Strontium isotopic composition (87Sr/86Sr) data from subfossil 14C-dated mollusk shells in raised beach sediments are used as a paleosalinity proxy in the brackish Baltic Sea, the precision (±5%) and accuracy (±0.7‰) of the method being judged from replicate analyses of modern shells. Paleosalinity data with an average time resolution of ∼200 yr for the period 7130–2775 calibrated 14C yr B.P. indicate maximum surface salinities of 10‰–11‰, 11‰–12‰, and 12‰–13‰ for the Bothnian Bay, Bothnian Sea, and Baltic Proper (the three major Baltic subbasins). The relative salinity differences between the basins were small (≤30%) compared to the as much as eightfold present-day relative salinity differences (Bothnian Bay 1‰–3‰; Bothnian Sea 4‰–5‰; Baltic Proper 6‰–8‰). Late Holocene freshening (ca. 3000 calibrated 14C yr B.P. to present) is most pronounced in the northernmost subbasin, the Bothnian Bay, consistent with the absence of a permanent halocline, sequestering of phosphorus in well-oxygenated bottom sediments, and phosphorus limitation of primary production in the present-day Bothnian Bay. This study suggests that paleosalinity data may be crucial to improving our understanding of the possible effects of any future, climate-induced freshening of the Baltic Sea.

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