Lisa Gerlinde Thekla Leist ^1* M Castrillejo ² J N. Smith ³ M Christl ⁴ C Vockenhuber ⁴ Nuria Casacuberta ¹

• ¹ Department of Environmental System Science, ETH Zürich, Zurich, Switzerland
• ² Université de Lausanne, Lausanne, Vaud, Switzerland
• ³ Bedford Institute of Oceanography (BIO), Dartmouth, Nova Scotia, Canada
• ⁴ Laboratory of Ion Beam Physics, Department of Physics, ETH Zurich, Zürich, Zürich, Switzerland

The Subpolar North Atlantic (SPNA) is crucial in the global ocean circulation system and one of the few regions where deep convection occurs. The intermediate and deep waters formed in the SPNA have long been investigated, yet their sources and pathways are not fully understood.In this study we employ a combination of two radionuclide tracers, namely 129 I and 236 U, to understand water mass provenance and mixing in the SPNA. The concentrations measured between Portugal and Greenland and across the Labrador Sea in 2020/21 agreed with previously observed tracer distributions. The highest tracer concentrations were measured in the East Greenland Current (EGC), Denmark Strait Overflow Waters (DSOW), and to a lesser extent, in the eastward-flowing Labrador Sea Water (LSW). In contrast, waters of southern origin such as the North East Antarctic Bottom Water and North East Atlantic Central Water (ENACW) carried comparably smaller amounts of 129 I. By using a binary mixing model we estimated that the EGC contains about 29-32% of the Polar Surface Waters outflowing the Fram Strait. DSOW was mainly derived from 20-35% Return Atlantic Water and mixed with LSW. The Iceland Scotland Overflow Water (ISOW) evolved into North East Atlantic Deep Water in the Irminger and Labrador seas primarily by mixing with LSW and to lesser extent with DSOW. The 129 I and 236 U binary mixing approach was less conclusive for LSW, reaching the current limitation of the model. This study suggests potential benefits and limitations of using 129 I and 236 U to investigate the mixing and provenance of water masses in the SPNA.