Spring 2004
Lecture 16: April 6
What drives evolution?
Similarly, the evolution of the Earth itself has been influenced by internal and external processes. Earth receives external energy for its exospheric cycle (outside of Earth, including biosphere) from the Sun; the exosphere is also affected by such processes as meteorite impact. Internal energy (e.g., driving plate tectonic processes) comes from the energy of decay of radioctive elements inside the Earth (mainly U, Th, K), and only very little from original heat content of the Earth (from gravitational energy during formation of Earth from planetesimals).
We do not have direct observational evidence on the composition of
the early Earth atmosphere, but there was no
free oxygen around at 3.5 Ga and
now
we have 20.9 % free O2 (the rest is mainly free nitrogen,
N2, and traces of other gases such as Ar, CO2,
water vapor). Oxygen is the most abundant element in the Earth
(remember that the average Earth crust composition was given as
CaSiO3 in the 'Wilson
Cycle'), but because of its very
high affinity for electrons, it is usually present bound to other
elements, as O2- (hence the word "oxidation",
see
below). There is no free oxygen
present within the Earth, as shown by the absence of free oxygen in
volcanic gases.
Two main processes have generated free O2 over the
billions of years of Earth history
The latter process (much less important than the first on present Earth) occurs in the upper atmosphere under the influence of short-wavelength, high energy UV light. The H2 escapes to outer space because it is not well retained by the gravitational forces of Earth (mother gravity has a hard time keeping track of her little children!). If the atmosphere contains O2, the H2 vented from deep within the Earth reacts with O2 back to H2O in the atmosphere, as happens presently.
How much oxygen do we have in today's atmosphere, and where did it
come from?
Assume that there was no free atmospheric oxygen in early Precambrian times, as indicated by the presence of pyrite and uraninite sand, minerals that are unstable in the present O2-rich atmosphere (you can read a very simple description of these processes in web page for E&ES 104).
To estimate how many moles of oxygen are present in today's atmosphere (where they are 20.9% by volume), we need to know the extent of the atmosphere (top of atmosphere is gradual, but taken on average at about 20 km above the Earth's surface), and how many moles of oxygen there are per m3. We can then calculate the number of moles of oxygen per area (e.g., 1 m2 ) throughout the atmosphere, and multiply that number by the surface area of the Earth. Since the Earth is a sphere, a "column" of oxygen with a specific area at the Earth's surface has a slightly greater area at the top of the atmosphere, but that volume difference from an ideal 'column' is only about 1.3%.
We use the ideal gas law: PV=nRT , or n=P V/R T, in which n is the number of moles, P is the atmospheric pressure (which varies as a function of the distance above the Earth's surface, h), T is the temperature (which also varies as a function of the distance above the Earth's surface, h), R is the gas constant, n the number of moles. The latter dependency is called the lapse rate, and is on average about 6oC per km elevation. In order to obtain n, we have to integrate over h (between 0 and 20 km).
We will not go into the details of this operation, and just say that we can calculate that 20.9 volume % of O2 (=23.3 weight %) adds up to 3.7 x 1019 moles O2 present.
CO2 + H2O => CH2O + O2.
The modern production rate of O2 can be derived from the production of biomass, because each mole oxygen produced must mean one mole of carbon dioxide used and one mole of carbon put into organic carbon, and equals about 5 x 1015 moles/yr. This is the Net Primary Production of oxygen (NPP), which is much less than the Gross Primary Production (GPP), because plants also respire (the reverse of the photosynthetic reaction).
If we divide the total amount of free oxygen in the atmosphere by the annual production, we get the atmospheric residence time of oxygen, which is equal to the time needed to produce all oxygen in the atmosphere if the productivity was equal to that of today. All oxygen in the atmosphere could have been produced in 7800 years, very short as compared to the age of the Earth (~4.65 Ga).
Figure: diagram of the oxygen cycle
Obviously, apart from the source of O2, there must be sinks, and the most obvious one is the oxidation of young, just-produced organic matter (leaves grow in spring but rot in fall). The sink is not living matter: live biomass (dominated by biomass on land) contains only about 0.5 x 1017 moles of carbon and thus produced that same number of moles of free O2 (see photosynthetic reaction above). It is also not the total amount of 'dead' biomass present in soils and lying around the Earth's surface: about 2.5 x 1017 moles of carbon. (Note that moles of oxygen was expressed in units of 1019 , 100 times as much as 1017 ).
Plants grow, then die and rot away (reversal of the photosynthetic reaction). We can calculate the net flux of O2 to the atmosphere by taking the difference between biomass production and biomass destruction. The simplest way of doing that is calculating the organic carbon burial rate: CH2O that was formed, produced its oxygen, but then was extracted from the exosphere by burial in sediments (lithosphere).
The organic carbon burial rate in the present world is about 3 x 10-3 mole of C per m2 per year, which gives an annual flux of about 1 x 1012 moles globally. It would take about 37 million years to accumulate all the present oxygen in the atmosphere if this was the only mechanism, clearly still quite little as compared to the age of the Earth. Remember: every mole of oxygen in the atmosphere MUST have a counterpart mole of carbon buried in the lithosphere (rocks).
