Biogeochemistry - Inorganic Carbon

What reaction forms bicarbonate from CO2 in water?

CO2(aq) + H2O ⇌ HCO3− + H+.

What reaction forms carbonate from bicarbonate?

HCO3− ⇌ CO32− + H+.

Dominant inorganic carbon species at pH ~8:

Bicarbonate (HCO3−).

Why rivers have different ions than seawater?

Weathering releases HCO3−, Ca2+, Si; seawater accumulates conservative ions like Na+, Cl−.

Definition of residence time?

Reservoir size divided by input/output rate.

Total alkalinity definition:

TA = 2[CO32−] + [HCO3−] + [B(OH)4−] + [OH−] − [H+].

Dissolved inorganic carbon (DIC) definition:

DIC = [CO2] + [H2CO3] + [HCO3−] + [CO32−].

What DIC measures:

Total inorganic carbon regardless of charge.

What a buffer is:

A system that resists pH change by consuming or releasing H+.

Why carbonate system buffers pH:

CO2, HCO3−, and CO32− interconvert to absorb or release H+.

Effect of photosynthesis on DIC and alkalinity:

Removes CO2 → decreases DIC; alkalinity unchanged.

Effect of respiration on DIC and alkalinity:

Produces CO2 → increases DIC; alkalinity unchanged.

Effect of CaCO3 precipitation on alkalinity:

Decreases alkalinity by 2 equivalents.

Effect of CaCO3 dissolution on alkalinity:

Increases alkalinity by 2 equivalents.

Why activity differs from concentration:

Activity accounts for non‑ideal behaviour; always ≤ 1.

Ion activity product (IAP) definition:

IAP = a(Ca2^+) × a(CO3^2−).

Saturation ratio (SR) definition:

SR = IAP / Ksp.

Saturation index (SI) definition:

SI = log10(SR).

Why calcite forms more easily than aragonite?

Calcite has lower Ksp (less soluble).

What is the silicate weathering feedback?

A long‑term negative feedback where CO2 enhances weathering, removing CO2 from the atmosphere.

Role of carbonic acid in weathering:

CO2 + H2O forms carbonic acid, which dissolves silicate minerals.

Main reaction removing CO2 via silicate weathering:

CaSiO3 + 2CO2 + H2O → CaCO3 + SiO2 + CO2 + H2O (net CO2 removal).

Why silicate weathering is temperature‑dependent?

Higher temperatures increase reaction rates and rainfall acidity.

Why continental crust matters:

It is rich in feldspars, which weather to release Ca2+ and HCO3−.

Source of bicarbonate in rivers:

Atmospheric CO2 dissolved in rainwater.

Role of organisms in CaCO3 formation:

Biomineralizers (coccolithophores, foraminifera) precipitate CaCO3 in oceans.

Effect of increased atmospheric CO2 on weathering:

More acidic rain → more weathering → more Ca2+ and HCO3− → more CaCO3 burial.

Why this is a negative feedback:

Higher CO2 increases weathering, which removes CO2 and stabilises climate.

Timescale of silicate weathering:

100,000 to millions of years.

Timescale of volcanic CO2 outgassing:

Millions to 100 million years.

Why organic carbon burial is not the main climate stabiliser:

Not strongly temperature‑dependent.

What happens to ocean DIC when CaCO3 burial increases:

DIC decreases, lowering CO2(aq) and atmospheric CO2.

What controls ocean pH long‑term:

Carbonate equilibria and CaCO3 burial/dissolution.