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

Abstract

Palaeoenvironmental and ¹⁴C 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 +300 ¹⁴C 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 ΔR values stabilise and show an increasingly marine environment, with ¹⁴C reservoir ages close to 400 years (ΔR=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.