• AMS Radiocarbon Measurements from the Swedish Varved Clays

      Wohlfarth, Barbara; Possnert, Göran (Department of Geosciences, The University of Arizona, 2000-01-01)
      The Swedish varve chronology, or Swedish Time Scale, is an annual chronology based upon the successive correlation of more than 1000 varve-thickness diagrams. The Late Glacial-Early Holocene varved clays were deposited as glaciolacustrine sediments in the Baltic Sea during the recession of the Scandinavian ice sheet. Formation of varved clays continued throughout the Holocene and is still going on in the estuary of River Angermanalven in northern Sweden. Accelerator mass spectrometry (AMS) radiocarbon measurements, which have been performed on terrestrial plant macrofossils extracted from the varved clays, show—in comparison with other annual chronologies—that several hundreds of varve years are missing in the varve chronology. These findings are supported by, among others, pollen stratigraphic investigations on time-equivalent varve year intervals. If an effort were undertaken to evaluate the erroneous parts, the Swedish Time Scale would have the potential of becoming a continuous annual chronology.
    • The Swedish Time Scale: A Potential Calibration Tool for the Radiocarbon Time Scale During the Late Weichselian

      Wohlfarth, Barbara; Björck, Svante; Possnert, Göran (Department of Geosciences, The University of Arizona, 1995-01-01)
      The Swedish Time Scale (STS) is a ca. 13,300-yr-long varve chronology that has been established for the Swedish east coast from >1000 overlapping clay-varve diagrams. We describe the present state of the STS and illustrate the application of this worldwide unique varve chronology for AMS radiocarbon measurements. The results are compared to other 14C-dated calendar-year chronologies: dendrochronology, laminated lake sediments and U/Th. Our data set agrees with the oldest part of the dendrochronological calibration curve, and with AMS 14C-dated lake lamination data and U/Th on corals down to ca. 12 ka calendar years BP. Further back in time, the AMS-dated part of the STS partly compares well with lake lamination chronologies and shows that the difference between 14C and calendar years decreases rapidly between 12,600 and 12,800 calendar years BP. Such a development seems to contrast with U/Th measurements on corals. We suggest that the cause for the divergence among three supposed calendar-year chronologies lies in the fact that the data points on the marine 14C-U/ Th curve are more widely spaced in time than the tightly grouped set of terrestrial AMS 14C dates, and thus are not able to reflect short-term changes in atmospheric 14C. Therefore, we argue that the use of the pre-Holocene part of the calibration program is premature and inadvisable.