* Our concept of the age of the earth is constantly undergoing revision as older and older rocks are discovered and as methods of study are refined. The oldest rocks now known in North America are in the Canadian Shield area. Their precise age has not been determined, but some from Manitoba and Ontario are believed to have been formed about 3 billion years ago. Even older rocks have been discovered in the Karelia Peninsula in the U.S.S.R., and in South Africa. Geologists are generally of the opinion that present concepts of geologic time will be considerably lengthened in the future. Tentative adjustments of the length of the various periods have already been made (see chart above) and the age of the Cambrian has been pushed back 100 million years compared with the dating assigned to it a decade ago. It is in that immense and shadowy time that preceded the Cambrian, however, that the greatest uncertainty exists. This is the time of the pre-fossiliferous rocks. Whatever life may have inhabited the earth during that time has left few traces, although by indirect evidence we may infer that life existed in some abundance before its record was written in the rocks.
By studies of the rocks themselves geologists have established a few good benchmarks standing out in those vast stretches of time indicated on the chart as the Proterozoic and Archeozoic Eras. These indicate a billion-year age for the ancient Grenville Mountains of eastern North America. Where these rocks are exposed at the surface, as in Ontario, they contain large amounts of graphite, giving silent testimony to the abundance of plant life when these rocks were forming, for plants are a common source of carbon. An age-reading of 1,700,000,000 years has been obtained in the Penokean Mountains of Minnesota and Ontario, formerly known to geologists as the Killarney Mountains. The remains of these once lofty mountains are still to be seen as low, rolling hills. The discovery of even older rocks in Canada, Russia, and Africa, dating back more than 3 billion years, suggests that the earth itself may have been formed about 4½ billion years ago.
The Pattern of the Surface
There is, one knows not what sweet mystery
about this sea, whose gently awful stirrings
seem to speak of some hidden soul beneath.
HERMAN MELVILLE
NOWHERE IN ALL the sea does life exist in such bewildering abundance as in the surface waters. From the deck of a vessel you may look down, hour after hour, on the shimmering discs of jellyfish, their gently pulsating bells dotting the surface as far as you can see. Or one day you may notice early in the morning that you are passing through a sea that has taken on a brick-red color from billions upon billions of microscopic creatures, each of which contains an orange pigment granule. At noon you are still moving through red seas, and when darkness falls the waters shine with an eerie glow from the phosphorescent fires of yet more billions and trillions of these same creatures.
And again you may glimpse not only the abundance but something of the fierce uncompromisingness of sea life when, as you look over the rail and down, down into water of a clear, deep green, suddenly there passes a silver shower of finger-long fishlets. The sun strikes a metallic gleam from their flanks as they streak by, driving deeper into the green depths with the desperate speed of the hunted. Perhaps you never see the hunters, but you sense their presence as you see the gulls hovering, with eager, mewing cries, waiting for the little fish to be driven to the surface.
Or again, perhaps, you may sail for days on end without seeing anything you could recognize as life or the indications of life, day after day of empty water and empty sky, and so you may reasonably conclude that there is no spot on earth so barren of life as the open ocean. But if you had the opportunity to tow a fine-meshed net through the seemingly lifeless water and then to examine the washings of the net, you would find that life is scattered almost everywhere through the surface waters like a fine dust. A cupful of water may contain millions upon millions of diatoms, tiny plant cells, each of them far too small to be seen by the human eye; or it may swarm with an infinitude of animal creatures, none larger than a dust mote, which live on plant cells still smaller than themselves.
If you could be close to the surface waters of the ocean at night, you would realize that then they are alive with myriads of strange creatures never seen by day. They are alive with the moving lamps of small shrimplike beings that spend the daylight hours in the gloom of deep water, and with the shadowy forms of hungry fish and the dark shapes of squid. These things were seen, as few men have seen them, by the Norwegian ethnologist Thor Heyerdahl in the course of one of the most unusual journeys of modern times. In the summer of 1947 Heyerdahl and five companions drifted 4300 miles across the Pacific on a raft of balsa logs, to test a theory that the original inhabitants of Polynesia might have come from South America by raft. For 101 days and nights these men lived practically on the surface of the sea, driven by the trade wind, carried on the strong drift of the Equatorial Current, as much a part of the inexorable westward movement of wind and water as the creatures of the sea. Because of his enviable opportunity to observe the life of the surface while living as an actual part of it for so many weeks, I asked Mr. Heyerdahl about some of his impressions, especially of the sea at night, and he has written me as follows:
Chiefly at night, but occasionally in broad daylight, a shoal of small squids shot out of the water precisely like flying fish, gliding through the air as much as up to six feet above the surface, until they lost the speed accumulated below water, and fell down helplessly. In their gliding flight with flaps out they were so much like small flying fish at a distance, that we had no idea we saw anything unusual until a live squid flew right into one of the crew and fell down on the deck. Almost every night we found one or two on the deck or on the roof of the bamboo hut.
