The impact of climate and shore-level displacement on the late-Holocene environmental development of Havstens Fjord and Koljö Fjord,Swedish west coast

Koljö Fjord and Havstens Fjord on the Swedish west coast are, like many silled fjords in Scandinavia, characterized by strong stratification and stagnant bottom water with periodically occurring low-oxygen and anoxic conditions. High organic production together with a stable water-column, very low tidal activity and existence of the sill create an ideal foundation for low-oxygen conditions to develop. The aim of this study was to find out how the fjord environments developed during the later part of the Holocene and, especially, how and when the low-oxygen conditions evolved. To achieve these goals, sediment cores were dated (₂₁₀Pb and ₁₄C) and x-rayed, the distribution of benthic foraminifera was analysed and their content of the stable isotopes d₁₈O and d₁₃C was investigated. In both fjords, the climate proved to be of importance for the environment and it seems that increased freshwater runoff increased the primary production between 500 bc and ad 500. Increasing freshwater runoff not only increased primary production but, at least since ad 1880, also made the water stratification stronger and the deep water more stagnant. Stagnant conditions are the main cause of the development of periodic anoxia and formation of laminated sediments in Koljö Fjord from ad 1930 to 1980 and in Havstens Fjord from ad 1950. In Koljö Fjord, the isostatic land rise and the shallow sill are the most important reasons for a general change from an almost normal saline deep-water environment to a brackish environment in ad 500. At that time the sill depth passed the pycnocline mean depth of 15 m. For Koljö Fjord, this is a threshold value of the depth for the quality of the deep water, marine or brackish.