Quaternary deep-thermocline cooling enhanced by southern Pacific Ocean tunnelling

Abstract Tropical climate is considered to be one of the decisive keys to global climate dynamics. The extent to which extratropical forcing mechanisms contributed to changes of tropical heat and thus global changes in the climate system, however, is only poorly understood. Here, we reconstruct early Quaternary hydrographic variability of the tropical deep thermocline of the eastern margin of the West Pacific Warm Pool using magnesium to calcium and oxygen isotope ratios of surface and deep-dwelling foraminifera. This approach allows to determine past ocean temperatures and relative salinity changes likely related to extratropical oceanographic reorganisations. Our proxy records point to a freshening of the tropical Pacific deep-thermocline and exhibit a strong coupling to the volume increase of the Antarctic ice shield and cooling of high/mid latitude surface water masses at 1.5 Ma. This temporal coincidence is accompanied by the progressive increase in Earth´s obliquity amplitude and suggests high latitude climate impact via ocean tunnelling of southern-sourced sub-surface water masses towards the equatorial Pacific. We hypothesize that ocean tunnelling of mid-latitude Southern Ocean/South Pacific water to the West Pacific Warm Pool effectively contributed to global climate between 1.7 Ma and 1.2 Ma prior the Mid-Pleistocene-Transition, particularly through changes in oceanic heat budget across vast latitudinal ranges.