Mid-Holocene Transition in the Eastern North Atlantic; Foraminiferal Stable Isotope Evidence from Western Norwegian Fjord Sediments

Instrumental data and stratigraphic studies suggest that basin water in fjords with deep thresholds are influenced by incoming Atlantic waters. These water masses are transported by the Norwegian-Atlantic Current on the adjacent shelf areas. Benthic foraminiferal stable isotopes have been measured with a time resolution of c. 17 year through the time interval of 6300 to 2400 cal yr BP from the western Norwegian fjord; Voldafjorden. The core investigated, HM102-04SC, was raised in 1994 from the deepest basin of the fjord system at 700 m depth and the shallowest threshold to the open ocean is c. 200 m. The stable isotope analyses were performed on the two benthic species Uvigerina mediterranea and Cassidulina laevigata. The oxygen isotope records from both species show high frequency variations superimposed on a millennial scale trend. Between 5000 and 4200 cal yr BP the oxygen isotopes show a general increase to higher mean δ18O values; equivalent to c. 1° C cooling. After c. 4400 cal yr BP the high frequency amplitude in the δ18O records increase in both species. Assuming that oxygen isotopes primarily reflects temperature, the data suggest a change in amplitude from 1-2° C before 4400 cal yr BP to amplitudes comparable to 1.5-4° C after. Comparison of the two oxygen isotope records suggest that not all the variability can be explained by temperature alone, i.e. the δ18O record from C. laevigata generally has larger high frequency amplitude than the δ18O record from U. mediterranea. The discrepancy becomes more apparent after 4400 cal yr BP. Our data do however indicate increased high frequency climate variability following the 800 year Mid-Holocene transition, reflected by a change in oxygen isotope composition corresponding to about 1° C cooling. The isotope records from Voldafjorden show a similar type of high frequency variability to oxygen isotopes from planktonic foraminifera in the Norwegian Sea. However some planktonic records in the region do not record a clear Mid-Holocene transition towards heavier oxygen isotopes. Possible explanations and mechanisms for this will be presented.