The Sea Around Us (13 page)

Before these techniques were developed, we could only guess at the thickness of the sediment blanket over the floor of the sea. We might have expected the amount to be vast, if we thought back through the ages of gentle, unending fall—one sand grain at a time, one fragile shell after another, here a shark's tooth, there a meteorite fragment—but the whole continuing persistently, relentlessly, endlessly. It is, of course, a process similar to that which has built up the layers of rock that help to make our mountains, for they, too, were once soft sediments under the shallow seas that have overflowed the continents from time to time. The sediments eventually became consolidated and cemented and, as the seas retreated again, gave the continents their thick, covering layers of sedimentary rocks-—layers which we can see uplifted, tilted, compressed, and broken by the vast earth movements. And we know that in places the sedimentary rocks are many thousands of feet thick. Yet most people felt a shock of surprise and wonder when Hans Pettersson, leader of the Swedish Deep Sea Expedition, announced that the
Albatross
measurements taken in the open Atlantic basin showed sediment layers as much as 12,000 feet thick.

If more than two miles of sediments have been deposited on the floor of the Atlantic, an interesting question arises: has the rocky floor sagged a corresponding distance under the terrific weight of the sediments? Geologists hold conflicting opinions. The recently discovered Pacific sea mounts may offer one piece of evidence that it has. If they are, as their discoverer called them, ‘drowned ancient islands,' then they may have reached their present stand a mile or so below sea level through the sinking of the ocean floor. Hess believed the islands had been formed so long ago that coral animals had not yet evolved; otherwise the corals would presumably have settled on the flat, planed surfaces of the sea mounts and built them up as fast as their bases sank. In any event, it is hard to see how they could have been worn down so far below ‘wave base' unless the crust of the earth sagged under its load.

One thing seems probable—the sediments have been unevenly distributed both in place and time. In contrast to the 12,000-foot thickness found in parts of the Atlantic, the Swedish oceanographers never found sediments thicker than 1000 feet in the Pacific or in the Indian Ocean. Perhaps a deep layer of lava, from ancient submarine eruptions on a stupendous scale, underlies the upper layers of the sediments in these places and intercepts the sound waves.

Interesting variations in the thickness of the sediment layer of the Atlantic Ridge and the approaches to the Ridge from the American side were reported by Ewing. As the bottom contours became less even and began to slope up into the foothills of the Ridge, the sediments thickened, as though piling up into mammoth drifts 1000 to 2000 feet deep against the slopes of the hills. Farther up in the mountains of the Ridge, where there are many level terraces from a few to a score of miles wide, the sediments were even deeper, measuring up to 3000 feet. But along the backbone of the Ridge, on the steep slopes and peaks and pinnacles, the bare rock emerged, swept clean of sediments.
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Reflecting on these differences in thickness and distribution, our minds return inevitably to the simile of the long snowfall. We may think of the abyssal snowstorm in terms of a bleak and blizzard-ridden arctic tundra. Long days of storm visit this place, when driving snow fills the air; then a lull comes in the blizzard, and the snowfall is light. In the snowfall of the sediments, also, there is an alternation of light and heavy falls. The heavy falls correspond to the periods of mountain building on the continents, when the lands are lifted high and the rain rushes down their slopes, carrying mud and rock fragments to the sea; the light falls mark the lulls between the mountain-building periods, when the continents are flat and erosion is slowed. And again, on our imaginary tundra, the winds blow the snow into deep drifts, filling in all the valleys between the ridges, piling the snow up and up until the contours of the land are obliterated, but scouring the ridges clear. In the drifting sediments on the floor of the ocean we see the work of the ‘winds,' which may be the deep ocean currents, distributing the sediments according to laws of their own, not as yet grasped by human minds.

