Oceans’ Origin

Oceanic waters were derived from the outgassing of hydrated minerals bound up during Earth’s formation. Subsequent evolution of the waters primarily involved ions dissolving in the fluid medium by interactions with the continental and oceanic bottom sediments to give the basic salinity of Earth’s oceans.

Earth’s Oceans

Of all the planets in the solar system, Earth stands out as a watery world, distinguished from the others by great quantities of liquid water. Encompassing more than 1.35 billion cubic kilometers, Earth’s seas contain enough salt to cover all of Europe to a depth of five kilometers. The salt content of the oceans is composed primarily of sodium chloride, some 86 percent of the ions by weight, in association with other ions of magnesium, calcium, potassium, sulfate, and carbonate groups, providing a salinity of 35,000 parts per million, with pH 8 for the hydrogen-ion concentration (slightly alkaline in nature). Of all the water on Earth, 97 percent is salty, the remainder being proportioned among ice (77 percent of total freshwater) and continental and atmospheric waters. The ice itself, principally in the Arctic-Greenland area (1.72 million square kilometers, 3,200 meters thick) and the Antarctic area (fourteen million square kilometers, 4,000 meters thick), provides effects ranging from climatic control to habitats for living organisms and sources of new seawater that, in the past, have caused sea levels to rise more than 100 meters.

Consideration of the oceans’ origin is twofold—the primordial origin of the water itself and the origin and rate of addition of the salt ions present in past and contemporary waters. The data sources for consideration include 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. Additionally, most researchers find that oceans and the atmosphere are linked in origin, providing even more data for analysis.

Numerous sources for the world’s water have been proposed, although a definitive resolution has not been achieved. Possible sources include the primordial solar nebula, the solar wind acting over time, bodies colliding with Earth, impact degassing, and outgassing from the planetary interior. A complete consideration of the origin of Earth’s oceans necessarily involves the investigation of factors controlling water on Earth, particularly the 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 (reflective power), temperature, alteration of mass, and other factors not clearly understood.

, Colliding Bodies, and Impact Degassing

The solar wind, as a primary source of terrestrial water, can be eliminated for several reasons. Its basic constituents, charged protons, can formulate water in the atmosphere by reactions with oxygen, but all evidence indicates that there was no free oxygen in the primordial atmosphere. The geologic record shows the presence of liquid water at least four billion years ago, which was substantially devoid of free oxygen. Astrophysical evidence suggests that the solar flux of energy, associated with the solar wind, was such that, early in Earth’s history, water on Earth should have been frozen, not liquid, if that was the primary source. The presence of liquid water through at least 80 percent of Earth’s history, however, has been established.

Colliding bodies would include two primary sources—meteorites and comets. The occurrence of cometary bodies striking Earth has yet to be documented. However, the basic chemical makeup of comets, consisting of various ions, metals, organic molecules, and dust grains in a mass of water ice, could theoretically have supplied enough water to account for the amount of water present on the planet, providing that large numbers of cometary objects struck the early Earth during the first half-billion years of its history. No evidence for such happenings is available, although a theory of Earth still being bombarded incessantly by small comets composed primarily of water ice has been fiercely debated.

Meteoritic impact, particularly during the earlier stages after final planetary accretion, would also have added water to the crust via two mechanisms. Through the study of carbonaceous chondrites (the oldest and most primitive meteorites), abundant volatiles, such as water, are found to be bound chemically to minerals such as serpentine. Additional waters, trapped in crustal and mantle rocks, would have been released during impact, particularly from large meteoritic rocks. 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 Earth. Remnants of such ancient astroblemes are lacking, however, because of subsequent erosion, filling in by molten magma, or shifting of the continental masses over four billion years.

Outgassing

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, under gravitational collapse, with silicates and metallic materials during the planetary accretion process, with the hydration of minerals assisted by the heating of Earth due to infalling bodies and radioactive elemental decay. Such “wet silicates” hold large quantities of bound water for indefinite periods. The primordial Earth is believed to have formed by cold accretion and would have trapped the water molecules. If it had been too hot during the accretion, all the minerals would have been dehydrated, and if too cold, no water would have been released. A delicate balance of temperature must, therefore, have been achieved. Further, the volatiles forming the atmosphere must have outgassed first because water must be insulated from solar radiation to form a liquid phase.

