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In the central Arctic Ocean, dissolved rare earth element concentrations ([dREE]) and the neodymium (Nd) isotope compositions are constant throughout the deep water column (>1000 m water depth), indicating unique conditions among the ocean basins and therefore requiring an investigation of seawater-particle interactions. Here, we present the first high resolution particulate REE and Nd isotope data from the Arctic Ocean and discuss the possible seawater-particle processes affecting the Arctic Ocean. Our results show that particulate [REE] are on the same order of magnitude as in other ocean basins, suggesting that particle composition is the main cause for a lack of pREE release to the dissolved pool. The lithogenic fraction dominates throughout the water column while the biogenic material contribution is very small. This paucity of biogenic material results in reduced particle-seawater exchanges of REEs and Nd isotopes. Moreover, we note only slight differences in the dissolved Nd isotope composition between the Eurasian and Canadian Basins. This is due to the different source regions supplying different dissolved and particulate Nd isotope signatures to both basins. The dissolved [REE] and Nd isotope composition of Atlantic waters are modified during their flow paths through contributions from the Kara Sea, lowering the salinity and increasing [dREE] and dNd isotope compositions. Hydrothermal influence from the Gakkel Ridge on dissolved and particulate [REE] and Nd isotopes could not be detected. Plain Language Summary The Arctic Ocean is strongly affected by climate change. Due to rising temperatures, the sea-ice is melting und the river input is increasing. This will also change the chemical composition of the Arctic Ocean and biological activity, since many trace elements added by rivers are essential for algal growth. In order to understand these changes and effects, the environmental conditions and processes need to be understood. We investigated rare earth e

A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river‐influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high‐resolution pan‐Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and ~25‐50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle‐reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 ± 0.4 Sv (106 m3 s‐1). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.

231Pa, 230Th and 232Th were analyzed in filtered seawater (n=70) and suspended particles (n=39) collectedalong a shelf-basin transect from the Barents shelf to the Makarov Basin in the Arctic Ocean during GEOTRACESsection GN04 in 2015. The distribution of dissolved 231Pa and 230Th in the Arctic Ocean deviates from the linearincrease expected from reversible scavenging. Higher 232Th concentrations were observed at the shelf, slope andin surface waters in the deep basin, pointing at lithogenic sources. Fractionation factors (FTh/Pa) observed at theNansen margin were higher compared to FTh/Pa in the central Nansen Basin, possibly due to the residual occurrenceof hydrothermal particles in the deep central Nansen Basin. Application of a boundary scavengingmodel quantitatively accounts for the dissolved and particulate 230Th distributions in the Nansen Basin.Modelled dissolved 231Pa distributions were largely overestimated, which was attributed to the absence of incorporation of water exchange with the Atlantic Ocean in the model. 231Pa/230Th ratios of the suspended particlesof the Nansen Basin were below the 231Pa/230Th production ratio, but top-core sediments of the Nansenmargin and slope have high 231Pa/230Th-ratios, suggesting that scavenging along the Nansen margin partly actsas a sink for the missing Arctic 231Pa., GEOTRACES

This study provides dissolved and particulate Th-230 and Th-232 results as well as particulate( )(234)Th data collected during expeditions to the central Arctic Ocean (GEO-TRACES, an international project to identify processes and quantify fluxes that control the distributions of trace elements; sections GN04 and GIPY11). Constructing a time series of dissolved Th-230 from 1991 to 2015 enables the identification of processes that control the temporal development of Th-230 distributions in the Amundsen Basin. After 2007, Th-230 concentrations decreased significantly over the entire water column, particularly between 300 and 1500 m. This decrease is accompanied by a circulation change, evidenced by a concomitant increase in salinity. A potentially increased inflow of water of Atlantic origin with low dissolved Th-230 concentrations leads to the observed depletion in dissolved Th-230 in the central Arctic. Because atmospherically derived tracers (chlorofluorocarbon (CFC), sulfur hexafluoride (SF6)) do not reveal an increase in ventilation rate, it is suggested that these interior waters have undergone enhanced scavenging of Th during transit from Fram Strait and the Barents Sea to the central Amundsen Basin. The Th-230 depletion propagates downward in the water column by settling particles and reversible scavenging.

