• 1. Our world in the beginning
• 2. Oxygenation of the atmosphere.

1. Our world in the beginning.

Origin of Earth and Atmosphere

Our planet and our solar system has a history of about 4.55 to 4.60 Ga (Ga = billion years). Our solar system originated LONG after the origin of the universe in the Big Bang about 10 to 15 billion years ago. All matter formed during the Big Bang consisted of the element hydrogen. The hydrogen atoms had to undergo nuclear reactions in stars, and the stars had to explode as supernovas, before heavier elements than hydrogen (such as carbon, the main building block of life on Earth) came into existence. Our Sun did thus not originate straight at the origin of the universe: several generations of stars went before. How did our planet look like shortly after its origin as part of our solar system, when a cluster of interplanetary dust and asteroids was blown together after the explosion of a supernova? Clearly, many things were very different from today at the time that the Earth first formed from the solar nebula, as discussed in the first lecture of this semester, where the following five points are discussed in more detail.

• The Sun was not as hot as it is today.
• The Moon was formed by the collision of a Mars-size asteroid with early Earth, probably at some time during the big meteor bombardment that ended about 4 billion years ago.
• The Earth's rotation rate was faster because tidal friction resulting from the presence of the Moon had not yet slowed it down.
• The continents were probably smaller than today's, with more ocean surface, especially more than 3 billion years ago.
• The atmosphere had a very different composition, as discussed in the first lecture of this semester, and has been strongly influenced by the evolution of Life on Earth (see below).

Early Earth environments:

Life does not originate spontaneously from non-life on Earth today (Pasteur): all organic matter is rapidly oxidized by the oxygen gas in the atmosphere, so that no simple 'organic' molecules stay preserved outside living organisms. If we want to explain the occurrence of life on Earth by natural processes, we must assume either that life came from somewhere else (outer space), or that it originated on an Earth that differed from the present Earth. The theory that life arrived on Earth from space is called panspermia (seeds everywhere).

We can derive information on the ancient environment on Earth from looking at ancient rocks. Metamorphic rocks (rocks that have been deep in the earth under high temperature and pressure) in British Columbia have been dated at 3.96 Ga, and the oldest sedimentary rocks that we know occur at Isua (Greenland) and are about 3.85 Ga. Presently, continental crustal rocks differ from oceanic crustal rocks in composition: they contain more silica (Si), and have a lower density. Therefore continental crust is not subducted and added to mantle rocks deeper in the Earth when plates of the Earth's crust collide- it is crumpled up against other pieces of crust and remains part of the crust. Ocean crust, however, is continually destroyed by subduction (and added back to the mantle).

Scientists generally agree on the hypothesis that continents (thus continental crust) formed very early although these early continents may have been covered by shallow seas. Large continents were present by 2.5 - 2.0 Ga. There is much discussion, however, as to the exact timing and processes of the formation of the continents. Specifically, there is major discussion on whether continents grew very rapidly over the first 500 million years or so, followed by a gradual slow-down (in which case the early continents would have been about the size of the present continents), or whether they grew gradually and slowly until about 2 Ga ago (in which case the early continents would have been much smaller than the present ones). The first hypothesis has been gaining recently.

Plate tectonic processes worked differently in the Archaean (the period before about 2.5 Ga): nowadays lavas that flow out at mid-oceanic ridges are basalt, with rather uniform composition over the whole world. In the Archaean , however, such lavas had a different composition and were very hot when molten: about 1600 ^oC, as compared to 1200 ^oC for present basalts. Such rocks are called komatiites. We thus assume either that the Earth as a whole was still hotter than today, or that the crust was much thinner, overlying hot rocks fairly close to the surface.

Sediments were formed in these times, therefore there was weathering and erosion (thus an atmosphere, consisting of dominantly CO₂, with some N₂, H₂O, minor CO, SO₂, and H₂S ). Deposition occurred in water, thus there was an ocean. As far as we know from these sediments, the ocean was salty, containing the common ions Cl^- (chlorine), SO₄^2-(sulfate), Na⁺ (sodium) and Ca²⁺ (calcium). Mg²⁺ (magnesium) may have been more abundant relative to Ca²⁺ than it is now, and sulfate (SO₄^2-) was probably much less abundant.

NOTE: if you want to read about the origin of life on earth, which is not really the topic of this class, click here.

2. Oxygenation of the atmosphere.

Photosynthesis, the reaction producing free oxygen gas, must have developed early on, because stromatolites have been recognized in rocks of about 3.5 Ga. Stromatolites are limestones secreted by the actions of photosynthesizing bacteria, not very simple semi-life forms, but fairly complex bacteria that could perform the difficult reaction of photosynthesis. The secretion of limestone (CaCO₃) was probably mediated by the coupling of reactions 1 and 2:

• H₂O + CO₂ <-> CH₂O + O₂ (Reaction 1)
• 2 HCO₃^- + Ca²⁺ <-> CaCO₃ + CO₂ + H₂O (Reaction 2)

If organisms use up CO₂ in photosynthesis (reaction 1), they drive at the same time reaction 2 towards the right, thus causing precipitation of calcite.

