Was the timing of the rise in Earth's atmospheric oxygen triggered by geological as opposed to biological processes

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EARTH SCIENCE

Was the timing of the rise in Earth's atmospheric oxygen triggered by geological as opposed to biological processes?

Viewpoint: Yes, the timing of the rise in Earth's atmospheric oxygen was triggered not by biological processes but by geological processes such as volcanic eruption, which transported elements (among them oxygen) from Earth's interior to its atmosphere.

Viewpoint: No, the theories based on geological principles accounting for the timing of the rise in Earth's atmospheric oxygen have insufficient data to supplant biological processes as the cause.

As most people know, oxygen is essential to most forms of life, with the exclusion of anaerobic or non-oxygen-dependent bacteria. But when, and from where, did this life-giving oxygen arise during the course of Earth's history? The first question, regarding the point at which oxygen appeared on the planet, is answered with relative ease by recourse to accepted scientific findings. According to the best knowledge available at the beginning of the twenty-first century, oxygen first appeared between 2.2 and 2.4 billion years ago. This would place the appearance of oxygen somewhere between 2.1 and 2.3 billion years after Earth's formation from a spinning cloud of gases that included the Sun and all the future planets and satellites. Yet though the "when" question is less fraught with controversy than the "how" question, there are still complications to this answer.

First of all, there is the fact that any knowledge of events prior to about 550 million years ago is widely open to scientific questioning. The term scientific is included here in recognition of the situation, which is unique to America among all industrialized nations, whereby a substantial body of the population rejects most scientific information regarding Earth's origins in favor of explanations based in the biblical book of Genesis. Despite what creationists might assert, the fact that there is dispute between scientists regarding the exact order of events in no way calls into question the broad scientific model for the formation of Earth, its atmosphere, its seas, and its life forms through extremely lengthy processes. In any case, most knowledge concerning these far distant events comes from readings of radiometric dating systems, which involve ratios between stable and radioactive samples for isotopes or elements such as potassium, argon, and uranium, which are known to have extremely long half-lives.

In addition, the answer to the "when" question is problematic due to the fact that scientists know only that Earth experienced a dramatic increase in oxygen levels 2.2 to 2.4 billion years ago. Logically speaking, this leaves open the possibility that oxygen may have existed in smaller quantities prior to that time—a position maintained by Hiroshi Ohmoto at Pennsylvania State University, whose work is noted in one of the essays that follow. Furthermore, there is evidence to suggest that oxygen levels may have fluctuated, and that the development of oxygen may not have been a linear, cumulative process; by contrast, like the evolution of life forms themselves, it may have been a process that involved many false starts and apparent setbacks. Certainly oxygen may have existed for some time without being in the form of usable atmospheric oxygen, since it is an extremely reactive chemical element, meaning that it readily bonds with most other elements and subsequently must undergo another chemical reaction to be released as usable atmospheric oxygen.

Given the fact that the "when" question is, as we stated earlier, the simpler of the two, this only serves to underscore the difficulty in answering the "how" question. Broadly speaking, answers to the question of how oxygen arose on Earth fall into two categories. On one side is the biological or biogenic position, based on the idea that the appearance of oxygen on the planet resulted from the growth of primitive life-forms. The bodies of the latter served as chemical reactors, releasing to the atmosphere oxygen previously trapped in various chemical compounds. On the other side is the geological position, which maintains that the growth of oxygen in the atmosphere resulted from processes such as volcanic eruption, which transported elements (among them oxygen) from Earth's interior to its atmosphere.

As we shall see from the two competing essays that follow, there is much to recommend each position, yet each faces challenges inherent to the complexity of the questions involved, and the reasoning and selection of data required to answer them. For example, there is the fact that changes in the atmosphere seem to have corresponded with the appearance of relatively complex multicellular life-forms on Earth. At first this seems to corroborate the biological position, but the reality is more complicated.

