Recent reservoir ages for Danish fjords and marine waters

AMS C-14 dates were measured for 28 mollusk shells collected live in Danish waters over the period AD 1885 to 1945. Fourteen samples were from fjords and 14 were marine samples from the Danish Skagerrak-Kattegat coastal area and from the Belts. Reservoir ages were calculated for all samples on the basis of the tree-ring calibration curve. For the marine samples, which cover the period AD 1885-1916, we found a weighted-average reservoir age of 377 +/- 16 yr. The marine Delta R values (the difference between the measured C-14 age and the age deduced from marine, mixed-layer model calculation of Stuiver, Pearson and Braziunas (1986)) were found to be uniform within the experimental uncertainty with a weighted average of Delta R = 13 +/- 16 yr. Based on the observed scatter, the standard deviation is 21 yr. This result shows that it is justified to use the marine calibration curve with standard parameters (Delta R = 0) when C-14-dating marina samples from the Danish area. Our value is consistent with the result Delta R = -33 +/- 27 yr previously found for the Norwegian and Swedish Skagerrak-Kattegat coasts. In contrast, reservoir ages for Danish fjords were found to vary from 400 to >900 yr, far beyond experimental uncertainty. We ascribe this to varying content of dissolved, old soil carbonate (hard-water effect). Therefore, dating of samples from such fjord environments is expected to be uncertain by several hundred years.