"Dinosaurs and the history of life" V1001y The Long Term Carbon Cycle


HOMEWORK ASSIGNED MONDAY 3/01/04 - DUE MONDAY 3/08/04

INTRODUCTION

Below are the basic components of the carbon cycle. You do not have to memorize all the details. However, you should be able to think about how the components of the carbon cycle interact. The questions at the end of this outline will show you the level of understanding that you should have. Try to answer these questions and discuss them with your classmates and with the TAs. Go to the bottom of the text for the homework.

 
1) Substances

a) main active gasses
O2 - Oxygen: which we need to breath. It is a product of photosynthesis in green plants and results from the breakdown of carbon dioxide with the addition of energy in the form of light.

 CO2 - Carbon dioxide: which we exhale. It is a result of respiration (literally the burning of fuel - carbon plus oxygen to yield energy for life). It is the main greenhouse gas even though it comprises less than 0.1% (0.0370 %, actually) of the atmosphere. The more CO2 in the atmosphere, the less heat can be lost to space, and the warmer it must become.

 CH4 - Methane: a simple hydrocarbon which results from the fermentation process by many kinds of bacteria in the absence of oxygen. It is a powerful greenhouse gas 20 times more effective than CO2.

b) main rock types
 
Calcium Silicate: "CaSiO3", a major constituent of continental crust composed of calcium, silicon, and oxygen. It is the source of the calcium which combines with carbon dioxide in soil to yield calcium carbonate. 

Basalt: basically asthenosphere come to the surface along zones where new crust is formed; it is composed mostly of Magnesium, Iron, Aluminum, Silicon, and Oxygen. When it is extruded there is also the liberation of carbon dioxide and water vapor into the atmosphere. 

Calcium Carbonate: (CaCO3 - limestone when a rock). An important mineral composed of calcium, carbon, and oxygen. It forms from the combination of CO2 with calcium silicates in the weathering process of rocks. It is highly soluble in water and is easily transported from the site of weathering to the sea. In the sea it is absorbed by living organisms and secreted as the solid mineral in the form of shells, skeletons, or coatings, and deposited to form rock. Deposits of calcium carbonate in the ocean sediments are very important as a place where much of the carbon in the world resides (e.g. a carbon sink).

Carbon: (C - in the form of organic matter). More or less the uneaten and unrespired remains of green plants and bacteria. It can be in the form of coal, or more commonly fine particles. A great deal of carbon escapes to sediments in the sea in the form of actual particles of carbon-rich organic matter. The amount of organic carbon that get buried is, however, only a very small fraction of what is present as living tissue at any one time.


2) Main sub-systems  
The lithosphere plate cycle: The lithosphere consists of relatively rigid plates floating on a more plastic asthenosphere. Most of the oceans are underlain by relatively thin (0 - 75 km thick) "oceanic" lithosphere, while continental areas are comprised of relatively thicker (75-250 km thick) "continental" lithosphere. Plates are often composed of both areas of oceanic and continental lithosphere. All lithosphere is composed of two layers of differing composition. The upper layer is called the crust and is generally made mostly of magnesium, iron, silicon, and oxygen (basalt) on oceanic portions of plates, and is composed mostly of silicon, oxygen, iron and calcium on continental portions of plates. Calcium in continental crust is what is important for the carbon cycle. The lower layer of the lithosphere is made up primarily of magnesium, silicon, and oxygen and comprises the upper mantle.
 
The asthenosphere consists of the lower mantle which is made up more or less of the same material as the upper mantle, but it differs from it because it is more plastic (because it is hotter). The plates move around, causing the effect called "continental drift", and sediments accumulating on them move passively around with the plate. The plates bang into each other and one plate often sinks below the other (subduction) where it eventually melts. The subducting plate often carries with it sediment laid down in the oceans and the sediment too eventually melts. Some of the melted, subducting crust and attached sediment rises up like lava-lamp-goo up through the overriding plate and comes to the surface in an explosive volcano like that which forms Mt. St. Helens.

