Changes in vertical geochemical gradients within the Western Pacific Warm Pool (WPWP) are important indicators of the region's upper ocean response to climate changes and to the oceanographic coupling between the thermocline and surface mixed layer. Here, we reconstruct temperature and delta O-18(sw) at International Ocean Discovery Program Site U1486 (1332 m water depth, 2.22' S, 144. 36' E) located in the Bismarck Sea in the southern sector of the WPWP. A 670-kyr record of Delta delta O-18 between the surface-dwelling foraminifera Globigerinoides ruber sensu stricto and the thermocline-dwelling foraminifera Pulleniatina obliquiloculata and Globorotalia tumida (when combined with Mg/Ca-based temperature and d18Osw estimates) suggests long-term thermocline shoaling and a progressively increasing vertical salinity gradient commencing near 240 ka. Through a detailed comparison to other Pacific records, it becomes clear this is not solely a local phenomenon, as we identify widespread cooling of the thermocline in the low-latitude Pacific after similar to 240 ka. After examining our temperature reconstructions alongside new delta O-18(sw) and constant flux proxy-derived focusing factor records, we potentially also validate previous models which find obliquity-induced strengthening of low-latitude Pacific currents. We extend this to support periods of increased transport of high-salinity thermocline water masses through the South Pacific low-latitude western boundary current system. These results indicate greater variability in thermocline circulation given amplified obliquity and strengthen previous evidence that variability in the WPWP thermocline is independent from the drivers of WPWP surface variability.

In much of the equatorial Pacific, surface nitrate is consistently available due to upwelling of sub-Antarctic Mode Water-derived nitrate from the thermocline. However, in the low-latitude western boundary current (WBC) region, surface nitrate is limited due to reduced upwelling and less nitrate-replete thermocline water masses. In order to explore South Pacific WBC paleoceanography via upper ocean nitrate dynamics, we present a new bulk sediment δ15N record from within this WBC region at International Ocean Discovery Program Site U1486 (2°22′S, 144°36′E) in the Bismarck Sea north of New Guinea. This record spans from 1420 ka to recent – surpassing nearby sediment δ15N reconstructions by over a million years and allowing for direct comparison to low-latitude Pacific sediment core δ15N records of similar length. Core-top and down-core bulk sediment δ15N, TOC/TN and δ13Corg values indicate minimal or no terrestrial influence on the organic fraction at Site U1486, which is consistent with previous studies of modern processes in the region. After comparison, differences in orbital variability and secular trends throughout the Middle and Late Pleistocene suggest that nitrate dynamics along the equator and in the WBCs were relatively disparate. We observe that δ15N at Site U1486 is consistent with patterns of eastern Pacific denitrification, while we suggest increasing δ15N after the mid-Pleistocene Transition (MPT; ∼1250–700 ka) at Sites 806 (0°19′N, 110°30′W) and 849 (0°19′N, 160°E) is linked to increasing Southern Ocean nitrate utilization. Enhanced nitrate utilization is a key indicator of the strengthened biological pump that contributed to a reduction in atmospheric pCO2 during the last glacial. Therefore, a post-MPT increase in nitrate utilization may bolster the role of the Southern Ocean biological pump in driving the deeper and longer glacial periods of the 100-kyr world.