Late-Holocene changes in ocean circulation and climate: foraminiferal and isotopic evidence from Sulafjord, western Norway

Abstract

A 278 cm long sediment core spanning the last 5500 years was collected from 440 m water depth in Sulafjorden, western Norway. Detailed analyses of benthonic foraminfera, stable isotopes and lithology have been performed on the core, and a chronology based on five AMS dates on benthonic foraminifera has been established. The foraminiferal assemblages consist of species that are common in the North Sea region today. The benthonic fauna seems to respond to changes in oxygenation of the bottom water in the fjord indicating relatively low dissolved oxygen contents prior to c. 2000 cal. bc followed by a gradual increase until c. 700 cal. bc. The oxygen levels in the water masses are assumed to reflect the ingress of oceanic water from the shelf. Downcore variations in the oxygen isotope ratios in the benthonic species Uvigerina mediterranea in Sulafjorden are interpreted to reflect temperature variations with a minimum amplitude of 2°C. The isotopic data indicate five cold periods at c. 2150–1800 cal. bc, c. 850–600 cal. bc, 150 cal. bc to cal. ad 150, c. cal. ad 500–650, and one in the top of the core corresponding to the ‘Little Ice Age’ ( c. cal. ad 1625). These periods are characterized by 1.5–2°C reduction in the bottom-water temperatures in Sulafjorden. Some of the cold periods in the fjord record are contemporaneous with the Holocene ice-rafting events in the North Atlantic and glacier fluctuations in western Norway and Northern Scandinavia. This implies that late-Holocene climate fluctuations in Scandinavia are caused by circulation changes in the North Atlantic region.