Lecture 08 – Lithosphere, Chemical Weathering, Carbon Cycle

Q: What is the lithosphere?

The lithosphere consists of the exposed parts of the Earth's crust; it interacts with the atmosphere and hydrosphere.

Q: What are the major elements in the lithosphere?

O (oxygen), Si (silicon), Al (aluminum), and also Fe, Ca, Na, Mg, and K.

Q: What percentage of the lithosphere do P, N, C, and S make up?

Less than 0.2%.

Q: What are the four main categories of rock types and minerals in the lithosphere?

Silicates, Carbonates, Sulfides, and Evaporites.

Q: What is the general formula for silicate minerals?

Cations–Si–O, for example (Ca,Mg)SiOn.

Q: What are the two main carbonate minerals and their formulas?

Calcite (CaCO3) and dolomite (CaMg(CO3)2).

Q: Which rock/mineral types consume carbon during weathering?

Silicates and carbonates.

Q: Which rock/mineral types do NOT consume carbon during weathering?

Sulfides and evaporites.

Q: What two broad types of weathering do exposed rocks undergo?

Physical weathering and chemical weathering.

Q: What acid primarily enhances chemical weathering?

Carbonic acid (formed from CO2 dissolving in water).

Q: How does the CO2 concentration in soil compare to the atmosphere?

CO2 concentration in soil can be 100 times greater than in the atmosphere.

Q: Why is there a significant supply of carbonic acid in soil seepage water?

Because the very high CO2 concentration in soil (100x atmospheric levels) dissolves into soil water, forming abundant carbonic acid that seeps down to the lithosphere.

Q: What does the formation of carbonic acid consume?

Atmospheric CO2.

Q: Besides carbonic acid, what other acids contribute to chemical weathering?

Organic acids (humic and fulvic acids) and sulfuric acid (H2SO4).

Q: What is congruent dissolution?

A type of chemical weathering in which the mineral dissolves completely into ions in solution (e.g., mafic silicates like olivine and amphibole dissolving entirely).

Q: What is incongruent dissolution?

A type of chemical weathering in which a mineral partially dissolves, releasing some ions into solution while leaving behind a secondary solid phase such as clay minerals (e.g., felsic silicate weathering).

Q: Give examples of mafic silicate minerals that undergo congruent dissolution.

Olivine and amphibole.

Q: What products are released during congruent dissolution of mafic silicates by carbonic acid?

Mg and silica (Si) in solution. Carbon from atmospheric CO2 is converted to HCO3− and exported with runoff.

Q: What products are released during incongruent dissolution of felsic silicates by carbonic acid?

Na and silica (Si) in solution. Carbon from atmospheric CO2 is converted to HCO3− and exported with runoff.

Q: What ions are released by silicate weathering reactions in general?

Na, Ca, K, Fe, Mg, and Si.

Q: Why is K concentration in rivers lower than expected from chemical weathering rates?

Because K is rapidly taken up by the biosphere.

Q: Why is Fe concentration in rivers lower than expected from chemical weathering rates?

Because Fe is rapidly oxidized and immobilized.

Q: What happens to Mg released during silicate weathering?

Mg either stays in solution or is incorporated into secondary clay minerals (incongruent dissolution).

Q: Which three ions (K, Fe, Mg) have lower-than-expected concentrations in rivers, and why?

K (rapidly taken up by biosphere), Fe (rapidly oxidized and immobilized), and Mg (partly incorporated into secondary clays). These processes remove them from solution.

Q: What does the dissolved load of river water from silicate weathering show?

A significant increase in Na, Ca, and HCO3− concentration, and an increase in Mg and K concentration.

Q: What is released during congruent dissolution of carbonates by carbonic acid?

High amounts of Ca (or Mg), and HCO3−, but no Na.

Q: How can you distinguish carbonate vs. silicate weathering contributions in river water?

Carbonate weathering releases Ca and HCO3− but no Na or Si. Silicate weathering releases HCO3−, Si, Na, and Ca. The presence of Na and Si indicates silicate weathering.

Q: In HCO3− produced by carbonate weathering, what fraction of the carbon comes from atmospheric CO2?

