Earth Science Quiz: Carbon Cycle

What this quiz covers

This quiz focuses on Carbon Cycle, giving you a quick way to practice the rules, question types, and explanations that matter most for Earth Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Uptake and consumption by methanotrophic bacteria in soils, which convert it to biomass.
2. Photodissociation by high-energy ultraviolet radiation in the stratosphere.
3. Oxidation by hydroxyl radicals (OH) in the troposphere, which converts CH4 to CO2 and H2O. (correct answer)
4. Dissolution into the ocean, where it is converted into stable methane clathrates in deep-sea sediments.

Explanation: The correct answer is C. The dominant sink for atmospheric methane is chemical reaction in the troposphere. Methane reacts with hydroxyl radicals (OH), which are highly reactive molecules often called the 'detergent' of the atmosphere. This initiates a series of reactions that ultimately oxidizes the methane, with the carbon atom ending up as carbon dioxide (CO2) and the hydrogen atoms ending up as water (H2O). This process has an average timescale of about a decade. A describes a valid but smaller sink for methane. B is a minor pathway. D describes a long-term storage mechanism for methane in sediments, but it is not the primary removal pathway for methane already in the atmosphere.

Question 2

1. It acts as a long-term positive feedback, as warmer and wetter conditions accelerate weathering, which releases stored carbon from rocks and amplifies warming.
2. It acts as a long-term negative feedback, as warming accelerates weathering, which consumes atmospheric CO2 and thus tends to counteract the initial warming. (correct answer)
3. It has no feedback role because the CO2 consumed by weathering is precisely balanced on an annual basis by CO2 released from global volcanic outgassing.
4. It functions as a short-term negative feedback, rapidly responding to seasonal temperature changes to stabilize atmospheric CO2 levels throughout the year.

Explanation: The correct answer is B. The weathering of silicate rocks (e.g., CaSiO₃) consumes atmospheric CO₂ and converts it to bicarbonate ions in solution. Because the rate of this reaction increases with temperature, it forms a stabilizing negative feedback loop over geological timescales. If the climate warms for any reason, weathering rates increase, which pulls more CO₂ out of the atmosphere, leading to cooling that counteracts the initial warming. Conversely, if the climate cools, weathering slows, allowing volcanic CO₂ to build up and warm the climate. A is incorrect because silicate weathering consumes CO₂, it does not release it. C is incorrect because while weathering and volcanism are balanced over millions of years, the temperature dependence of weathering allows it to act as a feedback to perturbations. D is incorrect because weathering is an extremely slow process and operates on timescales of thousands to millions of years, not seasonally.

Question 3

1. The net flux into the mature forest is even greater than during the growth phase because the larger trees have much higher rates of photosynthesis.
2. The mature forest becomes a net source of carbon to the atmosphere as the decomposition of old, dead trees outpaces the growth of new ones.
3. The net flux remains a large and constant sink throughout the life of the forest, as the total biomass determines the rate of carbon uptake.
4. The net flux is close to zero in the mature state, whereas it was a significant net sink during the decades of active growth. (correct answer)

Explanation: The correct answer is D. A forest acts as a net carbon sink when its total biomass is increasing. During the rapid growth phase, the amount of carbon taken up by photosynthesis is significantly greater than the amount released through respiration (by plants and soil microbes), resulting in a net flux of carbon from the atmosphere into the forest. In a mature, climax forest, the ecosystem is in a state of approximate equilibrium. The growth of new trees is balanced by the death and decomposition of old ones. Therefore, the total carbon uptake via photosynthesis is roughly equal to the total carbon release via respiration and decomposition. This means the net carbon flux is close to zero. A is incorrect because while gross photosynthesis is high in a mature forest, so is respiration. B is incorrect; a stable climax forest is generally considered carbon-neutral, not a net source. C is incorrect because it is the change in biomass, not the total amount, that determines the net flux.

