Bericht Internat. Geologenkongress, Stockholm, 1910. “Die Veränderungen des Klimas seit der Maximum des letzten Eiszeits.” Numerous papers, dealing with Europe and North America.
Munthe, H. “Studies in the Late-Quaternary history of southern Sweden.” Stockholm, Geol. Foren. i Forh., 22, 1910, pp. 1197-1292.
Antevs, E. On the late-glacial and post-glacial history of the Baltic. Geogr. Rev., New York, 12, 1922, p. 521.
CHAPTER XIV
THE POST-GLACIAL OPTIMUM OF CLIMATE
In most of the polar and temperate regions of the world the Glacial period seems to have been separated from the present by a short interval of slightly more maritime climate. The existence of this phase was the chief point brought out in the great collection of papers communicated to the Stockholm meeting of the International Geological Congress, which has frequently been referred to in this volume. The pioneer work on the subject has been done by the Scandinavian geologists, and we may commence with a discussion of this period in the countries bordering on the Baltic.
About 4000 B.C., at the conclusion of the continental phase referred to in the preceding chapter, a rapid movement of submergence set in over the whole of the southern Baltic, and shortly afterwards the land-bar which had formerly separated the fresh waters of the Ancylus lake from the Atlantic gave place to a wide strait, through which the waters of the ocean flowed into the Baltic across southern Sweden. Ultimately this channel became wider than the present outlet between Sweden and Denmark, and maritime influences penetrated to all parts of the Baltic. In recognition of this influence the period was termed by Blytt the “Atlantic stage.” The much greater freedom with which the waters of the Atlantic were able to enter is shown by a comparison of the “isohalines” of this period with those of the present day. Isohalines indicate the degree of saltness of the water; those of to-day can, of course, be measured directly, and show that in the Gulf of Bothnia the water becomes continually less salt as we go northward, for which reason many species of marine mollusca are unable to live. By studying the distribution of the mollusca in the Littorina Sea the isohalines of that period have been reconstructed also, and show that the salt content was much greater than at the present day, indicating a greater influx of oceanic waters.
If we take a map showing a reconstruction of the geography of Littorina time, and apply to it the formulæ given in the Appendix, comparing our results with the inferences of Scandinavian and north German geologists as to the temperature, we find that there is a remarkably good agreement. Many of the palæo-botanists comment on the prolongation of the autumn into the present winter, which is especially characteristic of a more insular climate. The amounts of change in each case are also in good agreement, except perhaps in the Christiania region and in north Denmark, where the geologists require a greater change than that calculated from the land and sea distribution; this is probably accounted for by a higher temperature in the waters of the Atlantic. The maximum change as calculated is shown in south-west Finland (winter 6° F. warmer, summer 2° F. cooler). Finland is described as having at that time the climate of western Europe, which we may take as meaning winter 8° warmer, summer 3-4° cooler. There was thus a great change from the extreme climate of the continental phase with its hot summers and severe winters and little rain, to an extremely temperate climate with cool summers, mild winters and a heavy rainfall. The warmth-loving plants which had begun to immigrate during the later part of the continental phase continued to spread, and probably the highest average temperatures were reached at the time of maximum submergence, but now they were accompanied by plants for which a large rainfall is necessary, and it seems that the average rainfall of southern Sweden must have been about 40 inches a year. The oak began to dominate the forests in place of the hazel, and the peat-bogs, which during the preceding dry period had hardened into a firm surface on which birch and pine were able to take root, again became moist, so that the trees were choked by growths of bog-plants. On the shores lived men of the Transition and Early Neolithic. As the land rose again and the Littorina Sea decreased in area the climate again became drier and more rigorous. In Denmark the forests of the Ancylus period gave place to oak as the land sank, and there are also remains of two water plants, the water-nut (Trapa natans), which is no longer found in Denmark, and Najas marina, still living in one isolated locality. Northern Denmark was broken up into islands, among which marine deposits were formed, containing the remains of southern mollusca, many of which are found in the kitchen-middings. Most of the wood used by Neolithic man was oak; there is little fir and no beech.
In Norway the work of C. Brögger has made us familiar with the Tapes beds, which correspond in point of time to the Littorina stage of the Baltic. Tapes decussatus is itself a southern species of mollusc, and it is associated with a very rich warmth-loving fauna. In southern Norway the geographical conditions were different from those in Sweden, for the land reached its lowest level relatively to the sea about the close of the Glacial period, and has been rising throughout the post-glacial. The seas show a progressive rise of temperature from 8° F. below present at the close of the Glacial period to 4° F. above the present in the older Tapes beds. The littoral climate at this stage resembled that prevailing at present on the coast of northern England. After this, as the land approached its present level, the temperature fell again, and in the upper Tapes stage was only 2° F. above the present.
The warm period represented by the Tapes beds is found at intervals along the west coast of Norway, and we again find evidence of a submergence of the land contemporary with the maximum temperature. These conditions extend even as far north as Tromsö, within the Arctic circle. In Spitzbergen there are raised beaches 30 to 80 feet above the sea, containing remains of molluscs and a species of Fucus, none of which are now living so far north. On the land there are old peat-bogs of great thickness, though peat-mosses cannot now grow, since the ground never thaws below a depth of 6-10 inches. It has been pointed out that a great number of the plants now living in Spitzbergen are unable to ripen their seeds under present climatic conditions, though they must have done so in the past. Ripe seeds of some species, in fact, have been found in the peat-bogs, which are contemporaneous with the raised beach. There is thus evidence of a very well-marked warm period associated with submergence in Spitsbergen.