Ocean Science Lecture Recap: Water, Plate Tectonics, Sediments and Ocean Chemistry

Posted on Oct 30, 2026 in Geology

Lecture 1 Recap

  • Water was present in the solar nebula from which our Solar System condensed.
  • Earth received water from icy asteroids, comets, and outgassing.
  • Oceans arose early in Earth’s history after the surface cooled (at least 3.8 billion years ago) and alongside the accumulation of a secondary atmosphere.
  • We can determine the ages of old materials using the principle of radioactive decay.
  • Other ocean worlds exist in our Solar System (and likely beyond), but our nearest neighbors, Venus and Mars, lost their oceans.

Lecture 1 Details

  • We don’t want it too cold or too hot for habitability, also known as the Goldilocks zone, where water can exist on the surface of a planet.
  • Closer to the Sun means more heat (energy), which creates higher temperatures.
  • The rapid decrease of water and carbon dioxide in the atmosphere over 4 billion years ago was a sign of the planet cooling down and water condensing into oceans. CO2 cools temperature, which allows water to condense. Why did CO2 decrease? Because CO2 dissolves into the ocean, becoming limestone, resulting in CO2 sequestration in the ocean.
  • Isotopes (different versions of an element’s atom) can decay radioactively. The measured rate of isotope decay from parent to child isotope, when reduced to 50%, is the half-life.

Lecture 2 Recap

  • The Earth has layers: solid crust, solid mantle, liquid outer core, and solid inner core.
  • There are two kinds of crust: oceanic (younger, denser) and continental (older, lighter).
  • Plate tectonics: Earth’s surface is broken up into plates that move due to convective currents in the mantle and interact at their edges through divergent, convergent, and transform boundaries.
  • Ocean margins are determined by tectonic activity, resulting in active margins and passive margins.
  • Ocean basins (and continents) are created and destroyed through time.

Lecture 2 Details

  • Plate boundaries:
    • Divergent: plates spread away from each other, leading to the creation of new seafloor.
    • Convergent: plates move toward each other; one subducts and goes underneath.
      • Ocean-ocean: volcanoes and new island arcs.
      • Ocean-continent: volcanoes form on the continent, but the oceanic plate will sink.
      • Continent-continent: neither plate subducts; forms mountains.
    • Transform: plates slide past each other; nothing is created or destroyed, but earthquakes occur.
  • Wilson cycle:
    • First stage: continents diverge, forming a valley in between that grows farther and farther apart as new material is deposited, creating seafloor spreading. The Red Sea is an example of the juvenile phase.
    • Maximum spread of the ocean basin: the Atlantic Ocean is an example.
    • Passive margins at the coast, where the plate boundary is far away, mark the mature phase.
    • Declining phases: ocean plates start converging.
    • Terminal phase: plates become very small; the Mediterranean Sea is an example.
    • Final stage: a mountain range forms where the ocean basin was; the Himalayan mountains are an example.
  • Seismic waves:
    • Order of arrival: P-waves, S-waves, surface waves.
    • Internal (body) waves:
      • Primary (P) waves propagate through compression.
      • Secondary (S) waves propagate sideways and up-and-down; they are slower due to friction.
    • Surface waves are large and the most destructive.

Lecture 3 Recap

  • Seafloor sediments are terrigenous, biogenous, hydrogenous, and cosmogenous.
  • Biogenous sediments are primarily siliceous and calcareous oozes.
  • Terrigenous sediments are mainly found near the continents; biogenous sediments show patterns in distribution based on ocean chemistry and productivity.
  • Sedimentation rate = change in depth / change in time.
  • Sediments can reconstruct changes in Earth’s climate and oceans through time.

Lecture 3 Details

  • CCD (Carbonate Compensation Depth):
    • Below the CCD there is NO CaCO3.
    • Above the CCD there is marine snow.
  • Four main types of sediments:
    • Two common:
      • Terrigenous: silt, clay, etc.; the most abundant by VOLUME.
      • Biogenous: marine snow; the most abundant by SURFACE AREA.
    • Two rare:
      • Hydrogenous: nodules formed by chemical precipitation from seawater, which sink to the bottom.
      • Cosmogenous: material from space, such as meteorites and asteroids.
  • Sedimentation rate:
    • Sediment age: volcanic ash can help date sediments through radiometric dating of the ash.

Lecture 4 Recap

  • Water has a high heat capacity because it is a polar molecule, allowing it to absorb or release energy without much change in temperature.
  • Melting ice requires 80 cal/g; vaporizing water requires 540 cal/g.
  • Evaporation cools the tropics, and the formation of sea ice warms the poles.
  • Colder, saltier water is denser and sinks to ocean depths, so the oceans are density stratified.
  • The oceans are blue because blue light travels deepest while other colors are absorbed near the surface.
  • Sound travels faster in water than in air.

Lecture 4 Details

  • Heat and temperature:
    • Latent heat is energy absorbed or released during a phase change without a change in temperature.
    • Latent heat of fusion = 80 cal/g: energy required to melt 1 gram of ice at 0°C. The same amount of energy is released when 1 gram of water freezes.
    • Latent heat of vaporization = 540 cal/g: energy required to vaporize 1 gram of water at its boiling point. The same amount of energy is released when 1 gram of water vapor condenses.
    • Temperature stays constant during phase changes because energy is used to change the state of matter rather than increase temperature.
    • Heating vs. cooling: phase changes work in reverse when heat is removed (melting and freezing; evaporation/boiling and condensation).
  • Density stratification:
    • Density = mass / volume.
    • Warm water is less dense (expands); cold water is more dense (contracts).
    • Higher temperature leads to greater volume and lower density; lower temperature leads to lower volume and higher density.
  • Salinity:
    • More salinity means more density, so it sinks to deeper water; less salinity means less density, so it stays in surface water.
    • Evaporation makes water saltier and denser; precipitation makes water fresher and less dense.
    • Brine is a salty liquid released from ice formation and is extremely dense.
  • Density layers:
    • Pycnocline: region of rapidly increasing density, making up about 18% of ocean volume (temperature and salinity control density).
      • Halocline: rapidly increasing salinity with depth.
      • Thermocline: rapidly decreasing temperature with depth.
  • Light:
    • Photic zone: where light reaches and photosynthesis occurs, so life exists here.
    • Aphotic zone: no light (except for some organisms), very little life.

Lecture 5 Recap

  • Typical salinity is 35‰, and the most abundant ion in the oceans is Cl−.
  • Salts come from erosion on land and outgassing; sediment formation and activity at hydrothermal vents remove salts from the ocean.
  • The ocean’s major element composition is at a steady state (chemical equilibrium), meaning input and removal fluxes are in balance.
  • Conservative elements have long residence times. Nonconservative elements have short residence times and are usually tied to biology.

Lecture 5 Details

  • Polarity (as well as temperature) makes water a good solvent for salts.
  • Solution = 2 solutes (solid/gas) + 1 solvent (liquid).
  • Seven main ions make up 99% of dissolved inorganic solids in seawater:
    • Chloride, sodium, sulfate, magnesium, calcium, potassium, bicarbonate.