3.3 Ocean Carbon Dynamics and Acidification

Oceans play a crucial role in regulating Earth's carbon cycle. They absorb CO2 through physical and biological processes, including the solubility pump and biological carbon pump. These mechanisms transport carbon from the surface to deep waters.

Ocean acidification, caused by increased CO2 absorption, poses significant threats to marine life. It impacts shell-forming organisms, alters ecosystems, and affects coral reefs. Understanding ocean-atmosphere interactions is vital for grasping global carbon dynamics and climate change impacts.

Ocean Carbon Dynamics

Carbon pumps in oceans

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• Physical carbon pump drives CO2 dissolution in cold surface waters and downwelling in polar regions
  • Solubility pump dissolves atmospheric CO2 in colder waters (North Atlantic, Southern Ocean)
  • Thermocline pump transports dissolved inorganic carbon vertically due to temperature-dependent CO2 solubility
• Biological carbon pump converts inorganic carbon to organic matter through phytoplankton photosynthesis
  • Sinking particulate organic carbon (marine snow) transports carbon to deep waters
  • Remineralization by bacteria breaks down organic matter, releasing CO2 at depth
  • Carbonate counter pump forms calcium carbonate shells (coccolithophores, foraminifera) which sink and dissolve

Ocean acidification and marine impacts

• Ocean acidification occurs as seawater absorbs increasing atmospheric CO2
  • Forms carbonic acid: $CO_2 + H_2O \rightarrow H_2CO_3$
  • Dissociates: $H_2CO_3 \rightarrow H^+ + HCO_3^-$, lowering seawater pH
• Marine organism impacts include:
  • Reduced calcification in shell-forming species (oysters, mussels)
  • Altered growth and reproduction across various taxa
  • Food web disruptions as plankton composition shifts
  • Coral reef degradation due to reduced aragonite saturation
  • Physiological stress in fish affecting behavior and sensory abilities

Ocean-Atmosphere Interactions

Oceans as CO2 regulators

• Oceans absorb ~25% of anthropogenic CO2 emissions, acting as a major carbon sink
• Air-sea gas exchange driven by partial pressure differences between atmosphere and surface waters
• Oceanic carbon storage occurs in:
  • Dissolved inorganic carbon pool (largest reservoir)
  • Organic carbon in marine biota and sediments
• Seawater's buffering capacity maintains pH through:
  • Carbonate system equilibrium reactions
  • Weathering of carbonate rocks (limestone) over geological timescales

Oceans for anthropogenic carbon absorption

• Ocean carbon uptake influenced by:
  • Air-sea interface area (70% of Earth's surface)
  • Wind speed and wave action increasing gas exchange
  • Temperature and salinity gradients affecting CO2 solubility
• Surface water saturation state decreasing with ocean warming, reducing CO2 absorption capacity
• Biological response to elevated CO2 potentially enhancing primary production (CO2 fertilization)
• Feedback mechanisms include:
  • Altered ocean circulation patterns (thermohaline circulation)
  • Marine ecosystem structure changes (phytoplankton community shifts)
• Long-term storage potential through:
  • Deep ocean carbon sequestration (centuries to millennia)
  • Sedimentary carbonate formation (geological timescales)