Question 4

1. In deep ocean sediments, where carbon was being buried and converted to rock at a much faster rate than previously estimated.
2. In the stratosphere, where CO2 has a longer residence time and was accumulating undetected by surface monitoring stations.
3. Through a dramatic acceleration in the chemical weathering of continental rocks due to the effects of acid rain.
4. In the terrestrial biosphere, due to factors such as CO2 fertilization of plant growth and the regrowth of forests in the Northern Hemisphere. (correct answer)

Explanation: The correct answer is D. The 'missing sink' puzzle was largely solved by recognizing that the terrestrial biosphere was taking up more carbon than initially thought. This uptake is attributed to several factors, including the 'CO2 fertilization effect' (where elevated CO2 levels stimulate photosynthesis), nitrogen deposition (which can act as a fertilizer), and significant regrowth of forests on abandoned agricultural land, particularly across North America and Eurasia. A is incorrect because sediment burial is a geologic process far too slow to account for the large, annual discrepancy. B is incorrect because the atmosphere is well-mixed over a few years, and CO2 does not stratify in this manner. C is incorrect because while weathering is a sink, its rate is too slow and the enhancement due to acid rain was not large enough to account for the missing billions of tons of carbon per year.

Question 5

1. Terrestrial plants, deep ocean, atmosphere, fossil fuels
2. Surface ocean, soil carbon, deep ocean, lithosphere
3. Atmosphere, terrestrial plants, deep ocean, lithosphere (correct answer)
4. Lithosphere, deep ocean, fossil fuels, atmosphere

Explanation: The correct answer is C. The residence time of carbon varies dramatically between reservoirs. In the atmosphere, a CO2 molecule's average residence time is only a few years (3-5 years) before it's exchanged with the ocean or biosphere. Carbon in terrestrial plants has a residence time of years to decades. Carbon in the deep ocean remains for centuries to over a thousand years due to slow circulation. The lithosphere, which includes rocks like limestone and fossil fuels, stores carbon for millions to hundreds of millions of years. Therefore, the correct order from shortest to longest residence time is Atmosphere < Terrestrial Plants < Deep Ocean < Lithosphere. The other options present incorrect orderings of these timescales.

Question 6

1. Cement production, through the chemical process of calcination where limestone (CaCO₃) is heated to produce lime (CaO) and CO₂. (correct answer)
2. The widespread use of nitrogen fertilizers, which directly react with organic matter in the soil to release geologically-sequestered CO₂.
3. The anaerobic digestion of waste in landfills, which primarily releases large quantities of CO₂ as organic matter breaks down.
4. The operation of nuclear power plants, through the fission of uranium atoms which releases carbon that was part of the atomic structure.

Explanation: The correct answer is A. Cement production is a significant source of global CO2 emissions (around 8% of the total). The process involves heating limestone (calcium carbonate, CaCO₃) to very high temperatures. This process, known as calcination, drives off CO2, leaving behind lime (calcium oxide, CaO), the key ingredient in cement. The chemical reaction is CaCO₃ → CaO + CO₂. This is a direct chemical release of CO2, separate from the CO2 released by burning fossil fuels to heat the kiln. B is incorrect; nitrogen fertilizers are a major source of nitrous oxide (N₂O), a different greenhouse gas. C is incorrect; anaerobic decomposition in landfills and rice paddies is a major source of methane (CH₄), not CO₂. D is incorrect; nuclear fission does not involve carbon and produces no direct CO2 emissions.

Question 7

1. The newly exposed land is rapidly colonized by hardy plant species, which absorb atmospheric CO2 through photosynthesis, creating a stabilizing negative feedback.
2. Meltwater transports the released organic matter to the Arctic Ocean, where it is quickly buried in marine sediments, leading to long-term carbon sequestration.
3. Increased microbial decomposition of the newly thawed organic matter releases large quantities of CO2 and methane, amplifying the initial warming in a positive feedback loop. (correct answer)
4. The additional freshwater runoff from melting permafrost alters ocean salinity, which strengthens the ocean's capacity to absorb atmospheric CO2.

Explanation: The correct answer is C. The thawing of permafrost exposes previously frozen organic matter to microbial decomposition. This process releases carbon dioxide (CO2) and methane (CH4), both potent greenhouse gases, into the atmosphere. This adds to the atmospheric greenhouse gas concentration, enhancing the greenhouse effect and leading to further warming, which in turn causes more permafrost to thaw. This is a classic example of a positive feedback loop. A is incorrect because while some plant colonization might occur, the rate of carbon release from decomposition is expected to far exceed the rate of uptake by new vegetation. B is incorrect because burial in marine sediments is a very slow process compared to the rapid release of gases from decomposition. D is incorrect because while freshwater runoff can affect ocean circulation, its direct impact on strengthening CO2 absorption is highly speculative and not the primary, direct effect of permafrost thaw.

