Week 11 - Carbon Cycle: Carbonates Flashcards
(18 cards)
What is the carbon cycle and its role in climate?
It’s the system of processes that regulate atmospheric carbon and climate, working with or against solar radiation changes.
What are the two parts of the carbon cycle and how do they differ?
Deep & slow cycle: Carbon stored in crust and mantle; huge reservoirs, tiny fluxes; stabilizes climate over millions of years.
Shallow & fast cycle: Carbon exchanges between ocean, atmosphere, and ecosystems; smaller reservoirs, fast fluxes; affects short-term climate.
What are the main processes that move carbon through the carbon cycle?
Volcanism: Releases CO₂ from mantle to atmosphere via eruptions; acts as a carbon source for the sulfur cycle.
Rock weathering: Chemical weathering of silicate/carbonate rocks absorbs CO₂, sequesters carbon, and helps stabilize climate.
Subduction & lithification: Tectonic processes subduct carbon into the mantle, locking it in rocks over geological timescales.
Burial: Marine organism carbon is deposited in sediments, slowly removing it from the fast cycle into long-term storage.
What are the key components of the shallow and fast carbon cycle, and how do they exchange carbon?
Atmosphere: Central hub for exchange; gains CO₂ via volcanism, loses it via photosynthesis and weathering.
Terrestrial ecosystems: Includes vegetation, soils, and organic matter; exchange carbon through photosynthesis and respiration.
Active reservoir for carbon cycling.
Shallow ocean: CO₂ dissolves in via gas exchange; carbon exits via sinking and mixing to deeper layers.
Deep ocean: Largest active reservoir in this cycle; receives carbon by sinking, mixing, and stores it via burial in marine sediments.
Rocks & fossil fuels: Gain carbon via subduction and lithification; release it through volcanism.
Note:
This fast cycle drives climate variability on short timescales.
Deep ocean: Although it’s deep, it’s still part of the fast cycle because it exchanges carbon on timescales of decades to centuries through ocean circulation — not millions of years like the mantle or rocks.
What are the main components of the slow and deep carbon cycle?
Rocks: Largest carbon reservoir (includes fossil fuels, carbonates, and carbon in crust/mantle); stores carbon for millions of years.
Mantle: Receives carbon via subduction; releases it via volcanism.
Marine sediments: Site of long-term carbon burial; carbon here can eventually form sedimentary rock.
The deep ocean stores carbon temporarily in the fast cycle. When carbon sinks and is buried in marine sediments, it exits the fast cycle and enters the slow cycle, eventually becoming part of rocks. Sediment burial links the fast and slow carbon cycles.
Note: These components cycle carbon slowly but play a major role in stabilizing climate over geological timescales.
How does carbon behave in the deep ocean, and what role does it play in the carbon cycle?
1.Carbon enters the deep ocean through:
-Physical mixing: Cold, dense surface waters rich in dissolved CO₂ sink (especially near poles) and carry carbon into the deep ocean (called the solubility pump).
-Biological sinking: Marine organisms fix carbon via photosynthesis → when they die or excrete waste, some of it sinks (the biological pump).
- While in the deep ocean:
-Much of the carbon remains dissolved in deep waters for decades to centuries.
-Some of the sinking organic carbon is decomposed and recycled back into CO₂, which may later upwell to the surface.
-A small fraction is buried in marine sediments — this is the gateway into the slow carbon cycle. - Why it matters:
-The deep ocean acts like a temporary storage buffer, pulling carbon out of the atmosphere and keeping it from rapidly contributing to climate change.
-This buffering effect helps regulate atmospheric CO₂ on human-relevant timescales.
How does atmospheric CO₂ get converted into calcium carbonate, and why is it important in the carbon cycle?
Hydration of aqueous CO₂.
