Evolution of Earth's Oceans

Earth’s ocean water was derived by outgassing from hydrated minerals bound up during the formation of the Earth. Subsequent evolution of the water primarily involved ions from continental and oceanic bottom sediments dissolving in the fluid medium to yield the basic saltiness characteristic of Earth’s oceans.

Overview

Of all the planets in the solar system, Earth stands out as basically a watery world, distinguished from the other planets by large quantities of liquid water. In all, Earth has about 1.36 billion cubic kilometers of water, and 97.2 percent is stored in the oceans. The remaining 2.8 percent of Earth’s water not in the oceans is apportioned among ice (77 percent of the total remaining water) and continental and atmospheric waters. The ice itself, now principally in the Arctic-Greenland area (1.72 million square kilometers, up to 3,200 meters thick) and the Antarctic area (12 million square kilometers, up to 4,000 meters thick), has effects ranging from climate control to providing habitats for living organisms to being a reservoir for water that, when added to or removed from the oceans in the past, has caused sea level to rise or fall more than 100 meters.

Ocean water is salty because it contains dissolved minerals; with a salinity of 35,000 parts per million, there is enough dissolved salt to cover the entire surface of the Earth to a depth of about 50 meters. This salty solution is composed primarily of sodium and chlorine ions (together constituting about 86 percent of the ions by weight), along with ions of magnesium, calcium, potassium, and sulfate and carbonate groups. Seawater is slightly basic, with a pH of 8 for the hydrogen-ion concentration.

The problem of the oceans’ origin is twofold: (1) the primordial origin of the water itself and (2) the origin and rate of addition of the ions that make the oceans salty. The database for solving these problems includes the chemistry of water, the amounts and types of runoff delivered by rivers into the sea, and the composition of volcanic gases, geysers, and other vents opening to the surface, since the oceans and atmosphere are linked in origin.

Numerous sources for the Earth’s water have been proposed, and the problem has not been resolved completely. Possible sources include the primordial solar nebula, the solar wind acting over time, delivery or impact degassing by bodies colliding with the Earth, and outgassing from the planetary interior. Processes controlling water on Earth involve rates, amounts, and types of outgassing, modes of planetary formation, possible chemical reactions providing water, loss rates of gases to space, and, finally, internal feedback mechanisms such as changes in Earth’s albedo (reflectivity), temperature, alteration of materials, and other factors not clearly understood.

The solar wind as a primary source can be eliminated for several reasons. The basic constituents, protons, may help form water in the atmosphere by reactions with oxygen, but all evidence points to no free oxygen in the primordial atmosphere. The geologic record shows the presence of liquid water at least 4 billion years ago, but Earth was substantially devoid of free oxygen then.

Colliding bodies would be from two primary sources: meteorites and comets. The basic composition of cometary nuclei, consisting of water ice, various ions, metals, organic molecules, and dust grains, would supply enough water, provided that gigantic numbers of cometary objects struck the Earth during the first half-billion years of history. No conclusive evidence for such happenings is available at present, although a theory that Earth is still being bombarded incessantly by small comets containing large quantities of water is supported by the detection of diffuse ice balls entering the atmosphere in the 1990s. Meteoritic impact, particularly during the early stages after final planetary accretion, would definitely add water to the crust via two mechanisms. Carbonaceous chondrites, the oldest and most primitive meteorites, contain abundant volatiles, such as water, chemically bound in various minerals. Additional water, trapped in crustal and mantle rocks since Earth’s accretion and differentiation, would have been released during impacts, especially by large impacting objects. It has been calculated that such impact degassing could have released 10²² kilograms of volatiles, quite close to the currently estimated value of 4 10²¹ kilograms for the Earth as a whole. Remnants of such ancient astroblemes are lacking, however, because of subsequent erosion, filling in by molten lava, or shifting of the continental masses over 4 billion years. Further research has suggested that many icy asteroids formed beyond Jupiter and were flung in many different directions by the gravity of Jupiter and Saturn. Researchers have suggested that if not for the presence of the Jupiter and Saturn, Earth may have acquired more water from comets and asteroids.

