Title: Pleistocene Nutrient, Thermocline, and Bottom Current Dynamics in the South Pacific Sector of the Western Pacific Warm Pool
Description: Located in the far western equatorial Pacific, the Western Pacific Warm Pool (WPWP) is a greater than 10 million km² area of the warmest water on the planet. The WPWP therefore facilitates intense atmospheric convection and participates in coupled ocean-atmosphere climate phenomena such as El Niño Southern Oscillation, regional monsoons, and the shifting Intertropical Convergence Zone. The WPWP is also a water mass crossroads where thermocline-depth western boundary currents (WBCs) such as the New Guinea Coastal Undercurrent (NGCUC) facilitate the transfer of mass, heat, and nutrients vertically, zonally, and meridionally in the ocean. In this dissertation I focus mostly on reconstructing WPWP upper ocean temperature, salinity, nutrient, and productivity dynamics via a suite of physical and geochemical paleoclimate proxies. I apply these proxies in bulk sediments and planktic foraminifera from International Ocean Discovery Program (IODP) Site U1486 over the Pleistocene (2580 ka to 11.7 ka) and Holocene (11.7 ka to present).

Site U1486 is located at 2°22’S, 144°36’E in the Bismarck Sea north of New Guinea in the southern WPWP, and is ideally situated to track changes to the WPWP upper water column forced by the South Pacific. The presence of glacial-interglacial (G-IG) variability within WPWP records is particularly important for determining local versus high-latitude climatic influences on the WPWP – with climate shifts such as the mid-Pleistocene Transition (MPT; ~1250 – 700 ka) and mid-Brunhes Event (MBE; ~430 ka) of particular interest in the long-term records I present. In chapter 1, I explore the paleoceanography of the low-latitude Pacific via upper ocean nitrate dynamics. I present a new bulk sediment ẟ¹⁵N record from Site U1486 that spans from 1420 to 0.67 ka – over a million years longer than any nearby records. Via analysis of orbital variability and secular trends at Site U1486 and in records directly along the equator in the Pacific, I find that nitrate dynamics were largely unrelated in the two regions in the Middle and Late Pleistocene.

Whereas ẟ¹⁵N at Site U1486 is in line with patterns of eastern Pacific denitrification, increasing ẟ¹⁵N after the MPT at sites located directly along the equator appears linked to increasing Southern Ocean nitrate utilization. Enhanced nitrate utilization is an indicator of a strengthened biological pump – a major contributor to the reduction of atmospheric 𝑝CO₂ during the last glacial. A post-MPT increase in nitrate utilization may therefore point to the Southern Ocean biological pump as a driver for the deeper and longer glacial periods of the 100-kyr world after the MPT. In Chapter 2, I investigate changes in the vertical temperature and salinity structure of the southern sector of the WPWP in relation to the upper ocean’s response to climate change. When combined with Mg/Ca paleotemperatures and δ¹⁸O_sw, my 670-kyr record of Δẟ¹⁸O between the surface-dwelling foraminifera Globigerinoides ruber (sensu stricto) and the thermocline-dwelling foraminifera Pulleniatina obliquiloculata and Globorotalia tumida suggests enhanced thermocline shoaling and a progressively increasing vertical salinity gradient commencing near 240 ka.

This secular change in upper water column dynamics does not appear to be associated with previously documented changes in G-IG variability such as the MPT or MBE. Via comparison to other records, I identify widespread cooling of the thermocline in the equatorial Pacific after ~240 ka. After combining these reconstructions with ²³⁰Th-derived focusing factors I validate previous model results indicating obliquity-driven strengthening of low-latitude ocean currents and extend this to imply the periodic increased transport of high-salinity thermocline water masses. These results strengthen previous evidence that the structure of the WPWP thermocline is relatively independent from the drivers of climate at the surface and support that variability in WPWP thermocline circulation is substantially influenced by obliquity. Because of the nitrate dynamics in the Bismarck Sea, bulk sediment ẟ¹⁵N cannot be used to reconstruct productivity. However, chapter 3 constrains variability in productivity via the analysis of new ²³⁰Th-normalized records of preserved biogenic flux and its components at Site U1486 over the last 138 kyr.

Here, I assess the drivers of variability in paleo-productivity by reconstructing paleo-stratification, as in the modern Bismarck Sea productivity is stimulated by the delivery of nutrients to the surface during increased upwelling (reduced stratification). Paleo-stratification is approximated by calculating upper ocean density gradients between the calcification depths of G. ruber, P. obliquiloculata, and G. tumida using Mg/Ca temperatures and δ¹⁸O_sw-estimated salinity. Decreased paleo-stratification (a reduced vertical density gradient) was associated with increased productivity and is generally in phase with maximum orbital precession. Paleo-productivity therefore appears to respond to monsoonal increases in coastal upwelling when the Intertropical Convergence Zone (ITCZ) was at its southernmost extent. This illustrates that the unique and more direct method of constraining stratification presented here, which is subject to greater uncertainty, yields results consistent with our current understanding of upper ocean dynamics.

I also identify a period between 100 and 60 ka during a potential reorganization of the upper water column in which variability in productivity occurs at a higher frequency than that of precession. Finally, while also related to ITCZ shifts, a nearby record closer to the equator is phase-lagged from Site U1486 – emphasizing the fine-scale regional differences in the drivers of primary productivity in the WPWP.