Strontium isotopic composition of modern and Holocene mollusc shells as a palaeosalinity indicator for the Baltic Sea

The 87Sr/86Sr isotope ratio in subfossil carbonate mollusc shells from raised-beach sediments is used as a palaeosalinity indicator for the Baltic Sea. The main purpose is to demonstrate the applicability of the method in a formerly glaciated area, using 87Sr/86Sr ratios found in modern shells (Mytilus edulis, Macoma baltica, and Radix balthica) to elucidate the parameters and conditions to be used for palaeosalinity determinations based on subfossil shells. Proxy salinity data are calculated assuming that the Sr concentration in Baltic Sea water is controlled by a two-component, conservative mixing between seawater and river water. Based on replicate determinations of the 87Sr/86Sr ratio in modern shells, proxy salinity data can be quantified with a relative precision of better than ± 5% for salinities up to ∼ 10‰. Comparison with measured, present-day salinities indicates that the accuracy is generally better than ± 5% for the proxy data. With this precision and accuracy, the present-day salinity differences between the major Baltic sub-basins (Bothnian Bay, Bothnian Sea, and Baltic Proper) can be detected. salinities were determined using subfossil shells (M. edulis and M. baltica) with calibrated 14C ages of 6770–3070 cal BP. The shell deposits occur from 65 m a.s.l. down to the present-day sea level. The palaeosalinities determined for the Bothnian Bay (4.8–10.3‰) and the Bothnian Sea (7.3–10.3‰) are in good agreement with earlier estimates of the maximum Littorina Sea stage palaeosalinity in these sub-basins (8–10‰). At one locality, two different shell species from the same shell deposit were dated (M. edulis and M. baltica). The shells differed in age by 460 ± 70 14C years, a possible indication that the ecological conditions at a locality may have been favourable to different faunas at different times.