231Pa, 230Th and 232Th were analyzed in filtered seawater (n=70) and suspended particles (n=39) collected along a shelf-basin transect from the Barents shelf to the Makarov Basin in the Arctic Ocean during GEOTRACES section GN04 in 2015. The distribution of dissolved 231Pa and 230Th in the Arctic Ocean deviates from the linear increase expected from reversible scavenging. Higher 232Th concentrations were observed at the shelf, slope and in surface waters in the deep basin, pointing at lithogenic sources. Fractionation factors (FTh/Pa) observed at the Nansen margin were higher compared to FTh/Pa in the central Nansen Basin, possibly due to the residual occurrence of hydrothermal particles in the deep central Nansen Basin. Application of a boundary scavenging model quantitatively accounts for the dissolved and particulate 230Th distributions in the Nansen Basin. Modelled dissolved 231Pa distributions were largely overestimated, which was attributed to the absence of incorporation of water exchange with the Atlantic Ocean in the model. 231Pa/230Th ratios of the suspended particles of the Nansen Basin were below the 231Pa/230Th production ratio, but top-core sediments of the Nansen margin and slope have high 231Pa/230Th-ratios, suggesting that scavenging along the Nansen margin partly acts as a sink for the missing Arctic 231Pa.

231Pa, 230Th and 232Th were analyzed in unfiltered sea water samples (n = 66) and suspended particles (n = 19) collected in the Mediterranean Sea during the MedSeA-GA04-S cruise along the GEOTRACES section GA04S and used to investigate mechanisms controlling the distribution and fractionation of Pa and Th in an ocean margin environment. 231Pa and230Th are particle reactive radionuclides and are often used astracers of processes such as boundary scavenging, particle transport and ocean circulation. The depth profiles of total 231Pa and 230Th concentrations in the Mediterranean Sea displayed non-linear shapes. Higher total 232Th concentrations were observed at the straits and in deepwaters pointing at lithogenic sources. Fractionation factors FTh/Pa ranged from 1.4 to 9. Application of a box-model illustrated that 94 % of the 231Pa and almost all of the 230Th (99.9 %) produced in the Mediterranean Sea is removed to the sediment by scavenging. The negligible export of 230Th to the Atlantic Ocean, leads to a reevaluation of the mean settling speed of the filtered particles, which is now estimated to 500-1000 m/y. The low FTh/Pa fractionation factors are attributed to the efficient scavenging and lack of transport of 231Pa to the Atlantic Ocean., GEOTRACES

In this study we present dissolved and particulate Th-230 and Th-232 results, as well as particulate Th-234 data, obtained as part of the GEOTRACES central Arctic Ocean sections GN04 (2015) and IPY11 (2007). Samples were analyzed following GEOTRACES methods and compared to previous results from 1991. We observe significant decreases in Th-230 concentrations in the deep waters of the Nansen Basin. We ascribe this nonsteady state removal process to a variable release and scavenging of trace metals near an ultraslow spreading ridge. This finding demonstrates that hydrothermal scavenging in the deep-sea may vary on annual time scales and highlights the importance of repeated GEOTRACES sections. Plain Language Summary This study presents new results of thorium isotopes from the central Arctic Ocean. Thorium-230 is produced continuously in seawater by radioactive decay of U-234 and subsequently removed by particle scavenging. We show that observed changes in Th-230 concentrations compared to earlier times are related to submarine volcanic eruptions. We use Th-230 data from three different expeditions conducted in 1991, 2007, and 2015. The Nansen Basin is part of the Eurasian Basin of the Arctic Ocean. It is divided from the Amundsen Basin by the Gakkel Ridge. The Gakkel Ridge is a region where the Eurasian and the North American plates spread apart, triggering volcanism. Submarine volcanos and hydrothermal vents release trace elements such as iron. Iron is known to be oxidized to particles that react with Th-230. Thus, when iron particles sink they remove Th-230 from the water column. In the Nansen Basin this process took place between 2007 and 2015, triggered by earthquake-induced volcanic eruptions in 2001. In this study, we present a conceptual hydrothermal scavenging process and plume dispersal by deep water circulation.

