1. Composition of Seawater Features of the Sea Floor
Oceans – Part I
Introduction
Composition of Seawater
Features of the Sea Floor
Sediments

2. Introduction The “Blue Planet”
Seventy-one percent of Earth’s surface is represented by oceans and marginal seas
Continents and islands comprise the remaining 29

4. The Oceans of Earth About 7% of the size of the Pacific
About half the size of the Pacific and not quite as deep
The largest and has the greatest depth
Slightly smaller than the Atlantic, largely a Southern Hemisphere body

5. Earth’s Northern & Southern Hemispheres

6. The Composition of Seawater

7. Composition of Seawater
Salinity
Salinity is the total amount of solid material dissolved in water
Typically expressed as 0/00 or parts per thousand (ppt)
350/00 is world average

8. Composition of Seawater
Salinity
Anion + cation = a salt
Salt = NaCl = Na+ + Cl-
Na+ has a positive charge (cation)
Cl- has a negative charge (anion)

9. Composition of Seawater
Salinity
What goes in must equal what goes out
Addition of sea salts
Chemical weathering of rocks on continents (cations)
Volcanic eruptions / outgassing (anions)

10. Composition of Seawater
Salinity
What goes in must equal what goes out
Addition of sea salts
Removal of sea salts
Sea spray
Evaporites
Biologic processes
Magma at M.O.R.

11. Important Factors:
Precipitation
Evaporation
Freezing
Run-off

12. The Ocean’s Layered Structure
Temperature and salinity change with depth in the oceans
A three-layered structure exists in the open ocean

13. The Ocean’s Layered Structure
Temperature and salinity change with depth in the oceans
Salinity variations with depth correspond to the general three-layered structure described for temperature
Thermocline = zone of rapid temperature change
Halocline = zone of rapid salinity change

14. Features of the Ocean Floor

15. Mapping the Ocean Floor
Bathymetry – Measurement of ocean depths and the charting of the shape or topography of the ocean floor

16. Mapping the Ocean Floor
Echo sounder (also called sonar)
Invented in the 1920s
Primary instrument for measuring depth
Reflects sound from ocean floor

17. Mapping the Ocean Floor
Multibeam sonar
Employs and array of sound sources and listening devices
Obtains a profile of a narrow strip of seafloor

18. Mapping the Ocean Floor
Measuring the shape of the ocean surface from space
Employs satellites equipped with radar altimeters

19. Mapping the Ocean Floor
Three major topographic units of the ocean floor

20. Continental Margins Classifications of ocean floor
Continental Margins – the submerged outer edge of a continent
Ocean Basin – the deep seafloor beyond the continental margin

21. Continental Margins There are two types of continental margins:
Passive margins
Also called Atlantic-type margins
Face the edges of diverging tectonic plates.
Very little volcanic or earthquake activity

22. Continental Margins There are two types of continental margins:
Active margins
AKA as Pacific-type margins
Located near the edges of converging plates.
Active margins are the site of volcanic and earthquake activity.

23. Passive Continental Margins
EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens fig 18.4
Continental Shelf
Shelf Break
Continental Slope
Submarine Canyons
Continental Rise

24. Passive Continental Margins
Continental Shelf
Found at margins of continents
Relatively flat
Average km in width
Formed by:
Thick accumulations of shallow-water sediments
Some areas are mantled by extensive glacial deposits
Ends at change in slope (Shelf break)
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25. Passive Continental Margins
Continental Slope
Boundary between continental crust and oceanic crust
Extends from shelf break to rise
Submarine canyons are major features
Submarine canyons formed by:
River erosion
Turbidity currents

26. Submarine Canyons and Turbidity Currents

27. Graded Beds
Each layer grades from coarse at its base to fine at the top.
EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens fig 6.21
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28. Passive Continental Margins
Continental rise
Found in regions where trenches are absent
Located where the continental slope grades into a more gradual incline as it meets the deep ocean floor
Deep-sea fans form here
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29. Active Continental Margins
Located primarily around the Pacific Ocean
Continental slope descends abruptly into a deep-ocean trench
Accumulations of deformed sediment and scraps of ocean crust form accretionary wedges
Some subduction zones have little or no accumulation of sediments

30. Active Continental Margins
Deep-ocean trenches
Long, relatively narrow features
Deepest parts of ocean
Most are located in the Pacific Ocean
Sites where moving lithospheric plates plunge into the mantle
Associated with volcanic activity
Volcanic islands arcs
Continental volcanic arcs

31. An Active Continental Margin

32. Deep-Ocean Basins Features of the deep-ocean floor Oceanic Ridges
Hydrothermal Vents
Abyssal Plains and Abyssal Hills
Seamounts and Guyots
Trenches and Island Arcs

33. Deep-Ocean Basins Ridges and Rises Underwater volcanic mountain chain
extends for 65,000 km
1,000 km wide
1,000-2,000 m high
South West Indian Ridge

34. Deep-Ocean Basins

35. Deep-Ocean Basins Ridges and Rises Ridges = steep slopes
Rises = gentle slopes
Contain central rift valley
15-50 km wide
m deep
Offset by fractures

36. Deep-Ocean Basins
Hydrothermal vents are sites where superheated water containing dissolved minerals and gases escapes through fissures, or vents.

