In one respect, planet Earth is a misnomer in that 71% of the earth is covered by water and only 29% is terra firma.
Indeed, the abundance of water on Earth is a unique feature that clearly distinguishes our "Blue Planet" from others in the solar system.
Not a drop of liquid water can be found anywhere else in the solar system. It is because the Earth has just the right mass, the right chemical composition, the right atmosphere, and is the right distance from the Sun (the "Goldilocks" principle) that permits water to exist mainly as a liquid.
However, the range of surface temperatures and pressures of our planet permit water to exist in all three states: solid (ice), liquid (water), and gas (water vapor).
Most of the water is contained in the oceans and the high heat capacity of this large volume of water (1.35 million cubic kilometers) buffers the Earth surface from large temperature changes such as those observed on the moon.
Water is the universal solvent and the basis of all life on our Planet. It is an essential life-sustaining resource which led Benjamin Franklin to comment "When the well's dry, we know the worth of water."
The hydrosphere (from Greek ὕδωρ hydōr, "water"[1] and σφαῖρα sphaira, "sphere"[2]) is the combined mass of water found on, under, and above the surface of a planet, minor planet or natural satellite. Although the Earth's hydrosphere has been around for longer than 4 billion years, it continues to change in size. This is caused by seafloor spreading and continental drift, which rearranges the land and ocean.[3]
It has been estimated that there are 1,386 million cubic kilometres (333,000,000 cubic miles) of water on Earth.[4] This includes water in liquid and frozen forms in groundwater, oceans, lakes and streams. Saltwater accounts for 97.5% of this amount, whereas fresh water accounts for only 2.5%. Of this fresh water, 68.9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic, and mountain glaciers; 30.8% is in the form of fresh groundwater; and only 0.3% of the fresh water on Earth is in easily accessible lakes, reservoirs and river systems.[4]
The total mass of the Earth's hydrosphere is about 1.4 × 1018tonnes, which is about 0.023% of Earth's total mass. At any given time, about 20 × 1012tonnes of this is in the form of water vapor in the Earth's atmosphere (for practical purposes, 1 cubic meter of water weighs one tonne). Approximately 71% of Earth's surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by ocean. The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5%).[5]
The water cycle refers to the transfer of water from one state or reservoir to another. Reservoirs include atmospheric moisture (snow, rain and clouds), streams, oceans, rivers, lakes, groundwater, subterraneanaquifers, polar ice capsand saturated soil. Solar energy, in the form of heat and light (insolation), and gravity cause the transfer from one state to another over periods from hours to thousands of years. Most evaporation comes from the oceans and is returned to the earth as snow or rain.[6]:27Sublimation refers to evaporation from snow and ice. Transpiration refers to the expiration of water through the minute pores or stomata of trees. Evapotranspiration is the term used by hydrologists in reference to the three processes together, transpiration, sublimation and evaporation.[6]
Marq de Villiers has described the hydrosphere as a closed system in which water exists. The hydrosphere is intricate, complex, interdependent, all-pervading and stable and "seems purpose-built for regulating life."[6]:26 De Villiers claimed that, "On earth, the total amount of water has almost certainly not changed since geological times: what we had then we still have. Water can be polluted, abused, and misused but it is neither created nor destroyed, it only migrates. There is no evidence that water vapor escapes into space."[6]:26
"Every year the turnover of water on Earth involves 577,000 km3 of water. This is water that evaporates from the oceanic surface (502,800 km3) and from land (74,200 km3). The same amount of water falls as atmospheric precipitation, 458,000 km3 on the ocean and 119,000 km3 on land. The difference between precipitation and evaporation from the land surface (119,000 - 74,200 = 44,800 km3/year) represents the total runoff of the Earth's rivers (42,700 km3/year) and direct groundwater runoff to the ocean (2100 km3/year). These are the principal sources of fresh water to support life necessities and man's economic activities."[4]
Water is a basic necessity of life. Since 2/3 of the Earth is covered by water, the Earth is also called the blue planet and the watery planet.[notes 1] Hydrosphere plays an important role in the existence of the atmosphere in its present form. Oceans are important in this regard. When the Earth was formed it had only a very thin atmosphere rich in hydrogen and helium similar to the present atmosphere of Mercury. Later the gases hydrogen and helium were expelled from the atmosphere. The gases and water vapor released as the Earth cooled became its present atmosphere. Other gases and water vapor released by volcanoes also entered the atmosphere. As the Earth cooled the water vapor in the atmosphere condensed and fell as rain. The atmosphere cooled further as atmospheric carbon dioxide dissolved in to rain water. In turn this further caused the water vapor to condense and fall as rain. This rain water filled the depressions on the Earth's surface and formed the oceans. It is estimated that this occurred about 4000 million years ago. The first life forms began in the oceans. These organisms did not breathe oxygen. Later, when cyanobacteria evolved, the process of conversion of carbon dioxide into food and oxygen began. As a result, Earth's atmosphere has a distinctly different composition from that of other planets and allowed for life to evolve on Earth.
According to Igor A. Shiklomanov, it takes 2500 years for the complete recharge and replenishment of oceanic waters, 10,000 years for permafrost and ice, 1500 years for deep groundwater and mountainous glaciers, 17 years in lakes and 16 days in rivers.[4]
"Specific water availability is the residual (after use) per capita quantity of fresh water."[4] Fresh water resources are unevenly distributed in terms of space and time and can go from floods to water shortages within months in the same area. In 1998 76% of the total population had a specific water availability of less than 5.0 thousand m³ per year per capita. Already by 1998, 35% of the global population suffered "very low or catastrophically low water supplies" and Shiklomanov predicted that the situation would deteriorate in the twenty-first century with "most of the Earth's population will be living under the conditions of low or catastrophically low water supply" by 2025. There is only 2.5% of fresh water in the hydrosphere and only 0.25% of water is accessible for our use.
A hydrosphere is the total amount of water on a planet. The hydrosphere includes water that is on the surface of the planet, underground, and in the air. A planet's hydrosphere can be liquid, vapor, or ice. On Earth, liquid water exists on the surface in the form of oceans, lakes and rivers. It also exists below ground—as groundwater, in wells and aquifers. Water vapor is most visible as clouds and fog.
The frozen part of Earth's hydrosphere is made of ice: glaciers, ice caps and icebergs. The frozen part of the hydrosphere has its own name, the cryosphere. Water moves through the hydrosphere in a cycle. Water collects in clouds, then falls to Earth in the form of rain or snow. This water collects in rivers, lakes and oceans. Then it evaporates into the atmosphere to start the cycle all over again. This is called the water cycle.
Catch a wave, and you're sitting on top of the hydrosphere.
Hydrosphere in Space
Some scientists believe a hydrosphere exists on Europa, a moon of Jupiter, that consists of a frozen outer layer and a giant, liquid ocean underneath it.
Hydrosphere, discontinuous layer of water at or near Earth’s surface. It includes all liquid and frozen surface waters, groundwater held in soiland rock, and atmospheric water vapour.
Water is the most abundant substance at the surface of Earth. About 1.4 billion cubic km (326 million cubic miles) of water in liquid and frozen form make up the oceans, lakes, streams, glaciers, and groundwaters found there. It is this enormous volume of water, in its various manifestations, that forms the discontinuous layer, enclosing much of the terrestrial surface, known as the hydrosphere.
