Understanding Ocean Salinity: Myths and Realities Explored
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Growing up on an island, I often tasted the ocean’s saltiness firsthand. The unpleasant experience of swallowing seawater led me to wonder about its origins. When I learned that urine is also salty, I became suspicious of the beachgoers around me, questioning if they were the source of that briny flavor.
Despite my aversion to further investigation, my curiosity got the better of me, prompting me to research. Most sources explain that rainwater, being slightly acidic, erodes rocks on land, releasing minerals that rivers transport to the ocean, accumulating over millions of years and creating salinity.
Even the U.S. Geological Survey supports this explanation. However, upon reflection, this logic falls short. Claiming that rivers are the primary contributors to ocean salinity is akin to suggesting that rivers carrying orange juice could eventually result in an ocean of tomato soup—it's simply not feasible, as rivers and oceans consist of different components.
To truly comprehend ocean salinity, I consulted an oceanography textbook, which clarifies that river runoff alone does not account for it: “seawater is not concentrated river water.”
A comparison of mineral compositions in oceans and rivers reveals stark differences. Seawater is predominantly made up of sodium (Na+) and chloride (Cl-), while river water mainly contains bicarbonate ions (HCO3-).
But if seawater isn't simply concentrated river water, where do its salts originate? First, let’s clarify what we mean by “salt.”
What is Salt, Exactly?
The term “salt” has two definitions:
- Common Definition: Refers to sodium chloride, the white crystalline substance we sprinkle on food.
- Chemical Definition: Refers to a compound formed from positive ions (cations) and negative ions (anions).
An ion is an atom or molecule with an imbalance in electrons: it becomes negative by gaining electrons or positive by losing them. This charge difference causes positive and negative ions to attract, forming neutral compounds we call salts.
A basic salt consists of two ions, and if those ions are sodium and chloride, we get sodium chloride (NaCl) or table salt. However, table salt consists of numerous sodium and chloride ions arranged in a repeating three-dimensional pattern, forming a crystal structure.
Surprising Truth: Seawater Contains No Salt
What? Seawater lacks salt?
Indeed, seawater does not contain salt in the sense of sodium and chloride ions clumped together. When salt is added to water, the ions separate due to their preference for water molecules, leading to a solution where ions are dispersed rather than bonded.
You might wonder why seawater tastes salty if it contains no salt. The answer lies in the fact that we don’t taste salt itself but rather the dissociated ions, primarily sodium, which are responsible for the salty flavor. Our taste buds respond to ions interacting with water molecules, not to salt as a compound.
Where Do Ocean Ions Come From?
Now that we understand that seawater comprises ions mingled with water, we can explore their origins. The ocean acts like a giant tea, where the planet’s rocks infuse minerals into the water over millions of years. This process, known as weathering, involves rain, rivers, groundwater, and ocean surf, which all contribute to ion dissolution.
However, additional ions are needed to create seawater. These extra ions, termed excess volatiles, come from the Earth’s interior and are released into the ocean through volcanic activity.
Ion Contribution Processes
Several natural processes contribute ions to seawater:
- Weathering: Dissolves minerals from rocks into water.
- Outgassing: Volcanoes release additional ions.
- Filtering: Various mechanisms, such as marine organisms and deep-sea hydrothermal vents, remove excess ions, particularly magnesium and potassium, while leaving sodium and chloride largely intact.
Is the Ocean Becoming Saltier?
This raises an intriguing question: Is the ocean's salinity increasing? According to my textbook:
> “If outgassing and chemical weathering of rocks are ongoing, shouldn’t the ocean become saltier over time? While landlocked seas and lakes often grow saltier, the ocean remains in a chemical equilibrium, with the concentration of dissolved salts stable over millions of years.”
Thus, the ocean is not becoming saltier because the processes adding and removing ions are balanced, similar to how airplanes don’t accumulate in the sky as landings and takeoffs are managed effectively.
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