The first cells to form were known as prokaryotic cells, or cells without a nucleus, which were little more than sacs of self-replicating DNA—much like bacteria today. These early forms of bacteria, which dominated Earth from about 3.7 billion to 2.3 billion years ago, were apparently anaerobic. By about 2.5 billion years ago, bacteria had begun to undergo a form of photosynthesis, as plants do today, and this would necessarily place great quantities of oxygen into the atmosphere over long stretches of time. These events produced two interesting consequences: first of all, the formation of "new" cells—spontaneously formed cells that did not come from already living matter—ceased altogether, because these were killed off by reactions with oxygen. Second, thereafter aerobic respiration would become the dominant means for releasing energy among living organisms.

But does this mean that biological rather than geological forms were responsible for the great increase of oxygen in the atmosphere? Not necessarily. After all, attempts to establish a biological connection may be a case of finding causation where none exists. (This is rather like a person issuing an order to a house cat that happens to subsequently display the demanded behavior—not, however, because of the command, but rather because it chose to do so of its own whim.) Furthermore, there is abundant evidence linking volcanic and other forms of tectonic activity—i.e., activity resulting from movement of plates in Earth's crust and upper mantle—with the spreading of carbon and other life-giving elements from Earth's interior to its surface. It is no accident that of all the planets in the Solar System, Earth is the only one with active volcanoes, and that the next-best candidate for sustaining life, Mars, happens to be the only other planet to have experienced volcanic activity in the past billion years.

—JUDSON KNIGHT

Viewpoint: Yes, the timing of the rise in Earth's atmospheric oxygen was triggered not by biological processes but by geological processes such as volcanic eruption, which transported elements (among them oxygen) from Earth's interior to its atmosphere.

The geophysical and fossil record presents abundant, clear, and convincing evidence that approximately 2.2 to 2.4 billion years ago, there was a dramatic increase in the oxygen content of Earth's atmosphere. The atmospheric changes correspond to the appearance of complex multicellular life forms in the fossil record. Scientists have argued for decades whether the increase in atmospheric oxygen resulted primarily from geophysical processes that then facilitated the evolution of complex multicellular life, or whether evolutionary processes produced organisms capable of biogenically altering Earth's atmosphere.

The scientific argument involves classic arguments of correspondence (the simultaneous occurrence of phenomena) versus causation (a dependence or linking of events where the occurrence of one event depends upon the prior occurrence of another). Although the extent (visible and existing) fossil record establishes correspondence between the rise of complex life-forms and higher oxygen content in the atmosphere, it does not conclusively establish a causative correlation between the two events.

In addition to providing evidence of the rise in atmospheric oxygen, the geological and fossil records also provide evidence of oxygen in the more primitive atmosphere prior to the precipitous rise in oxygen content levels. Sedimentary rocks believed to be approximately 2.2 billion years old contain evidence of changes in the oxygen and sulfur chemistry that are consistent with the rise in atmospheric oxygen. The global distribution of this evidence is consistent with a global atmospheric change. However, these chemical indicators of primitive atmospheric oxygen also extend farther back into the geological record to a time well before the established rise of oxygen content levels.

Because evidence exists that reactions creating free atmospheric oxygen operated before the global accumulation of atmospheric oxygen, it becomes essential to address the rate of oxygen and associated ozone build-up when assessing the relative contributions of biogenic and geophysical processes. Moreover, it can be fairly argued that biogenic contributions to atmospheric oxygen depended upon the development of a sufficient ozone layer to provide adequate protection from ultraviolet radiation and thus allow the development of more advanced organisms.

