2.2. Textural analysis and the age of an ice core
We examined ice thin sections to analyze ice growth processes. Ice sections 0.7 cm thick were attached to glass plates and planed down to 0.1-cm thickness with a microtome (Model SM2400, Leica Microsystems, Wetzlar, Germany). We photographed ice crystallographic structures by illuminating the thin sections placing on a light table in between the cross-polarized sheets.
Ice growth processes were examined based on our classification of the ice cores as granular, columnar, or mixed ice (Eicken & Lange, 1989). Granular ice was subdivided into frazil ice and snow ice (or superimposed ice) based on its δ18O value. We defined snow ice or superimposed ice as granular ice with a δ18O value below that of its parent seawater (see Granskog et al., 2004; Jeffries et al., 1994), taken as the δ18O value of the under-ice seawater sampled during JARE60 (−0.7‰). Although coarse-grained clear ice (large, polygonal, granular crystal textures) and fine-grained opaque ice originate from superimposed and snow ice, respectively (Kawamura et al., 1997), we hereafter refer to both as “snow-origin ice” unless obviously clear ice was observed in thin section, which was described as superimposed ice. We note that the analyzed snow was thawed and moistened, and refrozen ice was observed. The snow crystal structure was an aggregate of fine particles in thin section but did not contain salt (salinity = 0); the structure sometimes became sandwiched between superimposed ice, which we refer to as ”fine-grained superimposed-ice” (see details in Section 3.1).
The ice type was determined according to WMO (2014) after checking satellite imagery of the sampling point, visually observing the sea ice condition at Syowa Station, and confirming the vertical profile of textural parameters (including thin section analysis) in the core. First-year ice was collected in locations where fast ice had broken up and flowed out during the previous year (Figure 1d–f), and sea ice had reformed within the year prior to collection. According to Syowa Station records, multi-year ice has not been broken up and flowed out since March 1980 (Higashi et al., 1982; Ushio, 2006).