Decrease in 230Th in the Amundsen Basin since 2007: Far-field effect of increased scavenging on the shelf?

This study provides dissolved and particulate 230Th and 232Th results as well as particulate 234Th data collected during expeditions to the central Arctic Ocean (GEOTRACES, 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 230Th from 1991 to 2015 enables the identification of processes that control the temporal development of 230Th distributions in the Amundsen Basin. After 2007, 230Th 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 230Th concentrations leads to the observed depletion in dissolved 230Th 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 230Th depletion propagates downward in the water column by settling particles and reversible scavenging.


Introduction
The Arctic Ocean is one of the most rapidly changing parts of the Earth's ocean-atmosphere system as a result of climate change. Underlying the potential anthropogenic changes is a large natural variability of the Arctic. Due to the limited observations in this extreme environment, establishing datasets that allow an assessment of its variability is important.
Natural tracers of physical, chemical and biological processes provide an integrated description of the changing state of the 5 system. They are therefore key tools to investigate processes, monitor environmental changes, and provide an observational baseline against which models can be tested.

Hydrography and Circulation patterns of the central Arctic Ocean
The central Arctic Ocean is divided into the Amerasian Basin and Eurasian Basin by the Lomonosov Ridge (Fig. 1). The Gakkel Ridge separates the Eurasian Basin further into the Nansen Basin and the Amundsen Basin, while the Amerasian 10 Basin is separated into the Makarov and Canada Basin by the Alpha-Mendeleev Ridge.
Water masses of the Arctic Ocean are commonly distinguished as five layers (Rudels, 2009). The uppermost low salinity Polar Mixed Layer (PML) varies in thickness between winter and summer, due to melting and freezing of sea ice. Salinity ranges from 30 to 32.5 (Amerasian Basin) to [32][33][34]. Below the PML is a 100-250-m-thick halocline in which salinity increases sharply from approximately 32.5 to 34.5. The underlying Atlantic Layer is characterized in salinity 15 and temperature by waters of Atlantic origin and is usually found between 400 m and 700 m water depth. Its salinity is 34. . Intermediate waters down to 1500 m, with a salinity of 34. 87-34.92, are still able to exchange over the Lomonosov Ridge. In contrast, deep and bottom waters differ between the Eurasian ) and the Amerasian ) due to the topographic barrier.
Atlantic waters from the Norwegian Atlantic Current enters the Arctic Ocean via the Fram Strait and the Barents Sea. Fram 20 Strait Branch Water (FSBW) is supplied through the West Spitsbergen Current (WSC) (Rudels, 2012) (Fig. 1). Barents Sea Branch Water (BSBW) enters through the Barents Sea and consists of Atlantic water that undergoes strong modifications in Barents-and Kara Seas by cooling down and mixing with continental runoff and meltwater (Rudels et al., 2015). The BSBW enters the Nansen Basin through the Santa Anna Trough, where limited mixing with the FSBW occurs. Once in the polar ocean, surface waters follow wind driven ice motion (Aagaard et al., 1980), whereas deeper Atlantic water branches (FSBW 25 and BSBW) flow cyclonically to the east forming a boundary current along the continental slopes of the Nansen and Amundsen basins. BSBW (around approx. 1025 m depth, Tanhua, 2009) and FSBW (approx. 425 m) return in the Atlantic and Intermediate water layers along the Lomonosov Ridge towards Fram Strait (Rudels et al., 2013) (Fig. 1) and a second branch crosses the Lomonosov Ridge entering the Canada Basin following the Arctic Ocean Boundary Current (AOBC) (Rudels, 2009). 30 Deep waters of the Arctic Ocean have similar structure, with a thick intermediate layer stratified in temperature but with salinity almost constant with depth (Rudels, 2009). Yet, the Amerasian Basin deep water is warmer, saltier and less dense than the Eurasian Basin Deep Water (EBDW) (Aagaard, 1981;Worthington, 1953). The deepest exchange of Makarov Basin water, part of the Amerasian Basin, with Eurasian Basin water occurs through a depression of the ridge, called the Intra-Basin with sill depth of approximately 1800 m (Björk et al., 2007;Jones et al., 1995;Björk et al., 2010). Water from the Amundsen Basin flows over the Lomonosov Ridge into the deep Makarov Basin and in the reverse direction (Middag et al., 2009). 5 Another important component of the Arctic Ocean is the freshwater content, coming from the melting of sea-ice and from river runoff. The fresh water content of the central Arctic Ocean is currently at the highest level since the early 1980s, and is expected to increase in the future (Rabe et al., 2014) which could lead to a stronger stratification of the water column. This process is supported by sea ice decline, as observed in the Beaufort Gyre (Wang et al., 2018). Karcher et al. (2012)  and modelling. This could lead to a decoupling of flow regimes in the Canada and Eurasian Basins and reduce exchange times between the two major basins of the Arctic Ocean (Karcher et al., 2012).

