|
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 Boreal was divided into the Pre-boreal, a transitional phase, and the Boreal proper. The beginning of the Pre-boreal is the former beginning of the Boreal. 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 A date of 11,500 BP calibrated is generally accepted for the end of the Younger Dryas and the start of the Pre-Boreal. This 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. The start of the period is relatively sharply defined by a rise of 7 °C within a 50-year period. Agreement ends there. Dates can vary by as much as 1000 years for a number of reasons. Foremost is the tendency to redefine "Boreal". It can identify a paleoclimate, a pollen zone or a chronozone, but those three bases of definition allow quite different dates. Different methods of dating also 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. Whether variation from region to region will allow high-resolution standardization remains to be seen. There are some solid dates providing fixed points in the Pre-Boreal or Boreal. The Saksunarvatn tephra (an ash layer of volcanic fall-out), is dated in Greenland ice to 10,180 plus or minus 60 BP; in lakes deposits at Krakenes in Norway to 10,010-9,980 years BP calibrated; in northwest German lakes, to 10,090 years BP calibrated. The tephra occurs in early Boreal contexts. It seems certain, then, that the early Boreal (pollen zone V) includes the year 10,000 BP. By a similar line of reasoning, the late Boreal certainly includes the Kilian/Vasset tephra of Swiss and southwest German lakes at 8200 BP, all calibrated. Where the borders are to be drawn is not as certain. Studies conducted on 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 uncal. The scheme goes on to divide the Boreal (BO) into BO-1, 9300-9000, BO-2, 9000-8500, and BO-3, 8500-8000, uncal. CalPal used on these dates suggests overall boundaries of 11,500 and 10,500 BP for the Pre-Boreal with the end of the Boreal at 8900. Dating Discrepancies The dates and sources given above are enough to give the reader an idea of the type of work being done and the variation in the dates. These recent dates are generally earlier than dates given more than 10 years ago. For example, Iverson (1973) and Rud (1979) give dates of 10,000-9000 BP for the Pre-Boreal and 9000-8000 BP for the Boreal, which are stated to be calibrated C-14 dates based on Scandinavian pollen stratigraphy. Presumably the more recent dates are the more accurate, as technology improves with time, often quite rapidly. The reader should be aware of these discrepancies, while at the same time realizing that pollen and climate phases also to some degree may depend on latitude. No date can be regarded as wrong with 100% certainty. Scientists look for the overall pattern of the dates, but that technique is not 100% reliable, either. Description Prior to the sudden rise in temperature marking the start of the Pre-Boreal, Eurasia— locked in the chill of the Younger Dryas— was a more or less continuous tundra belt, with regions of taiga, covered with an herbaceous blanket of grasses and other low plants and shrubs typical of open land. The plant life supported large numbers of herbivores wandering in herds over vast distances. The blanket teemed with small, rapidly reproducing species, which supported food chains of larger predators. The largest predators hunted the mammals of the open tundra, as did man, the most successful predator of them all. This entire ecosystem came to an abrupt end with the sudden rise in temperature that marked the onset of the Boreal period. Replaced by forest, the open lands disappeared from Europe and with them numerous species of ice-age mammals. Lands were repopulated with species that spread from southern refugia, and new climax ecosystems developed. The old fauna persisted in Central Asia, but, not being replenished by the larger areas formerly nourishing the ecosystem, were soon hunted out. The sea brought about additional isolation by rising rapidly and drowning the entire coast. Ireland was cut off early in the Boreal, suffering an impovershment of species. It is home to only two-thirds of the species present in Britain. Britain also was cut off by the end of the Boreal. Forest had closed over the former European tundra. Man was offered two choices by nature: adapt to the encroaching forest or move eastward with the large mammals. Those that 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 that 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, man 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 B. verrucosa, 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. 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 (reeds) prevailed. Warm-weather species such as ivy and mistletoe were to be found in Denmark. Image:Coppice1.JPG|Corylus coppice Image:Typha latifolia norway.jpg|Typha latifolia Image:Bäke11.JPG|Oak forest Image:Lehto.JPG|An alder forest at Strömsinlahti, Roihuvuori, Helsinki 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 (pike) and Siluris glanis (catfish) could be found in inexhaustible abundance. The Boreal forests must surely have been the wonderful and frightening wilderness remembered in legend. Man Right from the outset, the Preboreal-Boreal was the time of transition from Palaeolithic to Mesolithic in Europe. The forests and drowned coastlands of the Boreal were a time and place of plenty. Settlements tended to avoid the deep forest in favor of streams, lakes and especially bays of the ocean. Settlements have been found in north central Europe dating from the Pre-Boreal, such as at Friesack. There an unusual find of net fragments made from plant fibers give an indication that fishing was an important part of life. Finds from another settlement at Vis near the Vychegda river in Russia fill out more of the details of living in a settlement of the Boreal. Plant fibers were used also for baskets and for hafting bone points to shafts. The fishermen crossed the waters in bark boats, plied by oars, setting nets. They also constructed hand-held nets from wood hoops and plant fiber. Food gathering did not cease in winter: skiis and sledge runners have been found. The reindeer was one of the animals not replaced by the climate change. It continued to be hunted, probably herded. Bows, arrows and spears have been found. All implements were likely to be embellished by sculpting in wood or bone. Only a few motifs were used: the elk's head, the snake, and man. In Europe the major culture was the Maglemosian (9000-6400 BC). It extended into Denmark and to the east into Russia. Some localized cultures were the Nieman of Lithuania, the Kunda of Latvia and Estonia, the Azilian of France and the Epi-Gravettian of Italy. Towards the end of the period Mesolithic local traditions began to multiply, perhaps due to influences from the south, or perhaps due to the general advancement of culture. | ||||||||
|
| |||||||||
 |
|
| |