Layers of
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 and 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 the 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. Geologists working in different regions are studying sea levels, peat bogs, and
ice core samples by a variety of methods, intending to further verify, and refine 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
Lake Kornerup in
Denmark identified 40 to 62 distinguishable layers of
pollen, respectively. However, no universally accepted replacement model has been proposed. ==Problems==