We have developed a new method to reconstruct surface water salinity of the North Atlantic Ocean, based on a comparison between transfer function estimates of sea surface temperature and the oxygen isotope ratio of two common planktonic foraminiferal species, N. pachyderma (left coiling) and G. bulloides. We first used core-top analyses to demonstrate that, under modern conditions, the paleotemperatures determined from the isotopic composition of foraminiferal shells are linearly linked to the summer sea surface temperature only within a certain temperature range, characteristic of each species, which is termed the "optimum temperature range". We then used this information to derive an estimate of the isotopic composition and salinity of the North Atlantic surface water during the last glacial maximum. The resulting reconstruction shows the presence of a sharp salinity gradient associated with the polar front. However, high-salinity water was present near 30-40-degrees-W, north of the polar front. This pattern would have provided the flux of salt required for increasing the density of surface water during winter and its subsequent sinking to the abyss.