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This study has developed a Mid- to Late Holocene isotope record, based on analyses of the benthic foraminiferal species Uvigerina mediterranea and Cassidulina laevigata. The oxygen isotope data from Voldafjorden has been interpreted to primarily reflect bottom water temperature variations. Voldafjorden reaches a depth of 700 m and is connected to the open ocean by a 190 m deep threshold. The basin water in the fjord is directly influenced by the Atlantic water on the adjacent continental shelf by estuarine circulation. These water masses are a part of the Norwegian Atlantic Current, which transports warm saline water northwards along the Norwegian coast. The present study is based on two cores from 694 m water depth and the main emphasis has been on the period between 6300 and 2500 cal yr BP, with a time resolution between 13 to 25 years.
The isotopic record shows high frequency variation superimposed on a millennial scale trend, where time series analyses indicate that the record has a periodicity of 90, 114 and 157-209 years. By assuming that the oxygen isotope record is solely determined by temperature changes, the record reveals temperature amplitudes of 1-2°C. The data suggest that the lowest temperatures occur at 4100 cal yr BP and between 2700-2500 cal yr BP. A clear transition to lower bottom water temperatures, equivalent to 1.5°C, is found between 5000 and 4300 cal yr BP representing the end of the Holocene Thermal Optimum. These results indicate that the fjord system is recording regional oceanographic changes that seem to be linked to variations in the oceanic heat transport.
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