2.1 Carbon cycle

The carbon cycle is a fundamental process in Earth's systems, involving the movement of carbon between various reservoirs. This topic explores the key components of the cycle, including carbon reservoirs, fluxes, and biogeochemical processes that drive carbon exchange.

Understanding the carbon cycle is crucial for geochemists studying climate change and environmental impacts. The notes cover natural and anthropogenic sources, carbon isotopes, and modeling techniques used to predict future scenarios and inform policy decisions.

Carbon reservoirs

• Carbon reservoirs play a crucial role in the global carbon cycle, storing and exchanging carbon between different components of the Earth system
• Understanding carbon reservoirs is essential in geochemistry for tracking carbon movement and predicting climate change impacts
• The four main carbon reservoirs interact dynamically, influencing atmospheric CO2 concentrations and global climate patterns

Atmosphere

Top images from around the web for Atmosphere

• 

  Heat Transfer in the Atmosphere | Physical Geography View original

  Is this image relevant?

• 

  Biogeochemical Cycles | OpenStax: Concepts of Biology View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

• 

  Heat Transfer in the Atmosphere | Physical Geography View original

  Is this image relevant?

• 

  Biogeochemical Cycles | OpenStax: Concepts of Biology View original

  Is this image relevant?

1 of 3

Top images from around the web for Atmosphere

• 

  Heat Transfer in the Atmosphere | Physical Geography View original

  Is this image relevant?

• 

  Biogeochemical Cycles | OpenStax: Concepts of Biology View original

  Is this image relevant?

• 

  Biogeochemical Cycles and the Flow of Energy in the Earth System | Sustainability: A ... View original

  Is this image relevant?

• 

  Heat Transfer in the Atmosphere | Physical Geography View original

  Is this image relevant?

• 

  Biogeochemical Cycles | OpenStax: Concepts of Biology View original

  Is this image relevant?

1 of 3

• Contains approximately 750 gigatons of carbon, primarily as CO2 and CH4
• Atmospheric carbon concentration has increased from ~280 ppm to over 410 ppm since the Industrial Revolution
• Serves as a critical link between other carbon reservoirs, facilitating rapid exchange
• Influences global climate through the greenhouse effect

Hydrosphere

• Largest active carbon reservoir, storing about 38,000 gigatons of carbon
• Carbon exists in various forms (dissolved CO2, carbonic acid, bicarbonate, carbonate ions)
• Oceans act as a significant carbon sink, absorbing about 25% of anthropogenic CO2 emissions
• Carbonate chemistry in oceans regulates pH and influences marine ecosystem health

Biosphere

• Stores approximately 2,000 gigatons of carbon in living and dead organic matter
• Terrestrial vegetation accounts for the majority of biospheric carbon storage
• Carbon flux in the biosphere driven by photosynthesis, respiration, and decomposition
• Plays a crucial role in short-term carbon cycling and climate regulation

Lithosphere

• Largest carbon reservoir, containing over 75,000,000 gigatons of carbon
• Carbon stored in sedimentary rocks (carbonates, fossil fuels) and Earth's mantle
• Exchanges carbon with other reservoirs through weathering, volcanism, and tectonic processes
• Operates on geological timescales, influencing long-term climate patterns

Carbon fluxes

• Carbon fluxes represent the movement of carbon between different reservoirs in the Earth system
• Understanding carbon fluxes is crucial in geochemistry for quantifying carbon cycle dynamics and predicting future climate scenarios
• Carbon fluxes occur at various spatial and temporal scales, from local ecosystems to global atmospheric circulation

Natural vs anthropogenic sources

• Natural sources include volcanic eruptions, wildfires, and respiration from living organisms
• Anthropogenic sources primarily stem from fossil fuel combustion and land-use changes
• Natural carbon fluxes have been in relative balance for millennia
• Anthropogenic emissions have disrupted this balance, leading to increased atmospheric CO2 concentrations
  • Current anthropogenic CO2 emissions exceed 35 gigatons per year
  • Deforestation contributes an additional 5-10 gigatons of CO2 annually

