Blytt-Sernander

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.

Description
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 grenzhorizont, 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 period can identify. For example, recent peat core samples at Roskilde Fjord and also Lake Kornerup in Denmark evidenced 40 and 62 distinguishable layers of pollen, respectively. However, no universally accepted replacement model has been proposed.

Dating and calibration
Today the Blytt-Sernander sequence has been substantiated by a wide variety of scientific dating methods, mainly C-14 dates obtained from pollen. Earlier C-14 dates are uncalibrated; that is, they were derived by assuming a constant rate of absorption of C-14 into the analyzed material over time. In fact the actual rate varies because of a number of factors.

The dates can to some degree be corrected by comparing the uncalibrated date to dates known by more certain methods, such as counting and comparing tree rings. Usually one looks up the uncalibrated date on a suitable graph, or “curve”, to obtain the calibrated date. Calibration assumes that the modern rate of absorption is the one known in 1950, when calibration began. Dates BP (before present) are actually before 1950: since then atmospheric testing of atomic weapons has altered the rate. Even with the standard of 1950, there is always a question of accuracy. No date from any source can be securely discarded as “wrong”. The best method of dating bases the estimated date on a trend of dates, unless one or some are known to be more accurate.

In the literature and on the Internet, therefore, the reader might see chronologies that are offset from each other by about a thousand years. The difference is caused by calibration or lack of it. The difference increases with time backward from the present. Often, however, whether the date is calibrated is not stated or, if stated, is wrong. Single dates or periods defined by terminal dates are estimated dates. The author may prefer to list the dates derived from analysis. Terminal dates should only be regarded as conventional end points; i. e., only rarely can you say with precision and accuracy that an event or series of events began or ended in that year, but single dates are more convenient than a scatter of dates.

Cross-discipline correlation
The Blytt-Sernander classification has been used as a temporal framework for the archaeological cultures of Europe and America. Some have gone so far as to identify stages of technology in north Europe with specific periods; however, this approach is an oversimplification not generally accepted. There is no reason, for example, why the north Europeans should stop using bronze and start using iron abruptly at the lower boundary of the Subatlantic at 600 BCE. In the warm Atlantic period, Denmark was occupied by Mesolithic cultures, rather than Neolithic, as climate would lead you to believe. Moreover, the technology stages vary widely globally.

The sequence
The Pleistocene phases and approximate calibrated dates (see above) are
 * Older Dryas stadial, 14,000–13,600 BP
 * Allerød interstadial, 13,600–12,900 BP
 * Younger Dryas stadial, 12,900–11,500 BP

The Holocene phases are
 * Preboreal
 * Boreal, cool, dry, rising temperature, 11,500–8,900 BP
 * Atlantic, warm, moist, maximum temperature, 8900–5700 BP
 * Subboreal, 5700–2600 BP
 * Subatlantic, 2600–0 BP

Marker species
Some marker plant genera/species studied in peat are
 * Sphagnum
 * Carex limosa
 * Scheuchzeria palustris, Rannock rush
 * Eriophorum vaginatum, cotton grass
 * Vaccinium oxycoccus, bog cranberry
 * Andromeda polifolia, bog rosemary
 * Erica tetralix, cross-leaved heather
 * Calluna vulgaris, heather
 * Pinus, pine
 * Betula, birch

More sphagnum appears in wet periods. Dry periods feature more tree stumps, of birch and pine.