Notes

Outline

Lecture 9: Faint Young Sun Paradox  Carbon Cycle As a Thermostat for Earth  Chemical Weathering and Continental Drift  Why No Similar Thermostat on Venus and Mars

Faint Young Sun Paradox Solar luminosity was much weaker (~30%) in the early part of  Earth’s history (a faint young Sun). If Earth’s albedo and greenhouse effect remained unchanged at that time, Earth’s mean surface temperature would be well below the freezing point of water during a large portion of its 4.5 Byr history. That would result in a “snowball” Earth, which was not evident in geologic record.

Earth’s Thermostat – Chemical Weathering Chemical weathering acts as Earth’s thermostat  and regulate its long-term climate. This thermostat mechanism lies in two facts:       (1) the average global rate of chemical weathering depends on the state of Earth’s climate,       (2) weathering also has the capacity to alter that state by regulating the rate which CO2 is removed from the atmosphere.

Negative Feedback From Chemical Weathering The chemical weathering works as a negative feedback that moderates long-term climate change. This negative feedback mechanism links CO2 level in the atmosphere to the temperature and precipitation of the atmosphere. A warm and moist climate produces stronger chemical weathering to remove CO2 out of the atmosphere è smaller greenhouse effect and colder climate.

Carbon Cycling Between Atmosphere and Earth

Plate Tectonics  Continental Drifting Theory     Alfred Wegener proposed that all the continents were once assembled into a supercontinent (Pangea) and then broke and slowly drifted to their current positions.  Plate Tectonics     The branch of tectonics that deals with the processes by which the lithosphere plates move and interact with each other is called plate tectonics.

Circulation of the Solid Earth

Twenty Rigid Plates What can happen to the cold boundary? The lithosphere has broken into a number of rocky pieces, called plates. There are six large plates plus a number of smaller one comprise the Earth’s surface (a total of 20 plates). The plates range from several hundred to several thousand kilometers in width.

The Theory of Plate Tectonics  A major problem of the continent drifting theory is: How could the continents drift through the rigid sea floor?  This problem is answered by the seafloor spreading hypothesis: Continents do not plow through the sea floor. Continents and segments of ocean floor are connected into plates that continuously move away from one another at mid-ocean ridges.

Seafloor Spreading

Plate Margins Interactions between plates occur along their edges. There are three types of plate margins: Divergent margins        form mid-ocean ridges (over oceans) and rift valleys (over lands) (2)  Convergent margins        form deep-sea trenches (two oceanic plates or ocean+continental plates) or high mountains (such as Tibetan Plateau) (two continental plates). Transform fault margins        form earthquake faults

Tectonic Control of CO2 Input – The Seafloor Spreading Rate Hypothesis During active plate tectonic processes, carbon cycles constantly between Earth’s interior and its surface. The carbon moves from deep rock reservoirs to the surface mainly as CO2 gas associated with volcanic activity along the margins of Earth’s tectonic plates. The centerpiece of the seafloor spreading hypothesis is the concept that changes in the rate of seafloor spreading over millions of years control the rate of delivery of CO2 to the atmosphere from the large rock reservoir of carbon, with the resulting changes in atmospheric CO2 concentrations controlling Earth’s climate.

Why No Thermostat on Mars?  Mars is too small in size Mars had no large internal heat Mars lost all the internal heat quickly No tectonic activity on Mars Carbon can not be injected back to the atmosphere Little greenhouse effect A very cold Mars!!

Why No Thermostat On Venus?  Venus is too close to the Sun  Venus temperature is very high  Very difficult for Venus’s atmosphere to get  saturated  Evaporation keep on bringing water vapor into Venus’s atmosphere  Greenhouse effect is very large  A “run away” greenhouse happened on Venus  Water vapor is dissociated into hydrogen and oxygen  Hydrogen then escaped to space and oxygen reacted with carbon to form carbon dioxide  No water left on Venus (and no more chemical weathering)