Central to any discussion of the hydrosphere is the concept of the water cycle (or hydrologic cycle). This cycle consists of a group of reservoirs containing water, the processes by which water is transferred from one reservoir to another (or transformed from one state to another), and the rates of transfer associated with such processes. These transfer paths penetrate the entire hydrosphere, extending upward to about 15 km (9 miles) in Earth’s atmosphere and downward to depths on the order of 5 km (3 miles) in its crust.
In the hydrologic cycle, water is transferred between the land surface, the ocean, and the atmosphere.Encyclopædia Britannica, Inc.
hydrologic cycleAn overview of how water in its various phases flows through the hydrologic, or water, cycle.Encyclopædia Britannica, Inc.
This article examines the processes of the water cycle and discusses the way in which the various reservoirs of the hydrosphere are related through the water cycle. It also describes the biogeochemical properties of Earth’s waters at some length and considers the distribution of global water resources and their use and pollution by human society. Details concerning the major water environments that make up the hydrosphere are provided in the articles ocean, lake, river, and ice. See alsoclimate for specific information about the impact of climatic factors on the water cycle. The principal concerns and methods of hydrologyand its various allied disciplines are summarized in Earth sciences.
Distribution And Quantity Of Earth’s Waters
Ocean waters and waters trapped in the pore spaces of sediments make up most of the present-day hydrosphere. The total mass of water in the oceans equals about 50 percent of the mass of sedimentary rocks now in existence and about 5 percent of the mass of Earth’s crust as a whole. Deep and shallow groundwaters constitute a small percentage of the total water locked in the pores of sedimentary rocks—on the order of 3 to 15 percent. The amount of water in the atmosphere at any one time is trivial, equivalent to roughly 13,000 cubic km (about 3,100 cubic miles) of liquid water, or about 0.001 percent of the total at Earth’s surface. This water, however, plays an important role in the water cycle.
Water masses at Earth's surface
reservoir
volume (in cubic kilometres)
percent of total
*As liquid equivalent of water vapour.
**Total surpasses 100 percent because of upward rounding of individual reservoir volumes.
Source: Adapted from Igor Shiklomanov's chapter "World Fresh Water Resources" in Peter H. Gleick (ed.), Water in Crisis: A Guide to the World's Fresh Water Resources, copyright 1993, Oxford University Press, New York, N.Y. Table made available by the United States Geological Survey.
oceans
1,338,000,000
96.5
ice caps, glaciers, and permanent snow
24,064,000
1.74
ground ice and permafrost
300,000
0.22
groundwater (total)
23,400,000
1.69
groundwater (fresh)
10,530,000
0.76
groundwater (saline)
12,870,000
0.93
lakes (total)
176,400
0.013
lakes (fresh)
91,000
0.007
lakes (saline)
85,400
0.006
soil moisture
16,500
0.001
atmosphere*
12,900
0.001
swamp water
11,470
0.0008
rivers
2,120
0.0002
biota
1,120
0.0001
total**
1,409,560,910
101.67
At present, ice locks up a little more than 2 percent of Earth’s water and may have accounted for as much as 3 percent or more during the height of the glaciations of the Pleistocene Epoch (2.6 million to 11,700 years ago). Although water storage in rivers, lakes, and the atmosphere is small, the rate of water circulation through the rain-river-ocean-atmosphere system is relatively rapid. The amount of water discharged each year into the oceans from the land is approximately equal to the total mass of water stored at any instant in rivers and lakes.
Soil moisture accounts for only 0.005 percent of the water at Earth’s surface. It is this small amount of water, however, that exerts the most direct influence on evaporation from soils. The biosphere, though primarily H2O in composition, contains very little of the total water at the terrestrial surface, only about 0.00004 percent, yet the biosphere plays a major role in the transport of water vapour back into the atmosphere by the process of transpiration.
As will be seen in the next section, Earth’s waters are not pure H2O but contain dissolved and particulate materials. Thus, the masses of water at Earth’s surface are major receptacles of inorganic and organic substances, and water movement plays a dominant role in the transportation of these substances about the planet’s surface.
About 107,000 cubic km (nearly 25,800 cubic miles) of rain fall on land each year. The total water in the atmosphere is 13,000 cubic km, and this water, owing to precipitation and evaporation, turns over every 9.6 days. Rainwater is not pure but rather contains dissolved gases and salts, fine-ground particulate material, organic substances, and even bacteria. The sources of the materials in rainwater are the oceans, soils, fertilizers, air pollution, and fossil fuelcombustion.
It has been observed that rains over oceanic islands and near coasts have ratios of major dissolved constituents very close to those found in seawater. The discovery of the high salt content of rain near coastlines was somewhat surprising because sea salts are not volatile, and it might be expected that the process of evaporation of water from the sea surface would “filter” out the salts. It has been demonstrated, however, that a large percentage of the salts in rain is derived from the bursting of small bubbles at the sea surface due to the impact of rain droplets or the breaking of waves, which results in the injection of sea aerosol into the atmosphere. This sea aerosol evaporates, with resultant precipitation of the salts as tiny particles that are subsequently carried high into the atmosphere by turbulent winds. These particles may then be transported over continents to fall in rain or as dry deposition.
Assuming equilibrium with the atmospheric carbon dioxide partial pressure (PCO2) of 10–3.5 (0.00035) atmosphere, the approximate mean composition of rainwater is in parts per million (ppm): sodium (Na+), 1.98; potassium (K+), 0.30; magnesium (Mg2+), 0.27; calcium (Ca2+), 0.09; chloride (Cl−), 3.79; sulfate (SO42−), 0.58; and bicarbonate (HCO3−), 0.12. In addition to these ions, rainwater contains small amounts of dissolved silica—about 0.30 ppm. The average pH value of rainwater is 5.6. (The term pH is defined as the negative logarithm of the hydrogen ion concentration in moles per litre. The pH scale ranges from 0 to 14, with lower numbers indicating increased acidity.) On a global basis, as much as 35 percent of the sodium, 55 percent of the chlorine, 15 percent of the potassium, and 37 percent of the sulfate in river water may be derived from the oceans through sea aerosol generation.
A considerable amount of data has become available for marine aerosols. These aerosols are important because (1) they are vital to any description of the global biogeochemical cycle of an element, (2) they may have an impact on climate, (3) they are a sink, via heterogeneouschemical reactions, for trace atmospheric gases, and (4) they influence precipitation of cloud and rain droplets. For many trace metals, the ratio of the atmospheric flux to the riverine flux for coastal and remote oceanic areas may be greater than one, indicating the importance of atmospheric transport. Figures have been prepared that illustrate the enrichment factors (EF) of North Atlantic marine aerosols and suspended matter in North Atlantic waters relative to the crust (that is, terrestrial sources), where
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and (X/Al)air and (X/Al)crust refer, respectively, to the ratio of the concentration of the element X to that of Al, aluminum (which is an easily observed terrestrial component of aerosols), in the atmosphere and in average crustal material. Comparing the enrichment factors in marine aerosols with those of suspended matter in the water column indicates qualitatively the marine aerosols’ importance as a source that alters the composition of marine suspended matter and, consequently, their importance to deep-sea sedimentation. Moreover, such comparisons help identify how significant terrestrial sources are for both the marine aerosols and the water below.
