Late Weichselian marine 14C reservoir ages at the western coast of Norway

A shallow marine Late Weichselian deposit on the outer coast of western Norway contains both terrestrial plant material and articulated marine shells. We have C-14 dated both types of material from eight different stratigraphic levels covering the time interval 12,300 to 11,100 C-14 yr B.P. The difference between C-14-dated terrestrial plant material and marine shell material (the marine reservoir age) ranges from 200 to 525 yr, with a weighted average of 380 +/- 32 yr, This is almost identical to the present reservoir age of 379 +/- 20 yr for southern Norway, In the mid-Younger Dryas (YD) interval the reservoir age in the North Atlantic (55 degrees N-65 degrees N) was 700-800 yr, considerably greater than the present reservoir age and the age we have measured for the Bolling-Allerod interval. The reason for this increase during the YD is probably a combination of reduced inflow of surface waters to the North Atlantic and more extensive sea-ice cover. Evidence from marine cores show that the southeastern Norwegian Sea experienced rapid fluctuations in the inflow of warm Atlantic surface water during the period 12,300 to 11,000 yr B.P, However, the reservoir age apparently did not increase during these colder periods (Older Dryas I and II), The reason is probably that, in contrast to the YD, these colder periods did not last long enough and/or were of too limited extent to alter the reservoir age of the ocean. A comparison of the obtained C-14 dates with the varve C-14 chronology from Lake Suigetsu indicates an age of 12,770 cal yr B.P, for the AL/YD boundary. (C) 1999 University of Washington.