🌊 AS Level Marine Science (9693) Study Notes: Chapter 1.3 Density and Pressure 🌊

Hello Marine Scientists! This chapter is fundamental to understanding the ocean's structure, currents, and how marine life survives at different depths. Don's worry if the physics seems intimidating—we'll break it down into simple, real-world examples.

The core concept here is density. Density differences drive ocean circulation (the global conveyor belt) and create distinct layers, which profoundly affects the distribution of organisms.

1. Understanding Density: Mass in a Space

Syllabus Outcome 1.3.2: Recall and apply the formula for density

Think of density as how tightly packed the "stuff" (mass) is inside a given space (volume).

Key Definition:

  • Density is the mass of a substance per unit volume.

The formula for density is simple and you must be able to recall and apply it:

Density Formula: \[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]

Units:

  • Mass is measured in kilograms (\(\text{kg}\)).
  • Volume is measured in cubic metres (\(\text{m}^{3}\)).
  • Therefore, density is measured in kilograms per cubic metre (\(\text{kg m}^{-3}\)).

Analogy: Imagine two buckets of equal size (same volume). One is filled with feathers, the other with lead weights. The bucket with lead has much greater mass, so it has a much higher density.

Typical Ocean Densities: Pure fresh water has a density of approximately \(1000 \text{ kg m}^{-3}\). Typical sea water has a density ranging from \(1020 \text{ kg m}^{-3}\) to \(1030 \text{ kg m}^{-3}\) because of the dissolved salts.

Key Takeaway:

Density is a ratio of mass to volume, and in the ocean, small changes in this ratio are incredibly important for marine environments.

2. Factors Affecting the Density of Sea Water

Syllabus Outcome 1.3.1: Explain how water temperature, water pressure and salinity affect the density of sea water

The density of seawater is primarily controlled by three factors, often referred to collectively as the TSD factors (Temperature, Salinity, Depth/Pressure).

Temperature (The Biggest Factor)

This factor generally has the largest effect on density.

  • When water temperature increases, the water molecules move faster and spread slightly further apart, causing the volume to increase. This leads to lower density (less mass packed into the space).
  • When water temperature decreases, the molecules slow down and move closer together, causing the volume to decrease. This leads to higher density.

This is why warm surface water floats on cold deep water!

Salinity (Dissolved Salts)

Salinity refers to the concentration of dissolved salts (in parts per thousand, ppt).

  • When salinity increases, there is more dissolved salt (mass) packed into the same volume of water. This leads to higher density.
  • When salinity decreases, the water is less salty, leading to lower density.

Did you know? The Great Salt Lake in Utah and the Dead Sea are so salty (high salinity) that humans can easily float on the surface because the water density is much higher than human body density.

Pressure (Water Depth)

Pressure increases significantly with depth in the ocean.

  • When pressure increases (deeper water), the water volume is slightly compressed. This subtle decrease in volume leads to a slight increase in density.

Important Note: While pressure increases dramatically with depth, its effect on density is minor compared to the effects of temperature and salinity. The ocean is mostly incompressible.

Quick Review Box: TSD Summary

  • Temperature \(\uparrow\) \(\rightarrow\) Density \(\downarrow\)
  • Salinity \(\uparrow\) \(\rightarrow\) Density \(\uparrow\)
  • Depth/Pressure \(\uparrow\) \(\rightarrow\) Density \(\uparrow\) (Slightly)

3. The Unique Importance of Floating Ice

Syllabus Outcome 1.3.3 & 1.3.4: State and explain why the density of ice is lower than sea water and its importance

This property of water is highly unusual and incredibly important for marine life!

Why Does Ice Float?

As water cools from 4 °C down to 0 °C, the structure of the water molecules changes due to hydrogen bonding. In ice, these bonds form an open, crystalline lattice structure. This structure takes up more volume than the tightly packed liquid water molecules.