Particle Fluxes, shelf input and biological productivity
Biological productivity in the central Arctic Ocean and related particle fluxes are lower than in other oceans due to the perennial sea ice cover (Clark and Hanson, 1983). This is expected to change in the future when light limitation is relieved 15 by sea ice retreat . Arctic sea-ice extent is declining (Serreze et al., 2016) and ice is becoming thinner (Serreze and Stroeve, 2015). Biological productivity may increase and begin earlier in the year, at least in the Pacific part of the Arctic, depending on nutrient supply (Hill et al., 2017). Recent studies show that productivity is still low in the central Arctic Ocean, limited by both light and nutrient availability (Arrigo and van Dijken, 2015). Highest net community production (NCP) is found at the ice edge of the Nansen Basin and over the shelves, while the Amundsen Basin shows the 20 lowest NCP (Ulfsbo et al., 2014). Apart from the possible effect on NCP, the declining sea-ice cover will also enhance ice derived particle fluxes Boetius et al., 2013). The Arctic Ocean has the largest relative amount of shelves of all World Ocean, approximately 50% of area in total (Jakobsson, 2002). Shelf sediments and large volumes of riverine input add trace metals and carbon among other terrestrial components to Arctic shelf areas, some of which are transported to the central Arctic by the Transpolar Drift (TPD) (Wheeler et al., 1997;Rutgers van der 25 Loeff et al., 1995). On the basis of an increase of 228 Ra supply to the interior Arctic Ocean, Kipp et al. (2018) suggested that the supply of shelf derived materials is increasing with a following change in trace metal, nutrient and carbon balances.

Th as a tracer of water circulation and particle fluxes 30
Thorium isotopes have been extensively used to study and model physical oceanographic processes, such as advection, water mass mixing and particle flux (Bacon and Anderson, 1982;Rutgers van der Loeff and Berger, 1993;Roy-Barman, 2009;Rempfer et al., 2017). In seawater, 230 Th (t 1/2 =75380 yrs) is produced by the radioactive decay of dissolved 234 U. Without lateral transport by currents, the vertical distribution of 230 Th in the water column is controlled by reversible exchange with sinking particles and increases with depth (Bacon and Anderson, 1982;Nozaki et al., 1981). Deviations from a linear increase with depth profile of 230 Th (Bacon and Anderson, 1982) suggest that oceanic currents transport 230 Th away from the production area, or that ventilation, upwelling, or depth-dependent scavenging processes play a role for the 230 Th distribution 5 in the water column (e.g., Rutgers van der Loeff and Berger, 1993;Moran et al. 1995;Roy-Barman, 2009). 232 Th (t 1/2 =1.405x10 10 yrs) is known as a tracer for shelf/continental derived signatures (Hsieh et al., 2011), while 234 Th (t 1/2 =24.1 d) serves as a tracer for particle flux (Moran and Smith, 2000).