- When atmospheric CO₂ dissolves in water (rain, oceans), a portion of it reacts w water molecules to form carbonic acid (H₂CO₃):
CO₂ + H₂O → H₂CO₃
(reversible reaction, could decompose back into CO2 and water)
- This weak acid reacts with rocks in two main weathering processes:
-Silicate weathering:
H₂CO₃ + silicate minerals → HCO₃⁻ + Ca²⁺/Mg²⁺ (dissolved ions)
-Carbonate weathering
H₂CO₃ + carbonate minerals → HCO₃⁻ + CO₃²⁻
Result: A solution rich in ions
-Bicarbonate (HCO₃⁻)
-Carbonate (CO₃²⁻)
-Calcium (Ca²⁺) and Magnesium (Mg²⁺)
- These ions (bicarbonate and carbonate) stay in solution and eventually combine with calcium ions released (Ca²⁺) to precipitate as solid calcium carbonate (CaCO₃).
-Marine organisms use this to build shells and skeletons, i.e coral reefs, marine shells (biogenic structures)
-When they die, CaCO₃ can accumulate as limestone or marine sediments (long-term carbon storage)
-This entire process helps remove CO₂ from the atmosphere and store it long-term in geological formations.
Plays foundational role in:
-Buffering pH –> Acid-base chemistry in natural waters
-Carbon transport in the ocean and atmosphere
-Support of marine ecosystems
-Note: The extent of this process depends on the equilibrium between CO₂ and H₂O.
What law governs the dissolution of CO₂ in water, and what factors affect it?
Henry’s Law:
[CO₂] = kH × pCO₂
where:
[CO2] = concentration of dissolved CO₂ in water
kH = Henry’s constant (gas-specific, temperature-dependent)
pCO2 = partial pressure of CO₂ in the atmosphere
-As temperature increases, kH decreases, causing CO₂ solubility to decrease (less dissolved). This means less CO2 is being removed from the atmosphere.
-Changes in temperature or pCO2 affect ocean CO₂ uptake and impact the global carbon cycle
What is the deprotonation of carbonic acid, and why is it important?
Carbonic acid (H₂CO₃) dissociates into a hydrogen ion (H⁺) and a bicarbonate ion (HCO₃⁻):
H₂CO₃ ⇌ H⁺ + HCO₃⁻
-H⁺ increases acidity of the solution
-HCO₃⁻ (bicarbonate ion) is a key intermediate carbon species in the carbon cycle
-The equilibrium constant (K₁) = [H⁺][HCO₃⁻] / [H₂CO₃]
-A higher K₁ means more dissociation and greater acidity (higher numerator)
What is the deprotonation of bicarbonate (HCO₃⁻), and what does it form?
Bicarbonate (HCO₃⁻) loses a proton (H⁺) to form carbonate ion (CO₃²⁻):
HCO₃⁻ ⇌ H⁺ + CO₃²⁻
H⁺ contributes to water acidification
CO₃²⁻ (carbonate ion) is the most stable, fully deprotonated form of carbon in water
The equilibrium constant (K₂) governs this reaction — higher K₂ → more CO₃²⁻ formation
What is dissolved Inorganic Carbon (DIC) the total of:?
Dissolved Inorganic Carbon (DIC) is the total of:
DIC = [H₂CO₃] + [HCO₃⁻] + [CO₃²⁻]
DIC = carbonic acid + bicarbonate ion + carbonate ion
How do the reactions between CO₂, H₂CO₃ (carbonic acid), HCO₃⁻ (bicarbonate ion), and CO₃²⁻ (carbonate ion) relate to the carbon cycle?
These reactions control how atmospheric CO₂ is:
-Absorbed by oceans
-Converted into dissolved forms (H₂CO₃, HCO₃⁻, CO₃²⁻)
-Buffered in ocean water, influencing pH
-Stored long-term as calcium carbonate (CaCO₃) in shells and sediments
How does ocean acidification affect calcium carbonate formation?
More H⁺ ions react with carbonate (CO₃²⁻), converting it to bicarbonate (HCO₃⁻). This reduces carbonate availability for combining with calcium (Ca²⁺), making it harder for marine organisms to form shells and skeletons.