A secondary problem examines how swiftly the fluids would have outgassed, whether all at once, as individual events, or in a continuous fashion. Most data suggest the continuous mode of emission, with the greatest reliance on data from still-active sources—mainly volcanoes, undersea vents, and associated structures. Fumaroles, at temperatures of 500 to 600 degrees Celsius, emit copious quantities of water, sulfur gases, and other molecules. These bodies grade gradually into hot spots and geysers, areas where water is moved upward through the crust 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 volcano Kilauea’s most active vent, emits—in terms of material—68 percent water, 13 percent carbon dioxide, and 8 percent nitrogen, with the rest mostly sulfurous gases. Similar types of values are found in ridge-axis black and white smokers, where hydrothermal accretions result in spectacular deposits of minerals falling out of solution from the emerging hot waters. Detailed studies show water trapped in the structures of altered minerals within the basaltic crustal rock of the oceanic plates, with 5 percent of the rocks, by weight, in the upper two to three 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, then acting as a further catalyst for melting the surrounding rocks.

In Earth’s earliest stages, the primordial atmosphere probably escaped from Earth’s gravitational pull because of overheating. In the second phase, gases released from molten rocks, with a surface temperature of 300 degrees Celsius, provided 70 percent water and large quantities of carbon dioxide and nitrogen. In stage three, the atmosphere and oceans gradually changed, with gases and liquid water ejected from volcanoes, resulting in more and more water deposited as liquid as the temperature fell. Oxygen was added to the atmosphere through thermal dissociation of water molecules, photochemical breakdown of high-altitude water, or photosynthetic alteration of carbon dioxide to oxygen in plants.

Ocean Salt

The saltiness of the oceans can be accounted for by the inordinately large dielectric constant of water, a property that essentially ensures that it does not remain chemically pure. Geologic evidence shows the general composition to be similar over time. The stability of water content is attributable to the continuous seawater-sediment interface. Geophysicist John Verhoogen determined that only 0.7 percent of the present ocean has been added since the Paleozoic era, primarily from lava materials. The salty quality is a product of acidic gases from volcanoes (forming hydrochloric, sulfuric, and carbonic acids) that act to leach out the common silicate rocks. Paleontological evidence indicates that 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. Ion concentrations in present-day river waters differ drastically from the ocean’s values, indicating a different atmospheric environment in the past. Geochemists Robert M. Garrels and Fred T. Mackenzie have divided the oceans into three historical periods. In the earliest, water and volcanic acidic emissions actively attacked the crustal rock, leaching out ions and leaving residues of alumina and silicates. During the next period, from 3.5 to 1.5 billion years ago, slow, continuous chemical action continued to attack sedimentary rocks, adding silica and ferrous ions. During the third period, from 1.5 billion years ago to the present, ions have accumulated to the point of modern concentrations, such that the composition is in apparent equilibrium with a mixture of calcite, potassium-feldspars, illite-montmorillonite clays, and chlorite.

Because it is known that, in an equilibrium state, the output of ions is equal to the input of ions, a new problem, that of geochemical “sinks,” has been identified. Calcium carbonate (limestone) is removed from solution by living organisms to form skeletons, as is silica for opaline skeletons. Metals are dropped from seawater as newly formed mineral clays, oxides, sulfides, and zeolites and as alteration products at the hot-water basaltic ridges. Sulfur is removed as heavy-metal sulfides precipitate 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 twenty-two million years, calcium in one million years, and silicon in 40,000 years. With such effective removal systems, it is truly a measure of the geochemical resistance of Earth’s ocean waters to change that has allowed their composition to achieve stability over four billion years.

