Boreal (period)

In paleoclimatology of the Holocene, the Boreal was the first of the Blytt-Sernander sequence of north European climatic phases that were originally based on the study of Danish peat bogs, named for Axel Blytt and Rutger Sernander, who first established the sequence. In peat bog sediments, the Boreal is also recognized by its characteristic pollen zone. It was preceded by the Younger Dryas, the last cold snap of the Pleistocene, and followed by the Atlantic, a warmer and moister period than our most recent climate. The Boreal, transitional between the two periods, varied a great deal, at times comprising within it climates like today's.

Subdividing the Boreal
Subsequent to the original Blytt-Sernander scheme, the first stage of the Boreal was divided off as a Pre-boreal transitional phase, followed by the Boreal proper. Some current schemes based on pollen zones also distinguish a pre-Boreal (pollen zone IV), an early Boreal (pollen zone V) and a late Boreal (pollen zone VIa, b, and c).

Dating
The generally accepted date for the end of the Younger Dryas and the start of the Pre-Boreal is 11,500 BP 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 incalibrated 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.

Description
Before the Pre-Boreal, Eurasia was locked in the chill of the Younger Dryas and was a mostly continuous tundra belt, with regions of taiga, covered with a blanket of grasses, shrubs and other low plants typical of open land. Large numbers of herbivores wandered in herds over vast distances. The blanket teemed with small, rapidly reproducing species, which supported food chains of larger predators. The largest predators and humans hunted the mammals of the open tundra.

The Pre-Boreal began with a sudden rise in temperature that abruptly changed this ecosystem. Forest replaced the open lands in Europe, and forest-dwelling animals spread from southern refugia and replaced the ice-age tundra mammals; new climax ecosystems developed. The old fauna persisted in Central Asia, but were soon hunted out, as they were not replenished by the larger areas formerly nourishing the ecosystem.

The sea brought additional isolation by rising rapidly and drowning the entire coast. Ireland was cut off early in the Boreal, suffering an impoverishment of species. It is home to only two-thirds of the species present in Britain. Britain was cut off by the end of the Boreal. Forest had closed over the former European tundra.

Humans had to adapt to the encroaching forest or move east with the large mammals. Those who stayed became hunter-gatherers of the forests and fishers of the numerous bays, inlets and shallow waters around the thousands of islands that now spangled the seas of Europe. They lived richly and were encouraged to enter the pre-productive phase that we call the Mesolithic. Those who moved east hunted out the last of wild big game and turned their best efforts into learning to herd what was left. In the Americas, humans had left the Paleoindian phase and were now in the Archaic.

Meanwhile humanity toward the south of the north temperate zone had already turned to food production in a number of widely separated locations and were on the brink of civilization. There is no evidence of any extensive contact with the cultures of the north during the Boreal. The producers tended to live in dense centers without any interest in moving from there except when motivated to find new lands. The gatherers ranged widely over their lands, building only temporary settlements in which to spend the winter.

Flora
During the Pre-Boreal pollen zone IV, large quantities of tree pollen began to replace the pollen of open-land species, as the most mobile and flexible arboreal species colonized their way northward, replacing the ice-age tundra plants. Foremost among them were the birches, Betula pubescens and Betula pendula, accompanied by Sorbus aucuparia and Quaking Aspen, Populus tremula. Especially sensitive to temperature changes and moving northward almost immediately were Juniperus nana and J. communis, the dwarf and shrub Juniper respectively, which reached a maximum density in the Pre-Boreal, before their niches were shaded out. Pine soon followed, for which reason the resulting open woodland is often called a birch or a pine-birch forest.

In the yet warmer early Boreal pollen zone V, Corylus avellana (hazel) and pine expanded into the birch woodlands to such a degree that palynologists refer to the resulting ecology as the hazel-pine forest. In the late Boreal it was supplanted by the spread of a deciduous forest called the mixed-oak forest. Pine, birch and hazel were reduced in favor of Quercus, Ulmus, Tilia and Alnus. The former tundra was now closed by a canopy of dense forest. In the marshland Typha latifolia prevailed. Less cold-tolerant species such as ivy and mistletoe were to be found in Denmark.

Fauna
The new forest was populated with animals from refugia in Italy, Spain and the Balkans. Animals such as Emys orbicularis (European pond tortoise), which require warmer temperatures, were to be found in Denmark. The Eurasian golden plover came as far north as Norway.

The plains Perissodactyla (horses, rhinoceros, etc.) were replaced by forest Cervidae: Cervus elaphus (red deer), Capreolus capreolus (roe deer) and Alces alces (elk). Sus scrofa (wild pig) rooted around in the oaks, while Bos primigenius (aurochs) haunted the glades and thickets, creating a state of awed unease for human travellers. The forest had its share of predators: Canis lupus (wolf), Ursus arctos (brown bear), Lynx lynx (lynx), Felis sylvestris (wildcat). Lepus europaeus (hare) provided a tasty meal for any who could catch them.

The inland waters were as today's European waters would be if they were left alone. Castor fiber (beaver) increased the wetlands by damming the streams and ponds. Lutra lutra (otter) hunted for fish there. Such fish as Esox lucius (northern pike) and Siluris glanis (catfish) could be found in inexhaustible abundance. The continuous Boreal forests must surely have been the wonderful and frightening wilderness remembered in legend.