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

A shallow marine Late Weichselian deposit on the outer coast of western Nor way contains both terrestrial plant material and articulated marine shells. We have C-14 dated both types of material from eight different stratigraph ic levels covering the time interval 12,300 to 11,100 C-14 yr B.P. The diff erence between C-14-dated terrestrial plant material and marine shell mater ial (the marine reservoir age) ranges from 200 to 525 yr, with a weighted a verage 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) int erval the reservoir age in the North Atlantic (55 degrees N-65 degrees N) w as 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 surfa ce waters to the North Atlantic and more extensive sea-ice cover. Evidence from marine cores show that the southeastern Norwegian Sea experienced rapi d fluctuations in the inflow of warm Atlantic surface water during the peri od 12,300 to 11,000 yr B.P, However, the reservoir age apparently did not i ncrease during these colder periods (Older Dryas I and II), The reason is p robably 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 chronol ogy from Lake Suigetsu indicates an age of 12,770 cal yr B.P, for the AL/YD boundary. (C) 1999 University of Washington.