Shell formation is integral because when they die, shells sink and may be buried in sediments, storing carbon in the deep ocean and eventually forming rocks — a key step in long-term carbon sequestration.
Why does water become more acidic as CO₂ dissolves?
The key reactions:
- Hydration of CO₂ (not acidic yet, but sets the stage):
CO₂ + H₂O ⇌ H₂CO₃ - First deprotonation — this is where acidity starts:
H₂CO₃ ⇌ H⁺ + HCO₃⁻ (bicarbonate)
→ Adds H⁺ → lowers pH (more acidic) - Second deprotonation — adds even more H⁺:
HCO₃⁻ ⇌ H⁺ + CO₃²⁻ (carbonate)
→ Also contributes to acidity, but to a lesser extent because this step occurs only significantly at higher pH levels (more basic conditions).
In acidic conditions (more H⁺):
-There are already a lot of hydrogen ions in the water.
-So, either the second deprontonation reaction wont happen much at all to favour equilibrium, or
-If it does happen, the H⁺ and CO₃²⁻ that are formed will quickly recombine to form HCO₃⁻ (bicarbonate) again
This is why, in more acidic water:
-Bicarbonate (HCO₃⁻) becomes the dominant form of dissolved inorganic carbon.
-Carbonate ions (CO₃²⁻) become much less abundant.
What does a Bjerrum plot represent in the context of the carbon cycle?
A Bjerrum plot shows the relative abundance of carbonate species (H₂CO₃, HCO₃⁻, CO₃²⁻) as a percentage of total dissolved inorganic carbon (DIC) across a range of pH values.
-Each curve represents one carbonate species.
-At low pH: H₂CO₃ dominates
-At neutral pH (~6–8): HCO₃⁻ is the most abundant
-At high pH: CO₃²⁻ dominates
This plot illustrates how carbon speciation shifts with pH, important for understanding buffering, acidification, and shell formation in the carbon cycle.
Which weathering processes are a CO₂ sink, source, or neutral in the carbon cycle?
Silicate weathering (by carbonic acid) is a CO₂ sink because it removes two carbon atoms from the atmosphere and stores them in CaCO₃.
Carbonate weathering (by carbonic acid) is CO₂-neutral because it removes one carbon from the atmosphere, but releases one from the rock, so no net gain or loss.
Carbonate weathering (by sulfuric acid) is a CO₂ source because it releases rock carbon as CO₂ without removing any from the atmosphere.
Silicate weathering (by sulfuric acid) is CO₂-neutral because no atmospheric CO₂ is involved or affected.
In what key ways do silicate and carbonate weathering differ in the carbon cycle?
Carbon source:
-Silicate weathering uses carbon entirely from the atmosphere (via carbonic acid).
-Carbonate weathering uses one carbon from the atmosphere and one from the rock.
Net CO₂ impact:
-Silicate weathering is a net sink — it removes CO₂ from the atmosphere and can store it long-term as CaCO₃.
-Carbonate weathering is carbon-neutral — it simply recycles carbon already present in the Earth system.
Timescale of effect:
-Silicate weathering acts over tectonic timescales (10⁶–10⁸ years) and plays a role in long-term climate regulation.
-Carbonate weathering operates over shorter timescales (10⁴–10⁶ years) and primarily buffers pH without reducing total CO₂.
How do carbonate and silicate weathering compare in their ability to buffer environmental pH?
Carbonate weathering releases bicarbonate ions (HCO₃⁻) quickly and abundantly, which directly buffer pH by neutralising excess H⁺ or OH⁻. It’s highly effective at stabilising pH in rivers, soils, and oceans over short timescales (days–years).
Silicate weathering also produces bicarbonate, but much more slowly due to low mineral solubility. It contributes to long-term ocean alkalinity but does not effectively buffer short-term pH changes.
