Ocean Chemistry

Composition of Seawater

Seawater contains approximately 35 grams of dissolved salts per kilogram, composed primarily of six ions that together account for 99.4 percent of total dissolved salts: chloride (55 percent), sodium (30.6 percent), sulfate (7.7 percent), magnesium (3.7 percent), calcium (1.2 percent), and potassium (1.1 percent). This composition remains remarkably constant across all ocean basins despite varying total concentration (salinity), a principle called the constancy of composition or Marcet's principle. Measuring any single major ion allows calculation of total salinity.

Trace elements present at parts-per-billion concentrations play critical biological roles despite their scarcity. Iron limits phytoplankton growth across one-third of the ocean surface. Zinc is essential for carbonic anhydrase enzymes used in carbon fixation. Copper is required for electron transport in photosynthetic and respiratory proteins. The distribution of these trace metals is controlled by biological uptake in surface waters and remineralization at depth, creating nutrient-like profiles where surface concentrations are depleted and deep concentrations are enriched.

Dissolved gases enter the ocean at the sea surface through air-sea exchange, with solubility determined by temperature (cold water dissolves more gas), salinity (fresher water dissolves more gas), and wind speed (stronger winds increase gas transfer rates). Oxygen, carbon dioxide, and nitrogen are the most important dissolved gases for marine life. Oxygen concentrations range from supersaturation at the surface to near-zero in oxygen minimum zones at intermediate depths.

The Ocean Carbon Cycle

The ocean contains approximately 38,000 gigatons of carbon, compared to 900 gigatons in the atmosphere and 2,000 gigatons in terrestrial biota and soils combined. Most oceanic carbon exists as dissolved inorganic carbon (DIC) in three forms: bicarbonate ions (roughly 90 percent), carbonate ions (roughly 9 percent), and dissolved CO2 (roughly 1 percent). These three species exist in chemical equilibrium, with their relative proportions determined by pH, temperature, and pressure.

The biological pump transfers carbon from surface waters to the deep ocean through three pathways. Primary production by phytoplankton converts dissolved CO2 into organic carbon. When organisms die or produce fecal pellets, this organic carbon sinks as particulate matter. Most particles are consumed or decomposed in the upper 1,000 meters, releasing CO2 back to dissolved form at depth. Only about 1 to 3 percent of surface production reaches the deep seafloor, where it may be buried in sediments and sequestered for millions of years.

The solubility pump operates through temperature-dependent CO2 dissolution. Cold polar waters absorb more CO2 than warm tropical waters because gas solubility increases at lower temperatures. When these cold, CO2-rich waters sink in deep water formation regions (North Atlantic, Southern Ocean), they carry dissolved carbon to depth where it remains isolated from the atmosphere for centuries to millennia. The solubility pump accounts for approximately 70 percent of the ocean's total carbon uptake from the atmosphere.

Ocean Acidification

When CO2 dissolves in seawater, it reacts with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate. This process, intensified by anthropogenic CO2 emissions, has decreased ocean surface pH from approximately 8.2 to 8.1 since the pre-industrial era, representing a 26 percent increase in hydrogen ion concentration. Projections indicate that continued emissions could reduce surface pH to 7.8 by 2100, a 150 percent increase in acidity compared to pre-industrial conditions.

Reduced pH decreases the saturation state of calcium carbonate minerals (aragonite and calcite) that shell-building organisms require. When saturation drops below 1.0, these minerals dissolve faster than organisms can produce them. Already, waters in the Arctic Ocean, deep North Pacific, and coastal upwelling zones periodically become undersaturated with respect to aragonite. Pteropods (swimming snails that form aragonite shells) show measurable shell dissolution in these regions, with implications for food webs that depend on them as prey.

Not all organisms respond negatively to acidification. Some seagrasses and non-calcifying algae show enhanced growth under elevated CO2 conditions. Certain fish species appear relatively tolerant, though behavioral effects (impaired predator detection, altered decision-making) have been demonstrated in laboratory experiments. The varied responses across species suggest that acidification will reshape marine community composition rather than uniformly suppressing all marine life, favoring non-calcifying organisms over those dependent on carbonate shells.

Nutrient Chemistry and Productivity

Macronutrients (nitrogen, phosphorus, silicon) and micronutrients (iron, zinc, manganese, cobalt) control rates of marine primary production. In the surface ocean, biological uptake depletes nutrients to near-zero concentrations in permanently stratified subtropical gyres. The Redfield ratio, the observed proportion of C:N:P in marine organic matter (106:16:1), reflects the average nutritional requirements of phytoplankton and provides a framework for predicting which nutrient limits production in a given region.

Nitrogen cycling in the ocean involves complex microbial transformations including nitrogen fixation (converting N2 gas to biologically available ammonium), nitrification (oxidizing ammonium to nitrate), denitrification (reducing nitrate to N2 gas in oxygen-depleted zones), and anammox (anaerobic ammonium oxidation). These processes maintain a global balance between nitrogen inputs and losses, though human activities (agricultural runoff, atmospheric deposition from fossil fuels) have significantly increased nitrogen inputs to coastal waters, causing eutrophication and dead zones.