Earth the hydrosphere

You have already studied the liquid outer part of Earth—the hydrosphere— and the gaseous atmosphere. Each of these parts has a distinct composition and environmental chemistry, as you have learned. 

On Earth, elements maybe found in the lithosphere (the rocky, solid part of Earth), the hydrosphere (the aqueous, or watery, part of Earth), or the atmosphere. Elements such as the noble gases, the rare earths, and commercially valuable metals like silver and gold occur in only trace quantities. Others, like oxygen, silicon, aluminum, iron, calcium, sodium, hydrogen, sulfur, and carbon are abundant. 

Water is one of the most abundant chemical components on Earth. The hydrosphere consists of more than 90% of water 50%-90% of the gas in the atmosphere is water vapor the lithosphere (the Earth s crust) contains, on average, about 15% water and living organisms on our planet typically contain 50%-90% water. Because of its ubiquity, water may be considered as very normal and common. Yet, from a physical-chemical point of view, water is a unique extraordinary substance. By virtue of its unique properties, water is the medium, par excellence, in which life has evolved and is sustained. To be more precise, in living organisms water fulfills the following functions ... 

Although water is not destroyed on Earth, the hydrosphere can certainly suffer damage by human activities as discussed in Chapters 4 and 5. One of the main human activities detrimental to the hydrosphere is excessive utilization of water in regions deficient in rainfall. Withdrawal of irrigation water from rivers in arid regions has reduced some once-mighty rivers to trickles by the time they reach the ocean. The water is not destroyed, but it evaporates and in some cases infiltrates the soil. 

Oxygen is the most abundant element on the earth s surface it occurs both as the free element and combined in innumerable compounds, and comprises 23% of the atmosphere by weight, 46% of the lithosphere and more than 85% of the hydrosphere ( 85.8% of the oceans and 88.81% of pure water). It is also, perhaps paradoxically, by far the most abundant element on the surface of the moon where, on average, 3 out of every 5 atoms are oxygen (44.6% by weight). 

About 80% of the earth s surface is covered with aqueous solution. This liquid layer, the oceans, is called the hydrosphere. The average depth of the hydrosphere is about three miles but at ocean deeps or trenches, it changes precipitously to depths over twice that. 

The composition of the earth s atmosphere differs from day to day, from altitude to altitude, and from place to place. The largest variation is in the concentration of water vapor. Water evaporates continually from the hydrosphere, from the soil, from leaves, from clothes drying, etc. At intervals, parts of the atmosphere become chilled until the dew point or frost point is reached and then any vapor in excess of the saturation amount is precipitated as rain or snow. 

Rubey, W. W. (1955). Development of the hydrosphere and atmosphere, with special reference to probable composition of the early atmosphere. In "Crust of the Earth" (A. Poldenvaart, ed.), pp. 631-650. Geological Society of America, New York. 

Just as in the case for the hydrosphere, the atmosphere participates in all of the major biogeochemical cycles (except for phosphorus). In turn, the chemical composition of the atmosphere dictates its physical and optical properties, the latter being of great importance for the heat balance of Earth and its climate. Both major constituents (O2, H2O) and minor ones (CO2, sulfur, nitrogen, and other carbon compounds) are involved in mediating the amounts and characteristics of both incoming solar and outgoing infrared radiation. 

While the hydrosphere has long been appreciated as essential to life on Earth, only in the past couple of decades have scientists expanded their exploration of the global hydrologic cycle and its roles across the spectrum of Earth science... 

Although it is one of the smallest reservoirs in terms of water storage, the atmosphere is probably the second most important reservoir in the hydrosphere (after the oceans). The atmosphere has direct connections with all other reservoirs and the largest overall volume of fluxes. Water is present in the atmosphere in solid, liquid, and vapor forms, all of which are important components of the Earth s natural greenhouse effect. Cycling of water within the atmosphere, both physically (e.g. cloud formation) and chemically, is also integral to other biogeochemical cycles and climate. Consult Chapter 17 for more details. 

This treatment of the carbon cycle is intended to give an account of the fundamental aspects of the carbon cycle from a global perspective. After a presentation of the main characteristics of carbon on Earth (Section 11.2), four sections follow 11.3, about the carbon reservoirs within the atmosphere, the hydrosphere, the biosphere... 