230 Th in the Arctic Ocean
Several studies have addressed the regional distribution of dissolved 230 Th in the Arctic Ocean in relation to particle fluxes 10 and water mass residence time over the past decades. Yet several key points related to removal processes of dissolved 230 Th are not entirely understood and the sensitivity of dissolved 230 Th to environmental changes is still not explained sufficiently. Bacon et al. (1989) hypothesized that scavenging of reactive elements in the central Arctic Ocean was significantly lower than in other parts of the world to explain the high 230 Th concentrations observed at the Alpha Ridge and the northern Makarov Basin (Bacon et al., 1989). Edmonds et al. (1998), later confirmed by Trimble et al. (2004), showed that 230 Th 15 activities in the deep southern Canada Basin were much lower, and residence times correspondingly shorter, than observed by Bacon et al. (1989) at the Alpha Ridge. Cochran et al. (1995) calculated residence times of dissolved 230 Th of 18-19 years in the central Nansen Basin and 10-12 years on the Barents Sea slope. 230 Th concentrations in the Nansen Basin were found to be lower than those from the Alpha Ridge reported by Bacon et al. (1989) and deep water in the central Nansen Basin had lower particulate and higher dissolved 20 230 Th concentrations than near the slopes (Cochran et al., 1995). Scholten et al. (1995) found that the shallower Eurasian Basin Deep Water (EBDW) is influenced by ventilation, in contrast to the deeper Eurasian Basin Bottom Water (EBBW) and suggested resuspension as the cause for the increased scavenging rates in the EBBW. Valk et al. (2018) showed that the deep Nansen Basin is influenced by volcanic and hydrothermal inputs that lead to scavenging removal of 230 Th over several years, at least episodically. 25 Sedimentary 231 Pa xs / 230 Th xs from the Canada Basin provided new insights into the relevance of scavenging removal and the horizontal redistribution of these tracers as well as the fractionation between the low productivity, sea ice covered interior basins and the seasonally high particle flux areas at the margins. Low surface sediment 231 Pa xs / 230 Th xs ratios were interpreted as a result of chemical fractionation of 230 Th and 231 Pa in the water column resulting in preferred 231 Pa export out of the Arctic. Almost all of the 230 Th produced in-situ (ca. 90 %) was estimated to be removed within the Arctic by scavenging onto 30 particles (Moran et al., 2005), while Hoffmann et al. (2013) suggested that the deep waters of the Arctic are exchanged through the Fram Strait on centennial timescales. (2009) presented a boundary scavenging profile model, showing that linear 230 Th concentration profiles do not necessarily imply that circulation is negligible. They suggested that the difference between the Arctic and other oceans is a considerable lateral transport of 230 Th from the interior to the margins.

Motivation
Global warming is triggering profound changes in the ocean, and the Arctic Ocean is especially vulnerable to such 5 environmental forcing. Summer ice cover is rapidly declining, while the supply of terrestrial material (Günther et al., 2013) and particle flux (Boetius et al., 2013) increases and ocean circulation is changing (Karcher et al., 2012). These developments are expected to leave an imprint on the distribution of particle-reactive radionuclides, such as Th isotopes. A central motivation for this GEOTRACES study is to use the Th isotopes to depict changes in circulation and particle fluxes in the Arctic Ocean from 1991 to 2015. The basis of this study is a time series consisting of natural radionuclide data from 10 various previous studies combined with new data from 2007 and 2015.

Sampling and analysis of Th in samples collected in 2007
Sea water samples were filtered directly from the 24 L CTD-Niskin ® bottles into acid cleaned cubitainers (LDPE) using 0.45 µm pore size Acropaks ® . Samples were collected in volumes of 1 L, 2 L, and 10 L and acidified with concentrated ultraclean 15 HNO 3 . Samples for the analysis of total 230 Th were taken without filtration. Analyses were performed at the University of Minnesota, Minneapolis, following methods from Shen et al. (2003). Measurements were done using Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Thermo Finnigan, Neptune) equipped with a Secondary Electron Multiplier (SEM) and a Retarding Potential Quadrupole (RPQ) energy filter.

