NITROGEN ISOTOPE CONSTRAINTS ON REDOX CONDITIONS IN THE NORTH PACIFIC OCEAN DURING THE WARM PLIOCENE EPOCH

The Pliocene is the most recent interval of Earth's history that experienced atmospheric pCO2 above 400 ppmv (present-day level) and higher than average modern global temperatures (by 3-4°C). The configuration of the continents was also similar to the present-day; hence, the Pliocene constitutes a potential analogue for predicting future climate change. Until recently, it was assumed that global warming would lead to sluggish ocean overturning currents and stagnation. However, recent model and proxy results suggest that during warm Pliocene conditions, the North Pacific Ocean (NPO) was characterized by a robust Pacific Meridional Ocean Circulation (PMOC), North Pacific deep-water formation, and a more ventilated seafloor between ~1000 and 3500 m. A decrease in the strength of the PMOC is then inferred to have occurred during the late Pliocene at ~2.73 Ma. These conclusions have been drawn using V and U concentrations in deep-sea sediments, with these redox-sensitive elements typically increasing in sediments with reduced ventilation and lower deep-sea oxygen concentrations. Some redox-sensitive trace metals such as Mo are only responsive to strongly reducing conditions (e.g., the presence of free H₂S in an anoxic water column), however, and these conditions are unlikely to have existed in the Pliocene Pacific Ocean. We therefore propose to use an alternative proxy—nitrogen isotopes (δ¹⁵N)—which are more sensitive to slight variations in seafloor oxygenation. This study hypothesizes that a shift from oxic to weakly reducing conditions occurred in the deep North Pacific Ocean during the late Pliocene, and that this shift was characterized by a shift to intensified denitrification as ocean ventilation decreased and the oxygen minimum zone (OMZ) expanded. This increase in denitrification should be recorded to by a shift towards heavier δ¹⁵N values in the North Pacific IODP core samples we are investigating. Ultimately, our results will demonstrate the potential of nitrogen isotopes to trace the strength of the PMOC in the Pliocene and through the final stagnation of deep waters in the NPO. This, in turn, will provide new linkages between climate warming and ocean circulation/ventilation, which serves as an potential analogue for studies on modern climate warming.