This was a project designed to develop high quality datasets from a suite of deep-sea sediment cores from the equatorial and North Pacific to investigate the sedimentary ²³¹Pa budget and oceanographic change during key climate periods in the past. These data were then placed within the context of regional and global climate variability to help assess the role of the Pacific Ocean in past climate change, including variations on sediment deposition, biological productivity, near-sea-surface hydrographic changes, ocean circulation and carbon storage in the deep ocean.

 The sediments of the study cores were analyzed for bulk sediment composition, and to determine the naturally occurring abundance of isotopes of several elements, including uranium, thorium and protactinium, in the sediments. We also analyzed the ratio of stable isotopes in the microfossil shells of single-celled organisms that lived at or near the sea surface. Much of the research was accomplished by junior scientists, including two PhD students, a Master’s student, and a number of undergraduates who all acquired skills and experience that can contribute to their academic and professional development.

Our findings include evidence that much of the equatorial and north Pacific Ocean was less biologically productive during the last ice age, ruling out any substantial contribution of productivity in this region to the lowering of the concentration of carbon dioxide in the atmosphere at that time. In contrast, our radiochemical evidence points to a strong inverse connection between deep ocean oxygenation and atmospheric carbon dioxide concentrations throughout the entirety of the last glacial cycle, indicating that the deep ocean sequestered carbon from the atmosphere during ice ages and released it during past warm interglacial intervals. We also found evidence for enhanced accumulation of calcium carbonate in the glacial north Pacific, suggesting a change in deep ocean chemistry at that time. Dust deposition was generally greater across the entire region during the last glaciation, and spikes of enhanced deposition near the equator during the last two deglaciations indicate a substantial southward migration of the mean annual position of the inter-tropical convergence zone (ITCZ) at those times of abrupt climate change. Our study also revealed a persistent pattern of shoaling and deepening of the thermocline in the eastern equatorial Pacific (EEP) on precessional timescales, with a timing that is consistent with the influence of warming across the tropics during late summer and early fall. The shallowest thermocline in the EEP was repeatedly associated with enhanced upwelling and productivity, along with diminished variability in the El Niño-Southern Oscillation (ENSO) phenomenon and more frequent / persistent La Niña conditions.

 This set of findings formed the bulk of a thesis for one PhD student and contributed to the research of a second PhD student, both supported by the project. A master’s student and several undergraduate students were given research experience opportunities, and thus developed skills that helped them in their career development. Five of the undergraduates who participated in the project made it part of their senior thesis work, and three of the students, all women, have now begun PhD studies at other institutions. Many of the project participants participated in the annual Open House at the Lamont-Doherty Earth Observatory, and the PI helped lead a project-related workshop for high school science teachers in the New York area.

Last Modified: 01/01/2024
Modified by: Jerry F Mcmanus