Blytt–Sernander system

The Blytt-Sernander classification, or sequence, is a series of north European climatic periods or phases based on the study of Danish peat bogs by Axel Blytt (1876) and Rutger Sernander (1908). The classification was incorporated into a sequence of pollen zones later defined by Lennart von Post, one of the founders of palynology.

Layers in peat were first noticed by Heinrich Dau in 1829. A prize was offered by the Royal Danish Academy of Sciences and Letters to anyone who could explain them. Blytt hypothesized that the darker layers were deposited in drier times; the lighter, in moister times, applying his terms Atlantic (warm, moist) and Boreal (cool, dry). In 1926 C.A. Weber noticed the sharp boundary horizons, or Grenzhorizonte, in German peat, which matched Blytt’s classification. Sernander defined subboreal and subatlantic periods, as well as the late glacial periods. Other scientists have since added other information.

The classification was devised before the development of more accurate dating methods, such as C-14 dating and oxygen isotope ratio cycles. Currently geologists working in different regions are studying sea levels, peat bogs and ice core samples by a variety of methods, with a view toward further verifying and refining the Blytt-Sernander sequence. They find a general correspondence across Eurasia and North America.

The fluctuations of climatic change are more complex than Blytt-Sernander periodizations can identify. For example, recent peat core samples at Roskilde Fjord and also Lake Kornerup in Denmark identified 40 to 62 distinguishable layers of pollen, respectively. However, no universally accepted replacement model has been proposed.

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