GROSS PRIMARY PRODUCTION 



gravity corer — Any type of corer tliat achieves 

 bottom penetration solely as a result of gravita- 

 tional force acting upon its mass. 



gravity instruments — Devices for measuring the 

 differences in the gravity force or acceleration at 

 two or more points. They are of three principal 

 types: (1) A static type in which a linear or 

 angular displacement is observed or nulled by an 

 opposing force. (2) A dynamic type in which 

 the i^eriod of oscillation is a function of gravity 

 and is the quantity directly observed. (3) A 

 gradient measuring type, for example, Eotvos 

 torsion balance. 



gravity meter — See gravimeter. 



gravity wave — A wave whose velocity of propa- 

 gation is controlled primarily by gravity. 

 Water waves of length greater than 2 inches are 

 considered gravity waves. (61) 



gravity wind — See katabatic wind. 



gray ice — A Eussian term for sea ice 10 to 30 

 centimeters (4 to 12 inches) in thickness. It is 

 roughly comparable to medium winter ice. 



gray mud — (or gray ooze). A deep sea deposit 

 of grayish color which is intermediate in com- 

 position between red clay and globigerina ooze. 



(2) 



gray ooze — See gray mud. 



grazing — The feeding of zooplanktonic organisms 

 ujion i:)hytoplanktonic organisms. Generally in 

 reference to the feeding of copepods upon 

 diatoms. 



grazing angle — The angle that the sound ray 

 path forms with the reflecting surface; usually 

 applies to sound rays reflected from the bottom. 

 Conventionally, the angle is measured from the 

 horizontal. 



grease ice — (also called ice fat, lard ice). A 

 sludge of ice crystals in the sea that gives the 

 sea surface a greasy appearance. (5) 



great circle — The curve on the surface of a sphere 

 traced by the intersection of the sphere and a 

 plane containing the center of the sphere. 



great diurnal range — (also called diurnal range) . 

 The average difference in height between all 

 mean higher high waters and all mean lower 

 low waters measured over a 19-yeai;^ period, 

 or its computed equivalent. 



greater ebb — The stronger of two ebb currents 

 occurring during a tidal -day, usually associated 

 with tidal currents of mixed characteristics. 

 (66) See mixed current. {See figure for 

 current curve.) 



greater flood — The stronger of two flood tidal 

 currents occurring during a tidal day, usually 

 associated with tidal currents of mixed char- 

 acteristics. (66) See mixed current. {See 

 figure for current curve.) 



great tropic range — (also called tropic range). 

 The average difference in height between all 

 tropic higher high waters and all tropic lower 

 low waters which occur twice monthly when 



the moon's north and south declination is 

 greatest. See lunar declination. 



green alga — One of a division or phylum (Chloro- 

 pliyta) (the Clilorophyceae of another scheme) 

 of grass-green, single-celled, filamentous, mem- 

 branous, or branchmg plants in wJiich the color, 

 imparted by chlorophylls a and 6, is not masked 

 by the accessory pigments. Green algae are 

 cosmopolitan in the upper littoral zone but are 

 most abundant in warmer waters. 



greenhouse effect — In the ocean where a layer 

 of low salinity water overlies a layer of more 

 dense water the short wavelength radiation of 

 the sun is absorbed in the deeper layers. The 

 radiation given off' by the water is in the far 

 infrared, and since this cannot radiate through 

 the low salinity layer, a temperature rise results 

 in the deeper layers. 



In the atmosphere the same effect is produced 

 by a layer of clouds and the long wave radiation 

 is trapped between the clouds and the earth. 



Greenland currents — See East Greenland Cur- 

 rent, West Greenland Current. 



green mud — 1. A fine-grained terrigenous mud or 

 oceanic ooze found near the edge of a continen- 

 tal shelf, and similar to other terrigenous muds 

 except for the greenish color and, in some cases, 

 less organic matter. It occurs at depths of 300 

 to 7,500 feet (91 to 2,286 meters). 



2. A deep sea terrigenous deposit character- 

 ized by the presence of a considerable proportion 

 of glauconite and CaCOs in variable amounts 

 up to 50 percent. 

 (2) 



green sand — (or glav^onitic sand). A sand con- 

 taining a considerable quantity of glauconite 

 which gives it a green color. (2) 



Greenwich argument — The equilibrium argu- 

 ment computed for the meridian of Greenwich. 

 (50) See equilibrium argument. 



Greenwich epoch — The phase difference between 

 a tidal constituent and its equilibrium argu- 

 ment referenced to the corresponding Greenwich 

 equilibrium argument. 



Greenwich interval — An interval referred to the 

 transit of the moon over the meridian of Green- 

 wich as distinguished from the local interval 

 which is referred to the moon's transit over the 

 local meridian. (50) 



Greenwich mean time — (abbreviated GMT). 

 Local mean time at the Greenwich meridian. 

 (68) 



gribble — See Limnoria. 



grit — Coarse sand or coarse-grained sandstone. 



groin — A low artificial wall-like structure of dur- 

 able material extending from the land to sea- 

 ward for a particular purpose, such as to protect 

 the coast or to force a current to scour a channel. 

 (30) 



gross primary production — See primary 

 production. 



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