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reading: Chapter 4

Lecture 24. Forces Stabilizing Climate, Carbonate-Silicate Cycle.

Forces Stabilizing Climate

1. Presence of an atmosphereWhat happens on the Moon?How does an atmosphere stabilize climate during day and night?

What happens when you have a ticker atmosphere?What happens when you have a thinner atmosphere?

Seasons are Caused by Tilt (also called Obliquity)

ecliptic = plane of thesolar system

More solar radiation in the summerLess solar radiation in the winter

Earth’s tilt angle: 23.5˚

Forces Stabilizing Climate, cont.

2. Having a large Moon

Moon very large compared to the Earth and other Moons.The Moon stabilizes the tilt of the Earth.This stabilizes the seasons.

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Forces Stabilizing Climate, cont.

Mars:Currently Mars’ tilt is 25˚No large Moons (2 tiny Moons: Deimos and Phobos)Tilt is highly variable - chaotic.Tilt varies between 0-60˚ on timescales of tens of

thousands of years/millions of years

ecliptic = plane of thesolar system

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Mars at High and Low Tilt Angles

today

high tilt angle

low tilt angle

Forces Stabilizing Climate, cont.

3. The Carbonate-Silicate Cycle

Unique feature of the Earth -is a result of:

a. plate tectonicsb. volcanismc. having oceans

Where does most CO2 come from?

} having a geologically active planet

Plate Tectonics

Forming Carbonate Rocks

CO2 + H2O ---> H2CO3 (carbonic acid, dissolved in rainwaterand in the oceans)

Ca2+ + H2CO3 ---> CaCO3 + 2H+ (limestone + acid)

Where is most of the CO2 in the Earth?1x tiny bit in atmosphere (270 ppm)50x dissolved in the oceans30,000x deposited as carbonate rocks (sedimentary rock)1.7 millionx dissolved in the mantle

If we were to put all carbonate rocks into the atmosphere, wouldhave an atmosphere similar to Venus.

Silicate/Rock Weathering

CaSiO3 + 2H2CO3 ---> Ca2+ + 2HCO3- + SiO2 + H2O

silicate carbonic ions quartz rock acid

Another type of silicate rock:

2KAlSi3O8 + 2H2CO3 + 9H2O ---> 2K+ + 2HCO3- + 4H4SiO4 + Al2Si2O5(OH)4

feldspar carbonic ions quartz clay acid

Acid weathering of rocks produces ions (Ca2+, K+, Fe2+, Mg2+, etc.).Ions washed into rivers and the oceans (ocean salinity).Clays and quartz are produced.Ions combine with H2CO3 to produce limestone.

Forming Carbonate Rocks

CO2 + H2O ---> H2CO3 (carbonic acid, dissolved in rainwaterand in the oceans)

Ca2+ + H2CO3 ---> CaCO3 + 2H+ (limestone + acid)

Today:Much of the limestone is biogenic (coral reefs, shells)Some of the limestone is abiogenic.

Subduction

CaCO3 + SiO2 ---> CaSiO3 + CO2

limestone quartz silicate rock

metamorphic reaction: occurs at high T and P

The Carbonate-Silicate Cycle

1. CO2 outgassing from volcanos (greenhouse gas)2. CO2 dissolves in rain, lakes, streams, turns into carbonic acid3. Carbonic acid reacts with rocks, making ions, quartz, and clay4. Ions and dissolved CO2 reacts to make carbonate rocks5. Carbonate rocks are subducted6. Subducted carbonate rocks turned into CO2

How Does this Cycle Stabilize Climate?

CO2 + H2O ---> H2CO3 (carbonic acid, dissolved in rainwaterand in the oceans)

Ca2+ + H2CO3 ---> CaCO3 + 2H+ (limestone + acid)

At high temperatures, more limestone is precipitated.More CO2 dissolves in the oceans.This cools climate and lower temperatures.

} negative feedback loop

CO2 Constantly Replaced By Subduction

Earth’s Climate Is Obviously Not That Stable

1. The tilt undergoes precession (spinning like a top)Alters how much solar radiation each hemisphere

receives during summer and winter.Cycles: 20,000 years

2. There are small variation in the Earth’s tilt21.5 to 24.5˚Cycles: 40,000 years

3. Small variation (5%) in Earth’s eccentricityChanges the distance to the Sun, higher heating when closer.Cycles: 100,000 years

Milankovitch cycles - couldtrigger ice ages.

reading: Chapter 4

Lecture 25. Snowball Earth vs. Slushball Earth..