HYDROSPHERE

discontinuous layer of water at or near the Earth's surface. It includes all liquid and frozen surface waters, groundwater held in soil and rock, and atmospheric water vapour. Current research strongly suggests that the total amount of water on the Earth has remained essentially constant for the past 1 billion years, notwithstanding changes in the volume content of the individual components of the hydrosphere over geologic time. Substantial quantities of oceanic water, for example, were transferred to continental ice sheets during the major glaciations. The Earth's total budget of water is estimated at about 1.4 billion cubic km (336 million cubic miles). The oceans constitute about 97 percent of this amount; the polar ice caps and glaciers hold slightly more than 2 percent; and freshwater bodies (e.g., lakes, streams, ponds), groundwater, vegetation, and atmospheric vapour account for the rest. Virtually all of these waters of the hydrosphere are in constant circulation. They move through the hydrologic cycle (q.v.). Broadly speaking, this cycle involves the transfer of water from the oceans through the atmosphere to the continents and back to the oceans over and beneath the land surface by means of evaporation, transpiration, precipitation, interception, infiltration, subterranean percolation, overland flow, runoff, and other complex processes. Although the components of the hydrosphere are undergoing such a continuous change of state and location, the total water budget remains in balance. The components of the hydrosphere and the hydrologic cycle itself have been seriously affected by the activities of modern society. The discharge of toxic chemicals and other industrial wastes have, for example, contaminated rivers and streams, while groundwater seepage of mineral fertilizers and dumping of sewage into lakes have caused eutrophication (q.v.), a gradual increase in the concentration of phosphorus, nitrogen, and other plant nutrients. The buildup of carbon dioxide and other so-called greenhouse gases in the atmosphere due to the burning of fossil fuels and the slash-and-burn activities attendant upon deforestation practices may cause global mean surface temperatures to rise by as much as 5 C (9 F) by the late 21st century. Such a temperature increase would accelerate the hydrologic cycle and thereby disrupt the water balance in various regions, particularly in the middle latitudes of the Northern Hemisphere. Global warming could further affect the hydrologic cycle by melting ice presently locked in the polar ice caps and mountain glaciers. The meltwater would be transferred to the oceans, possibly resulting in a substantial rise in sea level over several centuries. Melting of arctic sea ice might also occur; this would cause a northward shift in storm activity and a decrease in rainfall in the Northern Hemisphere during spring and autumn. Additional reading General introductory discusssions on the distribution of water on and around the Earth and the role of water in supporting life are found in Cynthia A. Hunt and Robert M. Garrels, Water: The Web of Life (1972); C.L. Mantell and A.M. Mantell, Our Fragile Water Planet: An Introduction to the Earth Sciences (1976); Elizabeth Kay Berner and Robert A. Berner, The Global Water Cycle: Geochemistry and Environment (1987); H.M. Raghunath, Ground Water, 2nd ed. (1987); Eberhard Czaya, Rivers of the World (1981; originally published in German, 1981); Mary J. Burgis and Pat Morris, The Natural History of Lakes (1987); and Neil Wells, The Atmosphere and Ocean: A Physical Introduction (1986).Biogeochemical properties of the hydrosphere are discussed in Ronald J. Gibbs, Mechanisms Controlling World Water Chemistry, Science 170(3962):10881090 (1970); R. Wollast and Fred T. Mackenzie, Global Cycle of Silica, in S.R. Aston (ed.), Silicon Geochemistry and Biogeochemistry (1983), pp.3976; P. Buat-Menard, Particle Geochemistry in the Atmosphere and Oceans, in Peter S. Liss and W. George N. Slinn (eds.), Air-Sea Exchange of Gases and Particles (1983), pp. 455532; Robert A. Berner, A.C. Lasaga, and Robert M. Garrels, The Carbonate-Silicate Geochemical Cycle and Its Effect on Atmospheric Carbon Dioxide Over the Past 100 Million Years, American Journal of Science 283(7):641683 (1983); Lawrence A. Hardie and Hans P. Eugster, The Evolution of Closed-Basin Brines, Mineralogical Society of America Special Paper 3:273290 (1970); James I. Drever, The Geochemistry of Natural Waters (1982); A. Lerman, Geochemical Processes: Water and Sediment Environments (1979, reprinted 1988); G. Evelyn Hutchinson, A Treatise on Limnology, vol. 1 (1975); Werner Stumm (ed.), Chemical Processes in Lakes (1985); and Georg Matthess, The Properties of Groundwater (1982; originally published in German, 1973).Analyses of the processes of the hydrologic cycle and its utilization are presented in Robert C. Averett and Diane M. Mcknight (eds.), Chemical Quality of Water and the Hydrologic Cycle (1987); R.A. Freeze, A Stochastic-Conceptual Analysis of Rainfall-Runoff Processes on a Hillslope, Water Resources Research 16(2):391408 (1980); T. Dunne, Field Studies of Hillslope Flow Processes, in M.J. Kirkby (ed.), Hillslope Hydrology (1978), pp. 227293; Ray K. Linsley and Joseph B. Franzini, Water-Resources Engineering, 3rd ed. (1979); Mark J. Hammer and Kenneth A. Mackichan, Hydrology and Quality of Water Resources (1981); and Alvin