The next consideration: although carbon is buried in the sediments, sediments are not an eternal reservoir, because they are accreted onto the continents (plate tectonics again) and weathered, so that their organic carbon is once more oxidized and returns to the atmosphere as CO2. So there is a carbon burial rate as well as a carbon exhumation rate. The latter is very hard to constrain, so we wil not consider this factor; but is ha been guessed to be on the same order of magnitude as the carbon burial flux or larger by a factor of 10.
If we can not use the estimate of present rates because the data are
not good enough, we need to use something else: we can make a simple
mass balance argument. All atmospheric O2 has its
counterpart of organic carbon buried in sediments. We have to
calculate the average content of organic matter in all sediments of
all ages worldwide. This would then give us the minimum amount of
O2 that we expect to be in the atmosphere.
Fossil fuels are one form of buried carbon (coal, oil , gas), and their total reserves are estimated to be about 3.3 x 1017 moles of carbon (between 0.5 and 5 x 1017 moles. Note that the decimal point really has no significance, in view of the large uncertainties; what is important it the order of magnitude. By the way, if we were to burn all fossil fuel on Earth in one day, we will deplete the atmosphere's oxygen (look above for numbers) only by about 1%!.
Where is the rest of all that organic carbon, that must have been generated to account for the atmospheric oxygen? It is present as finely dispersed organic carbon (kerogen) in sedimentary rocks: fossil fuel generation is a very inefficient process!. The total amount of this kerogen is estimated at about 1.3 x 1021 moles of carbon.
But this number is much larger than
that for the moles of oxygen produced: we should have had much more
O2 in the atmosphere than we actually see (about 12.6 x
1020 moles more), to be the counterpart of all these
moles of kerogen. This is called the 'excess O2 '.
The estimate of the total amount of globally buried organic carbon is
probably of the right order of magnitude: if we take the modern
organic carbon burial rate we see that we could have generated all
that buried carbon in about 1 billion years).
Where has all that oxygen gone?
It must have oxidized something else than organic carbon, in so-called redox reactions: which are reactions with electron transfer: one or more electrons are taken away from an atom or ion and taken up by another. Taking electrons away is "oxidation", donating electrons is "reduction". How oxidizing an environment is depends on the type of oxidizer (how eager it is to "rob" electrons) and its concentration. When you balance a redox reaction equation, you have to take care that you not only have element balance on both sides of the equation, but you also must have charge balance. Common oxidizers are O => O2- and F = > F-, whereas organic carbon is a common reductor, C => C4+, as are iron (Fe2+ => Fe3+) and sulfur (S2- => S4+).
To find that something else, we have to look for abundant elements that can occur in multiple valency states, and which occur in reduced state in the mantle of the Earth. Sulfur (S) and iron (Fe) come to mind. If all mantle S occurred as S2-, then we need to consider the following reaction:
S2- + 2O2 => SO42- (we need 2 O2 for each mole of sulfur oxidized).
The total inventory of sulfate in the world is about 2 x 1020 moles, which would have consumed 4 x 1020 moles of O2. That is still not enough to explain all the excess O2.
The other abundant element is Fe, which is predominantly present as Fe2+ in the Earth's mantle (Fe3+/Fe2+ = 0.07 in basalts erupted at mid ocean ridges, the most common volcanic rock formed from the mantle, called Mid Ocean Ridge Basalts or MORBs). The iron in the basalts is oxidized by the oxygen in the cold sea water into which the lavas are erupted. The most common oxidized iron minerals are haematite (Fe2O3) and magnetite (Fe3O4, having 2 Fe3+ and 1 Fe2+).
We have to convert about 3.4 x 1021 moles of Fe2+ to Fe3+ to make up for the remainder of excess O2 (about 8.6 x 1020 moles) because each Fe2+ needs 1/4 O2 to turn into Fe3+ .We can then calculate how much basalt (volume) we would have to oxidize to get that amount of O2. If we use the modern surface area of the oceans, we this can calculate how thick a layer we would have had to oxidize.
But the ocean floor is continuously subducted, and it takes about 200 million year to renew the complete ocean floor. We have had (in 3.5 Ga) about 15 cycles of ocean floor renewal. We can calculate that the upper 200-300 m of basaltic ocean floor (out of a total thickness of 5-6 km) must have been oxidized over all these 15 cycles in order to have used up the required amount of oxygen. This is a reasonable thicknes of oxidized basalt, if we look at the processes in the present oceans.
The oxidized lavas form a slab of ocean floor sinking deep into the Earth's mantle during subduction. It is thought to sink deep into the mantle, and finally reach the core mantle boundary, at about 2900 km depth (this boundary is called the 'slab grave yard').
What would happen to the biota at high oxygen levels (see reading by Graham et al.)? Energy intensive life styles (at high metabolic rates) would become possible on land. It has been argued that the evolution of vertebrates on land was made possible by the occurrence of high levels of oxygen as well as low levels of CO2 in the atmosphere. Diffusion of oxygen into, and carbon dioxide out of, organisms works easier at higher concentrations of the first, lower of the second. Insects, which use tracheae (tubes) to let oxygen diffuse into their bodies, are limited in size because diffusion is efficient only over short distances. At higher oxygen concentrations in the atmosphere insects thus can grow larger, and fossil giant dragonflies (wing span up to 70 cm) and cockroaches are indeed known from the Carboniferous and Permian.