Boreal (age) - Dating

Dating

The generally accepted date for the end of the Younger Dryas and the start of the Pre-Boreal is 11,500 Before Present calibrated. The start of the period is relatively sharply defined by a rise of 7 °C in 50 years. The date is based fairly solidly on Greenland ice cores, which give 11,640 BP for the late Younger Dryas and 11,400 BP for the early Pre-Boreal.

But estimates of other dates vary by up to 1000 years, for a number of reasons. First, "Boreal" can identify a paleoclimate, a pollen zone or a temporally-fixed chronozone, and those three bases of definition allow quite different dates. Second, different dating methods obtain different dates. The underlying problem is that climate and pollen vary somewhat from region to region. The scientists of each region use the methods available in their region, whether lake varves, the annual layers of sediment from ancient or modern lake bottoms, ice cores or counts of tree rings (dendrochronology).

Standardization has become of increasing concern to scientists everywhere. Dates from many methods continue to multiply as paleoclimatologists seek higher resolution. But it is unclear whether regional variation will allow high-resolution standardization.

Yet, there are some solid dates of the PreBoreal and Boreal. The Saksunarvatn tephra (an ash layer of volcanic fall-out), is dated in Greenland ice to 10,180±60 BP; in lake deposits at Krakenes in Norway, to 10,010–9,980 years BP calibrated; in northwest German lakes, to 10,090 BP calibrated. The tephra occurs in early Boreal contexts. So, it seems certain that the early Boreal (pollen zone V) includes the year 10,000 BP. Similarly, the late Boreal includes the Kilian/Vasset tephra of Swiss and southwest German lakes at 8200 BP, all calibrated. But the borders are less certain.

Studies of bogs in northwest Russia are the basis for a division of the PreBoreal (PB) into PB-1, 10,000-9800, and PB-2, 9800–9300 BP incal. The scheme goes on to divide the Boreal (BO) into BO-1, 9300–9000, BO-2, 9000–8500, and BO-3, 8500–8000, incal. CalPal used on these dates suggests overall boundaries of 11,500 and 10,500 BP for the Pre-Boreal, and the end of the Boreal at 8900.

Dates given recently are usually earlier than those given more than 10 years ago. For example, Iverson (1973) and Rud (1979) give dates of 10,000–9000 BP for the PreBoreal and 9000–8000 BP for the Boreal, which are uncalibrated C-14 dates based on Scandinavian pollen stratigraphy.

Presumably, more-recent dates are more accurate, as technology improves with time, often quite rapidly. Yet, pollen and climate phases also to some degree may depend on latitude, so no date can be regarded as certainly wrong. Scientists look for the overall pattern of the dates, but that technique is not 100% reliable, either.