The 1.4-Myr evolution of low- and high-latitude South Pacific influence on the Western Pacific Warm Pool as recorded via an amplified sediment δ15N record

Abstract:

The Western Pacific Warm Pool (WPWP) is a region of exceptionally high (>28°C) sea surface temperature (SST) in the equatorial Pacific. The WPWP greatly influences regional and global hydroclimate and is an oceanic crossroads that connects the broader Pacific to the Indonesian Throughflow (ITF) and Equatorial Undercurrent (EUC). A key question exists as to the relative influence of low- versus high-latitude processes on WPWP SST and nutrient dynamics, especially on orbital and longer time-scales. The New Guinea Coastal Undercurrent (NGCUC) is the primary oceanic conduit between the low- and high-latitude South Pacific and the broader WPWP, and is thus important for regulating conditions in the EUC (and to a lesser extent the ITF). We have measured bulk sediment δ¹⁵N at International Ocean Discovery Program Site U1486 (within the NGCUC ~160 km north of Papua New Guinea) to evaluate orbital variability and long-term trends in WPWP upper ocean conditions over the Pleistocene. At this site, δ¹⁵N increases from a mean of ~5‰ at 1.4 Ma to ~11‰ near present, with dominant precession-scale variability of 3-4‰ superimposed on this trend. At Ocean Drilling Program Site 806B (located in the EUC flowpath on the Ontong Java Plateau), δ¹⁵N also shows a secular increase, but U1486 displays higher amplitude variability, a greater overall increase, and a lack of obliquity forcing. Similar δ¹⁵N trends have been observed globally and are attributed to increasing denitrification within oxygen minimum zones (OMZs). The long-term trends at both U1486 and 806B suggest a consistent Pleistocene connection between the WPWP and the Eastern Equatorial Pacific OMZ, yet with important regional differences driven by orbital variability. Precessional forcing at U1486 indicates sensitivity of the NGCUC to variability in low-latitude processes such as the monsoon and/or subtropical-tropical cell circulation. Obliquity forcing at 806B suggests that conditions near the site may be sensitive to high-latitude processes affecting the properties of Subantarctic Mode Water, which is upwelled from the EUC along the equator.