The most widely accepted origin for the oceans and atmosphere combines the features of the primordial solar nebula and slow outgassing from within the solidifying Earth. Original water would have been combined with silicate minerals and metallic materials during the planetary accretion process, the hydration assisted by the heating of the Earth due to infalling bodies and the decay of radioactive elements. Such wet silicates appear to be able to hold large quantities of bound water for indefinitely long periods of time. The primordial Earth, believed to have accreted cold, trapped water molecules. If Earth had started too hot, all the minerals would have been dehydrated, and if too cold, no water would have been released; a delicate balance of temperature must have been achieved. Further, the volatiles forming the atmosphere must have outgassed first, in order to provide an insulating blanket under which water could form a liquid phase.

A secondary problem deals with how swiftly the fluids outgassed, either all at once, as many individual events, or in a continuous fashion. Most studies suggest the continuous mode of emission, with greatest reliance on data from currently active sources, such as volcanoes, undersea vents, and associated features. Fumaroles, at temperatures of 500—600° Celsius (800-900 kelvins), emit copious quantities of water, sulfur gases, and other molecules. These structures grade gradually into geysers and hot spots, areas where water is moved crustward from great depths. Magmatic melts rising in volcanoes release water and other gases directly to the surface. In Hawaii, for example, the Halemaumau Pit, the most active vent on the volcano Kilauea, emits 68 percent water vapor, 13 percent carbon dioxide, and 8 percent nitrogen, with the rest mostly sulfurous gases. Similar values are found for ocean-ridge-axis black and white smokers, where hydrothermal accretions result in spectacular deposits of minerals falling out of solution from the emerging hot mantle waters. Detailed studies show that water is trapped in the altered minerals within the basaltic crust of the oceanic plates, 5 percent of the rocks (by weight) in the upper 2 to 3 kilometers being water and hydroxide ions. Free water is known to be extremely buoyant, rising in the crust along shallow dipping faults. Bound water, subducted to great depths, would be expected to cook, moving upward as the rock density lessens and then acting as a further catalyst for melting the surrounding rocks.

In the Earth’s earliest stage, the primordial atmosphere was released, only to vanish from the Earth because of overheating. In the second stage, gases were released from molten rocks, with a surface temperature of 300° Celsius (600 kelvins), providing 70 percent water and large quantities of carbon dioxide and nitrogen. In stage three, the atmosphere and oceans gradually changed as a result of volcanoes and weathering action; more and more water was deposited as liquid as the temperature fell. Then the atmosphere added oxygen either by thermal dissociation of water molecules, photochemical breakdown of high-altitude water, or photosynthetic conversion of carbon dioxide to oxygen in plants.

The saltiness of the oceans can be accounted for by the extreme dielectric constant of water, essentially ensuring that ocean water does not remain chemically pure. Geologic evidence shows the general composition of seawater to be similar over time, the content stability attributable to the continuous seawater-sediment interface. John Verhoogen has shown that only 0.7 percent of the ocean has been added since the Paleozoic era, primarily from lava materials. The saltiness is a product of acidic gases from the volcanoes (hydrochloric, sulfuric, and carbonic acids) acting to leach ions out of the common silicate rocks. Paleontological studies indicate the change in ions must have been extremely slow, as demonstrated by the narrow tolerance of organisms then alive, such as corals, echinoderms, brachiopods, and radiolarians. Present river ion concentrations differ drastically from the ocean’s values, however, indicating a different atmospheric environment in the past. Robert M. Garrels and Fred T. Mackenzie have divided ocean history into three periods. In the earliest, until 3.5 billion years ago, water and volcanic acid emissions actively attacked the crust, leaching out ions and leaving residues of alumina and silicates. The next period, from 3.5 to 1.5 billion years ago, saw slow continuous chemical action attacking the sedimentary rocks, adding silica and ferrous ions. Period three, from 1.5 billion years ago onward, added ions until seawater composition reached apparent equilibrium with a mixture of calcite, potassium-feldspars, illite-montmorillonite clays, and chlorite.

