Coral reefs: Architects of the lithosphere and regulators of the planetary carbon cycle

“ Every one has heard of coral reefs. They are one of the best and most familiar examples of the way in which great masses of solid rock can be built up by the dead bodies of animals. In the warmer seas of the earth, and notably in the track of the great ocean currents, various kinds of coral polyps, as they are called, take root on the edges and summits of submerged rocks and peaks, as well -64- as on the shelving shores of islands. The coral polyp is a jelly-like creature, but it has a hard chalky skeleton inside its transparent body. ”

“ Perhaps the grandest reef to be found in any part of the world is the one extending along the northeast coast of Australia. It is nearly one thousand miles in length, and proves to us that the helpless coral polyps have played no trifling part in the formation of our earth. All they have accomplished has been done merely by their living and growing. ”

“ Those pyramids of coral rock are built upon a foundation which is itself formed by the deposits which the geologist has to deal with. If we go back in time and search through the series of the rocks, we find at every age of the world's history which has yet been examined, accumulations of limestone, many of which have certainly been built up in just the same way as those coral reefs which are now forming the bottom of the Pacific Ocean. ”

“ There are coral reefs and coral trees, domes and balls of coral, graceful vases, and all sorts and kinds of different plants and odd growths.
“You know that living coral cannot exist above the surface of the ocean, for exposure to the sun and air kills the polyps; yet it is always growing upward and outward, the living animals making their homes upon the tombs of their ancestors, so to speak, until they in their turn perish and add their skeletons to the growing structure. ”

“ In short, here we have evidence of new factors in world-building, of land and ocean, of atmospheric decay of rocks, of deoxidizing processes carried on by vegetable life on the land and in the waters, of limestone-building in the sea. To afford material for such rocks, the old Ottawa gneiss must have been lifted up into continents and mountain masses by bendings and foldings of the original crust. Under the slow but sure action of the carbon dioxide dissolved in rainwater, its felspar had crumbled down in the course of ages. ”

“ Darwin was the first to show how sure a proof of geological changes such coral reefs are. The polyps which build them die at a depth of thirty metres, and the contact with the atmosphere is fatal to them. Hence, very deep coral structures denote a sinking, those above the water, on the contrary, an upheaval of the earth's crust. ”

“ We have seen that in S. America an enormous area has been rising within the recent period; and in other quarters of the globe immense spaces appear to have risen contemporaneously. From my examination of the coral-reefs of the great oceans, I have been led to conclude that the bed of the sea has gone on slowly sinking within the present era, over truly vast areas: this, indeed, is in itself probable, from the simple fact of the rising areas having been so large. ”

“ In the bottom of the sea, while sand and mud and gravel were being deposited in successive layers in some portions of the ocean floor, in others great reefs of Eozoon were growing up in the manner of reefs of coral. If we can imagine the modern Pacific, with its volcanic islands and reefs of coral, to be deprived of all other forms of life, we should have a somewhat accurate picture of the Eozoic time as it appears to us now. ”

“ A Crocodile and the fleshless skeleton of the Ichthyosaurus remind us that Reptiles still occupied an important place in the animal creation. A few Insects, especially Dragon-flies, fly about in the air. Ammonites float on the surface of the waves, and the terrible Plesiosaurus, like a gigantic swan, swims about in the sea. The circular reef of coral, the work of ancient Polyps, foreshadows the atolls of the great ocean, for it was during the Jurassic period that the Polyps of the ancient world were most active in the production of coral-reefs and islets. ”

“ These ancient inland seas were seldom over five or six hundred feet deep, and sunlight can penetrate ocean water for more than six hundred feet.
And it was from such seashores of the mild and equable climes of a later age that primitive plant life found its way onto the land. There the high degree of carbon in the atmosphere afforded the new land varieties of life opportunity for speedy and luxuriant growth. Though this atmosphere was then ideal for plant growth, it contained such a high degree of carbon dioxide that no animal, much less man could have lived on the face of the earth. ”