In this study we present dissolved and particulate 230Th and 232Th results, as well as particulate 234Th data, obtained as part of the GEOTRACES central Arctic Ocean sections GN04 (2015) and IPY11 (2007). Samples were analyzed following GEOTRACES methods, and compared to previous results from 1991. We observe significant decreases in 230Th concentrations in the deep waters of the Nansen Basin. We ascribe this non-steady state removal process to a variable release and scavenging of trace metals near an ultra-slow spreading ridge. This finding demonstrates that hydrothermal scavenging in the deep-sea may vary on annual time scales and highlights the importance of repeated GEOTRACES sections

The naturally occurring isotopes 231Pa and 230Th are used as tracers of marine biogeochemical processes. They are both produced from the radioactive decay of their uniformly distributed uranium parents (235U and 234U) in seawater. After production, 231Pa and 230Th are removed by adsorption onto settling particles (scavenging) and subsequently buried in marine sediments. 230Th is more particle reactive compared to 231Pa. Consequently, 230Th will be removed from the open ocean by adsorption onto settling particles, while 231Pa tend to be laterally transported by currents and removed by scavenging in areas of high particle flux (e.g. ocean margins). The primordial 232Th indicates lithogenic supply via rivers and resuspension of sediments, which provides additional information about processes involved in the cycling of particle reactive elements in the ocean. The preferential deposition of particle reactive elements at ocean margins (boundary scavenging) has important implications for our understanding of the distribution and dispersion of micronutrients (e.g. iron) and pollutants in the ocean. It is therefore valuable to understand the nature of boundary scavenging processes in order to evaluate the relative contribution of circulation and scavenging behaviors.The major characteristics of thermohaline circulation in the Mediterranean are well known and have been studied for decades. This sea is an almost land-locked area, where limited water-exchange with the Atlantic Ocean only occurs through the Strait of Gibraltar. Therefore, this marginal sea is often referred to as a “miniature ocean” suitable as a “laboratory” for marine environmental research. In this licentiate thesis, distributions of 231Pa, 230Th and 232Th in seawater and marine particles collected during the GEOTRACES MedSeA-GA04-S cruise in 2013 are presented. Observed nuclide distributions indicate the impact of deep water formation processes, where observed differences can be linked to the type of deep wa, GEOTRACES, MeDSeA

In seawater, particle reactive 231Pa and 230Th are uniformly produced by decay of soluble 235U and 234U. Due to differences in particle reactivity, 230Th tends to be removed to the sediment close to its production site, while 231Pa is more prone to lateral transport and is removed to the sediments in areas of high particle flux (i.e. boundary scavenging). Due to a combination of perennial ice cover, large shelf areas and river runoff, boundary scavenging in the Arctic Ocean strongly impacts the distribution of 231Pa and 230Th in the water column. Here, 231Pa and 230Th were analyzed in seawater and particles from the Arctic GEOTRACES section GN04 along the Barents shelf and in the Nansen basin. Key observations include lower concentrations of dissolved 231Pa and 230Th on the Barents shelf compared to the Nansen basin, indicating enhanced removal of 231Pa and 230Th near the margin. The particulate nuclide/total nuclide ratio increases from the surface (0.2% for 231Paxs and 0.7% for 230Thxs) to the seafloor (11% for 231Paxs and 71% for 230Thxs), highlighting the role of deep scavenging. The fractionation factor between 231Pa and 230Th (FTh/Pa) ranges from ~3 to 25 and generally increases with depth. Comparison between suspended particles and sediments will be provided. Further, dissolved 231Paxs and 230Thxs concentrations in the Nansen basin are lower compared to concentrations measured 20 years ago1 . This might reflect changes in particle flux and/or changes in circulation patterns. 1 Scholten, J., Rutgers van der Loeff, M., 1995. Distribution of 230Th and 231Pa in the water column in relation to the ventilation of the deep Arctic basins. Deep. Res. Part II 42, 1519–1531.