We have known the general pattern of the sediment carpet, however, for a good many years. Around the foundations of the continents, in the deep waters off the borders of the continental slopes, are the muds of terrestrial origin. There are muds of many colors—blue, green, red, black, and white—apparently varying with climatic changes as well as with the dominant soils and rocks of the lands of their origin. Farther at sea are the oozes of predominantly marine origin—the remains of the trillions of tiny sea creatures. Over great areas of the temperature oceans the sea floor is largely covered with the remains of unicellular creatures known as foraminifera, of which the most abundant genus is Globigerina. The shells of Globigerina may be recognized in very ancient sediments as well as in modern ones, but over the ages the species have varied. Knowing this, we can date approximately the deposits in which they occur. But always they have been simple animals, living in an intricately sculptured shell of carbonate of lime, the whole so small you would need a microscope to see its details. After the fashion of unicellular beings, the individual Globigerina normally did not die, but by the division of its substance became two. At each division, the old shell was abandoned, and two new ones were formed. In warm, lime-rich seas these tiny creatures have always multiplied prodigiously, and so, although each is so minute, their innumerable shells blanket millions of square miles of ocean bottom, and to a depth of thousands of feet.

In the great depths of the ocean, however, the immense pressures and the high carbon-dioxide content of deep water dissolve much of the lime long before it reaches the bottom and return it to the great chemical reservoir of the sea. Silica is more resistant to solution. It is one of the curious paradoxes of the ocean that the bulk of the organic remains that reach the great depths intact belong to unicellular creatures seemingly of the most delicate construction. The radiolarians remind us irresistibly of snow flakes, as infinitely varied in pattern, as lacy, and as intricately made. Yet because their shells are fashioned of silica instead of carbonate of lime, they can descend unchanged into the abyssal depths. So there are broad bands of radiolarian ooze in the deep tropical waters of the North Pacific, underlying the surface zones where the living radiolarians occur most numerously.

Two other kinds of organic sediments are named for the creatures whose remains compose them. Diatoms, the microscopic plant life of the sea, flourish most abundantly in cold waters. There is a broad belt of diatom ooze on the floor of the Antarctic Ocean, outside the zone of glacial debris dropped by the ice pack. There is another across the North Pacific, along the chain of great deeps that run from Alaska to Japan. Both are zones where nutrient-laden water wells up from the depths, sustaining a rich growth of plants. The diatoms, like the radiolarians are encased in silicious coverings—small, boxlike cases of varied shape and meticulously etched design.

Then, in relatively shallow parts of the open Atlantic, there are patches of ooze composed of the remains of delicate swimming snails, called pteropods. These winged mollusks, possessing transparent shells of great beauty, are here and there incredibly abundant. Pteropod ooze is the characteristic bottom deposit in the vicinity of Bermuda, and a large patch occurs in the South Atlantic.

Mysterious and eerie are the immense areas, especially in the North Pacific, carpeted with a soft, red sediment in which there are no organic remains except sharks' teeth and the ear bones of whales. This red clay occurs at great depths. Perhaps all the materials of the other sediments are dissolved before they can reach this zone of immense pressures and glacial cold.

The reading of the story contained in the sediments has only begun. When more cores are collected and examined we shall certainly decipher many exciting chapters. Geologists have pointed out that a series of cores from the Mediterranean might settle several controversial problems concerning the history of the ocean and of the lands around the Mediterranean basin. For example, somewhere in the layers of sediment under this sea there must be evidence, in a sharply defined layer of sand, of the time when the deserts of the Sahara were formed and the hot, dry winds began to skim off the shifting surface layers and carry them seaward. Long cores recently obtained in the western Mediterranean off Algeria have given a record of volcanic activity extending back through thousands of years, and including great prehistoric eruptions of which we know nothing.

The Atlantic cores taken more than a decade ago by Piggot from the cable ship
Lord Kelvin
have been thoroughly studied by geologists. From their analysis it is possible to look back into the past 10,000 years or so and to sense the pulse of the earth's climatic rhythms; for the cores were composed of layers of cold-water Globigerina faunas (and hence glacial stage sediments), alternating with Globigerina ooze characteristic of warmer waters. From the clues furnished by these cores we can visualize interglacial stages where there were periods of mild climates, with warm water overlying the sea bottom and warmth-loving creatures living in the ocean. Between these periods the sea grew chill. Clouds gathered, the snows fell, and on the North American continent the great ice sheets grew and the ice mountains moved out to the coast. The glaciers reached the sea along a wide front; there they produced icebergs by the thousand. The slow-moving, majestic processions of the bergs passed out to sea, and because of the coldness of much of the earth they penetrated farther south than any but stray bergs do today. When finally they melted, they relinquished their loads of silt and sand and gravel and rock fragments that had become frozen into their under surfaces as they made their grinding way over the land. And so a layer of glacial sediment came to overlie the normal Globigerina ooze, and the record of an Ice Age was inscribed.

