Reconstructing the paleo-budget of 231Protactinium in the North Pacific

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Naturally occurring isotopes produced in seawater and delivered to deep-sea sediments by settling particles in the open ocean serve as powerful indicators of changes in physical, chemical and biological processes through time. The recent burial of 231Protactinium (Pa) in sediments of the North Pacific Ocean is observed to be in near balance with its production in the overlying water column. In contrast, the few existing longer sedimentary records suggest that significantly less 231Pa was buried than produced during past ice ages, raising the possibility of changes in biological productivity, ocean currents, or ocean chemistry. This project will produce high quality datasets from past time intervals to investigate changes in the sedimentary 231Pa budget and explore these oceanographic possibilities, using an array of deep-sea sediment cores from across the North Pacific. The resulting data will be used to test hypotheses regarding the distribution and burial of 231Pa in the ocean and to provide insights into past changes in biological productivity, chemical oceanography, and ocean circulation. The project will promote training and learning for students at multiple different educational levels. A graduate student will play a central role in the research, gaining technical training and professional development.Sedimentary 231Pa/230Th is a widely utilized proxy that can provide valuable insight into circulation, surface productivity and hydrothermal activity in the modern and past ocean. Although the modern 231Pa budget in the North Pacific appears to be in balance, almost twice as much 231Pa is buried along the basin margins than in the open ocean today. The few recently generated records from the Northeast Pacific suggest that this major sink along the margins may not have been in effect during the last ice age. In fact, 231Pa burial in the Northeast Pacific appears to vary in tandem with changing climate conditions, with excess burial only occurring during interglacial periods. This project will provide a systematic, quantitative assessment of how the North Pacific 231Pa budget has evolved over the last glacial cycle. The study will generate 231Pa/230Th time series and productivity records from each of the major biogeographic provinces in the Pacific, including the open ocean gyre and surrounding continental margins, to assess how the biogeochemical cycles of the Pacific Ocean have evolved over time. These data will determine whether and where the apparently missing marginal 231Pa was buried during the last glacial period, evaluate the mechanisms that drove this shift, and assess how the spatial distribution of 231Pa burial may have evolved over time. This will allow the testing of hypotheses regarding potential changes in biological productivity and deep ocean circulation in the North Pacific Ocean.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.