Sampling and analysis of dissolved Th samples collected in 2015 20
Samples were filtered directly from the 24 L CTD-Niskin ® bottles into cubitainers (LDPE) through 0.45 µm pore size Acropaks ® in volumes of 10 L (>2000 m) and 20 L (<2000 m), according to the expected concentrations (Nozaki et al., 1981). Acropaks ® were used for half of the cruise and then replaced by new ones. Subsequently water samples were acidified to a pH of 1.5-2 by addition of 1 mL (acid)/L (seawater) of concentrated double distilled HNO 3 .
Preconcentration and analysis of 230 Th and 232 Th were performed following GEOTRACES methods in clean laboratories of 25 the Alfred-Wegener-Institute (AWI), (Anderson et al., 2012).
Samples were spiked with 229 Th and 236 U, calibrated against the reference standard material UREM11, a material in radioactive equilibrium (Hansen et al., 1983), followed by addition of a purified Fe-carrier solution (FeCl 3 ). The next day, the pH of the samples was raised to 8.5 by adding double-distilled NH 4 OH, to induce Fe(OH) 3 precipitation. After 72 h, when the Fe(OH) 3 had settled to the bottom of the cubitainer, the precipitate was transferred from the cubitainers to acid 6 cleaned 1 L Teflon ® bottles, after syphoning off the supernatant water. After dissolution of the sample in concentrated HCl, the pH was raised again to 8.5 to allow the Fe(OH) 3 precipitate and settle. The supernatant water was syphoned off and the preticipate was transferred into acid cleaned 50 mL Falcon ® tubes the following day. The samples were then washed by centrifugation four times at 4000 rpm for 12 minutes, where the supernatant was decanted before addition of new ultrapure Milli-Q ® water. Finally, the precipitation was dissolved in concentrated HCl and evaporated to a drop (>10 µL) in an acid 5 cleaned 15 mL Savillex ® beaker. After evaporation, the fractions of Pa, Th, U and Nd were separated using chromatographic columns filled with anion exchange resin (AG1X8, 100-200 mesh) according to GEOTRACES methods (Anderson et al., 2012). All fractions were collected in acid cleaned 15 mL Savillex ® beakers and columns were washed and conditioned before the samples were loaded onto the columns using concentrated HCl and HNO 3 .
Procedural blanks for 230 Th and 232 Th were run with each batch of 10-15 samples. Average 230 Th and 232 Th blank corrections 10 are 0.24 fg/kg and 0.003 pmol/L, respectively. At station 81, a sample (2000 m) was divided into two samples and resulted in different dissolved 232 Th concentrations, probably due to Th attached to the walls of the original cubitainer. Here, an average value considering the volumes of both parts of the divided samples was calculated.

Sampling and analysis of particulate 234 Th samples collected in 2015
Particulate samples were taken using in-situ pumps (McLane and Challenger Oceanic). 268 L to 860 L seawater were 15 pumped through a 142 mm ᴓ, 0.45 µm pore size Supor ® (polyether sulfone) filter (Anderson et al., 2012). Filters were cut aboard for subsamples under a laminar flow hood using tweezers and scalpels. Subsamples (23 mm ᴓ) were dried, put on plastic mounts, covered with Mylar and aluminium foil and directly measured by beta decay counting of 234 Th for at least 12 h. Six months later, background measurements were performed at the AWI in Bremerhaven.

Model 20
The model of Rutgers van der Loeff et al. (2018) was used to analyse the downward propagation of a ventilation signal in the Atlantic layer by settling particles and radioactive ingrowth. The 230 Th model is based on the reversible exchange model of Bacon and Anderson (1982) and Nozaki et al. (1981) and solved with programming language R. We first let the 230 Th model run with the base parameters as given for the Amundsen Basin in Table 1 Table 1.

Dissolved 230 Th in 1991, 2007 and 2015 10
Data obtained in 1991 by Scholten et al. (1995) constitute the baseline for the time series presented in this study ( Fig. 2A).

Particulate 234 Th from 2015 5
Particulate 234 Th from 2015 is shown as the relative amount of particulate 234 Th (Fig. 2D) compared to total 234 Th, calculated from 238 U activities, assuming equilibrium of total 234 Th with 238 U in deep water (Owens et al., 2011). All profiles show rather low concentrations of particulate 234 Th in the Amundsen Basin. Especially below 2000 m particulate 234 Th is much higher in the Nansen Basins (Valk et al., 2018).  . This difference is larger than the concentration range for the three 2015 profiles ( Fig. 2A). The three stations from 2015 (81, 117 and 125) are distributed over a wide area of the Amundsen Basin 15 (Fig. 1). Because all stations show lower concentrations in 2015, this points to a temporal rather than a regional variability over the entire basin. The decrease in dissolved 230 Th in the Amundsen Basin started after 2007, considering the similar concentrations in 1991 and 2007. Dissolved 230 Th decreased by 0.32 fg/k/y in 300-500 m water depth, and by 0.52 fg/kg/y in 1000-1500 m. 230 Th is known to respond to particle fluxes as well as ocean circulation (Anderson et al., 1983b, a). A reduction in dissolved 230 Th concentrations can therefore be caused by either increased scavenging (Anderson et al., 1983b) 20 or by changing circulation (Anderson et al., 1983a).

