Ancient

Ancient Earth: The First Three Billion Years

May 20, 2012, 12:54 pm
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Cyanobacteria. Shown is the free-living, nitrogen-fixing cyanobacterium Nostoc sp. (Courtesy of J. Meeks.)

The Big Bang, the single point in space and time from which all matter and energy in the universe supposedly emanated, is thought to have occurred sometime around 13.7 Ga (gigayears ago; 1 Ga means 1000 million years ago). Our solar system began to coalesce about 4.5 Ga. When the surface of primitive Earth cooled to a temperature below the boiling point of water, sometime around 4.1 Ga, the atmosphere consisted primarily of gases released from volcanoes. [1] These gases included high concentrations of carbon dioxide (CO2), carbon monoxide (CO), water vapor (H2O), dinitrogen (N2), hydrogen chloride (HCl), and, perhaps, small amounts of methane (CH4). Within a relatively short time, perhaps as early as 3.8 Ga, photosynthetic bacteria (cyanobacteria) were present. [2]

Cyanobacteria proliferated widely during the next few billion years. Their photosynthesis depleted the CO2 concentration in the atmosphere to below 10% and released sufficient O2 to rust (oxidize) iron near Earth’s surface, thereby producing layers of rock called Banded Iron Formations. This release of O2 also exhausted minerals such as elemental sulfur near Earth’s surface and methane (CH4) in the atmosphere. At this time, energy emitted from our young sun amounted to only about 83% of the energy it emits today, and so, at about 2.5 Ga, the loss of CO2 and CH4 sent global temperatures plummeting because CO2 and CH4 are two of the major greenhouse gases that permit visible sunlight to pass freely through Earth’s atmosphere, but absorb a significant portion of the infrared radiation emitted by Earth and thereby raise its temperature. As its surface became covered with ice, Earth reflected more solar energy and became colder still until the entire planet froze, an episode known as Snowball Earth. [3], [4]

Recovery from this frozen state was slow. Volcanoes continued to release CO2 at a steady rate, but weathering of silicate rock and photosynthesis, processes that deplete CO2, virtually stopped at the low temperatures; thus, CO2 gradually accumulated in the atmosphere. After about 30 million years, atmospheric CO2 reached a level at which Earth retained sufficient solar energy to thaw ice at the equator. Once sunlight struck open water and bare ground instead of ice, Earth absorbed more solar energy. Additional water vapor, another greenhouse gas, entered the atmosphere and further accelerated warming.

Snowball Earth melted. Indeed, with this melting, Earth’s energy balance shifted, and a heat wave persisted from 2.4 Ga to 2.0 Ga. High temperatures plus high CO2 levels stimulated a cyanobacterial bloom that released massive amounts of O2. Sometime between 2.45 Ga and 2.22 Ga, atmospheric O2 concentrations jumped from less than 0.02% to around 3% in what has been called the Great Oxidation Event (Bekker et al. 2004; Canfi eld 2005; Holland 2006). The Great Oxidation Event had major biological repercussions. Oxygen, as the name implies, is a strong oxidizing agent (electron acceptor) that poisons several crucial biochemical reactions. Many simple life forms that had up to that time been prevalent on Earth could not tolerate exposure to O2.

Eukaryotes, organisms with cells that have nuclei and other specialized compartments (organelles)—some of which isolate reactions involving O2 from more sensitive parts of the cell, appeared after the height of the Great Oxidation Event. [5] Aerobic respiration, O2-based breakdown of organic compounds, generates far more energy than its anaerobic alternatives. The energy bonanza of aerobic respiration fostered the development of more complex organisms. Additionally, higher O2 levels gave rise to an ozone (O3) layer in the upper atmosphere that screened out ultraviolet radiation harmful to life; now eukaryotes could complete relatively long life cycles without extensive ultraviolet damage.

Earth’s landmasses were in motion and coalesced into one supercontinent named Rodinia (Russian; homeland) near the equator at about 1.0 Ga, uplifting vast amounts of silicate rock as well as burying vast amounts of organic matter. Rodinia disassociated into eight smaller continents at about 0.8 Ga. Algae (photosynthetic eukaryotes) proliferated in the nutrient-rich, equatorial continental shelves.

 

Supercontinent Rodinia. Breakup and dispersal of the Rodinia supercontinent in a 750 Ma reconstruction. The brown areas indicate mountain belts that arose between 1.3 Ga and 1.0 Ga (Torsvik 2003).

 

 

 

 

 

Together, these processes—weathering of silicate rock, burial of organic matter, and algal photosynthesis—depleted atmospheric CO2 concentrations to about 0.03%. [6] For a while, Earth’s climate remained uniform despite low CO2 concentrations because anaerobic respiration of the buried organic matter released lots of methane (CH4), and CH4 is 23 times more effective than CO2 per molecule as a greenhouse gas. The situation, given the transient nature of CH4, was unstable, and winter was knocking.

[1] Hopkins, M., T. M. Harrison, and C. E. Manning (2008) Low heat flow inferred from >4 Gyr zircons suggests Hadean plate boundary interactions. Nature 456:493-496.

[2] Buick, R. (2008) When did oxygenic photosynthesis evolve? Philosophical Transactions of the Royal Society B-Biological Sciences 363:2731-2743 doi:10.1098/rstb.2008.0041.

[3] Kopp, R. E., J. L. Kirschvink, I. A. Hilburn, and C. Z. Nash (2005) The paleoproterozoic snowball Earth: A climate disaster triggered by the evolution of oxygenic photosynthesis. Proceedings of the National Academy of Sciences of the United States of America 102:11131-11136.

[4] Kasting, J. F. and S. Ono (2006) Palaeoclimates: the first two billion years. Philosophical Transactions of the Royal Society B-Biological Sciences 361:917-929.

[5] Rasmussen, B., I. R. Fletcher, J. J. Brocks, and M. R. Kilburn (2008) Reassessing the first appearance of eukaryotes and cyanobacteria. Nature 455:1101-U9 doi:10.1038/nature07381

[6] Schrag, D. P., R. A. Berner, P. F. Hoffman, and G. P. Halverson (2002b) On the initiation of a snowball Earth. Geochemistry Geophysics Geosystems 3:10.1029/2001GC000219.

This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.

©2010 Sinauer Associates and UC Regents

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Citation

Bloom, A. (2012). Ancient Earth: The First Three Billion Years. Retrieved from http://www.eoearth.org/view/article/51cbeffa7896bb431f69fbe1

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