Roy-Barman, M., Pons-Branchu, E., Levier, M., Bordier, L., Foliot, L., Gdaniec, S., . . . Castrillejo, M. (2019). Barium during the GEOTRACES GA-04S MedSeA cruise: The mediterranean sea ba budget revisited. Chemical Geology, 511, 431-440. Available here

Valk, O., Rutgers van der Loeff, M. M., Geibert, W., Gdaniec, S., Moran, S. B., Lepore, K., ... & Paffrath, R. (2020). Decrease in 230Th in the Amundsen Basin since 2007: far-field effect of increased scavenging on the shelf?. Ocean Science, 16, 221-234. https://doi.org/10.5194/os-16-221-2020

Gdaniec, S., Roy-Barman, M., Foliot, L., Thil, F., Dapoigny, A., Burckel, P., ... & Andersson, P. S. (2017). Thorium and protactinium isotopes as tracers of marine particle fluxes and deep water circulation in the Mediterranean Sea. Marine Chemistry, 199, 12-23. doi:here.

The global mapping of the GEOTRACES program requires consistent and precise measurements of 232Th (pg/kg), 230Th and 231Pa (fg/kg) in seawater and suspended particulate matter. Concentrations of particulate and dissolved 231Pa, 230Th and 232Th were measured by four labs in samples collected at the Arctic crossover station in 2015. These samples were collected during two separate expeditions (HLY1502 GN01, Station 30 and PS94 GN04 ARK-XXIX/3, Station 101). Detailed descriptions of chemical procedures used by the participating labs are followed by a discussion focused on dissolved surface samples and particulate samples. Results demonstrated that participating labs can determine concentrations of dissolved 230Th and 231Pa in deep water (<500 m depth) that are internally consistent within 4 % of the mean values. However, the analysis of radionuclide concentrations in suspended particles still needs improvement. The pre-concentration of particulate material used at LSCE was proven to be unsuccessful. Aliquots of particulate samples at St. 101 were re-measured and it was concluded that the absence of HF in the leaching solution was the cause for the previously underestimated particulate concentrations. Large blank contributions are still a problem, especially for measurements of particulate 231Pa., GEOTRACES

The global mapping of the GEOTRACES program requires consistent and precise measurements of 232Th (pg/kg), 230Th and 231Pa (fg/kg) in seawater and suspended particulate matter. Concentrations of particulate and dissolved 231Pa, 230Th and 232Th were measured by four labs in samples collected at the Arctic crossover station in 2015. These samples were collected during two separate expeditions (HLY1502 GN01, Station 30 and PS94 GN04 ARK-XXIX/3, Station 101). Detailed descriptions of chemical procedures used by the participating labs are followed by a discussion focused on dissolved surface samples and particulate samples. Results demonstrated that participating labs can determine concentrations of dissolved 230Th and 231Pa in deep water (<500 m depth) that are internally consistent within 4 % of the mean values. However, the analysis of radionuclide concentrations in suspended particles still needs improvement. The pre-concentration of particulate material used at LSCE was proven to be unsuccessful. Aliquots of particulate samples at St. 101 were re-measured and it was concluded that the absence of HF in the leaching solution was the cause for the previously underestimated particulate concentrations. Large blank contributions are still a problem, especially for measurements of particulate 231Pa., GEOTRACES

