A new paradigm relating global climate and North Pacific Intermediate Water circulation

Abstract:

North Pacific Intermediate Water (NPIW) is the primary water mass associated with Pacific meridional overturning circulation, yet little is known about how NPIW prominence and formation are linked to climate change. Models have predicted that the closure of the Bering Strait, during past glacials when relative sea level dropped at least 50 m, may have had a significant impact on NPIW and the climate of the North Pacific. However, until now, proxy records of NPIW were limited to just the last glacial cycle and were insufficient to evaluate the links between NPIW, climate, and sea level. Here, we use new, long cores from Integrated Ocean Drilling Program Expedition 323 Site U1342 (818 m water depth) from the Bering Sea to reconstruct the first continuous records of NPIW over the past 35 Marine Isotope Stages (1.2 myrs). Records of benthic foraminiferal δ¹³C and δ¹⁸O from U1342 indicate that increased brine formation and NPIW ventilation occurred during extreme glacials when flow through the Bering Strait was cut off. During glacial climates in which the Bering Strait was still open, the region was as weakly ventilated as during interglacials. Additionally, cross-spectral analyses show that variations in NPIW are coherent and in-phase with variations in Upper North Atlantic Deep Water (UNADW), and are unrelated to changes in lower NADW. These results contradict many previous modeling studies that predict weaker NPIW influence with Bering Strait closure and an out-of-phase (see-saw) relationship between NPIW and NADW. We propose that NPIW formation was enhanced when the Bering Strait was closed due to changes in the position of the Aleutian Low that were conducive to increased sea ice formation and brine formation within the Bering Sea. These results offer a new paradigm relating global climate and North Pacific Ocean circulation and provide data to ground-truth model simulations of NPIW.