“ Nor have we a right to infer that the growth of the coal-plants was rapid. Stems, with woody axes and a thick bark, containing much fibrous and thick-walled cellular tissue, are not to be compared with modern succulent plants, especially when we consider the sparse and rigid foliage of many of them. Our conclusion should, therefore, be that geographical conditions and the abundance of carbon dioxide in the atmosphere favoured a moist climate and uniform temperature, and that the flora was suited to these conditions. ”

“ We have, moreover, no right to calculate on unlimited time for the accumulation of small organic bodies into great masses. Every fact in geology proclaims that neither the land, nor the bed of the sea retain for indefinite periods the same level. As well might it be imagined that the British Seas would in time become choked up with beds of oysters, or that the numerous small corallines off the inhospitable shores of Tierra del Fuego would in time form a solid and extensive coral-reef. ”

“ From the facts given in this section, it may be concluded, first, that considerable thicknesses of rock have certainly been formed within the present geological area by the growth of coral and the accumulation of its detritus; and, secondly, that the increase of individual corals and of reefs, both outwards or horizontally and upwards or vertically, under the peculiar conditions favourable to such increase, is not slow, when referred either to the standard of the average oscillations of level in the earth’s crust, or to the more precise but less important one of a cycle of years. ”

“ The forests of the Tertiary Era would steadily reduce it, but the extensive upheaval of the land at its close would be even more important. The newly exposed surfaces would absorb great quantities of carbon. The ocean, also, as it became colder, would absorb larger and larger quantities of carbon-dioxide. Thus the Pleistocene atmosphere, gradually relieved of its vapours and carbon-dioxide, would no longer retain the heat at the surface. We may add that the growth of reflective surfaces—ice, snow, cloud, etc.—would further lessen the amount of heat received from the sun. ”

“ The naturalists who have visited the Pacific, seem to have had their attention riveted by the lagoon-islands, or atolls,—those singular rings of coral-land which rise abruptly out of the unfathomable ocean—and have passed over, almost unnoticed, the scarcely less wonderful encircling barrier-reefs. The theory most generally received on the formation of atolls, is that they are based on submarine craters ”

“ The rising generation of naturalists and geologists have not had, and most probably will never have, such feelings of intellectual pleasure as fell to the lot of the readers of Charles Darwin's book on 'Coral Reefs,' which was offered to science more than thirty years ago. The recent researches into the nature of the deposits of the deep-sea, and the discoveries of bathymetrical zones of water of very different temperatures, are certainly full of vast interest, and will afford the data for the development of many a theory ”

“ I am, then, fully justified in repeating, that enormous spaces, both in the Pacific and Indian Oceans, are interspersed with islands, of which not one rises above that height, to which the waves and winds in an open sea can heap up matter. On what foundations, then, have these reefs and islets of coral been constructed? A foundation must originally have been present beneath each atoll at that limited depth, which is indispensable for the first growth of the reef-building polypifers. ”

“ This theory is supported by a vast amount of curious and ingenious reasoning. In the first place, there is nothing more remarkable in the physical conformation of the Globe, than the immense predominance of water over land throughout those extensive tracts of ocean where Coral Reefs abound. Now this is just what we should naturally expect if the hypothesis of Mr. Darwin were admitted; for wherever the Crust of the Earth has been subsiding for many ages on a large scale, the domain of the sea must of necessity have been considerably enlarged, and 138 that of the land contracted in proportion. ”

“ Turning now to the red colour; as on our map, the areas which have sunk slowly downwards to great depths are many and large, we might naturally have been led to conjecture, that with such great changes of level in progress, the coasts which have been fringed probably for ages (for we have no reason to believe that coral-reefs are of short duration) , would not have remained all this time stationary, but would frequently have undergone movements of elevation. ”

“ In many places the coral rocks were covered with vegetation—the beginning, as it appeared to us, of future islands. Thus, on this reef, we came to perceive how most of the small islands of those seas are formed. On one part we saw the spray of the breaker washing over the rocks, and millions of little, active, busy creatures continuing the work of building up this living rampart. At another place, which was just a little too high for the waves to wash over it, the coral insects were all dead ”

