Dataset abstract This data set contains oxygen and hydrogen isotope measurements from discrete seawater samples that were collected in the Southern Ocean (south of 30 deg S) during the Antarctic Circumnavigation Expedition (ACE). 637 samples were collected during the period December 24th, 2016 and March 18th, 2017 in the Southern Ocean from the surface ocean using the ship's underway line (UW; 338 samples) and in vertical profiles using Niskin bottles mounted on the CTD rosette (287 samples). A few additional samples were collected from a parallel cast with a trace-metal rosette, with a bucket, and from the surface of a tabular iceberg. All samples were analyzed for their oxygen isotopic composition (reported as permille deviation of the oxygen-18 to oxygen-16 ratio from VSMOW2: DEL18O) and a few samples (80) from the Pacific sector were analyzed for their hydrogen isotopic composition (reported as permille deviation of the hydrogen to deuterium ratio from VSMOW2: DELD) by mass spectrometry at the British Geological Survey. This circumpolar data set provides insights into the hydrological cycle of the Southern Ocean and the processes (precipitation, evaporation, sea-ice melting and freezing, iceberg and land-ice melting) that determine the salinity of a certain water mass. Dataset contents ace_18_data_d18o_dd_ctd_20190219.csv, data file, comma-separated values ace_18_data_d18o_dd_other_20190219.csv, data file, comma-separated values ace_18_data_d18o_dd_uw_20190219.csv, data file, comma-separated values README.md, metadata, markdown

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The Antarctic Circumnavigation Expedition was made possible by funding from the Swiss Polar Institute and Ferring Pharmaceuticals. This work was supported by the SNSF grant numbers 175162 and 142684. F.A.H. and I.V.G. were supported by the BNP Paribas Foundation.