chapter 4: ocean basins (ocean basins)

the ocean floor is mapped by bathymetry

  • previously people measured bathymetry using rope and a stone tied to it, by lowering the stone down into the water and waiting till it hit the bottom. after that people moved on to echosounding, where sound is bounced off of the seafloor and reflected back onto the ships computer. then people started using satellites to measure the dips and rises in the ocean surface to map out seabed topography

  • echosounders use 121 beams radiating from the ships hul, which are fanned out at right angles to the direction of travel. a ping is sent every 10 seconds toward the seabed and the refleected sound is recorded from narrow corridors

  • satellites measure divots and rises on the ocean surface to measure the topography of the seafloor. there is usally distrotion of the sea surface above the seabed when extra gravitational attraction of the feature pulls water toward it from the sides, forming a mound of water over itself.

ocean floor topography varies with location

  • the submerged outer edge of the continent is called the continental margin

  • the deep sea floor beyond the continental margin is properly called the ocean basin

  • the ocean basin would have the dip from the continental shelf tot eh rest of the sea bed. but in between the shelf and the floor there are sea mounts, ridges, canyons, and other strucutres, making it just a simple bowl strucutre, but more like how terrestrial topography is, but underwater

  • it is the continental margin and the ocean basin. the contitnetal margin is the outer edge of the continent and the ocean basin id teh deep sea floor beyond teh continental margin

  • seabed topography results from plate movement and isostatic balance

continental margins may be active or passive

  • passive margins

    • form where the plates are moving apart (diverging boundaries)

    • characterised by broad continental shelves, thick sediments, and gentle slopes

    • little tectonic activity

    • common around the atlantic ocean

    • features:

      • broad, gently sloping shelf extends far from shore

      • extends from shore ina gentle angle

      • 350 km in width and end at a depth of about 140m, where steeper drop-off begins

  • active margins

    • forms where plates collide (converging) or slide past one another (trasnfroms boundaries)

    • narrow shelves, steep slopes, deep ocean trenches often present high tectonic activity

    • common around teh pacific ocean (pacific ring of fire)

the topography of deep ocean basins

  • continental slopes connect the continnetal shleves to the deep ocean floor

    • continental slopes are formed by sediment accumulating at hte helf edge

    • sediments are trasnported dwonslope to the ocean floor

  • contintnetal rises form as sediments accumulate at the base of the continental slope

    • these sediments are transported by turbidity currents

    • earthquakes can trigger underwater landslides or sediment flows called turbidity currents

  • the sediments on the deep ocean floor reflect the history of the surrounding continents, the biological productivity, and the ages of the basins

  • ridges occur along divergent plate boundaries, where new ocean floor is created as teh earths tectonic plates spread apart

  • spreading rate of the plates control topography

    • as the plates separate, molten rocks rises to teh seafloor producing enormous volcanic eruptions of basalt

    • spreading speed shapes the ridge

    • slow spreading → steep rugged terrain

    • fast spreading → wide gentle slopes

  • hydrothermal vents are hot springs on active oceanic ridges

abyssal plains (and abyssal hills) cover most of earth’s surface

  • abyssal plains are flat featureless expanses of sediment covering the ocean floor

  • they lie between the contitnetla margins and the oceanic ridges

seamounts and oceanic trenches

  • seamounts → volcanic projections that do not rise above the surface of the sea

  • trench → an arc-shaped depression in the deep ocean floor. They occur where a converging ocenaic plate is subducted

trenches and trash

  • human impacts in the trench

    • platic bag

    • snailfish and drifiting into the mariana trench carry mercury pollution with them which has led to mercury pollution in this area which was thought to be pristine becuase of this depth

  • how would scinetists explore the deep and detect human impacts

    • sunmersibles and ROVs

major features of ocean basins

  • neritic zone: shallow water just above the continental shelf

  • oceanic zone: deep water beyond the shelf

    • epipelagic → sunlit surface layer

    • mesopelagic

    • bathypelagic

    • abyssopelagic

  • two major differences between neritic and oceanic zones that affects biota

    • nutrients are high in the neritic zone, coming in from river inputs, upwelling, and resuspension of seidments. in the oceanic zone there are lower amounts of nutrients because its farther from land + stronger stratification. the neritic zone has high primary production; ocenaic zone has low surface production

    • light is absorbed more quickly in neritic waters because of higher particle loads and turbidity. there is less blue light penetration, and a shallower photic zone. in oceanic waters, its more clear, so blue light penetrates the deepest. this leads toa deeper photic zone, affecting photosynthetic organisms and visual predators