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

This study explores for the first time the possibilities that the 233U/236U 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, 233U originates from global fallout while 236U carries both, a global fallout and a prominent nuclear reprocessing signal. Thus, the 233U/236U ratio provides a tool to identify water masses with distinct U sources. In this work, 233U and 236U 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 233U/236U signature; and (b) Deep and Bottom Waters show extremely low 233U and 236U concentrations close to or below analytical detection limits with isotopic ratios distinct from known anthropogenic U sources. The comparably high 233U/236U ratios are interpreted as a relative increase of naturally occurring 233U and 236U and thus for gradually reaching natural 233U/236U levels in the deep Arctic Ocean. Our results set the basis for future studies using the 233U/236U ratio to distinguish anthropogenic and pre‐anthropogenic U in the Arctic Ocean and beyond. Plain Language Summary: We study the presence of 233U and 236U in seawater samples taken in the Arctic Ocean from the GN01 GEOTRACES expedition in 2015. Both long‐lived manmade U isotopes were introduced worldwide during the open‐air testing of nuclear weapons (mainly in the 1960s). 236U is also linked to the civil uses of nuclear energy, particularly to the liquid effluents from Sellafield (United Kingdom) and La Hague (France) reprocessing plants (starting in the 1950s). Atlantic Waters flowing into the Arctic Ocean carry both the bomb‐tests and the reprocessing plants signals. Pacific Waters entering the Arctic Ocean carry the bomb‐tests signal only. Thus, they show different 233U and 236U compositions. We demonstrate that the 233U/236U ratio and the 236U composition can be used to identify water masses of Pacific and Atlantic origin in the upper waters of the western Arctic Ocean. Deep and Bottom Waters, where an anthropogenic influence is not expected, exhibit extremely low 233U and 236U concentrations but 233U/236U ratios above the ones observed in the upper Arctic Ocean. These features can be explained by the presence of naturally produced uranium isotopes of lithogenic origin. This study demonstrates the potential of the 233U/236U ratio to distinguish anthropogenic and natural U in the Arctic Ocean and beyond. Key Points: The 233U/236U ratio and the 236U composition unravel Pacific and Atlantic waters in Polar Surface Waters of the western Arctic OceanThe Atlantic Layer exhibit distinct 233U and 236U signals from Polar Surface WatersDeep and Bottom Waters show extremely low 233U and 236U concentrations with isotopic ratios distinct from known anthropogenic U sources [ABSTRACT FROM AUTHOR]