2.1. Sampling and sample preparation
Sea ice observations of multi-year fast ice were performed during the summers of 2015–2019 as part of the 56–60th JAREs. Sea-ice cores were collected near Syowa Station (Figure 1, Table 1). Ice cores were sampled using an ice corer (Geo Tecs Co., Ltd., Chiba, Japan) with an internal diameter of 0.09 m. Cores were immediately placed in tubular polyethylene bags and kept horizontal in a cooler box along with refrigerants to maintain below-freezing temperatures and minimize brine drainage from the core. Fresh snow samples were collected with an acid-washed polycarbonate shovel and placed into polyethylene zipper storage bags. For comparison, four cores of first-year fast ice were also collected (Table 1).
Note that in Table 1, cores JARE58_KU3 and JARE59_2 were sampled only the top 2.78 and 2.47 m of ice, respectively, because the ice was too thick to sample the entire column. As part of the ice monitoring at Syowa Station, an electromagnetic (EM) ice thickness survey was conducted near our coring site in January 2018; the ice thickness was approximately 6 m.
Under-ice water was collected through the coring holes with a Teflon water sampler (GL Science Inc., Japan) from 1 m below the bottom of the sea ice during JARE60 (2019). Water samples were collected approximately 30 minutes after the ice core at JARE60_KU2 was drilled to avoid potential artifacts associated with the disturbance of drilling. All collected water samples were subsampled into (1) a 120-mL glass vial (Maruemu Co., Ltd., Osaka, Japan) for salinity measurement, (2) a 15-mL glass screw-cap vial (Nichiden-Rika Glass Co. Ltd, Kobe, Japan) for δ18O analysis, (3) a 10-mL polyethylene screw-cap vial (Eiken Chemical Co. Ltd, Tokyo, Japan) for measurement of major inorganic nutrient concentrations (NO3 + NO2, NH4+, PO43−, and Si(OH)4), and (4) a 500-mL Nalgene polycarbonate bottle (Thermo Fisher Scientific Inc., Waltham, MA, USA) for measurement of chlorophyll a (chl.a) concentrations. All samples were returned to the laboratory of the Japan Maritime Self-Defense Force icebreaker Shirase immediately after sampling. Samples for salinity and δ18O analyses were stored in a refrigerator at +4°C. Samples for nutrient analyses were stored in a freezer at −30°C. Samples for chl.a measurements were immediately filtered through 25-mm-diameter Whatman GF/F filters. Pigments on the filters were then extracted withN,N -dimethylformamide (Suzuki & Ishimaru, 1990) for more than 24 h and stored at −30°C until analysis.
Sea ice cores were stored in a freezer at −30°C during their transport to the Institute of Low Temperature Science (ILTS), Hokkaido University, Sapporo, Japan. In the ILTS cold room (−15℃), cores were split lengthwise into two halves with an electric band saw: the first half for ice textural analysis and the second half for measurements of bulk ice salinity, δ18O composition, nutrient and chl.a concentrations, and cell counting for ice algae community assemblage.