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Earth's Proto-Atmosphere

 

The release of gases, especially water vapor, from our nascent Earth makes a fascinating scientific story. It’s also one with profound implications for our modern-day climate and what might result from the steady stream of carbon dioxide and other gases that we are pumping into it. Because this next section provides a foundation for other topics in this book, we’re going to spend some time with it and approach it with a fair amount of detail. So you may want to refresh your beverage, turn off the stereo and make sure your mind is fully engaged. Are you ready?

 

 

Outgassing is defined as the loss or removal of gases (also known as volatiles) from an object or mass usually by heating and/or pressure. A trip to Yogi and Boo-Boo’s home, Yellowstone National Park, provides many examples of outgassing. Though data are scant, volatiles released from the proto-Earth probably included helium, hydrogen, carbon dioxide, nitrogen gas, water and hydrogen sulfide (Chang, 1983) and may have included other compounds, like carbon monoxide, hydrochloric acid and methane (Kasting, 1993). Such conclusions are based on studies of volatiles released in modern-day volcanoes, mid-ocean ridges and volatiles present in meteorites, which may or may not be good analogues.

As the Earth accreted, volatile compounds trapped in planetesimals became part of the Earth. Subsequent heating and melting of the Earth mobilized these gases driving them towards the surface where they were released as the planet’s first (or early) atmosphere, called the proto-atmosphere. The molecular weight and composition of the gases determined whether they escaped back into outer space, condensed and precipitated to Earth or combined to form new compounds. Put another way, the formation of Earth’s proto-atmosphere was determined by

  • the source materials (the chemical composition of the trapped volatiles)
  • whether they escaped to the atmosphere or remained within the protoplanet’s interior
  • whether they escaped to outer space (Abe et al, 2000)

Light elements, like helium and hydrogen, would have escaped, although hydrogen likely combined with other elements on its way out (of the Earth or the proto-atmosphere) and was trapped. (A good treatment of molecular mass, temperature and escape velocity of molecules can be found here: http://www.astronomynotes.com/solarsys/s3.htm). Outgassed water vapor probably condensed into clouds, forming a steam atmosphere which may have blanketed the earth and maintained surface heating—see below). Carbon dioxide/monoxide, nitrogen and other “heavy” elements and compounds did not have sufficient escape velocities (even at the elevated temperatures of the proto-planet) and so became part of Earth’s proto-atmosphere.

New data and revised hypotheses support quite a different composition to our atmosphere than was thought in the later decades of the 20th century. The subject is still highly contentious but there are important stages in the evolution of Earth’s atmosphere so that allow you, as an inquiring mind oceanographer, to make some sense out of what you hear and read in other places.

Borrowing from several sources, I would propose the following stages in the evolution of Earth’s atmosphere:

  1. Accretionary atmosphere (Hadean): 4.56 to 4.4 Ga
  2. Prebiotic (post-impact) atmosphere (Archaean): 4.4 to 3.8 Ga
  3. Biotic O2-free atmosphere (Proterozoic): 3.8 to 2.2 Ga
  4. Biotic O2-richatmosphere (Phanerozoic): 2.2 Ga to present

Each of these stages represent discrete conditions at some point in Earth’s evolution and provide a good framework for understanding the evidence upon which our knowledge and ideas of Earth’s atmosphere are based. Life’s role here is profound, shaping not only the chemistry of the atmosphere but a number of other important processes as well. While we’ll talk more about the origins and evolution of life in the next chapter, we will begin our discussion of the interactions of living organisms with the physics, chemistryl and geology of our planet from the outset. But first, let’s explore conditions before life emerged.

The accretionary atmosphere, the atmosphere that developed during the formation of Earth, derives its importance for what it supplied or didn’t supply to the subsequent stages. Our inferences about the composition of volatiles at this stage come from observations on modern-day solar system matter. These include:

  • the protoplanetary disk
  • solar wind
  • meteorites
  • comets
  • unsampled planetary materials

You may recall that the clouds of dust and gases from which planets form are called protoplanetary disks or proplyds. Gases within this material may have been trapped within the accreting Earth. Similarly, particles and ions released from our Sun, i.e., the solar wind, may have been captured (adsorbed, occluded or dissolved) within the grains of the planetesimals that formed Earth.