Study of the Ocean

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. Using a petrographic microscope to view thin sections of rock under polarized light enables the identification of minerals and provides quantitative measurements of the water attached to the minerals themselves. Paleontological studies of fossil organisms and paleosols indicate the range of ions in the sea at diverse geologic periods, by the ions themselves left in the deposited soils and rocks, and through studies of the ion tolerance ranges for similar, twentieth-century organisms. Such studies, along with sedimentology investigations of the rates and types of river depositions, dissolved ion concentrations, and runoff rates for falling rain, provide determinants for comparing present-day ion concentrations with those of the past for continentally derived materials.

Chemical analysis reveals the various ions present in seawater and rocks via two principal methods. The use of the mass spectrometer identifies the types and quantities of ions present by using a magnetic field to accelerate the charged ions along a curved path with a radius that is strictly determined by the weight and charge of the ions. Collection at the end of the path provides a pure sample of the different ions present. For solid samples, electron beam probe studies provide analysis from an area that is only one micron in diameter. The electrons fired at the sample cause characteristic X-rays to arise from the point. The energy of each type of X-ray is characteristic of a specific element or compound. By using various optics to focus the X-rays, identification of even minute variations in concentration is possible.

Solubility studies provide residence times for geochemical analysis for cyclical research. Similar laboratory projects, testing the ability of water to dissolve and hold ions in solution, argue for a primordial atmosphere that was essentially neutral or mildly reducing in nature. Such reduction characteristics are based on the study of planetary composition for Earth and for other solar planets as supplemented by the various “lander missions” to Venus and Mars, and by modern astrophysical satellite observations. Chemical analysis from such missions in interplanetary space has also determined compositions for meteoritic gases, cometary tails and nuclei, and the mixing ratios for noble gases, all important for determining the origin of the solar system. The latter study, involving analysis of radioactive isotopes such as helium-3 (³He), an isotope of helium formed in the mantle, has prompted geophysicists to consider the mantle as a major source and sink for elements in various geochemical cycles.

Laboratory analysis reaches two other areas. Petrographic studies of returned lunar rocks suggest that the moon is essentially devoid of water, lacking even hydroxyl ions, at least on the surface. This discovery initially helps eliminate the solar wind and meteoritic impact as major factors in the initial formation of oceans. However, the identification of bound subsurface water on the moon is cause for reconsideration of that conclusion. Furthermore, high-temperature/high-pressure metallurgical and chemical studies indicate molten granite, at temperatures of 900 degrees Celsius 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 oceans’ waters can be accounted for, particularly if parts of the fluid, such as steam under pressure, are a result of oxidation of deep-seated hydrogen deposits trapped within or combined with mantle rocks. This supposition is, thus, considered an excellent likelihood based on evidence gathered on radioactive decay in Earth’s interior.

In April 2024, Northwestern University scientists discovered a large ocean 700 kilometers below the Earth's crust. The ocean was surrounded by ringwoodite, a blue rock, and posed new scientific questions about the planet's oceans and the hydrological cycle. Also advancing the body of knowledge on Earth's oceans, scientists in the Himalayas discovered mineral formations of water droplets from an ancient ocean in 2023. The dropletsdate back to 600 million years ago and offer information about the oxygenation on Earth over time.

Principal Terms

carbonaceous chondrites: a class of meteoritic bodies found to contain large amounts of carbon in conjunction with other elements; used to date the solar system and provide chemical composition assessments of the original solar nebula

geochemical sinks: the processes by which elements and compounds are removed from the crust and oceans to be recycled in active chemical cycles

outgassing: the process by which volatile materials trapped within rock formations are released into the atmosphere and the environment

primordial solar nebula: the original collection of dust and gases that constituted the basic cloud from which the solar system formed

smokers: undersea vents on the active rift areas that emit large amounts of superheated water and dissolved minerals from within the crust

solar wind: the stream of highly charged particles emitted into space from the surface of the sun

volatiles: chemical elements and compounds that become gaseous at fairly low temperatures

water of hydration: water that is bound to the crystal structure of minerals without being part of their actual molecular structure

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