The Potential of U-233/U-236 as a Water Mass Tracer in the Arctic Ocean

This study explores for the first time the possibilities that the U-233/U-236 atom ratio offers to distinguish waters of Atlantic or Pacific origin in the Arctic Ocean. Atlantic waters entering the Arctic Ocean often carry an isotopic signature dominantly originating from European reprocessing facilities with some smaller contribution from global fallout nuclides, whereas northern Pacific waters are labeled with nuclides released during the atmospheric nuclear testing period only. In the Arctic Ocean, U-233 originates from global fallout while U-236 carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the U-233/U-236 ratio provides a tool to identify water masses with distinct U sources. In this work, U-233 and U-236 were analyzed in samples from the GN01 GEOTRACES expedition to the western Arctic Ocean in 2015. The study of depth profiles and surface seawater samples shows that: (a) Pacific and Atlantic waters show enhanced signals of both radionuclides, which can be unraveled based on their U-233/U-236 signature; and (b) Deep and Bottom Waters show extremely low U-233 and U-236 concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high U-233/U-236 ratios are interpreted as a relative increase of naturally occurring U-233 and U-236 and thus for gradually reaching natural U-233/U-236 levels in the deep Arctic Ocean. Our results set the basis for future studies using the U-233/U-236 ratio to distinguish anthropogenic and pre-anthropogenic U in the Arctic Ocean and beyond.Funding Agencies|ETH Zurich Research GrantETH Zurich [ETH-06 16-1]; Swiss National Science FoundationSwiss National Science Foundation (SNSF)European Commission [PRIMA SNF PR00P2_193091]; Spanish Government (Ministerio de Ciencia, Innovacion y Universidades)Spanish Government [PGC2018-094546-B-I00]