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Abstract The physical and biogeochemical properties of the western Arctic Ocean are rapidly changing, resulting in cascading shifts to the local ecosystems. The nutrient‐rich Pacific water inflow to the Arctic through the Bering Strait is modified on the Chukchi and East Siberian shelves by brine rejection during sea ice formation, resulting in a strong halocline (called the Upper Halocline Layer (UHL)) that separates the cold and relatively fresh surface layer from the warmer and more saline (and nutrient‐poor) Atlantic‐derived water below. Biogeochemical signals entrained into the UHL result from Pacific Waters modified by sediment and river influence on the shelf. In this synthesis, we bring together data from the 2015 Arctic U.S. GEOTRACES program to implement a multi‐tracer (dissolved and particulate trace elements, radioactive and stable isotopes, macronutrients, and dissolved gas/atmospheric tracers) approach to assess the relative influence of shelf sediments, rivers, and Pacific seawater contribution to the Amerasian Arctic halocline. For each element, we characterized their behavior as mixing dominated (e.g., dCu, dGa), shelf‐influenced (e.g., dFe, dZn), or a combination of both (e.g., dBa, dNi). Leveraging this framework, we assessed sources and sinks contributing to elemental distributions: shelf sediments (e.g., dFe, dZn, dCd, dHg), riverine sources, (e.g., dCu, dBa, dissolved organic carbon), and scavenging by particles originating on the shelf (e.g., dFe, dMn, dV, etc.). Additionally, synthesized results from isotopic and atmospheric tracers yielded tracer age estimates for the Upper Halocline ranging between 1 and 2 decades on a spatial gradient consistent with cyclonic circulation. 

Radium is a useful tracer of sediment‐derived materials, improving our understanding of the geochemical cycling of elements at ocean boundaries. We have developed an autonomous in situ sampler to collect time series samples of radium isotopes on mooring deployments. Samplers were deployed for 2 yr in the Arctic Ocean, a region particularly hard to access outside of the summer season, and collected monthly samples to create the first annual time series of radium‐228 and radium‐226 in the Arctic. Results from the Laptev Slope show increased radium‐228 and radium‐228/radium‐226 ratios in spring/summer, concomitant with increased meteoric water and brine influence. Together, these tracers indicate seasonal periods of increased influence of shelf‐ and river‐derived materials, findings which would not be possible to discern from summertime shipboard surveys alone. The development of this in situ sampler has therefore expanded our capability to use radium as a tracer to discern temporal changes in the geochemistry of remote areas of the ocean. 

Abstract. The Siberian Arctic Ocean (SAO) is the largest integrator and redistributor of Siberian freshwater resources and acts to significantly influence the Arctic climate system. Moreover, the SAO is experiencing some of the most notable climate changes in the Arctic, and advection of anomalous Atlantic- (atlantification) and Pacific-origin (pacification) inflow waters and biota continue to play a major role in reshaping the SAO in recent decades. In this study, we use a large collection of mooring data to create a coherent picture of the spatiotemporal patterns and variability of currents and shear in the upper SAO during the past decade. Although there was no noticeable trend in the upper SAO's current speed and shear from 2013 to 2023, their seasonal cycle has significantly strengthened. The cycle reveals a strong relationship between upper ocean currents and their shear with sea ice conditions – particularly during transitional seasons – evidenced by a strong negative correlation (−0.94) between seasonal sea ice concentration and current shear. In the shallow (< 20–30 m) summer surface mixed layer, currents have increased because strong stratification prevents wind energy from propagating into the deeper layers. In this case, strong near-inertial currents account for more than half of the summertime current speed and shear. In the winter, a thicker surface layer is created by deep upper SAO ventilation due to atlantification, which distributes wind energy to far deeper (> 100 m) layers. These findings are critical to understanding the ramifications for mixing and halocline weakening, as well as the rate of atlantification in the region. 

