Question

Say that someone brings several water samples to your laboratory. His problem is that the labels are incomplete. He knows samples \(\mathrm{A}\) and \(\mathrm{B}\) are from the Atlantic Ocean and that one came from near the equator and the other from near the Tropic of Cancer. But he does not know which one is which. He has a similar problem with samples \(\mathrm{C}\) and \(D\). One is from the Red Sea and the other is from the Baltic Sea. Applying your knowledge of ocean salinity, how would you identify the location of each sample? How were you able to figure this out?

Short answer

Measure salinity: higher for equator (Atlantic) and Red Sea, lower for Tropic of Cancer and Baltic Sea.

Step-by-step solution

Understand Properties of Ocean Samples

Different oceans and seas have distinct properties, one of them being salinity. The Atlantic Ocean generally has high salinity levels, particularly near the equator where evaporation rates are high.

Analyze Samples A and B for Salinity

Measure the salinity of samples A and B. The sample with the higher salinity is likely from near the equator, while the sample with the lower salinity is likely from near the Tropic of Cancer.

Understand Properties of Sea Samples

Seas also have varying salinity: The Red Sea is known for its exceptionally high salinity, while the Baltic Sea is much fresher and has lower salinity levels due to the influx of freshwater.

Analyze Samples C and D for Salinity

Measure the salinity of samples C and D. The sample with higher salinity is most likely from the Red Sea, and the sample with lower salinity is likely from the Baltic Sea.

Conclude the Source of Each Sample

Assign locations to each sample based on the salinity analysis. Sample A or B with higher salinity originates from near the equator, while the other from the Tropic of Cancer. Sample C or D with higher salinity is from the Red Sea, and the other from the Baltic Sea.

Key concepts

Atlantic Ocean

The Atlantic Ocean is one of the major bodies of water on Earth, second in size only to the Pacific Ocean. It stretches from the eastern coast of the Americas to the western shores of Europe and Africa.
One of the distinctive characteristics of the Atlantic Ocean is its salinity, which is generally higher than that of many other oceans. This is due to several factors, including its location in tropical and subtropical regions, where evaporation rates are high.
As a rule, the salinity of ocean water is influenced by climatic conditions, freshwater influx, and evaporation rates. In the Atlantic, the higher salinity is particularly observed around the equator and the Tropic of Cancer.

• Evaporation: The warm air at these latitudes leads to higher rates of evaporation, which increases salinity as water evaporates but salts remain.
• Climate: The warm climate around the equator enhances the evaporation process, influencing the salinity levels.

In essence, a sample from the Atlantic Ocean near the equator will typically exhibit higher salinity compared to one from the Tropic of Cancer.

Red Sea

The Red Sea is an arm of the Indian Ocean, lying between Africa and Asia. It is renowned for its unique ecological and climatic conditions.
One of the most striking features of the Red Sea is its exceptionally high salinity, making it one of the saltiest bodies of water in the world. Several factors contribute to this high salinity:

• Lack of Rivers: Few, if any, rivers flow into the Red Sea, minimizing the influx of freshwater that would decrease salinity.
• High Evaporation Rates: The hot and arid climate leads to significant evaporation, leaving behind high concentrations of salt.
• Limited Water Exchange: The sea's limited connection to other water bodies, particularly through the narrow Bab el Mandeb Strait, means that less salt is flushed out.

Given its high salinity, when comparing samples, a sample from the Red Sea will show much higher salinity than one from a sea with more freshwater influx, such as the Baltic Sea.

Baltic Sea

The Baltic Sea is a brackish inland sea in Northern Europe, surrounded by countries like Sweden, Finland, and Russia.
Unlike the Red Sea, the Baltic Sea is characterized by its low salinity levels. This is primarily due to its unique environmental factors and geographic setting:

• Freshwater Influx: Numerous rivers, including the massive Neva River, flow into the Baltic Sea, bringing fresh water and diluting the salt concentration.
• Limited Evaporation: The cooler climate reduces evaporation rates, maintaining lower salinity.
• Partially Enclosed: The Baltic is almost enclosed, minimizing the exchange of saline ocean water, which further keeps salinity levels low.

When assessing water samples, one from the Baltic Sea will have noticeably lower salinity compared to seas with higher evaporation and less freshwater influx, like the Red Sea.

Equator

The equator is an imaginary line that divides the Earth into the Northern and Southern Hemispheres and is equidistant from the North and South Poles.
The equator is known for its warm climate, which significantly influences ocean salinity. In regions close to the equator, the sun's direct rays lead to high temperatures.

• Evaporation: The high temperatures result in increased evaporation, which raises the salinity of the surface ocean water.
• Climate Patterns: Tropical climates around the equator, with their typical low pressure, often lead to enhanced rainfall, but evaporation effects frequently dominate in determining salinity increases.

The Atlantic Ocean's higher salinity near the equator is a classic example. As such, samples taken from this region will typically register high salinity levels.

Tropic of Cancer

The Tropic of Cancer is one of the major circles of latitude on the Earth. It is positioned at approximately 23.5° north of the equator.
The Tropic of Cancer plays a significant role in explaining variations in ocean salinity, particularly in regions like the Atlantic Ocean. This region experiences different climatic and environmental conditions compared to the equatorial zone.

• Seasonal Changes: The Tropic of Cancer experiences seasonal shifts, with variations in solar radiation and weather patterns influencing evaporation and precipitation rates.
• Salinity Differences: While evaporation still occurs, the interplay with rainfall in this zone can result in relatively lower salinity compared to the equator.

Therefore, when considering samples from the Atlantic Ocean, a sample from the vicinity of the Tropic of Cancer might display a lower salinity reading compared to one sourced from the equator.