Popular Science Monthly/Volume 76/May 1910/Ancient Climates of the West Coast

From Wikisource
Jump to navigation Jump to search

ANCIENT CLIMATES OF THE WEST COAST
By Professor JAMES PERRIN SMITH

STANFORD UNIVERSITY

WE naturally look at things from the standpoint of the present, and regard the existing distribution of climates as the normal one. But even in our own times there are slight fluctuations of climatic conditions, for we hear wonderful stories from our elders of cold winters and hot summers, and tremendous storms of former years. The advances and retreats of existing glaciers give us surer testimony of recent fluctuations in temperature and moisture, as does also the shifting of the zones where wine grapes can be grown successfully in Europe.

If we go still further back into older history we find still stronger evidence of change, for in northern Africa and in central Asia there are remnants of ancient cities, evidently the flourishing capitals of prosperous peoples, where now is nothing but desert, and where even the most advanced modern skill in irrigation could not support the population of the old days.

And yet all the changes alluded to above may be only the secular variations in climate that we know are going on all the time. The climatic changes of the west coast which will be described in this paper are older than those fluctuations recorded in history, and much greater.

The old geological theory was that the earth cooled down slowly from the poles toward the equator, and that life first appeared at the poles. It was further thought that in the more remote geologic ages the interior heat of the earth was so great that there was little difference in temperature between the equator and the poles, and that, until Tertiary time, there was no differentiation into climatic zones. The Glacial epoch was supposed to be the culmination of this secular cooling off of the earth.

Then came the discovery of the old Tertiary fossil floras of Siberia, Alaska and Greenland, with abundant forests of trees that evidently lived in a temperate climate where it is now arctic. This was so remarkable that geologists had to invent some extraordinary explanation for the phenomenon. They rose to the occasion and invented the theory of the obliquity of the poles in early Tertiary time, to account, for the warm temperature under the arctic circle. This, however, did not agree with the known distribution of life at that time over the rest of the earth, and also the physicists declared this obliquity, or any obliquity, to be a mathematical impossibility.

Later it was discovered that there was a great glacial epoch in the late Paleozoic era, about the line between Carboniferous and Permian, and that too in the regions around the Indian Ocean, where it is now tropical or subtropical in temperature.

Before we had entirely recovered from the shock of that discovery, it was found out that in China and Australia there was another glacial epoch, or epochs, near the beginning of Cambrian time. Now the geologist's spirit is so broken, that when the supposed discovery of glacial epochs in Silurian and Devonian time is announced, he hardly raises a dissenting voice, and appears to be resigned to the occurrence of glacial epochs at almost any time in the history of the earth. The theory of ancient climatic uniformity is definitely abandoned, and we must accept fluctuations of climate from the earliest geologic record all through the history of our planet. The old idea was delightfully simple, but too good to be true.

 

Criteria of Ancient Climates

Physical Criteria.—Physical evidence as to ancient climatic conditions is limited to two classes—glacial deposits and ice-work, and sediments indicating desiccation, that is, saline and gypsiferous beds. These are both necessarily limited to continental areas, and tell us nothing of marine conditions. And as we go back in time they become more and more indefinite, so that there is much difference of opinion as to their value. The evidence of the recent Glacial epoch is positive enough to satisfy the most critical, but geologists are not yet united as to the glacial epochs in older periods of geologic history, because of the difficulty of determining whether the ice-masses were true sheets or whether they were mere local highland glaciers.

Also the sedimentary deposits indicating desiccation may have been merely local, and although they are positive as to prevalence of evaporation at that particular place, they can not tell positively of wide-spread dry climate, and certainly they do not indicate temperature.

Organic Evidence.—Fossil remains of animals or plants known to have lived in either warm or cold climates are more definite, and tell us equally well of land and water conditions, but they are authentic only when the fossils are animals or plants that have either lived on into our own time, or when the groups to which they belong have always had the same habits. This becomes more and more conjectural as we go back in geologic history, and have to deal not only with extinct species, but even with extinct genera, families and orders.

