Carbon-14 (14C) is an important contributor to the collective effective dose to the public due to releases from nuclear power plants (NPPs). In Sweden, only airborne emissions of 14C from NPPs are currently routinely monitored, and the existing data on waterborne 14C discharges are limited. A recent study of 14C in brown algae (Fucus spp.) in Swedish coastal waters showed higher F14C values collected at Ringhals NPP, on the Swedish west coast, than expected. Therefore, this study aimed at assessing if blue mussels (Mytilus edulis) could be used to retrospectively estimate the 14C concentration of dissolved inorganic carbon (DIC) in seawater at three sites. A method was developed to extract the fibrous layer that forms visible annual structures in the shells. All samples were analyzed with accelerator mass spectrometry and the results compared with 14C data from Fucus spp. For one of the analyzed shells (structures from 1974–1978), from the site Särdal, F14C in Fucus spp. and M. edulis agreed very well. For another shell (1972–1978), shell structures from some of the earlier years displayed up to 6% lower F14C than Fucus spp. F14C in one shell from a remote site, Båteviken, only had small annual variations (2017–2022: F14C = 1.070 ± 0.015 (1 σ)). Two shells from Ringhals NPP had higher average F14C, and a significant temporal variability (2014–2022: F14C = 1.427 ± 0.268 (1 σ)). Difficulties in unambiguous identification of the annual structures in the shells, as well as the future potential of this method, are discussed.

Oxygen isotope profiles along the growth axis of fossil bivalve shells of Macoma calcarea were established to reconstruct hydrographical changes in the eastern Laptev Sea since 8400 cal yr B.P. The variability of the oxygen isotopes (δ18O) in the individual records is mainly attributed to variations in the salinity of bottom waters in the Laptev Sea with a modern ratio of 0.50‰/salinity. The high-resolution δ18O profiles exhibit distinct and annual cycles from which the seasonal and annual salinity variations at the investigated site can be reconstructed. Based on the modern analogue approach oxygen isotope profiles of radiocarbon-dated bivalve shells from a sediment core located northeast of the Lena Delta provide seasonal and subdecadal insights into past hydrological conditions and their relation to the Holocene transgressional history of the Laptev Sea shelf. Under the assumption that the modern relationship between δ18Ow and salinity has been constant throughout the time, the δ18O of an 8400-cal-yr-old bivalves would suggest that bottom-water salinity was reduced and the temperature was slightly warmer, both suggesting a stronger mixture of riverine water to the bottom water. Reconstruction of the inundation history of the Laptev Sea shelf indicates local sea level ∼27 m below present at this time and a closer proximity of the site to the coastline and the Lena River mouth. Due to continuing sea level rise and a southward retreat of the river mouth, bottom-water salinity increased at 7200 cal yr B.P. along with an increase in seasonal variability. Conditions comparable to the modern hydrography were achieved by 3800 cal yr B.P.