Admittedly, there is strong evidence of biogenic alteration of the oxygen content of Earth's atmosphere. Recent issues of Science and Nature contained articles presenting evidence for two different mechanisms of biogenic alteration. Arguing in Science, David Catling and colleagues presented evidence that a loss of hydrogen and the consequential enrichment of Earth's atmospheric oxygen content was accomplished through the microbial splitting of hydrogen and oxygen in water molecules, with the subsequent loss of hydrogen in biogenically produced methane gas. Methane is made of carbon and hydrogen (CH4). So when the H2O is split, the hydrogen combines with carbon to form methane. In Nature, Terri Hoehler and colleagues presented a different mechanism of atmospheric oxidation through the action of the direct loss of hydrogen to space (hydrogen molecules have a mass too low to be held in Earth's atmosphere by Earth's gravitational field) and the physiochemistry of "microbial mats" that utilize hydrogen to produce methane. Importantly, however, both of these biogenic mechanisms, in part, rely on geophysical and inorganic chemical reactions.

Although most arguments assert that biogenic processes—especially photosynthesis—played a far more significant role in producing oxygen in Earth's primitive atmosphere than did the geophysical process associated with volcanic action and hydrothermal venting, geophysical processes likely contributed significant amounts of atmospheric oxygen.

Photodissociation

One such geophysical or chemical process is that of photodissociation. Photodissociation is a process driven by the influx of ultraviolet radiation—especially in Earth's upper atmosphere—that results in the liberation of oxygen from water molecules. Just as oxygen-oxygen bonds in ozone molecules are shattered, the oxygen-hydrogen bonds in water molecules are broken by exposure to intense ultraviolet radiation. A portion of the hydrogen ions produced from the breakage then combined to form hydrogen gas that, because of its low mass, can escape Earth's atmosphere into space. Hydroxyl ions (OH^-)—produced by the breaking of an oxygen-hydrogen bond and the accompanying loss of a proton from the water molecule—react as part of the photodissociation reaction to produce water and atomic oxygen that, because of the net loss of hydrogen, can then react with other atomic oxygen atoms to produce molecular oxygen gas (O2). The mass of oxygen molecules allows sufficient gravitational attraction so that the molecules do not escape into space and accumulate in the atmosphere.

Given that water vapor was present in Earth's primitive atmosphere, this process and the net gain of oxygen began long before the evolution of microbes capable of photosynthesis. Accordingly, the mechanism for the slow accumulation of oxygen was in place before the precipitous rise in oxygen levels. That such photodissociation mechanisms have been observed to occur in the upper atmosphere of Venus—a presumably abiotic environment—is often offered as evidence of the ubiquity of such reactions in a primitive atmosphere.

Why a Delay?

If geophysical mechanisms that created free atmospheric oxygen existed, it is fair then to ask why there was a delay in the accumulation of atmospheric oxygen until approximately 2.2 billion years ago. One answer may lie in the fact that oxygen is a highly reactive gas and vigorously enters into a number of reactive processes. Throughout Earth's early history, the existence of highly active oxygen sinks (reactions that utilize oxygen and therefore remove it from the atmosphere) prevented any significant accumulation of oxygen. Moreover, the ubiquitous existence of organisms interpreted to be anaerobic in the fossil record provide abundant evidence that some mechanism or mechanisms operated to remove free oxygen from the primitive atmosphere.

Analysis of carbon isotopes in geologic formations and microfossils provides evidence that cyanobacteria capable of photosynthesis reactions evolved long before the abrupt rise in atmospheric oxygen. More importantly to the assessment of the role of non-biogenic process in oxygen production, the same evidence argues that atmospheric oxygen production may not have greatly fluctuated over time. Accordingly, an alternative model to explain a rise in atmospheric oxygen (i.e., an alternative to cyanobacteria producing greater quantities of oxygen which then accumulated in the atmosphere) rests on rate variable geophysical processes that altered aspects of oxygen utilization, which, in turn, allowed for accumulation of atmospheric oxygen.

For example, if geophysical processes that utilized, and thus removed, atmospheric oxygen occurred at higher rates earlier in Earth's history, the slowing of these processes more than two billion years ago would then allow and account for the accumulation of atmospheric oxygen.