Terrestrial carbon exchange

• Involves carbon exchange between the atmosphere, vegetation, and soil
• Photosynthesis removes approximately 120 gigatons of carbon from the atmosphere annually
• Plant and soil respiration release about 60 gigatons of carbon back to the atmosphere
• Net terrestrial carbon uptake estimated at 3 gigatons per year
  • Influenced by factors such as temperature, precipitation, and land-use changes

Ocean-atmosphere exchange

• Oceans and atmosphere exchange about 90 gigatons of carbon annually
• Driven by differences in partial pressure of CO2 between air and sea surface
• Cold, high-latitude waters tend to absorb CO2, while warm, equatorial waters release CO2
• Biological pump transports carbon from surface to deep ocean through marine organism activity
  • Phytoplankton photosynthesis in surface waters
  • Sinking of organic matter and carbonate shells

Weathering and sedimentation

• Chemical weathering of silicate rocks consumes atmospheric CO2 over geological timescales
• Carbonate weathering temporarily sequesters CO2 in ocean waters
• Sedimentation of organic matter and carbonate minerals in marine environments
• Burial of carbon in sediments represents a long-term carbon sink
  • Estimated 0.2 gigatons of carbon buried annually in marine sediments

Biogeochemical processes

• Biogeochemical processes drive the cycling of carbon between different reservoirs in the Earth system
• These processes are fundamental to understanding carbon dynamics in geochemistry and their impact on global climate
• Involve complex interactions between biological, geological, and chemical systems across various spatial and temporal scales

Photosynthesis vs respiration

• Photosynthesis converts atmospheric CO2 into organic compounds using solar energy
  • $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$
• Respiration breaks down organic compounds to release energy, producing CO2 as a byproduct
  • $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$
• Balance between photosynthesis and respiration regulates atmospheric CO2 concentrations
• Influenced by factors such as temperature, water availability, and nutrient levels

Decomposition

• Microbial breakdown of dead organic matter releases CO2 back to the atmosphere
• Rate of decomposition affected by temperature, moisture, and organic matter composition
• Produces soil organic carbon, which can be stored for varying periods
• Contributes to nutrient cycling in terrestrial and aquatic ecosystems
  • Release of nitrogen, phosphorus, and other essential elements

Carbonate formation

• Precipitation of calcium carbonate in marine environments
  • $Ca^{2+} + 2HCO_3^- \rightarrow CaCO_3 + CO_2 + H_2O$
• Biogenic carbonate formation by marine organisms (corals, foraminifera, coccolithophores)
• Abiotic carbonate precipitation in warm, shallow marine environments
• Represents a significant long-term carbon sink in the ocean

Organic carbon burial

• Deposition and preservation of organic matter in sedimentary environments
• Occurs in both terrestrial (peatlands, permafrost) and marine (continental shelves, deep ocean) settings
• Influenced by sedimentation rates, oxygen availability, and biological productivity
• Forms the basis for fossil fuel formation over geological timescales
  • Coal, oil, and natural gas deposits

Carbon cycle timescales

• Carbon cycle operates on multiple timescales, from seconds to millions of years
• Understanding these timescales is crucial in geochemistry for interpreting past climate changes and predicting future scenarios
• Different processes dominate carbon cycling at various temporal scales, influencing atmospheric CO2 concentrations and global climate

Short-term carbon cycle

• Operates on timescales of days to decades
• Dominated by biological processes such as photosynthesis, respiration, and decomposition
• Rapid exchange between atmosphere, biosphere, and upper ocean
• Seasonal fluctuations in atmospheric CO2 concentrations
  • Northern Hemisphere growing season causes annual CO2 drawdown
  • Respiration and decomposition dominate during winter months

