Deep ocean circulation and uranium-series radionuclides in Arctic Ocean sediments

The Arctic Ocean remains one of the least explored realms on Earth, despite the increasing awareness of its importance and ongoing changes. This project is designed to increase our understanding of the cycling of relatively insoluble elements in seawater through the Arctic and either into the underlying deep-sea sediments or south through the Fram Strait into the Nordic Seas and possibly beyond. The element uranium is easily dissolved in the ocean, and decays predictably to thorium, which is essentially insoluble, and protactinium, which is only slightly soluble. This contrasting behavior of highly soluble uranium and its less soluble long-lived radioactive decay “daughters” provides a wide array of insights into global ocean processes including biological productivity, atmospheric dust deposition and the movement of deep currents, yet has been far less utilized in the Arctic. In this study, an array of deep-sea sediment cores from within and around the Arctic Ocean will be analyzed for evidence of the transport and burial of thorium and protactinium, with a particular focus on the variability of deep ocean currents through time. The project will provide research opportunities and student training at multiple levels. In partnership with a team of science educators, the researchers will also host a workshop for middle and high school science teachers to increase public awareness of the Arctic. Deep-sea sediment cores from Lomonosov Ridge, Northwind Ridge, Canada Basin, Nansen Basin and the Fram Strait, along with selected locations around the periphery of the Arctic Ocean will be analyzed for uranium, thorium and protactinium isotopes using bulk sediment digestion and inductively coupled plasma mass spectrometry (ICP-MS). The data will be examined to assess the inventories and Arctic budget of the radionuclides, and to explore evidence of changes in the preferential lateral transport of protactinium out of the basin through the Fram Strait by deep ocean currents since the last ice age. These results will be used to test the hypotheses that the seawater production and deep-sea sedimentary burial of thorium are in balance in the Arctic, as in the global ocean, and that there has been a persistent deficit in the burial of protactinium due to its transport out of the Arctic by deep ocean currents. 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.