| Summary: | Present-day (PD) and Last Glacial Maximum (LGM) simulations of the global ocean are presented, with the oxygen-18 isotope included as a passive tracer. The gradient of the PD North Atlantic surface 18O:salinity relationship is found to result from different processes at low and high latitudes. At low latitudes, the balance between surface 18O flux and oceanic advection and mixing sets the surface 18O:salinity gradient, whereas at high latitudes, mixing between 18O-depleted runoff and precipitation to the Arctic, Bering Strait inflow, and waters from lower latitudes, controls the 18O:salinity gradient. The importance of the Bering Strait contribution has not previously been recognised. These gradients change significantly at the LGM, and are found to be sensitive to both Arctic runoff 18O concentrations and changes in oceanic advection, particularly the rate of exchange of North Atlantic deep water with the global ocean. It is concluded that reconstructions of past climates from records of sea surface 18O based on analogues of the PD 18O:salinity relationship are likely to be in error
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