Lecture 11: Biogeochemical Cycles: Water and Carbon

Water Cycle Review and Biogeochemical Cycles Introduction

  • Water Cycle Activity Answers:
    • Atmosphere residence time: approximately 9.5 days (very short).
    • Ocean residence time: over 3000 years (very large reservoir).
    • Land residence time: approximately 371 years (includes water in ice, streams, rivers, lakes, soils, and biota).
    • These residence times vary depending on specific reservoirs within the land compartment.
  • Upcoming Focus: This week's discussion section will apply similar concepts to the carbon cycle, including human impacts.
  • Biogeochemical Cycles Overview:
    • Definition: Global patterns of chemical elements circulating between living (biotic) and nonliving (abiotic) compartments/reservoirs.
    • Components: Involve both nonliving (e.g., environment) and living (e.g., organisms) parts of an ecosystem.
    • Interactions: Affect life on Earth and are affected by life on Earth.
    • Climate Dependence: Rates of exchange (flows) of elements can be changed by climate conditions.
    • Human Impact: Humans have strongly perturbed some of these cycles.
    • Compartments (Spheres): Atmosphere, Biosphere, Hydrosphere, and Lithosphere.
      • Alternative terms: Atmosphere, Terrestrial organisms, Aquatic organisms (oceans/water bodies), Rock.

General Model of Biogeochemical Cycles

  • Purpose of the Model: A framework to understand how different elements move and cycle, highlighting differences between various cycles.
  • Key Compartments (Reservoirs):
    • Atmosphere: The top starting point for conceptualizing fluxes.
    • Water: Includes water in soil, rivers, lakes, and oceans.
    • Rock (Lithosphere): Geological formations.
    • Ocean Sediments: Material at the bottom of the ocean.
    • Terrestrial Communities: Organisms and ecosystems on land.
    • Aquatic Communities: Organisms in saltwater (ocean), freshwater, or brackish systems.
  • Fluxes (Movements of Material/Elements):
    • From Atmosphere:
      • Directly into terrestrial or aquatic communities (e.g., CO_2 uptake by organisms).
      • Directly into water (e.g., CO_2 dissolving into water).
    • Within Terrestrial Communities:
      • Biotic uptake and assimilation through water (e.g., from soil).
      • Processing through food webs (primary producers, consumers, decomposers).
      • Recycling within the community (a single molecule might pass through many individuals).
    • From Terrestrial Communities/Water:
      • Decomposition allows material to re-enter water or soil (e.g., runoff).
      • Streamflow transports material to aquatic communities.
    • Within Aquatic Communities:
      • Biotic uptake (e.g., dissolved CO_2 used by photosynthesizing algae).
      • Recycling through aquatic food webs (tightly cycled, especially for limiting nutrients).
    • Return to Atmosphere:
      • From terrestrial communities, water, or aquatic communities through respiration (e.g., CO_2 from organic molecules).
    • To Ocean Sediments:
      • Decomposition of dead terrestrial organisms leads to runoff into oceans.
      • Dead aquatic organisms settle to the ocean floor.
    • From Ocean Sediments to Rock and Back:
      • Material settles, gets buried, compressed, heated over geological timescales.
      • Geological uplift exposes rock and material to the environment.
      • Physical and chemical weathering liberates elements back into the environment.
  • Biological Mediation - Water Cycle Example:
    • The process in the water cycle mediated by a biological component is evapotranspiration (C).
    • Evapotranspiration is the combined process of evaporation and transpiration.
    • Transpiration is mediated by plants, which pump water vapor into the atmosphere.

Human Perturbations of Biogeochemical Cycles

  • Overarching Theme: Human activities are fundamentally changing many biogeochemical cycles.
  • Human Activities (represented as a 'big star' reactive element):
    • Increased Emissions to Atmosphere: Raising concentrations of elements (e.g., greenhouse gases).
    • Affecting Terrestrial Communities:
      • Land clearance, forestry, agriculture.
      • Leads to less carbon storage, excess nutrients, material movement.
    • Affecting Water Compartment:
      • Increased concentrations in water (pollution, excess nutrients from agriculture/fertilizers).
    • Harvesting:
      • Harvesting fish from aquatic communities transfers biomass (and elements like phosphorus) to land (e.g., human plates).

