Biogeochem - Chapt 5: Inorganic Carbon & Carbonates

Question 1

What is formed when carbon dioxide reacts with water?

Answer 1

Carbonic acid

Question 2

what is a pKa value?

Answer 2

a number that describes the acidity of a particular molecule

the lower the number, the stronger the acid and the greater its ability to donate protons

Question 3

Chemical formula for Carbonic Acid

Answer 3

H2CO3

Question 4

What does it mean if bicarbonate (HCO3-) has a pKa of 10.3

Answer 4

Means that at pH 10.3, 50% of the bicarbonate has become carbonate.

Question 5

What is carbonate most frequently present as in marine environments?

Answer 5

in marine environments and most freshwater lakes, it is present at bicarbonate

this is because at pH 8.3, only 1% of bicarbonate has become carbonate.

Question 6

what physiocochemical parameters do equilibrium (dissociation) constants depend on?

Answer 6

pressure
pH
temperature
salinity

Question 7

what is the solubility product?

Answer 7

the equilibrium constant for the dissolution of a solid substance into an aqueous solution.

Question 8

What concentration are solids given in the solubility product equation?

Answer 8

1

Question 9

What is one reason for the supersaturation of a solution

Answer 9

the lack of nuclei facilitating the formation of solids

Question 10

what is the Ion Activity Product (IAP)

Answer 10

the numerical product of ion activities in water.

Question 11

What solution is in a meta-stable state?

Answer 11

A supersaturated solution

Question 12

What can small changes to a supersaturated solution result in and what causes them?

Answer 12

can lead to significant precipitation

they are caused by slight changes in concentration (ie by biological processes like photosynthesis) or changes in temperature or pressure

Question 13

Give an example where temperature impacts the saturation of a lake

Answer 13

In the N and S Alps, lakes are supersaturated with respect to Calcium Carbonate.

on sunny days, the small increase in temperature in surface waters can increase the rate of photosynthesis, changing the concentrations of compouds in the carbonate system.

this causes the water to turn a turquoise colour and increase in turbidity due to the small calcite crystals in the water column

Question 14

Why does the removal of CO2 from equilibrium by primary production or decreased solubility of gases with increasing temperatures lead to the precipitation of calcite?

Answer 14

The production of CO2 from HCO3- requires H+

Bicarbonate is the main buffer in natural waters, releasing H+ in order to keep the pH. If bicarbonate releases a H+, it forms carbonate. If the waters are supersaturated for calcium carbonate, a small increase in carbonate concentration can lead to an abrupt precipitation of calcite (CaCO3)

this works in both directions… an increase in CO2 can cause a dissolution of CaCO3

Question 15

what happens if you remove CO2 from the system?

Answer 15

Equilibrium will try to be re-established by splitting carbonic acid

Question 16

What are the 3 steps that occur to re-establish equilibrium when CO2 is removed from the system

Answer 16

1. H2CO3 dissociates into H2O and CO2
2. H2CO3 is formed from a H+ and a HCO3-. These concentrations decrease
   - a decrease in H+ means increase in pH
3. As soon as protons are consumed or released, bicarbonate will either take it up or release a proton

Question 17

amphiprotic meaning

Answer 17

a proton-bearing molecule that is capable of donating its proton as well as accepting an additional proton.

Question 18

What are the conditions of lakes which are not buffered by bicarbonate (HCO3)

Answer 18

pH shows strong diurnal patterns with alkaline conditions (up to 11.5) during intense photosynthesis and almost neutral conditions (7.5-8) at night

Question 19

Give an example of a biogenic carbonate and what they do/form

(hint: vegetation)

Answer 19

Mosses can use an enzyme called carbonic anhydrase that consumes CO2 and precipitates calcium carbonate and is incrusted by travertine.

The mosses are responsible for building the dams. As the calcium carbonate became more stable it built up and up until it created the dams and blocked the river to create lakes

Question 20

Give an example of biogenic carbonates and what they do

(hint: algae)

Answer 20

Charophytes are multicellular algae that precipitate calcium carbonate. They are indicators of clean water

Question 21

What is the composition of dolomite?

Answer 21

Calcium - Magnesium Carbonate

Question 22

how do the sulphur reducing bacteria create dolomite

Answer 22

The bacteria consume formate (CHO2-) from fermentation products and organic assets and sulphate. They consume protons and create bicarbonate and sulphide.

The sulphide consumes protons while the bicarbonate produces the protons and precipitates as carbonate.

Question 23

Where is most of the carbonate precipitation presumed to take place?

(hint: water depth)

Answer 23

The production of biogenic carbonates are in shallow marine waters

Question 24

Give an example of marine biogenic carbonates?

Answer 24

reefs, atolls, platforms (tropical areas with water depths shallower than 50m) and shelves

Question 25

Examples of shallow water carbonate producers.

Answer 25

Photozoan association
- reefs and platforms
- they have green symbians which undergo photosynthesis

Halimeda bioherms
- thallus-forming algae with calcite crystals in its tissue

Heterozoan association
- coral purely heterotrophic, no symbionts
- live out of sunlights reach
- they filter the water
- cold water deep sea corals

Question 26

Why is calcium carbonate typically formed in the surface layers?

Answer 26

Due to the CO2 consumption

Question 27

Why is there not much carbonate in the deep sea?

Answer 27

The dominant algae are carbonate shelled algae. They have carbonate shells and when they die the carbonate sinks

Question 28

conditions in which the solubility product works

Answer 28

When the solubility product is reached, things are precipitated.

However, this only works if both partners are in the same concentration range

Question 29

Why is there not much carbonate in the deep sea?

Answer 29

The level of calcite supersaturation decreases.

At around 4000 meters depth, you reach the lysocline. Here, the saturation level for calcite matches its solubility product, causing calcite precipitates to start redissolving into calcium and carbonate ions.

At approximately 5000 meters depth, you reach the Calcite Compensation Depth. This is where calcium carbonate completely dissolves.

In the deep sea, there is more CO2 due to biological activity by heterotrophic organisms, but there is no photosynthesis to consume it.

Increased pressure in the deep sea increases the solubility of gases like CO2. This leads to a higher concentration of CO2 due to increased diffusion.
This disturbs the dynamic equilibrium, causing calcium carbonate to dissolve. In other words, the equilibrium equation shifts to the left.

Question 30

Why is there very little Red Clay sedimentation in the South Pacific?

Answer 30

You are far away from the land, where clay originates from, so very little sedimentation occurs.

Question 31

doline definition

Answer 31

a shallow usually funnel-shaped depression of the ground surface formed by solution in limestone regions

like a collapsed room in a cave

Question 32

What is the pH of rainwater?

Answer 32

5.5

Question 33

explain the formation of stalactites

Answer 33

Rain water is slightly acidic (pH of 5.5) due to CO2 in the atmosphere. The rain water percolates through the soil and takes up more CO2 due to the biological activity.

The water with a higher CO2 conc eventually reaches the limestone. The high CO2 levels cause the dissolving of limestone/calcium carbonate to occur

The water then travels through cracks and crevices until it reaches a cave opening/ceiling

The exposure to air causes the water supersaturated CO2 to release some into the atmosphere. This removal causes the precipitation of calcite, causing stalactites to form

Question 34

What are the biggest carbon reservoirs on the planet?

Answer 34

• Trapped organic carbon
• limestone and other fixed carbonates
• organic carbon in sediments and soils
• dissolved organics
• living biomass
• dissolved CO2
• atmospheric CO2