MEV 012: Unit 09 - Introduction to Oceanography

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 UNIT 9: INTRODUCTION TO OCEANOGRAPHY

9.0 Introduction

Oceanography is the scientific study of oceans — encompassing their physical structure, chemical composition, biological life, and geological formations. Oceans cover about 71% of Earth's surface and play a crucial role in regulating climate, supporting biodiversity, and sustaining global systems.

9.1 Objectives

After completing this unit, learners will be able to:

  • Understand the physiographic structure of ocean basins
  • Identify various components of oceanic margins
  • Describe the origin and evolution of ocean basins
  • Explain the nature of oceanic sedimentation
  • Analyze physical, chemical, and biological properties of seawater

9.2 Physiography of Ocean

9.2.1 Physiographic Subdivision of Ocean

The ocean floor is divided into distinct zones:

  • Continental Shelf: Submerged edge of the continent, relatively shallow.
  • Continental Slope: Steep slope transitioning to the deep ocean floor.
  • Continental Rise: Area of sediment accumulation between slope and abyssal plain.
  • Abyssal Plain: Flat, deep-sea floor extending across the ocean basin.
  • Oceanic Trenches: Deep depressions formed at subduction zones.
  • Mid-Ocean Ridges: Undersea mountain chains formed by tectonic activity.

9.2.2 Subdivision of Ocean

Global oceans are subdivided into major oceans:

  • Pacific Ocean (largest and deepest)
  • Atlantic Ocean
  • Indian Ocean
  • Southern Ocean
  • Arctic Ocean

Each of these oceans has its own basin, current systems, and unique sedimentation patterns.

9.2.3 The Continental Margin of the Ocean

The continental margin consists of three parts:

  1. Continental Shelf – shallow, biologically productive region.
  2. Continental Slope – steeper descent into deeper waters.
  3. Continental Rise – accumulation of sediments from land sources.

These areas are crucial for marine life, offshore resources, and sediment transport.

9.2.4 Oceanic Margin

The oceanic margin refers to the transitional area between continental crust and oceanic crust. Active margins (e.g., around the Pacific) are tectonically active with trenches and volcanoes; passive margins (e.g., along the Atlantic) are more stable and sediment-rich.

9.3 Origin and Evolution of Ocean Basins

Ocean basins formed through plate tectonics:

  • Continental Drift Theory by Alfred Wegener
  • Seafloor Spreading explains the creation of new oceanic crust at mid-ocean ridges.
  • Plate Tectonics describes the movement of lithospheric plates that shape ocean basins over millions of years.

Major events:

  • Break-up of Pangaea led to formation of Atlantic Ocean.
  • Subduction zones recycle oceanic crust into the mantle.

9.4 Shelf and Deep-Sea Sedimentation

Ocean sedimentation varies by depth and source:

  • Terrigenous Sediments – derived from land, carried by rivers.
  • Biogenic Sediments – from marine organisms (e.g., shells, plankton).
  • Hydrogenous Sediments – precipitated from seawater (e.g., manganese nodules).
  • Volcanogenic Sediments – from volcanic eruptions.

Continental shelves are sites of rapid sediment accumulation, while deep-sea basins collect fine pelagic sediments.

9.5 Physical, Chemical and Biological Aspects of Sea Water

9.5.1 Physical Aspects

Physical properties determine ocean behavior:

  • Temperature: Varies with latitude and depth; surface layers are warmer.
  • Salinity: Average ~35‰; influenced by evaporation, precipitation, river inflow.
  • Density: A function of temperature and salinity; affects ocean circulation.
  • Light Penetration: Limited to photic zone (~200 m); below is aphotic zone.
  • Sound Propagation: Oceans transmit sound efficiently — key for marine navigation.

9.5.2 Chemical Composition of Seawater

Seawater contains dissolved salts and gases:

  • Major ions: Sodium (Na⁺), Chloride (Cl⁻), Magnesium (Mg²⁺), Sulfate (SO₄²⁻)
  • Trace elements: Iron, copper, zinc, etc.
  • Gases: Oxygen (O₂), Carbon dioxide (CO₂), Nitrogen (N₂)

pH is mildly alkaline (~8.1). Oceans act as a carbon sink, absorbing atmospheric CO₂.

9.5.3 Biological Characteristics of Ocean

Oceans support diverse life forms:

  • Plankton – drifting microscopic organisms (phytoplankton and zooplankton).
  • Nekton – swimming creatures like fish, squid, and whales.
  • Benthos – organisms living on or in the ocean floor.

Marine ecosystems include coral reefs, deep-sea hydrothermal vents, and estuaries.

9.6 Let Us Sum Up

This unit introduced the fundamental concepts of oceanography. You explored the ocean's physical structure, margins, origin, sedimentation, and the physical, chemical, and biological properties of seawater. Understanding ocean systems is essential for marine studies, climate science, and resource management.

9.7 Keywords

  • Oceanography-Scientific study of oceans and marine environments.
  • Continental Shelf-Submerged, shallow extension of the continent.
  • Abyssal Plain-Deep, flat area of the ocean floor.
  • Mid-Ocean Ridge-Underwater mountain range formed by tectonic activity.
  • Subduction Zone-Area where oceanic crust sinks into the mantle.
  • Terrigenous Sediments-Land-derived sediments transported into the ocean.
  • Salinity-Concentration of dissolved salts in seawater.
  • Thermocline-Layer with a sharp temperature gradient in the ocean.
  • Biogenic Sediments-Sediments formed from biological material.
  • Plankton-Small drifting organisms that form the base of the ocean food chain.
  • Hydrothermal Vents-Openings on the seafloor emitting mineral-rich water, supporting unique life.
  • Ocean Basin-Large geologic basin under oceans formed by tectonic processes.
  • Carbon Sink-A reservoir (like oceans) that absorbs more carbon than it releases.
  • Nekton-Active swimmers in the ocean (e.g., fish, whales).
  • Pelagic Zone-Open ocean area away from the shore and ocean floor.

 

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