| 1. |
- Charette, M. A., et al.
(författare)
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The Transpolar Drift as a Source of Riverine and Shelf-Derived Trace Elements to the Central Arctic Ocean
- 2020
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Ingår i: Journal of Geophysical Research-Oceans. - : American Geophysical Union (AGU). - 2169-9275 .- 2169-9291. ; 125:5
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Charette, M, et al.
(författare)
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The Transpolar Drift as a Source of Riverine and Shelf‐Derived Trace Elements to the Central Arctic Ocean
- 2020
-
Ingår i: Journal of Geophysical Research - Oceans. - 2169-9275 .- 2169-9291. ; 125, s. 1-34
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Tidskriftsartikel (refereegranskat)abstract
- 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 openocean 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(106m3 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 hydrologicc ycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.
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| 3. |
- Gdaniec, Sandra, et al.
(författare)
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231Pa and 230Th in the Arctic Ocean : Implications for boundary scavenging and 231Pa-230Th fractionation in the Eurasian Basin
- 2020
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 532
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Valk, Ole, et al.
(författare)
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Decrease in Th-230 in the Amundsen Basin since 2007 : far-field effect of increased scavenging on the shelf?
- 2020
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Ingår i: Ocean Science. - : Copernicus GmbH. - 1812-0784 .- 1812-0792. ; 16:1, s. 221-234
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Valk, O., et al.
(författare)
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Importance of Hydrothermal Vents in Scavenging Removal of Th-230 in the Nansen Basin
- 2018
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 45:19, s. 10539-10548
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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