It was my own definite impression that the marine life in general went deeper down in the daytime than during the nights, and that the darker the night was, the more life we had around us. At two different occasions, a snake-mackerel, Gempylus, never before seen by man except as skeletal remains washed ashore on South America and the Galapagos, came jumping clear out of the water and right up on the raft (once right into the hut). To judge from the huge eyes and the fact that the fish has never before been observed, I am inclined to suspect that it is a deep-sea fish that comes to the surface only at night.
On dark nights we could see much marine life which we were unable to identify. They seemed to be deep-sea fishes approaching the surface at night. Generally we saw it as vaguely phosphorescent bodies, often the size and shape of a dinner plate, but at least one night in the shape of three immense bodies of irregular and changing shape and dimensions which appeared to exceed those of the raft (Kon-Tiki measured about 45 by 18 feet). Apart from these greater bodies, we observed occasionally great quantities of phosphorescent plankton, often containing illuminating copepods up to the size of a millimeter or more.
With these surface waters, through a series of delicately adjusted, interlocking relationships, the life of all parts of the sea is linked. What happens to a diatom in the upper, sunlit strata of the sea may well determine what happens to a cod lying on a ledge of some rocky canyon a hundred fathoms below, or to a bed of multicolored, gorgeously plumed seaworms carpeting an underlying shoal, or to a prawn creeping over the soft oozes of the sea floor in the blackness of mile-deep water.
The activities of the microscopic vegetables of the sea, of which the diatoms are most important, make the mineral wealth of the water available to the animals. Feeding directly on the diatoms and other groups of minute unicellular algae are the marine protozoa, many crustaceans, the young of crabs, barnacles, sea worms, and fishes. Hordes of the small carnivores, the first link in the chain of flesh eaters, move among these peaceful grazers. There are fierce little dragons half an inch long, the sharp-jawed arrow-worms. There are gooseberrylike comb jellies, armed with grasping tentacles, and there are the shrimplike euphausiids that strain food from the water with their bristly appendages. Since they drift where the currents carry them, with no power or will to oppose that
of the sea, this strange community of creatures and the marine plants that sustain them are called ‘plankton,’ a word derived from the Greek, meaning ‘wandering.’
From the plankton the food chains lead on, to the schools of plankton-feeding fishes like the herring, menhaden, and mackerel; to the fish-eating fishes like the bluefish and tuna and sharks; to the pelagic squids that prey on fishes; to the great whales who, according to their species but not according to their size, may live on fishes, on shrimps, or on some of the smallest of the plankton creatures.
Unmarked and trackless though it may seem to us, the surface of the ocean is divided into definite zones, and the pattern of the surface water controls the distribution of its life. Fishes and plankton, whales and squids, birds and sea turtles, all are linked by unbreakable ties to certain kinds of water—to warm water or cold water, to clear or turbid water, to water rich in phosphates or in silicates. For the animals higher in the food chains the ties are less direct; they are bound to water where their food is plentiful, and the food animals are there because the water conditions are right.
The change from zone to zone may be abrupt. It may come upon us unseen, as our ship at night crosses an invisible boundary line. So Charles Darwin on H.M.S. Beagle one dark night off the coast of South America crossed from tropical water into that of the cool south. Instantly the vessel was surrounded by numerous seals and penguins, which made such a bedlam of strange noises that the officer on watch was deceived into thinking the ship had, by some miscalculation, run close inshore, and that the sounds he heard were the bellowing of cattle.
To the human senses, the most obvious patterning of the surface waters is indicated by color. The deep blue water of the open sea far from land is the color of emptiness and barrenness; the green water of the coastal areas, with all its varying hues, is the color of life. The sea is blue because the sunlight is reflected back to our eyes from the water molecules or from very minute particles suspended in the sea. In the journey of the light rays into deep water all the red rays and most of the yellow rays of the spectrum have been absorbed, so when the light returns to our eyes it is chiefly the cool blue rays that we see. Where the water is rich in plankton, it loses the glassy transparency that permits this deep penetration of the light rays. The yellow and brown and green hues of the coastal waters are derived from the minute algae and other microorganisms so abundant there. Seasonal abundance of certain forms containing reddish or brown pigments may cause the ‘red water’ known from ancient times in many parts of the world, and so common is this condition in some enclosed seas that they owe their names to it—the Red Sea and the Vermilion Sea are examples.
The colors of the sea are only the indirect signs of the presence or absence of conditions needed to support the surface life; other zones, invisible to the eye, are the ones that largely determine where marine creatures may live. For the sea is by no means a uniform solution of water; parts of it are more salty than others, and parts are warmer or colder.
The saltiest ocean water in the world is that of the Red Sea, where the burning sun and the fierce heat of the atmosphere produce such rapid evaporation that the salt content is 40 parts per thousand. The Sargasso Sea, an area of high air temperatures, receiving no inflow of river water or melting ice because of its remoteness from land, is the saltiest part of the Atlantic, which in turn is the saltiest of the oceans. The polar seas, as one would expect, are the least salty, because they are constantly being diluted by rain, snow, and melting ice. Along the Atlantic coast of the United States, the salinity range from about 33 parts per thousand off Cape Cod to about 36 off Florida is a difference easily perceptible to the senses of human bathers.
