Mid- to late-Holocene reservoir-age variability and isotope-based palaeoenvironmental reconstruction in the Limfjord, Denmark

Palaeoenvironmental and14C reservoir age variability in the Limfjord, a sound through northern Jutland, Denmark, was investigated for the period 7300 to 1300 cal yr BP. Shells and bulk sediment samples from a core from a former inlet, Kilen, were analysed by radiocarbon dating and stable isotope (C/N) measurements. A strong correlation between the C/N ratios and δ13C values verifies that these are good carbon source indicators and thus allow environmental reconstructions. Furthermore, δ13C values are correlated with salinity in the photic zone, inferred quantitatively from diatom assemblages. They are therefore used to differentiate between brackish and marine palaeo-conditions.14C reservoir ages of shells vary from Δ R=−140 to +30014C years. Between 7300 and 5400 cal. yr BP, reservoir age and stable isotope values are highly variable and indicate mixing of marine water and brackish surface waters with hard water effects. After 5400 cal. yr BP, the Δ R values stabilise and show an increasingly marine environment, with14C reservoir ages close to 400 years (Δ R=0). After 2000 cal. yr BP, Kilen becomes brackish. Reservoir ages and stable isotope values are again highly variable and δ13C and C/N values are no longer correlated. Before 4000 cal. yr BP, δ15N values vary by only 1‰ and do not reflect the changes in the marine environment. A δ15N increase between 3500 and 2000 cal. yr BP signifies enhanced organic productivity or a change in agricultural practices.