Ventilation History of the Sulu Sea: Record of Abrupt Changes in the Tropical Western Pacific Thermocline

The potential impacts of rapid high-latitude ice sheet melting on sea level and oceanographic circulation in the coming several centuries are of increasing concern. There is evidence that the great ice sheets of the last glacial maximum did not melt at a uniform rate, but instead disintegrated most rapidly in one or two abrupt melting steps known as Meltwater Pulses 1A (MWP-1A; ~14,600-13,500 years BP) and 1B (MWP-1B; ~11,450 to 11,000 years BP). MWP-1A has been documented in several locations, but there remains controversy over whether deglacial MWP-1B existed and how much sea level rose across that time interval. Preliminary data from the Sulu Sea basin indicate that the subsurface ocean rapidly warmed and freshened during the several centuries of MWP-1B. These results suggest wide-spread oceanographic changes in the western Pacific but it is not known whether this change is related to sea level rise driven by melting ice or some other change in the ocean-climate system. This research will utilize the sediment archive in the Sulu Sea to further understand the origin of the ocean changes tied to the MWP-1B event and document whether similar events have occurred over the last 395,000 years, a period spanning three glacial-interglacial cycles. The product will be a new and unique reconstruction of thermocline conditions in Sulu Sea and sea level related ventilation history. Developing a better understanding of the ventilation history of the western tropical Pacific thermocline and the relationship to high-latitude ice sheet melting events will facilitate more accurate predictions about future changes. Broader impacts of the project include support for a Ph.D. student, incorporation of results into teaching, and continued practice of public outreach activities.

This project will reconstruct the timing of thermocline and deep basin ventilation events in the Sulu Sea in the western tropical Pacific using geochemical measurements of deep dwelling planktonic and benthic foraminifera. Earlier geochemical work on shallow dwelling foraminifera in the Sulu Sea has provided detailed records of millennial-scale surface ocean change over the last 395,000 years. Preliminary oxygen isotope data from the thermocline dwelling foraminifer Globorotalia tumida across the MWP-1B interval in a Sulu Sea sediment core indicates a larger decrease in delta-18O than in the surface dwelling species previously measured. This unexpected result raises the following questions: (1) is this evidence of glacial meltwater influx into the tropical Pacific thermocline or an Asian monsoon related signal at the same time as MWP-1B? (2) if this is an ice sheet meltwater signal, why is the response larger in the thermocline than at the surface? (3) is this change in thermocline properties at the time of MWP-1B related to pulses of rapid sea level rise or to changes in the western boundary current advection of surface and thermocline waters into the western Pacific? To address these and other questions this project will analyze delta-18O on G. tumida (calcifies ~160-200 m) at high resolution in samples from Sulu Sea sediment core MD97-2141 spanning the Holocene to Marine Isotope Stage 6 (~130,000 years BP), and across glacial-interglacial transitions back to ~395,000 years BP. These data will be paired with Mg/Ca measurements on selected intervals to isolate the water temperature contribution to measured changes in foraminifera delta-18O. The project will also utilize analyses of the planktonic foraminifera Globorotalia crassaformis that calcifies at ~700m and the benthic foraminifera Oridorsalis umbonatus to assess the timing of deeper ventilation events in the Sulu Sea.