Long-term carbon cycle

• Functions on timescales of centuries to millennia
• Involves slower processes such as ocean circulation and soil carbon accumulation
• Carbon exchange between atmosphere, deep ocean, and terrestrial reservoirs
• Influenced by climate feedbacks and changes in ocean chemistry
  • Ocean thermal stratification affects carbon uptake
  • Soil carbon dynamics respond to temperature and moisture changes

Geological carbon cycle

• Operates on timescales of millions to hundreds of millions of years
• Driven by tectonic processes, weathering, and sediment burial
• Regulates atmospheric CO2 over geological time through silicate weathering feedback
• Influences long-term climate trends and major climate transitions
  • Formation and breakup of supercontinents
  • Evolution of land plants and their impact on weathering rates

Carbon isotopes

• Carbon isotopes serve as powerful tools in geochemistry for tracing carbon sources and processes
• Understanding carbon isotope behavior is essential for reconstructing past environments and climate conditions
• Carbon isotope ratios provide insights into biological productivity, ocean circulation, and anthropogenic carbon emissions

Stable isotopes of carbon

• Two stable isotopes: carbon-12 (98.9%) and carbon-13 (1.1%)
• Isotopic composition expressed as δ13C in parts per thousand (‰) relative to a standard
  • $δ^{13}C = \left(\frac{^{13}C/^{12}C_{sample}}{^{13}C/^{12}C_{standard}} - 1\right) \times 1000$
• Fractionation occurs during various biological and physical processes
• Used to trace carbon sources and pathways in ecosystems and the global carbon cycle
  • C3 vs C4 plant discrimination
  • Marine vs terrestrial organic matter inputs

Radiocarbon dating

• Carbon-14 (14C) is a radioactive isotope produced in the upper atmosphere
• Half-life of 5,730 years, useful for dating materials up to ~50,000 years old
• Widely used in archaeology, paleoclimatology, and Earth sciences
• Calibration required to account for variations in atmospheric 14C production
  • Tree ring records
  • Marine reservoir effects

Isotopic fractionation

• Preferential use or incorporation of one isotope over another during physical, chemical, or biological processes
• Kinetic fractionation occurs during unidirectional processes (diffusion, evaporation)
• Equilibrium fractionation occurs during reversible processes (dissolution of CO2 in water)
• Biological fractionation results from enzymatic preferences (photosynthesis, methanogenesis)
• Fractionation factors vary with temperature, affecting paleoclimate reconstructions
  • Oxygen isotope fractionation in carbonate shells
  • Carbon isotope fractionation during photosynthesis

Climate change impacts

• Climate change significantly alters the global carbon cycle, affecting carbon reservoirs and fluxes
• Understanding these impacts is crucial in geochemistry for predicting future climate scenarios and developing mitigation strategies
• Carbon cycle changes can lead to positive feedbacks, potentially accelerating global warming

Greenhouse effect

• Increased atmospheric CO2 concentrations enhance the natural greenhouse effect
• CO2 absorbs and re-emits infrared radiation, trapping heat in the lower atmosphere
• Other greenhouse gases (methane, water vapor) also contribute to warming
• Positive feedback loops can amplify warming effects
  • Melting permafrost releases stored carbon
  • Increased water vapor in a warmer atmosphere

Ocean acidification

• Absorption of atmospheric CO2 by oceans leads to decreased pH and carbonate ion concentrations
• Threatens marine ecosystems, particularly calcifying organisms (corals, mollusks)
• Alters ocean carbon chemistry and potentially reduces CO2 uptake capacity
• Impacts marine food webs and biogeochemical cycles
  • Reduced calcification rates in coral reefs
  • Changes in phytoplankton community composition

Feedback mechanisms

• Positive feedbacks amplify initial warming, while negative feedbacks dampen it
• Carbon cycle feedbacks can significantly influence future climate projections
• Examples of positive feedbacks:
  • Permafrost thaw releasing stored carbon
  • Reduced ocean CO2 uptake due to warming and stratification
• Examples of negative feedbacks:
  • Increased plant growth and CO2 uptake in some regions
  • Enhanced weathering rates due to higher temperatures and CO2 levels

