🪨Biogeochemistry Unit 3 – Carbon Cycle: Earth's Interconnected Systems

Key Concepts and Definitions

• Carbon cycle involves the exchange of carbon between Earth's spheres (atmosphere, biosphere, hydrosphere, and lithosphere)
• Carbon reservoirs are places where carbon is stored for varying lengths of time
  • Examples include oceans, atmosphere, terrestrial ecosystems, and sedimentary rocks
• Carbon fluxes refer to the movement of carbon between reservoirs
  • Can occur through physical, chemical, and biological processes (photosynthesis, respiration, weathering)
• Photosynthesis is the process by which plants and other organisms convert sunlight into chemical energy, fixing atmospheric carbon dioxide into organic compounds
• Respiration releases stored carbon back into the atmosphere as CO2 through cellular processes in living organisms
• Weathering of rocks and minerals can consume atmospheric CO2 and store it in the lithosphere
• Anthropogenic activities such as fossil fuel combustion and land-use changes have significantly altered the natural carbon cycle

Carbon Reservoirs and Fluxes

• The global carbon cycle consists of four main reservoirs: atmosphere, biosphere, hydrosphere, and lithosphere
• Atmosphere contains approximately 750 gigatons of carbon (GtC), primarily as CO2
• Biosphere stores around 2,000 GtC in living organisms and decaying organic matter
  • Terrestrial biosphere holds ~1,500 GtC, while marine biosphere holds ~500 GtC
• Hydrosphere, particularly oceans, is the largest active carbon reservoir with about 38,000 GtC
  • Dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) are the main forms
• Lithosphere stores the vast majority of Earth's carbon, estimated at 60,000,000 GtC in sedimentary rocks and fossil fuels
• Carbon fluxes between reservoirs occur on various timescales, from short-term (photosynthesis and respiration) to long-term (rock weathering and sedimentation)

Terrestrial Carbon Cycle

• Terrestrial carbon cycle involves the exchange of carbon between the atmosphere and land-based ecosystems
• Photosynthesis by plants and other autotrophs fixes atmospheric CO2 into organic compounds
  • Globally, terrestrial photosynthesis removes ~120 GtC from the atmosphere annually
• Respiration by plants, animals, and microorganisms releases CO2 back into the atmosphere
  • Approximately half of the carbon fixed by photosynthesis is returned to the atmosphere through respiration
• Soil carbon pool, consisting of organic matter and microorganisms, holds around 1,500 GtC
  • Decomposition of organic matter by microbes releases CO2 and nutrients back into the soil and atmosphere
• Wildfires and biomass burning release stored carbon back into the atmosphere rapidly
• Land-use changes (deforestation, agriculture) can significantly alter terrestrial carbon storage and fluxes

Oceanic Carbon Cycle

• Oceans play a crucial role in regulating atmospheric CO2 levels through physical, chemical, and biological processes
• Physical carbon pump involves the dissolution of atmospheric CO2 into surface waters and its transport to the deep ocean via circulation patterns
  • Solubility of CO2 increases in cold, high-latitude waters, leading to increased absorption
• Biological carbon pump refers to the fixation of CO2 by marine photosynthetic organisms (phytoplankton) and the subsequent sinking of organic matter to the deep ocean
  • Approximately 50 GtC per year are fixed through marine photosynthesis
• Carbonate pump involves the formation and dissolution of calcium carbonate (CaCO3) shells by marine organisms, affecting ocean alkalinity and CO2 uptake
• Ocean acidification occurs when increased atmospheric CO2 dissolves in seawater, lowering pH and potentially impacting marine ecosystems
• Upwelling of deep, carbon-rich waters can release stored CO2 back into the atmosphere

Atmospheric Carbon Cycle

• Atmosphere acts as a central hub in the global carbon cycle, exchanging carbon with other reservoirs
• CO2 is the primary form of carbon in the atmosphere, with a current concentration of around 415 parts per million (ppm)
• Atmospheric CO2 levels are influenced by natural processes (photosynthesis, respiration, ocean-atmosphere exchange) and human activities (fossil fuel combustion, land-use changes)
• Greenhouse effect: CO2 and other greenhouse gases absorb and re-emit infrared radiation, warming Earth's surface
  • Increasing atmospheric CO2 concentrations due to human activities are the main driver of current climate change
• Carbon isotopes (12C, 13C, 14C) in the atmosphere can be used to trace carbon sources and sinks
  • Radiocarbon dating relies on the decay of 14C to determine the age of organic materials

Human Impact on the Carbon Cycle

• Anthropogenic activities have significantly altered the natural carbon cycle since the Industrial Revolution
• Fossil fuel combustion releases ancient carbon stored in the lithosphere back into the atmosphere
  • Burning of coal, oil, and natural gas emits ~9 GtC per year
• Land-use changes, particularly deforestation and agricultural expansion, reduce terrestrial carbon storage and increase atmospheric CO2
  • Deforestation contributes ~1-2 GtC per year to the atmosphere
• Cement production involves the calcination of limestone (CaCO3), releasing CO2 as a byproduct
• Anthropogenic CO2 emissions have led to an increase in atmospheric concentrations from ~280 ppm (pre-industrial) to ~415 ppm (present-day)
• Enhanced greenhouse effect resulting from human activities is the primary driver of observed climate change, including global warming, sea-level rise, and changes in precipitation patterns

Carbon Cycle Models and Measurements

• Carbon cycle models simulate the movement of carbon between reservoirs and the response to perturbations (e.g., increased emissions)
  • Models range from simple box models to complex Earth system models (ESMs) that incorporate multiple biogeochemical cycles and climate feedbacks
• Measurements of atmospheric CO2 concentrations began in the late 1950s at Mauna Loa Observatory (Keeling Curve)
  • Annual oscillations reflect seasonal changes in photosynthesis and respiration
  • Long-term trend shows a steady increase due to anthropogenic emissions
• Ice core records provide a longer-term perspective on atmospheric CO2 levels, extending back hundreds of thousands of years
  • CO2 concentrations varied between ~180-280 ppm during glacial-interglacial cycles
• Remote sensing techniques (e.g., satellite imagery) can monitor changes in terrestrial and marine carbon storage
• Eddy covariance towers measure local-scale carbon fluxes between the atmosphere and ecosystems

Connections to Other Biogeochemical Cycles

• Carbon cycle is closely linked to other biogeochemical cycles, particularly nitrogen and phosphorus
  • Nutrient availability can limit photosynthesis and carbon fixation in terrestrial and marine ecosystems
• Nitrogen fixation by microorganisms converts atmospheric N2 into biologically available forms, supporting plant growth and carbon storage
• Phosphorus weathering and transport by rivers deliver essential nutrients to marine ecosystems, influencing primary productivity
• Methane (CH4) is a potent greenhouse gas with both natural (wetlands, termites) and anthropogenic (agriculture, fossil fuels) sources
  • Methane oxidation in the atmosphere and soils acts as a sink
• Permafrost thaw due to warming temperatures can release stored carbon (CO2 and CH4) into the atmosphere, creating a positive feedback
• Ocean deoxygenation, linked to warming and stratification, can alter marine carbon storage and cycling
  • Expansion of oxygen minimum zones (OMZs) can increase anaerobic respiration and release of CO2 and CH4