Question 8

1. Satellite data shows that atmospheric CO2 is most concentrated in the industrialized Northern Hemisphere, not distributed evenly around volcanic zones.
2. The isotopic ratio of atmospheric carbon shows a significant decrease in both carbon-13 and carbon-14, which is characteristic of burning ancient plant-derived fossil fuels. (correct answer)
3. Volcanic eruptions also emit sulfur dioxide aerosols, which cause a temporary global cooling effect that opposes the observed warming trend.
4. Annual CO2 emissions from all global volcanic activity are estimated to be less than 2% of the CO2 emissions from human activities.

Explanation: The correct answer is B. This is a very direct and specific piece of evidence often called a 'chemical fingerprint'. Carbon from living organisms and fossil fuels (which are derived from ancient life) is depleted in the heavier carbon-13 isotope (¹³C) compared to carbon from volcanic or mantle sources. Furthermore, fossil fuels are so old that all their radioactive carbon-14 (¹⁴C) has decayed away. The observed decrease in the atmospheric ratios of both ¹³C/¹²C and ¹⁴C/¹²C is a clear signature that the source of the excess CO2 is the combustion of ancient organic matter. D is also a strong refutation based on comparing the magnitude of the fluxes and is a valid argument, but B is more direct as it identifies the specific type of carbon being added. A is supportive evidence but less conclusive, as atmospheric mixing complicates source attribution by location alone. C addresses the climate effect (warming vs. cooling) rather than the source of the CO2 itself.

Question 9

1. An increase, because the enhanced biological activity would lead to a corresponding increase in respiration by marine organisms.
2. No significant change, as the carbon sinking to the deep ocean would be quickly returned to the surface via upwelling.
3. A decrease, as more carbon would be fixed by phytoplankton and sequestered in the deep ocean for long periods. (correct answer)
4. An initial decrease, followed by a rapid increase as decomposition of sunken organic matter depletes ocean oxygen.

Explanation: The correct answer is C. The biological pump works because phytoplankton (microscopic marine plants) in the sunlit surface layer take up CO2 through photosynthesis. When these organisms die, a fraction of their organic matter sinks to the deep ocean. This process effectively 'pumps' carbon from the surface (which is in contact with the atmosphere) to the deep ocean, where it can be sequestered for hundreds to thousands of years. Enhancing this pump's efficiency would mean more carbon makes this journey, thus drawing down atmospheric CO2. A is incorrect because while respiration does occur, an enhanced pump implies that the net effect is increased sinking and sequestration, meaning photosynthesis outpaces surface respiration. B is incorrect because deep ocean circulation and upwelling operate on long timescales (typically ~1000 years), so the sequestration is effective over the centuries timescale specified. D describes a potential secondary effect related to ocean anoxia, but the primary and direct consequence of an efficient pump is sequestration.

Question 10

1. The deep ocean, due to seasonal changes in overturning circulation patterns.
2. The terrestrial biosphere, due to the annual cycle of photosynthesis and respiration, dominated by Northern Hemisphere vegetation. (correct answer)
3. The lithosphere, as seasonal temperature changes affect the rate of chemical weathering of carbonate rocks.
4. The surface ocean, due to the temperature-dependent solubility of CO2 in seawater, with more uptake in winter.

Explanation: The correct answer is B. The seasonal sawtooth pattern is a direct reflection of the 'breathing' of the planet's forests and plants. In the Northern Hemisphere spring and summer, the vast landmasses 'inhale' CO2 as plants grow, causing a drop in global atmospheric concentrations. In the fall and winter, plants respire and decompose, 'exhaling' CO2 and causing concentrations to rise. This cycle is dominated by the Northern Hemisphere simply because it has much more land area and terrestrial vegetation than the Southern Hemisphere. A and C involve processes that are far too slow to cause a noticeable seasonal signal. D is a real effect, but the magnitude of the terrestrial biosphere's seasonal flux is significantly larger and is the primary driver of the observed oscillation.