Other reactions also account for the lack of free oxygen in Earth's primitive atmosphere. For example, methane—a primitive atmospheric component—reacts with available atmospheric oxygen to form water and carbon dioxide. Ammonia gases react with oxygen to form water and nitrogen gas. Until the levels of methane and ammonia gases lessened to negligible rates, these reactions would have prevented any significant accumulation of atmospheric oxygen.

Atmospheric oxygen is also consumed during oxidative reduction of minerals and in the oxidative reduction of gases associated with volcanic activity (e.g., carbon monoxide, hydrogen, sulfur dioxide, etc.). Oxygen is also ultimately utilized in reducing the gases produced in hydrothermal vents (e.g., volcanic vents associated with the Mid-Atlantic Ridge). The vent gases include reduced species of iron and sulfur (e.g., FeII and SII-). Currently, the majority of free or net oxygen usage occurs in the weathering of rock via reduction of carbon, sulfur, and iron.

At current levels of volcanic activity, estimates of oxygen utilization indicate that oxygen not recycled in the respiration/photosynthesis cycle is consumed by these geophysical reactions and so atmospheric oxygen levels remain stable. Earlier in Earth's history, however, a vastly greater percentage of oxygen utilization existed, exceeding that used in current weathering reactions. Combined with evidence of increased levels of volcanic activity earlier in Earth's history, such an oxygen sink could have been sufficient to prevent oxygen accumulation.

Some scientists, including L. R. Kump and others, have recently suggested that a quantitative change in volcanic activity (including hydrothermal venting) alone would not account for increased oxygen consumption (i.e., a more vigorous oxygen sink) because the concurrent production of other gases, including carbon dioxide, would also have stimulated increased photosynthesis. Instead, they argued that possible qualitative changes in the reduction state of materials increased the required uptake of oxygen sufficiently to prevent atmospheric oxygen accumulation.

Oxidation-Reduction Reactions

The postulated qualitative shift is based upon evidence that indicates that the earth's mantle was far more reduced in Earth's early history. Accordingly, more oxygen would be required to complete oxidation-reduction reactions. The increased reduction in upper mantle materials would have also meant that volcanic gases arising from the upper mantle would have been more reduced and therefore utilized more oxygen in oxidative reaction processes. Kump and his colleagues estimate that "the potential O2sink from surface volcanism could therefore have been higher than today by a factor of 40 or more, allowing it to easily overwhelm the photosynthetic O2 source."

One possible explanation for differential states of reduction involves changing states of mantle materials that contributed the reduced gases. Moreover, there is geological evidence that mantle materials that contribute to volcanic and hydrothermal gases have not remained constant in terms of their state of reduction. The geologic record provides direct evidence that prior to the rise in atmospheric oxygen, mantle materials were more reduced and therefore would have utilized more oxygen in oxidation-reduction reactions. This oxygen sink then acted to prevent the accumulation of atmospheric oxygen. In addition, the geologic record provides evidence that oxidized material was transported (subducted) from the crustal surface to the lower lithosphere (a region of the crust and upper mantle). This movement of oxidized materials away from the crust-atmospheric interface only increased the exposure of more reduced materials to whatever atmospheric oxygen existed and thus increased the rate of atmospheric oxygen utilization.

If the exposure of oxidized materials changed—as the geologic record indicates happened near the rise in atmospheric oxygen—then once-buried, more oxidized materials would have became dominant in volcanic gases. The increased oxidation state of the volcanic and hydrothermal gases then reduced the utilization of atmospheric oxygen and allowed for the subsequent accumulation of atmospheric oxygen.

The existence of geophysical processes to produce atmospheric oxygen and evidence of variability in the vigor (reflected in oxygen consumption) and abundance of these reactions provide evidence of a substantially non-biogenic transition of Earth's primordial atmosphere that combined hydrogen, ammonia, methane, and water vapor to one dominated by nitrogen and oxygen (approximately 21% oxygen).

