During the occupation of ice stations, samples from the mixed layer were acquired approximately 150m from the ship to mitigate disruption of surface water properties. A hydrohole was cut through the ice, followed by the lowering of a portable conductivity, temperature, depth probe (CTD) to determine mixed layer depth. A weighted sampling hose was then deployed to the center of the observed mixed layer. Remotely operated vehicle (ROV) observations confirmed that the weighted hose attained accurate deployment-depth measurements from the ice surface. A gasoline generator powered a centrifugal pump which drew water through the sampling hose to the surface where it was passed through iron-oxide impregnated Acrylic fiber filters. A flow meter attached in-line to the filter compartment recorded the amount of seawater passed through each filter. To maximize 7Be collection, two fibers filtering approximately 600L of seawater apiece were collected from each ice station depth and later combined. The efficiency of the fiber for extraction of Be from seawater was determined by adding stable Be atomic absorption standards to a drum containing seawater, pumping the water through an iron fiber cartridge, and at every 100 L measuring the Be content of the cartridge effluent. Based on several trials, it was found that for sample volumes in the range 400-700L, extraction efficiencies are respectively, 82 ± 3% to 76 ± 2%.
The same filtering apparatus was deployed over the side of the ship to collect 7Be from waters below the mixed layer, and from open-ocean mixed layers when not in the ice. At each ice station, a submersible in-line pump was deployed over the side of the ship to a depth of 40m for sampling below the mixed layer. This was used to fill two 1000L holding tanks, from which up to 700L of seawater was passed through one fiber filter each as was done with the mixed-layer samples. For open-water sampling, the ship’s seawater system drew water from the mixed layer at a depth of 8m and filled two holding tanks for immediate filtration. All fibers were returned to the lab where they were dried, ashed, and placed in a Marinelli beaker, which in turn was placed over a low background germanium gamma detector. 7Be has a readily identifiable peak at 478keV. The detector is calibrated for these samples by adding a commercially prepared mixed solution of known gamma activities to an ashed fiber and counting it in the Marinelli geometry.
Melt pond water, ice, and snow samples were collected as well. Melt pond samples consisted of 50L of water from undisturbed ponds which were typically 50cm deep. Ice core samples were taken from the ice surface to seawater, producing a core 14cm in diameter and generally 2m in depth. When snow was present, it was removed from the surface of the ice prior to ice coring. All samples were melted and received 5mL of concentrated HCl, 5mL of FeCl in solution, and 0.5mL of a stable Be tracer. After 12 hours of equilibration, a concentrated NaOH solution was slowly added to coprecipitate the 7Be with Fe(OH)3. The precipitate was returned to the lab where it was dried, placed in Petrie dishes, and counted by gamma spectroscopy calibrated for this geometry. The precipitate was redissolved in dilute HCl, and analyzed for stable Be by atomic absorption to calculate recovery yields.
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