Photosynthesis (which generates free oxygen) originated in Prokaryotes (Eubacteria and Archaebacteria) very early in Earth history: we have evidence from stromatolites that photosynthesis occurred about 3.5 Ga ago. The photosynthesizing organisms were probably similar to the modern cyanobacteria (formerly called blue-green algae). When photosynthesizing organisms first became common, the free oxygen gas that they generated did not immediately start to accumulate in the atmosphere: there were many chemical compounds around in the oceans and on land that were not stable in the presence of free oxygen gas, and that became oxidized. The most common of 'things' to be oxidized were iron (Fe) on land, and sulfur (S) in the oceans. Presently, iron occurs in Fe₂O₃ (rust), sulfur occurs in sulfate (SO₄^2-) in the oceans. The free oxygen gas was thus used up in oxidation reactions for a considerable time, and during this considerable time we think that the oxygen concentrations in the atmosphere did not become higher than 1 or a few percent of the present atmospheric level (PAL). We do not know whether eukaryotes could become common at such low levels of oxygen.

How long was this 'considerable time'? We have geochemical evidence from several different sources. These are:

1. Paleosols (fossil soil horizons). Soil forms during weathering of bed rock. In present soils, the bed rock is reacting with water, taking up carbon dioxide as well as oxygen to form oxidized, water-containing minerals such as clay minerals. The composition of minerals in soils thus depends upon the chemical composition of the bed rock, as well as on which gases are present in the atmosphere. Soils formed on ancient lava flows on the African continent formed at various ages. The minerals present in the ancient soils fall into two groups: older than about 2.2 Ga and younger than about 1.9 Ga (no data between 2.2 and 1.9 Ga). The minerals present in the younger soils would be chemically stable in the presence of oxygen to concentrations of at least 15 - 20 % of PAL. The minerals in the older soils could not have been stable in the presence of more than 1-2 % of PAL oxygen gas.

2. Continental Red Beds: red, quartz- rich sands, which are red because grains are covered with iron oxide (such as in the much younger, Triassic-Jurassic rocks with dinosaur footprints along the roadcuts between Middletown and New Britain). These sediments form presently in regions without much vegetation (e.g., deserts), and they can form only when there is enough oxygen in the atmosphere to keep the iron oxidized. The oldest Red Beds found are between 2.1-2.0 Ga old.

3. Uranium ore pebbles: the mineral uraninite occurs in pebbly sediments that formed by the accumulation of gravel in ancient rivers, which are preserved in South Africa. These pebbles are of course in contact with the atmosphere. The mineral uraninite, however, can not survive prolonged contact with free oxygen: it oxidizes, and presently uraninite gravels do not occur. The youngest uraninite-bearing gravels are about 2.6 to 2.4 Ga.

4. Banded Iron Formation (BIF): these rocks are not formed on the present earth. They are very thick (thousands of feet), very widespread (hundreds of miles), and consist of finely-laminated rocks (laminae as thin as 1 mm). The rocks are very rich in iron, which occurs in both reduced (Fe²⁺) and oxidized (Fe³⁺) minerals in the BIFS; BIFS are widely used to mine iron. The oxidized form ALWAYS froms when there is oxygen around (it is very hard to keep metal objects from rusting). Interestingly, Fe³⁺ is extremely insoluble in water, whereas Fe²⁺ is very soluble. To form the widespread deposits, the oceans from which the deposits formed must have been enriched in dissolved iron, which is possible chemically speaking ONLY if the Fe occurred as Fe²⁺. And that is only possible if there was no or very little free oxygen in the oceans. We think that the BIFS formed at a time when oceans and atmosphere contained very little free oxygen, so that the waters could become very rich in iron, for instance, coming out of hydrothermal vents. The iron could then have precipitated (dominantly in Fe³⁺ minerals) ONLY at places where it came in contact with free oxygen, produced by floating bacteria during their photosynthesis.

The lamination could have been produced by episodic introduction of Fe by hydrothermal vents, episodic (e.g., seasonal) blooming of bacteria, or both. BIFS thus were taking the oxygen generated by photosynthesis out of the atmosphere and put it into the lithosphere. Almost all BIFS are older than 1.9 Ga, with a few exceptions that are 700-800 million years old.

As to the exceptions: one should keep in mind that even in the present oceans not all waters are oxygenated. In the Black Sea, water below about 200 m deep is fully anoxic, and hydrogen sulfide (H₂S) is present in the waters. (Click on Black Sea link for more information).

Most evidence thus suggests that the Earth's atmosphere (thought not necessarily its oceans) had a level of oxygen equal to about 10-15% of its present levels around 2 Ga. The oldest microfossils of unicellular eukaryotes (large, complex unicellular organisms, with a complexity similar to that of the modern foraminifera, diatoms, radiolarians and nannoplankton) have been dated at 1800 to 1900 million years (1.8-1.9 Ga). The emergence of common Eukaryotes (by symbiosis of different types of prokaryotes) has been connected to the evolution of the atmosphere: all known Eukaryotes must have relatively high concentrations of oxygen in order to run their cell metabolic reactions.