Relationship of oceanography, temperature and salinity to δ18Oequil.. δ18Oequil. is defined here as the δ18O of aragonite in equilibrium with seawater at observed temperatures and salinities. (a) Calculated δ18Oequil. climatology from climatological temperature and salinity values22 from vertical section across the GIN Seas at 75°N (see Methods), with water-masses annotated. (b) Temperature (contour colours) and salinity (contour lines) from vertical section across 75°N within the GIN Seas Regional Climatology22. The solid black lines depict the boundaries between water masses. The dashed black lines highlight where the water stratifies in summer, leading seasonally to the warm surface layer evident in panel b.