The lava from this melt of the subducting plate is rich in CO2, calcium and water vapor. The water vapor and CO2 (and other gases) tend to make this kind of volcanism explosive. When the lava, ash, and unerupted magma hardens, it adds to the continental crust, in part replacing the material which is lost from the continental crust by erosion. Because lithosphere, and hence crust, is continuously being lost by subduction, it must be replaced. The site of that replacement is along mid-ocean ridges where the asthenosphere makes it to the surface and spreads out. As it cools, it makes brand new oceanic crust, which moves away from the spreading ridge. This new crust makes up the deficit of crust lost by subduction. The cycle of the plates is like a conveyer belt moving sediment and crust from the spreading ridges in the oceans to the subduction zone where it gets recycled.
 
It is thought that the motive force driving the movement of the plates may be the result one of two sets of processes: 1) a pull on the plates that are being subducted coupled with a push at the mid-ocean ridges where new crust is being formed; or, 2) a drag beneath the plates caused by circulation (convection) of the asthenosphere. The former theory is most popular now.
 
 

The rock cycle: Both the collision of plates which produces mountains, and the subduction of plates adds to continental crust above sea level. Because it is above sea level, the continental crust can weather and yield material to streams and rivers which eventually make it to the oceans.

Weathering is the process by which rock is broken down to form soil and dissolved substances. For the carbon cycle, the most important products of weathering are particulate matter such as sand grains and clay, and most importantly, calcium carbonate derived from the combination of CO2 from the atmosphere (dissolved in water to form carbonic acid) plus calcium silicates (a major continental rock constituent).

Along with organic matter left over from uneaten plants, all of this material travels to the sea where the particulate matter drops down to the sea floor and produces sedimentary layers. The CaCO3 is sucked out of solution by marine invertebrates and made into their shells. The shells, along with the organic matter, get buried by successive layers of sediment. In this way CaCO3 and organic matter (mostly carbon) get incorporated into sediments in the sea floor and the contained carbon gets removed from the atmosphere.
 
Through the lithosphere plate cycle, the sea floor sediments either get subducted or get pushed up to form mountains. In any case, the result is that the material reappears to be weathered again.
 
 

The biosphere-atmosphere cycle: Green plants use the sunlight as a source of energy to convert CO2 to oxygen (O2) and organic carbon. Part of the latter is immediately burned for energy to run the plant, liberating CO2 again, and the rest is used by the plant to make plant tissue. Animals and decomposers, such as bacteria, use plant tissue as a source of energy by eating and "burning" which again results in using oxygen and releasing CO2. If consuming organisms were perfectly efficient, all of the carbon produced by plants would be consumed and put back into the atmosphere as CO2 and the cycle would be perfect. But the consumers are not that efficient and some of the plant tissue carbon escapes to be buried in the sea. Thus, carbon from the atmosphere gets buried in the oceans to return ultimately to the atmosphere via the rock cycle and the lithosphere plate cycle.
 
 

3) Connections between systems
  
a) The biosphere-atmosphere cycle is connected to the rock cycle by the roots of plants. Roots deliver CO2 deep into soil where its combines with water to make carbonic acid which attacks calcium silicate in rock to yield calcium carbonate and clay. The calcium carbonate can then be transported in solution to the oceans where it can be dumped out as limestone. Thus, carbon dioxide gets removed from the atmosphere by weathering of rock which is mediated by plant roots. The rate and density that plant roots penetrate soil is therefore one major control on weathering rates and the rate of CO2 removal from the atmosphere.

b) Another connection between the biosphere-atmosphere cycle and the rock cycle is through the production of plant tissue by the conversion of CO2 to carbon compounds using sunlight as an energy source. Plant matter that escapes being eaten, escapes to the oceans where it can be buried. Again, plants take CO2 from the atmosphere and deliver it to sediments in the sea. One kind of rock (coal) is made up entirely of plant matter which escaped being eaten.

c) The rock cycle and the lithosphere plate cycle are intimately connected, because, it is the latter which delivers material that was dumped out in the oceans back up to the surface where it can again be weathered. This happens either by the creation of mountain belts by the collision of the continental parts of plates, or by subduction which results in the melting of a plate with concomitant volcanism.

d) Volcanism connects the lithosphere plate cycle with the biosphere-atmosphere cycle. Volcanism due both to subduction and the creation of new oceanic crust at spreading centers results in the release of CO2 into the atmosphere, completing the cycle of carbon. It takes on average 60,000,000 years for carbon to get from plants to sediments in the oceans and back out into the atmosphere again.
 
