PYTR Class

Learning Objectives

1. Describe the properties of water
2. Describe the roles of water as C sink
3. Describe water stratification
4. Outline process of upwelling
5. Describe the thermohaline circulation system

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Part 1: Properties of Water

• Water (H₂O) is an inorganic compound made of hydrogen (H) and oxygen (O).
• It exists as a solid, liquid, or gas and covers over 70% of Earth’s surface.

Structure & Properties

• Polarity:
  • Oxygen draws electrons away from hydrogen, creating a partial negative charge on oxygen and partial positive charges on hydrogen.
  • This makes water a polar molecule.
• Hydrogen Bonding & Cohesion:
  • The attraction between partially charged hydrogen and oxygen atoms of different water molecules forms hydrogen bonds.
  • These bonds keep water molecules close together (cohesion).
• Specific Heat Capacity:
  • Water has a high specific heat capacity, meaning it takes longer to heat up and cool down.
  • This leads to moderate maritime climates near water bodies and extreme continental climates inland.
• Surface Tension:
  • Due to strong hydrogen bonds, water has high surface tension, allowing some insects to walk on its surface.
• Adhesion:
  • Water molecules stick to other surfaces due to their polarity.
  • On clean glass, adhesion > cohesion, forming a thin film.

Part 2: Water as C Sink

Carbon in Oceans & Seabed Sediments

• Over the long term, carbon is taken up by living organisms as biomass, which accumulates on the seabed.
• Seabed sediments contain inorganic carbonates and organic carbon compounds that are not fully decomposed.
• Over millions of years, these sediments can become fossil fuels.

Oceans as a Carbon Sink

• Oceans are the largest CO₂ sink on Earth.
• More than 90% of the world’s carbon has settled in the ocean over geological time.
• Photosynthesis converts CO₂ into organic material, which eventually settles in the deep ocean.
• The deep ocean has a higher concentration of carbon than the upper ocean.
• Oceans reduce atmospheric CO₂ by storing large amounts in seawater and deep-sea sediments.
• If carbon stored on the ocean floor were released, oceans could become a source of CO₂ instead of a sink.

Ocean Acidification

• Short-term carbon sequestration in oceans as dissolved CO₂ leads to ocean acidification.
• Cause of ocean acidification: Anthropogenic sources, such as carbon emissions from industries, power plants, cars, and planes.

Effects of CO₂ Absorption in Oceans

1. Up to half of the CO₂ released from fossil fuel burning in the last 200 years has been absorbed by the ocean.
2. Absorbed CO₂ in seawater forms carbonic acid (H₂CO₃), lowering the pH and making the ocean more acidic.
3. Increased H⁺ concentration in water:
   • Makes the ocean more acidic.
   • Reduces marine organisms’ access to carbonate ions, which are essential for forming shells and hard parts.

Part 3: Water Stratification

Water Stratification

• Lakes and reservoirs can develop layers of warmer, less dense water and colder, denser water due to thermal stratification.
• Water is most dense at 4°C and becomes less dense at temperatures above and below this point.

Seasonal Stratification Changes

• Autumn cooling:
  • Surface water cools, becomes denser, and sinks to the bottom.
  • When the whole lake reaches 4°C, it reaches its maximum density.
  • Further cooling makes the surface water colder but less dense, causing it to float on top of the denser 4°C water.
  • If the temperature reaches 0°C, ice forms on the surface, insulating the water below.
  • Water below the ice stays around 4°C, supporting freshwater ecosystems.
• Spring warming:
  • Ice melts as temperatures rise.
  • The surface water warms, increases in density, and sinks, mixing with deeper water.

Stratification Zones in Lakes

• Epilimnion: The warm, well-mixed upper layer.
• Hypolimnion: The colder, denser bottom layer.
• Metalimnion: The transitional layer between them, where temperature changes rapidly.

Factors Affecting Stratification Stability

• Lake depth, size, and shape influence stratification.
• Wind speed and water movements can disrupt or maintain stratification.
• Lakes with high water flow do not develop persistent stratification due to shorter water residence time.