“ Lurie said. "Why is an artificial supply of oxygen necessary?"
"The answer is very simple," Baker said. "This is a young planet, as planets go. The conditions here are just about what they were on Earth eons ago. The carbon dioxide exhaled from the interior of the planet and saturating the atmosphere is gradually being converted by the plant life—broken down, I should say, releasing free oxygen. At the present time, our oxygen content is about seven percent. Eventually, the oxygen will probably reach the level you find on the surface of the Earth—about twenty-one percent. ”

“ It should not be confused with the opposite action of plants under the influence of sunlight in taking in and decomposing carbon dioxide and expelling pure oxygen.
Carbon Dioxide. It forms the chief food supply of all green-leaved plants. It is as necessary to the life of vegetation as is oxygen in the supporting of animal life. In the ratio of seventy-seven to one hundred there is less of this gas present in the atmosphere in the winter than in the summer; there also is a diurnal maximum and minimum. In the open country the amount averages about 0.035 per cent. ”

“ The oxygen, though a permanent element in the atmosphere, tends constantly to enter into combinations which fix it temporarily or permanently in the earth, in which it forms, indeed, in its combined state about one half the weight of all the mineral substances we know. The carbon dioxide, or carbonic-acid gas, as it is commonly termed, is a most important substance, as it affords plants all that part of their bodies which disappear on burning. It is constantly returned to the atmosphere by the decay of organic matter, as well as by volcanic action. ”

“ Another potent factor that has wrought tremendously for evolution has been the changing environment. We may briefly consider the character of its work. For millions of years before the close of the Carboniferous period, some ten million years ago, the whole earth enjoyed a perpetual summer. During the last million years or more of that vast era, while the dense Carboniferous forests, overwhelmed again and again by the growing soil, were forming the coal deposits of the earth, many kinds of amphibians and low reptiles wandered over the marshes and among the trees. ”

“ When vegetation decays in open air the carbon of its tissues, taken from the atmosphere by the leaves, is oxidized and returned to it in its original form of carbon dioxide. But decomposing in the presence of water, as in a bog, where the oxygen of the air is excluded, the carbonaceous matter of plants accumulates in deposits of peat. ”

“ Zooplankton, mostly copepods, collected with automatic underwater sampling equipment on board the nuclear submarine Seadragon while cruising under the Arctic ice.
Other plankton research at Woods Hole uses radioactive carbon-14 and phosphorus-32 as tracers to evaluate rates of growth and nutrient assimilation by algae (floating green plants) . These investigations have revealed that the presence or absence of minute quantities of nutrient minerals in seawater affects the rate at which the algae produce oxygen by the process of photosynthesis. ”

“ With climate, however, it was a very different matter. With polyp corals building reefs almost to the pole (81° 50′) , [1] as far north nearly as man has yet by his utmost efforts succeeded in getting, while their fellows were busy at the like industry in the tropics, it is clear that latitude was laughed at and climate even lacked a name.
Another astronomic feature, then for the first time disclosed, was the full significance of the day and the revelation of its cause. While the Earth brooded under perpetual cloud, there could have been but imperfect recognition of day and night. ”

“ But I believe I very much understate both the height of some of these masses, and overstate the amount which these animals can form in the course of a year; so that you might very safely double the period as the time during which the Pacific Ocean, the general state of the climate, and the sea, and the temperature has been substantially what it is now; and yet that state of things which now obtains in the Pacific Ocean is the yesterday of the history of the life of the globe. ”

“ Where shall we look for the living analogues of the numerous fossil ferns, including arborescent species of great size, the sigillariæ, lepidodendra, and gigantic equisetaceæ, which fill the coal shales of England, the cycadeæ, coniferæ of the oolites, and the palms of the tertiary rocks of France?
In what climate grow the modern coral reefs comparable to the fossil zoophytic rocks? where live the parallels to thousands of echinida, crinoidea, trilobites, brachiopoda, cephalopoda, sauroid fishes, crocodiles, pachydermata, ruminantia, which characterise different geological periods? ”