Then the sea grew warmer again, the glaciers melted and retreated, and once more the warmer-water species of Globigerina lived in the sea—lived and died and drifted down to build another layer of Globigerina ooze, this time over the clays and gravels from the glaciers. And the record of warmth and mildness was again written in the sediments. From the Piggot cores it has been possible to reconstruct four different periods of the advance of the ice, separated by periods of warm climate.

It is interesting to think that even now, in our own lifetime, the flakes of a new snow storm are falling, falling, one by one, out there on the ocean floor. The billions of Globigerina are drifting down, writing their unequivocal record that this, our present world, is on the whole a world of mild and temperate climate. Who will read their record, ten thousand years from now?

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Now that the sediments have been measured over much greater areas of the ocean floor, the reaction of oceanographers is one of considerable amazement— but their surprise concerns the fact that on the whole the mantle of sediments is so much thinner than related facts would lead them to expect. Over vast areas of the Pacific the average thickness of the sediments (unconsolidated sediments plus sedimentary rock) is only about a quarter of a mile. It is little thicker over much of the Atlantic. (These are average figures; some much deeper deposits of course exist.) In some areas there has been almost no sedimentation. A few years ago several oceanographers obtained photographs of manganese nodules lying on the floor of the Atlantic at great depths and of others on the Easter Island Ridge of the southeast Pacific. Sharks' teeth dating from the Tertiary, hence possibly as much as 70 million years old, sometimes form the nuclei of these nodules. Certainly their growth, by deposit of successive layers around the nuclei, must be very slow. Hans Pettersson has estimated a growth of about 1 mm. per thousand years. Yet during the period these nodules have lain on the ocean floor, sediments deep enough to cover them have not been accumulated.

Some idea of the rate of sedimentation during post-glacial time has been gained by observation of the rate of radioactive decay of some of the components of the sediments. If this sedimentation rate had prevailed during the supposed life of the oceans, the average thickness of the sediments would be enormously greater than it now appears to be. Did much of the deposited sediments dissolve? Were most of the present land masses submerged for far greater periods than we now assume, with consequently long periods of slight erosion? These and other explanations of the mystery of the sediments have been suggested, but none seems wholly satisfying. Possibly the dramatic project of boring holes in the floor of the ocean down to the Mohorovicic discontinuity (Project Mohole; see Preface) will provide the explanation that is now lacking.

The Birth of an Island

Many a green isle needs must be
In the deep, wide sea …

SHELLEY

MILLIONS OF YEARS AGO
, a volcano built a mountain on the floor of the Atlantic. In eruption after eruption, it pushed up a great pile of volcanic rock, until it had accumulated a mass a hundred miles across at its base, reaching upward toward the surface of the sea. Finally its cone emerged as an island with an area of about 200 square miles. Thousands of years passed, and thousands of thousands. Eventually the waves of the Atlantic cut down the cone and reduced it to a shoal—all of it, that is, but a small fragment which remained above water. This fragment we know as Bermuda.

With variations, the life story of Bermuda has been repeated by almost every one of the islands that interrupt the watery expanses of the oceans far from land. For these isolated islands in the sea are fundamentally different from the continents. The major land masses and the ocean basins are today much as they have been throughout the greater part of geologic time. But islands are ephemeral, created today, destroyed tomorrow. With few exceptions, they are the result of the violent, explosive, earth-shaking eruptions of submarine volcanoes, working perhaps for millions of years to achieve their end. It is one of the paradoxes in the ways of earth and sea that a process seemingly so destructive, so catastrophic in nature, can result in an act of creation.