231Pa and 230Th in the Arctic Ocean 1991-2015: Changes in the Eurasian and Makarov Basins

230Th and 231Pa are produced in sea water by radioactive decay of Uranium isotopes (234U, 235U). Both are particle reactive and are scavenged onto settling particles. As 230Th is more particle reactive than 231Pa, their distribution in the water column and activity ratio give information about particle fluxes and circulation patterns and –intensities. Both particle fluxes and deep water circulation may respond to climatic changes in the Arctic Ocean. This study discusses temporal changes in radionuclide concentration in the context of climate change. We compare results from 1991 [1] 2007 and 2015. We present results of dissolved 231Pa and 230Th activities of samples collected in the Nansen-, Amundsenand Makarov Basins during GEOTRACES sections GIPY11 (2007, 4 stations), GN04 (2015, 10 stations) aboard RV Polarstern. Our discussion of factors controlling the 230Th and 231Pa distribution is supported by, dissolved CFC, dissolved iron and particulate 230Th and 231Pa (3 stations) collected during GEOTRACES section GN04. We find that distributions and concentrations of dissolved 231Pa and 230Th in the central Arctic Ocean have changed significantly since 1991. Dissolved 231Pa concentrations in the Makarov basin decreased by half within less than 20 years. These changes are discussed in the context of environmental changes, such as declining sea ice cover and related increase of particle fluxes or changing deep water circulation. [1] Scholten, J. C., et al. (1995). Deep-Sea Research II 42: 1519- 153

231Pa, 230Th and 232Th as tracers of deep water circulation and particle transport : Insights from the Mediterranean Sea and the Arctic Ocean

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 (FTh/Pa ~ 10), while higher values were observed in deep waters (FTh/Pa ~ 20). Additionally, the hydrothermal particles in the Nansen interior produce lower FTh/Pa values compared to FTh/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.geotracesMedSe

231Pa and Th isotopes as tracers of deep water ventilation and scavenging in the Mediterranean Sea

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 FTh/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.GEOTRACESMeDSe

230Th and 231Pa: Tracers for Deep Water Circulation and Particle Fluxes in the Arctic Ocean

230Th and 231Pa data from the central Arctic Ocean is very limited. 230Th and 231Pa are produced at a constant rate in the water column by radioactive decay of Uranium isotopes (234U and 235U respectively) (e.g. Anderson et al., 1983). They are both particle reactive and are scavenged on settling particles. As 230Th is more particle reactive than 231Pa, their distribution in the water column and activity ratio give us information about particle fluxes and circulation patterns and –intensities (Henderson et al., 1999; Scholten et al., 2001). The Arctic Ocean is an almost landlocked ocean with limited connections to the Atlantic and Pacific and a high input of river water. About 10 % of the global river run-off is delivered to the Arctic Ocean. Due to climate change the Arctic Ocean will undergo dramatic changes in sea ice cover and supply of fresh water, while increasing coastal erosion will cause an increased input of terrestrial material (Peterson et al., 2002). This will influence the biogeochemical cycling and transport of carbon, nutrients and trace elements (IPCC, 2007). We expect that the distribution of 230Th and 231Pa will reflect changes in particle fluxes and shelf-basin exchange (Roy-Barman, 2009). We will present the first results of 230Th and 231Pa, in combination with on board measured particulate 234Th, collected during the 2015 Polarstern section (GEOTRACES section GN04 2015) through the Nansen, Amundsen, and Makarov Basins

Seawater‐Particle Interactions of Rare Earth Elements and Neodymium Isotopes in the Deep Central Arctic Ocean

International audienceThe sensitive and marked response of the Arctic to current climate warming requires a robust understanding of the natural conditions and biogeochemical processes in this area to allow for future evaluations of the impact of these changes on this unique environment (e.g., IPCC, 2014). Particularly, the nutrient and trace metal composition of the Arctic Ocean, that is, expected to change due to changes in, for example, sea ice cover, river discharge, and melting of the Arctic permafrost, are of high interest due to their high relevance for Arctic ecosystems. Through waters emerging from the Arctic Ocean through the Fram Strait, an

Particulate rare earth element concentrations and neodymium isotope compositions in the central Arctic Ocean during FS Polarstern PS94 (GEOTRACES GN04)

We present spatially highly resolved distributions of particulate seawater rare earth element concentrations and neodymium isotopes along a transect in the central Arctic Ocean from FS Polarstern cruise PS94 (GEOTRACES GN04) in August-October 2015. Particle samples were taken using in-situ pumps and Supor filters (pore size 0.45 μm)

Dissolved rare earth element concentrations and neodymium isotope compositions in the central Arctic Ocean during FS Polarstern PS94 (GEOTRACES GN04)

We present spatially highly resolved distributions of dissolved seawater rare earth element concentrations and neodymium isotopes along a transect in the central Arctic Ocean from FS Polarstern cruise PS94 (GEOTRACES GN04) in August-October 2015. Seawater samples were collected using Niskin bottles and filtered through AcroPak 500 filter cartridges (pore size 0.8/0.2 µm) directly from the Niskin bottles onboard

231Pa and 230Th in the Barents Sea and the Nansen Basin: Implications for Shelf-Basin Interactions and Changes in Particle Flux

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

Thorium and Protactinium isotopes as tracers of marine particle fluxes and deep water circulation in the Mediterranean Sea

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.GEOTRACE

Barium during the GEOTRACES GA-04S MedSeA cruise: The Mediterranean Sea Ba budget revisited

International audienceThe distribution of barium (Ba) in seawater was determined in the main basins and straits of the Mediterranean Sea during the GEOTRACES GA-04S MedSeA cruise. In addition, the concentrations of Ba and radium (Ra) isotopes (228Ra) were determined in groundwaters discharging in the Mediterranean Sea. The dissolved Ba concentration increases from the inflowing Atlantic surface water (Ba ~ 45 nmol/kg) to the intermediate (Ba ~ 70 nmol/kg) and deep (Ba ~ 70–75 nmol/kg) Mediterranean Sea waters in general agreement with previously published profiles. We use these data to build a Ba budget of the Mediterranean Sea and to evaluate the role of different sources of Ba (dusts, rivers, sediments, submarine groundwater discharges, Messinian evaporites). Evaporation alone cannot account for the Ba concentration increase because salinity increases only by 8% between the Gibraltar Strait and the Eastern Basin, whereas the Ba concentration increases up to 75% between the Atlantic Water and the deep waters. Thus, the particulate Ba flux that maintains the Ba concentration gradient between the surface and the deep waters must be 10–100 times stronger than previously estimated and the net Ba loss at the Gibraltar Strait or out of the Western Basin must be balanced by external inputs to the Mediterranean Sea. Newly available data from Mediterranean rivers confirm that rivers are insufficient Ba sources to balance this budget. Using recent estimates of Saharan dust fluxes, we show that dust deposition is also a negligible Ba source, whereas it was previously considered as the main external source. Using Ba-228Ra data, we show for the first time that submarine groundwater discharges can significantly contribute to the Ba budget, but large uncertainties exist due to the high variability of the chemical composition of these groundwaters. The inputs from Messinian evaporites require to be further investigated. Ba transfer from the surface water to the intermediate and deep waters occurs through active particulate Ba cycling and/or Ba dissolution in the sediment. Our study suggests that the mean particulate Ba cycling is more intense than previously estimated