Circulation pathways, time scales, and water mass composition in the Arctic Ocean: Results from 25 years of tracer observations

The Arctic is a hotspot of global change. For example, changes caused by global warming are both amplified and are seen more rapidly in the Arctic (e.g., Serreze & Francis, 2006; Bekryaev et al., 2010; Serreze & Barry, 2011; Overland et al. 2015; Macdonald et al., 2015). Thus, the Arctic is an indicator of the state of the planet. Among the strongest changes that have been observed in the Arctic Ocean are changes in circulation regimes, hydrographic properties and freshwater content and composition. These changes have the potential of global impact through interaction with the deep-water formation regions of the North Atlantic Ocean, a major source of deep and bottom water in the global ocean. Although significant progress in understanding the signals of change in the Arctic Ocean and their causes has been made during the past decades there are still some fundamental questions unanswered. They include the stability of the circulation of the upper waters and changes in the freshwater budget and how these changes are connected to changes in the composition of the freshwater lens that covers the Arctic Ocean. In this thesis, we address these two topics using measurements of isotopes obtained during over three decades of Arctic Ocean section work.This dissertation is composed by three parts and its structure mimics the layered vertical structure of the Arctic Ocean water column. Chapter 1 is dedicated to the Atlantic waters, Chapter 2 to the halocline waters, and Chapter 3 to the freshwater sources and their distribution and variability in the surface layer. In the first two chapters, we present transient tracer (³H/³He) and hydrographic data from over 25 years of Arctic oceanographic campaign ranging from 1987 to 2013 to evaluate flow rates and circulation pathways in the Upper Halocline Water (UHW), Lower Halocline Water (LHW), and Atlantic Layer on a pan-Arctic scale. In agreement with previously established circulation schemes, tracer data show that the flow paths in the LHW and the Atlantic layer are typically topographically steered with the presence of a cyclonic boundary current along the continental shelf and separate circulation branches tracking major bathymetric features, such as the Lomonosov Ridge. Tracer data suggest that the general circulation of UHW is decoupled from the cyclonic regime observed in the deeper layer, and strongly influenced by surface stress forcing, such as the anticyclonic Beaufort Gyre. Within the limits of our method, tracer data show that the mean flow paths and spreading velocities have been more or less constant over the past three decades despite dramatic shifts in the Arctic system heat and freshwater balances from anthropogenic climate change over imposed to a high natural variability. The third and final chapter discusses the water-mass composition and the distribution of freshwater sources in Canadian Basin, the western section of the Arctic Ocean. Results are produced by performing a water-mass decomposition on the water samples collected during the 2015 Arctic GEOTRACES (GN01) oceanographic expedition. Stable isotope measurements (H₂¹⁸O/H₂¹⁶O and DHO/H₂O ratios) are used in combination with salinity and nutrients data to calculate the water-mass components for the upper 500 m Arctic Ocean (mixed layer through Atlantic Water layer). The sources of liquid freshwater into the Arctic Ocean include Pacific water, sea ice meltwater, river discharge and net precipitation. The topmost 50 meters of Canadian Basin contain the large fraction of freshwater from sea ice meltwater and meteoric water. Pacific water dominated the freshwater budget along the 2015 GN01 transects from 100 to 250 m. These depths are also characterized by a strong brine rejection signal, reflecting an enhanced annual sea ice cycle with more ice refreezing and melting each year, and an overall loss of multiyear ice. The 2015 results are compared with the overlapping 1994 and 2005 Arctic Ocean Sections (AOS94 and AOS05) and discussed in the context of regional and temporal variability of liquid freshwater and its components distribution. Our findings show significant increases in the Canadian Basin total liquid freshwater reservoir both compared to the 1994 and 2005 transects confirming a freshwater accumulation in the Canadian Basin already established by numerous observations and modeling studies (Gilles et al., 2012; Carmack et al., 2016; Proshutinsky et al. 2019; Solomon et al., 2021). The total freshwater reservoir increased by ca. 12,500 km³ from 1994 to 2015, of which ca. 5,000 km3 are within the Beaufort Gyre. Meteoric and Pacific freshwater components were the largest sources of the observed freshwater accumulation in the upper 500m of the western Arctic Ocean. An intensified Ekman transport in the Beaufort Gyre and increased availability of freshwater for accumulation are the two primary drivers for freshwater accumulation in the Canadian Basin. Within the limits of our analysis, it is not possible to quantitatively estimate the relative importance of the each forcing nor to resolve the seasonal to year‐to‐year variability. Our tracer-based analysis suggests that there is a significant variability in the freshwater components and UHL distribution while the major features of the circulation patterns and spreading velocities of the AW and the LHW have remained largely stable over the past decades. Future research should address whether in a fast changing Arctic, the dynamics of the surface layer will expand to the halocline and Atlantic layer substantially destabilizing the current Arctic Ocean water column with potentially dramatic consequences

