Week 13 - Carbon Cycle: Climate Feedbacks Flashcards
(9 cards)
What are the major carbon reservoirs in the geological carbon cycle, and how do their sizes compare?
-Rocks (including fossil carbon): ~75,000,000 PgC
-Ocean, atmosphere, ecosystems: ~40,000 PgC
-Marine sediments: ~150 PgC
→ Most carbon is stored in the crust and mantle, making the deep carbon cycle slow but crucial for long-term climate stability.
What are the four major dynamic fluxes that influence atmospheric pCO₂ on >1 million-year timescales?
- CO₂ sinks:
-Silicate weathering (J_sil-CO₂)
-Biospheric organic carbon burial (J_OCbio-burial)
J_OCbio-burial refers to the long-term burial of organic carbon produced by photosynthesis in plants and algae. When some of this organic matter escapes decomposition and is buried in marine or terrestrial sediments, it effectively removes CO₂ from the active carbon cycle. This acts as a long-term CO₂ sink, stabilizing climate over geological timescales.
- CO₂ sources:
-Carbonate + sulfide weathering (J_carb-sulf)
-Petrogenic organic carbon oxidation (J_OCpetro-ox)
J_OCpetro-ox is the oxidation of ancient organic carbon that was previously buried in sedimentary rocks (fossil organic matter). When these rocks are uplifted and exposed at the surface (e.g. by erosion), the organic carbon can react with oxygen to form CO₂, making it a source of atmospheric CO₂ in the geological carbon cycle.
Why is silicate weathering considered a negative feedback on climate?
Silicate weathering consumes atmospheric CO₂, forming bicarbonate ions that eventually precipitate as carbonate minerals, locking carbon away for geological timescales. An increase in temperature or CO₂ enhances weathering, which in turn removes more CO₂ — stabilizing climate.
What is the significance of “kinetically limited” vs “erosion-limited” weathering regimes in the context of chemical weathering?
- Kinetically Limited (Weathering-Limited) Regime
Occurs when fresh, unweathered minerals are abundant, due to sufficient erosion that exposes new rock at the surface.
The availability of reactants is not limiting — instead, the rate of chemical weathering is controlled by reaction kinetics (i.e. temperature, pH, water availability).
This regime is typically found in wet, tectonically active regions with high erosion and runoff.
As a result, weathering responds strongly to climate: higher temperatures and rainfall → faster reaction rates → more CO₂ drawdown.
It supports a strong negative feedback on atmospheric CO₂ — making it critical for climate regulation over geological timescales.
- Erosion-Limited (Supply-Limited) Regime
Occurs when physical erosion is too slow to supply fresh minerals to the surface.
Soils are often thick and deeply weathered, with minerals already largely altered.
In this case, weathering is limited by the supply of fresh primary minerals, not by chemical reaction rates.
Increasing temperature or rainfall does not significantly increase weathering, because there’s little left to weather.
Found in stable, low-relief landscapes like ancient cratons or humid tropics with low uplift.
Feedback to climate is weak or absent, because chemical weathering cannot respond effectively to changes in CO₂ or temperature.
The distinction between these regimes determines:
-How effective silicate weathering is at drawing down atmospheric CO₂
-Where and when Earth’s “thermostat” is functioning
-The global balance of carbon-climate feedbacks
-Only kinetically limited systems can provide a rapid, climate-sensitive CO₂ sink, making them crucial for long-term Earth system stability.
How does the erosion rate influence chemical weathering and the global carbon cycle?
Erosion rate is a key control on the supply of fresh, unweathered minerals to the surface, and therefore directly affects chemical weathering rates, particularly silicate weathering, which is a major long-term sink for atmospheric CO₂.
Moderate erosion creates an optimal balance:
-It exposes fresh primary minerals at the surface that can chemically weather.
-Maintains thin soils that support vegetation and allow water infiltration.
-Leads to kinetically limited weathering, where climate (temperature and precipitation) strongly controls CO₂ drawdown.