In some instances the ratios of ions in rainwater deviate significantly from those in seawater. Mechanisms proposed for this fractionation are, for example, the escape of chlorine as gaseous hydrogen chloride (HCl) from sea salt aerosol with a consequent enrichment in sodium and bubbling and thermal diffusion. In addition, release of biogenic gasessuch as dimethyl sulfide (DMS) from the sea surface and its subsequent reaction in the oceanic atmosphere to sulfate can change rainwater ionratios with respect to seawater. Soil particles also can influence rainwater composition. Rainfall over the southwestern United States contains relatively high sulfate concentrations because of sulfate-bearing particles that have been blown into the atmosphere from desert soils. Rain near industrial areas commonly contains high contents of sulfate, nitrate, and carbon dioxide (CO2) largely derived from the burning of coal and oil. There are two main processes leading to the conversion of sulfur dioxide (SO2) to sulfuric acid (H2SO4). These are reactions with hydroxyl radicals (–OH) and with hydrogen peroxide (H2O2) in the atmosphere:
and
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The sulfuric acid then dissociates to hydrogen and sulfate ions:
For the nitrogen gases nitric oxide (NO) and nitrogen dioxide (NO2) released from fossil fuel burning, their atmospheric reactions lead to the production of nitric acid (HNO3) and its dissociation to hydrogen ions (H+) and nitrate (NO3−). These reactions are responsible for the acid rainconditions that occurred in the northeastern United States, southeastern Canada, and western Europe during the second half of the 20th century (see belowAcid rain). The high sulfate values of the rain in the northeastern United States reflect the acid precipitation conditions of this region.
River and ocean waters
River discharge constitutes the main source for the oceans. Seawater has a more uniform composition than river water. It contains, by weight, about 3.5 percent dissolved salts, whereas river water has only 0.012 percent. The average density of the world’s oceans is roughly 2.75 percent greater than that of typical river water. Of the average 35 parts per thousand salts of seawater, sodium and chlorine make up almost 30 parts, and magnesium and sulfate contribute another four parts. Of the remaining one part of the salinity, calcium and potassiumconstitute 0.4 part each and carbon, as carbonate and bicarbonate, about 0.15 part. Thus, nine elements (hydrogen, oxygen, sulfur, chlorine, sodium, magnesium, calcium, potassium, and carbon) make up 99 percent of seawater, though most of the 94 naturally occurring elements have been detected therein. Of importance are the nutrient elements phosphorus, nitrogen, and silicon, along with such essential micronutrient trace elements as iron, cobalt, and copper. These elements strongly regulate the organic production of the world’s oceans.
A portion of the delta of the Mekong River as it flows through southern Vietnam and empties into the South China Sea.M. Gifford/De Wys Inc.
In contrast to ocean water, the average salinity of the world’s rivers is low—only about 0.012 percent, or 120 ppm by weight. Of this salt content, carbon as bicarbonate constitutes 58 parts, or 48 percent, and calcium, sulfur as sulfate, and silicon as dissolved monomeric silicic acid make up a total of about 39 parts, or 33 percent. The remaining 19 percent consists predominantly of chlorine, sodium, and magnesium in descending importance. It is obvious that the concentrations and relative proportions of dissolved species in river waters contrast sharply with those of seawater. Thus, even though seawater is derived in part by the chemical differentiation and evaporation of river water, the processes involved affect every element differently, indicating that simple evaporation and concentration are entirely secondary to other processes.
Water-rock interactions as determining river water composition
Generally speaking, the composition of river water, and thus that of lakes, is controlled by water-rock interactions. The attack of carbon dioxide-chargedrain and soil waters on the individual minerals in continental rocks leads to the production of dissolved constituents for lakes, rivers, and streams. It also gives rise to solid alteration products that make up soils or suspended particles in freshwater aquatic systems. The carbon dioxide content of rain and soil waters is of particular importance in weathering processes. The pH of rainwater equilibrated with the atmospheric carbon dioxide partial pressure of 10−3.5 (0.00032) atmosphere is 5.6. In industrial regions, rainwater pH values may be lower because of the release and subsequent hydrolysis of acid gases—namely, sulfur dioxide and nitrogen oxides (NOx) from the combustion of fossil fuels. After rainwater enters soils, its characteristics change markedly. The usual few parts per million of salts in rainwater increase substantially as the water reacts. The upper part of the soil is a zone of intense biochemical activity. The bacterial population near the surface is large, but it decreases rapidly downward. One of the major biochemical processes of the bacteria is the oxidation of organic material, which leads to the release of carbon dioxide. Soil gases obtained above the zone of water saturation may contain 10 to 40 times as much carbon dioxide as the free atmosphere, and in some cases carbon dioxide has been shown to make up 30 percent of the soil gases as opposed to 0.03 percent of the free atmosphere. In addition to the acid effects of carbon dioxide, there is a highly acidic microenvironment created by the roots of living plants. Values of pH as low as 2 have been measured immediately adjacent to root hairs. The combined length of a plant’s root hairs may be several kilometres, so their chemical effects on acidic water are formidable.
These acid solutions in the soilenvironment attack the rock minerals, the bases of the system, producing neutralization products of dissolved constituents and solid particles. Two general types of reactions occur: congruent and incongruent. In the former, a solid dissolves, adding elements to the water according to their proportions in the mineral. An example of such a weathering reaction is the solution of calcite (CaCO3) in limestones:
Here one of the HCO3− ions comes from calcite and the other from CO2(g) in the reacting water. The amount of carbon dioxide dissolved according to reaction (4) depends on temperature, pressure, original bicarbonate content of the weathering solution, and partial pressure of the carbon dioxide. The carbon dioxide and the temperature are the most important variables. Increases in one or both of these variables lead to increases in the amount of calcite dissolved. For example, for a carbon dioxide pressure of 10−3.5 (0.00032) atmosphere, the amount of calcium that can be dissolved until saturation is about 10−3.3 (0.0005) mole, or 20 ppm, at 25 °C (77 °F). For an atmospheric carbon dioxide pressure of 10−2(0.01) atmosphere and for a soil atmosphere of nearly pure carbon dioxide, the values are 65 and 300 ppm, respectively. The weathering of calcite leads to the release of calcium and bicarbonate ions into soil waters and groundwaters, and these constituents eventually reach lakeand river systems. The insoluble residue of quartz (SiO2), clay minerals, and iron oxides (e.g., FeOOH) in the limestone rock make up the deep-red soils that form from limestoneweathering. These particles may be carried into streams by runoff and hence to lakes and the oceans and become part of the suspended load of these systems.
An example of an incongruent weathering reaction—that is, one where only part of a solid is consumed—is that involving aluminosilicates. One such reaction is the aggressive attack of carbon dioxide-charged soil water on the mineral K-spar (KAlSi3O8), an important phase found in continental rocks. The reaction is
It should be noted that the K-spar changes into a new mineral—kaolinite(a clay mineral) in this case—plus solution, and acid is consumed. The total dissolved material per litre of soil solution released is about 60 ppm for a solution initially equilibrated with a typical soil carbon dioxide content. The water resulting from reaction (5) would contain bicarbonate, potassium, and dissolved silica in the ratios 1:1:2, and the new solid, kaolinite, would be a weathering product. These dissolved constituents and the solid alteration product would eventually reach rivers to be transferred possibly to lakes and ultimately to the sea. It has been demonstrated that the composition of river water is the product of a variety of mineral-water reactions such as (4) and (5). The dissolved load of the world’s rivers comes from the following sources: 7 percent from beds of halite (NaCl) and saltdisseminated in rocks, 10 percent from gypsum (CaSO4·2H2O) and anhydrite (CaSO4) deposits and sulfate salts disseminated in rocks, 38 percent from limestones and dolomites, and 45 percent from the weathering of one silicate mineral to another. Of the bicarbonate ions in river water, 56 percent stems from the atmosphere, 35 percent from carbonate minerals, and 9 percent from the oxidative weathering of fossil organic matter. Reactions involving silicate minerals account for 30 percent of the riverine bicarbonate ions.
