Proterozoic


Even if expense were no object, none of these [biosphere] services could be performed at such scales and with such efficacy by any anthropogenic means.  Our dependence on biosphere services is literally a matter of survival, and that’s why the integrity of the biosphere matters.” Vaclav Smil.

[T]he accumulation of atmospheric oxygen paved the way for significant leaps in biological evolution in the Paleoproterozoic with the rise of macroscopic oxygen-breathing organisms and in the Neoproterozoic-Cambrian with the emergence of animals.” Dominic Papineau

When economists try to put a value on the biosphere, they are kidding themselves and us.  As Vaclav Smil points out without a healthy biosphere, humans cannot survive [1].  Our dependence is existential.  Nor is this dependence limited to the current biosphere.  We owe our existence to the biosphere extending back through deep time.

 The Earth is 4.55 billion years old and its history is divided into four eons.  The earliest the Hadean Eon ended 3800 million years ago.  The Hadean Earth was dominated by the kinetic energy of constant collisions as it swept up debris scattered along its orbit in the young solar system.  A magma ocean bubbled on its surface, a frightening uninhabitable place.  The Archean Eon lasted from 3800 million years ago until 2500 million years ago.  The Archean Earth climate was temperate despite the faint young sun we’ve discussed in a previous article [2].  James Kasting proposed that it was moderated by carbon dioxide and the methane produced by methanogenic Archaea [3].  These methane-producing microorganisms kept the Earth from freezing solid, while the sun’s fusion reactor gradually intensified through the Archean, giving the rest of life a chance.

 The Proterozoic Eon began where the Archean left off and ran until the Cambrian Explosion 544 million years ago, the start of the present Eon, the Phanerozic.  But we are interested today in two remarkably similar events which bookend the Proterozoic.  These events have in common the breakup of a supercontinent, several snowball Earth episodes, where the Earth’s oceans may have frozen to the equator, interspersed between hothouse climates, a rise in atmospheric oxygen and a leap in biological evolution as described by Papineau [4].

 As the Archean Eon gave way to the Paleoproterozoic nickel isotope sedimentary deposits suggest that the productivity of the methanogens were winding down [5].  Methanogens use nickel in their metabolism to produce the atmospheric methane which along with the principle greenhouse gas carbon dioxide was keeping the Earth warm.  At the same time the supercontinent Kenorland was breaking up.  Rifting of supercontinents is accompanied by increased weathering of the newly exposed surfaces.  When the most recent supercontinent Pangaea broke up 200 million years ago the rift valley forming between South America and Africa became the Atlantic Ocean which is still spreading.  The rifting of the supercontinent Rodinia during the Neoproterozoic, about 700 million years ago gave rise to the Iapetus Ocean.

 Increased weathering released phosphorus into the seas.  Phosphorus is the most limiting element in the biosphere presently, as discussed by Dave Vaccari in a recent Sustainable Planet article [6, see also 4 and 11].  Even today plants concentrate phosphorus and can contain up to seven times the concentrations in the surrounding soils.  At the same time methanogens were becoming less productive at the end of the Archean, ancestors to present day cyanobacteria bloomed as a consequence of increased phosphorus which these microbes need for oxygen photosynthesis.  From Susan Gaines remarkable textbook on molecular fossils Echoes of Life [7] and from Plaxco and Gross’ Astrobiology [8] text book we learn that these bacteria may have been around for several hundred million years waiting for this opportunity. 

Oxygen photosynthesis increased the level of atmospheric oxygen after the breakup of Kenorland from essentially zero to about 2% of the atmosphere.  Oxygen is poisonous to methanogens so this turn of events created the opportunity for an entirely new biological regime.  But it also drew down the atmospheric carbon dioxide.  As a consequence of the loss of methane and carbon dioxide the Earth froze over.  Since the ice and snow which now blanketed the planet reflects most incoming short wave solar radiation rather than absorbing it, there is no known way which the Earth could have recovered except for volcanic activity and the release of carbon dioxide.  Enough carbon dioxide accumulated in the atmosphere over millions of years to melt back the ice and snow by trapping long wave heat radiation from the Earth surface.  Once the ice melted away completely, the huge quantity of carbon dioxide necessary to melt it in the first place now created a superheated greenhouse effect.  The subsequent increased weathering of silicate rocks [9] and additional bacterial blooms kick started the process all over again, drawing down the carbon dioxide leading to yet another snowball earth episode.  Each cycle may have pumped more oxygen into the atmosphere.