Carbon cycle modeling

• Carbon cycle models are essential tools in geochemistry for understanding and predicting carbon dynamics
• Models integrate various processes and feedbacks to simulate carbon fluxes and reservoir changes
• Continuous improvement of models is crucial for accurate climate projections and policy decisions

Box models

• Simplify the carbon cycle into discrete reservoirs (boxes) connected by fluxes
• Useful for conceptual understanding and exploring long-term carbon cycle dynamics
• Can be easily manipulated to test hypotheses and sensitivity to parameter changes
• Examples include:
  • GEOCARB model for geological carbon cycling
  • Ocean carbon cycle box models (Harvardton-Bear type models)

Coupled climate-carbon models

• Integrate carbon cycle processes with climate system components (atmosphere, ocean, land surface)
• Allow for simulation of feedbacks between climate and carbon cycle
• Used in global climate projections and Earth system studies
• Examples include:
  • Earth System Models (ESMs) used in IPCC assessments
  • Community Earth System Model (CESM)

Future projections

• Model scenarios based on different greenhouse gas emission pathways
• Incorporate uncertainties in carbon cycle processes and climate sensitivity
• Project future atmospheric CO2 concentrations and associated climate impacts
• Used to inform policy decisions and mitigation strategies
  • Representative Concentration Pathways (RCPs)
  • Shared Socioeconomic Pathways (SSPs)

Human perturbations

• Human activities have significantly altered the global carbon cycle, leading to rapid increases in atmospheric CO2
• Understanding these perturbations is crucial in geochemistry for assessing anthropogenic impacts on climate and ecosystems
• Mitigation strategies aim to reduce human-induced carbon emissions and enhance natural carbon sinks

Fossil fuel combustion

• Primary source of anthropogenic CO2 emissions, releasing ~35 gigatons of CO2 annually
• Burning of coal, oil, and natural gas for energy production and transportation
• Rapid release of carbon stored over millions of years in geological reservoirs
• Isotopic signature of fossil fuel-derived CO2 (depleted in 13C and 14C) traceable in the atmosphere
  • Suess effect: dilution of atmospheric 14C due to fossil fuel emissions

Land use changes

• Deforestation, agriculture, and urbanization alter terrestrial carbon storage and fluxes
• Contributes ~10% of annual anthropogenic CO2 emissions
• Reduces natural carbon sinks and can turn landscapes from net sinks to sources
• Impacts on soil carbon dynamics and ecosystem functioning
  • Tropical deforestation releases stored carbon and reduces CO2 uptake
  • Agricultural practices can lead to soil carbon depletion or sequestration

Carbon sequestration strategies

• Approaches to remove CO2 from the atmosphere or prevent its release
• Natural climate solutions utilize ecosystems to enhance carbon storage
  • Reforestation and afforestation
  • Wetland and peatland restoration
• Technological approaches to capture and store CO2
  • Carbon capture and storage (CCS) from point sources
  • Direct air capture (DAC) of CO2 from the atmosphere
• Ocean-based strategies to enhance carbon uptake
  • Iron fertilization to stimulate phytoplankton growth
  • Alkalinity enhancement to increase ocean CO2 absorption

Carbon cycle in Earth's history

• The carbon cycle has undergone significant changes throughout Earth's geological history
• Understanding past carbon cycle dynamics is crucial in geochemistry for interpreting long-term climate trends and predicting future scenarios
• Studying past events provides insights into the Earth system's response to major perturbations

Snowball Earth events

• Periods of global glaciation during the Neoproterozoic era (720-635 million years ago)
• Extreme cooling led to ice cover extending to low latitudes
• Carbon cycle played a crucial role in both initiating and terminating these events
  • Drawdown of atmospheric CO2 through silicate weathering initiated glaciation
  • Volcanic CO2 emissions accumulated over millions of years, eventually triggering deglaciation