Besides dissolved substances, rivers also transport solids in traction (i.e., bed load) and, most importantly, suspended load. The present global river-borne flux of solids to the oceans is estimated as 15.5 billion metric tons (about 17.1 billion tons) per year. Present elemental fluxes are estimated in millions of metric tons per year as silicon, 4,420; aluminum, 1,460; iron, 740; calcium, 330; potassium, 310; magnesium, 210; and sodium, 110. The total load of particulate organic carbon of the world’s rivers is 180 million metric tons (nearly 200 million tons) per year. The riverine fluxes of trace metals to the oceans are dominated by their occurrence in the particulate phase as opposed to the dissolved phase. The particulate matter in river water is an important source of silicon, aluminum, iron, titanium, rubidium, scandium, vanadium, the lanthanoids, and other elements for deep-sea sediments.
waters
Although lakewatersconstitute only a small percentage of the water in the hydrosphere, they are an important ephemeral storage reservoir for fresh water. Aside from their recreational use, lakes constitute a source of water for household, agricultural, and industrial uses. Lake waters are also very susceptible to changes in chemical composition due to these uses and to other factors.
temperate lake circulationAnnual circulation patterns in a dimictic lake. The typical dimictic lake has distinct layers that fully mix twice a year. It undergoes stratification in the summer and complete overturn in the autumn and spring. During winter, surface ice prevents further mixing by the wind. Small differences in density and temperature exist, with cooler water (0 °C [32 °F]) staying near the surface and warmer, denser water (4 °C [39.2 °F]) extending to the bottom.Encyclopædia Britannica, Inc.
In general, fresh waters at the continental surface evolve from their rocksources by enrichment in calcium and sodium and by depletion in magnesium and potassium. In very soft waters the alkalies may be more abundant than the alkaline earths, and in the more-concentrated waters of open river systems calcium > magnesium > sodium > potassium. For the anions, in general, HCO3− exceeds SO42− , which is greater in concentration than Cl−. It is worthwhile at this stage to consider some major mechanisms that control global surface water composition. These mechanisms are atmospheric precipitation, rock reactions, and evaporation-precipitation.
The mechanism principally responsible for waters of very low salinity is precipitation. These waters tend to form in tropical regions of low relief and thoroughly leached source rocks. In these regions rainfall is high, and volumes of fresh water (rivers, tributary streams, pools, etc.) within a watershed are usually dominated by salts brought in by precipitation. Such waters constitute one part of a chain of water volumes that begins with falling precipitation and ends with the release of water into the ocean, for which the final part of the chain represents water volumes dominated by contributions of dissolved salts from the rocks and soils of their basins. These waters have moderate salinity and are rich in dissolved calcium and bicarbonate. They are in turn the “end-member” of another series that extends from the calcium-rich, medium-salinity fresh waters to the high-salinity, sodium chloride-dominated waters of which seawater is an example. Seawater composition, however, does not evolve directly from the composition of fresh waters and the precipitation of calcium carbonate; other mechanisms that control its composition are involved. Such factors as relief and vegetation also may affect the composition of the world’s surface waters, but atmospheric precipitation, water-rock reactions, and evaporation-crystallization processes appear to be the dominant mechanisms governing continental surface water chemistry.
Continental fresh waters evaporate once they have entered closed basins, and their constituent salts precipitate on the basin floors. The composition of these waters may evolve along several different paths, depending on their initial chemical makeup.
Biological processes strongly affect the composition of lake waters and are responsible to a significant degree for the compositional differences between the upper water layer (the epilimnion) and the lower water layer (the hypolimnion) of lakes. The starting point is photosynthesis, represented by the following reaction:
The reversal of this reaction is oxidation-respiration leading to the release of the nutrientsnitrogen and phosphorus, as well as carbon dioxide. In a stratified lake, carbon, nutrients, and silica are extracted from the upper layer during photosynthesis. This process leads to reduced concentrations of nitrate, phosphate, and silica in these waters and, during times of maximum daylight organic production, to supersaturation of the upper layer with respect to dissolved oxygen. The organic matter produced by phytoplankton may be either grazed upon by zooplankton and other organisms or decomposed by bacteria. Some of it, however, sinks into the lower layer. There it is further decomposed, especially by bacteria, resulting in the release of dissolved phosphorus and nitrogen and the consumption of oxygen. Oxygen concentrations therefore are reduced in these lower lake waters, because stratification prevents oxygen exchange with the atmosphere. Furthermore, the inorganic carbonate and siliceous skeletons of the dead organisms sinking into the lower layer may dissolve, giving rise to increased concentrations of dissolved silica and inorganic carbon in the deep waters of stratified lakes. This dissolution is a result of undersaturation of the waters of the lower layer with respect to the opaline silica and calcium carbonate that make up the skeletons of the dead and sinking plankton. These natural biological processes have been accelerated in some lakes because of excess nutrient input by human activity, resulting in the eutrophication of lake waters and marine systems (see belowEutrophication).
Processes affecting the major chemical components of groundwater
component
origin*
*The sources for each constituent are given in approximate order of decreasing importance.
Source: Adapted from Elizabeth Kay Berner and Robert A. Berner, The Global Water Cycle: Geochemistry and Environment, copyright 1987, Table 4.6, p. 170. Reproduced by permission of Prentice Hall, Inc., Englewood Cliffs, N.J.
sodium ion
sodium chloride dissolution (some pollutive)
plagioclase weathering
rainwater addition
potassium ion
biotite weathering
K-feldspar weathering
biomass decreases
dissolution of trapped aerosols
magnesium ion
amphibole and pyroxene weathering
biotite (and chlorite) weathering
dolomite weathering
olivine weathering
rainwater addition
calcium ion
calcite weathering
plagioclase weathering
dolomite weathering
dissolution of trapped aerosols
biomass decreases
bicarbonate ion
calcite and dolomite weathering
silicate weathering
sulfate ion
pyrite weathering (some pollutive)
rainwater addition
chloride ion
sodium chloride dissolution (some pollutive)
rainwater addition
hydrogen silicate
silicate weathering
The water table is the top level of groundwater. Surface water is an exposed part of the water table.Encyclopædia Britannica, Inc.
The biological processes of greatest importance are microbial metabolism, organic production, and respiration (oxidation). By far the most important overall process for the major constituents of groundwater is that of mineral-water reactions, which were briefly described above in River and ocean waters. Thus, the composition of groundwaters strongly reflects the types of rockminerals that the waters have encountered in their movement through the subsurface.
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In general, the most mobile elements in groundwater—i.e., those most easily liberated by the weathering of rock minerals—are calcium, sodium, and magnesium. Silicon and potassium have intermediate mobilities, and aluminum and iron are essentially immobile and locked up in solid phases.
Groundwaters are highly susceptible to contamination because of human activities and the fact that their dissolved constituents are derived to a large extent from the leaching of surface materials. Some of the nitrogen and phosphorus applied to soils as fertilizers and organic pesticides may be leached and leak into groundwater systems, leading to increased concentrations of ammonium and phosphate. Radioactive wastes, industrial chemicals, household materials, and mine refuse are other anthropogenic sources of dissolved substances that have been detected in groundwater systems.
