Evaluating Tropical and Extratropical Influences on the Western Pacific Warm Pool Thermocline via 18O and Mg/Ca of Three Planktic Foraminifera (670 ka Recent)

The structure of the upper water column in the Western Pacific Warm Pool (WPWP) is intimately connected to the Pacific mean climate state. Because of this relationship, surface-to-thermocline temperature and geochemical gradients can be used to assess the upper oceans response in the WPWP to basin-scale climate changes. Here, we present new thermocline proxy data from International Ocean Discovery Program Site U1486 (1332 m, 222S, 14436E), which is located in the heart of the WPWP directly within the flow path of the New Guinea Coastal Undercurrent. These data are mostly comprised of three 670-kyr orbital-resolution 18O records for the planktic foraminifera Globigerinoides ruber (mixed-layer), Pulleniatina obliquiloculata (upper thermocline), and Globorotalia tumida (mid-thermocline). From this, we calculated 18O between each thermocline-dwelling species and G. ruber. Based on evaluation of modern vertical gradients of water temperature and salinity in this sector of the WPWP, it is apparent that changes in 18O are mostly temperature-driven. Late Pleistocene Mg/Ca-based temperature estimates for all three species at Site U1486 are used to further corroborate this assumption. A secular increase in 18O from ~250 ka to recent therefore likely corresponds to a steeper temperature gradient, and thus progressive shoaling of the thermocline toward present. Similar results from the tropical South Pacific suggesting thermocline shoaling over longer Pleistocene time intervals have been connected to greater South Pacific shallow overturning and/or enhanced penetration of extratropical intermediate waters. We also detect strong half-precession variability within the record, which is potentially related to insolation-forced variability within the inter-connected tropical-subtropical meridional shallow overturning circulation. These findings suggest that both tropical and extratropical processes play a role in regulating WPWP thermocline structure. Comparison between the record at Site U1486 and others throughout the WPWP is also likely key to deciphering the interplay between these processes on a regional scale.