Extensive fossil beds of deciduous trees point to moist climates, and usually to temperate conditions. But deciduous trees extend back only to the middle of the Cretaceous, and beyond that time we have no positive criteria for temperate climate.

Cycads and palms are the best evidence as to tropical climates on the land. At present, cycads are almost exclusively tropical, ranging outside only a short distance in eastern Asia. Palms are not quite so delicate, ranging outside the tropics to 34° N. lat. on the west coast of America and to 36° N. on the east coast. But in any case, abundant remains of either point to tropical or subtropical conditions.

Beef-building corals are even more definite in their testimony concerning tropical temperature of the water. They are now found only in the tropics, where the winter temperature does not fall below 68° F. (20° C), and in general between 26° N. and 26° S. But since this temperature zone may be extended by marine currents, coral reefs may sometimes reach beyond 26° N. lat., as in the Bermudas, but more often they fail to reach this geographic limit, as on the west coast of America.

The principal reef-builders, the Madreporidæ and the Astræidæ, are confined to the hottest part of the tropical belt,[1] within 18° of the equator, and where the temperature does not fall below 74° F. (23° 20′ C). Between this line and the isotherm of 68° F. coral reefs occur on both sides of the equator, but they are composed largely of Poritidæ and Milleporidæ.

On the west coast of America the minimum isotherm of 68° F. runs north of the equator, and the Galapagos Islands have no reefs, for the temperature there often falls below 68° F. Reef building corals occur in patches from Panama to Magdalena Bay on. the coast of Lower California, but they do not form any reefs, and are composed almost entirely of Poritidæ.

Fossil deposits of Astrgeidæ, in any age and anywhere, indicate with a reasonable degree of certainty that the sea had a temperature of not less than 74° F., and corals of any of the modern reef-building groups show that the temperature was not less than 68° F.

But the reef building Hexacoralla are not known below the Triassic, and for the Paleozoic era we must use other criteria. From the Cambrian to the upper part of the Carboniferous coral reefs are known, but they are formed by Favositidæ and Tetracoralla, both wholly extinct, so that we can only infer their habits. It is, however, nearly certain that these ancient reef-forming corals lived under the same conditions as the modern groups, and that the temperature of the sea where they lived was tropical.

Absence of coral reefs from any formation does not prove that the temperature of that time was not tropical, for even now coral reefs are lacking in many parts of the tropics, on account of unfavorable conditions other than low temperature. Also the corals of the ancient reefs have often been obliterated by metamorphism, and only massive limestone left. Now while one swallow does not make a summer, one reef-building coral, or one palm, or one cyead does, since neither one of these organisms now lives outside of a warm climate.

 

Paleozoic Climate of the West Coast

All the Paleozoic sediments on the west coast are marine, and while the record is fragmentary, the evidence points uniformly to warm temperature of the sea, and, thus by inference, of the land. The Lower Cambrian, or Pre-Cambrian, glaciation of China and Australia has not been recognized in this part of the world, but this is merely negative, since land formations of that period are unknown here.

The Lower Cambrian limestones of Inyo County, California, and the adjacent region of Nevada, have extensive coral reefs of Archæocyathidæ; similar reefs are known in Europe and Australia, but not in the Arctic region.

In the Silurian of Plumas County, and the Devonian of Shasta County, California, there are coral reefs composed of Favositidse and Tetracoralla, and in both these ages similar reefs are known in Siberia and Alaska, which may show that the temperature of the sea had grown warmer in the middle Paleozoic, with a northward extension of the isotherms.

The Carboniferous of Shasta and Plumas counties, California, has great limestone masses full of reef-building Tetracoralla, and similar reefs are known up to 82° N\ lat., and down to the equator. Whatever the temperature was, it was remarkably uniform. The flora of the Coal Measures[2] in the northern hemisphere indicates a warm and equable climate for the land, extending up into the Arctic region, and without evidence of any trace of climatic zones.

