4.1 Chemical composition of seawater

Seawater's chemical composition is dominated by major ions like chloride and sodium. These ions maintain constant ratios throughout the ocean, simplifying analysis and enabling the study of ocean circulation patterns. This consistency is known as Marcet's Principle.

The ocean's dissolved ions come from various sources, including weathering, volcanic activity, and biological processes. They're removed through biological uptake, chemical precipitation, and other processes. Understanding ion residence times helps us grasp oceanic chemical cycles and predict environmental impacts.

Major Ions and Chemical Composition of Seawater

Major dissolved ions in seawater

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• Chloride (Cl-) dominates seawater composition accounts for 55% of total dissolved solids
• Sodium (Na+) second most abundant ion makes up 30% of total dissolved solids
• Sulfate (SO4^2-) comprises 8% of total dissolved solids plays crucial role in marine biogeochemical cycles
• Magnesium (Mg^2+), calcium (Ca^2+), and potassium (K+) collectively contribute 7% of total dissolved solids essential for marine life processes
• Minor ions include bicarbonate (HCO3-) regulates ocean pH, bromide (Br-) involved in atmospheric chemistry, strontium (Sr^2+) used in paleoceanography, and boron (B) influences marine productivity
• Trace elements like iron (Fe), zinc (Zn), and copper (Cu) present in minute quantities but vital for biological functions (enzyme cofactors)

Concept of constant proportions

• Relative ratios of major ions maintain consistency throughout global ocean enables simplified analysis of seawater composition
• Marcet's Principle states major constituents in seawater maintain constant ratios facilitates oceanographic research
• Allows salinity determination using single ion measurement streamlines oceanographic field work
• Enables use of chlorinity as proxy for total salt content simplifies salinity calculations
• Facilitates study of ocean circulation and mixing patterns aids in understanding global ocean dynamics
• Exceptions occur in coastal areas with significant freshwater input (estuaries), deep-sea hydrothermal vents, and isolated basins (Dead Sea)

Sources and sinks of dissolved ions

• Sources:
  1. Weathering of terrestrial rocks releases ions through chemical and physical processes
  2. Volcanic activity introduces ions via submarine volcanoes and hydrothermal vents
  3. Atmospheric deposition contributes ions through sea spray and dust particles
  4. Biological processes release ions through decomposition of organic matter and shell dissolution
• Sinks:
  1. Biological uptake removes ions for incorporation into shells, skeletons, and cellular processes
  2. Chemical precipitation forms evaporite deposits and authigenic minerals
  3. Adsorption onto particles like clay minerals and organic matter removes ions from solution
  4. Hydrothermal alteration exchanges ions with oceanic crust modifying seawater composition

Residence time of seawater ions

• Average time an ion spends in ocean before removal indicates its reactivity and cycling rate
• Calculated using formula $Residence time = \frac{Total amount of ion in the ocean}{Rate of input or removal}$
• Influenced by chemical reactivity, biological uptake rates, and physical removal processes
• Longer residence times generally correlate with higher concentrations (sodium: 260 million years)
• Shorter residence times indicate more dynamic cycling (iron: few hundred years)
• Helps understand oceanic chemical cycles and predict impacts of anthropogenic inputs
• Indicates sensitivity of ions to environmental changes crucial for climate change studies