Which one among the following sequences of water bodies, from lower to higher salinity concen- tration, is correct?

1. Introduction to Ocean Salinity (basic)

When we talk about the salinity of the ocean, we are essentially looking at the total content of dissolved mineral salts present in the water. It isn't just about the "saltiness" you taste; it is a critical physical property that dictates how ocean water moves and behaves. At a fundamental level, salinity is calculated as the amount of salt (in grams) dissolved in 1,000 grams (1 kg) of seawater. This is expressed as parts per thousand, denoted by the symbol ‰ or simply as ppt FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.104.

While we often think of salt as just the table salt (Sodium Chloride) we use in our food, seawater contains a complex mixture of many dissolved salts. However, Sodium Chloride (NaCl) is indeed the most abundant, often separated from seawater or found as "rock salt" in dried-up ancient seabeds Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29. In the vast open oceans, salinity typically stays within a narrow range of 33 to 37 ppt, with the global average often cited around 35 ppt Certificate Physical and Human Geography, GC Leong, The Oceans, p.107. For geographical classification, scientists use a specific threshold: a salinity of 24.7 ppt is considered the upper limit for defining "brackish water," which is water that is saltier than fresh water but not as salty as the open sea Physical Geography by PMF IAS, Ocean temperature and salinity, p.518.

Understanding salinity is vital for a UPSC aspirant because it doesn't exist in a vacuum. It directly influences the density of water, its freezing point, and how much evaporation can occur. Higher salinity makes water denser, causing it to sink and drive deep-ocean currents. It also affects marine biodiversity; different species are adapted to specific salinity ranges. To map these variations across the globe, geographers use isohalines—lines on a map connecting points that have the same salinity levels Certificate Physical and Human Geography, GC Leong, The Oceans, p.107.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Water (Oceans), p.104; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29; Certificate Physical and Human Geography, GC Leong, The Oceans, p.107; Physical Geography by PMF IAS, Ocean temperature and salinity, p.518

2. Primary Factors Controlling Salinity (intermediate)

Welcome back! Now that we understand what ocean salinity is, let’s look at the "engine" behind it. Think of ocean salinity as a delicate balance between the water that leaves the ocean and the water that enters it. The concentration of salt changes whenever the ratio of water to dissolved solids is altered. At the surface layer, this is primarily dictated by the tug-of-war between evaporation and precipitation NCERT Class XI: Fundamentals of Physical Geography, Water (Oceans), p.104. When water evaporates, it leaves the salt behind, increasing salinity. Conversely, rain or snow adds fresh water, which dilutes the salt and lowers salinity.

Geography plays a massive role here. For instance, the highest salinity levels aren't found at the Equator, even though it's hot. Why? Because the Equator receives heavy daily rainfall and has high humidity, which dilutes the seawater GC Leong: Certificate Physical and Human Geography, The Oceans, p.107. Instead, the "salt peaks" are found in the subtropical high-pressure belts (around 20°–30° North and South). These regions experience high temperatures, low humidity, and very little rain, creating a perfect storm for high evaporation and rising salinity levels.

Beyond the sky, the land and ice also intervene. In coastal regions, massive freshwater influx from rivers like the Amazon, Ganges, or Congo significantly lowers salinity near the river mouths PMF IAS: Physical Geography, Ocean temperature and salinity, p.518. In polar regions, we see a seasonal cycle: when seawater freezes, it excludes salt (a process called brine rejection), increasing the salinity of the remaining liquid water. When that ice melts in the summer, it releases fresh water, lowering the salinity again.

Finally, we must consider the dynamic factors: wind and ocean currents. Winds don't just create waves; they physically push surface water from one area to another. If wind moves saltier water into a fresher zone, the salinity there rises. Furthermore, because salinity, temperature, and density are interrelated, any change in one usually triggers a change in the others, driving the complex circulation of our global oceans NCERT Class XI: Fundamentals of Physical Geography, Water (Oceans), p.104.

Process	Effect on Salinity	Primary Reason
Evaporation	Increase (↑)	Removes H₂O molecules, leaving salts behind.
Precipitation	Decrease (↓)	Adds fresh water, diluting the concentration.
River Influx	Decrease (↓)	Discharge of fresh terrestrial water into coasts.
Ice Formation	Increase (↑)	Salt is excluded from the ice crystal lattice (Brine Rejection).

