Marine Carbon Cycle Flashcards

Characteristics and Properties of Carbon

  • General Information:

    • Carbon is a non-metallic element found in the periodic table.

    • Atomic Number: 66

    • Atomic Mass: 12.0111512.01115

    • Etymology: The name originates from the Latin word "carbon," meaning coal or charcoal.

    • Natural Abundance: It is a naturally abundant element that occurs in many inorganic compounds and in all organic compounds.

  • Physical and Chemical Properties:

    • Melting Point: 3500 ∘C3500\,^\circ\text{C}

    • Boiling Point: 4827 ∘C4827\,^\circ\text{C}

    • Density: 2.2 g cm−32.2\,g\,cm^{-3}

    • Chemical Formula: CC

    • Reactivity: Carbon undergoes oxidation when it combines with O2O_2 to produce Carbon Dioxide (CO2CO_2) and Carbon Monoxide (COCO).

    • Compounds: There are several million known carbon compounds.

    • Structural Ability: Carbon has the unique ability to make long strings or chains of atoms.

The Carbon Cycle as a Biogeochemical Process

  • Definition and Transport:

    • Carbon is transported through different components of the environment, moving between living (biotic) and non-living (abiotic) components.

    • It is considered a biogeochemical cycle because its movement involves biological interactions, geological activities, and chemical reactions.

  • Biological Importance:

    • Carbon is a vital nutrient for life as it is a fundamental component of:

      • Fats

      • Carbohydrates

      • Proteins

      • Nucleic acids

Reservoirs of Carbon on Earth

  • Atmosphere:

    • Acts as a dynamic reservoir influenced by both natural and anthropogenic (human) activities.

    • Examples: Methane gas (CH4CH_4) and Carbon Dioxide (CO2CO_2).

  • Biosphere:

    • Terrestrial Ecosystem: Plants, animals, and microorganisms store carbon as organic matter.

    • Marine Ecosystem: Phytoplankton absorb CO2CO_2 during photosynthesis, forming the foundational base of the oceanic food web.

    • Examples: Living and dead organisms.

  • Hydrosphere:

    • Oceans hold dissolved inorganic carbon primarily as bicarbonate and carbonate.

    • The hydrosphere serves as a significant carbon sink, absorbing approximately 25%25\% of anthropogenic CO2CO_2 emissions.

    • Examples: Bicarbonate and carbonate ions.

  • Lithosphere:

    • This reservoir includes fossil fuels, sedimentary rocks (such as limestone), and soil organic matter.

    • Carbon can be stored for millions of years in geological formations.

    • Examples: Soil and rocks.

Quantitative Major Stores of Carbon

Sink

Amount in Billions of Metric Tons

Atmosphere

578578 (as of 1700) to 766766 (as of 1999)

Soil Organic Matter

1,5001,500 to 1,6001,600

Ocean

38,00038,000 to 40,00040,000

Marine Sediments & Sedimentary Rocks

66,000,00066,000,000 to 100,000,000100,000,000

Terrestrial Plants

540540 to 610610

Fossil Fuel Deposits

4,0004,000

Fundamentals of the Marine Carbon Cycle

  • Definition: Also known as the oceanic carbon cycle, it refers to the continuous processes by which carbon is exchanged within the ocean and between the ocean and the atmosphere.

  • Capacity: Oceans are an extensive reservoir containing roughly 36,000 gigatons36,000\text{ gigatons} of carbon, which is approximately 6060 times more than the atmosphere.

  • Role in Climate: The marine carbon cycle is essential in mitigating global warming by absorbing excess CO2CO_2 emissions.

  • Uncertainty: Anthropogenic climate change impacts ocean circulation and ecosystem dynamics, making the future of the ocean as a carbon sink increasingly uncertain.

  • Time Scales: Due to the ocean's vast size and rapid surface exchange, it controls atmospheric CO2CO_2 concentrations over timescales of centuries or longer. Slight changes in natural components significantly affect the Earth's climate system.

Classification of Carbon in the Ocean

  • Inorganic Carbon:

    • Found in simple compounds: CO2CO_2, carbonates, carbonic acid, and bicarbonate.

    • Dissolved Inorganic Carbon (DIC): Includes dissolved phases.

    • Particulate Inorganic Carbon (PIC): Includes solid forms like coccolithophores, foraminifera, and coral fragments.

  • Organic Carbon:

    • The main component of proteins, carbohydrates, nucleic acids, and lipids.

    • Dissolved Organic Carbon (DOC): Organic molecules dissolved in seawater.

    • Particulate Organic Carbon (POC): Includes detritus and living organisms.

Atmosphere-Ocean Exchange and Chemical Reactions

  • Diffusion Process: CO2CO_2 diffuses into sea surface waters and dissolves via chemical processes. This depends on variables such as wind, water temperature, sea surface mixing, and CO2CO_2 concentration gradients.

