Surface-water freshening: A cause for the onset of North Pacific stratification from 2.75 Ma onward?

Paleoceanographic data indicate a sudden stratification in the North Pacific Ocean around 2.75 Ma. Understanding the triggering mechanism for this sudden North Pacific stratification is one key to resolving forcing mechanisms for the Northern Hemisphere glaciation around 2.75 Ma, one of the most perplexing problems in paleoclimate research. The trigger for this sudden stratification has not been fully explored. Changes in deep-water or surface-water temperature and/or salinity can cause stratification. Here we re-examine paleoceanography data from the North Pacific Ocean and find evidence that the stratification was preceded by a surface-water salinity decrease between 2.83 and 2.75 Ma. This paleoceanographic result, in combination with recent model simulation results, suggests that fresh water input to the North Pacific via an intensified East Asian summer monsoon and/or an intensified Polar Westerly winter jet stream associated with a phase of accelerated uplift of the northeastern Tibetan Plateau from 4–2.6 Ma might be one important forcing for the onset of North Pacific stratification. Onset of North Pacific stratification around 2.75 Ma is closely related to the intensification of Northern Hemisphere glaciation around the same time via its effects on atmospheric CO2 concentration and sea surface temperature seasonality. Forcing the onset of the North Pacific stratification and Northern Hemisphere glaciation via surface-water freshening has not been emphasized previously.