Hydrosphere origin

The second important source for the hydrosphere and the oceans are asteroids and comets. Estimating the amount of water which was brought to Earth from outer space is not easy. Until 20 years ago, it was believed that the only source of water for the hydrosphere was gas emission from volcanoes. The amount of water involved was, however, unknown (Rubey, 1964). First estimates of the enormous magnitude of the bombardment to which the Earth and the other planets were subjected caused researchers to look more closely at the comets and asteroids. New hypotheses on the possible sources of water in the hydrosphere now exist the astronomer A. H. Delsemme from the University of Toledo, Ohio, considers it likely that the primeval Earth was formed from material in a dust cloud containing anhydrous silicate. If this is correct, all the water in today s oceans must be of exogenic origin (Delsemme, 1992). 

The natural cycle of carbon involves compounds of the atmosphere, hydrosphere, lithosphere, and biosphere. A certain difference in the 13C isotope content exists between the samples, depending on their origin. To estimate the deviation from the average value of 13C isotope contents 8(%o) scale is used. The deviation may be calculated by Equation 5.14 ... 

Silicon is next to oxygen the most abundant element in the lithosphere the average content amounts about 30% by weight. Inorganic silicon compounds such as silica and silicates form the basis of most of the rocks forming the earth s crust. In the atmosphere, there is no silicon present, except as dust of cosmic and terrestrial origin. The silicon content in the hydrosphere, mainly in form of dissolved silica (silicic acid), is also very small. 

Whatever the real time sequence of eukaryote evolution may be, origin and development of the nucleated cell has implications for the understanding of the evolution of 02 in hydrosphere and atmosphere since even the few anaerobic eukaryotes that presently exist require molecular oxygen for synthesis of some essential biochemicals47 2 Thus all eukaryotic cells are basically aerobic and it is most likely that they evolved from an aerobic protoeukaryote stem cell. 

Reactions of this nature are very important for the return of CO2, originally sequestered in CaCC>3 deposition on the sea floor, to the atmosphere-hydrosphere system (see Chapter 10). The chemical reaction of equation 8.7 can also occur in the contact aureoles associated with the intrusion of igneous bodies into limestone. 

Synopsis of the origin and evolution of the hydrosphere-atmosphere-sedimentary lithosphere... 

SYNOPSIS OF THE ORIGIN AND EVOLUTION OF THE HYDROSPHERE-ATMOSPHERE-SEDIMENTARY LITHOSPHERE... 

If the scheme of a steady-state ocean is adopted, in which volatile components (mainly HCl and CO2) occurred from the beginning in the same proportions in which they are now found in the lithosphere and hydrosphere, then the original solution would have been hydrochloric acid saturated with CO2 at high pressure. On the basis of calculations of mass made by Garrels... 

Table IV gives the composition of the Precambrian ocean after neutralization of HCl, calculated under these assumptions. The pH values correspond to the end of congruent solution of the original rocks of the Earth s crust and beginning of deposition of the carbonate and silicate facies of the BIF (amorphous sediments). The values of correspond to the beginning of deposition of FeCOj (first value) and the beginning of deposition of Fe3Si205(OH)4 (second value). The values of the sum of carbonate ions in solution (2C02. ) are consistent with the values of co, in the atmosphere. Questions of the proportion of the masses of CO2 m the atmosphere, hydrosphere, and carbonate sediments will be considered in more detail later.

Assuming that the sedimentary rocks were derived from rocks with an original value of +1.0%c (original basalts, meteorites), and that the total amount of oxygen in the ocean is 1.2 10 " g and in sedimentary rocks 0.9 10 " g, the value found for the of the water of the primordial ocean is + 12.0%o (for the water of the present ocean, was taken as 0.0%o — SMOW standard). If granitized rocks are taken into account, then the of the ancient hydrosphere increases to + 16.2%o. These calculations cannot be considered reliable, as Silverman himself remarks, inasmuch as the composition and amount of primary rocks of the Earth s crust remain uncertain nor was juvenile water introduced from the mantle taken into account. 

Typical BIF are metamorphosed chemogenic cherty iron sediments, deposition of which was typical only of the Precambrian and ceased in subsequent geologic epochs. The formation of these rocks reflects a certain stage in the irreversible process of evolution of the Earth s crust, atmosphere, hydrosphere and biosphere. The period of intensive deposition apparently was preceded by a long period of accumulation of dissolved iron and silica in the waters of the original sedimentary basins. 

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