Comparison of Figs 13-6a and 13-6b clearly demonstrates the degree to which human activity has modified the cycle of sulfur, largely via an atmospheric pathway. The influence of this perturbation can be inferred, and in some cases measured, in reservoirs that are very distant from industrial activity. Ivanov (1983) estimates that the flux of sulfur down the Earth s rivers to the ocean has roughly doubled due to human activity. Included in Table 13-2 and Fig. 13-6 are fluxes to the hydrosphere and lithosphere, which leads us to these other important parts of the sulfur cycle. 

The carbon cycle is complicated by several reactions that involve CO2. These reactions transfer carbon between the atmosphere, the hydrosphere (Earth s surface waters), and the lithosphere (Earth s crastal solids). The processes that move carbon from one sphere to another are illustrated schematically in the figure below. 

Almost all the Earth s carbon is found in the lithosphere as carbonate sediments that have precipitated from the oceans. Shells of aquatic animals also contribute CaC03 to the lithosphere. Carbon returns to the hydrosphere as carbonate minerals dissolve in water percolating through the Earth s crust. This process is limited by the solubility products for carbonate salts, so lithospheric carbonates represent a relatively inaccessible storehouse of carbon. 

As a compound water is remarkable. It is the only inorganic liquid to occur naturally on earth, and it is the only substance found in nature in all three physical states, solid, liquid and vapour (Franks, 1983). It is the most readily available solvent and plays a vital role in the continuation of life on earth. Water circulates continuously in the enviromnent by evaporation from the hydrosphere and subsequent precipitation from the atmosphere. This overall process is known as the hydrologic cycle. Reports estimate that the atmosphere contains about 6 x 10 litres of water, and this is cycled some 37 times a year to give an annual total precipitation of 224 X 10 litres (Franks, 1983 Nicholson, 1985). 

The hydrosphere (the Greek prefix hydro means water) is the great mass of water that surrounds the crust of the earth. Water is one of a few substances that, at the temperatures normal on the surface of the earth (which range between about -50 and 50°C), exists in three different states liquid, gas, and solid. Liquid water makes up the oceans, seas, and lakes, flows in rivers, and underground streams. Solid water (ice) occurs in the polar masses, in glaciers, and at high altitudes, and gaseous water (moisture) is part of the atmosphere (O Toole 1995). Liquid and solid water cover over 70% of the surface of the earth. 

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 building block molecules may have been synthesized in the atmosphere, the hydrosphere or on the lithosphere of the young Earth from species such as CO, CO2, CH4, H2O, N2 and NH3 (endogenous synthesis). 

A detailed theoretical study of the properties of the redox system FeS/FeS2 was carried out in the Department of Geosciences of SUNY Stony Brook (Schoonen et al., 1999). The authors conclude that the hypothetical reduction of CO2 (by the FeS/FeS2 redox pair) formulated in Wachtershauser s early work, and the carbon fixation cycle on the primeval Earth associated with it, probably could not have occurred. This judgement is made on the basis of a theoretical analysis of thermodynamic data other conditions would naturally have been involved if CO had reacted rather than C02. It is not known whether free CO existed in the hydrosphere, or if so, at what concentrations. 

Among the oldest rocks on Earth are those on Isua, an island off the coast of Greenland they are 3.8 Gyr old, formed some 0.7 Gyr after accretion of the Earth. The rocks mark the beginning of the Archean period of geological time. The Isua rocks suggest that there was an extensive hydrosphere at this time, with erosion, transportation and deposition of minerals from water solution. The oldest lunar rocks, however, record an earlier high-temperature event - the Earth-Moon capture event. 

As the planet acquires a volatile molecule inventory it begins to develop an atmosphere, and in the case of the Earth this also includes the extensive circulation of water in the hydrosphere. The weight of the volatiles trapped in the atmosphere of a planet leads to a mean surface pressure, po, given by ... 

Liposphere The circulation of hydrocarbon-based materials in Titan s atmosphere, which is analogous to the hydrosphere (water cycles) on Earth... 

The nitrogen cycle is the complex series of reactions by which nitrogen is slowly but continually recycled in the atmosphere, lithosphere (earth) and hydrosphere (water). Atmospheric nitrogen is made accessible to us and other life-forms in mainly two ways. 

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