The global mapping of the GEOTRACES program requires consistent and precise measurements of 232Th (pg/kg), 230Th and 231Pa (fg/kg) in seawater and suspended particulate matter. Concentrations of particulate and dissolved 231Pa, 230Th and 232Th were measured by four labs in samples collected at the Arctic crossover station in 2015. These samples were collected during two separate expeditions (HLY1502 GN01, Station 30 and PS94 GN04 ARK-XXIX/3, Station 101). Detailed descriptions of chemical procedures used by the participating labs are followed by a discussion focused on dissolved surface samples and particulate samples. Results demonstrated that participating labs can determine concentrations of dissolved 230Th and 231Pa in deep water (<500 m depth) that are internally consistent within 4 % of the mean values. However, the analysis of radionuclide concentrations in suspended particles still needs improvement. The pre-concentration of particulate material used at LSCE was proven to be unsuccessful. Aliquots of particulate samples at St. 101 were re-measured and it was concluded that the absence of HF in the leaching solution was the cause for the previously underestimated particulate concentrations. Large blank contributions are still a problem, especially for measurements of particulate 231Pa., GEOTRACES

The global mapping of the GEOTRACES program requires consistent and precise measurements of 232Th (pg/kg), 230Th and 231Pa (fg/kg) in seawater and suspended particulate matter. Concentrations of particulate and dissolved 231Pa, 230Th and 232Th were measured by four labs in samples collected at the Arctic crossover station in 2015. These samples were collected during two separate expeditions (HLY1502 GN01, Station 30 and PS94 GN04 ARK-XXIX/3, Station 101). Detailed descriptions of chemical procedures used by the participating labs are followed by a discussion focused on dissolved surface samples and particulate samples. Results demonstrated that participating labs can determine concentrations of dissolved 230Th and 231Pa in deep water (<500 m depth) that are internally consistent within 4 % of the mean values. However, the analysis of radionuclide concentrations in suspended particles still needs improvement. The pre-concentration of particulate material used at LSCE was proven to be unsuccessful. Aliquots of particulate samples at St. 101 were re-measured and it was concluded that the absence of HF in the leaching solution was the cause for the previously underestimated particulate concentrations. Large blank contributions are still a problem, especially for measurements of particulate 231Pa., GEOTRACES

The global mapping of the GEOTRACES program requires consistent and precise measurements of 232Th (pg/kg), 230Th and 231Pa (fg/kg) in seawater and suspended particulate matter. Concentrations of particulate and dissolved 231Pa, 230Th and 232Th were measured by four labs in samples collected at the Arctic crossover station in 2015. These samples were collected during two separate expeditions (HLY1502 GN01, Station 30 and PS94 GN04 ARK-XXIX/3, Station 101). Detailed descriptions of chemical procedures used by the participating labs are followed by a discussion focused on dissolved surface samples and particulate samples. Results demonstrated that participating labs can determine concentrations of dissolved 230Th and 231Pa in deep water (<500 m depth) that are internally consistent within 4 % of the mean values. However, the analysis of radionuclide concentrations in suspended particles still needs improvement. The pre-concentration of particulate material used at LSCE was proven to be unsuccessful. Aliquots of particulate samples at St. 101 were re-measured and it was concluded that the absence of HF in the leaching solution was the cause for the previously underestimated particulate concentrations. Large blank contributions are still a problem, especially for measurements of particulate 231Pa., GEOTRACES

The global mapping of the GEOTRACES program requires consistent and precise measurements of 232Th (pg/kg), 230Th and 231Pa (fg/kg) in seawater and suspended particulate matter. Concentrations of particulate and dissolved 231Pa, 230Th and 232Th were measured by four labs in samples collected at the Arctic crossover station in 2015. These samples were collected during two separate expeditions (HLY1502 GN01, Station 30 and PS94 GN04 ARK-XXIX/3, Station 101). Detailed descriptions of chemical procedures used by the participating labs are followed by a discussion focused on dissolved surface samples and particulate samples. Results demonstrated that participating labs can determine concentrations of dissolved 230Th and 231Pa in deep water (<500 m depth) that are internally consistent within 4 % of the mean values. However, the analysis of radionuclide concentrations in suspended particles still needs improvement. The pre-concentration of particulate material used at LSCE was proven to be unsuccessful. Aliquots of particulate samples at St. 101 were re-measured and it was concluded that the absence of HF in the leaching solution was the cause for the previously underestimated particulate concentrations. Large blank contributions are still a problem, especially for measurements of particulate 231Pa., GEOTRACES