The Permian, or Upper Carboniferous, glaciation, which was so widespread in India, Australia, South Africa and South America, has not been recognized in North America. But this event is now recognized as the greatest catastrophe in geologic history, and its effects probably extended far beyond the limits of glaciation. With the accompanying lowering of oceanic temperature, near the end of the Paleozoic era, the ancient types of reef-building corals, the Favositidae and Tetracoralla, disappeared. Hardly anything but solitary corals, that may have been deep-water forms, are left in the Permian, and in the Lower Triassic no corals of any sort are known.

The Hexacoralla, the modern reef-builders, had already originated in the Paleozoic, but were then little developed, unspecialized types. They escaped the general catastrophe either by being distributed in regions where the destruction did not take place, or by being then deep-sea forms, habituated to lower temperatures. When the amelioration of the earth's climate took place, near the beginning of the Mesozoic era, they found a free field on the coasts, and at once took possession. In the epoch of the Middle Triassic they had already become widely distributed, but as yet had formed no known reefs.

The distribution of the cephalopods in time shows a strong contrast to that of the corals. There is an unbroken genetic series of ammonoids and nautiloids from the Coal Measures, through the Permian, and extending into the Lower Triassic, several genera ranging through the interval. This does not necessarily mean that the cephalopods were hardier, for they probably were not. But they were very widely distributed, and must have lived on in some region, or regions, where great catastrophe had little or no effect, and by their superior facility in locomotion got back into the regions affected by glaciation, when the temperature of the seas had risen again.

 

Mesozoic Climates of the West Coast

Since corals are wholly unknown in the Lower Triassic, and since the flora of that epoch is as yet little known, it is not possible to determine the temperature of either the land or the water. It is, however, certain that the oceanic temperature in India, in western America and in northern Siberia, was the same, for there is a remarkable similarity of the cephalopod faunas in all three regions.

It is also known that in the Permian and the Lower Triassic a dry climate prevailed over large areas, for products of desiccation, such as gypsum and saline deposits are common in many parts of the world, and even in regions that are now rainy, as in western Europe.

In the Upper Triassic there are great limestone masses and coral reefs in the Alps, the Himalayas and in California, with many species common to the three regions. Certainly the epoch of the Tropites subbullatus fauna was tropical up as far as Shasta County, California, for there reefs of Astræidæ are extensive. We may even be justified in assuming that the isotherm of 74° F. extended that far north. Also in the Blue Mountains of northeastern Oregon there are coral reefs in the Upper Triassic, but no Astræidæ were found in them, only extinct genera. This outlying occurrence may correspond to the isotherm of 68° F., in which now corals may form reefs, but Astræidæ can not flourish.

After the formation of the coral reefs in northern California and Oregon the facies changed suddenly from limestones to clay shales, and with this came an abrupt change in the marine fauna. The Indian types of cephalopods disappeared entirely, and in their stead came in a fauna of which the home seems to have been the boreal region. Pseudomonotis ochotica was the commonest species in this fauna, and was widely distributed around the North Pacific. It has also been found as far south as Peru, on one side, and down to the equatorial part of the Indian Ocean on the other. This wide dispersion does not necessarily mean a lowering of the oceanic temperature during this epoch, for this species may have lived in deep water, and therefore could easily find uniform temperature from the equator to the Arctic region. But the sudden change of facies and impoverishment of the fauna over such an enormous area are suggestive. A slight drop in temperature, below 68° F. would account for it.

The last epoch of the Triassic, the Rhætic, has no marine faunas anywhere in America, but the flora, with its abundant cycads, is widely distributed in both the northern and the southern hemisphere. Coal deposits are common in this epoch, and this points to a very uniform and mild climate far beyond the present temperate zones.

At the opening of the Jurassic period we find a Mediterranean marine fauna established in western America; this same fauna also extended from the equatorial regions to Alaska, so that we are without evidence as to climatic zones, and can only infer that the temperature was uniform.

