North Pacific Deep Water Formation Termination over the Northern Hemisphere Glaciation

Modern ocean circulation is driven by formation of deep waters in the North Atlantic and Southern Oceans. Despite the cold temperatures at high latitudes, the North Pacific exhibits no modern deep-water formation, due to the strong halocline in the subpolar North Pacific Ocean. However, modelling results suggest the halocline was weak during the Pliocene, allowing the formation of North Pacific Deep Water (NPDW). Here we compare the oxygen isotopes from benthic foraminifera at two sites in the Northwest Pacific Ocean, ODP Sites 1208 (3346 m) and 1209 (2387 m) to understand how NPDW varied over the Plio-Pleistocene (3.3 to 2.6 Ma). While in the modern the water that occupies the deep Pacific is homogenous, during the Pliocene there was a ~0.5 per mill gradient at times between sites 1208 and 1209, with, surprisingly, more positive δ¹⁸O values observed at the shallower 1209. The disparity in oxygen isotopes suggests different water masses over the two sites over the Plio-Pleistocene. Oxygen isotopes of foraminifera record temperature and the δ¹⁸O of seawater which loosely correlates with salinity in the modern surface oceans, with more positive δ¹⁸O values generally representing more saline, colder, and therefore more dense waters. High latitude processes like sea ice formation could result in a decoupling of salinity from δ¹⁸O of seawater. We suggest that the divergences in oxygen isotopes are the result of Site 1208 being bathed in southern-sourced waters, while Site 1209 is bathed in deep waters formed in the North Pacific. Modelling results suggest NPDW is a warm, saline water mass. That the divergence in δ¹⁸O values between the two sites is not constant throughout this period suggests that the strength of NPDW export may have fluctuated over this interval. Future work includes Mg/Ca analyses at Site 1209 to compare with the existing record at 1208 to examine the temperature and δ¹⁸O of seawater differences between these sites.