Subboreal

History and stratigraphy

The composite scientific term Subboreal, i. e. below the Boreal, is derived from the Latin sub (below, under) and the Greek Βορέας - Boreas, the God of the North Wind. It was first introduced in 1889 by Rutger Sernander to distinguish it from Axel Blytt's Boreal, established in 1876. The Subboreal replaces the Atlantic and is followed by the Subatlantic.

The Subboreal is equivalent to W. H. Zagwijn's pollen zones IVa and IVb and T. Litt's pollen zone VIII. In the pollen scheme of Fritz Theodor Overbeck it occupies pollen zone X.

In paleoclimatology the Subboreal is divided into an Older Subboreal and a Younger Subboreal. Historically the Subboreal is equivalent to most of the Neolithic and the entire Bronze Age which started 4200 to 3800 years ago.

Dating

The beginning of the Subboreal is usually defined as 3710 years BCE or 5660 years BP. This lower limit is flexible, as some authors prefer to use 4400 BCE or 6350 BP or like in northwestern Poland even 4830 BC or 6780 BP, whereas others use 5000 calendar years or 3050 BCE. The upper limit of the Subboreal (and therefore the beginning of the Subatlantic) is also flexible and can be attributed to the interval 1170 to 830 BCE, but is usually fixed at 450 BCE. In varve years the Subboreal corrsponds to the interval 5660 till 2750 years BP.

The boundary between the older and the younger Subboreal is equated as 1350 BCE.

Climatic evolution

During the subboreal the climate was generally dryer and slightly cooler (by about 0.1 °C) than in the preceding Atlantic, but still was warmer than today. The temperatures were 0.7 °C higher than during the following Subatlantic. Consequently, in Scandinavia the lower limit of the glaciers was a 100 to 200 metres higher than during the Subatlantic. On the whole the oscillating temperatures slightly receded in the course of the Subboreal by about 0.3 °C.

In the Aegean the beginning of the Subboreal was marked by a pronounced drought centered around 5600 years BP. Yet of far greater importance at this time was the coming to an end of the African Humid Period, which is reflected in lakes of subtropical Africa (like Lake Chad for example) experiencing a rapid fall in their water levels. During the interval 6200 to 5000 years BP more arid conditions were also on the fore in southern Mesopotamia causing great demographic changes and probably instigating the end of Uruk.

In Northwestern Europe (Germany) a drastic climatic cooling can be observed around 5000 varve years BP in the maars of the Eifel. In the preceding interval lasting from 8200 till 5000 varve years (Holocene Climatic Optimum) the July temperatures were on average still 1 °C higher. Yet at the same time the January temperatures were rising and the yearly precipitation increased.

In North Africa and in the Near East the interval lasting from 4700 to 4100 years BP is characterized by renewed, lasting dry conditions as indicated by lake level minima. Between 4500 and 4100 years BP monsoonal precipitations weakened, a possible cause for the upheavals leading to the end of the Old Kingdom of Egypt.

The region of the Levant shows a similar climatic evolution. The dry conditions prevailing in Mesopotamia around 4200 years BP probably resulted in the downfall of the Akkadian Empire.

Carbon dioxide

The greenhouse gas carbon dioxide had reached right at the beginning of the Subboreal its Holocene minimal value of 260 ppm. During the Subboreal this value started rising and arrived at 293 ppm at the end of the period. As a comparison, today's value is over 400 ppm.

Vegetation history

In Scandinavia the Atlantic/Subboreal boundary shows a distinct vegetational change. This is less pronounced in Western Europe. Yet its typical mixed oak forest does show quite a fast decline in elm and linden. The linden decline is not fully understood, it might be due to the climatic cooling or due to human interference. The elm decline was most likely due to elm disease caused by the ascomycete Ceratocystis ulmi, but climatic changes and anthropogenic pressure on the forests certainly have to be considered as well. The elm decline (with a recession from 20 to 4%, as observed in Eifel maar pollen) has been dated in Central and Northern Europe as 4000 years BC, but more likely was diachronous over the interval 4350 to 3780 years BC.

Another important event was the immigration of European beech (Fagus sylvatica) and hornbeam (Carpinus betulus) from their retreats on the Balkan and south of the Apennines. This happened also diachronously – beech pollen are found for the first time in the interval 4340 to 3540 years BC and hornbeam pollen somewhat later between 3400 and 2900 years BC. With the start of the Younger Subboreal coincides the massive spreading of beech. The establishment of beech and hornbeam was accompanied by indicator plants for human settlements and agriculture like cereals and plantain (Plantago lanceolata), at the same time hazel was receding.

The relatively dry climate during the subboreal furthered the spreading of heath plants (Ericaceae).

Sea Level

Like in the preceding Atlantic the global sea level kept on rising during the Subboreal, yet at a much slower rate. The increase amounted to about 1 meter which corresponds to a rate of 0.3 millimeters per year. At the end of the Subboreal the sea level was about 1 meter below the actual value.

Evolution in the Baltic

In the Baltic the Litorina Sea had already established itself before the onset of the Subboreal. During the Older Subboreal the second Litorina transgression augmented the sea level to 1 Meter below the actual value. After an intermediate Post-litorine Regression the third Litorina transgression reached 60 centimeters below zero and later during the beginning Subatlantic already today's value.

Evolution in the North Sea region

In the North Sea region the Flandrian transgression of the Atlantic was followed by a slight regression or standstill at the beginning of the Subboreal.