In the Middle Jurassic reef-building corals lived in the waters of the Great Basin Sea, and their remains are quite common in Plumas County, California, but in that province they formed no reefs, for the waters were not clear, and much disturbed by the deposition of volcanic ash. Abundant cycads lived on the land in California at this time, adding their testimony to the warmth of the climate. This same Middle Jurassic marine fauna extended up to Queen Charlotte Islands, and to southern Alaska, in the latter place with cycads interbedded with the salt-water fossils. Here, as was often the case, the cycads extended some distance north of the corals, a coral reef with Astræidæ being known in this epoch on Queen Charlotte Islands, in 53° N. lat., while cycads occur as far north as 57° N. lat. In this same epoch the northern limit for coral reefs in the Atlantic region was 53° N., in southern England, while the other invertebrates and cycads ranged up to 80° N. lat. A mild climate must have extended up nearly to the pole.

The Upper Jurassic of California shows a sharp contrast to the preceding epoch; its marine fauna is scanty, and what little there is belongs to the boreal type, the Aucella fauna, which is characteristic of Russia, northern Siberia and Alaska. For a short time this fauna ranged down into the edge of the tropics in Mexico. This does not mean that the climate was cold, but merely that the temperature was lower than that at which reef-building corals and the other sensitive invertebrates could flourish. In the Lower Cretaceous we find the same boreal type still persisting as far south as middle California. But here, as in the Upper Jurassic, the evidence is conflicting, for cycads are known in both formations.

In the Lower Cretaceous epoch there was a sharp contrast between conditions on the Pacific and those on the Atlantic side of America. In the Atlantic waters coral reefs extended as far north as Texas, while no corals at all are known in the Pacific waters of America in California. In the Upper Cretaceous, on the other hand, coral reefs extended to Ensenada, Lower California, lat. 31° 30′ N., while in the Atlantic waters they did not reach so far north. In other words, the Pacific waters on the western side of America became warmer in Upper Cretaceous time than they were in the preceding epoch, while in the Atlantic the conditions were reversed, as was the case also in southern Europe, where coral reefs extended much further north in the Lower Cretaceous than they did in the Upper Cretaceous.

The change in faunal geography in western America about the middle of the Cretaceous period is very remarkable. The Knoxville epoch had a boreal fauna, while with the opening of the Horsetown epoch the facies changed rather abruptly, and an Indian fauna came in. Swarms of ammonites of Indian type occupied the shallow marginal sea, showing at least a great change in geographic connections, if not in climate. It has been suggested by the writer that the opening of the Bering Sea passage during the Mariposa epoch of the Upper Jurassic and the Knoxville epoch of the Lower Cretaceous would account satisfactorily for the change of facies and the lowering of the temperature at that time. The closing of this passage near the end of the Knoxville epoch explains the change of facies from the boreal to the Indian type of fauna, and also the accompanying rise of oceanic temperature on the coasts of western America.

The favorable conditions, inaugurated in the middle of the Cretaceous, continued throughout the Chico epoch, during which coral reefs extended up to Ensenada, Lower California, N. lat. 31° 30′, and a warm climate prevailed even in Alaska. Beef-building corals extended up to the middle of California, but they formed no reefs, since there were no stretches of clear sheltered waters in which they could flourish.

 

Neozoic Climates of the West Coast

The Eocene climate of the west coast was nearly the same as that of the Upper Cretaceous. The marine deposits have numerous molluscan genera that are now confined to the tropics, and on the land palms abounded in California, Washington and Alaska. No reef-building corals of this age are yet known anywhere on the west coast, and it is probable that the marine temperature was slightly below that necessary for their existence in this region. The climate of the coast, from California to Alaska, was probably very much like that of the states bordering the Gulf of Mexico. There to-day many tropical molluscan genera are found in the waters, and on the marginal coastal plain there is a mixture of palms, deciduous trees and conifers. This is just what we find in the fossil Eocene flora of California and Puget Sound; laurels, figs, sycamores, chestnuts, elms, liquidambar, oaks, palms and sequoias lived together. From this association we should infer that the climate of the west coast was no longer tropical, but subtropical, and very rainy.