S. Goodman, Principles of Water Resources Planning (1984).For the evolution of the hydrosphere, see James C.G. Walker, Evolution of the Atmosphere (1977); J. Veizer,The Evolving Exogenic Cycle, in C. Bryan Gregor et al. (eds.), Chemical Cycles in the Evolution of the Earth (1988), pp. 175220; Heinrich D. Holland, The Chemical Evolution of the Atmosphere and Oceans (1984); and Robert M. Garrels and Fred T. Mackenzie, Evolution of Sedimentary Rocks (1971).The impact of human activities on the hydrosphere is studied in Robert M. Garrels, Fred T. Mackenzie, and Cynthia A. Hunt, Chemical Cycles and the Global Environment: Assessing Human Influences (1975); Arthur N. Strahler and Alan H. Strahler, Environmental Geoscience: Interaction Between Natural Systems and Man (1973); W.D. Bischoff, V.l. Paterson, and Fred T. Mackenzie, Geochemical Mass Balance for Sulfur- and Nitrogen-Bearing Acid Components: Eastern United States, in Owen P. Bricker (ed.), Geological Aspects of Acid Deposition (1984), pp. 121; S.C. Chapra, Simulation of Recent and Projected Total Phosphorus Trends in Lake Ontario, Journal of Great Lakes Research 6(2):101112 (1980); Brian Henderson-Sellers and H.R. Markland, Decaying Lakes: The Origins and Control of Cultural Eutrophication (1987); Thomas D. Brock, A Eutrophic Lake: Lake Mendota, Wisconsin (1985); G. Dennis Cooke et al., Lake and Reservoir Restoration (1986); Sven Olof Ryding and Walter Rast (eds.), The Control of Eutrophication of Lakes and Reservoirs (1989); Louis Thibodeaux, Chemodynamics, Environmental Movement of Chemicals in Air, Water, and Soil (1979); Stanley E. Manahan, Environmental Chemistry, 4th ed. (1984); Howard S. Peavy, Donald R. Rowe, and George Tchobanoglous, Environmental Engineering (1985); James L. Regens and Robert W. Rycroft, The Acid Rain Controversy (1988); and Daniel D. Chiras, Environmental Science: A Framework for Decision Making, 2nd ed. (1988). Fred T. Mackenzie Origin and evolution of the hydrosphere It is not very likely that the total amount of water at the Earth's surface has changed significantly over geologic time. Based on the ages of meteorites, the Earth is thought to be 4.6 billion years old. The oldest rocks known date 3.8 billion years in age, and these rocks, though altered by post-depositional processes, show signs of having been deposited in an environment containing water. There is no direct evidence for water for the period between 4.6 and 3.8 billion years ago. Thus, ideas concerning the early history of the hydrosphere are closely linked to theories about the origin of the Earth. The Earth is thought to have accreted from a cloud of ionized particles around the Sun. This gaseous matter condensed into small particles that coalesced to form a protoplanet, which in turn grew by the gravitational attraction of more particulates. Some of these particles had compositions similar to that of carbonaceous chondrite meteorites, which may contain up to 20 percent water. Heating of this initially cool, unsorted conglomerate by the decay of radioactive elements and the conversion of kinetic and potential energy to heat resulted in the development of the Earth's liquid iron core and the gross internal zonation of the planet (i.e., differentiation into core, mantle, and crust). It has been concluded that the Earth's core formed over a period of about 500 million years. It is likely that core formation resulted in the escape of an original primitive atmosphere and its replacement by one derived from the loss of volatile substances from the planetary interior. At an early stage the Earth thus did not have water or water vapour at its surface. Once the planet's surface had cooled sufficiently, water contained in the minerals of the accreted material and released at depth could escape to the surface and, instead of being lost to space, cooled and condensed to form the initial hydrosphere. A large, cool Earth most certainly served as a better trap for water than a small, hot body because the lower the temperature, the less likelihood for water vapour to escape, and the larger the Earth, the stronger its gravitational attraction for water vapour. Whether most of the degassing took place during core formation or shortly thereafter or whether there has been significant degassing of the Earth's interior throughout geologic time remains uncertain. It is likely that the hydrosphere attained its present volume early in the Earth's history, and since that time there have been only small losses and gains. Gains would be from continuous degassing of the Earth; the present degassing rate of juvenile water has been determined as being only 0.3 cubic kilometre per year. Water loss in the upper atmosphere is by photodissociation, the breakup of water vapour molecules into hydrogen and oxygen due to the energy of ultraviolet light. The hydrogen is lost to space and the oxygen remains behind. Only about 4.8 10-4 cubic kilometre of water vapour is presently destroyed each year by photodissociation. This low rate can be readily explained: the very cold temperatures of the upper atmosphere result in a cold trap at an altitude of about 15 kilometres, where most of the water vapour condenses and returns to lower altitudes, thereby escaping photodissociation. Since the early formation of the hydrosphere, the amount of water vapour in the atmosphere has been regulated by the temperature of the Earth's surfacehence its