🌊 Average Ocean Salinity
Most seawater has a salinity of about 35 ppt. This means that in every 1000 grams of seawater, 35 grams are dissolved salts and 965 grams are pure water. That's roughly 3.5% salt!
pH and Salinity » Seawater Composition and Salts
What you'll learn this session
Study time: 30 minutes
- Understand what makes seawater different from freshwater
- Learn about the main salts dissolved in seawater
- Discover how pH affects marine life
- Explore how salinity varies around the world's oceans
- Examine the impact of human activities on ocean chemistry
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Unlock This CourseIntroduction to Seawater Composition
Seawater is much more than just water with salt added. It's a complex solution containing dozens of different dissolved substances that make life in the ocean possible. Understanding what's in seawater and how acidic or basic it is helps us understand how marine ecosystems work and how they're changing.
Key Definitions:
- Salinity: The total amount of dissolved salts in seawater, measured in parts per thousand (ppt) or practical salinity units (PSU).
- pH: A scale from 0-14 that measures how acidic or basic a solution is. 7 is neutral, below 7 is acidic, above 7 is basic.
- Dissolved salts: Minerals that have broken down into tiny particles (ions) and mixed completely with water.
- Ocean acidification: The process where oceans become more acidic due to absorbing carbon dioxide from the atmosphere.
The Major Salts in Seawater
Seawater contains many different dissolved substances, but six main ions make up over 99% of all the salts. These have remained in roughly the same proportions for millions of years, even though the total salinity can vary from place to place.
The Big Six Ions
Think of seawater as a recipe that's been perfected over millions of years. The main ingredients are always in the same proportions, no matter where you sample the ocean.
🧾 Chloride (Cl⁻)
Makes up 55% of all dissolved salts. This is the same chloride found in table salt and gives seawater its salty taste.
🧾 Sodium (Na⁺)
About 31% of dissolved salts. Combines with chloride to make sodium chloride - common table salt.
🧾 Sulphate (SO₄²⁻)
Around 8% of salts. Important for some marine organisms that use it to build shells and skeletons.
The remaining three major ions are magnesium (Mg²⁺) at about 4%, calcium (Ca²⁺) at 1% and potassium (K⁺) at 1%. Together, these six ions account for 99.4% of all dissolved salts in seawater.
Amazing Ocean Fact
If you could remove all the water from the oceans and spread the remaining salt evenly over all the land on Earth, it would create a layer about 150 metres thick! That's enough salt to bury a 50-storey building.
Where Do These Salts Come From?
The salts in seawater come from two main sources that have been working for billions of years to create the ocean's unique chemistry.
🌋 Weathering of Rocks
Rain is slightly acidic and slowly dissolves minerals from rocks on land. Rivers carry these dissolved minerals to the sea. Over millions of years, this has built up the ocean's salt content.
🌋 Underwater Volcanoes
Volcanic activity on the ocean floor releases minerals directly into seawater. Mid-ocean ridges and hydrothermal vents are constantly adding new dissolved substances to the oceans.
Understanding pH in Seawater
The pH of seawater is crucial for marine life. Most ocean water has a pH between 7.5 and 8.4, making it slightly basic (alkaline). This might not sound like much variation, but small changes in pH can have huge effects on marine organisms.
Why pH Matters for Marine Life
Many sea creatures depend on the ocean's pH being just right. Shellfish, corals and many tiny floating organisms need to build shells and skeletons from calcium carbonate. When seawater becomes more acidic, it becomes much harder for them to do this.
🐠 Shell-Building Problems
More acidic water can actually dissolve the shells and skeletons of marine animals. It's like trying to build with sugar cubes in the rain - the building materials keep dissolving away.
Case Study Focus: The Great Barrier Reef
Scientists studying the Great Barrier Reef have found that as ocean pH drops, coral growth slows down significantly. Some corals are growing 15% slower than they did 20 years ago. This makes the reef more vulnerable to storms and other damage because the corals can't repair themselves as quickly.
Ocean Acidification - A Growing Problem
Since the Industrial Revolution began around 1750, humans have been burning more fossil fuels and releasing carbon dioxide (CO₂) into the atmosphere. About 30% of this extra CO₂ has been absorbed by the oceans, making them more acidic.
How Carbon Dioxide Changes Ocean Chemistry
When CO₂ dissolves in seawater, it forms carbonic acid. This is the same process that makes fizzy drinks acidic. The more CO₂ the oceans absorb, the more acidic they become.
Ocean pH has already dropped from about 8.2 to 8.1 since pre-industrial times. This might seem small, but because pH is measured on a logarithmic scale, this represents a 30% increase in acidity!
Salinity Variations Around the World
While the proportions of different salts stay the same, the total amount of salt (salinity) varies depending on location and local conditions.
🌞 High Salinity Areas
The Mediterranean Sea and Red Sea have salinity up to 40 ppt due to high evaporation and limited freshwater input.
🌦 Low Salinity Areas
Near river mouths and in polar regions, salinity can drop to 20-30 ppt due to freshwater dilution from rivers and melting ice.
🌊 Open Ocean
Most open ocean areas maintain the average salinity of about 35 ppt, with small variations due to currents and weather patterns.
Factors Affecting Salinity
Several natural processes constantly change the salinity of seawater in different regions:
Evaporation and Precipitation
Hot, dry areas lose water through evaporation, leaving the salt behind and increasing salinity. Areas with lots of rainfall become less salty as freshwater dilutes the seawater.
Freshwater Input
Rivers, melting glaciers and ice sheets add freshwater to the oceans, reducing salinity in coastal areas and polar regions.
Ocean Currents
Currents mix waters of different salinities, helping to distribute salt around the globe and maintain relatively stable conditions in most ocean areas.
Case Study Focus: The Dead Sea
The Dead Sea isn't actually part of the ocean, but it shows what happens when salinity gets extreme. With a salinity of about 340 ppt (nearly 10 times saltier than seawater), it's so salty that almost nothing can live in it - hence the name "Dead Sea". The high salt content makes the water so dense that people can easily float on the surface!
Measuring Salinity and pH
Scientists use several methods to measure these important ocean properties:
📈 Conductivity Meters
These measure how well seawater conducts electricity. Saltier water conducts electricity better, so scientists can calculate salinity from conductivity measurements.
📈 pH Electrodes
Special sensors that measure the concentration of hydrogen ions in seawater to determine pH levels accurately.
Why This Matters for Marine Life
Understanding seawater composition helps us protect marine ecosystems. Many marine organisms are adapted to very specific salinity and pH conditions. Even small changes can affect their ability to survive, reproduce and find food.
Adaptation and Survival
Fish and other marine animals have special organs and processes to deal with the salt in seawater. Marine fish constantly drink seawater and have special gills that remove excess salt. If salinity changes too much, these systems can't cope.
Climate change and human activities are altering ocean chemistry faster than many marine species can adapt. By understanding how seawater composition works, we can better predict and prevent damage to marine ecosystems.