Although the current evolutionary record reveals that organisms capable of photosynthesis existed at least a half billion years before the rise in Earth's atmospheric oxygen content, the contributions of cyanobacteria and botanical photo-synthesis to free net atmospheric oxygen do not stand up to quantitative scrutiny. A review of the biogenic oxygen cycle reveals that more than 98% of the oxygen produced biogenically is reused in photosynthesis/respiration process. This strongly argues against a purely biogenic source of rapid oxygen accumulation approximately 2.2 billion years ago.

—K. LEE LERNER

Viewpoint: No, the theories based on geological principles accounting for the timing of the rise in Earth's atmospheric oxygen have insufficient data to supplant biological processes as the cause.

Most of us love a good mystery, especially scientists. The rise of oxygen in Earth's atmosphere approximately 2,200 to 2,400 million years ago engenders a mystery worthy of Sherlock Holmes. Cyanobacteria are simple organisms that produce oxygen when they harnesses the power of the Sun, which is called photosynthesis. They have been around for at least 2,700 million years; some scientists estimate that cyanobacteria have thrived for nearly 3,500 million years. Nevertheless, even at the younger age estimate, it took at least 300 to 500 million years for Earth's atmospheric oxygen levels to rise significantly and enable more complex life to evolve. The mystery lies in why there is such a big gap between the emergence of cyanobacteria and the rise of atmospheric oxygen.

Although biological processes have long been considered the primary source for increased oxygen in the atmosphere, some scientists propose that geological processes, such as volcanic gases, initiated the timing of the precipitous rise in oxygen levels. However, the evidence to support a geological theory is minimal at best, and biological processes remain the best possible explanation for controlling the evolution of the atmosphere's oxygen content.

One of the most popular geological theories behind the rise in oxygen was proposed by James F. Kasting and colleagues at the Department of Geosciences and Astrobiology Research Center at the Pennsylvania State University. According to these scientists, it was escaping volcanic gases that most influenced the sharp rise in atmospheric oxygen. This theory is based on composition aspects of the earth's mantle (the region below the crust and above the core), where some volcanic gases originate. Before oxygen, the mantle contained an abundance of "reducing" components like iron silicates. These components, when brought to, or near, the surface by volcanic processes react with oxygen and extract most of the oxygen produced by photosynthesis. However, according to this theory, about 2.7 billion years ago the mantle underwent a change in which oxidized material that had "sunk" to the base of the mantle was pushed to the top, resulting in the volcanic gases emitted becoming less reducing. The bottom line is that as volcanic gases became more oxidized over time, their ability to "mop up" atmospheric oxygen decreased. As a result, the net photosynthetic production of oxygen was greater than the volcanic gases' ability to react with the oxygen. This resulted in an oxygen-rich atmosphere. However, studies of ancient basalts and komatiites (rocks made of Fe-and Mg-rich silicate) show that the mantle's oxidation state did not change appreciably with time, indicating that the volcanic gases theory is not correct.

New Theories Bolster Biological Causes

NASA scientists David Catling and colleagues recently proposed that because the early atmosphere had low oxygen content, methane (which is normally broken down in high oxygen environments) accumulated in the atmosphere. Made up of one carbon atom and four hydrogen atoms, the methane eventually ascended to the upper atmosphere, where intense ultraviolet rays broke it down. The extremely light hydrogen atoms drifted from Earth's gravitational field and off into space. The basic result was that the hydrogen atoms and oxygen atoms (or the organic matter made from hydrogen) were kept apart, thus creating more oxygen. Because there were less hydrogen atoms to combine with oxygen and make it disappear, Earth's oceans and rocks became saturated with oxygen, which then could accumulate in larger amounts in the atmosphere.

