Abstract

We present a new 3‐dimensional 1° × 1° gridded data set for the annual mean seawater oxygen isotope ratio (δ 18 O) to use in oceanographic and paleoceanographic applications. It is constructed from a large set of observations made over the last 50 years combined with estimates from regional δ 18 O to salinity relationships in areas of sparse data. We use ocean fronts and water mass tracer concentrations to help define distinct water masses over which consistent local relationships are valid. The resulting data set compares well to the GEOSECS data (where available); however, in certain regions, particularly where sea ice is present, significant seasonality may bias the results. As an example application of this data set, we use the resulting surface δ 18 O as a boundary condition for isotope‐enabled GISS ModelE to yield a more realistic comparison to the isotopic composition of precipitation data, thus quantifying the ‘source effect’ of δ 18 O on the isotopic composition of precipitation.