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Lecture 6

Earth's Cycles

There is a small homework project assigned for this lecture.

I. We have seen how a few principles could allow us to deduce history from geometry, fossils, and radiometric dates; how evolution works, and how we can assess the relationships of organisms.

II. So, we see that organisms have evolved and diversified and that indeed produces the order in the fossil record, which we can use to deduce relative time.

Much of the rest of the course will deal with the relationships of organisms, notably dinosaurs.

But organisms also interact with each other and with their surrounding environment. This aspect, which is ecology, has a very special set of effects.

IV. What is Life?

What special properties does life have? 1. Life is active chemical system separated from its surroundings and out of chemical equilibrium with its surroundings. Chemical equilibrium = death. 2. Life reproduces with heredity. 3. There is variation in that heredity. 4. Living things grow and develop. 5. There is evolution of populations, adaptation, and innovation. 6. There are species that are reproductively isolated from other species and on their own evolutionary trajectories. 7. Species make societies of kin, and parasitic to symbiotic relationships with other species.

These properties of life produce the world as we now know it. As we shall see, the composition of the atmosphere, and our climate, and perhaps even the structure of the Earth's surface itself is due to life processes within the Earth System.

V. FIRST, LET'S LOOK AGAIN AT HUTTON'S CYCLIC VIEW OF THE EARTH - Largely correct, he postulated the Rock Cycle. This cycle produces the major rock types: 1). Sedimentary Rocks - Rocks formed from sediment, precipitation from solution, or the products of organisms. Examples: sandstone, rock salt, coal. 2). Igneous Rocks - Rocks formed from hot molten material. Examples: basalt, diabase, granite. 3). Metamorphic Rocks - Rocks formed by the profound alteration of other rocks by the effects of heat and pressure. Examples: marble, slate, schist.

VI. We now know that the reason that the rock cycle happens as a cycle - indefinitely - is because of "plate tectonics". Plate tectonics are a consequence of the existence of large sheets or "plates" of solid rock crust, more-or-less floating on a more liquid or plastic mantle. As a consequence of deep movements within the mantle the plates move, separate from each other and crunch into each other. The overall structure of the earth is to have this thin rind, the crust, under which is the more liquid mantle of hot silicate rock, followd by a liquid iron outer core and then a solid iron inner core at the Earth's center. We know about the Earth's interior, not through direct sampling, but rather by inferring its structure mostly by the way earthquake waves behave when passing through the earth. Here is an easy way to remember some of the basic dimensions of the Earth - thanks to Enrico Fermi. If we look in detail at a near-surface cross section of the Earth, we see that the crust is not everywhere of the same thickness.

In the above diagram; 1 is asthenospheric mantle, 2 is lithospheric mantle, 3 is continental crust (mostly granite in composition), 4 is oceanic crust (mostly basalt), 5 is sediment in the ocean basins, 6 is erupting igneous material (lava when it is at the surface), and 7 is ocean water. A, B, C, and D show the direction of plate motion. C is an oceanic rifting zone where new oceanic crust is created by upwelling asthenospheric mantle which reaches the surface and turns horizontally. An example is the Mid-Atlantic Ridge. D is a subduction zone, where old, cold crust sinks below an overriding plate. Erosion and sedimentation (E & S) transfers sediments weathered from the continental crust to the ocean basins where it accumulates. At D, the old crust and the sediments that were deposited are being subducted. As they sink they heat up and eventually melt. The melt contains a lot of water and gas and rises to the surface to erupt as volcanoes. Mt. Fuji in Japan is such a volcano riding above the subducting Pacific plate. Another example is Mt. St Helens. Some material gets scraped off by the overriding plate and added to the continental crust in mountains that bears the volcano. The cycle of the creation of new oceanic crust at C (a so-called spreading ridge) and addition to continental crust by scraping off material from old oceanic crust at the subduction zones (above D) and that added to continental crust by subduction-produced igneous material (6 at D) about balances the loss of crust by subduction. The movement of the plates themelves is thought to be due to circulation of the asthenosphere and/or pull of the sinking, subducting plates.

The motion of the plates has occured throughout earth history and changed the configuration of the continents and oceans though time.

VII. Thus the plate tectonic cycle is responsible for the Rock Cycle and is why it makes sense.

SEDIMENTARY ROCKS

- are produced by the burial of material eroded from the continents

- are produced either by mantle upwelling, such as at spreading ridges, or by subduction-related volcanos

- are produced mostly by the heating and deformation of sedimentary or igneous rocks where subduction is taking place, or where burial depths get very great

The Atmosphere consists of:

Mostly Nitrogen = N2 (78%)
Second Oxygen = O2 (21%)
Third Water Vapor = H2O (<1% to 4%)
Fourth Argon = Ar (1%)
Fifth Carbon Dioxide = CO2 (<0.1%) (0.0353%) now
Sixth Methane = CH4 (1.7 ppm = 0.00017%)

1. O2 we need to breathe
2. CO2 plants need

CO2 is a greenhouse gas

IX. WHERE DOES THE O2 COME FROM AND WHAT CONTROLS ATMOSPHERIC CO2 ??

Plants through photosynthesis convert CO2 to C in Carbohydrates (CH2O)

CO2 + H2O + E = CH2O + O2 (where E is energy from light)

the reverse process is respiration: we eat carbohydrates and use O2 to get energy

CH2O + O2 = CO2 + H2O + E

Photosynthesis on land and in the water is important.

