10.3 Open Ocean Biogeochemistry

The open ocean is a vital player in Earth's biogeochemistry. Phytoplankton drive primary production, converting inorganic carbon to organic matter. Nutrient limitation, especially by nitrogen, phosphorus, and iron, controls productivity. The biological pump transports organic matter from surface to deep waters.

Ocean circulation shapes nutrient distribution. Upwelling brings nutrient-rich deep waters to the surface, while deep water formation ventilates the ocean interior. Climate change impacts ocean biogeochemistry through acidification, deoxygenation, and shifts in primary production. The ocean acts as a crucial carbon sink, absorbing CO2 through solubility and biological pumps.

Open Ocean Biogeochemical Processes

Biogeochemical processes in open oceans

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• Primary production
  • Photosynthesis by phytoplankton converts inorganic carbon to organic matter
  • Net primary production measures carbon fixed minus respiration
  • Gross primary production accounts for total carbon fixed without subtracting respiration
  • Phytoplankton responsible for ~50% of global primary production (diatoms, coccolithophores)
• Nutrient limitation
  • Liebig's Law of the Minimum states growth limited by scarcest resource
  • Major limiting nutrients nitrogen, phosphorus, iron control productivity
  • Redfield ratio $C:N:P = 106:16:1$ reflects elemental composition of marine organic matter
  • Iron limitation common in high-nutrient, low-chlorophyll (HNLC) regions (Southern Ocean)
• Biological pump
  • Vertical transport of organic matter from surface to deep ocean
  • Sinking particles and fecal pellets carry carbon downward
  • Remineralization in water column releases nutrients and CO2
  • Export production measures organic matter escaping surface layer
  • Carbon sequestration occurs when organic matter reaches seafloor

Ocean circulation and nutrient distribution

• Upwelling
  • Wind-driven coastal upwelling brings nutrient-rich deep waters to surface
  • Equatorial upwelling driven by trade winds and Ekman transport
  • Upwelling regions highly productive (Peruvian coast, Benguela Current)
• Deep water formation
  • Thermohaline circulation driven by temperature and salinity differences
  • North Atlantic Deep Water (NADW) forms in Nordic and Labrador Seas
  • Antarctic Bottom Water (AABW) forms in Weddell and Ross Seas
  • Deep water formation ventilates ocean interior and transports nutrients
• Nutrient distribution
  • Surface waters depleted in nutrients due to biological uptake
  • Deep waters enriched in nutrients from remineralization
  • Nutrient spiraling describes repeated uptake and release along circulation path
  • Vertical profiles show low surface concentrations, increasing with depth
• Dissolved gases
  • Oxygen minimum zones form where respiration exceeds ventilation
  • Carbon dioxide solubility pump driven by temperature-dependent solubility
  • Methane hydrates found in cold, high-pressure environments (continental slopes)
  • Gas seeps release methane from seafloor (hydrothermal vents, cold seeps)

Climate Change Impacts and Ocean-Climate Interactions

Climate change impacts on ocean biogeochemistry

• Ocean acidification
  • Atmospheric CO2 dissolves in seawater, lowering pH
  • Carbonate system equilibrium: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+ \leftrightarrow CO_3^{2-} + 2H^+$
  • Impacts calcifying organisms by reducing carbonate ion availability
  • Changes carbonate saturation state, affecting shell formation (corals, pteropods)
• Ocean deoxygenation
  • Warmer waters hold less dissolved oxygen
  • Enhanced stratification reduces vertical mixing and ventilation
  • Oxygen minimum zones expand, threatening marine ecosystems
  • Alters biogeochemical cycles, particularly nitrogen and phosphorus
• Changes in primary production
  • Shifts phytoplankton community structure (smaller species favored)
  • Alters nutrient availability and limitation patterns
  • Impacts biological pump efficiency and carbon export
  • Regional variations expected (increases in polar regions, decreases in tropics)

Ocean's role as global carbon sink

• Ocean carbon sink
  • Solubility pump dissolves CO2 in cold, dense waters
  • Biological pump exports organic carbon to deep ocean
  • Ocean absorbs ~25% of anthropogenic CO2 emissions annually
• Carbon cycle feedbacks
  • Changes in ocean circulation affect CO2 uptake and storage
  • Alterations in marine ecosystems impact carbon cycling
  • Potential release of methane hydrates amplifies warming
• Ocean-atmosphere interactions
  • Air-sea gas exchange driven by partial pressure differences
  • El Niño-Southern Oscillation (ENSO) influences CO2 fluxes
  • Long-term changes in ocean heat content affect CO2 solubility
• Future projections
  • Ocean carbon sink may saturate, reducing uptake capacity
  • Tipping points in ocean-climate system (thermohaline circulation shutdown)
  • Uncertainties in climate models and biogeochemical feedbacks complicate predictions