Department of Earth Sciences
Indian Institute of Technology-Kanpur
Course ES451A-Environmental Geology
Lecture 4
(Fundamental concept #3 – Earth as a
System – contd.)
Evolution of atmospheric oxygen
- Third most abundant element in the universe (after H and He); second
most abundant on Earth (after Fe); most abundant in Earth Crust (~47%)
- Very little oxygen occurs in the free (elemental) form - as O2
- Anoxic conditions prevailed during the first 2 billion years (Ga)
- Anoxygenic photosynthesis began ~3.4 Ga
- Oxygenic photosynthesis ~2.7 Ga. O2 produced initially used to oxidize
reduced gases (e.g. H2) and other reduced species
- Until ¬0.85 Ga atmospheric O2 content remained low – Boring Billion –
after which it increased rapidly
Oxygenic
photosynthesis
Anoxygenic BORING
photosynthesis BILLION
Time (billion years before present)
Oxygen sources and sinks
- By product of (oxygenic) photosynthesis
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CO2 + H2O CH2O + O2
Chlorophyll
- Consumed during aerobic respiration
CH2O + O2 CO2 + H2O
Also for oxidation of other reduced chemical species
2H2 + O2 2H2O
2H2S + 5O2 + 4H+ 2H2SO4 + 2H2O
4Fe2+ + O2 + 4H+ 4Fe3+ + 2H2O
NH4+ + 2O2 NO3- + H2O + 2H+
Atmospheric temperature and CO2 contents
- CO2 the most important https://en.wikipedia.org/wiki/Geologic_temperature_record
greenhouse gas; Variability
similar to that of temperature
- Atmospheric concentration
determined by the balance
between carbon sequestration
(plant uptake, sedimentary
burial, weathering) and
emission (volcanic activity and A B C D
decomposition)
(A) Very low CO2 and
atmospheric temperature Long-term decrease due to chemical
weathering
during Carboniferous-Early CaSiO3+H2O+2CO2Ca(HCO3)2+SiO2
Permian were caused by carbon
removal as coal deposits
(B) Paleocene-Eocene Thermal
Maximum (PETM) ~55 Ma due
to volcanism
(C) Orbitally-forced glacial- https://e360.yale.edu/features/how-the-world-passed-a-carbon-
interglacial cycles threshold-400ppm-and-why-it-matters
(D) Steep increases in Dynamic system: Steady state where inputs equal
temperature and CO2 due to
human activities outputs achieved only on short time scales
Role of Life: The Gaia Hypothesis
Earth is like an organism with the ocean serving as its heart and
forests its lungs (James Hutton, 1785).
Idea revived by Lovelock and Margulis (1974) who proposed the
Gaia Hypothesis according to which Earth and its biological
systems behave as a huge single entity. This entity has closely
controlled self-regulatory negative feedback loops that keep the
conditions on the planet within boundaries that are favorable to
life.
Does life consciously control Earth’s living environment?
Humans dominate Earth today, but have no control on processes
such as plate tectonics, volcanism, etc. – key contributors to the
evolution of the Earth’s environment. The Earth will outlive Homo
sapiens, and the human perturbations will be eventually mitigated.