Until recently, the distinct differences in the abundance and isotopic ratios of rare gases in our modern atmosphere from proplyds and the solar wind led scientists to conclude that the contribution of these sources to Earth’s atmosphere were minor. However, Porcelli and Pepin (2000, OEM) and others present strong arguments that at least some of the gases within Earth’s modern-day mantle (e.g. Neon and possibly Argon and Xenon) resemble solar-derived materials. Thus, there is still some debate about the contribution of protoplanetary and solar components to Earth’s atmosphere.

The contribution of meteorites and comets holds more promise as a solvable scientific problem but is not without its share of difficulties. A group of meteorites known as carbonaceous chondrites (see http://www.saharamet.com/meteorite/gallery/Ctypes/CAIs.html for pics and further information) contain ample amounts of water (<1 to ~9% H2O by weight). Comets hold even more water, upwards of 40% or more (http://www.jpl.nasa.gov/sl9/news28.html). Both meteorites and comets contain other volatile compounds as well. However, there are two observations that cast doubt on carbonaceous chondrite and comet origins for the bulk of Earth’s atmosphere. Drake and Righter (2002) make a strong geochemical case against the current crop of meteorites, arguing instead for an “Earth chondrite” or “Earth achondrite” that have not yet been sampled (i.e., not yet found) as the material from which Earth formed. A similar case is made against comets, as the ratio of heavy hydrogen (deuterium) to normal hydrogen (the D/H ratio) in comets is substantially different from the D/H ratio of seawater. Scientists estimate that no more than 20% of Earth’s water (and by inference, other volatiles within comets) could have come from comets.

As a result, the parent material for Earth’s volatiles remains to be identified. That also means that we are not certain where Earth’s water came from, although we will discuss this question in more detail in the next section. Despite the uncertainty, progress is being made and perhaps by the time you are reading this book, scientists will have achieved a breakthrough in our understanding.

Fortunately, when we get to Earth’s prebiotic atmosphere, we are on more solid ground. Earth’s prebiotic atmosphere offers clues to its properties in the chemistry of volcanoes, rocks and sediments found in modern times.

Analyses and modeling of the chemistry of volatiles released from modern-day volcanoes and materials released from mid-ocean ridges, affectionately known as MORBS (mid-ocean ridge basalt systems) have led scientists to speculate that Earth’s prebiotic (before life), post-impact (after formation of the moon) atmosphere consisted of:

  • hydrogen (H2) and water vapor (H20)
  • carbon monoxide (CO) and carbon dioxide (CO2)
  • nitrogen as N2
  • hydrogen sulfide (H2S)

Take home this clear and important message about the composition of the prebiotic atmosphere: there was little or no free oxygen and carbon dioxide and nitrogen were the dominant gases.

Professor Abrajano at Rensselaer Polytechnic Institute in Troy, New York, has an excellent treatise based on simple chemical principles that demonstrates the effects of extremely low oxygen concentrations on several once-thought-important compounds, such methane (CH4+), ammonium (NH4+) and sulfate (SO4-), in Earth’s prebiotic atmosphere (see http://ees2.geo.rpi.edu/abrajanoCourses/public_html/AtmosphereHistory.html and http://ees2.geo.rpi.edu/abrajanoCourses/public_html/DegassingChemistry.html). His conclusion (now generally accepted) is that these compounds, if present, would only have existed in very low concentrations. For decades, scientists believed these compounds were the mainstay of Earth’s prebiotic atmosphere. In fact, you will still find textbooks that include methane and ammonia as significant components of Earth’s prebiotic atmosphere because these compounds in the presence of lightning give rise to amino acids (as demonstrated in the 1950s by Miller and Urey) which were once thought to be precursors to life. The science has moved well beyond these conclusions in modern times and scientists now accept the above composition as the most likely.

