shelf-and river-derived elements to the central Arctic Ocean • The TPD is rich in dissolved organic matter (DOM), which facilitates long-range transport of trace metals that form complexes with DOM • Margin trace element fluxes may increase with future Arctic warming due to DOM release from permafrost thaw and increasing river discharge

The first full transarctic section of 228 Ra in surface waters measured during GEOTRACES cruises PS94 and HLY1502 (2015) shows a consistent distribution with maximum activities in the transpolar drift. Activities in the central Arctic have increased from 2007 through 2011 to 2015. The increased 228 Ra input is attributed to stronger wave action on shelves resulting from a longer ice-free season. A concomitant decrease in the 228 Th/ 228 Ra ratio likely results from more rapid transit of surface waters depleted in 228 Th by scavenging over the shelf. The 228 Ra activities observed in intermediate waters (<1,500 m) in the Amundsen Basin are explained by ventilation with shelf water on a time scale of about 15-18 years, in good agreement with estimates based on SF 6 and 129 I/ 236 U. The 228 Th excess below the mixed layer up to 1,500 m depth can complement 234 Th and 210 Po as tracers of export production, after correction for the inherent excess resulting from the similarity of 228 Ra and 228 Th decay times. We show with a Th/Ra profile model that the 228 Th/ 228 Ra ratio below 1,500 m is inappropriate for this purpose because it is a delicate balance between horizontal supply of 228 Ra and vertical flux of particulate 228 Th. The accumulation of 226 Ra in the deep Makarov Basin is not associated with an accumulation of Ba and can therefore be attributed to supply from decay of 230 Th in the bottom sediment. We estimate a ventilation time of 480 years for the deep Makarov-Canada Basin, in good agreement with previous estimates using other tracers. RUTGERS VAN DER LOEFF ET AL. 4853

The Fram Strait is the only deep connection between the Arctic and Atlantic Oceans. The main water and mercury (Hg) fluxes between these oceans occur via the Fram Strait and Barents Sea Opening. Several Hg mass balance studies indicated a net Hg export from the Arctic to the Atlantic Ocean. However, in the absence of Hg measurements in the Fram Strait and Barents Sea Opening, these estimates were based on North Atlantic and central Arctic Ocean data alone. Here, we refine the Arctic total Hg (tHg) and methylated Hg (MeHg) mass budgets using new data acquired during the 2015 GEOTRACES (section GN04) TransArcII cruise in the Barents Sea Opening and the 2016 GEOTRACES (section GN05) GRIFF cruise, which covered the Fram Strait and Northeast Greenland Shelf. Total Hg increased westward along the Fram Strait transect, reaching the highest concentrations on the Northeast Greenland Shelf. Concentrations of tHg averaged 1.29 ± 0.43 pM in the East Greenland Current, while core waters of the West Spitsbergen Current had average values of 0.80 ± 0.26 pM. Using our new data, we estimate that 43 ± 9 Mg y-1 of tHg is transported to the Arctic Ocean in the core of the West Spitsbergen Current, while 54 ± 13 Mg y-1 of tHg is exported from the Arctic Ocean in the East Greenland Current and Recirculated Atlantic Water. This results in a net tHg export of 11 ± 8 Mg y-1 via the Fram Strait. We find a shallow MeHg maximum (at 150 m depth) in the East Greenland Current, in agreement to what was reported for the central Arctic Ocean and Canadian Arctic Archipelago. The West Spitsbergen Current is characterized by lower MeHg concentrations and a deeper MeHg maximum, that is located at approximately 1000 m depth. We estimate a net MeHg export of 6 ± 2 Mg y-1 from the Arctic Ocean via the Fram Strait, which is nearly half of the exported tHg. Most of the exported MeHg is in the form of DMHg (2:1 ratio of dimethylmercury to monomethylmercury). Previous studies reported lower MeHg proportions. Our observations show that the Arctic Ocean is producing and exporting MeHg to the Atlantic Ocean. In total, the Arctic Ocean exports about 18 Mg y-1 of tHg to the Nordic Seas and North Atlantic via the Fram Strait and Davis Strait, of which 7.5 Mg y-1 is in the MeHg form.

In this study we present dissolved and particulate 230 Th and 232 Th results, as well as particulate 234 Th 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 230 Th 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 234 U and subsequently removed by particle scavenging. We show that observed changes in 230 Th concentrations compared to earlier times are related to submarine volcanic eruptions. We use 230 Th 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 230 Th. Thus, when iron particles sink they remove 230 Th 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.

High mercury (Hg) levels have been observed for arctic biota despite limited local sources of anthropogenic Hg in the Arctic. Scavenging of Hg exerts an important control on the residence time of Hg in surface waters. The downward Hg export flux, as well as Hg burial rates in bottom sediments, are not well constrained due to the lack of particulate Hg (pHg) observations in the Arctic Ocean. Here we estimated downward Hg export flux based on Hg concentrations in suspended particulate matter (SPM) and by using the radionuclide pair 234 Th/ 238 U, coupled to pHg/ 234 Th ratios in particles. Using new observations made during the German GEOTRACES TransArcII (GN04) and the U.S. Arctic GEOTRACES (GN01) cruises in August-October 2015, we estimated the pHg export flux in the central Arctic Ocean and the outer shelf. We find that 81 ± 58 Mg y-1 pHg are exported from the upper 100 m, of which 16 ± 10 Mg y-1 are ultimately buried in marine sediments. An extrapolation to the entire Arctic Ocean, including the inner shelf, results in 156 Mg y-1 pHg export from the surface ocean and 28 Mg y-1 Hg burial rate. Our study shows that the pHg export flux could be higher than previously thought and this should be taken into consideration for future arctic Hg budget estimations.