Shelf-Basin Interactions and Water Mass Residence Times in the Western Arctic Ocean: Insights Provided by Radium Isotopes

Radium isotopes are produced through the decay of thorium in sediments and are soluble in seawater; thus, they are useful for tracing ocean boundary-derived inputs to the ocean. Here we apply radium isotopes to study continental inputs and water residence times in the Arctic Ocean, where land-ocean interactions are currently changing in response to rising air and sea temperatures. We present the distributions of radium isotopes measured on the 2015 U.S. GEOTRACES transect in the Western Arctic Ocean and combine this data set with historical radium observations in the Chukchi Sea and Canada Basin. The highest activities of radium-228 were observed in the Transpolar Drift and the Chukchi shelfbreak jet, signaling that these currents are heavily influenced by interactions with shelf sediments. The ventilation of the halocline with respect to inputs from the Chukchi shelf occurs on time scales of 19-23years. Intermediate water ventilation time scales for the Makarov and Canada Basins were determined to be similar to 20 and >30years, respectively, while deep water residence times in these basins were on the order of centuries. The radium distributions and residence times described in this study serve as a baseline for future studies investigating the impacts of climate change on the Arctic Ocean. Plain Language Summary The rapid rate of climate change in the Arctic is impacting the exchange of elements between the continental shelf and the Arctic Ocean. Radium isotopes are naturally produced in sediments and are added to the ocean when water comes into contact with the seafloor, so they are useful for identifying shelf- or sediment-influenced water parcels in the open ocean. Once removed from their sediment source, these radioactive isotopes decay at known rates and can therefore be used to determine the time elapsed since water was in contact with the shelf, or to estimate the time a water parcel spends in a certain region of the ocean. In this study, we use the levels of radium measured in the Western Arctic Ocean to determine how rapidly water moves from the surrounding shelf seas into the open Arctic Ocean, and to estimate how long water resides in the deepest parts of the Western Arctic Ocean. These time scales provide important baseline estimates that can be used in future studies to assess how climate change is affecting sediment-ocean interactions in the Arctic.