  • Chemical Reactions in Seawater:

    1. CO2(gas)+H2O⇌H2CO3(aq)CO_{2(gas)} + H_2O \rightleftharpoons H_2CO_{3(aq)} (Formation of Carbonic acid)

    2. H2CO3⇌H++HCO3−H_2CO_3 \rightleftharpoons H^+ + HCO_3^- (Bicarbonate dissociation)

    3. HCO3−⇌CO32−+H+HCO_3^- \rightleftharpoons CO_3^{2-} + H^+ (Carbonate dissociation)

  • Net Reaction: H2O+CO2(gas)+CO32−⇌2HCO3−H_2O + CO_{2(gas)} + CO_3^{2-} \rightleftharpoons 2HCO_3^-

  • Species in Seawater: Carbon occurs as CO2CO_2 gas, H2CO3H_2CO_3, HCO3−HCO_3^-, and CO32−CO_3^{2-}.

Inputs and Outputs of the Oceanic Carbon Cycle

  • Inputs:

    • Rivers: Transport organic carbon via weathering and erosion of minerals and carbonate rocks.

    • Atmosphere: Historically balanced at roughly 280 ppm280\,ppm before the Industrial Revolution. Now rapidly increasing.

    • Temperature Factor: The capacity of oceans to absorb CO2CO_2 decreases as sea surface temperatures rise.

  • Outputs:

    • Particulate Organic Matter (POM): Sinking of organic material.

    • Calcium Carbonate Preservation: Burial in sediments.

    • Geological Cycling: Plate tectonics recycle deep-sea rocks to the surface for weathering or subduction into the mantle. Carbon is then outgassed via volcanic eruptions over time scales exceeding 500,000500,000 years.

Marine Carbon Pumps

Ocean carbon pumps maintain vertical gradients where DIC concentration is low at the surface and high at depth, regulating the partitioning of carbon between the ocean and atmosphere.

1. Solubility/Physical Pump

  • Mechanism: Physicochemical process transporting carbon from the surface to the deep ocean as DIC.

  • Driver 1: Solubility: CO2CO_2 is more soluble in cold water.

  • Driver 2: Global Ocean Conveyor Belt (Thermohaline Circulation): Cold waters at high latitudes sink (downwelling), bringing dissolved CO2CO_2 into the deep ocean. At warmer equatorial latitudes, upwelling brings deep cold water to the surface, and CO2CO_2 is returned to the atmosphere.

2. Biological Pump

  • Mechanism: Biologically mediated process starting with photosynthetic uptake of CO2CO_2 by phytoplankton in the euphotic zone.

  • Phases:

    • Fixation of carbon by planktonic phototrophs.

    • Sinking of dead organisms or waste.

    • Decomposition/remineralization by bacteria.

    • Sequestration in sediments for millennia.

  • Impact: Transfers approximately 10.2 gigatonnes10.2\text{ gigatonnes} of carbon to the deep ocean yearly (1300 gigatonnes1300\text{ gigatonnes} over 127127 years). Without it, atmospheric CO2CO_2 would be about 400 ppm400\,ppm higher.

3. Carbonate Pump

  • Mechanism: Also known as the "carbonate counter pump" or "hard tissue pump."

  • Process: Begins at the surface where organisms (like coccolithophores) generate PIC in the form of calcium carbonate (CaCO3CaCO_3) shells.

  • Counter-Intuitive Effect: Calcification/shell formation actually increases atmospheric CO2CO_2 in its immediate chemical environment through the shift in carbonate chemistry, even as it facilitates the downward transport of carbon to deep sediments.

Human Impacts and Ocean Acidification

  • Drivers of Increase: Fossil fuel combustion (releasing CO2CO_2 and CH4CH_4), deforestation, agriculture (methane from livestock, carbon from tillage), and industrial activities (e.g., cement production).

  • Ocean Acidification: Excess CO2CO_2 absorption decreases ocean pH.

    • Chemical Impact: Lowers carbonate saturation levels.

    • Biological Impact: Calcifying organisms struggle to build shells/skeletons. Biological consequences include depressed metabolic rates, reduced energy for essential functions, and lowered immune responses.

    • Socioeconomic Impact: Affects ecosystems providing food and livelihoods.

Atmospheric CO2 Monitoring: Mauna Loa Observatory

  • Location: Hawaii, approximately 3,400 meters3,400\text{ meters} (11,141 feet11,141\text{ feet}) above sea level. It is isolated from significant pollution sources.

  • History: Direct measurements began in March 1958 by C. David Keeling (Scripps Institution of Oceanography). NOAA began parallel measurements in May 1974.

  • Keeling Curve Data:

    • Pre-Industrial: roughly 280 ppm280\,ppm.

    • April 2024: 426.51 ppm426.51\,ppm.

    • April 2025: 429.64 ppm429.64\,ppm.

    • Observed trends correlate increased atmospheric CO2CO_2 (ppm) with decreased seawater pH and increased seawater pCO2pCO_2 ($\mu atm$).