The middle Tertiary faunas are very like the present in the association of genera, and the flora on the land agrees with this. The palms have disappeared, but laurels still occur. It is probable that the climate of the upper Miocene had about the same temperature as that of the present in California, but it had, apparently, a much greater rainfall, or one much more evenly distributed.

The Tertiary flora of the west coast was immensely richer than the present. No elm, liquidambar, nor true laurel lives wild on the west coast now, and many other types that flourished here are gone. The impoverishment of the present tree flora of California, as compared with that of the Tertiary, has been ascribed to volcanic activity, bur this is absurd. In the first place the great extinction of the old types took place in the lowering of temperature near the end of Eocene time, while the era of great lava outbursts on the west coast was after the middle of the Miocene. The climate continued to cool off in the Pliocene, as is shown by the northern types of mollusca that then ranged as far south as Los Angeles, and by the freshwater lake deposits of middle California, which contain a fauna at present confined to the Klamath region of northern California and southern Oregon. The flora of the Pliocene in California is very scanty, composed largely of willows, alders and conifers, very much like that of the Olympic Peninsula in Washington.

The constantly decreasing temperature throughout the Tertiary is sufficient to account for the reduction of the flora. The tropical and finally the warm-temperate types were killed off locally, and such as were confined to this region were wholly extinguished. Some of the forms that lived in more favored regions to the south returned after the Glacial epoch. But most of the region to the south of California is not favorable to the extensive growth of forests, and many of the types have never returned to California, except when brought in by man.

In the early Quaternary there were extensive ice-sheets in the Sierra Nevada, and probably the climate of the sea-coast was cool. The glaciers came down the slopes to a line that is now about 3,500 feet above sea-level; it is thought, however, that California stood considerably higher than now, and that conditions here were more like those of the present on the Olympic Peninsula.

After the Glacial epoch was past the climate became warmer, and many mollusca crept slowly up the coast, from the warm waters of Lower California. This southern type reached as far north as Santa Barbara in the upper San Pedro epoch of the Quaternary, during which time the sea probably had a temperature as warm as it now is on the shores of Lower California.

This warming up of the west coast was no mere local phenomenon, for the same thing occurred at the same time on the eastern coast of America, when a warm-water fauna ranged up to the Champlain district. And also in Europe the climate after the Glacial epoch was, for a little while, warmer than it is at present. After the San Pedro epoch on the west coast, and the Champlain in the east the climatic conditions became approximately what they now are, although it may well be that the Terrace epoch had a larger rainfall than that of the present.

 

Summary

In the foregoing pages it will be noted that during all the known Paleozoic the west coast enjoyed a warm and probably tropical climate, with some suggestion of a northward march of the isotherms, reaching a culmination in the Upper Carboniferous. There is then some indication of a southward recession of the isotherms in the Permian, and a renewed northward advance in the Lower Triassic. This continued until the middle of the Jurassic, but the farthest north was never again reached in the Pacific waters.

In the Upper Jurassic and the Lower Cretaceous another considerable southward recession of the isotherms is indicated, followed by a renewed northward advance in the middle of the Cretaceous. But this advance did not reach so far north as that of the Middle Jurassic. The Eocene epoch shows the temperature of the west coast nearly holding its own, but with a probable slight reduction. The Miocene climate had grown considerably cooler than that of the Eocene, and by the Pliocene it was already rather cold as far south as California. The early Quaternary climate was probably even colder than the Pliocene, for there we have the local ice-sheets in the high mountains of California. The post-Glacial amelioration of climate is as distinct here as it was in eastern America, and in Europe, and probably as short-lived. Middle and late Quaternary time was probably much longer than we have been accustomed to consider it, and there have doubtless been considerable fluctuations in our climate in that period, but we have as yet been unable to decipher these in the geologic record of the west coast.

  1. J. D. Dana, "Corals and Coral Islands," 3d ed. (1890), pp. 108-114.
  2. David White, Jour. Geol, Vol. XVII., No. 4 (1909), p. 338.