Atlantic (period)

The Atlantic in palaeoclimatology was the warmest and moistest Blytt-Sernander period, pollen zone and chronozone of Holocene north Europe. The climate was generally warmer than today. It was preceded by the Boreal, with a climate similar to today’s, and was followed by the Sub-Boreal, a transition to the modern. Because it was the warmest period of the Holocene, the Atlantic is often referenced more directly under Holocene climatic optimum, or just climatic optimum.

Subdividing the Atlantic
The Atlantic is equivalent to Pollen Zone VII. Sometimes a Pre-atlantic or early Atlantic is distinguished, on the basis of an early dividing cold snap. Other scientists place the Atlantic entirely after the cold snap, assigning the latter to the Boreal. The period is still in the process of definition.

Dating
As is true of the Boreal (refer to the presentation there), the dates for the Atlantic are somewhat variable, the more recent being (by presumption) the more accurate. Papers presented at the XVI INQUA Congress, sponsored by the Geological Society of America, in Reno, Nevada, 2003 (abstracts available on the Internet) give us some conclusions based on recent data:
 * S.J. Johnsen and others, after an analysis of some Greenland ice cores, define a period of optimum temperatures approximately between 8600 and 4300 BP calibrated, divided by the 8.2 kiloyear event.
 * H. Seppä and others, based on carbon-dated pollen stratigraphy in the sediments of lakes in Estonia, Finland, and Sweden, define the Holocene Thermal Maximum (HTM) to be after the 8200 BP event in the window 8000–4500 BP calibrated, during which the mean temperature was 2.5 °C higher than today. It was ended by cooling at about 4500. The 8200 BP event appears in the pollen stratigraphy as a transient cooling of 1.5 to 2.0 °C between about 8500 and 8100 BP.

The PreAtlantic, or early Atlantic if distinguished, is dated roughly 9000-8000 calibrated in earlier sources, with the Atlantic at approximately 8900–5700 BP calibrated, but more recent dates show that it lasted a little over a thousand years longer, at least in some locations. The difference might be accounted for by methodology and location of samples. Carbon-14 readings on oysters in Roskilde Fjord indicate 8000–5800 BP is appropriate for the full Atlantic of Denmark. Whether the complexity of the factors will ever allow a more precise definition remains uncertain.

Kul’kova and others abandon the 8200 BP date to the Late Boreal and, focusing on lake levels, supported by C-14 dates, define the Atlantic as running from 8000 to 5000 BP. Early Atlantic, or AT1, was a time of high lake levels, 8000–7000; in Middle Atlantic, AT2, lakes were at a lower level, 7000–6500; and in Late Atlantic I, 6500–6000, and II, 6000–5700, levels were on the rise. Each period has its distinctive ratios of species.

The Atlantic was hottest toward its end, in the range 6000–5500 BP. In summary, the mean temperature of Europe plunged at about 6000 BC, only to rise within a few hundred years of that time to a maximum at about 4000 BC. Climate was at its balmiest and life for man at its most convenient until about 3500 BC, when cooling began. By 2500 BC the warm spell was over.

Description
The Atlantic was a time of rising temperature and marine transgression on the islands of Denmark and elsewhere. The sea rose to 3 m above its present level by the end of the period. The oysters found there required lower salinity. Tides of up to 1 m were present. Inland, lake levels in all north Europe were generally higher, with fluctuations.

The temperature rise had the effect of extending southern climates northward in a relatively short period. The treelines on northern mountains rose by 600 to 900 m (2000–3000 feet). Thermophilous (“heat-loving”) species migrated northward. They did not replace the species that were there, but shifted the percentages in their favor. Across middle Europe, the boreal forests were replaced by climax or “old growth” deciduous ones, which, though providing a denser canopy, were more open at the base.

The dense canopy theory, however, has been questioned recently by F. Vera. Oak and hazel require more light than is allowed by the dense canopy. Vera hypothesizes that the lowlands were more open and that the low frequency of grass pollen was caused by the browsing of large herbivores, such as Bos primigenius and Equus ferus.

Flora
During the Atlantic period the deciduous temperate zone forests of south and central Europe extended northward to replace the boreal mixed forest, which found refugia on the mountain slopes. Mistletoe, Water Chestnut (Trapa natans) and Ivy (Hedera helix) were present in Denmark. Grass pollen decreased. Softwood forests were replaced by hardwood. Quercus, Tilia, both cordata and platyphyllos, beech, oak, hazel, linden, Ulmus glabra, alder, and ash replaced Betula and Pinus, spreading to the north from further south. The period is sometimes called “the alder-elm-lime period”.

In northeast Europe, the Early Atlantic forest was but slightly affected by the rise in temperature. The forest had been pine with an underbrush of hazel, alder, birch, and willow. Only about 7% of the forest became broad-leaved deciduous, dropping to Boreal levels in the cooling of the Middle Atlantic. In the warmer Late Atlantic, the broad-leaved trees became 34% of the forest.

Along the line of the Danube and the Rhine, extending northward in tributary drainage systems, a new factor entered the forest country: the Linear Pottery culture, clearing the arable land by slash and burn methods. It flourished about 5500–4500 BC, falling entirely within the Atlantic. By the end of the Atlantic, agricultural and pasture lands extended over much of Europe and the once virgin forests were contained within refugia. The end of the Atlantic is signaled by the "Elm decline", a sharp drop in Elm pollen, thought to be the result of human food-producing activities. In the subsequent cooler Sub-Boreal, forested country gave way to open range once more.

Fauna
The best picture of Atlantic Period fauna comes from the kitchen middens of the Ertebølle culture of Denmark and others like it. Denmark was more of an archipelago. Humans lived on the shorelines, exploiting waters rich in marine life, marshes teeming with birds, and forests where cervids and suids as well as numerous small species were plentiful.

The higher water levels offset the effects of the submarine toxic zone in the Baltic Sea. It contained fish now rare there, such as the anchovy, Engraulis encrasicolus, and the three-spined stickleback, Gasterosteus aculeatus. Also available were pike, whitefish, cod, and ling. Three kinds of seals were found there, the ringed, harp and grey. Mesolithic man hunted them and whales in the estuaries.

The main birds were maritime, as you might expect: the red-throated diver, the black-throated diver, the gannet, and capercaillie. The Dalmatian pelican (Pelecanus crispus) has been found in Denmark, which today is only in south Russia.

In the lofty canopy could be found a continuous zone of smaller animals, such as the ubiquitous squirrel, Sciuris vulgaris. Daubenton's bat (Myotis daubentonii) was common. In and around the big trees hunted the wildcat, pine marten, polecat (Mustela putorius), and wolf. other wild animals that can be found include the mountain lions and leopards.

The forest floor was prolific with larger browsers and rooters as well: the red deer, roe deer, and wild boar. Not all the former plains mammals had abandoned the country. They remained in the open forest and meadows. These include the aurochs, ancestor of cattle, and the wild horse, which is something of a revelation. The horses were not entirely hunted out, were not confined to the plains further east, and were not entirely the property of the Indo-European cultures there. The Mesolithic Ertebölle people were hunting them in Denmark.