SCIENCE AND THE SEA 



may be deposited also by dust, which is carried far into the oceans 

 by the wind. For example, the dust originating from the Sahara 

 Desert has been found in the bottom sediments west of the Cape 

 Verde Islands. Sediments of volcanic origin, the fine ash from sub- 

 aerial eruptions, may be transported by the wind over wide areas. 

 Biogenous sediments are of organic origin, and are divided into two 

 types — the remains of animals and plants living on the ocean 

 floor, or the remains of animal and plant plankton. Halmyrogenous 

 sediments are new direct formations of minerals which are deposited 

 when the water is over-saturated with soluble material. Deposits of 

 iron and manganese oxides on the ocean floor belong in this category. 

 Cosmogenous sediments originate from outer space and consist of 

 small balls about 0.2 mm in diameter of magnetic iron or silica. 



Since 1947 M. Ewing and his co-workers of the Lamont Geologi- 

 cal Observatory have raised more than 3,000 cores from sediments 

 in all oceans and adjacent seas during 44 oceanographic expeditions. 

 The study of these cores taken from the bottom of the deep-sea floor 

 revealed that deposits of the glacial periods were present in these 

 sediments. Layers of gravel and stones, deposited by drifting ice- 

 bergs during the periods of the ice ages, alternate with layers 

 deposited during warmer stages of the climate. After investigating 

 numerous cores, it was concluded that of the four ice periods the 

 first one, called the Nebraskan Glacial period, started about 1,500,000 

 years ago, the beginning of the Pleistocene, the last epoch of geo- 

 logic time. Using the radio-carbon method of dating, the last two 

 major maxima of ice ages were fixed at about 60,000 and 18,000 

 years ago. 



Sand has been found to cover large parts of the ocean floor. 

 There was a general belief that these parts were great submergences 

 and that these areas had been located close to the surface. After 

 long cores had been obtained from these regions, it was observed 

 that the sand layers are imbedded in layers of deep-sea deposits. 

 From the map made by David Ericson, it was learned that these 

 sandy areas stretch out from the coast in great sea fans and must 

 have been transported by density currents. 



MAGNETIC MEASUREMENTS 



Because of the universal use of the magnetic compass for many 

 centuries, systematic magnetic measurements at sea were already 

 conducted during the voyage of Joao de Castro from 1538 to 1541. 



After the introduction of iron and steel ships, accurate magnetic 

 measurements at sea could no longer be taken except in non-mag- 

 netic ships, specially constructed for this purpose. An example of a 

 non-magnetic vessel was the Carnegie, which conducted magnetic 

 measurements from 1909 to 1929. 



Recently the United States Navy has been carrying out "Project 

 Magnet", covering almost the whole ocean area, using aircraft as 

 well as ships. These measurements are essential to comprehend the 

 whole magnetic field, which is to be divided into parts of internal 

 and external origin. Complicated equipment is necessary to conduct 

 these observations. 



Much geological information can be obtained from a much sim- 

 pler device towed behind a ship or aircraft, measuring the total mag- 

 netic force, which is influenced by the magnetic properties of the 

 different types of rock lying beneath the ocean floor. 



One of the first attempts to make a detailed magnetic map of 

 an extensive ocean area was based on the survey conducted by the 

 Scripps Institution of Oceanography, in conjunction with the U. S. 

 Coast and Geodetic Survey, off the west coast of the United States. 

 The results showed major structural trends of which there is little 

 or no indication in the topography. 



GRAVITY MEASUREMENTS 



A body suspended above the earth's surface when free to move 



will travel towards the earth with an acceleration mainly dependent 

 upon the attractive force of the earth and the centrifugal force 

 caused by the earth's rotation. From the many observations with a 

 pendulum taken on land to determine this acceleration, it was found 

 that the gravitational pull varies in different locations on the earth's 

 surface. One of the reasons causing this phenomenon is the unequal 

 distribution of the earth's masses containing various materials. 



To acquire a more complete picture of the location of these 

 masses, gravity measurements at sea became a necessity. However, 

 gravity observations taken with a pendulum required a stable plat- 

 form. To satisfy this condition, the steamship Fram of the Nansen 

 Polar Expedition (1893-1896) took observations when the ship was 



Figure 3. Sensor streamed behind the ship and magnetometer on board are 

 used for taking magnetic meaeurements. 



fast in the ice. Later on, F. A. Vening Meinesz perfected the pen- 

 dulum apparatus and conducted many observations around the globe 

 from submerged submarines. In recent years J. Lamar Worzel of 

 Lamont Geological Observatory has made many investigations in 

 this field. 



During the International Geophysical Year, instrumentation had 

 improved so much that gravity meters installed on surface ships of 

 more than 1,000 tons displacement provided sufficiently accurate 

 measurements under reasonably favorable sea conditions. However, 

 a correction has to be applied to oceanic gravity measurements for 

 depth of water to determine the relative heaviness of material 

 underlying the ocean floor. 



From the many gravity measurements in the ocean it was 

 learned that the oceanic crust as a whole is probably balanced 

 against the continents. However, local variations in the corrected 

 gravity measurements are caused by density variations beneath the 

 sea bed. Therefore, gravity surveys are useful for making recon- 

 naissance to locate features which cannot be detected otherwise. 



Deficient gravity (negative anomaly) has been found above the 

 ocean trenches caused, presumably, by a thin crust underneath. 

 Seismic shooting, however, indicated that the crust underneath the 

 trenches is relatively thick. Further investigation will, no doubt, 

 resolve these differences of opinion 



A lack of balance and changes in level have been observed on 

 the coasts of the Hudson Bay, which have been rising because the 

 layer of ice diminishes throughout the years; the Atlantic Coast, 

 however, has been sinking through the build-up of deltas and shelves 

 deposited by sediments transported by the rivers. 



SEISMIC MEASUREMENTS 



Our knowledge of the structure and thickness of the sediment 

 and rock layers of the deep-sea floor has been advanced considerably 

 by seismic measurements. The methods are based upon the measure- 

 ment of the time required for sound waves to travel from the place 

 of origin, where the vibration is created, to a receiving station. If 

 the path and travel time are known, the speed of travel of the sound 

 waves can be computed. Because many experiments by laboratories 

 have given estimates of the sound velocities in various rocks and 

 unconsolidated sediments, a reasonable guess can be made about the 

 type of texture in a layer traversed by the sound waves. 



If the underocean faces between the bottom layers are good 

 reflectors, and the speed of sound in these strata are known, a 

 measurement of the time needed for sound waves to travel through 

 each layer and back by reflection indicates the thickness of each 

 layer. 



Because conventional echo sounders operate on relatively-high 

 sonic frequencies which cannot penetrate far into water-saturated 

 sediments, a sounding apparatus, called "Sonoprobe", has been 

 developed transmitting low-frequency sound waves. Penetration 

 through a sediment layer up to 300 feet, returning an echo from the 

 underlying rock floor, has been achieved by sound waves produced 

 by this device. However, an expert has to control continually the 

 proper output of sound into the water to detect minor reflections 

 from the rock lying beneath the sediment. 