Strong margin influence on the Arctic Ocean Barium Cycle revealed by pan‐Arctic synthesis

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Whitmore, L., Shiller, A., Horner, T., Xiang, Y., Auro, M., Bauch, D., Dehairs, F., Lam, P., Li, J., Maldonado, M., Mears, C., Newton, R., Pasqualini, A., Planquette, H., Rember, R., & Thomas, H. Strong margin influence on the Arctic Ocean Barium Cycle revealed by pan‐Arctic synthesis. Journal of Geophysical Research: Oceans, 127(4), (2022): e2021JC017417, https://doi.org/10.1029/2021jc017417.Early studies revealed relationships between barium (Ba), particulate organic carbon and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. But, what controls the distribution of barium (Ba) in the oceans? Here, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba ratio (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ∼50% of the dBa inventory (upper 500 m of the Arctic water column) was supplied by nonconservative inputs. Calculated areal dBa fluxes are up to 10 μmol m−2 day−1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. The Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin Bay-derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a sedimentary remineralization and terrigenous sources (e.g., submarine groundwater discharge or fluvial particles) are contributors.This research was supported by the National Science Foundation [OCE-1434312 (AMS), OCE-1436666 (RN), OCE-1535854 (PL), OCE-1736949, OCE-2023456 (TJH), and OCE-1829563 (R. Anderson for open access support)], Natural Sciences and Engineering Research Council of Canada (NSERC)-Climate Change and Atmospheric Research (CCAR) Program (MTM), and LEFE-CYBER EXPATE (HP). HT acknowledges support by the Canadian GEOTRACES via NSERC-CCAR and the German Academic Exchange Service (DAAD): MOPGA-GRI (Make Our Planet Great Again—Research Initiative) sponsored by BMBF (Federal German Ministry of Education and Research; Grant No. 57429828)

Strong Margin Influence on the Arctic Ocean Barium Cycle Revealed by Pan‐Arctic Synthesis

Laura M. Whitmore et al, 2022, Strong margin influence on the Arctic Ocean barium cycle revealed by Pan‐Arctic synthesis, Journal of Geophysical Research: Oceans, Citation number, 10.1029/2021JC017417. To view the published open abstract, go to https://doi.org/10.1029/2021JC017417

Gallium: A New Tracer of Pacific Water in the Arctic Ocean

©2020. American Geophysical Union. All Rights Reserved. Determining the proportions of Atlantic and Pacific Ocean seawater entering the Arctic Ocean is important both for understanding the mass balance of this basin as well as its contribution to formation of North Atlantic deep water. To quantify the distribution and amount of Pacific and Atlantic origin seawater in the western Arctic Ocean, we used dissolved Ga in a four-component linear endmember mixing model. Previously, nutrients, combined in their Redfield ratios, have been used to separate Pacific- and Atlantic-derived waters. These nutrient tracers are not conservative in practice, and there is a need to find quantities that are conserved. Dissolved Ga concentrations show measurable contrast between Atlantic and Pacific source waters, shelf-influenced waters show little impact of shelf processes on the dissolved Ga distribution, and dissolved Ga in the Arctic basins is conserved along isopycnal surfaces. Thus, we explored the potential of Ga as a new parameter in Arctic source water deconvolution. The Ga-informed deconvolution was compared to that generated with the NO3:PO4 relationship. While distributions of the water masses were qualitatively similar, the Ga-based deconvolution predicted higher amounts of Pacific water at depths between 150 and 300 m. The Ga-based decomposition yields a smoother transition between the halocline and Atlantic layers, while nutrient-based solutions have sharper transitions. A 1-D advection-diffusion model was used to constrain estimates of vertical diffusivity (Kz). The Ga-based Kz estimates agreed better with those from salinity and temperature than the nutrient method. The Ga-based approach implies greater vertical mixing between the Pacific and Atlantic waters