Essentially two chemical reactions take place which draw down carbon dioxide in a hot house climate.  The first is inorganic and involves the weathering of silicate rocks.  This is the Earth’s thermostat [9] and is given by the following simplified equation.

CO2 + CaSiO3 -> CaCO3 + SiO2

 In a hot house climate more water evaporates off the oceans and forms carbonic acid with the carbon dioxide in the atmosphere.  This weak acid rains out onto rocks weathering them.  Note that in this equation carbon dioxide is drawn down when the calcium carbonate and silica are “buried in marine sediments and eventually into the geological record [9].”  This process extracts excess carbon dioxide from the atmosphere but does not create free oxygen.

 However, oxygenic photosynthesis performed by the bacterial blooms, encouraged by the newly releases phosphorus performs the following reaction.

 CO2 + H2O -> CH2O + O2

 When organic compounds, here represented by CH2O, are buried as sediment without being oxidized and consumed by other organisms there is a net draw down of carbon dioxide and atmospheric oxygen is created.  Note that while rock weathering can act as the Earth’s thermostat by controlling the amount of carbon dioxide in the atmosphere, it cannot create atmospheric oxygen.  We need life for that.  The free oxygen created an opportunity for heterotrophic bacteria and eukaryotes to exploit and they did.  It also relegated methanogens, the heroes of the Archean, to anoxic hideouts such as deep ocean sediment, swamps and cow stomachs. 

 The Neoproterozic rifting resulting in the breakup of the supercontinent Rodinia about 750 million years ago had the same effect.  This rifting forming the Iapetus Ocean is recorded in the geological record of Loudoun County [10].  The sun was much warmer now, about 94% of today’s sun and the Earth was kept warm by its blanket of carbon dioxide.  With the breakup of Rodinia, the events of the Paleoproterozic were repeated.  Increased weathering of the continents increased burial of both inorganic and organic carbon with a subsequent rise in atmospheric oxygen, this time from about 2% to 20% of the atmosphere.   Again the Earth’s climate oscillated between a snowball and a hothouse several times between 750 and 580 million years ago [4].  While Eukaryotes were certainly already around, it was this rise in oxygen, due to photosynthesis, which allowed the evolution and radiation of metazoans; complex life. 

So we are alive today, not just because of the other inhabitants of our biosphere, the Earth’s environment but we also owe a debt of gratitude to the biospheres in Earth’s past.

 Tony Noerpel

 [1] Vaclav Smil, Global Catastrophes and Trends, the next fifty years, 2008

 [2] http://brleader.com/?p=1414

[3] Kastings, J., “When Methane made Climate”, Scientific American, 2004.

[4] Papineau, D., “Global Biogeochemical Changes at Both Ends of the Proterozoic: Insights from Phosphorites,” Astrobiology, Vol 10, Number2, 2010.

[5] Konhauser, O., et al.  “Ocean nickel depletion and a methaogen famine before the Great Oxidation Event,” Nature vol 458, April 9, 2009.

 [6] http://brleader.com/?p=1217

[7] Gaines, S., Eglinton, G. and J. Rullkotter, Echoes of Life, Oxford, 2009.

[8] Plaxco, K., and Gross, M., Astrobiology, Johns Hopkins University Press, 2006.

 [9] Berner, R., The Phanerozic Carbon Cycle, Oxford University Press, 2004.

 [10] Southworth, S. et al.  Geologic map of Loudoun County, Virginia, U.S. Department of the Interior, to accompany map OF-99-150 U. S. Geological Survey.

 [11] Filippelli, G., “The global phosphorus cycle: past, present and future,” Elements, Vol. 4, pp 97-104, April, 2008.

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