WELL-OXYGENATED CONDITIONS IN THE NORTH PACIFIC SUPPORT PACIFIC MERIDIONAL OVERTURNING CIRCULATION (PMOC) DURING THE PLIOCENE

To improve our understanding of oceanic circulation to a warming climate, we can look to time intervals in Earth’s past that were warmer than today. One such period is the Pliocene Epoch (2.58 to 5.33 Ma), when the global continental configuration was similar to the present, and atmospheric CO₂ concentrations reached levels comparable to modern values. Pliocene ocean circulation in the Atlantic Basin has been well-studied, but less is known about the Pacific. Today, only intermediate waters are formed in the North Pacific. However, during the Pliocene, i.e., pre-2.73 Ma, recent studies have put forward the hypothesis for North Pacific Deep Water formation, and hence an active Pacific Meridional Overturning Circulation (PMOC). Deep water formation and an active PMOC are expected to result in well-oxygenated bottom waters in regions where more reducing conditions existed after 2.73 Ma. To test this hypothesis, inorganic geochemical studies were conducted at three locations on sediment cores (ODP Sites 882A, 883B, and 1151A) from different depths (3244, 2384, and 1302 meters below sea level, respectively). Redox proxies (U, V, Mo, Zn, Ni, Cu) and bulk sediment δ¹⁵N were measured to assess redox conditions and productivity. The results indicate that before 2.73 Ma, there was no significant enrichment of U, V or Mo at any of the sites, and δ¹⁵N values from Sites 883B and 1151A are tightly constrained near +2‰, consistent with pre-2.73 Ma values observed previously at Site 882B. These low δ¹⁵N values indicate nitrate replete conditions and a lack of active water column denitrification, both consistent with well-oxygenated conditions. However, the interval after 2.73 Ma shows authigenic enrichment in redox-sensitive elements (Site 882A) and higher δ¹⁵N values (all three sites), consistent with conditions of reduced oxygen availability. These findings support the hypothesis that North Pacific Deep Water formation and a PMOC were present in the North Pacific before 2.73 Ma.