Question 11

1. It would release geologically ancient carbon stored within the silicate mineral structures, increasing atmospheric CO2.
2. It would have no net effect, as the CO2 consumed during weathering is re-released when carbonate minerals later precipitate in the ocean.
3. It would provide essential mineral nutrients to soils, stimulating plant growth that draws down atmospheric CO2 through photosynthesis.
4. It would chemically convert atmospheric CO2 into dissolved bicarbonate ions in water, which are then transported to the ocean for long-term storage. (correct answer)

Explanation: The correct answer is D. The chemical weathering of silicate minerals consumes atmospheric CO2. For example, the weathering of wollastonite (CaSiO3) can be represented as CaSiO3 + 2CO2 + H2O → Ca²⁺ + 2HCO₃⁻ + SiO₂. This reaction takes CO2 from the atmosphere and converts it into stable, dissolved bicarbonate ions (HCO₃⁻). These ions are eventually washed by rivers into the ocean. In the ocean, they can be used by organisms to form calcium carbonate (CaCO₃) shells. While the precipitation of CaCO₃ releases one CO2 molecule (Ca²⁺ + 2HCO₃⁻ → CaCO₃ + CO2 + H2O), the overall process from silicate weathering to carbonate burial results in a net removal of one molecule of CO2 from the atmosphere. A is incorrect; silicate minerals do not contain significant amounts of carbon. B is a common misconception that ignores the net effect of the full cycle. C describes a potential secondary benefit (fertilization), but the primary intended mechanism is the chemical conversion of CO2.

Question 12

1. Increased acidity enhances the dissolution of calcium carbonate sediments on the seafloor, releasing buffer agents and increasing the ocean's CO2 uptake capacity.
2. Ocean acidification directly harms photosynthetic phytoplankton, weakening the biological pump and thus reducing the ocean's net uptake of CO2.
3. As ocean pH drops, the concentration of carbonate ions (CO₃²⁻) decreases, which reduces the chemical capacity of seawater to react with and absorb additional CO2. (correct answer)
4. The lower pH of the surface ocean increases the density of the water, causing it to sink faster and more efficiently transport dissolved CO2 to the deep ocean.

Explanation: The correct answer is C. When CO2 dissolves in seawater, it forms carbonic acid, which then dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺), lowering the pH. These hydrogen ions also react with carbonate ions (CO₃²⁻) to form more bicarbonate. The overall reaction can be simplified as CO₂ + H₂O + CO₃²⁻ → 2HCO₃⁻. This reaction is a primary way the ocean absorbs CO2. As we add more CO2, we consume the available carbonate ions. A lower concentration of carbonate ions reduces the ocean's chemical buffering capacity, making it less efficient at absorbing further CO2 from the atmosphere. This is a positive feedback on atmospheric CO2. A describes a real but very slow process (seafloor dissolution) that cannot keep pace with current changes. B is a possible biological feedback, but the chemical feedback described in C is more direct and certain. D is incorrect; changes in pH have a negligible effect on seawater density compared to temperature and salinity.

Question 13

1. The CCD would become shallower, as increased acidity (lower carbonate ion concentration) makes the water more corrosive to CaCO₃. (correct answer)
2. The CCD would become deeper, as the higher concentration of total dissolved inorganic carbon promotes the formation of CaCO₃ shells.
3. The CCD would remain at the same depth, as it is primarily controlled by pressure and temperature, not by water chemistry.
4. The CCD would bifurcate into two separate depths, one for calcite and one for aragonite, but the average depth would remain the same.

Explanation: The correct answer is A. Ocean acidification, caused by the uptake of atmospheric CO2, lowers the pH and, more importantly, reduces the concentration of carbonate ions (CO₃²⁻) in seawater. The saturation state of seawater with respect to calcium carbonate minerals (like calcite and aragonite) depends on this carbonate ion concentration. As the water becomes more undersaturated (more corrosive), CaCO₃ dissolves more readily. This means that the depth at which dissolution outpaces supply (the CCD) will move upward, becoming shallower. B is incorrect because while total dissolved inorganic carbon increases, the crucial carbonate ion concentration decreases, which inhibits shell formation and promotes dissolution. C is incorrect because water chemistry is a critical control on the CCD. D is partially true in that calcite and aragonite have different saturation depths, but the overall effect of acidification is to make both depths shallower.

