The naturally occurring U and Th-series radionuclides have shown to have a considerable importance for the understanding of biogeochemical processes on Earth and in the ocean. In this thesis, the isotopes 230-thorium (²³⁰Th), 232-thorium (²³²Th) and 231-protactinium (²³¹Pa) are used as tracers of the transport and scavenging of marine particles and water circulation. Pa and Th are particle reactive elements, which makes the production, transport and distribution of Pa and Th key factors for our understanding of the origin, fate and distribution of marine particles in the oceans. 

This thesis explores the distribution of ²³¹Pa, ²³⁰Th and ²³²Th in two different ocean continental margin environments. In particular, the relative influence of water circulation and particles on the ²³¹Pa, ²³⁰Th and ²³²Th distributions in the Arctic Ocean and Mediterranean Sea was investigated. ²³¹Pa, ²³⁰Th and ²³²Th were analyzed in particles and seawater collected in the Mediterranean Sea during the MedSeA-GA04-S cruise along the GEOTRACES section GA04S and in the Arctic Ocean during the PS94 GN04 ARK-XXIX/3 along the GEOTRACES section GN04.

One of the important findings of this thesis was the low fractionation between ²³¹Pa and ²³⁰Th in the Mediterranean Sea, contrasting what is observed in the open ocean. Additionally, the observed depth profiles of Pa-Th allowed the identification of deep water convection and ventilation in the Western and Eastern Basins, respectively. Moreover, the particle settling speed was reevaluated to ~500 – 1000 m/y.

In the Arctic Ocean, scavenging onto particles derived from hydrothermal activity was producing relatively low F-factors (F_Th/Pa ~ 10), while higher values were observed in deep waters (F_Th/Pa ~ 20). Additionally, the hydrothermal particles in the Nansen interior produce lower F_Th/Pa values compared to F_Th/Pa observed at the Nansen continental margin. Application of a boundary scavenging model revealed the importance of ²³⁰Th scavenging at the continental margin along the Nansen Basin, hereafter the Nansen margin, and advocate for the advection of ²³¹Pa into the Atlantic Ocean. As the ocean margin was included in this model, a particle settling speed of 600 m/y was obtained at the Nansen margin.

Moreover, this thesis includes an inter-comparison of dissolved and particulate ²³¹Pa, ²³⁰Th and ²³²Th measurements between four laboratories of the GEOTRACES community. This comparison was conducted to provide detailed descriptions of various chemical procedures used for Pa-Th analysis and to provide a measure of consistency between the laboratories. Results demonstrated that participating labs can determine concentrations of dissolved ²³⁰Th and ²³¹Pa in deep water (below 500 m depth) that are internally consistent within 4 % of the mean values. Analysis of particulate ²³¹Pa, ²³⁰Th and ²³²Th allowed the highlighting of an incomplete Pa dissolution problem with our initial leaching procedure, a problem solved by measuring aliquots of particulate samples at two labs. However, in the present work, consistent particulate ²³¹Pa concentrations as low as ~ 0.002 fg/kg were obtained. Overall, it suggests an improvement of the results consistency compared to the previous GEOTRACES intercalibration exercise.

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 (F_Th/Pa) observed at theNansen margin were higher compared to F_Th/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.

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.

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 similar to 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 (10(6) m(3)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. Plain Language Summary A major feature of the Arctic Ocean circulation is the Transpolar Drift (TPD), a surface current that carries ice and continental shelf-derived materials from Siberia across the North Pole to the North Atlantic Ocean. In 2015, an international team of oceanographers conducted a survey of trace elements in the Arctic Ocean, traversing the TPD. Near the North Pole, they observed much higher concentrations of trace elements in surface waters than in regions on either side of the current. These trace elements originated from land, and their journey across the Arctic Ocean is made possible by chemical reactions with dissolved organic matter that originates mainly in Arctic rivers. This study reveals the importance of rivers and shelf processes combined with strong ocean currents in supplying trace elements to the central Arctic Ocean and onward to the Atlantic. These trace element inputs are expected to increase as a result of permafrost thawing and increased river runoff in the Arctic, which is warming at a rate much faster than anywhere else on Earth. Since many of the trace elements are essential building blocks for ocean life, these processes could lead to significant changes in the marine ecosystems and fisheries of the Arctic Ocean.

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.

²³¹Pa, ²³⁰Th and ²³²Th were analyzed in unfiltered seawater samples (n =66) and suspended particles (n =19)collected in the Mediterranean Sea during the MedSeA-GA04-S cruise along the GEOTRACES section GA04S andused to investigate mechanisms controlling the distribution and fractionation of Pa and Th in an ocean marginenvironment. ²³¹Pa and ²³⁰Th are particle reactive radionuclides and are often used as tracers of processes suchas boundary scavenging, particle transport and ocean circulation. The depth profiles of total ²³¹Pa and ²³⁰Thconcentrations in the Mediterranean Sea displayed non-linear shapes. Higher total ²³²Th concentrations wereobserved at the straits and in deep waters pointing at lithogenic sources. Fractionation factors F_Th/Pa ranged from1.4 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 tothe Atlantic Ocean, leads to a reevaluation of the mean settling speed of the filtered particles, which is nowestimated to 500–1000 m/y. The low F_Th/Pa fractionation factors are attributed to the efficient scavenging andlack of transport of ²³¹Pa to the Atlantic Ocean.

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 water formation process that occurs in respective basin. Essentially all in-situ produced ²³⁰Th is buried in Mediterranean Sea sediments. Despite lower affinity of ²³¹Pa for marine particles, most ²³¹Pa is also scavenged and deposited in Mediterranean Sea sediments. The efficient scavenging of ²³¹Pa produces a relatively low fractionation between ²³¹Pa and ²³⁰Th in terms of the fractionation factor F_Th/Pa. This licentiate thesis presents a summary of the methods used for the analysis of ²³¹Pa and Th-isotopes with details on the exchange chromatography method and the treatment of mass spectrometric data. The study of ²³¹Pa, ²³⁰Th and ²³²Th in the Mediterranean Sea has important implications for our understanding of processes that control their water column distributions and how their behavior can be utilized to trace chemical flux in modern and past ocean environments.

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.