Ice is nearly a pure solid and, as such, accommodates few foreign ions in its structure. It does contain, however, particulate matter and gases, which are trapped in bubbles within the ice. The change in composition of these materials through time, as recorded in the successive layers of ice, has been used to interpret the history of Earth’s surface environmentand the impact of human activities on this environment. The increase in the lead content of continental glacial ice with decreasing age of the ice up to the middle of the 1970s, for example, reflects the progressive input of tetraethyl lead into the global environment from gasoline burning. (Stringent environmental regulations that appeared in the 1970s regarding the use of leaded gasoline has led to a fall in lead concentrations in ice laid down since that time.) Also, atmospheric carbon dioxide and methane concentrations, which have increased significantly during the past century because of anthropogenic activities, are faithfully recorded in ice bubbles of the thick continental ice sheets. By 2016 atmospheric carbon dioxide and methane concentrations had increased by more than 43 percent and more than 150 percent, respectively, higher than their concentrations 200 years ago; the latter concentration values were obtained from measurements of the gases in air trapped in ice.
Perito Moreno Glacier, Los Glaciares National ParkPerito Moreno Glacier, Los Glaciares National Park, Argentina. Although many other South American glaciers have declined in mass since the middle of the 20th century, the mass of Perito Moreno Glacier has remained relatively steady.Jeremy Woodhouse—Digital Vision/Getty Images
Origin And Evolution Of The Hydrosphere
It is not very likely that the total amount of water at Earth’s surface has changed significantly over geologic time. Based on the ages of meteorites, Earth is thought to be 4.6 billion years old. The oldest rocksknown are 3.9 billion to 4.0 billion years old, 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 billion and 3.9–4.0 billion years ago. Thus, ideas concerning the early history of the hydrosphere are closely linked to theories about the origin of Earth.
Earth is thought to have accreted from a cloud of particles around the Sun. This gaseous matter condensed into small particles that coalesced to form a protoplanet, which in turn grew by the gravitational attractionof 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 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 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 (seeevolution of the atmosphere).
Earth's interiorEarth's major structural layers displayed in cross section. Near-surface and surface features, such as ocean basins and continents, are also shown in cross section.Encyclopædia Britannica, Inc.
At an early stage, Earth thus did not have water or water vapour at its surface, and the topic of how water arrived on Earth’s surface is a matter of substantial debate. Some scientists argue that much of the water on the planet was delivered by comet and meteor impacts. Both of these celestial objects have been shown to contain ice. Other scientists claim that most of Earth’s water came from chemical reactions within the planet’s interior. 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 planetary mass, 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 Earth’s interior throughout geologic timeremains uncertain. It is likely that the hydrosphere attained its present volume early in Earth’s history, and since that time there have been only small losses and gains. Gains would be from continuous degassing of Earth; the present degassing rate of juvenile water has been determined as being only 0.3 cubic km (about 0.07 cubic mile) 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 km (about 0.0005 cubic mile) 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 km (about 9.3 miles), 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 Earth’s surface—hence 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 Earth during its early history, including water vapour, have been called excessvolatiles because their masses cannot be accounted for simply by rock weathering. These volatiles are thought to have formed the early atmosphere of Earth. At an initial crustal temperature of about 600 °C (about 1,100 °F), almost all of these compounds, including water (H2O), would have been in the atmosphere. The sequence of events that occurred as the crust cooled is difficult to reconstruct. Below 100 °C (212 °F) 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 chemical 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 (that is, the temperature above which a compound cannot remain as a liquid—i.e., 374 °C [705 °F]). 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.
Earth's early and modern atmospheresComparison of Earth's prebiotic and modern atmospheres. Before life began on the planet, Earth's atmosphere was largely made up of nitrogen and carbon dioxide gases. After photosynthesizing organisms multiplied on Earth's surface and in the oceans, much of the carbon dioxide was replaced with oxygen.Encyclopædia Britannica, Inc.
This is one of several possible pathways for the early surface of Earth. Whatever the actual case, after 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 Earth’s surface had cooled to temperatures lower than 100 °C by this time, and neutralization of the original acid volatiles had taken place (seeacid-base reaction). It is possible, however, that, because of increased greenhouse gasconcentrations early in the Archean Eon (4.0 billion to 2.5 billion years ago), Earth’s surface could still have been warmer than today (seeclimate change: climate change through geologic time).
If most of the degassing of primary volatile substances from 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 H2O were released early in the history of 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 here—namely, 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 constituent released from the cooling crust. Early production of oxygen was by the photodissociation of water in Earth’s atmosphere, a process that was triggered by the absorption of the Sun’s ultraviolet radiation. The reaction is H2O (liquid) + hν → H2(gas) + O2(gas) in which hν 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 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).
The transitional hydrosphere
The nature of the rock record from the time of the first sedimentary rocks (approximately 3.8 billion years ago) to about one to two billion years ago suggests that the amount of oxygen in Earth’s atmospherewas significantly lower than it is today and that there were continuous chemical trends in the sedimentary rocks formed and, more subtly, in the composition of the hydrosphere. The chemistry of rocks shifted dramatically during this transitional period. The source rocks of sediments during this time may have been more basaltic than subsequent ones. Sedimentary debris was formed by the alteration of such source rocks in an oxygen-deficient atmosphere and accumulated primarily under anaerobic (oxygen-free) marine conditions. The chief difference between reactions involving mineral-ocean equilibria at this time and the present day was the role played by ferrous iron (i.e., reduced state of iron). The concentration of dissolved iron in modern oceans is low because of the insolubility of oxidized iron oxides. During the transition stage and earlier, oxygen-deficient environments were prevalent, and these favoured the formation of minerals containing ferrous iron from the alteration of rocks slightly more rich in basalt than those of today. Indeed, iron carbonatesiderite and iron silicategreenalite, in close association with chert and iron sulfidepyrite, are characteristic minerals that occur in iron formations of the middle Precambrian (about 2.5 billion to 1.6 billion years ago). The chert originally was deposited as amorphoussilica; equilibrium between amorphous silica, siderite, and greenalite at 25 °C (77 °F) and a total pressure of one atmosphere requires a carbon dioxide pressure of about 10−2.5 (0.00316) atmosphere, or 10 times the present-day value.
A banded-iron formation (BIF) rock recovered from the Temagami greenstone belt in Ontario, Canada, and dated to 2.7 billion years ago. Dark layers of iron oxide are intercalated with red chert.Prof. Dr. Michael Bau/Jacobs University Bremen
The oceans of this transitional period can be thought of as a solutionthat resulted from an acid leach of basaltic rocks, and, because the neutralization of the volatile acid gases was not restricted primarily to land areas as it is today, much of this alteration may have occurred by submarine processes. Anaerobic depositional environments with internal carbon dioxide pressures of about 10−2.5 atmosphere prevailed, and the oxygen-deficient atmosphere itself may have had a carbon dioxide pressure close to 10−2.5 atmosphere. If so, the pH of early ocean water was lower than that of modern seawater and the calciumconcentration was higher; moreover, the early ocean water was probably saturated with respect to amorphous silica—roughly 120 parts per million (ppm).