Question 14

1. 427 ppm
2. 481 ppm (correct answer)
3. 415 ppm
4. 551 ppm

Explanation: The correct answer is B. This problem requires a few steps. First, calculate the net annual increase in atmospheric carbon: Net Increase = Emissions - Sink Uptake = 10 GtC/year - 5 GtC/year = 5 GtC/year. Second, calculate the total carbon added to the atmosphere over 30 years: Total Added Carbon = 5 GtC/year * 30 years = 150 GtC. Third, convert this mass of carbon to a concentration in ppm. We can establish a ratio from the initial conditions: 410 ppm / 870 GtC. The increase in ppm will be (150 GtC) * (410 ppm / 870 GtC) ≈ 70.7 ppm. Finally, add this increase to the starting concentration: Final Concentration = 410 ppm + 70.7 ppm ≈ 481 ppm. Distractor D (551 ppm) is a common trap, representing the calculation if one incorrectly ignores the role of natural sinks (10 GtC/year * 30 years = 300 GtC added).

Question 15

1. The world's oceans are considered a major carbon flux because they are constantly exchanging large amounts of CO2 with the atmosphere.
2. Fossil fuel combustion is a carbon reservoir that has grown significantly since the industrial revolution began.
3. Photosynthesis is a flux that transfers carbon from the atmospheric reservoir to the terrestrial biosphere reservoir. (correct answer)
4. A reservoir, such as soil, has a fixed and unchanging capacity for carbon, while a flux, like decomposition, has a variable rate.

Explanation: The correct answer is C. This statement correctly identifies the key terms. A reservoir is a component of the Earth system where carbon is stored (a stock), such as the atmosphere or the biosphere. A flux is the process or rate of transfer of carbon between reservoirs. Photosynthesis is a process (a flux) that moves carbon from one place (atmosphere reservoir) to another (biosphere reservoir). A is incorrect because the ocean is the reservoir; the exchange of CO2 is the flux. B is incorrect because fossil fuel combustion is the process (flux) of moving carbon from the fossil fuel reservoir to the atmospheric reservoir. D is incorrect because reservoirs are not fixed; their size (the amount of carbon they hold) changes as a result of imbalances in fluxes.

Question 16

1. Cement production, through the chemical process of calcination where limestone (CaCO₃) is heated to produce lime (CaO) and CO₂. (correct answer)
2. The widespread use of nitrogen fertilizers, which directly react with organic matter in the soil to release geologically-sequestered CO₂.
3. The anaerobic digestion of waste in landfills, which primarily releases large quantities of CO₂ as organic matter breaks down.
4. The operation of nuclear power plants, through the fission of uranium atoms which releases carbon that was part of the atomic structure.

Explanation: The correct answer is A. Cement production is a significant source of global CO2 emissions (around 8% of the total). The process involves heating limestone (calcium carbonate, CaCO₃) to very high temperatures. This process, known as calcination, drives off CO2, leaving behind lime (calcium oxide, CaO), the key ingredient in cement. The chemical reaction is CaCO₃ → CaO + CO₂. This is a direct chemical release of CO2, separate from the CO2 released by burning fossil fuels to heat the kiln. B is incorrect; nitrogen fertilizers are a major source of nitrous oxide (N₂O), a different greenhouse gas. C is incorrect; anaerobic decomposition in landfills and rice paddies is a major source of methane (CH₄), not CO₂. D is incorrect; nuclear fission does not involve carbon and produces no direct CO2 emissions.

Question 17

1. The newly exposed land is rapidly colonized by hardy plant species, which absorb atmospheric CO2 through photosynthesis, creating a stabilizing negative feedback.
2. Meltwater transports the released organic matter to the Arctic Ocean, where it is quickly buried in marine sediments, leading to long-term carbon sequestration.
3. Increased microbial decomposition of the newly thawed organic matter releases large quantities of CO2 and methane, amplifying the initial warming in a positive feedback loop. (correct answer)
4. The additional freshwater runoff from melting permafrost alters ocean salinity, which strengthens the ocean's capacity to absorb atmospheric CO2.

Explanation: The correct answer is C. The thawing of permafrost exposes previously frozen organic matter to microbial decomposition. This process releases carbon dioxide (CO2) and methane (CH4), both potent greenhouse gases, into the atmosphere. This adds to the atmospheric greenhouse gas concentration, enhancing the greenhouse effect and leading to further warming, which in turn causes more permafrost to thaw. This is a classic example of a positive feedback loop. A is incorrect because while some plant colonization might occur, the rate of carbon release from decomposition is expected to far exceed the rate of uptake by new vegetation. B is incorrect because burial in marine sediments is a very slow process compared to the rapid release of gases from decomposition. D is incorrect because while freshwater runoff can affect ocean circulation, its direct impact on strengthening CO2 absorption is highly speculative and not the primary, direct effect of permafrost thaw.