37. Deep-Ocean Basins Abyssal hills Seamounts
Flat areas of sediment-covered ocean floor found between the continental margins and oceanic ridges.
Also small, extinct volcanoes or rock intrusions near the oceanic ridges.
Seamounts
Volcanic projections from the ocean floor that do not rise above sea level.
Flat-topped seamounts eroded by wave action are called guyots.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

38. Seamounts & Guyots
Thurman Essentials of Oceanography 6/e

39. Deep-Ocean Basins: Trenches
Mariana trench the deepest (11,020 m)
Peru-Chile trench the longest ( 5900 km)
EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens
Distribution of the world's oceanic trenches, ridge system, fracture zones, and transform faults. Where transform faults offset ridge segments, they permit the ridge to change direction (curve) as can be seen in the Atlantic Ocean.

40. EARTH: An Introduction to Physical Geology 7th ed / Tarbuck & Lutgens

41. Ocean Sediments

42. Seafloor Sediments Particles entering the ocean Accumulate
Rapidly on continental margin (neritic)
Slowly in the deep ocean (pelagic)
Reflect ocean history

43. Seafloor Sediments Thickness varies
Thickest in trenches—Accumulations may approach 10 kilometers
Pacific Ocean—About 600 meters or less
Atlantic Ocean—From 500 to 1000 meters thick
Mud is the most common sediment on the deep-ocean floor

44. Seafloor Sediments Sediment can be classified by particle size.
Waves and currents generally transport smaller particles farther than larger particles.

45. Seafloor Sediments Types of seafloor sediments Lithogenous sediment
Material weathered from continental rocks
Sands and muds from continental margins
Glacial deposits
Clays
Fine particles remain suspended for a long time
Oxidation often produces red and brown colored sediments

46. Sediment Sources: Terrigenous Sediments
Thurman Essentials of Oceanography 6/e fig 4.06
Most clay-sized quartz in deep-sea sediments is believed to have been transported by wind. Blue arrows indicate prevailing wind. After Leinen, M., et al Distribution of biogenic silica and quartz in recent deep-sea sediments. Geology 14:3,

47. Seafloor Sediments Types of seafloor sediments Lithogenous sediment
Biogenous sediment
Ooze = greater than 30% biogenous sediment
Distribution related to sediment supply, rate of dissolution and sediment dilution
Siliceous fossils at all depths, calcareous not below Calcium Carbonate Compensation depth
Diatoms and radiolarians = major siliceous microfossils
Calcareous = foraminifera, pteropods, coccolithophores

48. Biogenous Sediment Accumulation
Thurman Essentials of Oceanography 6/e fig 4.15
Relationships among carbonate compensation depth, the mid-ocean ridge, sea floor spreading, productivity, and destruction allow calcareous ooze to be found below the CCD.

49. Seafloor Sediments Siliceous Oozes Fine-grained pelagic deposit
Composition:
30% siliceous (SiO2) material of organic origin
Diatoms (phytoplankton) and Radiolaria (zooplankton)
Siliceous particles dissolve more slowly than calcareous particles
siliceous ooze
Fine-grained pelagic deposit of the deep-ocean floor with more than 30% siliceous material of organic origin. Radiolaria and diatom remains are the major constituents of the siliceous oozes, which tend to occur at depths in excess of 4500 m.
A Dictionary of Earth Sciences, © Oxford University Press 1999 (

50. Siliceous Oozes Thurman Essentials of Oceanography 6/e
Siliceous ooze accumulates on the ocean floor beneath areas of high productivity, where the rate of accumulation of siliceous tests is greater than the rate at which silica is being dissolved.

51. Diatoms Composed of SiO2 Phytoplankton Base of food chain

52. Seafloor Sediments Calcareous Oozes
Wide-spread in relatively shallow areas of the deep sea
CaCO3 particles dissolve at “Carbonate Compensation Depth” = (CCD)
Atlantic: ~ 4,000 m
Pacific: ~ ,500 m

53. Biogenous Sediment Accumulation
Thurman Essentials of Oceanography 6/e fig 4.15
Relationships among carbonate compensation depth, the mid-ocean ridge, sea floor spreading, productivity, and destruction allow calcareous ooze to be found below the CCD.

54. Radiolaria Composed of SiO2 Zooplankton Base of food chain

55. Foraminifera
Composed of calcium carbonate (CaCO3)
Zooplankton

56. Seafloor Sediments Types of seafloor sediments Terrigenous sediment
Biogenous sediment
Hydrogenous sediment
Minerals that crystallize directly from seawater
Most common types include
Manganese nodules
Calcium carbonates
Metal sulfides
Evaporites

57. Seafloor Sediments Ferromanganese Nodules
First sub-marine ferromanganese concretions were discovered in 1868 on the Kara Sea (Russia)
Characteristics - small balls (lightly flattened), dark-brown, and cm in diameter
Found at depths of 4,000 to ,000 m
Not clear how these nodules form

58. Distribution Of Manganese Nodules
Thurman Essentials of Oceanography 6/e fig 4.23
This generalized map shows the distribution of manganese nodules on the sea floor. After Cronan, D. S Deep sea nodules: Distribution and geochemistry, in Glasby, G. P., ed., Marine Manganese Deposits, Elsevier Scientific Publishing Co.

59. Seafloor Sediments
Evaporites are precipitates that form as water evaporates or as the conditions in the water change
include many salts with economic importance.
currently form in the Gulf of California, the Red Sea, and the Persian Gulf.
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

61. Studying Sediments How do scientists study sediments?
Deep-water cameras
Clamshell samplers
Dredges
Piston Corers
Core libraries
Seismic profilers

62. Studying Sediments What can scientists learn by studying sediments?
Historical information
Location of natural resources, especially crude oil and natural gas

63. End