The cessation of North Pacific Deep Water formation over Northern Hemisphere Glaciation.

Global ocean circulation is controlled by deep water formation in the high latitude Atlantic and Southern Oceans. There is no deep water formation in the modern North Pacific due to a strong salinity gradient (or halocline) which makes the deep Pacific relatively homogenous. There is evidence to suggest that this halocline was weaker in the Late Pliocene (3.3 – 2.6 Ma) which allowed for active deep water formation. Coupled stable isotope and trace element records from benthic foraminifera at the Northwest Pacific ODP Sites 1208 (3346 m depth) and 1209 (2387 m depth) indicate deep water formation in the North Pacific during the Late Pliocene. Heavier oxygen isotopes at the shallower site 1209 require that the two sites were bathed in deep waters formed in different locations. Trace metal analysis (Mg/Ca) shows that there was a marginally colder, and thus fresher, water mass at the shallower site 1209 which is partially consistent with modelling results showing a fresher North Pacific Deep Water reaching intermediate depths in the Late Pliocene, while the deeper site was bathed in southern sourced waters. The benthic isotope values at the two sites converged during the glacials of the Early Pleistocene after the intensification of Northern Hemisphere Glaciation (iNHG, c. 2.7 Ma). This convergence was coincident with a global drop in sea levels suggesting that sea level changes, potentially by constricting water mass transport through the Indonesian Gateway, may have modulated the strength of North Pacific Deep Water formation in the Pliocene. This would mean that the complete cessation of North Pacific Deep Water does not occur until considerably after the iNHG.