File:Climatic variation in historic and prehistoric time ((1914)) (20630615636).jpg

Title: Climatic variation in historic and prehistoric time
Identifier: Climaticvariati00Pett (find matches)
Year: (1914) ((190s)
Authors: Pettersson, Otto
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Publisher: Göteborg, W. Zachrissons boktr
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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sted as regards the other sea-fish: plaice, flounder, cod and sprat, etc. Southern forms of molluscs, too, such as Tapes, Littorina, Ost- rea, came with the Atlantic waters into the Baltic. Oyster-banks existed everywhere in Isefjord, and around the Danish Islands. This was the period of the »kitchen-middens»; the great fish-migration period, when the gates of the Baltic stood wide open to the fish from the Atlantic. All the species of fish found in the Baltic, except the eel, sea-pike, and mackerel, are to be considered as relicts from the Littorina period, at the close of which began the rise of the land which restored the depth-conditions of the Sund and the Belt to about their present position. This geological alteration, which was completed about 3,000 years ago, had a far-reaching influence on the fish-races of the Baltic, which are now in a greater or less degree separated from their relations in the ocean and live und other hydrographical conditions,
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Surface-map (present time). Surface-map (Littorina-epoch). which have gradually altered their physiological life-conditions and have even commenced to set their stamp on the bodily and exterior form of the fish. The expression »relict» must, however, be understood relatively, The ancient relicts from the days of the Polar Sea consist, at present, of some lower animal forms, mostly inhabiting the great depths and including too the arctic gray-seal, which still lives in the Baltic and in lake Ladoga in Russia. These are relicts in the original sense of the word, from a period dating 50,000—100,000 years back. Amongst them there is only one certain arctic species of fish, the Cottus quad- ricornis. Among the relicts of the Littorina period we reckon, in the first place, the stromling or small herring, the indigenous herring race of the central and northern Baltic, which now lives isolated from the North Sea-herring, whose migrations nowadays do not extend past the portals of the Baltic Sea. From the first, the stromling was a local race — a relict of the great herring migrations of a couple of thousand years ago, which has survived and gradually differentiated physiologically and even morphologically into a new species. After the maximum of the Littorina epoch, which occurred before the beginning of the Bronze-age, in the time of the »Kjok- kenmoddinger» (ancient refuse-deposits) 4000—5000 years ago, an upheaval of the landsurface occurred which lessened the depth of the Oresund to something like the present. This elevation of the land was almost complete at the close of the Bronze-age 600—500 b. C. From then, for the last 2500 years the bottom of the Oresund has remained nearly constant, its elevation increasing only some 0.25 meter since that time. The atmospheric conditions also changed into the cold climate of the early Iron-age. The transition from the warm climate of the Bronze-age was according to Sernander accomplis- hed in a few centuries, 650—400 b. G the last stage of the Bronze-age. The temperature of this epoch (»the »Fimbul-winter» of the Sagas) must have been considerably lower, for in the peat-layers from that time we find deposits of subarctic forms. From the end of the Bronze- age a gradual elevation of the land has been and is still in progress. Sernander estimates the elevation for the last 2,000 years to: In the environments of Upsala 10 meter On the West-coast of Sweden 5 » In Oresund 0.24 » On the Bohuslan-shore are found sub-fossil remains of Ostrea edulis, Tapes decussatus a. d. which formed the ancient oyster-beds of the Littorina epoch, but which now are either extinct or survive only as relicts in a few protected localities. They bear testimony both to the climatic deterioration and to the upheavel of land during and after the Bronze-age. In Greenland, Spitzbergen, Frans Josefs land and on the coast of North America subfossil deposits are found of molluscs which must once have lived in warmer waters. This proves that the land- elevation and the deterioration of climate have passed over the entire North Atlantic coast. I will give some examples in proof of this: The Swedish expeditions found in numerous places on Spits- bergen sub-fossil deposits of Mytilus eclulis1)Among such subfossil relicts from a more temperate sea found on Spitzbergen there are, beside Mytilus, also Cyprina Islandica, Littorina littorea and Anomia squamula, all of which can not live in the fjords of Spitzbergen under the present conditions. On the shores of the southeast coast of the Disco-bight on Green- land A. S. Jensen2 has found fossil shells of Anomia squamula and Zirphea crispata. The present limit of these molluscs is the south coast of Labrador and the St. Lawrence bight, which shows that during some part of the post-glacial time a warmer climate must have prevailed than now. The chief representative of this warmer postglacial period in the mollusc-fauna of East-Gronland and of Franz Josefs Land is Mytilus edulis3. On Iceland it is represented by Purpura lapillus4 a. o. It is probable that all these southern species of molluscs lived simultaneously in the North Atlantic ocean and its ramifications during this warm postglacial period. For the more distant parts of the ocean this view still must be regarded as a hypothesis but in Sweden and Denmark we have archseologic finds excavated from the shore-deposits which enable us to discern between the warmer and the colder period and to fix the time-limit with some approach to certi- tude. The southern mollusc-species are found together with remains from the Bronze-age. As to the next period, the rarity of archeeologic finds in the graves from the early Iron-age about 400 b. C. to 100—200 a. G. shows that the high stage of civilisation in the Bronze-age had for some reason or other declined and thatthepopulationhad decreased and 1 G. Andersson found Mytilus also on King Charies island about 40 M. above the sea-level; the Norwegian geologist Staxrud found the same mollusc at 60 M. on Spitzbergen. V. Nordrnann: Anomia squamula som Kvartser-fossil paa Spitzbergen Meddel. Dansk Geol. Forening Bd 4 Kobenhavn 1912. 2 Ad. S. Jensen: On the Mollusca of East Greenland I. With an introduction on Greenlands fossil Mollusc-fauna from the quaternary time. Medd. om Gronland, Bd 29. 1909 (Reprint 1905). Ad. S. Jensen and Poul Harder: Post-glacial changes of climate in arctic regions as revealed by investigations on marine deposits. Postglaziale Klima ver&nderungen seit dem Maximum der letzten Eiszeit. Stockholm 1910. 3 A. G. Nathorst. Bidrag till nordostra Gronlands geologi. Geol. Foreningens Forh. Bd. 23. Stockholm 1901. * G. Bardarson, Purpura lapillus i hsevede Liig paa Nord Kysten af Island. Vidensk. Meddel. Naturhist. Foren. i Kopenhavn 1906—1907. — 24 —

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