Dataset: Water column Beryllium-7 from samples collected on the MOSAiC expedition, PS122, on R/V Polarstern in the Central Arctic Ocean during 2019-2020

Methods of ⁷Be collection and analyses
Pumps were used to collect seawater within the upper 60 m of the ocean during legs 2-3. Typically ~1400-2100 liters were taken from a hydrohole through the ice at 1-2 depths below the mixed layer. Unfiltered seawater was drawn through the sampling hose to the surface where it was passed through iron-oxide impregnated acrylic fiber filters which adsorb the ⁷Be (Lal et al., 1988; Krishnaswami et al., 1972; Lee et al., 1991). A flow meter attached in-line to the filter compartment recorded the amount of seawater that passed through each filter. To maximize ⁷Be collection, two fibers filtering approximately 600- 700 L 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 (bringing the stable Be concentration to ~1 ppm), 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 400–700 L, extraction efficiencies are respectively, 82 ± 3% to 76 ± 2%. The uncertainty of the extraction efficiency (4%) and the detector efficiency (2%) was, in all cases, smaller than the statistical counting error and the uncertainty in the blank. For the mixed layer samples, water was collected from the ship's seawater intake (~8m) and collected in barrels from which the water was passed through the iron-oxide filters as described above. The fibers were dried and shipped to Florida International University. There, the fibers were ashed, placed in a Marinelli beaker and analyzed with a high purity germanium (HPGe) gamma detector (Kadko et al, 2016). The ⁷Be has a readily identifiable gamma peak at 478 keV. The detector is calibrated by adding a commercially prepared mixed solution of known gamma activities to an ashed fiber and counting it in the Marinelli geometry.