Background and aim of the project

About one third of the world’s soil carbon is accumulated within the Arctic region. Arctic rivers make up about 10% of the global river water runoff and contain high concentrations of dissolved and particulate organic carbon. A climate change-driven increase in the terrestrial organic carbon input to the Arctic Ocean – through permafrost thawing, increased river runoff and/or accelerated coastal erosion – could dramatically alter the carbon budget and biogeochemical cycles, causing positive/negative feedback to global change. We proposed to study the transport and fate of biogeochemically important constituents transported from the Eurasian continent into the Arctic Ocean, on the original planned route of leg 1 along the Northeast Passage, passing the major Siberian Rivers. This project included six cruise participants and aimed at using multiple techniques, including mineralogy, major and minor elements, carbon forms, molecular biomarkers of vascular plants, stable and radiogenic isotopes (of C, N, O, Fe, Nd, and Sr) and compound-specific isotope analysis (¹⁴C) of plant biomarkers. This multiproxy approach aimed at obtaining a detailed  picture of both degradation and transport processes of organic and inorganic material released from the tundra and taiga to the Russian-Siberian continental shelf. Given the last minute changes in the expedition, altering the leg 1 route to go through the Northwest Passage and the Canadian Archipelago on the way to the Beringia region, our original project could not be undertaken and a substantially scaled down project during legs 1 and 2 was quickly launched.

The unusually large fresh water input to the Arctic Ocean, in relation to its size, makes it an ideal area for examining the fate of terrestrial weathering products transported via rivers to the ocean in the dissolved phase or associated to particles. Particles in rivers can have a size range from a few nanometres (colloids) up to several micrometres. Large particles mostly undergo sedimentation in the shelf areas, whereas small colloids and dissolved ions may be transported long distances in the ocean. Thus speciation (dissolved ions or incorporated in particles) directly affects the bioavailability of major and trace elements in the ocean. The study of speciation involves determination of the size distribution and chemical composition of material suspended or transported in water.

Long-lived radioisotopes (e.g. Nd and Sr) can be used to determine the terrestrial sources of both dissolved and particulate material in water as well as sources for sediments found on the Arctic Sea ice. Short lived radionuclides (e.g. ²¹⁰Po, ²¹⁰Pb, and ⁷Be, and ²³⁴Th) are also used to examine transport and fate of water transported particles and sediments. Dr. Mark Baskaran, who has extensive experience in short lived radionuclide analysis, participated in leg 2 and sampled both water and ice rafted sediments in order to investigate the behavior of these nuclides in the Arctic.

The primary objectives of the project during leg 1 and leg 2 include:

• Source apportionment using trace elements, stable isotopes and long-lived radioisotopes in particles and water from along the Northwest Passage and shelf region around Bering Strait

• Investigating the biogeochemical cycling of carbon and particle-reactive nuclides in the Arctic shelf

• Examining the bioavailability (dissolved fraction) of trace elements on the Arctic shelf

• Investigating the role of sea ice sediments in the removal of particle-reactive radionuclides from the surface waters of the Arctic Ocean.

Fieldwork

During the cruise water was sampled using the seawater intake on the icebreaker Oden, which continuously can sample seawater from 8 m depth below the ships hull. Collection of water from depth profiles was done using the CTD rosette equipped with water sampling bottles (picture 1). After sampling the water was transferred to the ship’s laboratory for different types of filtration processes and experiments. The methods for speciation during this expedition included:

• Membrane filtration – Large particles (> 0.22 μm) are separated from the water. The filters can be analyzed for the particulate concentration, whereas the filter passing is analyzed for dissolved phase and colloidal nanoparticles.

• Ultrafiltration – This method enables a fine pore-sized membrane of 3 000 Dalton (molecular weight) nominal cut-off to enrich colloidal particles in a solution.

• Diffusive gradients in thin films (DGT) – This method is a passive in situ sampling device for dissolved metals. The concentration measured by the DGT represent dissolved ions and very small complexes. Large particles and colloids are excluded.