Spatial complexity in dissolved organic matter and trace elements driven by hydrography and freshwater input across the Arctic Ocean during 2015 Arctic GEOTRACES expeditions

This study traces dissolved organic matter (DOM) in different water masses of the Arctic Ocean and its effect on the distributions of trace elements (TEs; Fe, Cu, Mn, Ni, Zn, Cd) using fluorescent properties of DOM and the terrigenous biomarker lignin. The Nansen, Amundsen, and Makarov Basins were characterized by the influence of Atlantic water and the fluvial discharge of the Siberian rivers with high concentrations of terrigenous DOM (tDOM). The Canada Basin and the Chukchi Sea were characterized by Pacific water, modified through contact with productive shelf sediments with elevated levels of marine DOM. Within the surface layer of the Beaufort Gyre, meteoric water (river water and precipitation) was characterized by low concentrations of lignin and terrigenous DOM fluorescence proxies as DOM is removed during freezing. High-resolution in situ fluorescence profiles revealed that DOM distribution closely followed isopycnals, indicating the strong influence of sea-ice formation and melt, which was also reflected in strong correlations between DOM fluorescence and brine contributions. The relationship of DOM and hydrography to TEs showed that terrigenous and marine DOM were likely carriers of dissolved Fe, Ni, Cu from the Eurasian shelves into the central Arctic Ocean. Chukchi shelf sediments were important sources of dCd, dZn, and dNi, as well as marine ligands that bind and carry these TEs offshore within the upper halocline (UHC) in the Canada Basin. Our data suggest that tDOM components represent stronger ligands relative to marine DOM components, potentially facilitating the long-range transport of TE to the North Atlantic. Key Points Dissolved Organic Matter (DOM) distribution in the Arctic Ocean is largely controlled by sea ice formation and melt processes DOM distribution in the Arctic Ocean reveals its potential as a tracer for halocline formation and freshwater source assignments Terrigenous and marine DOM are carriers of trace elements from shelves to the open Arctic Ocean, but terrigenous DOM represent stronger ligand

Dissolved and Particulate Trace Elements in Late Summer Arctic Melt Ponds

Melt ponds are a prominent feature of Arctic sea ice during the summer and play a role in the complex interface between the atmosphere, cryosphere and surface ocean. During melt pond formation and development, micronutrient and contaminant trace elements (TEs) from seasonally accumulated atmospheric deposition are mixed with entrained sedimentary and marine-derived material before being released to the surface ocean during sea ice melting. Here we present particulate and size-fractionated dissolved (truly soluble and colloidal) TE data from five melt ponds sampled in late summer 2015, during the US Arctic GEOTRACES (GN01) cruise. Analyses of salinity, δ18O, and 7Be indicate variable contributions to the melt ponds from snowmelt, melting sea ice, and surface seawater. Our data highlight the complex TE biogeochemistry of late summer Arctic melt ponds and the variable importance of different sources for specific TEs. Dissolved TE concentrations indicate a strong influence from seawater intrusion for V, Ni, Cu, Cd, and Ba. Ultrafiltration methods reveal dissolved Fe, Zn, and Pb to be mostly colloidal (0.003–0.2 μm), while Mn, Co, Ni, Cu, and Cd are dominated by a truly soluble (\u3c0.003 μm) fraction. Isotopically light dissolved Fe in some melt ponds suggests that photochemical and/or biologically driven redox cycling also takes place. Comparisons of particulate TE/Al ratios to mean crustal values indicate influences from lithogenic sources, including natural aerosols and/or sedimentary material, with significant enrichments for some elements, including Ni, Cu, Zn, Cd and Pb, that may result from anthropogenic aerosols, biogenic material, and/or in situ scavenging of dissolved TEs. Our results indicate that melt ponds represent a transitional environment in which some atmospherically-derived TEs undergo physical and/or chemical changes before their release to the surface ocean. As a result, the ongoing changes in sea ice areal extent, thickness, and melt season length are likely to influence the bioavailability of atmospheric TE input to the surface Arctic Ocean, with material released from snow and sea ice via melt ponds earlier in the summer and with more extensive direct deposition to the ocean surface