Reconstruct Ocean Circulation and Carbon dioxide Across 2 Seas (CROCCA-2S) expedition in November – December of 2018. These data fill a gap in the δ¹⁸O sampling coverage of the southern Indian basin, providing new insights into the hydrologic and oceanographic processes influencing the δ¹⁸O distribution of the region and its relationship to salinity in the upper ocean. Our surface ocean data (<100 m)—in combination with decades of observations from the broader south Indian Ocean—show distinct δ¹⁸O – salinity characteristics on either side of ∼85°E. The balance between evaporation and precipitation yields a strong, robust δ¹⁸O – salinity relationship west of 85°E (δ¹⁸O = 0.50(±0.01) * S – 17.2(±0.22)). However, within the mesoscale eddy field initiated by the Leeuwin Current further east (∼85–120°E), our observations fall along a mixing line between the southwest Indian Ocean and data collected from the Australian coastal margin, illustrating for the first time how the unique eastern boundary system of the south Indian Ocean drives regional-scale variability in the δ¹⁸O – salinity relationship of the surface ocean. A comparison between our observations in the shallow subsurface (100–1000 m) and those from neighboring surveys reinforces this upper ocean connection across the Indo-Australian basin. Antarctic Intermediate Water from the Indian Ocean can be isotopically distinguished from the more regional Tasman Intermediate Water occupying the South Australian Bight, suggesting exchange between the two regions is most prevalent at surface and mode water depths. In deeper waters (> ∼1500 m), we observe a notable 0.87‰ spread in δ¹⁸O. This variability may represent interactions between distinct deep water masses in the region, although additional data are needed to confirm. Overall, our data provide a new look at the hydrography and isotopic chemistry of the southeast Indian Ocean, emphasizing the impact of the region's mesoscale eddy field and its interconnectivity with neighboring basins.