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To simulate what might have occurred, it is helpful to imagine emptying the Pacificbasin, throwing in great masses of broken basaltic material, filling it with hydrogen chloride dissolved in water so that the acid becomes neutralized, and then carbonating the solution by bubbling carbon dioxide through it. Oxygen would not be permitted into the system. The hydrochloric acid would leach the rocks, resulting in the release and precipitation of silica and the production of a chloride ocean containing sodium, potassium, calcium, magnesium, aluminum, iron, and reduced sulfur species in the proportions present in the rocks. As complete neutralization was approached, the aluminum could begin to precipitate as hydroxides and then combine with precipitated silica to form cation-deficient aluminosilicates. As the neutralization process reached its end, the aluminosilicates would combine with more silica and with cations to form such minerals as chlorite, and ferrous iron would combine with silica and sulfur to produce greenalite and pyrite. In the final solution, chlorine would be balanced by sodium and calcium in roughly equal proportions, with subordinate amounts of potassium and magnesium; aluminum would be quantitatively removed, and siliconwould be at saturation with amorphous silica. If this solution were then carbonated, calcium would be removed as calcium carbonate, and the chlorine balance would be maintained by abstraction of more sodium from the primary rock. The sediments formed in this system would contain chiefly silica, ferrous iron silicates, chloritic minerals, calcium carbonate, calcium-magnesium carbonates, and small amounts of pyrite.
If the hydrogen chloride added were in excess of the carbon dioxide, the resultant oceans would have a high content of calcium chloride (CaCl2), but with a pH still near neutrality. If the carbon dioxide added were in excess of the chlorine, calcium would be precipitated as carbonate until it reached a level roughly that of present-day ocean waters—namely, a few hundred parts per million.
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If this newly created ocean were left undisturbed for several hundred million years, its waters would evaporate and be transported onto the continents (in the form of precipitation); streams would transport their loads into it. The sediments produced in this ocean would be uplifted and incorporated into the continents. The influence of the continental debris would gradually be felt and the pH might change somewhat. Iron would be oxidized out of the ferrous silicates to yield iron oxides, but the composition of the water would not vary substantially.
The primary minerals of igneous rocks are all mildly basic compounds. When these minerals react in excess with acids such as hydrogen chloride and carbon dioxide, they produce neutral or mildly alkaline solutions as well as a set of altered aluminosilicate and carbonate reaction products. It is improbable that seawater has changed through time from a solution approximately in equilibrium with these reaction products—i.e., with clay minerals and carbonates.
The modern hydrosphere
It is likely that the hydrosphere achieved its modern chemical characteristics about 1.5 billion to 2 billion years ago. The chemical and mineralogical compositions and the relative proportions of sedimentary rocks of this age differ little from their counterparts of the Paleozoic Era(from 541 million to 252 million years ago). Calcium sulfate deposits dated to the late Precambrian (about 1.6 billion to 541 million years ago) attest to the fact that the acidsulfurgases had been neutralized to sulfate by this time. Chemically precipitated ferric oxides in late Precambrian sedimentary rocks indicate available free oxygen, whatever its percentage. The chemistry and mineralogy of middle and late Precambrian shales are similar to those of Paleozoic shales. The carbon isotopic signature of carbonate rocks has been remarkably constant for more than three billion years, indicating exceptional stability in size and fluxes related to organic carbon. The sulfur isotopic signature of sulfur phases in rocks strongly suggests that the sulfur cycle involving heterotrophic bacterialreduction of sulfate was in operation 2.7 billion years ago. It therefore appears that continuous cycling of sediments similar to those of today has occurred for 1.5 billion to 2 billion years and that these sediments have controlled hydrospheric, and particularly oceanic, composition.
rain and dark cloudPrecipitation results when rising moisture-laden air cools and condenses into water droplets or ice crystals. When these structures become large enough to be acted upon by gravity, they fall to Earth.AdstockRF
It was once thought that the saltiness of the modern oceans simply represents the storage of salts (that is, compounds that form when part of an acid is replaced by a metal or something like a metal) derived from rock weathering and transported to the oceans by fluvial processes. With increasing knowledge of the age of Earth, however, it was soon realized that, at the present rate of delivery of salts to the ocean or even at much reduced rates, the total salt content and the mass of individual salts in the oceans could be attained in geologically short time intervals compared with the planet’s age. The total mass of salt in the oceans can be accounted for at today’s rates of stream delivery in about 12 million years. The mass of dissolved silica in ocean water can be doubled in just 20,000 years by the addition of stream-derived silica. To double the sodium content would take 70 million years. It then became apparent that the oceans are not merely an accumulator of salts. Rather, as water evaporates from the oceans, together with some salt, the salts introduced must be removed in the form of minerals deposited in marine sediments. Accordingly, the concept of the oceans as a chemical system changed from that of a simple accumulator to that of a steady-state system in which rates of inflow of materials equal rates of outflow. The steady-state concept permits influx to vary with time, but the inflow would be matched by nearly simultaneous and equal variation of efflux.
In such a steady-state conceptual view of the oceans, it has been found necessary to treat components of ocean water in terms of all their influxes and effluxes and to be more cognizant of the time scale of application of the steady-state concept. Indeed, the recent increase in the carbon dioxide concentration of the atmosphere due to the burning of fossil fuels has been linked to a changed pH and dissolved inorganic carbon concentrations of surface ocean water on a time scale measured in hundreds of years. If fossil-fuel burning were to cease, return to the original state of seawater composition could take thousands of years. Ocean water is not in steady state with respect to carbon on these time scales, but on a longer geologic time scale it certainly could be. Even on this longer time scale, however, oceanic composition has varied because of natural changes in the carbon dioxide level of the atmosphere and because of other factors.
It appears that the best description of modern seawater composition is that of a chemical system in a dynamic quasi-steady state. Changes in composition may occur over time, but the system always seems to return to a time-averaged steady-state composition. In other words, since 1.5 billion to 2 billion years ago, evolutionary chemical changes in the hydrosphere have been small when viewed against the magnitude of previous change.
It should be noted that rivers supply dissolved constituents to the oceans, whereas high- and low-temperature reactions between seawater and submarine basalts and reactions in sediment pore waters may add or remove constituents from ocean water. Biological processes involved in the formation of the opaline silica skeletons of diatoms (a type of algaewith a shell made of silica) and radiolarians (planktonicprotozoans) and the carbonate skeletons of planktonic foraminiferans (shelled unicellular organisms possessing pseudopods) and coccolithophorids (marine planktonic biflagellated organisms that secrete a type of minute calcium carbonate platelet or ring) chiefly remove calcium and silica from seawater. Exchange reactions between river-borne clays entering seawater are particularly significant for sodium and calcium ions. Most of the carbon imbalance in ocean water represents carbon released to the ocean-atmosphere system during precipitation of carbonate minerals; i.e.,
In the case of iron, it has been documented that “dissolved” iron carried by rivers is rapidly precipitated as hydroxides in the mixing zone with seawater and that the reduced dissolved iron released from anaerobic sediments also is rapidly precipitated under the oxic conditions (i.e., those with oxygen present) prevailing in the water column. Iron is also precipitated as iron smectites, hydrated iron oxides, and nontronite (iron-rich montmorillonite) in the deep sea. It is thus likely that iron is removed by these processes.
The imbalance in sodium is large: 45 percent of the river input is not accounted for in the mass balance calculations. There are, however, major uncertainties in the estimation of the pore-water flux of sodium ions. An important sink for sodium on a geologic time scale is the formation of evaporites. If the amount of unbalanced sodium is expressed in terms of halitedeposition, it would correspond to 1.6 × 1014grams of sodium chloride per year as compared with a potential total depositional rate of 3.3 × 1014 grams annually. There are no important sodium chloride deposits forming today. Thus, one possibility is that sodium is accumulating in the oceans. If so, in 6 × 106 years at an accumulation rate of 63 × 1012 grams of sodium annually, the average salinity of the oceans would increase less than one part per thousand. The chlorine balance for the oceans, however, indicates that it is likely that the major problem in the imbalance for sodium lies in the flux estimates for sediment pore waters and perhaps submarine weathering processes.