Sources: NCERT Class XI: Fundamentals of Physical Geography, Water (Oceans), p.104; GC Leong: Certificate Physical and Human Geography, The Oceans, p.107; PMF IAS: Physical Geography, Ocean temperature and salinity, p.518

3. Latitudinal and Vertical Distribution (intermediate)

The horizontal or latitudinal distribution of salinity follows a distinct pattern driven by the balance between evaporation (which removes water and leaves salt behind) and precipitation (which adds fresh water). While you might expect the Equator to be the saltiest due to heat, it actually has lower salinity (around 35 ppt) because of heavy daily rainfall and high humidity which dilutes the surface water GC Leong, The Oceans, p.107. The maximum salinity is found in the subtropics (20° to 30° N and S), where high-pressure belts create dry, windy conditions with intense evaporation and very little rain Physical Geography by PMF IAS, Ocean temperature and salinity, p.519. As we move toward the poles, salinity dips again because of low evaporation and the influx of fresh water from melting ice Fundamentals of Physical Geography NCERT, Water (Oceans), p.104. Vertically, salinity changes with depth, creating a layered structure in the ocean. In the surface 'mixed layer,' salinity is highly variable depending on local weather and river runoff. However, as we go deeper, we encounter the Halocline—a distinct transition zone where salinity changes sharply with depth Physical Geography by PMF IAS, Ocean temperature and salinity, p.520. Generally, high-salinity water is denser and tends to sink below fresher water, leading to stratification. Interestingly, this vertical trend differs by latitude: in high latitudes, surface water is fresh (from ice melt), so salinity increases with depth; in many tropical regions, surface water is saltier (from evaporation), so salinity may actually decrease with depth after passing the initial surface layer.

Latitudinal Zone	Salinity Level	Primary Reason
Equator	Moderate (34-35 ppt)	Heavy Rainfall & Cloudiness
Sub-tropics (20°-30°)	Highest (36-37 ppt)	High Evaporation & Low Rainfall
Polar Regions	Lowest (20-32 ppt)	Melting Ice & Low Evaporation

Sources: Certificate Physical and Human Geography, GC Leong, The Oceans, p.107; Physical Geography by PMF IAS, Ocean temperature and salinity, p.519-520; Fundamentals of Physical Geography NCERT, Water (Oceans), p.104

4. Connected Concept: Seawater Density and Temperature (intermediate)

In the study of oceanography, understanding the relationship between density, temperature, and salinity is fundamental to grasping how our oceans move and breathe. Density refers to the mass of water per unit volume; in simpler terms, it tells us how "heavy" a specific amount of water is. In the ocean, density is primarily controlled by two variables: how hot or cold the water is (temperature) and how much salt it contains (salinity).

The relationship between temperature and density is inverse. As water warms up, its molecules move faster and spread further apart, making the water less dense (lighter). Conversely, as water cools, the molecules slow down and pack more tightly together, making it denser (heavier). This is why warm surface waters tend to float on top of colder, deeper layers. However, salinity plays a critical "weighting" role. When salt is dissolved in water, the mass of the water increases significantly without a proportional increase in volume. Therefore, high salinity water is denser than low salinity water. When these two factors work together—such as in the freezing poles where water is both very cold and saltier (due to salt rejection during ice formation)—the water becomes exceptionally dense and sinks toward the ocean floor Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.487.

This vertical movement of water—where denser water sinks and lighter water rises—creates a massive, three-dimensional "conveyor belt" known as thermohaline circulation. The term itself combines 'thermo' (temperature) and 'haline' (salinity), indicating that these density differences are the engine driving global deep-sea currents Physical Geography by PMF IAS, Ocean temperature and salinity, p.516. While surface currents are largely pushed by the wind, this deep-water circulation is governed by gravity and the subtle shifts in water chemistry and heat FUNDAMENTALS OF PHYSICAL GEOGRAPHY, NCERT 2025 ed., Water (Oceans), p.104.

To help you visualize these interactions, consider this comparison:

Factor	Change	Effect on Density	Resulting Movement
Temperature	Increase (Warmer)	Decreases	Water Rises/Floats
Temperature	Decrease (Colder)	Increases	Water Sinks
Salinity	Increase (Saltier)	Increases	Water Sinks
Salinity	Decrease (Fresher)	Decreases	Water Rises/Floats

Sources: Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.487; Physical Geography by PMF IAS, Ocean temperature and salinity, p.516; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, NCERT 2025 ed., Water (Oceans), p.104

5. Connected Concept: Ocean Currents and Mixing (exam-level)

In our previous steps, we looked at how evaporation and precipitation set the baseline for ocean salinity. However, the ocean is not a collection of static ponds; it is a dynamic system of moving water. Ocean currents act like a global conveyor belt, transporting water of different temperatures and salinities across the planet. For example, the North Atlantic Drift brings warmer, more saline water from the tropics toward the higher latitudes of Europe. This is why the North Sea maintains a higher salinity than other regions at the same latitude, which would typically be fresher due to lower evaporation rates NCERT Class XI, Fundamentals of Physical Geography, p.105.

While horizontal currents redistribute salt across the surface, vertical mixing is driven by density. Salinity and temperature together determine the density of seawater—salty water is denser and heavier, while fresher water is lighter. This leads to stratification, where low-salinity water floats on top of high-salinity water. In the open ocean, there is often a distinct vertical layer called the Halocline, where salinity changes sharply with depth PMF IAS, Physical Geography, p.520. This vertical movement is crucial because when high-salinity water sinks, it often drives deep-ocean circulation patterns.

