Carbon sequestration Study Guide

📖 Core Concepts Carbon sequestration – permanent storage of carbon in a carbon pool (soil, biomass, rock, ocean) that removes CO₂ from the atmosphere. Carbon pool – any Earth‑system reservoir (biosphere, geosphere, hydrosphere, atmosphere) where carbon resides for a measurable time. Biosequestration – biological pathways (forests, wetlands, soils, seaweed) that fix carbon via photosynthesis and store it as organic matter. Geologic sequestration – injection of captured CO₂ into deep porous rocks (depleted reservoirs, saline formations, coal beds) where it is trapped physically and/or mineralized. Oceanic sequestration – physical (solubility pump), biological (marine pump), and engineered (fertilization, alkalinity enhancement) mechanisms moving CO₂ into deep seawater or sediments. Carbon capture and storage (CCS) – technology that captures CO₂ from industrial sources, transports it, and stores it underground; sequestration is the storage component of CCS. Blue carbon – carbon stored in coastal ecosystems (wetlands, mangroves, seagrasses, seaweed). Mineral carbonation – chemical conversion of CO₂ into stable carbonate minerals (e.g., calcite, magnesite). --- 📌 Must Remember Forests store 25 % of annual human CO₂ emissions; protecting existing forest gives faster climate benefit than new planting. Wetland soils hold 20–30 % of global soil carbon while covering only 5–8 % of land. Peatlands (3 % of land) contain 30 % of terrestrial carbon; drainage releases large CO₂ pulses. Biochar can persist for centuries in soils and reduces mineralization rates. Geologic injection pressure: 100 bar to reach supercritical CO₂. Caprock requirement: high‑porosity reservoir + low‑permeability caprock (e.g., shale) to prevent upward migration. Basalt storage depth: > 2,700 m ensures CO₂ is denser than seawater → negative buoyancy. Cost ranges: Soil carbon farming: US $3–130 t⁻¹ CO₂. On‑shore geologic storage (easy sites): < $10 t⁻¹. Carbfix basalt mineralization: ≈ $25 t⁻¹. Forest‑based sequestration (including capture): $35–$280 t⁻¹. Potential caps: Full geological capacity limits warming to ≈ 0.7 °C; additional tree canopy could offset ≈ 205 Gt C (20 yr of current emissions). --- 🔄 Key Processes Forest Carbon Sequestration Photosynthesis: $CO2 + H2O \rightarrow CH2O + O2$ (biomass growth). Carbon allocation: Root, stem, branch, leaf storage. Long‑term retention: Mature wood → decades of storage; wood in construction → centuries. Soil Carbon Farming Residue retention (leave stubble, apply manure). Reduced tillage → less oxidation of organic matter. Perennial crops → deeper, more stable root carbon. Biochar addition → stable carbon matrix, reduces mineralization. Geological CO₂ Injection Compression to 100 bar (supercritical). Transport via pipelines to injection site. Injection into porous formation. Trapping mechanisms: Structural (caprock lid). Residual (CO₂ trapped in pore throats). Dissolution (CO₂ → aqueous $HCO3^-$). Mineral (reaction with silicates). Mineral Carbonation (e.g., basalt) CO₂ + H₂O → HCO₃⁻ + H⁺ (dissolution). Release of Ca²⁺/Mg²⁺ from silicate weathering. Precipitation: $Ca^{2+} + CO2 → CaCO3\downarrow$ (stable solid). Ocean Alkalinity Enhancement Add base (e.g., crushed limestone). Increase alkalinity → more $HCO3^-$ capacity. CO₂ uptake: $CO2 + H2O + Alkalinity → HCO3^- / CO3^{2-}$. Seaweed (Macroalgae) Sequestration Photosynthetic uptake in farms. Harvest → sink to deep ocean (or process to biomethane). Deep‑sea burial → millennial‑scale storage. --- 🔍 Key Comparisons Forest vs. Wetland carbon storage Forest: large above‑ground biomass, vulnerable to fire/drought. Wetland: soil carbon dominates, waterlogged → slow decomposition, but may emit CH₄. Physical trapping vs. Mineral carbonation (geologic) Physical: relies on caprock seal; risk of leakage if faults present. Mineral: converts CO₂ to solid carbonates → essentially permanent; depends on reactive rock availability. Seaweed burial vs. Land biochar Seaweed: rapid carbon turnover unless buried; storage depth critical. Biochar: chemically recalcitrant, stable in soils for centuries. Ocean fertilization vs. Artificial upwelling Fertilization: adds nutrients (iron) → boosts phytoplankton growth; side effects include potential harmful algal blooms. Upwelling: brings deep nutrients to surface, also stimulates blooms but uses energy for pumping. Carbon farming cost vs. CCS cost Carbon farming: $3–130 t⁻¹ (wide range, site‑specific). On‑shore CCS: often > $10 t⁻¹, higher if deep drilling needed. --- ⚠️ Common Misunderstandings “All wetlands are net carbon sinks.” Many emit methane (CH₄) and nitrous oxide (N₂O), offsetting CO₂ uptake. “CCS = cheap, easy carbon removal.” Capture, transport, and site‑specific storage costs can exceed $50 t⁻¹. “Seaweed carbon stays sequestered forever.” Only if the biomass is buried in deep, anoxic zones; otherwise it decomposes quickly. “Mineral carbonation needs no energy.” Rock grinding (e.g., olivine, basalt) and CO₂ compression require significant energy input. “Higher pressure always means safer storage.” Excess pressure can induce seismicity or fracture caprock. --- 🧠 Mental Models / Intuition Carbon pools = bank accounts. Deposits (photosynthesis, injection) increase balance; withdrawals (fire, decay, leakage) decrease it. Caprock = airtight lid on a jar; if the lid is cracked (fault), CO₂ can escape. Ocean pumps = conveyor belts moving carbon‑rich water down (solubility pump) or bringing nutrients up (upwelling). Biochar = charcoal time‑capsule – locks carbon in a lattice that microbes can’t easily break down. Mineral carbonation = “rock cementing” – CO₂ becomes part of the stone, just like building a permanent wall. --- 🚩 Exceptions & Edge Cases Forests turning into sources under heat stress, drought, or pathogen attack. Peatland drainage → rapid CO₂ release; rewetting needed to restore sink function. Basalt injection shallower than 2,700 m may not achieve negative buoyancy; CO₂ could rise. Biochar from wildfires can reduce humus, potentially net‑negative for soil carbon. Ocean fertilization may cause harmful algal blooms or deoxygenation if not carefully managed. --- 📍 When to Use Which | Situation | Preferred Pathway | Reasoning | |-----------|-------------------|-----------| | Immediate, large‑scale removal needed | Protect/restore forests & wetlands | Fast carbon uptake, low tech, co‑benefits (biodiversity). | | Long‑term, irreversible storage | Geologic mineral carbonation (basalt, ultramafic) | CO₂ locked as solid carbonate for > 10⁴ yr. | | Limited land availability | Ocean alkalinity enhancement or deep‑sea basalt injection | Uses ocean volume, minimal land use. | | Agricultural region | Carbon farming (no‑till, biochar, cover crops) | Improves soil health while sequestering carbon. | | Coastal community seeking income | Seaweed farming + biomethane | Generates product revenue, adds carbon sink. | | High‑pressure, low‑fault site | Physical CCS (structural trapping) | Lower cost than mineralization if caprock is excellent. | | Need for rapid climate benefit with existing infrastructure | Wood product substitution (use timber instead of steel/concrete) | Stores carbon for centuries without new tech. | --- 👀 Patterns to Recognize High porosity + low‑permeability caprock → good CCS candidate. Waterlogged, anaerobic soils → likely long‑term carbon storage (wetlands, peat). Low temperature + high pressure (deep ocean) → CO₂ negative buoyancy → stable storage. Disturbance (fire, drought, disease) → look for forest carbon source signals. Presence of reactive silicates (basalt, olivine) → potential for rapid mineral carbonation. --- 🗂️ Exam Traps “Wetlands always increase atmospheric CO₂.” Wrong – they are major carbon sinks despite occasional CH₄ emissions. “Biochar always raises soil carbon.” Only when added carbon exceeds any loss of humus from fire‑derived charcoal. “All geological sites are safe from leakage.” Faulted or poorly sealed sites can leak; site characterization is essential. “Ocean fertilization has no side effects.” Can trigger harmful algal blooms and alter marine ecosystems. “Seaweed carbon sequestration requires no processing.” Only deep‑sea burial guarantees long‑term storage; otherwise it decomposes. “Carbon farming costs are uniformly low.” Costs vary $3–130 t⁻¹ depending on practice, region, and verification. ---