Only 50%. The other 50% comes from the carbonate minerals themselves.

Q: Why is the Ca and HCO3− concentration in river water higher than expected from silicate weathering alone?

Because carbonate dissolution also contributes Ca and HCO3− to rivers.

Q: What solutes does chemical weathering of carbonates supply to rivers?

HCO3− and Ca.

Q: What solutes does chemical weathering of silicates supply to rivers?

HCO3−, Si, Na, and Ca.

Q: Is silicate weathering fast or slow? What percentage of river solutes does it deliver?

Silicate weathering is slow; it delivers 3% to 40% of solutes in rivers.

Q: How does carbonate weathering compare to silicate weathering in terms of speed and influence?

Carbonate weathering is fast and influences all rivers, whereas silicate weathering is slow.

Q: What percentage of the dissolved load in rivers comes from atmospheric deposition (rain, snow)?

Less than 5%.

Q: Of the HCO3− in river water, what percentage is from soil/atmospheric CO2 and what percentage is from carbonate minerals?

67% is from soil/atmospheric CO2; 33% is from carbonate minerals.

Q: What three key solutes are produced by both silicate and carbonate weathering, and where are they transported?

Si, Ca, and HCO3−. They are transported via rivers to the oceans.

Q: What happens to the DIC (HCO3−) from carbonate weathering once it reaches the ocean?

It is incorporated into CaCO3 shells that sink to the ocean floor and form carbonate sediments.

Q: Does carbonate weathering result in long-term carbon storage? Why or why not?

No. There is no long-term (over 1 million years) storage effect for carbon from carbonate weathering because the carbon originally came from the carbonate mineral and is simply recycled back into carbonate sediments.

Q: How many atmospheric carbon atoms does silicate weathering consume?

Two atmospheric carbons (2 CO2 molecules are consumed).

Q: In silicate weathering, what happens to the two consumed atmospheric carbons?

One carbon is transferred to carbonate shells and deposited on the ocean floor. The other carbon is returned to the atmosphere.

Q: Does silicate weathering result in long-term removal of carbon from the atmosphere?

Yes. Silicate weathering results in long-term removal of carbon from the atmosphere on time scales of millions of years.

Q: On what time scale does silicate weathering remove carbon from the atmosphere?

On time scales of millions of years.

Q: Compare the long-term carbon cycle effects of silicate weathering vs. carbonate weathering.

Silicate weathering results in a net long-term removal of CO2 from the atmosphere (one C permanently stored on the ocean floor per weathering reaction). Carbonate weathering has no net long-term carbon storage effect; carbon is recycled between carbonate minerals and carbonate sediments.

Q: Where do acids for chemical weathering originate?

From the soil zone. CO2 in soil forms carbonic acid; organic matter produces humic and fulvic acids; and oxidation of sulfide minerals produces sulfuric acid.

Q: What is the relationship between carbonic acid formation and the carbon cycle?

The formation of carbonic acid consumes atmospheric CO2, linking chemical weathering to the carbon cycle. The consumed CO2 is converted to HCO3− during weathering reactions.

Q: What type of dissolution does Mg undergo during silicate weathering when it forms secondary clay minerals?

Incongruent dissolution (the original mineral does not fully dissolve; part of it is transformed into a new secondary mineral).

Q: Which spheres of the Earth system interact through chemical weathering and the carbon cycle?

The lithosphere, hydrosphere, and atmosphere.

Q: What form does carbon take when it is exported from weathering sites via runoff?

Bicarbonate (HCO3−), which is a form of dissolved inorganic carbon (DIC).

Q: What is the fate of Si, Ca, and HCO3− produced by weathering after they reach the ocean?

Ca and HCO3− are used by marine organisms to build CaCO3 shells, which sink and form carbonate sediments on the ocean floor. Si is used by organisms such as diatoms or remains dissolved.

Q: Why is silicate weathering considered the most important long-term regulator of atmospheric CO2?

Because it is the only weathering process that results in a net, permanent transfer of atmospheric carbon to ocean-floor carbonate sediments over millions of years.