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Transverse sections of Baltic Sea cod otoliths from four different time periods: the Neolithic (A), late Iron Age/Middle Age (B), early 1990s when the areal extent of hypoxia was low (C), and 2001 when it was high (D). Regions in the core correspond to the juvenile stage. Note that Mn is high in the Neolithic and Iron Age otoliths in small cracks as well as along the edge, where it was in contact with soil pore water. The 1991 otolith is portrayed in a partial map. (Scale bar: 1 mm.)
