Neogene–Quaternary variations of the ‘Pole–Equatorʹ temperature gradient of the surface oceanic waters in the North Atlantic and North Pacific

The deep sea drilling core samples and published data of the Deep Sea Drilling Project and Ocean Drilling Program serve as the bases for this study. Original methods for paleotemperature interpretation of foraminiferal and oxygen isotopic data are suggested. For several Neogene–Quaternary time-slices, the surface and near-surface paleotemperatures of the high and low latitudes in the North Atlantic and North Pacific are evaluated. During the Neogene, the ‘Pole–Equatorʹ gradient of the surface water mass at different depths tended to increase. It increased by 8°C at the oceanic surface, by 11.5°C at the depth of 50 m, by 15.5°C at the depth of 100 m and by 11.5°C at the depth of 200 m. This is related to the progressive cooling in the high latitudes during Neogene–Quaternary. Maximum gradient variations are observed at the depth of 100 m resulting from thermocline onset and evolution. Our data suggest that the first sharp increase of gradient by 4–6°C occurs between 10–5 Ma, and the second one (also 4–5°C) during the Middle–Late Pliocene. These abrupt changes correspond to the intensification of the oceanic circulation caused by the evolution of Neogene glaciation. In the Quaternary, the temperature gradient slightly decreased. During the Last Glacial maximum, the gradient was less than those observed during the Last Interglacial optimum and now.