The first map of ocean salinity taken by the Aquarius/SAC-D spacecraft, August–September 2011.GSFC—JPL-Caltech/NASA
Modern seawater chemistry has been characteristic of roughly the past 600 million years of ocean history. Evaporite sediments provide strong evidence that the composition of seawater has not varied a great deal during this interval of geologic time. Nonetheless, it seems likely that fluctuations did occur, particularly in the concentrations of calcium, magnesium, and sulfate ions. The isotopic composition of sulfur in seawater, as recorded in evaporites, has varied dramatically during the past one billion years. Although it is difficult to relate these isotopic fluctuations to the calcium and sulfate concentrations of seawater, some scientists believe that the fluctuations do in fact imply changes in the latter. Furthermore, the major features of the sulfur isotopic curve for evaporites versus Phanerozoic time (the last 541 million years) is similar to that of the strontium-87/strontium-86 ratio, and perhaps the strontium/calcium ratio, of sedimentary materials during this time interval. Such covariation is consistent with a model in which fluxes related to alteration of seafloor basalts and continental river runoff vary with time, resulting in variation in seawater composition.
The chemistry of the atmosphere has certainly changed significantly during the past one billion years of Earth history. A modification of this kind implies changes in the chemistry of the hydrosphere as well. Oxygen in the atmosphere rose substantially between two billion years ago and the beginning of the Phanerozoic Eon (i.e., 541 million years ago), whereas atmospheric carbon dioxide levels probably decreased. This change led in general to a progressively more oxygenated and less acidic hydrosphere. It is likely that the development of higher land plantsduring the Devonian Period (from 419 million to about 359 million years ago) resulted in an increase in atmospheric oxygen and a decrease in carbon dioxide. Air trapped within bubbles in Arctic and Antarcticiceshows that the carbon dioxide content of the atmosphere during the climax of the last ice age was about 180 parts per million by volume (ppmv), and atmospheric CO2 levels reached approximately 280 ppmv during the last great interglacial of 120,000 years ago, long before modern society initiated its extensive fossil-fuel burning and deforestation activities (see belowBuildup of greenhouse gases).
These atmospheric changes in themselves can influence the chemistry of the hydrosphere, but they also appear to be coupled with other changes in the rock-ocean-atmosphere-biota system that strongly affect hydrospheric chemistry. For example, though surface waters probably remained oxygenated during the Cretaceous and Devonian periods, there is evidence that intermediate and deep ocean waters were more anoxic (oxygen-depleted) than today. These “anoxic events” are characterized by dramatic changes in Earth’s ocean-atmosphere system, including changes in the rates of the cyclic transfer of elements at Earth’s surface and in atmospheric composition.
The probable impact of a bolide (either an asteroid or comet) and increased volcanism at the end of the Cretaceous (about 66 million years ago), though still the subject of hot debate, certainly could have caused short-term changes in the chemistry of Earth’s atmosphere-hydrosphere. Scientists speculate that such an event could have had any of several results, including (1) changes in Earth’s radiant-energy balance because of vast amounts of particulate and gaseous input into the atmosphere and subsequent cooling, (2) acid rain stemming from the input into the atmosphere of nitrogen gases generated by the bolide impact, (3) increased trace metal fluxes to the hydrosphere brought on by the destruction of the bolide and increased volcanism, and perhaps (4) increased “anoxia” of the hydrosphere as a result of the death of land and marine organisms.
Whatever the case, evidence of change is present in the rock record, albeit the composition of the modern hydrosphere has not varied greatly for the past one billion years. Moreover, as will be seen in the following section, humans are modifying the chemistry not only of local and regional water bodies but also of the entire global atmosphere-hydrosphere by increasing the rates of input of natural substances and by introducing new synthetic substances to the environment.
The present discussion focuses on three major problems—eutrophication, acid rain, and the buildup of the so-called greenhouse gases. Each exemplifies human interference in the hydrologic cycle and its far-reaching effects.
Historically, aquatic systems have been classified as oligotrophic or eutrophic. Oligotrophic waters are poorly fed by the nutrientsnitrogenand phosphorus and have low concentrations of these constituents. There is thus low production of organic matter by photosynthesis(equation [6]) in such waters. By contrast, eutrophic waters are well supplied with nutrients and generally have high concentrations of nitrogen and phosphorus and, correspondingly, large concentrations of plankton owing to high biological productivity. The waters of such aquatic systems are usually murky, and lakes and coastal marine systems may be oxygen-depleted at depth. The process of eutrophication is defined as high biological productivity resulting from increased input of nutrients or organic matter into aquatic systems. For lakes, this increased biological productivity usually leads to decreased lake volume because of the accumulation of organic detritus. Natural eutrophication occurs as aquatic systems fill in with organic matter; it is distinct from cultural eutrophication, which is caused by human intervention. The latter is characteristic of aquatic systems that have been artificially enriched by excess nutrients and organic matter from sewage, agriculture, and industry. Naturally eutrophic lakes may produce 75–250 grams of carbon per square metre per year, whereas those lakes experiencing eutrophication because of human activities can support 75–750 grams per square metre per year. Commonly, culturally eutrophic aquatic systems may exhibit extremely low oxygen concentrations in bottom waters. This is particularly true of stratified systems, as, for instance, lakes during summer where concentrations of molecular oxygen may reach levels of less than about one milligram per litre—a threshold for various biological and chemical processes.
Aquatic systems may change from oligotrophic to eutrophic, or the rate of eutrophication of a natural eutrophic system may be accelerated by the addition of nutrients and organic matter due to human activities. The process of cultural eutrophication, however, can be reversed if the excess nutrient and organic matter supply is shut off.
Not only do freshwater aquatic systems undergo cultural eutrophication, but coastal marine systems also may be affected by this process. On a global scale, the input by rivers of organic matter to the oceans today is twice the input in prehuman times, and the flux of nitrogen, together with that of phosphorus, has more than doubled. This excess loading of carbon, nitrogen, and phosphorus is leading to cultural eutrophication of marine systems. In several polluted eastern U.S. estuaries (e.g., Chesapeake and Delaware bays) and in some estuaries of western Europe (e.g., the Scheldt of Belgium and the Netherlands), all of the dissolved silica brought into the estuarine waters by rivers is removed by phytoplankton growth (primarily diatoms) resulting from excess fluxes of nutrients and organic matter. In the North Sea there is now a deficiency of silica and an excess of nitrogen and phosphorus, which in turn has led to a decrease in diatom productivity and an increase in cyanobacteriaproductivity—a biotic change brought about by cultural eutrophication.
The emission of sulfur dioxide and nitrogen oxides to the atmosphere by human activities—primarily fossil-fuel burning—has led to the acidification of rain and freshwater aquatic systems. Acid rain is a worldwide problem and has been well documented for eastern North America and the countries of western Europe.
sulfur cycleMajor sulfur-producing sources include sedimentary rocks, which release hydrogen sulfide gas, and human sources, such as smelters and fossil-fuel combustion, both of which release sulfur dioxide into the atmosphere.Encyclopædia Britannica, Inc.