This lack of oxygen in the atmosphere persisted for at least two (2) billion years on our planet, well after the origin of life and perhaps after the origin of photosynthetic organisms (but this is currently being challenged). We’ll discuss the origins of life in our next chapter. For now, let’s examine the evidence for Earth’s anoxic (without oxygen) state at this stage of its development.

Evidence for biotic reducing atmosphere from ~4.45 to 2.4 Ga during a period of geologic time known as the Proterozoic comes from a number of sources. The absence of iron oxidation in sediments and paleosols (ancient soils) prior to ~2.4 Ga and the presence of compounds in rocks and sediments that can only exist under reducing conditions affirm the lack of oxygen at this stage of Earth’s development.

And if that doesn’t convince you, take a look at the evidence for mass-independent sulfur isotope fractionation in pyrites (sedimentary iron sulfides) presented by J. Farquhar and others in Science, December 20, 2002. You’ll have to read the article a dozen times like I did to understand it but basically they looked at “pieces” of sulfides (called inclusions) in diamonds (expensive science!) from the Orapa kimberlite pipe (a type of geological formation) within the Kaapvaal-Zimbabwe craton (an ancient piece of continental crust) in the Republic of Botswana, “nestled” between South Africa, Namibia, Zimbabwe and Zambia, and found that the ratio of sulfur isotopes in those inclusions was consistent with their formation in a reducing atmosphere. That’s hard core science, eh?

While scientists still debate the extent of anoxia or hypoxia (reduced oxygen concentration) during the first couple billion years of Earth’s existence, the majority favor a highly reducing atmosphere so we’ll go with them. Yet important changes of another kind appeared as life made its grand entrance onto our planet which led to a third stage in the evolution of Earth’s atmosphere called the biotic reducing atmosphere, also known as the Archaean atmosphere. (I favor the more descriptive titles as they are unambiguous.)

Like Saturn’s large moon, Titan (another offspring of Gaia and Uranus and, of course, the namesake for that most infamous ship, Titanic), Earth’s first biotic atmosphere may have contained methane. In fact, understanding Earth’s early atmosphere is part of the rationale behind the Cassini-Huygens mission to Saturn, which is currently in full swing (see http://saturn.jpl.nasa.gov/index.cfm).

Carl Sagan (as a posthumous author) and Christopher Chyba made arguments for a methane atmosphere in their 23 May 1997 Science article titled The Early Faint Sun Paradox: Organic Shielding of Ultraviolet-Labile Greenhouse Gases. You see, at this time, the Sun produced only about 60-75% of its current energy output so that greenhouse gases (of which methane and carbon dioxide are prime examples) must have been present if we are to explain the liquid water (as opposed to frozen water) in the Archean as evidenced in sediments and rock formations.

Catling, Zahnle and McKay weigh in on the early atmosphere-methane hypothesis with their calculations and discussion in the 3 August 2001 issue of Science. In their paper, the authors discuss the splitting of atmospheric methane (by photolysis) into hydrogen and carbon dioxide, leading to the irreversible escape of hydrogen and eventual increases in the concentration of oxygen at ~2.4 Ga (see their paper for the details). Thus, coming at it from a different angle, they argue that methane was likely present and its presence (and the reactions they present in their paper) explains the transition from a reducing atmosphere to an oxidizing atmosphere.

James Kasting and Janet Siefert argue in the 10 May 2002 issue of Science that methane (CH4+) may have been generated by an “ancient” group of anaerobic bacteria (living in the absence of oxygen) called methanogens. These members of the kingdom Archaea are among the oldest bacteria on Earth, appearing between 3.5 to 3.8 billion years ago (see Mojzsis et al., Nature 384, 1996). They also utilize hydrogen as an energy source for “fixing” carbon dioxide into organic matter, making them, technically speaking, autotrophs (organisms that produce their own organic matter...you’ve heard of plants?).

Despite their well-constrained arguments, none of these papers presents direct evidence for methane in Earth’s biotic reducing atmosphere. The obvious presence of methanogens at this time and some geochemical evidence from Archean dolomites and limestones (Dix et al, Precambrian Research, 1995) provide data that favor a methane atmosphere.

