The ocean is salty because of the natural process of erosion that transports dissolved minerals from land to sea. The main minerals that make seawater salty are sodium, chloride, magnesium, sulfate, calcium, and potassium. The ocean has had a relatively stable salinity level of around 3.5% for billions of years due to the balance of inputs and outputs of salts.
The Sources Of Salt In The Ocean
The salinity of the ocean is determined by four major inputs and outputs of salt. The main inputs are the weathering and erosion of rocks on land, atmospheric fallout, hydrothermal vents, and inflow from rivers. The main outputs are sea spray, interaction with oceanic crust, and mineral precipitation. Overall, the inputs and outputs are balanced, creating an equilibrium of ocean salinity.
The primary source of salt in the ocean is the weathering and erosion of continental rocks and soil. Rainwater is slightly acidic from dissolved carbon dioxide which slowly weathers rock material. The dissolved ions are carried away in runoff and empty into the ocean. The most abundant elements in seawater reflect the composition of the oceanic crust, rich in sodium, chloride, magnesium, and sulfur.
Another input is atmospheric fallout. Seawater obtains salt from particles of sea salt, dust, volcanic ash, and other aerosols that settle on the ocean surface. Winds carry these salty particles from land sources out over the seas where they fall out of suspension.
Hydrothermal vents on the seafloor provide concentrated inputs of dissolved minerals arising from the Earth’s crust. When seawater percolates down into mid-ocean ridges it is heated and reinjected into the ocean with high salinity. This accounts for significant amounts of calcium, sulfate, and magnesium.
Rivers that drain the continents also deliver salts washed out of soils and rocks. The dissolved load in rivers is relatively small compared to erosion and runoff directly off the land surface, but rivers play a key role in transporting minerals from inland areas out to coastal regions.
The Outputs Of Salt From The Ocean
In terms of outputs, the evaporation of seawater into water vapor leaves salt behind, increasing surface salinity. However, the effect is offset by precipitation over the ocean which dilutes the surface salinity.
Interactions between seawater and the oceanic crust at mid-ocean ridges also withdraws and deposits salts. High temperature hydrothermal fluids circulate through the crust exchanging various elements. Overall, this geological output is estimated to balance the input, maintaining stable salinity over geologic timescales.
The formation of calcium carbonate shells by marine organisms removes some calcium, carbon, and strontium from the ocean. When the organisms die, the shells dissolve and reenter solution. However, some accumulate in sediment deposits on the seafloor. Over millions of years, this can compensate for salt inputs from weathering.
In summary, the balance of the major inputs of salt to the ocean from erosion, atmospheric fallout, hydrothermal vents, and rivers is counterbalanced by outputs from sea spray, exchange with oceanic crust, and mineral precipitation. This gives the ocean a stable salinity over long timescales. However, over hundreds to thousands of years, salinity can naturally fluctuate.
Past Variations In Ocean Salinity
Researchers have examined paleoclimate records over the past hundreds of thousands of years to reconstruct changes in ocean salinity. Several factors can cause the salinity to vary beyond its average range. These include continental ice volume, meteoric water flux, ocean circulation, and seawater chemistry.
During past ice ages, continental ice sheets trapped huge volumes of freshwater that caused sea levels to drop by over 100 meters. So even though more salts were present in the oceans due to higher erosion rates, the salinity increased substantially due to the reduction in ocean volume. Interglacial periods like the current Holocene then experienced lower ocean salinity as melting glaciers raised sea levels and diluted the salt concentration.
Differences in temperatures and precipitation between glacial and interglacial climates also altered the meteoric water flux. Times of increased rainfall and freshwater runoff would lower ocean salinity by adding more dilution. Drier periods leave more salts behind allowing salinity to rise. Models suggest runoff may have varied by 15-20% between peak ice age conditions and the current warm period.
Variations in ocean circulation patterns also impact salinity over centuries to millennia. During ice ages, oceans were more stratified with less overturning of deep water to the surface. This allowed more salt to build up at depth raising the overall ocean salinity even though surface waters were fresher. Deep ocean currents can also store more dissolved minerals affecting total seawater chemistry.
On shorter decadal timescales, oscillating current systems like the Pacific Decadal Oscillation influence regional precipitation and evaporation patterns. This drives fluctuations in surface salinity in areas like the North Pacific and around Australia. El Nino Southern Oscillation events also alter freshwater flux from the tropics producing detectable salinity anomalies.
Anthropogenic Effects On Ocean Salinity
Human activities over the past century have also begun altering ocean salinity in measurable ways. The effects are both regional and global in scale and include:
– Altered freshwater runoff from dams and water extraction
– Melting of land ice and warming seas
– Increased rainfall and floods
– Deforestation changing evapotranspiration
– Sea level rise from melting glaciers
For example, salinity has decreased strongly near the mouths of major rivers with significant regulation like the Nile and Mississippi. Meanwhile, warming has increased ocean evaporation and precipitation over the tropics lowering and raising salinity respectively.
Changes are also occurring over broader ocean basins. Increasing ice melt and river runoff has freshened parts of the Arctic Ocean. Warming has raised salinity in the subtropical Atlantic from more evaporation. The Pacific has freshened near Asia and become saltier near North America reflecting precipitation shifts.
Future Projections For Ocean Salinity
Climate models looking ahead at the rest of the 21st century broadly project that:
– High latitudes will become fresher from sea ice melt, increasing precipitation, and runoff.
– The tropics and subtropics will become saltier due to higher evaporation in a warmer climate.
– Freshening from land ice and thermal expansion will slow Atlantic overturning circulation.
– Precipitation extremes like droughts and floods will intensify salinity anomalies.
The models forecast the global ocean average salinity to decline by 0.15 to 0.25% over the next 80 years. However, regional changes will vary with latitude and basin. Decreased salinity lowers seawater density which can affect currents and stratification. Some impacts may propagate through marine ecosystems.
A major uncertainty is how the hydrologic cycle will respond. If precipitation increases more than evaporation, the oceans could freshen faster. The amount of melt from land ice like Greenland is also unclear. Enhanced monitoring of ocean salinity changes is ongoing and may improve projections.
Some impacts of changing salinity like rising sea levels will persist for centuries. But for ocean salinity itself, if anthropogenic climate change is reversed, models suggest much of the perturbations could stabilize within decades to centuries as the marine system returns to equilibrium. However, lingering effects would still propagate through the slower moving abyssal ocean.
Conclusion
In summary, the ocean has an average salinity of around 3.5% maintained by the balance of inputs from continental weathering and outputs through geologic processes. This balance has kept seawater salty for billions of years through natural climate cycles. However, human activities over the last century have begun altering regional and global salinity patterns through changes to the water cycle and ice melt. Climate change projections indicate ocean salinity will decrease on average as more freshwater enters, but with regional increases in the subtropics. Monitoring and adapting to changing saltiness will be an ongoing challenge for marine ecosystems and human communities along the coast.
| Input Source | Output Sink |
|---|---|
| Continental weathering and erosion | Sea spray and salt aerosols |
| Atmospheric fallout | Mid-ocean ridge hydrothermal circulation |
| Hydrothermal vents | Calcium carbonate mineral precipitation |
| River inputs |