  • Since density = mass/volume, if the mass stays the same but the volume increases, the density decreases.
  • The density of ice (\(920 \text{ kg m}^{-3}\)) is significantly lower than the density of liquid seawater (\(\approx 1025 \text{ kg m}^{-3}\)), causing ice to float.
Importance of Floating Ice

The fact that ice floats is vital, especially in polar regions:

1. Thermal Insulator (The Blanket Effect):

  • The floating ice layer acts like a blanket, insulating the water beneath it from the extremely cold air temperature above.
  • This prevents the entire water body from freezing solid, allowing marine organisms (fish, plankton, invertebrates) to survive beneath the ice.

2. Habitat for Marine Organisms:

  • The undersides of sea ice provide a crucial habitat for specialized algae, bacteria, and small crustaceans (like krill).
  • This forms the base of the food web in the extreme polar environment when open water resources are scarce.

Imagine the catastrophe if ice sank! If ice were denser than water, lakes and oceans would freeze from the bottom up, killing most benthic life and preventing the water body from ever completely thawing.

Key Takeaway:

Due to its unique crystalline structure, ice is less dense than liquid water, allowing it to float and act as a critical thermal insulator and habitat in polar ecosystems.

4. Ocean Layers and Mixing

Syllabus Outcome 1.3.5: Describe how temperature and salinity gradients form in water columns to produce ocean layers, and how subsequent mixing of these layers may occur

Because density is affected by temperature and salinity, these factors create layers in the ocean, a process called stratification. Density differences act like walls, preventing water masses from easily mixing.

Understanding Gradients (The 'Clines')

A gradient is a change in a measurement (like temperature or salinity) over a distance, typically depth in the ocean. The layers defined by these gradients are known as 'clines'.

  1. Thermocline: A layer where the temperature changes rapidly with depth.

    The water goes from warm (surface) to cold (deep) very quickly in this zone. This rapid temperature change usually creates a rapid density change.

  2. Halocline: A layer where the salinity changes rapidly with depth.

    Haloclines are common where fresh water meets sea water (e.g., estuaries) or in polar regions where ice melting dilutes the surface water.

  3. Pycnocline: A layer where the density changes rapidly with depth.

    The pycnocline often combines the effects of both the thermocline and the halocline. The stronger the pycnocline, the more stable the layering and the less mixing occurs.

The Three Main Ocean Layers (Based on Density)

The water column can be simplified into three main density-driven layers:

1. The Surface Layer (Mixed Layer):

  • Characteristics: Warmest, lowest density (in tropical/temperate areas), and well-mixed by wind and wave action.
  • Depth: Varies significantly, but typically shallow (up to a few hundred meters).

2. The Transition Layers (Thermocline & Halocline):

  • Characteristics: Contains the pycnocline, where density increases rapidly. This layer acts as a barrier, separating the surface water from the deep ocean.

3. The Deep Ocean Layer:

  • Characteristics: Coldest, highest density, and highly stable. Temperature and salinity are almost uniform throughout this vast layer.
  • Depth: Makes up the majority of the ocean volume.
Mixing of Ocean Layers

In stratified oceans (tropical and temperate regions), the layers generally do not mix easily due to the strong density differences. However, mixing is crucial for moving oxygen and nutrients around.

Mixing primarily occurs in two ways:

1. Wind and Waves: Mechanical mixing breaks down the surface layer, keeping the surface water uniform (the "mixed layer").

2. Convection/Overturn (Usually in Polar Regions):

  • In cold polar seas, the surface water is chilled by freezing air or by the process of ice formation (which removes fresh water, leaving dense, salty brine behind).
  • This dense, cold water sinks deep into the ocean (convection).
  • This sinking process, known as thermohaline circulation, causes the entire water column to overturn, mixing nutrients from the deep sea up towards the surface and carrying oxygenated surface water down. This mixing is essential for global ocean health.
Key Takeaway:

Temperature and salinity gradients create density barriers (pycnoclines/thermoclines/haloclines) that separate warm surface water from cold deep water, preventing mixing in most areas. Deep mixing primarily occurs in polar regions where surface water becomes cold and dense enough to sink.