Recent research by another NASA scientist focuses on the methane production theory and attempts to answer the primary question inherent in the theory: Where did the methane come from? Terri Hoehler and colleagues measured the gases released from microbial mats in Baja, Mexico. These mats, which are aggregates of microorganisms composed mainly of bacteria and algae, are closely related to the ones that covered much of Earth's early biosphere and were found living in similar conditions. The group discovered that the mats released large quantities of hydrogen during the night. "If the Earth's early microbial mats acted similarly to the modern ones we studied," said Hoehler in a NASA news release, "they may have pumped a thousand times more hydrogen into the atmosphere than did volcanoes and hydrothermal vents, the other main sources."

Some of this hydrogen might have escaped directly to space, and the remainder could have provided "food" for microbes producing methane. The elevated levels of hydrogen within the mats favors the biological production and release of methane and supports the premise of Catling's work. "The bizarre implication," said Catling, "is that we're here as a result of these gases that came from microbial scum, if you like, back on early earth."

Although microbial mats can grow in various environments, including shallow seas, lakes, and rivers, the discovery of microbial mats intermixed with soil indicates that life possibly emerged from the sea to land as far back as 2.6 billion years ago, which contradicts what many scientists think about the timing when atmospheric conditions were favorable to life on land. Since the 2.6 billion year date is 200 to 400 million years earlier than previously estimated for the rise in atmospheric oxygen, the finding suggests that the estimates were possibly wrong and that life was already flourishing on land in an oxygen-rich atmosphere. It also indirectly supports the theory that these microbial mats were prolific enough to produce the gases needed to cause an increase in atmospheric oxygen.

New Information on the Age of Oxygen-Rich Atmosphere

One way to resolve the mystery of why it took oxygen so long to accumulate after the appearance of cyanobacteria is to say that the paradox does not actually exist. A Pennsylvania State University geochemist, Hiroshi Ohmoto, says that his research shows that an oxygen-rich atmosphere did not emerge as late as evidence suggests. One of the pieces of evidence about the timing of the rise in atmospheric oxygen is based on iron levels found in paleosols (ancient soils that are now rock). The evidence supporting the theory about when the rise in atmospheric oxygen occurred centers around insoluble iron hydroxides, which results when oxygen converts iron silicates. Iron hydroxides have only been found in paleosols less than 2.3 billion years old, indicating that the atmosphere did not have high oxygen levels before this time. However, according to Ohmoto and colleagues, just because the iron is not present in the paleosols doesn't mean that it was never there. There are a number of explanations for its disappearance. It could have been removed by hot water from volcanoes or by organic acids produced by cyanobacteria. As a result, the microorganisms might have confused the biological record by both creating oxygen that placed iron in the soils while producing the very acids that worked to remove it. As a result, it makes the supposed timing of the jump to high oxygen levels less certain.

Most of the early oxygen produced by cyanobacteria is still "imprisoned" in massive banded iron formations (sedimentary deposits of iron oxides), which were created on ocean floors in a process involving early reducing iron ions (Fe²⁺) that took up the oxygen produced by shallow water cyanobacteria during oxidizing reactions. Abundant oceanic Fe²⁺ suggests a lack of atmospheric oxygen, and banded-iron formations of ferric oxide would have formed an oxygen sink for any oxygen in the environment. So far, the vast majority of these banded iron formation have been found in rocks more than 2.3 billion years old, again indicating that oxygen levels were extremely low. In answer to this evidence, Ohmoto points out that similar banded iron formations have been found dating back to only 1.8 billion years, indicating that these structures can also form in the presence of high oxygen levels.

The idea that oxygen may have been around a lot longer than originally thought is further supported by research performed at Yale University by geochemist Antonio Lasaga. He has modeled the cycles of oxygen, carbon, sulfur, and iron from the early Earth and determined that within 30 million years after the appearance of cyanobacteria, the atmosphere would have contained very high oxygen content and that the atmosphere would have maintained this basic content.