One of the significant consequences of a methane-filled atmosphere results from its role as a greenhouse gas. Methane permits short-wave radiation from the sun to pass through it but traps long-wave radiation (otherwise known as heat). If methane was present, then we can expect the earth to be tolerable warm during this period, warm enough to maintain liquid water, as concluded by Sagan and Chyba. (We’ll explore the greenhouse effect in much greater detail in a later chapter).

The final stage in our tour of Earth’s early atmosphere gave rise to our modern atmosphere. The evolution of oxygenic photosynthesis (photosynthesis that yields oxygen as a by-product) and the eventual rise of atmospheric oxygen ~2 Ga wreaked havoc on Earth. The presence of oxygen in the atmosphere shifted Earth’s surficial chemistry from a reducing one to an oxidizing one. Iron-bearing rocks began to rust, dissolved iron converts to an insoluble form and many other types of oxidation reactions are accelerated. But the most significant effects of oxygen were felt by obligate anaerobic organisms, those bacteria who cannot tolerate even the slightest concentration of oxygen. These organisms were permanently banned to the netherworld, so to speak, regions of our planet where oxygen cannot penetrate or where chemical or biological processes have reduced its concentrations. Anaerobic organisms now inhabit anoxic muds, subterranean regions of the seafloor, interior regions of the earth and similar environments.

This catastrophic impact of oxygen on Earth’s original inhabitants—remember, these single-celled anaerobes ruled the planet for well over a billion years—represents the first mass extinction of life on Earth (if indeed large numbers of species ceased to exist). Mass extinctions punctuate the evolution of life on Earth. This event—brought about by the appearance of oxygen-producing microbes—caused what is known as the oxygen revolution (also termed the oxygen holocaust).

Our emphasis in this chapter rests with evidence supporting the rise of atmospheric oxygen. The most commonly cited observation of a rise in atmospheric oxygen is the presence of banded iron formations, sedimentary rocks with well-defined layers or ferric iron (oxidized iron, Fe3+). These bands were thought to correspond to periods when oxygen was present (generating the iron oxide stripes) or absent (producing silica-rich, less colorful layers). This interpretation of BIFs was central to scientists’ arguments regarding when and how oxygen began to increase Earth’s atmosphere.

However, new studies (Konhauser et. al, 2002) have shown that banded iron formations may result from the activity of chemolithoautotrophic bacteria under anoxic or microaerophilic (very low oxygen) conditions. These bacteria can perform photosynthesis in the absence of oxygen (called anoyxgenic photosynthesis) and use iron (from rocks, hence the litho prefix) as an oxidant in the reduction of carbon dioxide to make sugars. (Got that?). The conclusion is that bacteria may be responsible for the creation of BIFs complicating any interpretation of atmospheric oxygen concentrations derived from an analysis of BIFs.

Despite their diminished role as a proxy for atmospheric oxygen, BIFs may provide insights into the chemistry of the early ocean. A Danish scientist, David Canfield, has proposed a transitional state for the oceans from an anoxic to intermediate to oxic environment. Named the Canfield ocean, this two-part model of the ocean with oxygenated surface waters and anoxic deeper waters provides a plausible mechanism for the rise of atmospheric oxygen and helps bridge much of the chemical and geological evidence cited to date.

Fortunately, less equivocal data mark the appearance of oxygen at this stage of Earth’s history. As noted above, Farquhar et al (2000) demonstrated a difference in the isotopic ratios of sulfur in rocks younger than ~2 Ga that supports the idea of a rise in atmospheric oxygen at that time. Earlier studies on paleosols and microfossils hinted at such a change, but Farquhar’s was one of the first to provide convincing geochemical evidence. Further studies confirmed their results and subsequently, Catling (2001) and Habicht (2002) have presented compelling arguments and additional data. As Uwe Weichert (20 December 2002, Science 298)) puts it, “The recent discovery of mass-independent fractionation in the isotopic composition of sulfur in sedimentary iron sulfides and sulfates makes a change in the oxygen content of the atmosphere between 2400 to 2100 million years ago almost inevitable.”

I believe him.

 

   
   
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