The Atlantic in palaeoclimatology was the warmest and moistest Blytt–Sernander period, pollen zone and chronozone of Holocene northern 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 Subboreal, a transition to the modern. Because it was the warmest period of the Holocene, the Atlantic is often referenced more directly as the 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
Beginning of the Atlantic period
It is a question of definition and the criteria: Beginning with the temperatures, as derivable from Greenland ice core data, it is possible to define an 'Early' or 'Pre-Atlantic' period at around 8040 BC, where the 18O isotope line remains above 33 ppm in the combined curve after Rasmussen et al. (2006), which then would end at the well-known 6.2 ka BC (8.2 ka calBP)-cold-event.
Or one single Atlantic period is defined, starting at that just mentioned cold-event.
After a lake-level criterion, Kul’kova and others define the Atlantic as running from 8000 to 5000 (cal?) BP. Early Atlantic, or AT1, was a time of high lake levels, 8000–7000 BP; in Middle Atlantic, AT2, lakes were at a lower level, 7000–6500 BP; and in Late Atlantic I, 6500–6000 BP, and II, 6000–5700 BP, levels were on the rise. Each period has its distinctive ratios of species.
End of the Atlantic period
According to the ice-core criterion it is extremely difficult to find a clear boundary, because the measurements still differ too much and alignments are still under construction. Many find a decline of temperature significant enough after 4800 BC. Another criterion comes from bio-stratigraphy: the elm-decline. However, this appears in different regions between 4300 and 3100 BC.
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. 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 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 aurochs and wild horses, a thesis referred to as the wood-pastury hypothesis.
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. Birch and pine were replaced by oak, linden (lime, both small and large leaf species), beech, oak, hazel, elm, alder, and ash, 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.