Another group of scientists have also challenged the long-held theory of when Earth's atmosphere became enriched with oxygen. According to iron-rich nodules found in the deep strata of South Africa's Witwatersrand, Earth's atmosphere may have been rich in oxygen nearly 3 billion years ago. The researchers say the iron-rich nodules contain ferric iron produced by exposure to an oxygen-rich atmosphere. The nodules were dated at 2.7 to 2.8 billion years old. This theory is bolstered by evidence found in Western Australia's Pilbara region that found the presence of sulfates in rocks up to 3.5 billion years old. These sulfates also could not have formed without an oxygen-rich atmosphere.

If proven, these theories about a much earlier origin for an oxygen-rich atmosphere would make the debate over the geological versus the biological processes involved in the rise of an oxygen-rich atmosphere moot. If the higher oxygen levels existed hundreds of millions of years earlier, then the precipitous rise in atmospheric oxygen content did not even occur.

The Final Analysis

Scientists have long been tantalized by the great mystery of the rise in Earth's atmospheric oxygen levels between 2,200 and 2,400 million years ago. Many theories have been suggested, but none have become widely accepted. In fact, the geochemical and biological data can be interpreted in many ways. Some scientists are beginning to propose that the quick (in geological terms) transition from an atmosphere with little or no oxygen to an oxygen-rich atmosphere never occurred. Rather, they say that evidence shows that Earth had higher oxygen content far longer than scientists have thought. If this is the case, then the geological hypotheses proposed are wrong, and the biological processes remain the most likely reason for the rise in oxygen.

Barring a momentous discovery, the debate over whether the timing of the rise in atmospheric oxygen was triggered by geological or biological processes will continue for some time to come. Most likely, ancient geological processes have played some part in determining the amount of oxygen in the atmosphere. For example, tectonic plates may have opened up deep basins on the ocean floor that were free of oxygen and, as a result, served as oxygen sinks where organic material could have settled. Once these sinks were filled, oxygen began to accumulate rapidly. Nevertheless, there is not enough evidence to supplant biological processes as the dominating influence in the evolution of Earth's atmosphere. For now, the generally accepted theory is that cyanobacteria, as the only life on Earth for about two billion years, single-handedly saturated the atmosphere with oxygen.

—DAVID PETECHUK

KEY TERMS

Key Terms

BASALTS:

A type of volcanic rock that is composed of Fe-and Mg-rich silicates.

CYANOBACTERIA:

Photosynthetic bacteria. They are prokaryotic and represent the earliest known form of life on Earth.

KOMATIITES:

A very magnesium-rich volcanic rock.

METHANE:

The primary component of natural gas and the simplest member of the alkane series of hydro-carbons.

OXIDATION:

A chemical reaction that combines oxygen with other chemical species. Also, used to describe the loss of electrons from an atom or ion.

OXIDATION-REDUCTION REACTIONS:

Termed "redox" reactions, these are chemical reactions involving atoms or ions that result in changes in electron configurations. During oxidation-reduction reactions the oxidation state of the atoms (and of ions subsequently produced) changes and electrons are lost or gained. Electron loss increases oxidation (i.e., the oxidation state) and produces a positive oxidation state. In contrast, the capture or addition of electrons results in reduced or negative oxidation state. As the electrons are not destroyed in chemical reactions (i.e., the net number of electrons must remain the same) oxidation-reduction reactions are coupled so that oxidation cannot occur without a corresponding reduction.

PALEOSOL:

A stratum or soil horizon formed as a soil in a past geological age.

REDUCTION:

A term used variously to describe the transformation of a compound into component molecular or atomic species. Also used to describe the gain of electrons by an atom or ion during a chemical reaction.

SILICATES:

Any of a number of minerals consisting of silica combined with metal oxides; they form a chief component of the rocks in volcanic sinks, and are the minerals that comprise most of Earth.

TECTONIC PLATES:

A massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere (the rigid outer part of Earth, consisting of the crust and upper mantle). Plate size can vary greatly, from a few hundred to thousands of kilometers across.

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