Title: Investigating Climate Variability over the Last Four Glacial Cycles using Surface and Thermocline Dwelling Foraminifera from the Sulu Sea in the Far Western Pacific
Description: The geographic location of the Sulu Sea in the far western equatorial Pacific results in the basin’s oceanographic sensitivity to: 1.) the East Asian Monsoon strength, 2.) the Kuroshio Current and open western Pacific conditions, 3.) Indonesian Strait geometry, 4.) past sea level, and 5.) the Indonesian Throughflow (ITF). Due to the sea’s unique bathymetry as a deep basin (~5,000 m maximum depth) surrounded by shallow sills with maximum depths of 440 m, Sulu Sea bottom water is relatively warm with minimum temperature of ~10°C, low dissolved oxygen, high carbonate concentrations, excellent CaCO3 preservation, and low bioturbation which combine to produce a remarkable sedimentological record. Sulu Sea sediments have previously been utilized to generate high-resolution paleoclimate records of past variability of surface ocean conditions using the planktonic foraminifera Globigerinoides ruber. However, past changes in the Sulu Sea subsurface and thermocline have been largely unexplored until now.

Here I will show that reconstructing subsurface upper ocean conditions in the Sulu Sea reveals a rich archive of paleoclimatic information relating to past upper ocean dynamics. In this dissertation I seek to understand Indo-Pacific and global glacial-interglacial variability over the last four glacial cycles by generating foraminiferal δ18O, Mg/Ca-based temperature, and seawater δ18O (δ18Ow) records using the thermocline calcifying foraminifera Globorotalia tumida from high-resolution sediment core MD97-2141 from the Sulu Sea. Additionally, I extend existing G. ruber Mg/Ca-based temperature and δ18Ow paleoclimatic results to now span intervals of the last four glacial cycles. Sulu Sea thermocline reconstructions monitor regional paleoclimate on their own while also providing insight into surface variability when the surface and thermocline depth paleoclimatic records are compared.In Chapter 1 of this dissertation, I use Sulu Sea G. tumida reconstructions to investigate the North Equatorial Current (NEC) bifurcation latitude.

After flowing east to west across the Equatorial Pacific, the NEC bifurcates into the northward flowing Kuroshio Current and the southward flowing Mindanao Current. The latitude of bifurcation migrates seasonally and interannually, controlling the partitioning of water between the two currents and the transfer of energy to the North Pacific. Using modern instrumental data, I show that by controlling the strength of the Kuroshio Current, the NEC bifurcation latitude also controls the leakage of relatively salty western Pacific water into the South China and Sulu Sea thermoclines, thus playing a major role in the salinity of the thermocline in both basins. I then use Sulu Sea thermocline δ18Ow generated using the foraminifera G. tumida from core MD97-2141 to show that the NEC bifurcation latitude was north during Heinrich Stadial I (17.5-15 ka), the Younger Dryas Chronozone (12.9-11.6 ka), and from ~9.5-8.5 ka and south during the Bølling-Allerød and early Holocene. Understanding the calcification depth of foraminifera is crucial for interpreting paleoclimate records.

In Chapter 2 of this dissertation, I present δ18O records from six foraminiferal species and size fraction combinations from core MD97-2141 spanning ~12.75-10.75 ka. I calculate the δ18O difference between those records and previously published G. ruber data from the same core and use modern instrumental temperature and salinity data from the Sulu Sea to estimate calcification depths for each species and size fraction combination. I estimate that Sulu Sea G. tumida in the 400-600 μm size fraction calcify at a mean depth of 115m with the middle 95% of foraminifera geochemically analyzed yielding calcification depths between 99-131 m. G. tumida with no size constraint and in the >600 μm size fraction calcify at mean depths of 112 m and 107 m with the middle 95% of individual samples at 99-120 m and 88-117 m respectively. Globorotalia menardii was found to calcify at a mean depth of 106 m and the middle 95% of samples between 92-117 m. Neogloboquadrina dutertrei calcifies at a mean depth of 112 m (middle 95% between 102-120 m).

Lastly, Pulleniatina obliquiloculata calcifies at a mean depth of 93 m (middle 95% between 68-108 m). Estimated calcification depth ranges for the all six species and size fraction combinations overlap. My results suggest that larger G. tumida calcify shallower in the Sulu Sea. I also show evidence that calcification depths in the Sulu Sea did not change for any of these species through time. Chapter 3 focuses on Marine Isotope Stage 3 (MIS3) (60-26 ka). I present results which demonstrate that the opening and closing of the ~36 m deep Karimata Strait at the southern terminus of the South China Sea with rising and falling sea level plays a substantial role in Sulu Sea surface salinity. I generated surface and thermocline δ18Ow records spanning ~125-20 ka in order to determine when the Karimata Strait was open and closed. I then use the depth history of the Karimata Strait to constrain maximum sea level during MIS 3 to -22±6 m relative to modern sea level and minimum possible sea level during MIS 5a and 5c (117-72 ka) to -12±7 m relative to modern.

My record is the first to unequivocally demonstrate the Karimata Strait was subaerial during MIS 3 and suggests it could have facilitated the first human migration to the island of Borneo which occurred during that time. During the Younger Dryas paleoclimatic event (~12.9-11.6 ka), there was a return to near glacial conditions during the transition from the Last Glacial Maximum to the early Holocene (Termination I) (~18-11 ka). This was the most significant climate event during Termination I, but it is still unknown if it was a one-off event or if such millennial-scale events are intrinsic to glacial terminations. In Chapter 4 of this dissertation, I present Sulu Sea thermocline δ18O and Mg/Ca-based temperature records and a surface ocean Mg/Ca record to investigate millennial-scale events during Terminations II (~135-125 ka), III (~252-240 ka), and IV (~341-330 ka) in order to determine if the Termination II-IV events in the Sulu Sea demonstrate the same structure as the event during the Younger Dryas Chronozone.

My results show that the millennial-scale event in the Sulu Sea thermocline during Termination III was most similar to the Younger Dryas Chronozone event, while the Termination II and IV events were largely similar to the Younger Dryas Chronozone with some differences. My reconstructions of these events are very consistent with the Termination III event in the Sulu Sea and somewhat consistent with the Termination II and IV events in the Sulu Sea being driven by the same mechanism as the Sulu Sea Younger Dryas Chronozone event. As a whole, my dissertation takes a unique approach to characterizing millennial-scale climatic events by combining surface and thermocline foraminiferal δ18O and Mg/Ca reconstructions and attributing mechanistic drivers to each. In doing so, I am able to draw the most possible information from a core. Such an approach that utilizes both the surface and the subsurface of the water column also allows interpretation of the differences between the surface and thermocline and these differences can improve our interpretations of previously existing surface records.

Furthermore, my dissertation develops a new approach to constraining sea level using foraminiferal δ18O results by tying them to physical structures such as the Karimata Strait that have a direct relationship to sea level. The conclusions I reach in my dissertation shed light on North Equatorial Current bifurcation latitude variability since the Last Glacial Maximum, furthering our understanding of the controls of Indo-Pacific and western Pacific climate variability and the Indonesian Throughflow. They also improve our understanding of foraminiferal calcification depth in the Sulu Sea and through time, help constrain sea level during Marine Isotope Stages 3 and 5, and help develop our understating of millennial scale events during glacial terminations.