The evolution of climate

by C. E. P. Brooks

In Franz Josef Land, Nansen found raised beaches with mussels 10 to 20 feet above the present level; this shell does not now live so far north. In the White Sea and on the Murman coast there are also raised beaches with a southern fauna. The warm period shown in the beds of the New Siberian Islands has already been referred to (p. 79).

Returning to the British Isles we find that in the south the land was above its present level throughout the whole of the post-glacial period. On the other hand, a 25-foot beach is found in north-west England (Formby and Leasowe marine beds), but without any evidence as to climate; the same applies to the 25-foot beach of Scotland. It is only when we come to the north-east of Ireland that we find evidence of conditions appreciably warmer than the present, in the section of the Alexandra Dock, Belfast, where marine clays overlie beds of grey sand and peat. The lower estuarine clay is essentially a littoral clay, known as the Scrobicularia zone. It is brownish-blue and sandy, and contains in abundance the roots and leaves of the grass-wrack (Zostera marina), and a vast number of shells of a few species which live between tide-marks, indicating that the land stood 10 feet or so above its present level at first, while the climate cannot have differed greatly from that prevailing at the present day. It must have been formed during a period of gradual depression, for throughout its six feet or more of thickness it preserves identical littoral characters. After a time this depression became more rapid, and the upper estuarine clay began to form—a light blue clay, very pure and unctuous, with a very rich and well-preserved fauna, known as the Thracia zone. The fauna has a decidedly southern aspect, and indicates that the coasts of north-east Ireland had the present temperature of Bantry Bay—an increase of at least 3° F. in the mean annual temperature. The Thracia zone is followed by a bed of yellow shore sand, indicating re-elevation to about seven feet above the present level.

Corresponding to the upper estuarine clay are raised beaches at a height of 25 feet in north-east Ireland, falling to 15 feet at Dublin, and to only 6 or 7 feet in western Donegal and Sligo. The mollusca indicate a somewhat higher temperature than the present. In the beaches have been found flint scrapers and arrowheads of early Neolithic type.

Looking further westwards, we find that Iceland, which had undergone a slight elevation during the continental phase, so that peat was formed below present sea-level, again subsided, falling to 10 or 12 feet below its present level. During this subsidence the temperature rose, the greatest warmth coinciding with the lowest level of the land. Species from the south-west shores, where the temperature of the water is directly influenced by the Gulf Drift, extended to the cold northern coast. In some places the marine clays of this period have been ploughed up by a subsequent readvance of the glaciers.

From Greenland comes abundant evidence of a post-glacial warm period coincident with a subsidence of about 80 feet. Raised beaches all along the west coast contain mollusca, some species of which are not now living north of the St. Lawrence estuary. On the other hand, some northern species which lived off the west coast during the glacial maximum retreated northwards during this period, and have not re-established themselves, though the climate is now suitable. Further, K. Steenstrup describes the occurrence of “dead ice” at several places in North Greenland—masses of ice which have become separated from their parent glaciers owing to rapid recession, and are now buried in morainic matter. Subsequently the ice again advanced, and in some cases a new glacier has advanced over these masses of “dead ice.”

Passing to the mainland of North America, we find in eastern Canada colonies of southern mollusca, especially oysters and quohogs, separated from their main area of distribution south of Cape Cod by a wide area of cold seas—the Gulf of Maine and Bay of Fundy. At the beginning of the warm phase the land lay slightly below its present level, but subsequently rose above it. The climate became still warmer, until its temperature resembled that of the middle New England States. At the same time the rainfall diminished and the peat-bogs were replaced by forests of hardwood trees. In the basin of the Great Lakes the warm period is represented by gravel beds in the Niagara gorge, which from their position must, according to the most recent determinations, have been formed about 4000 to 3000 B.C. These gravels contain shells of fresh-water mollusca, especially species of Unio, which are not now living in the St. Lawrence system, but are found in tributaries of the Mississippi further south. Further south on the eastern coast of the United States there are marine deposits indicating a slight submergence, with a climate somewhat warmer than the present.

Passing to South America, we find in southern Patagonia and Tierra del Fuego exactly similar evidence of a post-glacial subsidence with a warmer climate than the present. Raised beaches at a height of 50 feet contain mollusca, some of which are now rare or extinct in that locality, and in sheltered situations plants are found still living whose nearest neighbours are some way to the north.

In the same way, in southern and eastern Australia there are beaches a few feet above present level, containing warmth-loving species of mollusca and indicating a post-glacial warm period. There is some evidence in the distribution of plants and marine mollusca that this warm period extended to New Zealand. Raised beaches at a height of 50 to 180 feet are also known from many places in Antarctica, and these contain mollusca, some of which are not now living south of the sub-antarctic islands. An interesting confirmation of this has been given by E. Philippi, from the results of an examination of the sea-floor at four points in about 63° S., 75-95° E., all within the present limit of pack-ice. The deposit at present forming is poor in pelagic foraminifera, and consequently contains little lime, but this deposit is very thin, and beneath it is a much more calcareous clay especially rich in Globigerina. The latter deposit is still forming north of the limit of pack-ice, and Philippi concludes that at no very distant date the limits of ice were further south, indicating warmer conditions. It is interesting to note that a similar sequence has been found in the Norwegian North Sea, the brown foraminiferous deposit (in this case containing Biloculina) being known to be underlain as well as overlain by an unfossiliferous grey clay attributed to the Glacial period. Finally, with regard to Cape Colony, A. W. Rodgers says: “It is possible that the presence of marine mollusca belonging to species that are only known in the living state from the coast north of Pondoland, in the raised beaches of Mossel and Algoa Bays, indicates that the sea on the south coast was formerly warmer than now.”

Thus we have evidence of a period of submergence and climates warmer than the present from a large number of places, including the Arctic Ocean and Greenland, the temperate coasts of North America and Europe, the Southern Ocean and Antarctica. The stage appears to be missing on the temperate coasts of the Pacific, on both the Asiatic and North American sides, and from the whole of the Tropics. It is fairly certain that the warm period occurred at the same time in eastern North America and western Europe; in the case of the southern hemisphere we have no direct proof of this, but in all cases the deposits are comparatively recent, and since they obviously refer to a similar state of affairs we may assume that they are of the same date.