Permanent Stratification (Meromictic Lakes)

• Some lakes, like Lac Pavin (France), experience permanent stratification.
• Layers in meromictic lakes:
  • Mixolimnion (top layer)
  • Chemocline (middle layer)
  • Monimolimnion (bottom layer) – hypoxic (low O₂), saltier, and denser

Stratification in Freshwater Lakes & Marine Areas

• Stratification occurs in deep freshwater lakes and oceans, forming distinct temperature layers.
• A thermocline is the transition layer between the warmer mixed surface layer and the cooler water below.
• Warm and cold water differ in:
  • Dissolved oxygen (O₂) concentration
  • Nutrient availability

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Thermoclines

Thermoclines in Oceans

• Sunlight energy is mostly absorbed in the top few centimetres of the ocean.
• Waves mix the surface water, distributing heat down to about 100 m depth.
• Below the mixed layer, temperature remains relatively stable between day and night.
• Ocean temperature declines with depth.
• Thermoclines vary by latitude:
  • Steepest in the tropics
  • Variable in temperate regions
  • Lowest in polar areas (where sea ice insulates the water below)

Thermoclines in Lakes & Seasonal Stratification

• Summer stratification:
  • Warm, less dense surface water sits above cold, dense deep water.
  • The thermocline separates the layers, preventing mixing.
  • Oxygen levels are low below the thermocline, as organisms deplete it.
• Autumn & Winter mixing:
  • Surface temperature drops, making water denser.
  • Dense surface water sinks, mixing the layers and bringing nutrients to the surface.
  • This may trigger an algal/phytoplankton bloom.
• Freezing conditions:
  • As temperatures fall, the surface may freeze.
  • The densest water sinks to the bottom.
  • Less dense water near 0°C rises to the surface, remaining until spring melts the ice.

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Ocean Stratification & Climate Change

• Global warming and salinity changes have intensified ocean stratification.
• The greatest changes occur in the upper 200 m of the ocean.
• Over the past 50 years, increasing global temperatures have increased the temperature difference between warm and cold water, reducing mixing.
• Reduced ocean mixing affects:
  • Heat transport from the surface to deeper layers.
  • Oxygen (O₂) and carbon dioxide (CO₂) exchange between layers.
  • Global warming, as warmer surface temperatures lead to further heating.
• Oceans have absorbed excess heat since the 1880s due to human-induced climate change.
• Stratification increase (1960–2018):
  • 7% in the upper 200 m.
  • 5.8% from the surface down to 2000 m.
  • Largest increases:
    • Southern Ocean (9.6%)
    • Pacific Ocean (5.9%)
• Stratification reduces ocean mixing:
  • Wind, currents, and tides usually mix surface and deeper waters.
  • Increased density differences between warm and cold water slow down mixing.
  • Oceans become more stable, reducing nutrient and gas exchange.
• A warming climate further increases stratification:
  • Warmer water expands (steric effect).
  • Melting ice adds freshwater, decreasing surface salinity.
• Consequences of stronger stratification:
  • Less CO₂ absorption by surface waters, increasing atmospheric CO₂ and global warming.
  • Reduced oxygen levels in deeper waters due to limited mixing.

Part 4: Upwelling

• Upwelling brings cold, nutrient-rich water to the surface.
• Upwelling occurs due to:
  • Wind displacement of surface water.
  • Coriolis effect (Earth’s rotational deflection).
  • Ekman transport (spiraling water movement due to wind and Earth’s rotation).
• Coastal upwelling happens when winds blow parallel to the coastline, replacing warm surface water with colder, nutrient-rich water.
• Main coastal upwelling areas:
  • Benguela Current
  • California Current
  • Canary Current
  • Humboldt Current
  • Somali Current
• Upwelling varies with ENSO (El Niño-Southern Oscillation) events:
  • Weaker trade winds reduce upwelling in equatorial regions.
  • Less upwelling = lower productivity due to reduced nutrient supply.

Part 5: Thermohaline Circulation System

• Driven by temperature and salinity differences in seawater.
• Temperature differences:
  • Heating and cooling at the sea surface.
• Salinity differences:
  • Evaporation and sea ice formation → increase salinity.
  • Precipitation, runoff, and melting ice → decrease salinity.
• Deep water formation & movement:
  • In polar regions, cold salty water sinks and moves toward the equator.
  • The densest water is in Antarctica, where seawater freezes at -2°C, leaving behind salty, dense water.
  • This water circulates into the deep Atlantic, Pacific, and Indian Oceans.
• Surface currents transport warm water to the North Atlantic from the Indian and Pacific Oceans.
• North Atlantic water loses heat to cold winds from Canada → water cools, becomes denser, and sinks.
• Denser water moves through deep ocean currents, eventually reaching the Pacific where it mixes and becomes less dense.
• Density factors in ocean layering:
  • Temperature: Higher temp = lower density.
  • Salinity: Higher salinity = higher density.
  • Pressure: Higher pressure = higher density.
• Freshwater from land runoff is less dense than seawater and stays at the surface.

Exercises