Scavenging in the central Amundsen Basin
Biological production in the central Arctic Ocean in 2011 was not higher than in 2007 (Ulfsbo et al., 2014). Therefore, enhanced biological production in the Amundsen Basin and subsequent sinking particles can be excluded as a reason for the changing Th distributions. Enhanced scavenging by lithogenic material at these stations can also be excluded because for all 25 three stations from 2015, dissolved 232 Th values at 1000 m are in the same range or lower than observed in 2007 (Fig. 2C).
Low dissolved 232 Th is taken here as an indicator of low amounts of lithogenic material. Enhanced particle loads would result in high concentrations of particulate 234 Th, as observed in the deep Nansen Basin where particulate 234 Th ranges between 3.3 and 9.1 % of total 234 Th (Valk et al., 2018). In the Amundsen Basin only station 125 (2015), located at the slope of the Lomonosov Ridge shows relatively high values of particulate 234 Th in the deep water from 1500m downwards (Fig.  30  2D). This feature could be explained by the resuspension of slope sediments along the Lomonosov Ridge, as no increased scavenging was observed in the deep Amundsen Basin (Slagter et al., 2017). Slagter et al. (2017) argue that similar riverine surface influence of humic substances in the Amundsen Basin and in the Makarov Basin did not lead to increased scavenging at depth in the Amundsen Basin, even at stations influenced by the TPD (e.g. station 125) (Slagter et al., 2017;. This is in contrast to the Makarov Basin, where they observed a slight increase of 5 dissolved Fe-binding organic ligand concentrations, and reduced dissolved Fe concentrations that may point to more intense scavenging or lower Fe inputs (Slagter et al., 2017;Klunder et al., 2012), while the high 232 Th observed at the surface of station 125 points to a notable continental component, a signal that is not observed below (Fig. 2C)

500-1500 m: Intermediate Water mass changes
The decrease of dissolved 230 Th at depths between 500 m and 1500 m for stations 81, 117 and 125 in the Amundsen Basin 15 (2015) is most prominent at 1000 m, where concentrations decreased to half of the value in 2007 ( Fig. 2A). This depth range in the Amundsen Basin is ventilated on considerably shorter time scales than in the Nansen and Makarov Basin by a westward boundary circulation (Tanhua et al., 2009).
The drop in dissolved 230 Th at 1000 m corresponds to an increase in the 129 I/ 236 U ratio (Figure 3), implying a higher Atlantic influence of younger waters   Edmonds et al., 2004;this study). Moreover, intrusion of Canada Basin water would not match the ventilation age estimated by , since the Canada Basin water is known to be much older than Amundsen Basin water at this depth (Tanhua et al., 2009). Hence, it is suggested that the changes in the Amundsen Basin cannot be explained by interaction with the Makarov Basin. On the contrary, salinity distributions imply that the influence of Atlantic waters in the Amundsen Basin has increased at 500-1500 m by 2015 indicating that water masses have 5 changed after 2007 (Fig. 2B). Figure 2B shows salinity profiles for three stations from the Amundsen Basin from 2007 (Schauer andWisotzki, 2010), three from 2015 (Rabe et al., 2016), one from 1994 (Swift, 2006a) and one from 1991 (Rudels, 2010). In 2015, the intermediate waters of the Amundsen Basin have a stronger Atlantic contribution (Polyakov et al., 2017;Rabe et al., 2016). This change is correlated with the decrease in dissolved 230 Th.
Anthropogenic tracers can help determine whether the increased Atlantic water contribution had resulted in increased

230 Th removal process in intermediate waters on circulation pathways
In order to judge the scavenging intensity it is useful to compare dissolved 230 Th concentrations at various locations along the flow paths of the Atlantic waters. Arctic Intermediate Water (AIW) is comprised of water from the Greenland Sea and the Nordic Sea via the West Spitzbergen Current (WSC) (Rudels, 2009) (BSBW). These pathways are influenced by an increased input of terrestrial matter (Günther et al., 2013) and/or increased primary production at the shelf and the ice edge compared to previous years (Arrigo and van Dijken, 2015;Ulfsbo et al., 2018). Relatively high concentrations of Fe at the margin indicate the possibility of enhanced scavenging by iron oxides (Rijkenberg et al., 2018). 30 At station 400, located at the south eastern margin of the Eurasian Basin, the deepest water is in the influence of BSBW, downstream of the Barents and Kara Sea shelf and slope. At the largest depth of ~1200m, 230 Th the concentration is low and similar to concentrations in the central Amundsen Basin in 2015. This is consistent with the hypothesis that Atlantic waters that were depleted in 230 Th on the shelf contribute to the decrease in dissolved 230 Th in the central Amundsen Basin. Such a relic scavenging signal implies that scavenging occurs on pathways of inflow waters along the shelves rather than locally within the central basin. The high surface values of dissolved 230 Th at station 400 are in line with low export production at this station compared to shallower stations over the shelf (Cai et al. 2010).
Hence, the observed reduction in dissolved 230  deeper Atlantic inflow. The closer the stations are to the Lomonosov Ridge, the younger the ventilation age (Fig. 5), and the more the salinities are shifted towards Atlantic values. Variability in temperature and salinity plots indicates that this branch interacts with ambient waters (Rudels et al., 1994). This is consistent with dissolved 230 Th concentrations observed at stations 81, 117 and 125 (2015), with station 125, located in the TPD and closest to the Lomonosov Ridge, showing the lowest concentrations. The low 230 Th concentrations at station 125 may also be affected by additional scavenging due to 15 resuspension on the slope of the Lomonosov Ridge.

