Biogeochem - Chapt 5: Inorganic Carbon & Carbonates Flashcards
What is formed when carbon dioxide reacts with water?
Carbonic acid
what is a pKa value?
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
Chemical formula for Carbonic Acid
H2CO3
What does it mean if bicarbonate (HCO3-) has a pKa of 10.3
Means that at pH 10.3, 50% of the bicarbonate has become carbonate.
What is carbonate most frequently present as in marine environments?
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.
what physiocochemical parameters do equilibrium (dissociation) constants depend on?
pressure
pH
temperature
salinity
what is the solubility product?
the equilibrium constant for the dissolution of a solid substance into an aqueous solution.
What concentration are solids given in the solubility product equation?
1
What is one reason for the supersaturation of a solution
the lack of nuclei facilitating the formation of solids
what is the Ion Activity Product (IAP)
the numerical product of ion activities in water.
What solution is in a meta-stable state?
A supersaturated solution
What can small changes to a supersaturated solution result in and what causes them?
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
Give an example where temperature impacts the saturation of a lake
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
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?
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
what happens if you remove CO2 from the system?
Equilibrium will try to be re-established by splitting carbonic acid
What are the 3 steps that occur to re-establish equilibrium when CO2 is removed from the system
- H2CO3 dissociates into H2O and CO2
- H2CO3 is formed from a H+ and a HCO3-. These concentrations decrease
- a decrease in H+ means increase in pH - As soon as protons are consumed or released, bicarbonate will either take it up or release a proton
amphiprotic meaning
a proton-bearing molecule that is capable of donating its proton as well as accepting an additional proton.
What are the conditions of lakes which are not buffered by bicarbonate (HCO3)
pH shows strong diurnal patterns with alkaline conditions (up to 11.5) during intense photosynthesis and almost neutral conditions (7.5-8) at night
Give an example of a biogenic carbonate and what they do/form
(hint: vegetation)
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
Give an example of biogenic carbonates and what they do
(hint: algae)
Charophytes are multicellular algae that precipitate calcium carbonate. They are indicators of clean water
What is the composition of dolomite?
Calcium - Magnesium Carbonate
how do the sulphur reducing bacteria create dolomite
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.
Where is most of the carbonate precipitation presumed to take place?
(hint: water depth)
The production of biogenic carbonates are in shallow marine waters
Give an example of marine biogenic carbonates?
reefs, atolls, platforms (tropical areas with water depths shallower than 50m) and shelves
Examples of shallow water carbonate producers.
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
Why is calcium carbonate typically formed in the surface layers?
Due to the CO2 consumption
Why is there not much carbonate in the deep sea?
The dominant algae are carbonate shelled algae. They have carbonate shells and when they die the carbonate sinks
conditions in which the solubility product works
When the solubility product is reached, things are precipitated.
However, this only works if both partners are in the same concentration range
Why is there not much carbonate in the deep sea?
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.
Why is there very little Red Clay sedimentation in the South Pacific?
You are far away from the land, where clay originates from, so very little sedimentation occurs.
doline definition
a shallow usually funnel-shaped depression of the ground surface formed by solution in limestone regions
like a collapsed room in a cave
What is the pH of rainwater?
5.5
explain the formation of stalactites
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
What are the biggest carbon reservoirs on the planet?
- Trapped organic carbon
- limestone and other fixed carbonates
- organic carbon in sediments and soils
- dissolved organics
- living biomass
- dissolved CO2
- atmospheric CO2