High erosion rates, often due to tectonic uplift or landslides, further expose fresh rock but may:
-Prevent soil development, reducing biological activity and reaction time.
-Lead to increased export of organic carbon (OC) to marine sediments (J_OCbio-burial), enhancing carbon sequestration.
-Increase oxidation of petrogenic organic carbon (J_OCpetro-ox), which acts as a CO₂ source.
Low erosion rates (e.g. in stable, low-relief areas):
-Lead to thick, highly weathered soils with little fresh mineral input.
-Result in supply-limited weathering, where the chemical weathering rate is constrained by the limited availability of unweathered minerals.
-Reduces the effectiveness of silicate weathering as a climate feedback mechanism.
How do low erosion rates reduce the effectiveness of silicate weathering as a climate feedback mechanism?
Low erosion rates limit the supply of fresh, unweathered silicate minerals to the surface, leading to supply-limited (or erosion-limited) weathering conditions. In such systems:
Thick, deeply weathered soils accumulate, and most reactive minerals have already been altered to clays and oxides.
The chemical weathering rate becomes independent of climate variables like temperature or rainfall, because there are few reactive minerals left to weather.
As a result, even if atmospheric CO₂ increases (causing warming and more rainfall), the silicate weathering rate cannot increase significantly, since the substrate is already exhausted.
This decouples silicate weathering from climate, weakening its role as a negative feedback mechanism in the long-term carbon cycle.
In contrast, in kinetically limited systems with higher erosion rates, fresh silicate minerals are continually exposed, and weathering rates respond strongly to climatic conditions — allowing silicate weathering to act as an effective “thermostat” that regulates atmospheric CO₂.
Discuss the role of mountain erosion in both enhancing and limiting carbon dioxide drawdown in the geological carbon cycle.
Model Answer Structure:
Define key processes: silicate weathering, OC burial, OC_petro oxidation:
Mountain erosion plays a complex and dynamic role in the geological carbon cycle by amplifying both carbon sinks and sources. Key processes include silicate weathering, organic carbon burial, and the oxidation of petrogenic organic carbon. These processes are intensified in active orogenic belts due to high erosion rates driven by tectonic uplift and exhumation.
Explain how mountain uplift increases erosion, exposing fresh minerals,
Show how it leads to CO₂ sinks: Silicate weathering, OC_bio burial:
Mountain uplift enhances exposure of fresh rock surfaces, accelerating chemical weathering, particularly of silicate minerals, which consume atmospheric CO₂ and lead to the long-term sequestration of carbon as marine carbonates. Additionally, enhanced erosion can increase the delivery of biospheric organic carbon (OC_bio) to depositional environments, promoting organic carbon burial, another important carbon sink.
CO₂ sources: OC_petro oxidation:
However, erosion also exposes previously buried petrogenic organic carbon, which is ancient organic matter stored in sedimentary rocks.
Upon exposure through erosion, petrogenic organic carbon undergoes oxidative weathering, converting stable organic compounds into dissolved inorganic carbon (e.g., CO₂), which may then be emitted to the atmosphere, acting as a long-term geological source of CO₂.
Sulfide oxidation and carbonate weathering:
Tectonic uplift and erosion expose sulfide-bearing rocks such as pyrite, which oxidizes to produce sulfuric acid. This acid can drive the weathering of carbonate minerals, releasing CO₂ — a process that offsets the CO₂ drawdown typically associated with silicate weathering. Thus, in certain lithological settings, erosion can lead to a net CO₂ source despite increased weathering activity.
Evaluate how lithology and erosion rate control whether a mountain range is a net CO₂ sink or source:
The overall carbon balance depends heavily on lithology. In regions where the bedrock is rich in carbonate minerals and sulfides like pyrite, weathering driven by sulfuric acid may dominate, potentially turning mountain belts into net CO₂ sources. Conversely, in silicate-rich terrains with low petrogenic carbon and sulfide content, mountain erosion can act as a net CO₂ sink.