Question 18

1. The net flux of CO2 into the ocean will decrease because warmer water has a lower capacity to hold dissolved gases. (correct answer)
2. The net flux of CO2 into the ocean will increase because the higher kinetic energy of molecules facilitates a more rapid rate of gas exchange.
3. There will be no net change in the flux, as increased photosynthesis by phytoplankton in warmer waters will balance the reduced solubility.
4. The equilibrium will shift to favor the formation of carbonic acid, thereby drawing more CO2 into the ocean to be stored as bicarbonate.

Explanation: The correct answer is A. Henry's Law states that the solubility of a gas in a liquid is inversely proportional to the temperature of the liquid. As the ocean's surface warms, its ability to dissolve CO2 from the atmosphere decreases. This reduces the concentration gradient driving CO2 from the air into the sea, thereby decreasing the net flux into the ocean. This represents a positive feedback that can weaken one of the Earth's primary carbon sinks. B is incorrect because it confuses the rate of exchange with the total solubility or capacity; while molecules move faster, the water can hold less gas at equilibrium. C is incorrect because while biological activity is affected by temperature, it is not guaranteed to perfectly compensate for the physical change in solubility, and in many areas, warming may lead to nutrient limitation that reduces productivity. D is incorrect because the formation of carbonic acid and bicarbonate depends on the initial dissolution of CO2, which is hindered by warmer temperatures.

Question 19

1. An increase, because the enhanced biological activity would lead to a corresponding increase in respiration by marine organisms.
2. No significant change, as the carbon sinking to the deep ocean would be quickly returned to the surface via upwelling.
3. A decrease, as more carbon would be fixed by phytoplankton and sequestered in the deep ocean for long periods. (correct answer)
4. An initial decrease, followed by a rapid increase as decomposition of sunken organic matter depletes ocean oxygen.

Explanation: The correct answer is C. The biological pump works because phytoplankton (microscopic marine plants) in the sunlit surface layer take up CO2 through photosynthesis. When these organisms die, a fraction of their organic matter sinks to the deep ocean. This process effectively 'pumps' carbon from the surface (which is in contact with the atmosphere) to the deep ocean, where it can be sequestered for hundreds to thousands of years. Enhancing this pump's efficiency would mean more carbon makes this journey, thus drawing down atmospheric CO2. A is incorrect because while respiration does occur, an enhanced pump implies that the net effect is increased sinking and sequestration, meaning photosynthesis outpaces surface respiration. B is incorrect because deep ocean circulation and upwelling operate on long timescales (typically ~1000 years), so the sequestration is effective over the centuries timescale specified. D describes a potential secondary effect related to ocean anoxia, but the primary and direct consequence of an efficient pump is sequestration.

Question 20

1. It would release geologically ancient carbon stored within the silicate mineral structures, increasing atmospheric CO2.
2. It would have no net effect, as the CO2 consumed during weathering is re-released when carbonate minerals later precipitate in the ocean.
3. It would provide essential mineral nutrients to soils, stimulating plant growth that draws down atmospheric CO2 through photosynthesis.
4. It would chemically convert atmospheric CO2 into dissolved bicarbonate ions in water, which are then transported to the ocean for long-term storage. (correct answer)

Explanation: The correct answer is D. The chemical weathering of silicate minerals consumes atmospheric CO2. For example, the weathering of wollastonite (CaSiO3) can be represented as CaSiO3 + 2CO2 + H2O → Ca²⁺ + 2HCO₃⁻ + SiO₂. This reaction takes CO2 from the atmosphere and converts it into stable, dissolved bicarbonate ions (HCO₃⁻). These ions are eventually washed by rivers into the ocean. In the ocean, they can be used by organisms to form calcium carbonate (CaCO₃) shells. While the precipitation of CaCO₃ releases one CO2 molecule (Ca²⁺ + 2HCO₃⁻ → CaCO₃ + CO2 + H2O), the overall process from silicate weathering to carbonate burial results in a net removal of one molecule of CO2 from the atmosphere. A is incorrect; silicate minerals do not contain significant amounts of carbon. B is a common misconception that ignores the net effect of the full cycle. C describes a potential secondary benefit (fertilization), but the primary intended mechanism is the chemical conversion of CO2.