Ocean temperatures vary from about 28° F. in polar seas to 96° in the Persian Gulf, which contains the hottest ocean water in the world. To creatures of the sea, which with few exceptions must match in their own bodies the temperature of the surrounding water, this range is tremendous, and change of temperature is probably the most important single condition that controls the distribution of marine animals.
The beautiful reef corals are a perfect example of the way the inhabitable areas for any particular class of creatures may be established by temperatures. If you took a map of the world and drew a line 30° north of the equator and another 30° south of it, you would have outlined in general the waters where reef corals are found at the present time. It is true that the remains of ancient coral reefs have been discovered in arctic waters, but this means that in some past ages the climate of these northern seas was tropical. The calcareous structure of the coral reef can be fashioned only in water at least as warm as 70° Fahrenheit. We would have to make one northward extension of our map, where the Gulf Stream carries water warm enough for corals to Bermuda, at 32° north latitude. On the other hand, within our tropical belt, we would have to erase large areas on the west coasts of South America and Africa, where upwelling of cold water from lower ocean levels prevents the growth of corals. Most of the east coast of Florida has no coral reefs because of a cool inshore current, running southward between the coast and the Gulf Stream.
As between tropical and polar regions, the differences in the kinds and abundance of life are tremendous. The warm temperatures of the tropics speed up the processes of reproduction and growth, so that many generations are produced in the time required to bring one to maturity in cold seas. There is more opportunity for genetic mutations to be produced within a given time; hence the bewildering variety of tropical life. Yet in any species there are far fewer individuals than in the colder zones, where the mineral content of the water is richer, and there are no dense swarms of surface plankton, like the copepods of the Arctic. The pelagic, or free-swimming, forms of the tropics live deeper than those of the colder regions, and so there is less food for large surface-feeders. In the tropics, therefore, the sea birds do not compare in abundance with the clouds of shearwaters, fulmars, auks, whalebirds, albatrosses, and other birds seen over far northern or far southern fishing grounds.
In the cold-water communities of the polar seas, fewer of the animals have swimming larvae. Generation after generation settle down near the parents, so that large areas of bottom may be covered with descendants of a very few animals. In the Barents Sea a research vessel once brought up more than a ton of one of the siliceous sponges at a single haul, and enormous patches of a single species of annelid worm carpet the east coast of Spitsbergen. Copepods and swimming snails fill the surface waters of the cold seas, and lure the herring and the mackerel, the flocks of sea birds, the whales, and the seals.
In the tropics, then, sea life is intense, vivid, and infinitely varied. In cold seas it proceeds at a pace slowed by the icy water in which it exists, but the mineral riches of these waters (largely a result of seasonal overturn and consequent mixing) makes possible the enormous abundance of the forms that inhabit them. For a good many years it has been said categorically that the total productivity of the colder temperate and polar seas is far greater than the tropical. Now it is becoming plain that there are important exceptions to this statement. In certain tropical and subtropical waters, there are areas where the sheer abundance of life rivals the Grand Banks or the Barents Sea or any antarctic whaling ground. Perhaps the best examples are the Humboldt Current, off the west coast of South America, and the Benguela Current, off the west coast of Africa. In both currents, upwelling of cold, mineral-laden water from deeper layers of the sea provides the fertilizing elements to sustain the great food chains.
And wherever two currents meet, especially if they differ sharply in temperature or salinity, there are zones of great turbulence and unrest, with water sinking or rising up from the depths and with shifting eddies and foam lines at the surface. At such places the richness and abundance of marine life reveals itself most strikingly. This changing life, seen as his ship cut across the pathways of the great currents of the Pacific and the Atlantic, was described with vivid detail by S. C. Brooks:
Within a few degrees of t
he equator, the scattered cumulus clouds become thicker and grayer, a confused swell makes up, rain squalls come and go, and birds appear. At first there is only a greater abundance of storm petrels, with here and there petrels of other kinds hunting along utterly indifferent to the ship, or small groups of tropic birds flying along with the ship, off to one side or high overhead. Then scattered groups of various petrels appear, and finally for an hour or two there are birds on every hand. If one is not too far from land, a few hundred miles perhaps, as in the case of the south equatorial drift north of the Marquesas, one may also see multitudes of sooty or crested terns. Occasionally one sees the grayish blue form of a shark gliding along, or a big purplish-brown hammerhead lazily twisting around as though trying to get a better view of the ship. Flying fish, while not so closely localized as the birds, are breaking the water every few seconds, and bewitch the beholder by their myriad sizes, shapes, and antics, and their bewildering patterns and shades of deep brown, opal blue, yellow and purple. Then the sun comes out again, the sea takes on its deep tropical blue, the birds become more and more scarce, and gradually, as the ship moves on, the ocean resumes its desert aspect.
If it were daylight all the time, this same sequence might be seen in a more or less striking fashion twice or perhaps even three or four times. Inquiry soon reveals that this sequence marks the time of passing the edge of one of the great currents …
The Sea Around Us Page 3