Because the composition of ocean water has remained similar over much of geologic time, generally output must equal input of ions, so geochemical “sinks” must balance geochemical sources. Calcite (calcium carbonate) and silica (silicon dioxide) are removed by marine organisms to form skeletons and shells. Metals are dropped from seawater as newly formed mineral clays, oxides, sulfides, zeolites, and as alteration products at the hot-water basaltic ridges. Sulfur is removed as heavy-metal sulfides precipitating in anaerobic environments, while salts are moved in pore waters trapped in sediments. Residence times for many of the ions have been determined: for example, sodium cycles in 210 million years, magnesium in 22 million, calcium in 1 million, and silicon in 40,000. With such effective removal systems, it is truly a measure of the geochemical resistivity of the Earth’s oceans to change that allows the composition to remain so stable for 4 billion years.

Methods of Study

Numerous avenues of approach have been used to investigate the ocean and its ions, including geological, chemical, and physical means. Geology has supplied basic data on the types and makeup of rocks from the earliest solidified materials to present depositional formations. Use of the petrographic microscope, involving thin sections of rocks seen under polarized light, allows the identification of minerals, providing quantity measurements of water attached to the minerals themselves. Paleontological studies of fossil organisms and paleosoils indicate the range of ions in the sea at diverse geologic periods, both by the ions themselves left in the deposited soils and rocks and through studies of the tolerance ranges for similar, twentieth century organisms. Such studies—along with sedimentological investigations of rates and types of river depositions, dissolved ion concentrations, and runoff rates for falling rain—provide determinants for comparing ion concentrations with those in the past for continentally derived materials.

Chemical analysis reveals the various ions present in seawater and rocks via two principal methods. The mass spectrometer identifies types and quantities of ions present by use of a magnetic field to accelerate the charged ions along curved paths, the curvature of the paths based on the weight and charge of the ions. Collection at the end of the paths provides pure samples of the different ions present. For solid samples, electron beam probes analyze an area only one micron in diameter. The electrons, fired at the sample, cause characteristic X rays to be emitted. The energies of the X rays identify the elements or compounds present in the sample.

Solubility studies provide residence times. Similar laboratory projects, testing the ability of water to dissolve and hold ions in solution, argue for a primordial Earth atmosphere that was essentially neutral or mildly reducing in nature. Such reduction characteristics are based on studies of the composition of Earth, supplemented by the composition of Venus and Mars as revealed by various “lander missions.”

Missions in interplanetary space have also provided chemical compositions for meteoritic gases, cometary tails and nuclei, and the mixing ratios for noble gases, important for determining the origin of the solar system. Analysis of radioactive isotopes such as helium 3, an isotope of mantle origin, has allowed geophysicists to treat the Earth’s mantle as a major elemental source and sink for the various geochemical cycles.

Laboratory analysis reaches two other areas. Petrographic studies of returned lunar rocks reveal that the Moon is devoid of water, lacking even hydroxyl ions. This discovery helps eliminate the solar wind and meteoritic impact as major factors in forming Earth’s oceans. Furthermore, high-temperature, high-pressure metallurgical and chemical studies indicate that molten granite, at temperatures of 900° Celsius (1200 kelvins) and under 1,000 atmospheres of pressure, will hold 6 percent water by weight, while basalt holds 4 percent. Based on geochemical calculations of the amounts of magma in the planet and lavas extruded over the first billion years, all the ocean’s waters can be accounted for, particularly if parts of the fluid, as steam under pressure, are a result of oxidation of deep-seated hydrogen deposits trapped within or combined with mantle rocks.

