Ocean Salinity Lesson: Understanding Seawater Salts

Lesson Overview

• Why is the Ocean Salty? (Sources of Ocean Salt)
• Factors That Affect Ocean Salinity
• How Do We Measure Salinity?
• Variations in Salinity: Freshwater, Brackish, and Saltwater
• What Happens if You Drink Ocean Water?

Have you ever wondered why the ocean is salty? Ocean salinity refers to how much salt is dissolved in seawater. In simple terms, it's the "saltiness" of the ocean. If you taste seawater, it's obviously salty – in fact, about 3.5% of the weight of ocean water is salt. This is often expressed as 35 parts per thousand (35 ppt), meaning in 1000 units of seawater, 35 units would be salt. 

For example, roughly 35 grams of salt per 1000 grams of seawater is the global average. The main chemical making the ocean salty is sodium chloride – the same table salt we use on food. But how did all that salt get into the ocean, and why isn't all water on Earth equally salty? Let's explore where ocean salt comes from, what factors affect salinity, how we measure it, and how salinity varies in different waters.

Why is the Ocean Salty? (Sources of Ocean Salt)

The ocean wasn't always salty; it became salty over millions of years through natural processes. The major source of the salt in the sea is the land itself. Here's how it works:

• Weathering and Rivers (Salt from Land): When rain falls on the land, it isn't pure water – it picks up small amounts of carbon dioxide from the air and soil, becoming slightly acidic. This rainwater slowly erodes rocks on land, dissolving minerals from them. Among these minerals are salts (like compounds containing sodium, chlorine, magnesium, calcium, etc.). The rain and streams carry these dissolved salts into rivers, and rivers eventually flow into the oceans. 

Over millions of years, this process (called weathering) has transported vast amounts of dissolved salts to the oceans. Rivers themselves aren't very salty because the concentration is low and they continuously flow, but all the tiny bits of salt they carry accumulate in the ocean basin (which has nowhere else to go). Imagine each river as a conveyor belt delivering a little salt to the sea; after a very long time, the ocean water became salty as we know it. In fact, most of the salt in seawater "came from" rocks on land that were broken down and carried by rivers to the ocean.

• Underwater Sources (Hydrothermal Vents and Volcanoes): In addition to rivers, some salt comes from inside the Earth. Under the oceans, especially along mid-ocean ridges, there are cracks in the Earth's crust called hydrothermal vents. These are like underwater hot springs or geysers. Seawater seeps into cracks, gets heated by hot magma beneath the ocean floor, and then shoots back out of the vents carrying dissolved minerals and metals. 

Hydrothermal vents continuously release mineral-rich, hot water into the ocean, which adds some salts and chemicals to seawater. (They also create unique deep-sea ecosystems with strange life forms!) Undersea volcanic eruptions can also contribute minerals and gases that form salts. However, these underwater sources are secondary compared to the enormous contribution from rivers – the majority of ocean salt still originates from land weathering.

By these processes, over geologic time, the oceans have collected enough dissolved minerals to reach the salinity they have today. Importantly, the ocean's salinity has likely stabilized: salts are also removed from the ocean when organisms use minerals to build shells or when water evaporates and leaves salt behind (forming salt deposits). So the ocean isn't getting endlessly saltier; it's in a kind of balance. But at any given time and place, the salinity can vary due to other factors, as we'll see next.

Factors That Affect Ocean Salinity

Not all seawater is equally salty. Salinity can change from place to place and over time. Several factors influence the salinity of ocean water in a region:

• Evaporation: When ocean water evaporates (turns from liquid into water vapor), the salt is left behind. Only water vapor rises, so the remaining water gets saltier. High evaporation rates (for example, in hot, dry climates) increase the salinity of the surface ocean. This is why some warm, dry regions of the ocean have higher salinity – water is removed faster than it's replaced, concentrating the salt.
  
• Precipitation (Rain/Snow): When freshwater falls into the ocean as rain or snow and melts, it dilutes the salt. Heavy rainfall decreases salinity by adding more fresh water. For instance, near the equator where it rains a lot, surface seawater tends to be a bit less salty because the rainwater freshens it.
  
• River Input and Freshwater Runoff: As mentioned earlier, rivers add salts in the long term, but locally when a river pours freshwater into the ocean (like at a river mouth or delta), it lowers the salinity of the nearby seawater (since that area is mixing with lots of fresh water). Areas near big river outflows (e.g., the Amazon River plume in the Atlantic) have lower salinity compared to the open ocean.
  
• Freezing and Melting of Ice: In polar regions, when seawater freezes into sea ice, the ice crystals exclude most of the salt (the salt doesn't freeze with the water). This makes the remaining liquid water saltier. Conversely, when ice melts, it releases fresh water that can dilute the salinity of the surface. So, around the poles, salinity can seasonally change: higher when ice forms, lower when ice melts. (Overall, polar surface waters tend to be less salty than mid-latitude waters because of melting ice and lower evaporation.)
  
• Ocean Circulation and Mixing: Currents and mixing can redistribute salinity. For example, highly saline water can sink (because saltier water is denser) and mix with deeper water, or currents can carry salty water to regions of fresher water. While this doesn't create or remove salt, it evens out some differences and can affect local salinity levels.
  

All these factors work together to create the salinity patterns we see in the oceans. Where evaporation dominates over rainfall, water gets saltier; where freshwater input (rain, rivers, melting ice) dominates, water gets fresher. This is why salinity is not uniform across the globe.

Take This Quiz:

Ocean Salinity quiz

How Do We Measure Salinity?