X. CO2 IS ALSO CONSUMED BY THE WEATHERING PROCESS:

Water reacts with CO2 to produce carbonic acid. Carbonic acid reacts with Calcium and Magnesium silicate rocks to yield Calcium Carbonate (CaCO3 - limestone!) (or Magnesium Carbonate = MgCO3 = Dolomite) and Silicon oxide (SiO2 - quartz!).

CO2 + CaSiO3 = CaCO3 + SiO2 or

CO2 + MgSiO3 = MgCO3 + SiO2

These carbonates and the quartz get buried as sediments, mostly in the ocean basins.

But where does the CO2 come from in the first place?

It comes from the lava and gasses coming up at spreading ridges and subduction zones.

In fact most of the CO2 at subduction zones comes from the metamorphism of the very materials that consumed the CO2 in the first place - Carbonates reacting with Quartz, for example:

CaCO3 + SiO2 = CO2 + CaSiO3

The Calcium Silicates (CaSiO3) come back out as lava adding to continental crust, and the CO2 comes out of the volcanos as explosive gas!

We can see this by adding the Carbon cycle to the plate tectonic diagram:

In the above diagram, Corg is organic carbon, primarily the breakdown products of carbohydrates produced by photosynthesis.

THUS, THE ATMOSPHERIC CO2 IS JUST WHAT REMAINS BETWEEN THE OUTGASSING CO2 FROM IGNEOUS AND METAMORPHIC SOURCES AND CONSUMPTION OF CO2 BY PHOTOSYNTHESIS AND WEATHERING.

XI. IT IS SENSITIVE TO THE RATE AT WHICH WEATHERING OCCURS. WHAT KEEPS THE SYSTEM STABLE?

The long term carbon cycle is a system with built-in feedback mechanisms. One that has long been proposed is quite simple.

Suppose the plate tectonic cycle sped up so that more CO2 was being released into the atmosphere? If weathering rates and photosynthesis remained constant, CO2 would "pile up" in the atmosphere. But CO2 is a green house gas - the more CO2 there is the more heat gets trapped in the atmosphere and the warmer it gets.

Chemical reactions, however, run faster as it gets warmer.

So, as it gets warmer, the chemical reactions of weathering increase and more CO2 gets consumed, so lower atmospheric CO2 levels result. It is a negative feedback.

Suppose, now, that the plate tectonic system slows down so very little CO2 is added to the atmosphere; what happens?

As it gets colder because of the reduced CO2 in the atmosphere, the chemical weathering reactions slow down.

CO2 now "piles up" because its consumption has dropped.

If photosynthesis increased wildly for some reason, O2 would go up because more organic carbon would be buried.

But if O2 goes up things burn easily (forests are close to self-imolation all the time now). The increase in forest fires would decrease photosynthesis.

Thus the more land plant photosynthesis there is, the faster the weathering - all again subject to negative feedback. If too much CO2 is removed it gets too cold for plants to live, plant-mediated weather decreases and the CO2 can build up again.

Doesn't this suggest that whatever affects the long-term distribution of plants will also effect atmospheric CO2? - it sure does, as we will see in subsequent lectures.

OBVIOUSLY LIFE IS CRITICAL TO THE MAINTENANCE OF OUR ATMOSPHERE

Now we can seen what I mean by life being an active chemical system separated from its surroundings and out of chemical equilibrium with its surroundings.

If there were no photosynthesis, all of the O2 would combine with reactive minerals and organic matter. CO2 would build up in the atmophere in exact proportion to the lack of carbon burial. We would end up with a super-greenhouse effect and end up more like Venus.

Our own Greenhouse problem from the burning of fossil fuels has a quite interesting relationship to the long term carbon cycle.

Presently nearly all of the organic carbon produced by photosynthetic organisms gets respired by animals, fungi, and microbes. Only the tiniest bit gets buried. But this is enough not only to maintain the O2 levels at a more or less steady state, but over geological time it is directly responsible for the formation of fossil fuels.

EVERY BIT OF OIL, GAS, AND COAL IN THE WORLD IS THE RESULT OF TINY AMOUNTS OF ORGANIC CARBON BEING BURIED BY ESCAPING BIOLOGICAL RESPIRATION.

We are increasing CO2 in the atmosphere by burning the accumulated carbon of 3.5 billion years of Earth's history. This is resulting in an increase in global temperature called " global warming". There is still much debate however.

GO TO THE LONG TERM CARBON CYCLE FOR A MORE THROUGH DISCUSSION OF THESE PRINCIPLES. I recommend that you surf the web for more on these subjects!

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