radiation balance. Higher temperatures imply higher concentrations of atmospheric water vapour, while lower temperatures suggest lower atmospheric levels. The early hydrosphere The gases released from the Earth during its early history, including water vapour, have been called excess volatiles because their masses cannot be accounted for simply by rock weathering. An estimate of the excess volatiles is given in Table 6. These volatiles are thought to have formed the early atmosphere of the Earth. At an initial crustal temperature of about 600 C, almost all of these compounds, including H2O, would have been in the atmosphere. The sequence of events that occurred as the crust cooled is difficult to reconstruct. Below 100 C all of the water would have condensed, and the acid gases would have reacted with the original igneous crustal minerals to form sediments and an initial hydrosphere that was dominated by a salty ocean. If the reaction rates are assumed to have been slow relative to cooling, an atmosphere of 600 C would have contained, together with other compounds, water vapour, carbon dioxide, and hydrogen chloride (HCl) in a ratio of 20:3:1 and cooled to the critical temperature of water (i.e., 374 C). The water therefore would have condensed into an early hot ocean. At this stage, the hydrogen chloride would have dissolved in the ocean (about one mole per litre), but most of the carbon dioxide would have remained in the atmosphere, with only about 0.5 mole per litre in the ocean water. This early acid ocean would have reacted vigorously with crustal minerals, dissolving out silica and cations and creating a residue composed principally of aluminous clay minerals that would form the sediments of the early ocean basins. This is one of several possible pathways for the early surface of the Earth. Whatever the actual case, after the Earth's surface had cooled to 100 C, it would have taken only a short time for the remaining acid gases to be consumed in reactions involving igneous rock minerals. The presence of cyanobacteria (e.g., blue-green algae) in the fossil record of rocks older than three billion years attests to the fact that the Earth's surface had cooled to temperatures lower than 100 C by this time, and neutralization of the original acid volatiles had taken place. It is possible, however, that, because of increased greenhouse gas concentrations (see below) in the Early Archean era (about 3.8 to 3.4 billion years ago), the Earth's surface could still have been warmer than today. If most of the degassing of primary volatile substances from the Earth's interior occurred early, the chloride released by the reaction of hydrochloric acid with rock minerals would be found in the oceans or in evaporite deposits, and the oceans would have a salinity and volume comparable to that of today. This conclusion is based on the assumption that there has been no drastic change in the ratios of volatiles released through geologic time. The overall generalized reaction indicative of the chemistry leading to the formation of the early oceans can be written in the form: primary igneous rock minerals + acid volatiles + H2O sedimentary rocks + oceans + atmosphere. It should be noted from this equation that, if all the acid volatiles and H 2O were released early in the history of the Earth and in the proportions found today, then the total original sedimentary rock mass-produced would be equal to that of the present, and ocean salinity and volume would be close to those of today as well. If, on the other hand, degassing were linear with time, then the sedimentary rock mass would have accumulated at a linear rate, as would have oceanic volume. The salinity of the oceans, however, would remain nearly the same if the ratios of volatiles degassed did not change with time. The most likely situation is the one presented herenamely, that major degassing occurred early in Earth's history, after which minor amounts of volatiles were released episodically or continuously for the remainder of geologic time. The salt content of the oceans based on the constant proportions of volatiles released would depend primarily on the ratio of sodium chloride locked up in evaporites to that dissolved in the oceans. If all the sodium chloride in evaporites were added to the oceans today, the salinity would be approximately doubled. This value gives a sense of the maximum salinity that the oceans could have attained throughout geologic time. One component absent from the early Earth's surface was free oxygen; it would not have been a constitutent released from the cooling crust. Early production of oxygen was by the photodissociation of water in the Earth's atmosphere, a process that was triggered by the absorption of the Sun's ultraviolet radiation. The reaction is in which hn represents the photon of ultraviolet light. The hydrogen produced would escape into space, while the oxygen would react with the early reduced gases by reactions such as 2H2S + 3O2 2SO2 + 2H2O. Oxygen production by photodissociation gave the early reduced atmosphere a start toward present-day conditions, but it was not until the appearance of photosynthetic organisms approximately three billion years ago that oxygen could accumulate in the Earth's atmosphere at a rate sufficient to give rise to today's oxygenated environment. The photosynthetic reaction leading to oxygen production is given in equation (6).