Oxygen concentrations [O 2 ] in the oceans have been decreasing and oxygen minimum zones (OMZ) expanding since 1960 (Schmidtko et al., 2017). Climate models also show this decrease and predict that this trend will not only continue but accelerate in the future (Oschlies et al., 2008(Oschlies et al., , 2018. To better understand the longer-term oxygen cycle in the past, on time scales typically exceeding centuries to millennia, we can use proxy reconstructions (Moffitt et al., 2015). Most proxy reconstructions assess in-situ bottom water [O 2 ] and rely on sedimentary samples (e.g.,

In this study, we used stable isotopes of oxygen (δ18O), deuterium (δD), and dissolved inorganic carbon (δ13CDIC) in combination with temperature, salinity, oxygen, and nutrient concentrations to characterize the coastal (71°–78°W) and an oceanic (82°–98°W) water masses (SAAW—Subantarctic Surface Water; STW—Subtropical Water; ESSW—Equatorial Subsurface water; AAIW—Antarctic Intermediate Water; PDW—Pacific Deep Water) of the Southeast Pacific (SEP). The results show that δ18O and δD can be used to differentiate between SAAW‐STW, SAAW‐ESSW, and ESSW‐AAIW. δ13CDIC signatures can be used to differentiate between STW‐ESSW (oceanic section), SAAW‐ESSW, ESSW‐AAIW, and AAIW‐PDW. Compared with the oceanic section, our new coastal section highlights differences in both the chemistry and geometry of water masses above 1,000 m. Previous paleoceanographic studies using marine sediments from the SEP continental margin used the present‐day hydrological oceanic transect to compare against, as the coastal section was not sufficiently characterized. We suggest that our new results of the coastal section should be used for past characterizations of the SEP water masses that are usually based on continental margin sediment samples.