The Carbon Cycle

  • Focus: Carbon as an element and material, distinct from energy transfer in ecosystems.
  • Forms of Carbon:
    • Organic Carbon (O, purple O):
      • Characterized by Carbon-Hydrogen (C-H) or Carbon-Carbon (C-C) covalent bonds.
      • Examples: Sugars, carbohydrates, lignin, DNA, fossil fuels.
      • Often carries chemical energy; also called fixed or reduced.
    • Inorganic Carbon (I, orange I):
      • Does not have bonds with hydrogen.
      • Examples: Carbon dioxide (CO2), calcium carbonate (CaCO3 in limestone, shells).
      • Also called oxidized.
  • Why is the Carbon Cycle Important?
    • Basis of Life: All life on Earth is carbon-based.
    • Global Climate: Determines atmospheric concentrations of greenhouse gases (CO2, CH4), impacting climate.
    • Ocean Acidification: Dissolved CO_2 in ocean waters contributes to this issue.
    • Energy System: Fossil fuels (ancient sunshine – energy stored in covalent bonds from a long time ago) are a primary energy source.
    • Human Impact: Human actions have significantly altered the carbon cycle, leading to global warming and other impacts.

The Carbon Cycle: Major Pools and Slow Processes

  • Illustration Differences: Certain reservoirs (ocean sediments, lithosphere) are shown with dotted lines in the carbon cycle model.
  • Dotted Lines Significance: Indicate part of the slow processes; fluxes in and out are very slow and small compared to reservoir size.
    • Amount of carbon in ocean sediments is small and slow to flux.
    • Lithosphere contains a very large amount of carbon, which leaves through slow chemical weathering.
  • Question: Is the carbon cycle a closed system?
    • On Earth, the elements (including carbon) are finite, making it a closed system.
    • Ecosystems, however, are open systems where carbon can enter and leave.

Specific Carbon Fluxes (Natural Cycle)

  • Atmosphere: Carbon present as CO2 and methane (CH4).
  • Flux from Atmosphere (Inorganic Carbon):
    • Photosynthesis (Terrestrial): Primary producers (plants) uptake CO_2 from the atmosphere into terrestrial communities.
    • Dissolution (Aquatic): CO_2 dissolves into water, then taken up by aquatic communities.
  • Within Terrestrial Communities (Organic Carbon):
    • Recycled through the ecosystem via food webs.
    • Dead organic matter (e.g., pieces of cells) becomes soil organic carbon.
    • Decomposers consume dead organic matter.
    • Respiration: Terrestrial communities release CO_2 back to the atmosphere.
  • From Terrestrial to Aquatic (Organic/Inorganic Carbon):
    • Soil organic carbon can be leached or drained into rivers, lakes, and oceans as dissolved organic carbon.
    • Stream flow transports carbon to aquatic communities.
  • Within Aquatic Communities:
    • Primary Producers: Dissolved CO_2 used by photosynthesizing algae and aquatic plants.
    • Heterotrophs: Dissolved organic carbon feeds heterotrophic organisms (e.g., microbes).
    • Recycled through aquatic food webs.
    • Respiration: Aquatic communities release CO_2 back to the atmosphere (especially from surface waters).
    • Calcium Carbonate Formation: Marine organisms accumulate carbon in their shells (CaCO_3).
  • To Ocean Sediments (Organic/Inorganic Carbon):
    • Dead aquatic organisms, shells (limestone) settle to the ocean floor.
    • Only about 0.1\% of organic matter makes it to the deep ocean; most is used up by microbial processes along the way.
  • Geological Cycle (Slow Processes):
    • Geological Uplift: Buried ocean sediments (now rock, e.g., limestone) are uplifted and exposed.
    • Weathering: Exposed rock undergoes physical and chemical weathering (e.g., CaCO_3 dissolved by slightly acidic raindrops), returning inorganic carbon to water compartments.

Terrestrial Carbon Cycle in Detail

  • Photosynthesis: CO_2 (inorganic) from atmosphere taken up by plants, assimilated into organic carbon.
  • Plant Respiration: Plants release CO_2 (inorganic) back to atmosphere.
  • Dead Organic Matter: When organisms die or are consumed and die, this matter enters the soil.
  • Heterotrophic Respiration: Microbes (decomposers) in soils consume organic matter, releasing CO_2 (inorganic) back to atmosphere.
  • Absorption into Minerals: Some carbon is bound into inorganic mineral forms.
  • Transfer to Aquatic Ecosystems: Dissolved organic carbon (DOC) is washed out by rain to rivers, lakes, and oceans.
  • Anaerobic Respiration: In waterlogged soils (bogs, swamps, wetlands), decomposition occurs without oxygen, producing methane (CH4) and CO2 (inorganic), which escapes to the atmosphere (e.g.,