Atlantification—the northward inflow of anomalous waters and biota from the Atlantic into the polar basins—has wide-ranging climatological ramifications. We present previously unknown observational evidence that the atlantification processes are strengthening in the eastern Eurasian Basin. The primary example is the diminishing sea ice, which is related to a powerful ocean-heat/ice-albedo feedback, which accelerates sea-ice losses. Furthermore, we observe that atlantification is extending far beyond the Lomonosov Ridge into the Makarov Basin of the Arctic Ocean where upper ocean ventilation creates a new and unique ecological environment. The eastern part of the Siberian Arctic Ocean is still strongly stratified, but the atlantification-driven shoaling of warm, salty, and nutrient-rich intermediate waters already has important ecological consequences there. Disentangling the role of atlantification in multiple and complex high-latitude changes should be a priority in future modeling and observational efforts. 

Radium isotopes, which are sourced from sediments, are useful tools for studying potential climate‐driven changes in the transfer of shelf‐derived elements to the open Arctic Ocean. Here we present observations of radium‐228 and radium‐226 from the Siberian Arctic, focusing on the shelf‐basin boundary north of the Laptev and East Siberian Seas. Water isotopes and nutrients are used to deconvolve the contributions from different water masses in the study region, and modeled currents and water parcel back‐trajectories provide insights on water pathways and residence times. High radium levels and fractions of meteoric water, along with modeled water parcel back‐trajectories, indicate that shelf‐ and river‐influenced water left the East Siberian Shelf around 170°E in 2021; this is likely where the Transpolar Drift was entering the central Arctic. A transect extending from the East Siberian Slope into the basin is used to estimate a radium‐228 flux of 2.67 × 10⁷atoms m⁻² d⁻¹(possible range of 1.23 × 10⁷–1.04 × 10⁸atoms m⁻² d⁻¹) from slope sediments, which is comparable to slope fluxes in other regions of the world. A box model is used to determine that the flux of radium‐228 from the Laptev and East Siberian Shelves is 9.03 × 10⁷atoms m⁻² d⁻¹(possible range of 3.87 × 10⁷–1.56 × 10⁸atoms m⁻² d⁻¹), similar to previously estimated fluxes from the Chukchi Shelf. These three shelves contribute a disproportionately high amount of radium to the Arctic, highlighting their importance in regulating the chemistry of Arctic surface waters. 

Radium isotopes are continuously produced at ocean boundaries and are soluble in seawater. Radium therefore serves as an analogue for similarly sourced shelf-derived materials, including biologically important elements such as carbon, nutrients, and trace metals. To test the hypothesis that climate change is leading to increased delivery of terrestrially-derived solutes to the Arctic Ocean, radium levels will be measured on bi-annual cruises along the Laptev and East Siberian Sea margins (to capture interannual changes) and on a first-of-its kind in situ radium isotope sampler (to capture seasonal changes). These sampling efforts will be complemented by an international network of collaborators that will contribute data to create an Arctic Radium Isotope Observing Network (ARION) that spans the Arctic Ocean and will serve the greater scientific community. This dataset contains the results of the water column measurements made on the first ARION cruise, which took place in Sept-Oct 2021 along the slopes of the East Siberian and Laptev Seas in collaboration with the Nansen and Amundsen Basin Observational Systems (NABOS) program. Parameters measured include radium-228, radium-226, oxygen-18, deuterium, and salinity. 

Radium isotopes (radium-228 and radium-226), water isotopes (oxygen-18 and deuterium), and salinity were measured on the slope of the East Siberian Sea in coordination with the 2018 Nansen and Amundsen Basins Observational System (NABOS) expedition. Radium is continuously produced at ocean boundaries and is soluble in seawater, thus it serves as an analogue for similarly sourced sediment-derived materials. Because the Eastern Arctic shelves are the origin of the Transpolar Drift, monitoring the radium levels in this region improves our understanding of potential climate-driven changes on the transport of shelf-derived materials offshore.