Context

Water is a ubiquitous and by far the most important molecule on Earth. All living organisms require it as a basic component of cellular structure and for numerous functions inside the body. The origin of Earth’s water is highly significant, because the very presence of water may have set the scheme for all subsequent evolution, both geological and biological, on the planet. During the formation of the solar system, the accretion of various materials trapped water by hydration. Tied to the minerals, the water molecules were released through outgassing by volcanoes and other vents acting as pressure escape valves for the molten interior of the Earth. The water and other volatile gases that were released formed the atmosphere and subsequent oceans. A vital interchange was established between the ground and the atmosphere, one replenishing elements and compounds as they were lost through geochemical sinks in the normal course of history. Water, at first in the atmosphere, then as liquid seas, apparently helped to mediate the greenhouse effect, a mechanism which, if allowed to act unhindered, would have trapped infrared radiation from the sun and overheated the early Earth. Such actions would have given the Earth the characteristics of the planet Venus: enormously hot and totally inhospitable for life’s occurrence.

The outgassed water, settling as rain, also played the dominant role in shaping the landforms of Earth. As a mechanism for fluidization of rocks, it controls to a large extent the motions of magmas, helping them rise to the surface. As a weathering agent, water, in the forms of rain, snow, and ice, carves away the landscape, removing elements, as ions, to the sea. In that location, these elements became usable by early organisms for fulfilling their biological needs, such as home building or metabolism. Water acts as a transport mechanism, a mixing agent, and ultimately a removal tool for maintaining a delicate ionic concentration range within the ocean itself. Evaporating seawater, falling as rain, breaks up rocks and forms soils with nutrients available for land-based plant life, and it provides the freshwater so necessary to non-ocean-dwelling organisms. Without the initial interplay of water on Earth, our planet, instead of being the home of countless billions of creatures, would undoubtedly be a desolate ball, revolving forever around the sun as an improbable abode of life.

Bibliography

Brancazio, Peter J., ed. The Origin and Evolution of Atmospheres and Oceans. New York: Wiley, 1964. Print.

Chamberlain, Joseph W. Theory of Planetary Atmospheres: An Introduction to Their Physics and Chemistry. New York: Academic, 1978. Print.

Crockett, Christopher. "How Did Earth Get Its Water?" Science News. Society for Science and the Public, 6 May 2015, www.sciencenews.org/article/how-did-earth-get-its-water. Web. 31 Dec. 2015.

Consolmagno, Guy. Worlds Apart: A Textbook in Planetary Sciences. Englewood Cliffs: Prentice 1994. Print.

Frakes, L. A. Climates Throughout Geologic Time. New York: Elsevier, 1980. Print.

Garrison, Tom. Oceanography: An Invitation to Marine Science. Florence: Brooks, 2007. Print.

Greene, Kelly. "Earth's Water: Where Did It All Come From?" SciTech Institute, 4 Apr. 2022, scitechinstitute.org/earths-water-where-did-it-all-come-from/. Accessed 19 Jan. 2023.

Henderson-Sellers, A. The Origin and Evolution of Planetary Atmospheres. Bristol: Hilger, 1983. Print.

Holland, Heinrich D. The Chemistry of the Atmosphere and Oceans. New York: Wiley, 1978. Print.

Knauss, John. Introduction to Physical Oceanography. 2d ed. Long Grove: Waveland, 2005. Print.

McElhinny, M. W., ed. The Earth: Its Origin, Structure, and Evolution. New York: Academic, 1979. Print.

Pickard, George L. Descriptive Physical Oceanography: An Introduction. 5th ed. New York: Pergamon, 1990. Print.

Ponnamperuma, C., ed. Cosmochemistry and the Origins of Life. Dordrecht: Reidel, 1982. Print.

Seibold, E., and W. Berger. The Sea Floor. New York: Springer, 1982. Print.

Trujillo, Alan, and Harold Thurman. Essentials of Oceanography. 9th ed. Upper Saddle River: Prentice, 2007. Print.