Vertical transport of circulation derived 230 Th scavenging signal and effects in deep waters
Intermediate waters in the central Amundsen Basin have a lower dissolved 230 Th in the depth range up to 1500 m, due to increased scavenging during transport of Atlantic water over the shelves and along the slope. The time series data also reveal changing conditions below the intermediate waters, indicated by a decrease of dissolved 230 Th in the deeper water column 20 ( Fig. 2A).
This raises the question as to whether a change, as observed for 500-1500 m, might cause a decrease in concentrations in the water column below that depth within just 8 years. Theoretically, such a decreasing signal could be manifest by sinking particles via reversible scavenging of sinking particles. With particle settling rates of 582 m/y (Rutgers van der Loeff et al. exchange process used to introduce the ventilated water mass is not meant to reproduce the actual ventilation with water from Kara/Barents Seas, but merely serves the purpose to create a rapid reduction of 230 Th in the upper 1500m in order to model the downward propagation of such a signal by reversible scavenging. The model results in figure 6 show how fast a decrease of 230 Th in the ventilated layer (500-1500 m) is propagated into the deep water. Uncertainties in the model assumptions, such as particle sinking speed and exchange between dissolved and particulate phases might cause the difference between model and data. This may also explain why the downward penetration of the ventilation signal is slower 5 in the model, where it has not yet reached the seafloor after 8 years (Fig. 6) than in the observed data. But the model results underpin the notion of a dissolved 230 Th decrease due to circulation and scavenging along the circulation pathways, and account for the reduction of dissolved 230 Th below the circulation influence. This temporal change can therefore be explained by a significant reduction in the input of low-230 Th waters from shallower depths, even if the scavenging rate in the deep basin remains constant. 10 Hydrothermal plumes released by volcanoes at the Gakkel Ridge could also decrease dissolved 230 Th efficiently and periodically, as suggested by Valk et al. (2018) for the deep Nansen Basin. However, these plumes probably do not affect the Amundsen Basin as much as the Nansen Basin, due to recirculation in the Nansen Basin that retains most of the hydrothermal plume affected waters in the Nansen Basin (Valk et al., 2018). Additionally, the depths where the major changes occurred in the Amundsen Basin are too shallow (the hydrothermal scavenging starts below 2000 m) and the deep 15 water decrease of dissolved 230 Th in the Amundsen Basin since 2007 is much weaker than in the Nansen Basin (Valk et al., 2018).

Conclusion
Concentrations of dissolved 230 Th throughout the entire water column in the Amundsen Basin decreased since 2007. There is no indication of increased scavenging removal of 230 Th due to increased particle flux within the Amundsen Basin. An 20 increase in salinity of intermediate water (at 500 -1500m) points to the influence of Atlantic derived waters, though SF 6 data suggest that the ventilation of this layer has not increased. The reduction in dissolved 230 Th concentration in the Amundsen Basin intermediate waters is therefore attributed to increased scavenging from source waters and transport of this relict scavenging signature by advection. Thus, these downstream waters reflect a scavenging history over the Siberian shelves and slope that results in a reduction of 230 Th relative to Atlantic source waters and, in turn, reduced dissolved 230 Th in the central 25 Amundsen Basin. The low-230 Th signal is propagated to deeper central Arctic Ocean waters by reversible scavenging. These findings highlight the close interaction of horizontal transport by advection and particle scavenging removal, which combine to generate far-field distributions of reactive trace elements.