The degree of mixing also depends on the physical geography of the area. In enclosed or semi-enclosed seas, the balance between freshwater influx (from rivers) and oceanic inflow (from currents) determines the final salinity. You can see this contrast clearly in the table below:

Water Body	Salinity Trend	Primary Reason
North Sea	Higher Salinity	Influence of the saline North Atlantic Drift GC Leong, Certificate Physical and Human Geography, p.110.
Baltic Sea	Lower Salinity	Massive freshwater influx from rivers and limited mixing with the open ocean NCERT Class XI, Fundamentals of Physical Geography, p.105.

Finally, do not forget the role of planetary winds. Winds do more than just generate waves; they physically push surface waters from one region to another. This can cause "piling up" of water or lead to upwelling, where deep, nutrient-rich (and often saltier) water is brought to the surface to replace water pushed away by the wind NCERT Class XI, Fundamentals of Physical Geography, p.104.

Sources: NCERT Class XI, Fundamentals of Physical Geography, Water (Oceans), p.104-105; PMF IAS, Physical Geography, Ocean temperature and salinity, p.520; GC Leong, Certificate Physical and Human Geography, The Oceans, p.110

6. Salinity Profiles of Specific Water Bodies (exam-level)

To understand why salinity varies so drastically across different seas, we must look at the Global Salinity Balance Sheet. At its core, salinity is determined by the tug-of-war between dilution (via river discharge, rainfall, and ice melt) and concentration (via evaporation). When a water body is partially enclosed or landlocked, these local factors become much more dominant than the global average salinity of 35 ppt FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Water (Oceans), p.105.

On one end of the spectrum, we have the Baltic Sea and the Arctic Ocean. The Baltic Sea is a classic example of a 'diluted' sea; it receives an enormous influx of fresh water from numerous rivers and melting ice, bringing its salinity down to as low as 7 ppt in some areas Certificate Physical and Human Geography, The Oceans, p.107. Similarly, the Arctic Sea maintains lower surface salinity (around 32-33 ppt) because the seasonal melting of ice caps injects fresh water into the system, effectively 'thinning' the salt concentration.

On the other end, we find enclosed seas in arid regions like the Red Sea and the Gulf of California. These bodies of water are located in 'hot zones' with intense solar radiation and minimal rainfall. The Red Sea, for instance, is a landlocked basin with almost no significant river discharge to replenish it, leading to extreme evaporation and salinity levels reaching 39 to 41 ppt Physical Geography by PMF IAS, Ocean temperature and salinity, p.519. The Gulf of California (Sea of Cortez) follows a similar trend, where the surrounding desert environment drives high evaporation, keeping its salinity higher than the open Pacific.

Water Body	Salinity Trend	Primary Reason
Baltic Sea	Very Low (~7 ppt)	Massive river influx and melting ice.
Arctic Sea	Low (~33 ppt)	Seasonal ice melt and low evaporation.
Gulf of California	High (~35-36 ppt)	High evaporation in an arid setting.
Red Sea	Very High (~40 ppt)	Extreme evaporation and lack of river discharge.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Water (Oceans), p.105; Certificate Physical and Human Geography, The Oceans, p.107; Physical Geography by PMF IAS, Ocean temperature and salinity, p.519

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental drivers of ocean chemistry—namely evaporation, precipitation, and riverine discharge—this question asks you to synthesize those building blocks geographically. To solve this, you must apply the Net Fresh Water Budget to each basin. Start by identifying the extremes: the Baltic Sea is the world's largest pool of brackish water because it receives massive freshwater influx from over 200 rivers and has very low evaporation due to its cold climate. Conversely, the Red Sea is the textbook example of high salinity; it is surrounded by deserts with intense evaporation and possesses no permanent river inflow, pushing its concentration to nearly 40 ppt.

Walking through the middle of the sequence, the Arctic Sea follows the Baltic; while cold, its surface salinity is significantly diluted by seasonal ice melt and large Siberian river discharges, keeping it lower than tropical bodies. The Gulf of California (Sea of Cortez) sits in an arid environment with high evaporation, making it more saline than the polar waters, yet it remains slightly less concentrated than the landlocked heat-trap of the Red Sea. Therefore, the logical progression from lowest to highest salinity is (B) Baltic Sea-Arctic Sea-Gulf of California-Red Sea.

UPSC often sets traps by swapping the Arctic Sea and the Baltic Sea, as seen in Option (D). Many students mistakenly assume that cold polar water is always more saline due to density, forgetting the dilution effect of melting glaciers and massive river runoff in the Baltic. Another trap is underestimating the Gulf of California; however, as noted in Environment and Ecology by Majid Hussain, enclosed or semi-enclosed seas in arid zones will almost always outrank open or polar seas in salinity concentration.