Paleocene-Eocene Thermal Maximum

• Rapid warming event ~56 million years ago, lasting ~200,000 years
• Characterized by a large carbon isotope excursion, indicating massive carbon release
• Possible sources include methane hydrate destabilization or extensive volcanism
• Resulted in significant environmental and ecological changes
  • Ocean acidification and carbonate dissolution
  • Major shifts in plant and animal distributions

Quaternary glacial cycles

• Periodic alternation between glacial and interglacial periods over the past 2.6 million years
• Driven by variations in Earth's orbit (Milankovitch cycles)
• Carbon cycle amplified and modulated these orbital forcings
  • CO2 concentrations varied between ~180 ppm (glacial) and ~280 ppm (interglacial)
  • Ocean circulation changes and iron fertilization influenced atmospheric CO2 levels

Carbon cycle measurement techniques

• Accurate measurement of carbon cycle components is essential in geochemistry for understanding current dynamics and reconstructing past conditions
• Various techniques are employed to quantify carbon fluxes, reservoirs, and isotopic compositions
• Continuous improvement and integration of measurement methods enhance our understanding of the global carbon cycle

Atmospheric CO2 monitoring

• Direct measurement of atmospheric CO2 concentrations and isotopic composition
• Continuous monitoring at fixed stations (Mauna Loa Observatory)
• Global network of monitoring sites (NOAA's Global Greenhouse Gas Reference Network)
• Satellite-based remote sensing of atmospheric CO2 (OCO-2, GOSAT)
  • Provides global coverage and insights into regional carbon sources and sinks

Ice core analysis

• Extraction and analysis of trapped air bubbles in polar ice cores
• Provides records of atmospheric composition spanning hundreds of thousands of years
• Measurement of CO2, CH4, and other trace gases
• Isotopic analysis of trapped gases and ice for paleoclimate reconstruction
  • Deuterium and oxygen-18 in ice as temperature proxies
  • Carbon-13 in CO2 for insights into carbon cycle processes

Sediment core analysis

• Examination of marine and lacustrine sediment cores for carbon cycle information
• Organic carbon content and isotopic composition
• Carbonate content and isotopic composition of foraminifera shells
• Biomarkers and microfossils for reconstructing past productivity and environmental conditions
  • Alkenones for sea surface temperature reconstruction
  • Leaf wax n-alkanes for terrestrial vegetation changes

Global carbon budget

• The global carbon budget quantifies carbon fluxes between major reservoirs and tracks changes over time
• Understanding the carbon budget is crucial in geochemistry for assessing the Earth system's response to anthropogenic perturbations
• Regular updates of the global carbon budget inform climate policy and mitigation strategies

Carbon sinks vs sources

• Carbon sinks remove CO2 from the atmosphere, while sources release CO2
• Major natural sinks include:
  • Terrestrial ecosystems (forests, soils)
  • Oceans (dissolution, biological pump)
• Significant anthropogenic sources:
  • Fossil fuel combustion
  • Land-use changes (deforestation)
• Net balance between sinks and sources determines atmospheric CO2 trends
  • Currently, sinks absorb ~50% of anthropogenic emissions

Anthropogenic carbon emissions

• Fossil fuel combustion and cement production: ~35 gigatons CO2 per year
• Land-use changes: ~5 gigatons CO2 per year
• Emissions have increased dramatically since the Industrial Revolution
• Geographical distribution of emissions has shifted over time
  • Historical dominance of North America and Europe
  • Recent rapid growth in emissions from Asia

Carbon cycle uncertainties

• Uncertainties in carbon fluxes and reservoir sizes affect budget calculations
• Major sources of uncertainty include:
  • Land carbon sink strength and variability
  • Ocean carbon uptake and its future evolution
  • Permafrost carbon feedback magnitude
• Improving measurements and models to reduce uncertainties
  • Enhanced monitoring networks and remote sensing capabilities
  • Development of high-resolution Earth system models