Conclude that erosion intensifies all C fluxes, but net effect depends on balance of sources vs sinks:
In conclusion, mountain erosion enhances all geological carbon fluxes. Its net effect on atmospheric CO₂ depends on the interplay between uplift-driven weathering and erosion processes, and the underlying lithology, particularly the abundance of silicates, carbonates, and pyrite.
What are the limitations of the “silicate weathering thermostat” as a mechanism for climate regulation?
Define the “weathering thermostat” concept:
The silicate weathering thermostat is a long-term negative feedback mechanism thought to regulate Earth’s climate over millions of years. It relies on the climate-sensitive chemical weathering of silicate minerals. When atmospheric CO₂ increases, global temperatures rise, enhancing silicate weathering. This weathering consumes CO₂ (through reactions with carbonic acid formed from CO₂ and water), converting it into bicarbonate ions that are eventually buried as marine carbonates. This draws down atmospheric CO₂, cooling the climate and forming a self-regulating feedback.
Describe how CO₂ increases → temp ↑ → weathering ↑ → CO₂ drawdown → negative feedback.
Explain that this mechanism is limited when:
Erosion is too low (no fresh minerals), system becomes supply limited:
One limitation arises when erosion rates are low, particularly in tectonically stable, low-relief regions. Without uplift and physical erosion to expose fresh silicate minerals, weathering becomes limited by the availability of reactive surfaces. In these cases, even if temperatures and CO₂ rise, the feedback weakens because there are no fresh minerals left to weather.
Becomes “supply-limited”, where the weathering rate becomes insensitive to climate, breaking the thermostat-like feedback.
Water supply is limited:
A second limitation is water availability. Chemical weathering requires liquid water to facilitate mineral dissolution and transport ions. In arid climates or during periods of low precipitation, the lack of runoff restricts weathering, limiting the capacity for CO₂ drawdown even if temperatures are elevated.
Silicate weathering by sulphuric acid:
An additional geochemical limitation arises from the identity of the acid driving weathering. While the thermostat depends on weathering by carbonic acid, in some mountain environments with high sulfide (e.g. pyrite) content, ARD occurs, which refers to the oxidation of sulphide minerals which produces sulphuric acid. This could become the dominant weathering agent. If silicate weathering is driven by sulfuric acid rather than carbonic acid, no atmospheric CO₂ is consumed in the process. The reaction is CO₂-neutral, meaning it does not contribute to CO₂ drawdown, even though minerals are being weathered.
In some cases, acid rock drainage may also enhance carbonate weathering, which can result in net CO₂ release — further complicating the role of silicate weathering in climate regulation, though carbonate weathering itself does not directly contribute to long-term CO₂ drawdown.
This further undermines the effectiveness of the silicate weathering thermostat in such settings.
Conclude that the thermostat requires erosion and rainfall to function effectively:
In conclusion, while the silicate weathering thermostat is central to Earth’s long-term climate stability, its operation is constrained by several environmental and geochemical factors. These include the availability of fresh mineral surfaces through erosion, adequate water supply, and critically, the presence of carbonic acid as the primary weathering agent. In regions affected by acid rock drainage, where weathering is driven by sulfuric acid, the system becomes decoupled from atmospheric CO₂, undermining its feedback function. The thermostat is therefore only effective under specific conditions where climate, hydrology, tectonics, and lithology align.
Explain how erosion controls both CO₂ sinks and CO₂ sources in the geological carbon cycle.
Introduce erosion as the main process linking lithosphere to atmosphere.
CO₂ sinks enhanced by erosion:
Exposes silicates → weathering
Exports OC_bio to sediments → burial
CO₂ sources enhanced by erosion:
Brings OC_petro to surface → oxidation
Exposes pyrite → sulfide oxidation → carbonate weathering by sulfuric acid –> CO₂ release
Explain how erosion amplifies total fluxes, but balance determines net climate effect.