Scientists and mariners measure salinity as the amount of salt in a given amount of water. The traditional unit for ocean salinity is "parts per thousand" (ppt), also called the practical salinity unit (PSU) in modern oceanography (numerically the same value). Ppt stands for parts per thousand, meaning how many parts of salt are present in 1000 parts of water. As mentioned earlier, average ocean water is about 35 ppt. To put it another way, if you took 1 kilogram of seawater and evaporated it completely, you would get about 35 grams of salt residue.

Using ppt is convenient for seawater because the numbers come out around the tens. For comparison, freshwater from a river might be around 0.1–0.5 ppt (almost no salt), while extremely salty brine might be 200+ ppt. Sometimes salinity is also expressed as a percentage: 35 ppt is 3.5% salt.

In practice, salinity can be measured by collecting water samples and evaporating them to weigh the salt, but more often scientists measure properties like the water's electrical conductivity (saltier water conducts electricity better) or its density or refractive index. These properties correlate with how much salt is present. Modern instruments and even satellites can estimate sea surface salinity by these methods. But for our purposes, knowing the ppt unit and typical values is most important.

Variations in Salinity: Freshwater, Brackish, and Saltwater

Not all water has the same salinity. We classify water based on how salty it is:

• Freshwater – This is water with very little dissolved salt (salinity less than about 0.5 ppt). Fresh drinking water, rivers, lakes, and rainwater fall in this category. Essentially, for every 1000 parts water there's less than half a part salt. 

Freshwater is what land organisms (including humans and most terrestrial animals) require to drink because our bodies are not adapted to high salt content. Most freshwater actually has some minerals, but the amount is so low we consider it salt-free for practical purposes.

• Brackish water – This term describes water that is in-between fresh and ocean saltiness. Brackish water ranges roughly from 0.5 ppt up to around a dozen or so ppt. (Some definitions say up to ~17 ppt; others extend brackish up to around 30 ppt – essentially it covers anything noticeably salty but not as salty as seawater.) Brackish water is found in places where freshwater mixes with seawater. Estuaries (where rivers meet the sea) are classic brackish environments. Mangrove swamps and coastal lagoons can be brackish. 

The water in a brackish area might taste slightly salty, but far less salty than the open ocean. Organisms that live here have to tolerate changing salinity levels as freshwater and seawater mix. A good example is the Baltic Sea in northern Europe, which is almost enclosed and gets a lot of river inflow – its salinity is only around 6–8 ppt in many areas, which is practically brackish. Another example is a coastal bay that receives river water; near the river mouth it might be a few ppt salinity, gradually increasing toward the ocean.

• Saltwater (Seawater) – When we say saltwater usually we mean normal ocean water. Typical open ocean salinity is around 35 ppt (as a global average). Most of the world's oceans range between about 33 ppt and 37 ppt. This is the saline water category. Marine organisms are adapted to this level of salinity. Within the oceans, there are variations: for instance, the Atlantic Ocean is a bit saltier in some parts than the Pacific because of differences in rainfall and evaporation. Closed seas or gulf areas in hot climates can become saltier than the open ocean because of extra evaporation and little freshwater input. 

For example, the Red Sea and the Persian Gulf have salinities in the 40 ppt range (very salty!) due to intense heat and evaporation in those regions combined with limited circulation. The Mediterranean Sea is around 37–39 ppt in parts, higher than the Atlantic, because it's almost landlocked with more evaporation than rainfall. On the other hand, areas with lots of rain or river water can be a bit less salty: for instance, the surface of the ocean near the Amazon River outflow or in the Bay of Bengal (where the Ganges and Brahmaputra rivers flow) is lower than 35 ppt due to huge freshwater input.

• Brine and Hypersaline Water – If water is even saltier than typical seawater, we often call it brine or hypersaline. Briny water would be 50 ppt or more. These conditions occur in special locations like isolated salt lakes or brine pools. A famous example is the Dead Sea in the Middle East – it's not actually an ocean, but a landlocked salt lake. 

The Dead Sea's salinity is around 250–300 ppt (around 10 times saltier than normal seawater!). You could practically crystallize salt out of it. Such extreme salinity occurs because water evaporates from these lakes faster than new water enters, leaving the salts behind to accumulate to very high levels. Only highly specialized life can survive in such briny conditions.

What Happens if You Drink Ocean Water?

Considering how salty the ocean is, a practical question arises: Can humans drink saltwater? What if you were stranded at sea with no fresh water – could you quench your thirst by sipping from the ocean? The answer is no – drinking ocean water will dehydrate you instead of hydrating you.

Let's explain why. Human kidneys can only produce urine that is less salty than seawater. Normal seawater at 35 ppt (3.5% salt) has a much higher salt concentration than our bodies can handle. If you drink ocean water, the salt content of that water is greater than what your body can integrate into its fluids.

To excrete the excess salt, your kidneys must use water from your body's cells to flush it out. In other words, the body will urinate more water than the water you drank, just to get rid of the salt. This leads to a net loss of water from your body – which is the definition of dehydration.

On a cellular level, if there's too much salt outside your cells (in your blood and digestive system after drinking seawater), water will leave your cells to try to balance the concentration (through a process called osmosis). Your cells actually shrivel as they lose water, and you become extremely thirsty. Essentially, the more saltwater you drink, the thirstier you get, because you're drying yourself from the inside. 

Humans lack specialized mechanisms to excrete salt without losing water. Some animals have adaptations for high salinity: for instance, seabirds can drink saltwater because they have special salt glands that expel excess salt from their nostrils, and marine fish constantly regulate salt through their gills and urine.