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Deep-Sea Life

water, bottom, ocean, currents, sea and fathoms

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DEEP-SEA LIFE. In this account of or ((pelagic]) life the writer purposes •to restrict himself to the open ocean, that is the spaces of deep water covering the real oceanic basins. The great land-masses are bordered by a submerged rim of varying width, where the water does not exceed an average depth of 100 fathoms, beyond which the bottom falls or slopes to the plain of the original ocean-basin. The sum of these continental borders and simi lar shallows elsewhere, as in the archipelagic region of the western Pacific, forms only about 15 per cent of the 140,000,000 square miles of salt water, leaving more than four-fifths of it deeper than 100 fathoms, while nearly 10, 000,000 square miles exceeds 3,000 fathoms in depth.

We speak of aoceans,s but in reality it is all one body of water, which by a constant inter change through currents both horizontal and vertical, maintains a virtual uniformity, yet not a complete one, since distinct variations are ob servable in temperature, density (salinity), pres Sure and other physical characteristics that in fluence animal being. Therefore some parts of the ocean are more populous than others; some are more populous in summer and less so in winter, and there is a definite (although as yet undetermined) geographical distribution of marine life, both horizontal and vertical.

Ocean Floors.—The character of the bot tom must be considered. The rivers discharge into the ocean daily an enormous amount of land-material, which is sorted out by weight, readiness to dissolve, etc., and by the action of waves and currents, and is spread out on the bottom to a greater or less distance according to circumstances, but nowhere goes far from the coast. This outer border of deposits from rivers shows a bottom of bluish mud, or, in places, of coral sand or of volcanic dust. It is rich in food-material, and • supports in the water above it a far larger assemblage of plants and animals than does the sea outside of it We have no further concern• with this littoral rim further than to point out that, by means of currents, it contributes considerable food to pelagic animals. The floor of the vast

spaces of ocean-bottom shows none of this land mad, but is covered with deposits of three kinds. The most extensive of these is a red clay, which covers the bottom in the great bight of the Atlantic between North and South America west of the 50th meridian, a large part of the mid-Atlantic, and most of the floor of the Pacific and Indian Oceans; it is believed to have been derived from the decomposition of submarine rocks, and of pumice and other vol canic debris showered into the sea, mainly in past ages.

Scarcely less widespread is a loose deposit called ((ooze,n composed almost exclusively of the remains of minute animals and plants that have silicious or lime-carbonate skeletons which have sunk down from the surface. All over the bed of the Atlantic, except where the red clay appears, the ooze shows more tests of the protozoan Globigerina than of anything else, and it is therefore known as globogerina ooze. Pteropod ooze, patches of which occur here and there, has a predominance of pteropod shells; and so on. Bottom of this kind is far more thickly inhabited by benthonic (deep-sea) ani mals than is the clay bottom; and it lies under the paths of the great oceanic currents, whereas the red day underlies chiefly the areas of com paratively stationary water — the great eddies in the oceans.

These oozes imply an abundant life in the open sea. What is its character and how does it exist in a world of only sea-water? Factors of Deep-Sea Life.— First we must glance at certain conditions. The pressure exerted by water on anything lowered into it increases at a rapid rate as you go down, so that at a depth of only 500 fathoms it equals about 100 times that at the surface. This con tributes to the density of the underlying water. The saltiness of the sea also contributes to its density, but varies somewhat according to local conditions, and decreases slightly from the sur face downward.

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