The Baltic sea-level oscillations in the Atlantic and Subboreal periods are known from sedimentary records on slightly uplifting coasts in Denmark and southern Sweden (e.g., Berglund, 1971; Digerfeldt, 1975). The periodicity of those oscillations is in close correlation with recent data on climatic cycles with a periodicity of 1500, 1000, and 550 yr (e.g., Stuiver et al., 1995; Chapman and Shackleton, 2000). The effects of regional eustatic oscillations in the coastal area of the Southern Baltic are poorly known because of the slightly subsiding coast and the magnitude of barographic and storm surges, which are larger than those of eustatic oscillations.

To solve these problems, sediment sequences on the western coasts of the Puck Lagoon (northwestern part of the Gulf of Gdańsk) were investigated. Recent vertical movements of Earth's crust are ∼0.0 to −0.5 mm/yr. According to ¹⁴C datings, pollen and diatom analyses, the earliest marine influences occurred in the Middle Atlantic period, while at the end of the Atlantic period almost all of the area was occupied by brackish waters (Kramarska et al., 1995; Uścinowicz and Miotk-Szpiganowicz, 2003). Several sediment cores were taken along the western coast of the Puck Lagoon. Ordinates of the collected cores were geodetically determined to ± 1 cm relative to the mean sea level. Peat, plant remains, and marine shells (64 samples) were dated using the classic ¹⁴C and AMS (accelerator mass spectrometry) methods. The plot showing the altitude of peat and marine mollusk shells versus their radiocarbon age shows that during the Subboreal and Subatlantic periods water levels in the Puck Lagoon were as follows: 5 ka B.P.—2.8 m, 4 ka B.P.—1.8–1.9 m, 3 ka B.P.—1.3–1.4 m, 2 ka B.P.—0.8–0.9 m, and 1 ka B.P.—0.4–0.5 m b.s.l. The rises in sea level during the Subboreal and Subatlantic periods may have been cyclical and related to climatic oscillations. The periodicity of water-level changes was ca. 1000 yr and their amplitude was ∼0.3–0.5 m. This is also confirmed by pollen and diatom analyses.