These three methods are used to separate out different size fractions. The separated fractions are preserved and brought back to onshore laboratories for determination of organic carbon, isotopes and trace elements.

Alaska through the Northwest Passage, a unique opportunity was offered to examine a rarely studied, and to a large extent unknown, part of the Arctic: the Canadian Archipelago. It is known that water from the Central Arctic Ocean and the Beaufort Sea is transported through the archipelago (e.g. via Lancaster Sound) to Baffin Bay. In these narrow and shallow straits, water flow may cause re-suspension and transport of deposited sediments to other locations. However, no detailed quantification for particle transport and metal speciation is currently available. Depth profiles were collected both in the north part of the Baffin Bay and in the central Lancaster Sound. The Northwest Passage also presented a much appreciated chance to sample the Mackenzie River fresh water plume. The Mackenzie is one of the largest rivers in the Arctic in terms of discharge. This could be clearly observed during the passage of the Mackenzie as the salinity at 8 m depth decreased from around 30‰ to a minimum of about 12‰ at a distance of more than 100 km from the river mouth (figure).

Although only about 50 m deep, Bering Strait is important for water exchange between the Arctic and the Pacific Ocean. Water exchange through the strait is complicated and believed to be controlled mainly by passing weather systems. Currents can alter their direction and can also be in different directions on the two sides of the strait. In general the currents are northerly in the eastern strait and southerly in the western. In addition a low salinity coastal current in the eastern strait supplies the Arctic with a large input of fresh water. During leg 2 sampling of water from vertical profiles on the shelf on both sides of Bering Strait was conducted. A vertical profile with samples down to 1 450 m was collected in the more central part of the Arctic Ocean (~77°N, north of Wrangel Island). The water samples from the Bering Strait area will be analyzed for Nd-isotopes and the influence of Pacific Water in the Arctic Ocean will be estimated.

During leg 2 large volume water samples were filtered for studies of the partitioning of ²¹⁰Po, ²¹⁰Pb, and ⁷Be and ²³⁴Th between the particulate and dissolved phases. Pre-concentration of the water samples was conducted onboard using one of the three methods:

1. Passing water samples through a pre-filter and MnO₂-coated polypropylene cartridge filters (for particulate and dissolved Th isotopes, particulate ⁷Be and ²¹⁰Pb and activity ratios of ²²⁸Ra/²²⁶Ra in the dissolved phase).
2. The Fe(OH)₃ precipitation method (for ²¹⁰Po, ²¹⁰Pb, and ⁷Be).
3. Filtering through “fluffy fiber” (acrilon coated with MnO₂ for specific activity of ²²6Ra).

In addition, sea ice samples from two stations were collected. At the second station we collected from seven sites for the measurements of ⁷Be, ²¹⁰Po, ²¹⁰Pb, and ²³⁴Th.

Preliminary results

The samples collected during the Beringia 2005 will be analyzed and the result interpreted by Dr. Ralf Dahlqvist during a two-year Marie Curie Fellowship at the Department of Earth Sciences, University of Oxford.

Preliminary results for the short lived radionuclides that must be analyzed soon after collection indicate the following:

1.  The activities of excess ²¹⁰Pb are about 1–3 orders of magnitude higher than what we find in the bottom sediments, similar to an observation reported recently (Baskaran 2005).
2. The activities of ⁷Be in the sea ice sediments are about 2–4 orders of magnitude higher than those found in bottom sediments, again indicating a large portion of ⁷Be are derived from surface water and direct atmospheric deposition. 
3. There is considerable inhomogeneity on the activities of ⁷Be and ²¹⁰Pb in ice-rafted sediments collected from a given station.
4. A comparison of the concentrations of these nuclides in the water column to that in the ice-rafted sediments and suspended particulate matter provides insight into the mechanisms of transport of sea ice sediments.