Acid rain is defined as precipitation with a pH of less than 5.2 that results from reactions involving gases other than carbon dioxide. The overall reactions that produce such precipitation are those of equations (1), (2), and (3) and
Average pH can be calculated as the −log aH+ (aH+ is activity of the hydrogen ion). A water chemistry study examining precipitation over the eastern United States for the period October 1979 through September 1980 revealed that low pH values were a result of equilibration of rainwater with the atmospheric acid gases of carbon, nitrogen, and sulfur. Equilibration only with atmospheric carbon dioxide would give a pH of 5.7. The significantly lower values are a result of reactions with nitrogen- and sulfur-bearing gaseous atmospheric components derived primarily from fossil-fuel burning sources.
Map of precipitation pH in the continental United States in 1994.Encyclopædia Britannica, Inc.
Map of precipitation pH in the continental United States in 2008.Encyclopædia Britannica, Inc.
Nitrate and sulfate concentrations in precipitation over the eastern United States are strongly correlated with pH—the lower the pH of rain, the higher the concentrations of nitrate and sulfate. Such low pH values and increased nitrate and sulfate concentrations were observed in the rains of western Europe and North America until the late 20th century. The pH values of precipitation in these regions have increased significantly since then because of strict air quality regulations. Other parts of the world that have industrialized since the late 20th century without enacting adequate air pollution controls, such as China, experienced similar pH declines in precipitation.
Areas affected by acid deposition contrasted with regions of high acid sensitivity.Encyclopædia Britannica, Inc.
Wet and dry deposition also removes the hydrogen ion produced in the rain by the oxidation and hydrolysis of these acid gases. This excess hydrogen ion can bring about the acidification of freshwater aquatic systems, particularly those with little buffer capacity (e.g., lakes situated in crystalline rock terrains). Furthermore, the lower pH values of rainwater, and consequently of soil water, can lead to increased mobilization of aluminum. Acidification of freshwater lakes in the eastern United States and southeastern Canada and increased aluminum concentrations in their waters are thought to be responsible for major changes in the ecosystems of the lakes. In particular, many lakes of this region lack substantial fish populations today, even though they supported large numbers of fish in the early 1900s. Acid rain also may be among the factors responsible for damage to the major forestsand soils of the eastern United States and western Europe.
One problem that was brought about by human action and is definitely affecting the hydrosphere globally is that of the greenhouse gases (so called because of their heat-trapping “greenhouse” properties) emitted to the atmosphere. Of the greenhouse gases released by anthropogenicactivities, carbon dioxide has received much attention. Measurements of carbon dioxide in air bubbles trapped in ice and the continuous measurement of carbon dioxide concentrations in air samples collected at Mauna Loa, Hawaii, since 1958 show that the atmospheric concentration of more than 400 ppmv is roughly 45 percent higher than its late 1700s value of 275 ppmv (see alsoKeeling curve). Much of this increase is due to carbon dioxide released to the atmosphere from the burning of coal, oil, gas, and wood and from the slash-and-burn activities that accompany deforestation practices (as, for example, those adopted in the Amazon River basin). The component of the hydrosphere most greatly affected by this emission of carbon dioxide is the ocean.
Before human activities had substantially affected the carbon dioxide cycle, there was a net flux of carbon dioxide from the oceans through the atmosphere to the land, where the gas was used in the net production of organic matter and the chemical weathering of minerals in continental rocks. Because of fossil-fuel burning and land-use practices, the net transfer from the ocean to the land has been reversed, and the ocean has now become an important sink of carbon dioxide. The net chemical reaction of adding carbon dioxide to the ocean (provided there is no reaction with carbonate solids) is
The generalized carbon cycle.Encyclopædia Britannica, Inc.
and a lowering of the pH of surface seawater. Such a pH effect has not been observed but conceivably could occur if carbon dioxide continues to be released to the atmosphere by human activities.
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Based on greenhouse climate models and other considerations, it is possible that atmospheric carbon dioxide concentrations may double from their late 1700s level of 275 ppmv by the years 2030–50. By 2015, atmospheric carbon dioxide concentrations had surpassed 400 ppmv for the first time in 800,000 years. Climate models, which also consider the long-term warming potential made by other greenhouse gases (e.g., methane and nitrous oxide) in addition to that of carbon dioxide, project a rise in global mean surface temperature of 0.3 to 4.8 °C (0.5 to 8.6 °F) by 2100. This projected temperature increase would be two to three times greater at the poles than at the Equator and greater in the Arctic than in the Antarctic. At present there is no systematic worldwide program to decrease greenhouse gas emissions, except for that affecting chlorofluorocarbon (Freon) releases. Thus, it is conceivable that atmospheric carbon dioxide concentrations in the late 21st and early 22nd centuries might reach levels greater than twice their 1700s value. Whatever the case, the effect of the potential rise in surface temperaturewould be to speed up the hydrologic cycle and probably the rate of chemical weathering of continental rocks. Increases in the global mean evaporation and precipitation rates are expected from a doubling of the carbon dioxide level and a few degrees rise in global mean temperature. The effect on the water balance would be regional in nature, with some places becoming wetter and others drier. In general, there would be a trend toward greater and longer periods of summer dryness induced by lower soil moisture content and higher evaporation rates in the mid-latitudes of the Northern Hemisphere. In the arid western regions of the United States, which depend on irrigation for growing plants, severe water shortages could occur. By contrast, precipitation and runoff might increase, except in summer, at latitudes beyond 60° N because of a greater poleward transport of moisture. In summer, in a zone centred around 60° N, greater dryness might occur as a result of an earlier end of snowmelt.
The Keeling Curve, named after American climate scientist Charles David Keeling, tracks changes in the concentration of carbon dioxide (CO2) in Earth's atmosphere at a research station on Mauna Loa in Hawaii. Although these concentrations experience small seasonal fluctuations, the overall trend shows that CO2 is increasing in the atmosphere.Encyclopædia Britannica, Inc.
Global warming could further affect the hydrologic cycle by the melting of ice and snow in the Greenland and Antarctic ice caps and in mountainglaciers, resulting in the transfer of water to the oceans. This process, together with thermal expansion of the oceans because of global warming, resulted in a slow rise in sea level during the 20th century, and it is expected to continue throughout the 21st century. If the West Antarctic Ice Sheet were to disintegrate, a much larger and more rapid rise in sea level of more than 3 metres (nearly 10 feet) could occur over the next several hundred years. The melting of all glacial ice would raise the sea level more than 66 metres (about 216 feet).
Significant reductions in the areal extent and thickness of sea ice in the Arctic occurring during the early 21st century have been attributed to global warming. Complete melting of the Arctic sea ice might occur, causing a northward shift in storm tracks and a reduction in Northern Hemispheric precipitation during the spring and fall. Furthermore, a worldwide reduction in sea ice might lead to increased evaporation from the ocean and increased low-altitude cloudiness, which would reflect solar radiation and cause cooling.
The potential changes in the hydrologic cycle induced by global warming resulting from anthropogenic emissions of greenhouse gases do not seem great. Yet, their consequences could be severe for ecosystems and human populations, especially since the latter are so sensitive to and dependent on such changes. A global rise in sea level of 1 metre (3.3 feet), for example, would almost completely inundate the coastal areas of Bangladesh. Agricultural lands could be displaced, just as patterns of arid, semiarid, and wet lands might become modified. It is essential that society plan for such potential changes so that, if they do occur, appropriate adjustments can be made to ameliorate them.