 

4) Main things which can change over long periods of time
 
 
a) Rate of plate subduction and production: Fast rates of plate motion and plate production at spreading centers results in faster rates of recycling of oceanic sediments and a greater release of CO2 into the atmosphere. This is a very important control on the CO2 on the atmosphere and it seems independent of life, and is presumably controlled by processes deep within the earth.

b) Rate of weathering due to topographic changes: Mountain building creates large topographic gradients resulting in much greater mechanical weathering by running water. The products of this mechanical weathering can be broken down more quickly by weathering by plant roots than solid rock. Therefore times of mountain building should be times of greater loss of CO2 from the atmosphere and greater limestone deposition in the oceans. Many scientists think the uplift of the Himalayas create so much of this rock flower, that the global balance of CO2 consumption by weathering has been shifted to such high levels and that we have been drawn into the ice ages by a reduced green house effect.

c) Rate of weathering due to plant root penetration: The rate of weathering increases with an increases in the density and depth of penetration by roots. The evolution of new plant types thus has a major impact on rates of weathering. Imagine the Precambrian with no plants on land. Weathering must have been very slow. Today, there are diverse plants with deep roots almost everywhere and weathering rates are high. In between, there were probably in between rates of weathering. 

d) Rate of consumption of plants by herbivores: Forests first appeared in the Devonian about 370 million years ago. There were no large herbivores for another 80 to 100 million years which could eat the plants. Colonial insects specializing in eating plants did not appear until another 100 million years after that. The rate of consumption of plants by herbivores obviously controls how much plant matter escapes being eaten and hence how much could be buried. It is not surprising then, that the Carboniferous was the age of coal (thus the name), because although there were vast forests, there was virtually nothing to eat the plants! It is also not surprising that an Ice Age followed the coal age, since so much CO2 in the atmosphere ended up buried in coal. 

Herbivores also disrupt the ability for plants to weather rocks since they prevent plants from sending down as many roots as they might do if they were not being eaten. Herbivory thus also controls weathering by controlling roots. The more herbivores, the less weathering. The less weathering the less CO2 ends up as limestone in the oceans. 

Herbivores also directly change the atmosphere by adding methane (CH4) to it from the fermentation of plant matter in their guts.

Perhaps it is not surprising then that the age of the giant herbivores - the Mesozoic - would also be a hot house age. Note that this is still extremely speculative. 

e) Amount of sunlight reaching earth: It is probable, judging from the pattern observed in other stars, that our sun's output of light and heat has increased about 30% during the 3.5 billion years during which life has been on the earth. The record of life on earth and the geological record, however, strongly suggest that the temperature of the earth has varied by much less than it would if it passively responded to solar output.
 
 

One popular theory, called the Gaia hypothesis, is that life itself actively cooperates somehow in regulating the temperature of the earth though the systems we have described above. However, another theory argues that because chemical reactions operate faster under warmer conditions, there would be a negative feedback relationship between heat and the release of CO2 back to the atmosphere. If it is very warm because of high CO2, weathering rates are higher and more CO2 gets taken out of the atmosphere and delivered to the sea where it get buried as limestone. The effect would be to decrease CO2 in the atmosphere and cool the surface. Conversely, if it got too cool, weathering rates would be low and CO2 would accumulate in the atmosphere from volcanism. These theories are some of the hottest topics in paleobiology and geochemistry.
 
 

QUESTIONS FOR HOMEWORK ASSIGNED MONDAY 3/01/04 AND DUE MONDAY 3/08/04.
 
 

1. Describe some possible implications of the evolution of super-large herbivorous dinosaurs during the early Mesozoic.
 
 

2. How might our problem of global warming related to the evolution of dinosaurs?
 
 

3. What might the evolution of grasses (essentially super plants) during the middle Cenozoic have to do with the glacial ages of the last 3 million years?
 
 

4. What effect on global climate do you think the following biological events might have had?: 1) evolution of termites; 2) extinction of the dinosaurs; 3) evolution of man; 4) appearance of photosynthetic life; 5) evolution of land plants.